Control & Inst_ V-I

CONTENTS

SI. No Topic Page No.

1. Concept of Instrumentation in TPS 1

2. Pressure Measurement and Measuring Instruments 17

3. Level Measurement and Measuring Instruments 51

4. Flow Measurement and Measuring Instruments 107

5. Temperature Measurement and Measuring Instruments 147

6. Pneumatic Instruments 171

PAGE 3

1.1 INTRODUCTION

Thermal Power Stations employ a great number of equipment performing number of

complex processes, the ulthnate ahn being the conversion of chemical energy into

Electricity. In order to have stable generating conditions, always a balance is

maintained that Heat input = Electricity output + losses. But this balance is frequently

disturbed due to (i) grid troubles external to the process and machines, (ii) the troubles

in the process itself or (iii) the troubles in the equipments. When the balance is

disturbed, all the process variables deviate from their normal valves thus creating the

necessity for the following:

i) Instruments: To measure and indicate the amount of deviations.

ii) Automatic Control: To correct the deviation and bring back to normalcy.

iii) Annunciation: To warn about the excessive deviations if any.

iv) Protection: To isolate the equipments or process from dangerous operating

conditions caused due to such excessive deviations.

The scope of this write up is to acquaint the Power Station personnel with the above

systems.

1.2 POWER STATION INSTRUMENTATION

1.2.1 The proportionate cost of instrumentation during seventies was about 2.3 to 2.5% of

the total cost of boiler, turbine and their Auxiliaries. When the unit sizes were 60/100

M.W. But this has become about 7% when the unit size has become 210 M.W. and is

expected to reach about 10-12% or even higher in the near future for the same

capacity units. This in'-rease in instrumentation cost is due to -

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i) Increase in unit capacity, making the unit to operate at higher parameter for

economic reasons.

ii) New inventions, improvements, modernization of instruments and equipments.

iii) Expected change in the duty cycles of the boiler and turbine facilitating two shift

operation, quick run up etc.

iv) Improved awareness among the personnel about the utility of the instruments.

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1.2.2 TYPES OF INSTRUMENTS

This discussion is only on the process instrumentation measuring the physical quantities

such as temperature, pressure, level flow etc. The other type of instruments are the

electrical instruments measuring electrical quantities such as current, voltage etc. The

different type of instruments normally in use are given below.

1.2.2.1 INDICATORS

Indicators are of two categories local and remote. Local indicators are self contained

and self operative and are mounted on the site. The remote indicators are used for

telemetering purposes and mounted in the centralised control roomor control panel.

The indicators both local and remote are sometimes provided with signalling contacts

whereever required. The remote indicators depend upon electricity, electronics,

pneumat 1ic or hydraulic system for their operation and accordingly they are named.

The indicators can be classified as analogue or digital on the basis of final reading.

Indicators are available for single point measurement or can be connected to a number

of points through selector switches or automatic scanner system. This multipoint

system considerably reduces the number of instruments without affecting the

measurements much.

1.2.2.2 RECORDERS

Recorders are necessary wherever the operating history is required for analysing the

trends and for any future case studies or efficiency purposes. Recorders can be of

single point measuring a single parameter or multipoint measuring a number of

parameters by a single instrument. Multipoint recorders are again categorised as

multipoint continuous recorders or multipoint dot recorders. The multipoint dot

recorders select the point one after the other in sequence whereas the continuous

recorders measure simultaneously all the points.



1.3 PRESENTATION OF INFORMATIONS

Enormous amount of information measured and received from the various parts of the

plant/process are to be presented to the operators giving appropriate hnportance to each

one. In order to have an easy and effective presentation, the information is generally

grouped into the following three categories.

1.3.1 Vital informations which is required by operators at all times for the safe operation of

the plant. The information is presented through single point indicators/recorders,

placed on the front panels. Main steam pressure, temperature, condenser level, vacuum,

drum level, furnace pressure etc. are some such parameters.

1.3.2 The second group of information is generally not vital under the normal operation of

the plant. But this becomes vital whenever some sections of the plant start

malfunctioning. Such needs are met through multipoint indicators/recorders placed in

the front panels. Temperature and draft across the flue gas path, bearing temperatures

of the motors of fans etc. are some such examples.

PAGE 5

1.3.3 The last group of information is not required by the operators but for the occasional

need of the efficiency engineers. These informations are given by recorders mounted

on back panels or local panels. D.M. make up quantity, fuel oil flow quantities etc. are

some examples.

1.4 CODING OF INSTRUMENTS

In order to distinguish the parameters required from the other instantly, a shape coding

of instrument face is being adopted in some advanced countries. This is an useful

practice and may find place in other power stations also shortly. Coding may vary as

per the practices of the organisation. A general approach could be as below:

Level instruments - Horizontal edgewise

Temperature instruments - Horizontal edgewise

Pressure instruments Circular

1.5 ARRANGEMENT OF INSTRUMENTS

Another welcome feature of instrumentation in advanced countries is the 'Master Panel

arrangement. In this arrangement, instruments measuring important parameters are

provided in one panel. All instruments in this panel will be of circular shape with



normal rated value marked at 12 o'clock position. Any deviation from the rated values

will be quickly noticed by the operators.

1.6 SELECTION OF INSTRUMENTS

Instrument engineers are required to work in close association with the system design as

well as the equipment design engineers in selecting instruments and sensing systems.

After deciding the capacity of the Thermal Power Station the designs of Boiler turbine

and auxiliary equipments such as mills, pumps, fans deaerator, feed heaters etc. are

taken up.

Based on the design of the main and auxiliary equipments, the parameter values for

efficient and economic operation at determined load are specified. The instrument and

system design engineers decide the location for the measurement of various parameters

such as level, pressure, flow, differential pressure, temperature etc. based on the system

design and layout conditions.

Then the instrument engineers select the appropriate instruments influenced by the

following factors:

i) Range of measurement

ii) Required accuracy of measurement

iii) The form of final data display required

iv) Process media

v) Cost

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vi) Calibration and repair facilities -equired/available

vii) Layout restriction,,;

viii) Maintenance requirements/availability

1.7 CONCEPT OF INSTRUMENTATION IN THERMAL POWER STATION

The concept of instrumentation is:-

i) Instruments should be independent for their working.

ii) The total instrumentation should be interdependent to each other in assessing

the process conditions.

iii) Instrumentation should be sufficient to provide adequate information to the

operators for -

a) Cold start of the unit



b) Warm/hot start of the unit

c) Shut down, both planned and emergency shut down.

1.8 POWER STATION INSTRUMENTATION

The process conditions and the equipment conditions are to be assessed by the operators

from the information received from the various instruments. The instruments and range

vary very widely as per the process media. The following section deals with these

instruments. The interdependence and inter relations of the instrument readings play

very significant roll in the stability and the efficiency of the heat balance.

1.8.1 TEMPERATURE MEASURING INSTRUMENTS

Accurate measurement of temperature is required to assess the material fatigue, heat

balance, heat transfer etc. The measurement ranges from ambient temperature viz. of

air at inlet to F.D. fan to 13000C to 14000C inside the furnace zone. Temperature

measurement is to be made for many media such as, water/steam, oil (fuel oil and

lubricating oil), air, flue gases, hydrogen gas, metal temperatures of bearing babbits,

turbine top and bottom, generator winding and core,-,, S.H. tube metal etc.

Filled system thermometry such as mercury in glass, mercury in steel, vapour filled or

gas filled areused for local indication of temperature. Bimetallicthermometers can also

be used for local indication. The selection of thermometer depends upon the range of

the temperatures to be measured. These instruments are available with electrical

contacts for setting up annunciation and protection system wherever required.

PAGE 7

measurement of temperatures depending upon the range. Resistance thermometers are

of platinum and copper resistance type. Platinum resistance thermometers are.

calibrated to have 46 ohms or 100 ohms at OOC while copper resistance thermometers

of 53 ohms at OOC. The secondary instruments used in conjunction are cross coil

indicators or electronic bridges. These instruments indicate temperature by measuring

the nature of resistance which changes with the change in temperature. Resistance

thermometers are generally used upto 300"C.

Above 3000C, thermocouples are used as primary sensors. The common type of

thermocouples used in thermal power stations are chromel-alumel or chromel-copel

depending upon the temperature. Iron-constantan is another therino couple in use. The

secondary instruments for thermocouple sensors are pyrometric miwvolt meters or



electronic potentiometers. Null balance method is used for the very accurate

measurement of millivolts generated by thermocouples sensing the process

temperatures.

The electronic bridges and potentiometers can be either indicators, or indicator cum

recorders with alarm/protection contacts and with remote transn-dssion facilities.

1.9 PRESSURE MEASURING INSTRUMENTS

The pressure measurement in Thermal Power Station ranges from 1 kg/ci& (nearly) at

condenser to hydraulic test pressure of boiler. Here again many media exist such as

steam/water, lubricating oil, fuel oil, air, flue gases, hydrogen etc.

For local indication of pressure and differential pressure, bourdon tube and diaphragm

type gauges or liquid manometers are used. Remote measurement of pressure is done

by transmitters either electric/electronic or pneumatic coupled with a secondary

instrument indicator/recorder. Many varieties of transmitters are in use. In these

transmitters the mechanical movement of sensing elements such as bourdon, bellows,

diaphragm etc. due to the pressure causes an electrical property change such as current,

voltage, resistance, capacitance, reluctance inductance etc. which is utilised to measure

pressure through the secondary instruments. The secondary instruments are either

indicators or recorders. Which may incorporate signalling contacts.

1.10 LEVEL MEASUREMENT

Level measurement is generally carried out as differential pressure measurements. In

powerstations, level measurement in open tanks such as, D.M. water storage and fuel

oil and lub oil tanks and in closed tanks such as deaerator, condenser hot wall. boiler

drum and L.P. & H.P. heaters are to be made. Gauge glasses and floats are used for

local indication of levels and the transn-dtters used for measuring the differential

pressure along with the secondary instruments are used for remote level measurements.

The measurement of boiler drum poses many problems because of varying pressures

and temperatures which continuously change the density of media. Thus many

computations and corrections are necessary to be made in order to get correct levels. A

recent development in this area is thehydra step', though it is very costly it hnproves

PAGE 8

the accuracy and reliability of this,, @ asurement.



Other problem area is the level measurement pertaining to solid media, such as the level

of the raw/pulverised coal bunkers and dust collectors hoppers. In these cases.

continuous level measurement is not possible. However, fairly reliable and accurate

provisions are available to indicate the extreme levels on either directions (low or high).

The nucleonic level gauges or the capacitance and resistance type sensors serve these

areas very well.

1.11 FLOW MEASUREMENT

Flow measurements of solids, liquids and gases are required in Thermal Power Stations

for carrying out safe and optimum operation. The liquid flow measurements can be

made within reasonabe accuracy, but for the gas flow measurement cannotbe so carried

out accurately. The water flow measurements are done fairly easily and accurately

whereas steam flow measurement requires density correction under varying pressures.

The air and flue gas flow measurements suffer accuracy and reliability due to variation

in pressure, temperature, duct leakage, dust accumulation etc. The flow measurement

of solids is very difficult and only a rough idea is arrived (such as the P.F. flow)

through inferential means.

In power stations flow measurements are based on inferential principles. Differential

pressures are created by placing suitable throttling devices in the flow path of the fluids

in the pipes/ducts. The throttling devices are suitably selected depending upon the

media, flow quantity etc. from among orifice, venturi, flow nozzle, dall tube etc. The

differential pressure developed across such sensing devices is proportional to the square

of the flow quantity. The differential pressure is measured by the devices discussed in

(section) 4.2.1 with additional square root extraction facilities.

1.12 ANALYRICAL INSTRUMENTS

Apart from the above there are a few special measurements necessary in thermal power

generation plants of high capacities. These include feed water quality control

instruments such as -for measurements of conductivity, PH, dissolved oxygen, and

sodium parameters, steam quality control instruments, such as conductivity, silica and

PH analysers. The combustion quality is assessed by the measurement of the

percentage of oxygen, carbon monoxide or carbon dioxide in the flue gases. The purity

of hydrogen inside the generator housing is measured by utilising the thermal

conducting capacity of the hydrogen gas.



The water and steam purity is measured as the electrolytic conductivity by electronic

bridge method in which one arm form the electrodes of conductivity cell dipped into the

medium.

The percentage volume of oxygen in combustion gases is measured by utilising the

paramagnetic properties of oxygen. The carbon monoxide percentage is measured by

the 'Absorption of Electromagnetic radiation principle. Both these gas analysers require

elaborate sampling and sample conditioning system resulting in poor reliability and a

bilability of these measurements. Recent developments in these field

PAGE 9

have brought out on line 'in situ' instruments for these two parameters where the

problem of sampling is dispensed with.

The 'Analytical Instruments' as the above instruments are called had been the

neglected lot so far in the power stations. But now the authorities seem to think their

importance for the optimum operation of the stations.

1.13 TURBOVISORY INSTRUMENTS

The turbov,isory instruments have become very important for modem day turbines

where the n-taterials have been stressed nearer to the yield points and the internal

clearances have become minhnum. Shaft eccentricity, vibration (both shaft and bearing

pedestal), differential expansion of shaft and cylinders, over all thennal expansion of

the cylinder, speed, & axial shift etc. are some of the turbovisory measurements. These

all measurements are interrelated and interdependent.

1.14 LIST OF INSTRUMENTS

All the measurements discussed above and their correct interpretation enables the

operators to check and watch the behaviour of the process and the equipments and take

necessary corrective actions in time.

A typical list of important measurements carried out in thermal power station is given

below.

1.14.1 TEMPERATURE

a) Steam temperatures at boiler outlet, super heater stages, steam legs before ESVS,

IVS at ESVS, IVS and at turbine curtis wheel-indicators/indicator

recorders with alarm and protection ,in control room.



b) Steam temperature at turbine H.P. cylinder outlet, hot reheat and exhaust hood

temperatures.

c) Metal temperature of turbine casing and metal temperatures of super-heaters and

reheaters-indicators, indicator cum recorder in U.C.B. with multipoint selection.

d) Temperature of condensate/feed water along the flow path from condenser to

Drum.

e) Flue gas temperature in various zones of boiler-indicator and indicator cum

recorder in control room.

f) Air temperature at inlet and outlet of air preheater.

g) Turbine bearing oil drain temperature-indicator cum recorder in U.C.B.

PAGE 10

h) Generator winding and core temperature-indicator cum recorders in control

room.

i) Temperature of auxiliary equipments bearings such as mill ID, FD and PA fans

etc. indicator cum recorder in U.C.B.

1.14.2 PRESSURE

a) Condensate pressure after condensate pumps and before the ejectors-indicator in

U.C.B.

b) Deaerator pressure-indicator ckim recorder in U.C.B. with electrical contacts for

interlocking facilities.

c) Feed water pressure after feed pumps - individual indicators for each pump.

d) Feed water pressure before and after feed regulating stations-indicators in U.C.B.

e) Drum pressure-indicator cum recorders in U.C.B. with alarm signalling facili6M

f) Super-heater steam pressure at boiler outlet 2 Nos. indicators one for each side in

U.C.B. and at local with alarm protection facilities. Measurement is done at

the ou of superheater and before boiler stop valves.

g) Steam pressure - 1 No. indicator cum recorder one of the lines before turbine

stop valve in U.C.B.

h) Steam pressure at emergency stop valves and IVS.

i) Steam pressure after control valves indicators in local panel for each valve.

j) Steam pressure at curtis wheel-indicator cum recorder in U.C.B. with alarm

contacts.



k) Steam pressure in H.P. turbine exhaustindicatorinU.C.B. for cold reheat steam.

l) Vacuum in condenser indicator cum recorder in U.C.B. with alarm facilities an(

separate vacuum relay for protection.

m) Hot reheat pressure indicator in U.C.B. with signalling contacts.

n) Steam pressure at the exhaust of I.P. cylinder-indicators in local panel.

1.14.2.1 PRESSURE, FUEL AND LUBRICATING OIL

a) Heavy oil pressure-indicators in U.C.B. with signalling contacts. Measurement

is made before and after pressure regulating valves.

PAGE 11

b) Light wann up oil pressure indicators in U.C.B. with signalling contacts.

Measurement is made before and after the flow control valves.

c) Ignitor oil pressure-indicator in U.C.B.

d) Governing oil pressure-indicator in U.C.B. with signalling contacts.

e) Lubricating oil pressure-indicator in U.C.B. Measurement is made after oil

coolers.

1.14.2.2 PRESSURE: AIR FLUE GAS

a) , Air pressure-indicators in U.C.B. before and after air heaters for secondary air.

b) Indicators in U.C.B. before and after air heaters for primary air.

c) Wind box pressure indicators in U.C.B.

d) Furnace draft-indicators and recorders in U.C.B. (Measurement is made

averaging left and right side drafts).

e) Flue gas draft before and after econon-dser-indicators in U.C.B.

f) Draft after air-heaters indicators in U.C.B.

g) ID fan suction-indicators in U.C.B.

1.14.3 LEVEL MEASUREMENT

a) Drum level-indicators and indicators cum recorders (total 3 Nos. from different

tapping) in U.C.B. with alarm and protection facilities. Normally 3 types of

measurement are adopted.

i) Local gauge glass

ii) Remote gauge glass and

iii) Remote indirect measurement



b) Drip level in H.P. and L.P. heaters-indicators in U.C.B. with alarm and

protection facilities.

c) Condensate level in condenser-indicator in Lf.C.B. with alarm facilities.

d) Deaerator level-indicator in U.C.B. with signalling contacts for alarm and

protection.

e) The various storage tank level such as D.M. water fuel oil, lubricating oil etc. are

measured by local direct gauge glasses.

PAGE 12

1.14.4. FLOW

a) Condensateflowtodeaerator-indicator/recorderinU.C.B.withintegratorunit for

totalizing in two locations (i) between air ejectors and L.P. heater No. 1 and

(ii) between the final L.P. heater and deaerator.

b) Feed water flow indicator/recorder in U.C.B. with integrator unit. Measurement

is made between final H.P. heater and feed regulating valves.

c) Superheated steam flow 2Nos. indicators cum recorders one for eachpipewith

integrator unit in U.C.B.

d) Re-heater steam flow-2 Nos. indicators cum recorders one for each side of the

boiler. Measurement is made at the inlet to reheater.

e) Air flow-2 Nos. indicators cum recorders one for each FD fan in U. B.

Measurement is made at the discharge of the FD fans.

f) Fuel Flow

The fuel oil flow to the unit is given by two indicators cum recorders in U.C.B. one

measuring the oil in the incoming line and the other in the return line. Normally the

coal flow is measured for the whole station by the belt conveyor weighers.

1.15 AUTOMATIC CONTROL

The importance of maintaining a balance in the process was discussed under para 1

whenever the process gets disturbed due to the deviation of process elements, these are

to be brought back to the balance condition. Since lot of process elements are involved,

and disturbances are very frequent, the correction can be carried out efficiently and

quickly only by the introduction of automatic control system eliminating any possible

human error. The following are the important automatic control loops in the Thermal

Power Station.



1.15.1 AUTOMATIC BOILER CONTROL

i) Steam pressure always called as Boiler Master Control.

ii) Combustion control.

iii) Furnace draft control.

iv) Boiler feed regulation or drum level control.

v) Super heater/ reheater steam temperature control.

vi) Auxilliary steam pressure control.

vii) MW group control

viii) Feed pump speed control.

PAGE 13

1.15.2 TURBINE AUTOMATICS

i) Condenser hot well level regulation.

ii) Drip level control in L.P. and H.P. heater.

1.16 SEQUENCE CONTROL AND INTERLOCKING SYSTEM

A power station is a combination of many individual equipments and systems and for

better performance itrelies upon the performance of these individual equipments. These

equipments are interdependent and interrelated with each other, and therefore they are

to operate in coordination with each other. Electrical interlock system connect these

individual equipments and operate then with required sequences. For example boiler is

a system comprising mifling-plant, ID fans, FD fans, PA fans etc. These equipments

are interlocked in such a way that they are started /shut down in specific sequences in

order to avoid damage to equipments and men. For example in a n-dlling system the

coal feeder is interlocked such a way that it will not start unless its succeeding system

to crush and discharge the coal into the furnace such as exhauster/ P.A. fan & one mill

are in operation. These schemes may vary little with different manufacturers but

generally all p.f. and oil fired boilers have common sequences.

Also equipments are so interlocked that in case the failure of the running equipment to

discharge its functions, the reserve one is automatically put into service. For example

in case a feed pump which is running fails to meet the demand of the boiler, the

interlock system will put the reserve pump into service to meet the demand. As the unit

size increases the number of interdependency of operations increases. A system of

automatic sequence control simplifying the operator's duty has come into existence.



1.17 REMOTE CONTROL AND OPERATION OF EQUIPMENTS

As discussed earlier that power station comprises many equipments, it became

necessary to operate them from a centralised room. The indirect way to remote

operations came into practice. A very low voltage level such as 1 1 0 V or 240 V AC /

D C is used to dose a breaker of the electrical motors. The low voltage switches are

usually provided on the operating desks in the control room. Where D.C. is used for

station batteries are provided Ss standby.

1.18 DATA ACQUISITION AND DATA LOGGING

The conventional central control room is rathera cumbersome system. Large number of

instruments are to be observed to know what is happening inside the plant. The data

acquisition simplifies this job by collecting all the measurements as transmitted from

the process, converting them into digital term and storing in the memory bank. The

periodic logging of parameter by the operators are dispensed with after the introduction

of data acquisition system, which prints out the periodic conditions on predetermined

time intervals. An the important measurements at one time are printed along a row.

Data loggers thus reduce the use of graphical recorders.

PAGE 14

Since data logging gives too many measurements at a time, it cannot be easily digested

by the control staff. Now data reduction systems are finding their use where only the

process quantity deviated from normal value is shown.

1.19 VISUAL DISPLAY UNIT (V.D.U.)

Visual display units go along with the data acquisition system. In V.D.U. preselected

schemes, flow paths with parameters, running alarm conditions etc. can be brought on

colour television tubes on demand. This gives the life picture of the happening inside

the plant making the operation easy and effective.

1.20 AUTOMATIC TURBINE RUN UP

Turbine start up is the most difficult operation. Each start up will be different. Many

parameters and procedures have to be scrupulously adhered to as an error in human

decision will result in heavier damage to the unit. Therefore modem day machines of

higher capacity or machines which are to be started frequently are to be provided with

automatic run up, synchronizing loading gears.



Under normal condition this gear will accelerate the. turbine from barring gear speed to

full speed uninterrupted at a rate determined by the initial turbine temperature

conditions. Under hot start conditions this run up period may be of the order of 5 to

10minutesandcomprisetwozones,thefirsthalfisrelativelyslowrateuptolOO0rpm, the

second half is a fast rate from 1000 to 3000 rpm (a range which includes critical

speeds). In the event of abnomial conditions this programme of acceleration is

temporarily held (except in the regions of critical speed), or in more severe conditions,

revers& or tripped.

The abnormaxl conditions are monitored by the turbovisory instruments earlier in

section 13.

Automatic synchronising is also effected by the gear if selected for the function.. This

scheme matches the frequency voltage and time phase of the generator output to that of

existing busbar and close the circuit breaker.

Automatic loading gear enables the machine to be loaded automatically at the selected

rate, through the control of governor speed motor. The supervisory gear also will be in

use during this flmction. The rate of loading varies widely from 5% per minute during

the initial block loading to 20% M.C.R. and also conditions of the plant when the cold

or hot start.

1.21 SCANNING SYSTEMS

In a complex process extending over a considerable area lot of messages are transmitted

to and from the process. These transmission channels are quite expensive and there

may be danger of loss of data owing to confusion of signals by extraneous electrical

noises. Digital coded transmission less sensitive to such noise, is found useful Hence

all the process signals are in analogue form scanned in sequence one

PAGE 15

at a time converted into digital form and transmitted to the central information system

for display or control purposes.

1.22 BURNERMANAGEMENT

For higher capacity boilers, fuel firing rate is also higher. Explosion can occur within 1

to 2 secs of fuel accumulation. Therefore leaving the management of fuel firing to the

operators will not be appropriate because human reflexes are slower. A complete



automatic burner management system called furnace safeguard supervisory system

(FSSS in short) has been introduced to manage the present day boilers.

This system takes care that every increment of fuel input corresponds to the available

ignition energy inside the furnace.

The following functions are entrusted to such an automatic burner management

System:

i) Fuance purge supervision

ii) Ignitor control

iii) Warm up oil control

iv) Pulverizer control

v) Secondary air damper control

vi) Flame scanner intelligence

vii) Boiler trip protection.

viii) And also condition of the plant whether the cold or hot start

The above discussion gives some synopsis of the instrumentation in Thermal Power

Station. Detailed descriptions of working principles and philosophy are discussed in

the subsequent sections.

PAGE 19

PRESSURE MEASUREMENT 2.1 SCOPE:

This note presents a brief description and discussion of different Industrial type

Pressure/Draft measuring devices with particular reference to those used in modern

Power Stations.

2.2 GENERAL

Pressure measurements are one of the most common measurements required in the

Power Stations. These range from very low, i.e. condenser vacuum to very high i.e.

hydraulic pressures. Between these two limits i.e. from 30-40 millibar absolute to 300

bar are to be the measurements of different process media-steam, water, fuel oil, lub &

sealing oils, air, gas etc., each with varying degree of accuracy and reliability.

2.2.1 UNIT OF PRESSURE

Pressure is defined as the force per unit area. Thus the unit of pressure in the metric

system presently being followed in our country is kg f/Cml which is simply written or



spoken as Kg/CM2, omitting 'F' which denotes force. But the unit in the SI system is

'Bar.

1Bar= l05N/M2=1.097Kg/CM2

where 'N' is Newton and

M is meter.

Pressure in higher range is measured in units mentioned above. But lesser range

pressures are measured in column of water or mercury or in millibars which can be

explained as below.

1 Atmosphere pressure = 760 mm of mercury column

or

1 Atmosphere = 1013.3 m bar

2.2.2 In industrial practice, the pressure measurement is either in terms of 'Gauge Pressure'

orintermsof'AbsolutePressure'therelationshipisshownbelowaswellasintheFig.

2.1 attached.

Absolute Pressure = Gauge Pressure (shown by gauge

+ Atmospheric pressure (for pressure above atmosphere)

Absolute Pressure = Atmospheric Pressure - Gauge Pressure (for vacuum or draft)

PAGE 20

2.3 PRESSURE MEASURING DEVICES:

The common pressure measuring devices are:

1. Manometers using water, mercury and other liquids of known density for low

pressure measurement.

2. Diaphragm, Capsule bellows for measuring medium pressures.

3. Bourdon tube gauges for measuring medium and high pressures.



4. Transducers of different types for measuring pressures of all ranges for

telemetering purposes.

Of these above, the manometers are mainly used in laboratories for calibration purposes

and as such, the diaphragms, Capsule bellows have taken its place for site use.

2.3.1 MANOMETER ELEMENTS

The manometers are mostly used in laboratory for calibration purposes as these are the

fundamental type of instruments. At site these are mainly used for test purposes, in the

low range 0-1000 mm.

The manometer liquid normally used is water. Sometime coloured water is used to

distinguish the column. The other liquids used are:

i) Transformer oil having specific gravity 0.864

ii) Mercury having specific gravity 13.56

iii) Blended paraffin liquid having specific gravity

2.3.1.1'U' Tube Manometers

If a glass tube of uniform cross section is bent to form a 'U' and partly filled with a

liquid of known density and both limbs are open it becomes a simple manometer and

responds to the pressure applied to one of the limbs as below.

PAGE 21

If P2 is the pressure (Fig. 2.2 a, and 2.2 b) applied and PI is the atmospheric pressure,

then the differential pressure working on the fluid is P2-P, which will raise the column

of fluid in the low pressure limb to a height 'H' from the new surface of separation, such

that -

P2-P1 = HPG where P is the density of the manometer liquid 'C' is accn, due to gravity.



2.3.1.3 'U' Tube with One End Sealed:

If one end is sealed & evaculated, then the manometer can be used for absolute pressure

measurement.

2.3.1.3 Cistern Manometer

If the area of one of the limb is made considerably greater than the other, then the

measurement of the differential pressure is represented by the height of the liquid

column in the smaller tube with negligible error. Such system is called the single limb

manometer or cistern manometer since the larger area pipe is in the form of a metal

cistern. (Fig. 2.2.c, 2.2 d and 2.2 (e).

Applied Pressure P, = HPG

If ratio of area A, of Cistern to A 2that of limb is high.

2.3.1.4Industrial Type High Pressure Manometer

Industrial type high pressure 'U' tube manometer are available having metallic tubing. These

manometers employ a secondary system of linkages/liverages for indication purposes. Fig. 2.2

(f).

PAGE 22



2.3.1.5 Inclined Tube Manometer

Inclined tube manometer are the special development to give increased length of

column for less differential pressure. The inclined tube carries the scale. Manometers

are available with adjustable inclination depending upon the range required. Fig.

2.2(g), 2.2(h).



2.3.2 DIAPHRAGM, CAPSULE AND BELLOWS:

The present days low pressure to medium pressure applications are met with

diaphragms. Also the introduction of these elements has greatly helped in remote

measurement and control of pressures even of very low range (0-4 mm wcl).

2.3.2.1The simplest form of these elements is the single diaphragm instrument Fig. 2.3. In

this a thin flat plate of circular shape is fixed firmly round its edges. On applying

PAGE 24

pressure to one side greater than the c lie existing on the other, the diaphragm deflects

away from the higher pressure sid @ , -iie maximum deflection occurring at the centre.

It is possible to link the diaphragm centre to a pointer a pen a slider on a scale or

through beverages to make an indicator recorder or a transmitter.

The deflection depends upon the following factors.



a) The radius of the diaphragm

b) The modulus of elasticity of the material

c) The thickness of the material

However, in this simple arrangement the deflection is related to the pressure only for a

small movement. This linear relationship is extended to a larger deflection by the

following provisions.

2.3.2.2CORRUGATED DIAPHRAGM Fig. 2.4 & 2.5

By introducing corrugations in the circular diaphragms the deflection-pressure

relationship is maintained for larger movement. Here the deflection is related to

a) The radius of the diaphragm

b) The radius of each corrugation

c) The depth of the corrugation

d) The thickness of the material

The cross section of the other two types of diaphragm gauges namely Slack

Diaphragm and Stack Diaphragm are shown in Fig. 2.6 & 2.7.

2.3.2.3. CAPSULES

Further improvement in the deflection was achieved by forming a.capsule out of two

circular diaphragms jointed at the edges. The specific area of application of these

capsule gauges is boiler draft measurements.

2.3.2.4CAPSULES FILLED WITH LIQUID Fig. 3.7

The capsules were further improved upon by filling the inside space with a non corn

pressible liquid the most common being the Silicon Oil.

2.3.2.5MULTI STACK DIAPHRAGMS

Where more deflection is desired for a given differential pressure, the number of

diaphragms are increased. This increases the number of joints thereby increasing the

cost and complexity of production. (Fig. 2.6).

PAGE 25



2.3.2.6BELLOWS Fig. 2.8 to Fig. 2.10

Bellows are the substitutes for multi stack diaphragms because of their simplicity and

cheap manufacturing cost. A thin walled tube is taken and formed into corrugated

shape by special hydraulic pressure. The corrugations are called convolutions. The

flexibility of the bellows depends upon.

i) The number of convolutions (direct proportion)

ii) The square of the outside diameter of the bellows (direct proportion)

iii) The cube of the wall thickness (Inversely proportion)

iv) The young's Modulus of elasticity of the materials (Inverse proportion)

2.3.2.7MATERIAL AND RANGE OF MEASUREMENT



The various types of diaphragm and bellow elements are made of, steel of special

composition, phospher bronze, nickel silver and beryllium copper etc.

Bellows and multi stack are made from 80-20 brass, phospher bronze, stainless steel

and beryllium copper.

For very low pressures, the diaphragms are required to be extremely flexible. For these

applications materials like colon leather, gold beater skin nylon rubberised fabrics etc.

are used.

These groups of sensors are used for the measurement of very low pressure upto 2025

Kg/C@.

2.3.2.8WORKING PRINCIPLES

Diaphragms either single or multi stack and bellows may be used to measure gauge

pressures. Fig. 2.6 and 2.8 explains the principle. Here P, becomes (P, + @) where P is

the gauge pressure to be measured and P. the atmospheric pressure. P2 is atmospheric

pressure Pa. The deflection is proportional to the pressure difference (P. + Pa)-Pa i.e.

proportional to the gauge pressure.

Diaphragm stacks and bellows can also be used to measure absolute pressure. Fig. 2.9

explains this measurement. If the bellow'A' is evacuated to perfect vacuum and both

bellows are matched that the ambient temperature and atmospheric pressure causes the

same effect in both this system will measure absolute pressure.

2.3.3 BOURDON TUBE GAUGES (Fig. 2.11)

This is the oldest instrument introduced initially to measure pressures from medium to

high ranges. But present days these are used almost for every range of pressure

measurement. However, their application is limited to measure "Gauge Pressures"

only.

PAGE 27



It consists of a metal tube approximately elliptical in cross section formed into a 'C'

shape, a long spiral (helical) or to a flat spiral by special machines, one end of the tube

is closed and sealed and the opposite end is left open and terminated to a block where

the process pressure is applied. If the pressure inside the tube is more than that,

existing outside, the elliptical section changes its shape and it begins to straighten out,

with the result that the free end deflects is an arc. The deflection is proportional to the



pressure difference between inside and outside pressures since the outside pressure is

atmospheric. These gauges measure 'Gauge pressure'. The other factors affecting the

deflection of the free end are -

i) The radius of the bend

ii) The total tube length

iii) The wall thickness of the tube

iv) The major and minor axes of cross section

v) The material of the bourdon tube.

The usable part of the movement of the 'C' shaped bourden is of the order of 3 mm or 6

mm length of arc in order to make this small movement a measurable amount, same

sort of multiplying mechanism is introduced between the tube and the indicating points.

Most commonly used mechanism is the sector and pinion movement. This method

produces error in the magnifying process. This error, however, can be minhnised by

increasing the bourdon deflection by way of increased number of 'C' (Spiral bourdons).

Fig. 2.11 explains the bourdon tube gauge and its components.

2.3.3.1PRESSURE RANGES

Bourdon tube gauges are in use from the range 0-0.5 Kg/CM2 to 6,000 Kg/C@ and

even higher ranges occasionally. The practical range for each type is listed below.

Helical bourdon - 0-05 Kg/CM2 Upto 0.6000 Kg/CM2

'C' Type bourdon - upto 700 Kg/Cm'

Flat spiral bourdon - low ranges upto 0-70 Kg/CM2

2.3.3.2MATERIALS

Material slike phospher bronze, steel beryllium copper. etc. are used depending up on

the pressure range and the media s corrosiveness. The chart given in Table 1 gives

more details of bourdon tube materials and their pressure ranges.

PAGE 29

Table 1 Bourdon Tube Material

material Composition Joints Heat Pressure range

Percentage Treatment Kg/Cm2

Phosphor Copper95 Soft None 1-70

Bronze Tin 5 Soldered



(Drawn) Phosphorus trace

Copper 97-9

Beryllium Beryllium 1.8 Brazed Precipitation

Copper Cobalt 0.3 Hardened 08-350

(Machined)

Alloy steel Carbon 0.26/0.32 Screw Quenched 70-6000

(,machined) Chromoum 0.8/0.1 Welded and

Molybdenum 0.15/0.25 Soft soldered tempered

K Model Nickel 66 Screwed Precipitation

(drawn) Copper 29 hardened 70-1350

Aluminium 2.75

Iron 0.9

Stainless Chromium 16/18 Welded Stress 2-70

Steel Nickel 10/14 relieved

(drawn) Molybdenum 2/3

2.4 BOURDON GAUGE (Fig. 2.11)

The simplicity and ruggedness of abourd on gauge makes it the most frequently used

pressure gauge. The reference pressure in a Bourdon gauge is atmospheric pressure.

Hence, the dial reading gives gauge pressure.

2.4.1 CONSTRUCTION

The gauge consists of a shank, tube anchorage (block) bourdon tube, quardrant pinion,

hairspring and case (with bezel, glass and cover) pointer and dial.

Bourdon tube the pressure responsive element is made of the material(s) shown in the

Table No. 1. The angle subtended at the centre by the tube is around 2600 The end open

to applied pressure is soldered brazed welded or screw connected to the tube anchorage.

The tube and tube anchorage (block) material must match. The other end which is

sealed is connected to the quardrant via link.

The linkage has a screw for adjusting angularity. The linkage also allows for

calibration of the gauge.



Bezel ring is a removable ring fitted to the case, to hold the glass window to the case. It

is generally made of mild steel, brass or aluminium.

PAGE 30

Shank contains screw connection, spanner flat, spigot. The shank has parallel pipe

screw threads for connection to pipe links.

Hairspring eliminates backlash in the pinion quardrant arrangement and holds the

pinion in contact with the quardrant at all t@mes. The hair-spring does not have

mechanical strength to force the pointer to zero.

2.4.1.1Quardrant and Pinion:

The tip movement of the bourdon tube is small and is amplified by the pinion quardrant

and pinion mechanism. The quardrant is provided with a screw for adjusting range

errors.

2.4.2 ERRORS IN BOURDON TUBE GAUGES (Fig. 2.12)

Errors that may occur in Bourdon gauge are zero error, range error, angularity error,

and hysteresis. For zero error the pointer is adjusted, for range errors the quardrant

screw is adjusted, for angularity error the linkage screw is adjusted. In case of error

due to hysteresis the tube should be replaced if the error goes beyond the specified

value.

Hysteresis is the difference in the indicated value of the gauge for an applied pressure

during the increasing cycle and during the decreasing cycle of pressure.

FIG. 2.12 BOURDON TUBE GAUGE ERRORS

2.4.3 OVERHAUL, HAIRSPRING ADJUSTMENT, CALIBRATION, TESTING AND

ERROR (BOURDON GAUGE)

2.4.3.1OVERHAUL



The bezel glass and pointer are removed after cleaning the external surface. The

PAGE 31

pointer should be removed with the pointer remover. The dial or rear of the case is

removed depending on the type of gauge. If the gauge is in good condition, on visual

examination light oiling is done and then calibrated. The tube should form a part of the

circle approximately 2600. If the condition of the gauge is doubted, it should be

stripped and reconditioned.

Wear of pivots, screw damage, tube deformation, blockages connector, spigot damage

should be checked. Damaged parts must be replaced. After this the gauge is

reassembled. Hair Spring should not be kinked. The mechanism should move freely

2.4.3.2 HAIR SPRING ADJUSTMENT.

The movement is disconnected from the tube. The pinion runs off the end of the

quadrant and wind up the hair spring so that it will take up its correct free position just

ovr 3/4 of the way along the quardrant for an anticlockwise spring.

2.4.3.3CALIBRATION:

For calibration following a strip down the case is marked at 10% and 100% point.

Rough calibration is done in span. Then the testing is done.

Testing a Bourdon Pressure Gauge

1. Gauge is tested at 5 points up and down before adjusting anything. Divisions

corresponding to about 10%, 30%, 50%, 70%, 90% are chosen.

2. About 10% pressure is applied then zero is set by removing and replacing the

pointer to read the pressure applied.

3. About 90% scale pressure is applied if necessary the range is adjusted by

loosening the shoulder screw and moving the linkage along the slot in the

quadrant (towards the pivot to increase the range, away from pivot to decrease the

range).

4. Step 1 and 2 are repeated until gauge is correct at both points.

5. When zero and range are correct then angularity is adjusted if necessary. Half full

scale pressure is applied to the gauge, angularity adjustment screw is loosened and

adjustable linkage is slided until the angle formed by the quardrant and linkage is

a right angle.



6. Approximately five points of the scale are checked with pressure increasing, the

readings are tabulated.

7. The same five points with pressure decreasing are checked and result tabulated.

8. Result from 6 and 7 are used to check for hysteresis.

9. Gauge is assembled. Pointer should not foul the glass over any part of its travel.

10. Result sheet is made the final condition of the gauge error as a @@ of full scale.

The error should be within 1 % of full scale.

PAGE 32

2.4.3.4 ERROR:

ZERO

A zero error can be observed easily by quickly testing at the cardinal points. A zero

error will have exactly the same amount of deviation at all points. In this type of error

the pointer is reset.

Applied Pressure

Kg/Cm' 10 30 50 70 90

Gauge Reading UP 9 29 49 69 89

Down 9 29 49 69 89

RANGE/SPANIMULTIPLICATION

As the pointer travels the dial it will show an increasing deviation from the test pressure

this is known as a fast error. If the pointer showed a decreasing amount this is known

as a slow error. The position of the link is adjusted in relation to the quadrant in this

type of error.

Applied Pressure

Kg/C@ 5 10 15 20 25


Down 5 11 17 23 29

ANGULARITY:

(When the zero and span errors have been corrected to within the specification 1%) for

general purpose gauges a check for angularity is carried out. Angularity shows up in

the form of a Centre scale error, that is the gauge is correct at the 10% and the 100%

points but it is incorrect at the Centre. Adjust the screw on the correcting link.

Applied Pressure



Kg/C@ 50 100 150 200 250


Down so 105 160 205 250

NO ERROR..

If gauge reads up and down by not more than the permissible error it is considered to be

calibrated correctly. A calibration certificate should now be made out on the standard

form.

Applied Pressure

Kg/Cm' 10 100 150 200 250


Down 52 100 151 200 251

PAGE 33

HYSTERESIS:

If after trying to calibrate a gauge the up and down readings differ beyond the specified

value it will be from one of the following:

Hysteresis

Worn Movement

Broken Hairspring

The last two can be repaired assuming the gauge is having its initial check, they should

not appear on the final check after a renovation. However the hysteresis will appear.

This shows the end of life of the tube.

2.4.4 CALIBRATION OF BOURDON TUBE GAUGES:

Pressure gauges in industrial process must be accurate so that any time the process

pressure is known. This helps to achieve accurate control of the industrial processes.

The pressure gauges described till now require regular calibration. The calibration is

possible if one is sure what pressure is being applied to the gauge.

Hydraulic calibrator is one such device which is used in calibrating the pressure gauges.

The hydraulic units dealt with in this chapter use oil for application of pressure.

The principle of operation, setting up and maintenance of two calibrators viz.

comparator and deadweight tester is given here. In both the devices the pressure that is

being applied is known. In a comparator the applied pressure is indicated in a standard



test gauge. In a deadweight tester the pressure is indicated by the standard weights

placed on the instrument.

2.5 SPECIAL APPLICATION GAUGES

INTRODUCTION:

This chapter gives a brief idea about the special pressure gauges, precautions to be

taken while working with them, measurement of pressure for steam and corrosive

fluids, protection against pressure surges, installation of gauges with alarm contacts.

SPECIAL GAUGES

2.5.1 OXYGEN GAUGES:

A gauge used for oxygen service should be marked "Oxygen-Use No Oil". Testing of

oxygen gauges must be done with alcohol, oil free air or nitrogen, since oil and oxygen

form an explosive combination.

PAGE 34

2.5.2 HIGHER PRESSURE GAUGES:

These have ranges upto 2 Kilobar (typical). The design of a high pressure gauge must

incorporate safety measures. A rupture in high pressure gauge can explode pushing the

glass outward which may harm personnel. A typical high pressure gas gauge consists

of a thin back plate is an integral part of the body. The glass is manufactured from

laminated glass.

High pressure gauges are mounted in such a manner that any escaping product is

reduced in force, & is directed away from impinging upon personnel or other objects.

2.5.3 DIFFERENTIAL PRESSURE GAUGE:

Fig. 2.13 shows a differential pressure gauge with a dual bourdon system. The two

tubes are connected to a single pointer. Tube 1 forms port 1 and tube 2 forms port 2.



Ports of tubes 1 and 2 are connected to the process whose differential pressure is to be

measured. The deformation of tube 1 causes the pointer to rotate anticlockwise via

link, cradle and quadrant. The deformation of tube 2 causes the pointer to rotate

clockwise via link and quadrant. The movement of the pointer is opposite for the

individual pressures and hence the gauge reads the differential pressure.

A typical application of this unit is the measurement of differential across filters to

indicate blockages or end of life of filter.

During installation of this unit due consideration must be given to overloads. A

differential unit must always be accompanied by an equalizing unit shown in Fig. 2.14.



To remove the gauge, first valves A and B are closed, valve C is opened to equalize

pressure in both the parts P, and P2. The gauge is removed now. To check zero on

plant .the same procedure is followed, but the gauge is now in position -

To restart, the unit is installed with valves A, B and C closed. After installation valve

C and B are opened. Then C is closed and A is opened.

2.5.4 STANDARD TEST GAUGES:

These units have a large dial size usually 8" to 10". This large dial size increases the

accuracy in reading. Gauges are chosen so that the full process pressure is about 70%

of the span of the gauge. In case of test gauge the choice must be near to the range to

be measured, to obtain greater accuracy. Further, during use the pressure applied to the

gauge will be under the control of the technician, so a standard test gauge need not be

overranged. Standard test gauges have an accuracy of 1/4%. They should be calibrated

periodically and the calibration certificate must be preserved.

PAGE 36

2.5.5 WORKSHOP TEST GAUGE:

These are secondary calibrated gauges. This type of gauge has lower accuracy than the

standard test gauge.

The accuracy of these units is +/- 1/2%. These gauges are used for general workshop

testing.

2.6 SEALS:

It is very often required to isolate the gauge from the fluid whose Pressure is to be

measured. The reason can be that, the fluid is corrosive and the gauge material can be



attacked by the fluid, or the fluid may get clogged in the gauge. In such cases seals

must be used to prevent the metered fluid from entering the gauge.

Seals may be of two kinds: they may be solids or liquids.

2.6.1 SOLID SEALS:

These are used with Bourdon tube pressure gauges in processes where:-

a) Process fluid would normally clog the Bourdon tube.

b) Materials capable of withstanding corrosive effects of process fluids are not

available for Bourdon tube.

c) Process fluid in the Bourdon tube might freeze due to changes in ambient

temperature. Two types of solid seals viz. diaphragm seals and bellow seals are

shown in Fig. 2.15 and 2.16 respectively.

PAGE 37

The space between the seal and the gauge tube is generally filled with liquid. Any

movement in the seal caused by a change in process pressure is transmitted via the tube

liquid. In some installations compressed air is used. The liquids used in the tube of the

gauge is chosen to meet the operating temperature. The liquid should not vapourize at

the operating temperature.

The material chosen for the seal must resist corrosive action.

2.6.2 LIQUID SEALS FOR STEAM AND VAPOUR

Many bourdon gauges are not meant to be used at high temperatures. In order to keep

live steam out of the pressure measuring instrument shown in Fig. 2.17(A) or a U tube



shown in Fig. 2.17(B) is often used. These shapes help to retain the condensed water

when the instrument is shut down. In this way the water from the condensed steam is

used as a seal and the Bourdon tube is not subjected to high temperature.

2.6.2.1 LIQUID SEALS FOR CORROSIVE LIQUIDS OR LIQUIDS WHICH WOULD

SOLIDIFY IN THE PIPES

When it is desired to keep the process fluid out of pressure pipes and instruments, a

chamber is fitted near the tapping of the main. This chamber is connected to the

instrument. A part of the chamber, the pressure piping and the instrument are filled

with the sealing liquid, while the process fluid fills the remainder of the sealing

chamber.

If the process fluid is less dense than the sealing fluid, the line from the pipe is

brought into the upper part of the seal chamber as shown in Fig. 2.18A, when the

pressure increases there is more in flow of process fluid on top of the seal chamber as

shown in Fig. 2.18(A). When the pressure increases there is more inflow of process

fluid on top of the sealing fluid. The process fluid pushes the sealing fluid into the

gauge

PAGE 38

through the gauge extension.

In case of process fluids which are denser than sealing fluid the pressure line extension

is bent and brought near the bottom. This is shown in Fig. 2.18(B). Any increase in

process pressure forces more of the sealing liquid into the gauge.



(A) FIG. 2.18 LIQUID SEAL (B)

2.7 SNUBBER

This is a protection device for pressure measuring instrument from violent pressure

surges and pulsations. Snubbers also known as deadners reduce the effects of pulsating

pressure. They result in the instrument indicating or recording an average pressure,

instead of recording each individual surge or pulse. Snubbers are used in pipe lines

leading to the instrument.

In general, these snubbers reduce the velocity of fluid to the instrument and thus

prevent sudden extreme changes in pressure from reaching the measuring element too

rapidly. The reduction in velocity can be achieved by several methods. Fig. 2.19

illustrates the ray siiubber. The body consists of two parts, the lower part and an upper

part. The lower part is connected to the pipe line. It contains a piston. The pin piston

assembly rises and fills with the pressure impulses and absorbs the effect of shock and

surge. Owing to the rise and fall of the piston the snubber is self cleaning. The upper

part of the snubber is screwed to the lower part, on one side and to the pressure

instrument on the other. The upper part has a stop for the piston. The stop has a hole in

the Centre for the process fluid to pass to reach the instrument from the pipe line and

viceversa.

PAGE 39



2.8 INSTALLATION OF PRESSURY' "-WAUCES:

To measure the steam pressure the pressure take off pipe should be at the top of the

main pipe and must be fitted. There must be suitable isolating valve, so that the gauge

may be removed for repair or maintenance without affecting the flow in main line.

2.8.1 SITE INSTALLATION:



For site gauges if the gauge is above the main line, a water seal must be provided by

means of pig tail or U-tube syphon. An isolating cock should be at the gauge end.

Syphonsarepr one to blockages and the cureis to steam them out orusea solvent. The

material of syphon and the cock are chosen for the pressure encountered and must

withstand any corrosion.

If the gauge is below the level of steam pipe water will form and protect the gauge.

There is no need to use a pig tail or syphon. The gauge must be mounted upright.

2.8.2 REMOTE INSTALLATIONS FOR GAUGES:

For remote installation with long runs of pipe it is desirable to have two cocks one at

the tapping point and one near the gauge. This provides gauge as well as source

isolation.

2.8.3 INSTALLATION OF GAUGES FOR GAS:

The pipe should slope upward from the source for gas installation. This allows the

drain of any liquid to the source. This is shown in Fig. 2.20(A).

If it is not possible to slope the gas pipe line, a catchpot must be fitted at the lowest

point in the installation to drain away liquid. This is shown in Fig. 2.20(B). The liquid

that has collected can be blown down at regular intervals.

3.8.4 INSTALLATION OF GAUGE FOR LIQUID:

In this case the pipe line should have a slope downward from the source so that gases

will not collect in the gauge as in Fig. 2.20(C).

If it is not possible to have a slope a chamber and a vent at the highest point as in Fig.

2.20(D) must be,provided. The collected gas must be vented out at regular intervals.

2.9 GAUGES WITH ALARM CONTACTS:

These types of gauges give alarm when the pressure reaches a set level. The working of

gauge in which the alarm contact is to be made when the pressure reaches a set higher

limit is given here.

The gauge has a pressure setting needle. This needle can be set a t any value on the dial

PAGE 41



of the gauge. The needle is set via a knob through the centre of the glass of the gauge.

The glass is generally of acrylic. The needle has a projection where contact will be

made. The needle is connected to a wire.

The pointer is also connected. to a wire. It has a projection. The projection when it

touches the projection on the setting indicator contact is made. When the pressure

reaches the set value then the gauge pointer touches the projection on the setting needle.

The pointer and setting needle behave like an open switch till the set limit is reached.

The gauge can be connected to relay, hooter or lamp for alarm or control. Sometimes a

magnet is provided on the needle to enable quick closing of contact thereby avoiding

chances of sparking.

In a similar manner alarm contacts can be made at two point one for a low pressure

limit and the other for a higher pressure limit. If the working pressure reaches the low

set limit or the high set limit alarm contact is made.

2.10 DIGITAL PRESSURE GAUGES:

Digital pressure gauges working with integralorre mote pressure sensing transducers are

now be coming moreeasily available and these are usually of a very high accuracy.

This means of course that these can be used for calibration purposes or for efficiency

monitoring. The following pages describe two such devices and they have proved in

practice to be very valuable calibration standards. In particular the device is used for

checking the transmitters which measure the condenser absolute pressure.

Calibration over the full range 0-100 m bar absolute can be achieved by use of the

condenser's own vacuum when the T/A is on load and the use of a vacuum pump in

series with the gauge tapping.

2.11 DRAUGHT GAUGES:

Draught gauges are used extensively throughout the Power Station to measure air and

gas pressures through the boiler and mills. The draught gauge is basically a diaphragm

pressure gauge with an elongated scale.

The readings are all transmitted as (0-1 OmA) standard signals, if the process medium

is not allowed in the control room.

The foregoing comments on pressure gauge installation is appropriate to draught gauges

also, and since they are measuring relatively low pressures it is important that the pipe



work is installed very carefully with the added provision of a blow down facility to

clear the lines from dust.

For suction gauges it has been found that the drilling of a small hole in a draught gauge

line near the tapping point can give an automatic cleaning of the lines without loss of

reading or sensitivity.

Calibration of draught gauges is best achieved with a manometer. Manometers of

PAGE 43

Reasonable accuracy and the correct range can be obtained from various

manufacturers. It is important when calibrating a draught gauge in this way that the

correct type of Manometer is used.

TRANSMITTERS FOR PRESSURE AND DIFFERENTIAL PRESSURE

MEASUREMENT

2.12.1 A transmitter has a process signal such as pressure, flow, level or temperature as its

input and an electric or pneumatic signal as its output.

PRESSURE

FLOW

LEVEL TRANSMITTER ELECTRIC / PNEUMATIC

TEMP OUTPUT SIGNAL

INPUT SIGNAL

Thus the basic function of a transmitter is to proportionally modulate an electric/

pneumatic signal in response to the process parameters. We shall confine our

discussion to electric transmitters. These transmitters sense the change in process

variable within a certain range and produce an output current within the range. The

output range is standardized to bring uniformity in the construction of secondary

instruments as well as to facilitate the testand calibration work. The provalent output

signal ranges are:

i) 4-20 mA DC

ii) 0-20 mA DC

iii) 10-50 mA DC

2.12.2 Transmitters are generally connected in a measurement loop according to one of the

following methods:

a)- Four Wire Transmitters (Fig. 2.21)



In this method four conductors are led to the transmitter. One pair is used to carry the

power supply, which may be 220 VAC or 24 VDC. The other pair is used for signal

transmission.

b) Two Wire Transmitters (Fig. 2.22)

This is presently the most widely used method for transmitter connections. There are

three basic elements in this loop, namely a / c power supply, transmitter and the

receiving instrument. They are connected in series and the transmitter acts as a

current regulator in the series circuit. The current in the series circuit changes with

respect to change in process parameters.

Thus only two wires are needed for connecting one element to another. This

simplifies cabling and reduces erection end cable costs.

PAGE 44



Being a series circuit, the input resistance of the receiving instrument plays an

important role, as higher input resistance will generally limit the loop current. For this

purpose transmitter manufacturers generally provide a load drive capacity curve for the

transmitter. Referring to Fig..2.23 we find that, this curve gives the maximum value of

input resistance that can be connected at the operating power supply voltage, without

effecting the output current of the transmitter.

2.12.2 OPERATING PRINCIPLES OF TRANSMITTERS

a) Manometric Type (Pressure, Flow, Level)

Transmitters for pressure, flow, and level generally use a transducer which converts the

pressure (or differential pressure in case of flow or level) to mechanical movement and

then finally to an electric signal. This signal is then amplified and conditioned to

produce 4 to 20 mA DC output.

This transducer typically can be bourdon tube, diaphragm, or diaphragm capsule or

bellows assembly. Three basic types of transmitters are in use in present day industries.

i) Force Balance Type (Fig. 2.24)

A pressure applied to each side of diaphragm capsule results in a force being applied

(via the C-fiexure) to the transmission bar. This force is proportional to the differential

pressure across the diaphragm capsule. The transmission bar has its fulcrum at the

flexible pressure seal and is connected at its opposite and (force bar) to the vector

linkage at point 'A'. The vector linkage is attached to the unit via the span adjustment

mechanism at point 'C'. This force applied to point 'A' will produce a further related

force at 'B'. The direction of these forces are shown by arrows. The relationship

between forces A and B is determined by'Tan U'. Thus by adjusting'Q' this relationship

and instrument span can be varied. At the free end of the linkage is mounted one

element of a position detector (a coil with a movable core). The detector senses very

minute movement of the secondary beam which then changes the current in the coil.

This is amplified by the electronic amplifier and converted to a standard 4-20 mA

signal.

At the opposite end of the secondary beam is the force coil which acts with in thepole

of a permanent magnet. The output current of transmitter flows through the coil which

then provides a feed-back force to the beam to restore balance. Greater the output



current derived from movement of force bar assembly, greater the feed-back force

opposing these movements.

PAGE 46



ii) Capacitance Type (Fig. 2.25)

Process pressure is transmitted through isolating diaphragm and silicon oil fill fluid to a

sensing diaphragm in the Centre of the differential pressure cell. The sensing diaphragm

deflects in response to differential pressure. The position of the sensing diaphragm is

detected by capacitor plates on both sides of tliesensing diaphragm. The differential

capacitance between the sensing diaphragm and the capacitor plate is converted

electronically to a 4-20 mA signal. Transmission on 2 wire system.

iii) Strain Gauge Type (Ref. Fig. 2.26(a) and 2.26(b)

In this type the sensing element of the transmitter consists of a beam and diaphragm

assembly, with the thin film,-,train gauge bridge circuit located on the bending beam. A

ceramic material is deposited on the back to provide electrical insulation for the bridge

elements. The four strain gauges are vacuum deposited onto the insulator material and

connected electricity into a wheatstone bridge circuit. A displacement in the sensing

diaphragm is transmitted to the beam assembly. A change in pressure causes the sensing

diaphragm to defect and thereby bending the beam. This changes the strain resistance

and is then converted to 4-20 mA DC.

iv) Inductive Transducers

Fig. 2.27 shows a principle of an inductive transducer. It comprises a transformer

having a primary supplied from a low voltage A.C. source and two secondary windings

electrically equal and would be adjacent to each other. When the cord is symmetrically



placed with the electrical Centre of the transformer, the voltages E, and E, in the

secondary windings

PAGE 48



are equal. Thus the output becomes 'O' as they are connected in series opposition.

When the pressure is applied, the core moves towards a side more voltage is induced in

the winding nearer to the present core position as compared to the other winding and

there exists a net voltage which is proportional to the pressure applied. The transducer

of this type is called L.V.D.T. (Linear Variable Differential Transformer).

Secondary measuring circuits are connected to measure this voltage as a measure of the

pressure. Different forms of secondary system are in use.

2.1.2.3COMPARISON OF DIFFERENT TRANSMITTERS

The force balance transmitter is one of the early designs of two wire transmitters and is

still used widely. It has inherently rugged construction and is of simple design.

However, it has a very large number of mechanical linkages and moving parts. This

inherently reduces its accuracy and makes it prone to errors due to hysterises &

deadband etc. It also makes it very bulky and heavy. The zero and span adjustments

are also mechanical and as such achieving good calibration accuracy is not very easy.

The capacitance type and strain gauge type are definitely of superior design. They have

virtually no moving parts and hence are very accurate and have a good repeatability.



Thev are lightweight in construction and much smaller in size. Also all the adjustments

such as zero, span, damping are electronic therefore calibration becomes very easy.

PAGE 53

LEVEL 3 INTRODUCI'ION

In power station applications level can be defined as 'the height of liquid or solid media

above or below a reference line', usually the base. Levels, below the reference line may

also be required in dynamic condition, such as for boiler dnnn etc.

If the diirnensions of a vessel are known then the volume or mass of its contents can be

determined by measuring the level. Hence the vessel contents can be directly displayed

in units of level (meters), volume (liters) or mass (kilograms).

There are three main reasons viz safety, economy & monitoring for making level me

asurements of solid particles or liquid level. The safety of personnel and plant is the

most important.

3.1 SAFETY

The boiler drum level monitoring and control system is probably the most important

level measurement made in a power station.

Factories Act places a statutory duty on the boiler operators to monitor drum level and

inspector of factories is legally entitled to enter a power station at any time to confirm

that the boiler operators are complying with the Act.

Irrespective of statutory requirements the boiler drum level must be controlled within

pre-defined limits.

Obviously if water level is too low then there is a danger that riser tubes will overheat

and burst, if too high, water droplets may be carried over to the super heaters or H.P.

turbine with equally disasterous results.

3.2 ECONOMY

Some plant units operate more effectively when levels are maintained within clearly

defined values. An example of this is in a coal fired station the fuel level in the

pulverising mills; the effectiveness of the pulverising process and the overall efficiency

of the Unit will be influenced by incorrect fuel levels.

3.3 MONITORING

The monitoring of levels in bulk storage and process vessels is necessary in order that:



a) Plant efficiency may be assessed and optimised.

b) Stock records may be kept.

c) Costs may be correctly allocated.

3.4 LEVEL MEASUREMENT - PRACTICAL CONSIDERATIONS

Before deciding on a level measurement system from the wide selection available t

following practical considerations should be taken into account:

1) Is the display device to be in units of height, volume or mass?

2) Does the size and shape of the vessel create display scaling problems?

3) Is one display device or several required?

4) Is display to be local or remote or both?

5) If remote display is required what is the length of the transmission path?

6) Is pneumatic or electronic operation required?

7) Is the level measurement system to be used for display only or display/contro or

control only?

8) If control, is the control system pneumatic or electrical/electronic?

9) What accuracy of display and/or control is required?

10) Are redundancy techniques for increased system integrity to be used?

3.5 LEVEL MEASUREMENT - METHODS

There are many methods of measuring level, the selection of a particular system is

largely determined by the practical considerations already mentioned together with

capital cost (equipment and installation), reliability, maintenance cost and degree of

expertise required by maintenance personnel.

The methods to be considered can be classified as follows:

1) Direct dipping

2) Direct viewing

3) Floats and liquid displacers

4) Head pressure measurement

5) Electrical/electronic

PAGE55

6) Ultrasonic

7) Nucleonic



3.5.1 DIRECIDIIPPING METHODS

3.5.1.1 DIPSTICK (Fig. 3.1)

Probably the simplest method of all level measuring devices. It is shnply a calibrated

scale inserted into the liquid until the zero of the scale hits the bottom of the tank, the

level is then read directly off the scale.

For accuracy the base of the stick is usually coated in metal and the stick is inserted in

the vessel perfectly vertically.

For level measurement in a dosed tank the stick must be withdrawn and level

determined by the wet/dry line junction on the scale.

If required, the stick scale can be calibrated in terms of volume if the shape of the

vessel is taken into consideration.

3.5.1.2HOOK GAUGE (Fig. 3.2)

When the level of liquid in an open tank is read off a dipstick it is sometimes difficult to

read accurately because it is difficult to focus one's eyes on the same level as the liq7

uid due to parallax. In this case a hook gauge type can be used. It consists simply of a

wire cord'of corrosion resisting alloy, such as gun metal or stainless steel, about 4"

PAGE56

diameter, bent into a 'U' shape with one arm longer than the other. The shorter arm is

pointed with a 600 taper, while the longerone is attached to a slider, havinga vernier.



The hook is then pushed below the surface of the liquid and gradually raised until the

point is just about to break through the surface. It can then be clamped and the level

read from the scale.

3.5.2 DIRECT VIEWING

3.5.2.1SIGHT GLASS (Fig. 3.3)

The sight glass is very useful as a simple arrangement wherein a section of the liquid is

brought outside the vessel and displayed alongside a main scale. If the diameter of the

bore of the sight glass is not small enough to introduce errors due to capillary action,

the liquid will stand at the same level in the sight glass and the vessel, provided the top

of the sight glass is subjected to the same pressure as the top of the vessel. It can be

used for open or closed vessels.

The system is analogous to a U-tube manometer where the vessel is one limb and the

sight glass the other limb.

Sight glasses are usually installed with two isolating valves and a blow down valve for

cleaning purposes. The tube material is generally pyrex or armoured glass. Reflex -

glass is sometimes used to improve readability, the division between the liquid

and gas region being made very pronounced. Other sight glasses include a float marker

to improve readability especially if the meniscus at the liquid interface is obscured by

scum or scale.

In high pressure installation such as a, boiler drum the gauge is usually fitted with an

automatic cut-off so that if the sight glass breaks, the dan_aer of anyone being injured

by tlhe content,5 of the vessel win be minimised. The cut-off device usually consists of

two ball bearings which normally hang loose in the connecting pipes, but if the glass

breaks the flow of steam and hot water forces the balls against valve seats so cutting off

the escaping flow.

Impurities in the liquid are one of the problems of sight glass systems as the glass

becomes discoloured and obscures the liquid meniscus.

Regular cleaning of the gaugeglassis the common maintenance task- Other problems

are broken glass tubes or leaks and/or blockages at the connections.

The range of sight glasses largely depends on the nature of the liquid, the static pressure

and the temperature involved. Ranges of 0.2 to 2 metres are typical.



If the density of the liquid is constant then sight glasses are simple, accurate devices

for level measurement. They can be calibrated by comparison with a dipstick or the

addition of a known volume of liquid.

PAGE57



3.5.2.2 WATER GAUGE WITH CLOSED CIRCUIT TELEVISION

(C.C.T.V.) REMOTE DISPLAY (Fig. 3.4)

One of the obvious problems with a simple sight glass system for, say boiler drum level

measurement is that only local indication is available. The use of a special type of sight

glass (water gauge) with an associated c.c.t.v. system allows level display to be

remotely located in the Control Room.

The water gauge works on the principle that different substances have different

refractive indexes i.e. they bend light by different degrees.

The gauge is a vertical tube of triangularorwedgecross-section. Two faces of the three

sides are made up of glass and mica divided up into small compartments. Illumination

is projected through the gauge and the light is bent by the medium. The degree of bend

depends on whether the medium is water or steam. Because of the prismatic

arrangement either thewater'windows or'Steam'windows are illuminated thus the level

of water in the drum can be determined.

A c.c.t.v. camera is mounted a few feet away from the gauge and is carefully aligned

with the light path through it. The camera and lens system being fully protected against

fuel dust and ash. The c.c.t.v. monitor is located in the Control Room.

This system is more difficult to operate with high steam pressure (120 bars) because the

refractive index of water closely approaches that of steam thus the angular deflection of

the light paths is very small making level indication difficult. At pressures of 166 bars

it is almost impossible to accurately determine water level by direct viewing means.

The problem can be overcome by gauge rear illuminators using quartz iodine lamps and

by using an optical magnifier to enlarge the small differential in the refractive index of

the two mediums.

The main problei-ns with this system are that of faulty alignment, hostile environmental

conditions which affect the reliability of the camera and high degree of technical

expertise required for c.c.t.v. maintenance.

3.5.3 FLOATS AND LIQUID DISPLACEMENT (Fig. 3.5)

The use of a float enables the level of liquid to be measured when direct viewing is

impracticable. The float material may be of hollow metal, a plastic material, or

moulded rubber. The size and shape will vary with different designs, but in general



they are made to reduce to a minimum the possibility of dirt or any other matter

building up on the top. If there is any large deposit of dirt, it will cause the float to sink

deeper into the liquid than it should and give a false indication. The float is usually

arranged to operate with about half its depth submerged in order to give some degree of

stability.

Calibration will be for one liquid only, because different liquid densities will cause the

float to sink to different depths.

PAGE 59



3.5.3.1FLOAT AND COUNTERWEIGHT TYPE

This method consists sirnply of a large area float connected by a chain type or cable to a

counterweight which passes in front of a scale and acts as an index.

The float should have the largest possible area in order to reduce the error owing to

friction and out of balance forces of the cable or chain. If the surface of the liquid

under measurement is turbulent, a guide will have to be set up to stop the float moving

around in the tank and causing errors.

3.5.3.2RIGID ARM FLOAT (Fig. 3.6)

With the further development of attaching the float to a pivoted arm, the total energy

available can be increased due to the moment of force of the buoyance factor acting on

the float at a distance from the pivot point. With this arrangement the total force can



therefore be regulated by changing the size of the float and also the length of the float

arm.

PAGE61

FIG. 3.6 RIGD ARM FLOAT LEVEL GAUGE WITH MAGNETIC COUPLING

A rigid arm float has the advantage of being completely self-contained and may be

fitted to open or closed tanks in virtually any position, though itsrange is much smaller

than that of the cable-and-pulley type. Remote indication may be built in as in the other

type.

The float movement is limited to about 1200 as a maximum, the motion being

transmitted to the pointer by a worm drive or similar arrangement. If the gauge is



mounted below the liquid level, there must be some seal between the gauge and the

tank. Some gauges use a magnetic method or pointer transmission.

The scale shape will depend to some extent on the position of the gauge and may not be

truly linear over the whole range. Hence calibration should be for one particular

installation. The range will depend on the length of the arm, about 1.5 m being an

upper limit.

PAGE62

3.5.3.2.1 "MOBREY" FLOAT OPERATED SWITCHES (Fig. 3.7)

Where it is required to initiate an alarm, start or stop a pump or open or shut a valve at a

high or low level the magnetically operated switch or air pilot may be used. The float

assembly carries with it a permanent magnet which is opposed by a similar magnet

which operates the switch, or air pilot valve. The adjacent poles of the two magnets are

of the same polarity so that they repel each other, thus giving the mechanism a snap

action. In the level switch mechanism the contacts change over with a snap action

when the float passes the mid position. In the air pilot valve, a compressed air supply is

led into the unit, and when the float is in its highest position the air valve permits the

passage'of air to the diaphragm or piston-operated valve causing the valve to close. A

fall in liquids level causes the air valve to change over, shutting off the air supply and

venting the air in the diaphragm valve to atmosphere, permitting the valve to open. If it

is required to reverse the action of the pilot so that the diaphragm valve closes on fall in

liquid level it is only necessary to change over the air supply and exhaust connections.

3.5.3.3LIQUID DISPLACER SYSTEMS (Fig. 3.8)

An object can be made in such a manner that it is either capable of floating on the

surface of a liquid, at some point below the surface, or may completely sink. By

adjusting the density of a float it will take up one of these conditions.

This gauge, embodying a displacer, relies on Archimedes principle for its operation.

According to this principle if an object is weighed in air and then in a liquid there is

apparent loss of weight which is equal to the weight of the displaced liquid. The

displacer is a long hollow cylinder loaded to remain partially submerged, and is

suspended in the liquid in the vessel or in an adjacent small diameter chamber

connected to the vessel. The apparent weight of the displacer will decrease as the level



of the liquid rises. The. change in apparent weight can be interpreted in terms of levels

& scale can be calibrated accordingly.

3.5.3.3.1 FISHER "LEVEL-TROL'SYSTEM (Fig. 3.9)

When the vessel is empty the whole weight of the displacer is carried by the free end of

the displacerarm. This causes a turning moment, equal to the product of the weight of

the displacer and the effective length of the arm, to be applied to the torsional stress set

up in the torque tube. Thus the angle through which the torque tube spindle is turned

will be proportional to the apparent weight of the displacer. As the liquid level in the

vessel rises the apparent weight of the displacer will get less and so win the turning

moment of the torque tube. The torsional stress in the torque tube will then rotate the

spindle to a new angle proportional to the apparent weight of the diaplcer.

A pointer or pen arm driven by the torque tube spindle will indicate or record the liquid

level. The spindle movement may be used to operate a pneumatic transmitter or

pneumatic controller.

Theu . thrust on the displacer depends upon the density of the liquid in the vessel and

PAGE63



PAGE66

the gauge reads correctly only when the displacer is immersed in a liquid for which it

was designed.

This system can be 1 used for enclosed vessels with a working temperature of up to

SOOOC and a static,pressure up to 340 bars.

3.5.3.3.2 FLOAT SYSTEMS - POSSIBLE FAULTS

1. Float and counterweightty-pes

a) Change to length of cord or tape causing zero error.

b) Dirty or worn pulley bearing.



c) Punctured float.

d) Cord or tape tension faults.

2. Rigid arm types

a) Change in position of float on the arm causing range error.

b) Bent float arm causing zero and range errors.

c) Punctured float.-

d) Friction at the glands and pivots.

e) Leakage at the glands.

f) Follower "Pulled out" of the influence of the magnet.

3. Displacer types

a) Heavy fouling on displacer and in displacer chamber restricting free movement

and causing zero error.

b) Damaged torque tube assembly causing zero and/or range error.

c) Dirty air supply.

d) Dirt, wear and friction on indicating, recording or transmission mechanism.

e) Leaks and blockages in associated pipe work.

f) Hole in displacer.

3.5.4. HEAD PRESSURE MEASUREMENT SYSTEMS (Fig. 3.10)

These systems use the principle that a column of liquid will exert a pressure whose

value depends only on the height of liquid, density of liquid and acceleration due to

gravity and is totally independent of the cross-sectional area of the column.

If the density of the liquid remains constant then the height of liquid above a datum

(tapping) point is directly proportional to the pressure measured at that datum point.

Thus a pressure measuring device can be used suitably scaled in units of level.



FIG.3.10 HEAD PRESSURE LEVEL MEASURMENT SYSTEM

PROOF

Let h = Height of liquid above datum point.

P = Pressure due to liquid

F = Force of column of liquid

A = Area of column at datum point

m = Mass of liquid

g = Acceleration due to gravity

V = Volume of liquid

p = Density of liquid

P = F

A

but F = mg

∴ P = mg

A but m = Vp

∴ P = Vpg

A

But V = Ah

PAGE68

P = Ahpg

A

P = hpg



h = P

pg

So, h ∝ P

3.5.4.1MEASUREMENT OF LIQUID LEVEL IN OPEN VESSELS (Fig. 3.11)

Since the static pressure at a chosen point of measurement (datum line) will vary

directly with the head of liquid above it, it can be seen that this pressure can be

measured and the gauge calibrated directly in head of liquid. The tapping,point is

always taken above the sediment level.

It may be noted that the gauge will only respond to the head of hquid'h'above it. The

actual depth is, however, 'h + x' where x is the distance from the datum line to the

bottom of the tank. To overcome this the gauge is usually "set fast". Fig. 3.1 1 a.

Another way which is not always physically convenient is to mount the gauge at the

bottom of the tank whilst still taking the tapping point from above the sediment level.

Fig. 3.1 lb.

The gauge will read directly the total depth of liquid in the tank.

The gauge can be any pressure measuring device, for example, bourdon tube, bellows,

U tube, enlarged leg manometer etc.

As they stand the above types are only suitable for measuring lev@?1 in open tanks,

since the pressure acting on the gauge (atmospheric)-must be the same as that acting on

the surface of the liquid in the tank, so that the. only differential is that due to the head

of liquid in the tank.

3.5.4.2MEASUREMENT OF LIQUID LEVEL IN CLOSED VESSELS (Fig. 3.12)

With closed vessels in most cases the vessel is closed because the system is to be

pressurised, or to operate at conditions other than atmospheric.

In these cases it is necessary to see that the same conditions exist on the reference side

of the indicator as inside the container, so the reference limb is fed back into the top of

the vessel.

In all cases the point of measurement should be made just above the base of the vessel

to prevent settlement of sediment into ganometer casing.

With level measurement, this form of indication is usually required for vessels sited in

difficult positions requiring the measuring unit to be located below the tank. With



FIG. 3.11 MEASUREMENT OF LIQUID LEVEL IN OPEN TANKS

PAGE70



this condition on the connecting line to the meter will at all times contain a heat of

liquid above the metering fluid through which pressure changes, due to level changes,

will be transmitted to the meter.

The pressure due to this connecting length of pipework must be allowed for in any

calculation and this is commonly referred to as a static or suppression head.

3.5.4.3CLOSED VESSEL WITH CONDENSIBLE VAPOUR (Fig. 3.13)

With closed vessels a further condition that may produce errors is when the pressure in

the tank contains vapour and these vapours condense on top of metering fluid in

reference limb, again causing additional pressure factor which must be taken into

account.

A CONDENSIBLE VAPOUR (NEOLIGIBLEPRESSURE)

B LIQUID P1 (DENSITY)

C CONDFMMG CHAMBER

(AREA WRY MUCH GREATER THAN MANOMETER LRAB

CONTATMING COWDENSING LIQUID.)

D MEASURTNO LIQUID P2 (DENSITY)

FIG.3.13 LEVELMEASUREMENTINCLOSEDVESSELWITHCONDENSIBLEVAPOUR



To offset this condition condensing chambers are used. These are chambers with a

considerably greater area than the meter chamber areas, so that the level of liquid in it

does not change much when the metering liquid moves in the manometer. The whole

line will thus be filled with condensate, thus forming a pressure head of relatively

constant value, any additional condensation now overflowingbackinto- the vessel.

If a U-tube manometer is used as the differential pressure measuring device then the

head of liquid (H) in the tank can be related to the manometer reading (h) as follows:

PAGE 72

Pressure at X = Pressure at Y

gp1H + gp1H1 + gp2h = gp1Hc

p1H + p1H1 - p1Hc = - P2h

p1 (H + H1 - Hc) = - P2h

H+H1-Hc = -h p2 / p1

So,H = Hc-H1-h p2 / p1

This system will introduce an error if the temperature of the liquid in the tank is differnt

from that of the surrounding air. The condensate in the right hand limb will have a

different temperature and consequently a different density from that in the tank.

It may be desirable to modify the system by returning the condenser leg to the inside of

the vessel.

Errors can also be reduced by the use of insulation or heat jackets.

EQUALISING CONSTANT HEAD VESSELS:

Specially designed condensing vessels are used where the condensible vapour is at very

high pressure and temperature such as in boiler drums. These vessels are meant to

provide both the columns of liquids (Reference and variable) at equal temperatures at

all time and all possible operating conditions to the measuring devices such as

transmitters etc.

Fig. 3.13'A'shows two of the most common types of equalising vessels used in Drum

Level measurements. These vessels are also used for condenser hot well levels and also

for some high pressure heater shells.

3.5.4.4DIFFERENTIAL PRESSURE TRANSMITTERS FOR LEVEL

MEASUREMENT



It is much more common to use differential pressure transmitters instead of manometers

as the level measuring device.

Refer to the pressure Measurement notes for details of pneumatic and electronic types

of differential pressure transmitters.

3.5.4.5LIQUID SEALS

When there is a danger that the liquid whose level is being measured will, due to its

PAGE 73

PAGE 74

nature, adversely effect the manometer fluid or transmitter diaphragm material then

liquid seals should be used.

The sealing liquid must not mix with the vessel liquid, be attacked by it, or absorb

corrosive elements from it. Of course it also must have no adverse effects on the

manometer i or iap ragm materia.

4.5.4.6ZERO SUPPRESSIONIELEVATION SPAN AND RANGE CONSIDERATIONS

When using a differential pressure transmitter for level measurement the location and

type of system (i.e. open or closed tank, dry or wet leg) must be considered when



specifying the range, span and zero offset facilities to the manufacturer and when

calibrating the system.

3.5.4.7GAS PURGE SYSTEM OF LEVEL MEASUREMENT (Fig. 3.14)

Basically this method consists of a tube which is inserted into a liquid whose depth is to

be measured. An air pressure is applied to the tube and the air pressure is built up until

bubbles just begin to escape from the bottom of the tube. Bubbles will escape only

when tli@ pressure in the tribe is just above the pressure exerted on the bottom of the

tube by the height of liquid above the bottom of the tube.

When bubbles escape, the pressure in the pipe is p = pgh. when the density of the liquid

is known the pressure will be proportional to the height of the liquid above the bottom

of the tube. Therefore if the pressure in the tube is measured by a pressure gauge or U-

tube the scale can be calibrated in terms of depth of liquid or into anyunits required

such as volume or weight of the liquid in the tank.

This is a widely used.system employed in a wide range of industries. It is satisfactory

for nearly all corrosive liquids and liquids with suspended solids. Practically the only

limitation to the system is that clogging of the bubble pipe may occur on a few semi

solids such as chemical slurries.

The tube is never inserted up to the bottom of the tank since sediment may form at the

bottom and block the tube. Because of this the tube is usually inserted to a depth about

60 mm above the sediment level. The tube end isusually mitred to 450 to provide an

accurate datum point.

The range of this system will vary from about 250 mm up to an upper level which is

limited only by the pressure of the gas supply.

3.5.4.7.1 GAS PURGE SYSTEM - OPEN OR VENTED TANK (Fig. 3.15)

The system consists of.

(A) Regulator valve set at a pressure slightly higher than that exerted by the

maximum head of liquid in the tank.

(B) Needle valve for controlling flow of gas.



(C) Bubbler or sight glass to indicate flow rate.

(D) Standpipe or dip tube.

The regulator valve is set so as to give a pressure slightly higher than that exerted by

the head of liquid in the tank, when the level of liquid is at its maximum level. To set

this, the level in the tank is adjusted to its maximum level, the needle valve is fully

opened and the regulating valve is adjusted until bubbles issue freely from thebottom

of the standpipe, the regulating valve is then clamped. The level in the tank is then set

at its minimum level and the needle valve is adjusted until the rate of bubbling is

about 60 bubbles per minute. The pressure at the bottom of the standpipe will always

be approximately the same as that exerted by the head of liquid above it. Suppose the

level ishalfway, the pressure in the stand-pipe willbe equal to the head of liquid above

the bottom of standpipe. Assume the level drops, the excess pressure in the standpipe

will bubble away until the pressure in the standpipe is equal to the head of liquid.

Assume the level rises, the pressure in the tube will build up until the pressures are

equal, when all excess pressure again bubbles to atmosphere.



The rate of bubbling is kept to a maximum of about 60 bubbles per n-dnute because, if

the flow went above this, a pressure drop would occur down the line and the indicator

would show a pressure higher than that at the bottom of the standpipe.

3.5.4.7.2 GAS PURGE SYSTEM - CLOSED TANK (Fig. 3.16)

A typical installation for a closed tank comprises of.

A. Reducing valve, set at a pressure greater than that in the tank.

PAGE 77

FIG. 3.16 TYPICAL INSTALLATION FOR CLOSED TANK

BB. Needle valves, used to adjust the rate of flow of purge gas.

CC. Bubblers or sight feeders.

M. Manonfeters or any other differential pressure measuring instrument.

S. Standpipe.

The differential pressure indicated by the manometer will indicate the level of liquid

in the tank. Air flowing into the open part of the tank will ensure that the static

pressures on both sides of the manometer are equal and thus ensure that the only

differential pressure that the manometer will indicate will be that due to the head of

liquid above the bottom of the standpipe.

3.5.4.8 AIR TRAP SYSTEM (Fig. 3.17)

In some cases where measurement of level is required, such as strong corrosives or at

working temperatures unsuitable for diaphragms, the air trap system can be used.



The box is covered by a plate with a small hole just large enough to allow liquid to

enter. As the level of liquid in the tank rises, the pressure on the air trap increases,

liquid flows into the trap and compresses the gas in the trap. When the air pressure plus

the head of liquid in the trap is equal to the head of liquid above the trap, no more liquid

enters the trap. The air pressure set up can be measured.by a suitable indicator or

recorder which can be calibrated directly in terms of level.

PAGE 78

PAGE 79

3.5.4.8.1BELLOWS TYPE (Fig. 3.18)

In the above type the diaphragm box is replaced by a box containing a bellows of

synthetic material. Changes of pressure within the bellows due to changing levels are

communicated to the measuring bellows by copper tubing having a fine bore. The

bellows are filled with air at a pressure slightly above atmospheric pressure. As the

level increases the measuring bellows is compressed - this increases the pressure in the

system and the detecting bellows detects the change in pressure and indicates it on a

gauge, calibrated directly in units of level. In common with other instruments, its



reading of depth will be in error due to the change in density of the tank contents with

change in temperature.

If thebellows is truly flexible, then increase of temperature of the gas within the sealed

system will not influence the reading for, as soon as the pressure within the system

increases due to the increase temperature, thebellows will allow it to expand until its

pressure again balances that due to the liquid. The same kind of thing happens in the

diaphragm box.

FIG.3.18 BELLOWSTYPETANKCAUGE

3.5.4.8.2 DIAPHRAGM STACK SYSTEM

As the level increases the diaphragm stack is compressed, this compresses the air in

the system which creates an increase in pressure. This increase is detected by a

suitable indicator which is calibrated in terms of liquid level, volume or weight.

PAGE 80

ELECTRICAL/ELECTRONIC METHODS OF

LEVEL MEASUREMENT AND CONTROL

Electrical methods for level measurement are very useful as generally there is minimum

limitation on transmission distances between transducer and display or control devices.



Their speed of response is often better than pneumatic systems and they are very useful

when measuring the levels of vessels containing solids.

Depending on the medium it is sometimes important to ensure that the electrical

system is intrinsically safe should there be a danger of fire or explosion as a result of an

electrical spark.

3.5.5.1CONDUCTIVITY METHODS

If the liquid or solid in a vessel is a reasonably good conductor of electricity it is

possible to utilise this property in several ways for level measurement/ control

purposes.

3.5.5.1.1CONDUCTIVITY METHODS-LEVEL MEASUREMENT ( Fig.3.19)

The system consists of number of electrodes of different lengths connected together by

a series of resistors. As the level increases more and more conductors are shorted

together. The shorting of the resistors joins them, thus the overall resistance will

decrease.

If a constant voltage is applied across the terminals, then as level increases, resistance

decreases, hence the current flowing in the circuit will increase. Therefore, current will

be proportional to the level. If an ammeter is placed in series with the circuit, then it

will indicate the current flowing in the circuit. Since the current is proportional to level

the ammeter can be calibrated directly in terms of level.

PAGE 81



This method can be adapted for use in a manometer level measurement system by

locating the electrodes in the mercury of one of the limbs.

3.5.5.1.2 CONDUCRIVITYMETHODS - HYDRASTEP SYSTEM

(Fig. 3.20,3.20 a to Fig. 3.20c)

The Hydrastep system is probably the most common system used for boiler drum level

measurement. It has three main advantages over traditional gauge type systems:

a) Smaller errors incurred due to change in liquid densities.

b) The output supplied to a conventional analogue level controller can be easily

checked for error using the level indicating lamps.

c) Digital outputs for computer or microprocessor control logging can be readily

provided.

Density errors

In sight gauge and head pressure manometer systems it has been assumed that the

density of the liquid remains constant throughout, but this is not necessarily true. If

the temperature of the liquid in the limbs varies then its density varies thus errors in

level indication will occur.

For example, if we consider a simple U-tube manometer with an equal pressure

applied to each limb then the levels will be thesame. If, however, the liquid in one

limb is heated its density will change and the level in that limb will rise giving an

erroneous reading.

Let's consider a typical gauge installation system such as a visual gauge with c.c.t.v.

display or a simple manometric gauge.

The top of the gauge is normally heated to saturation temperature from the steam

side, but the part of the gauge below the water/steam interface draws its heat from

the water column only, the mean temperature of which falls by some 900C for a

boiler operating at 183 bar. (T sat 358OC).

As shown in the graph the density of the water at 3580C (in the drum is 0.54 g/CM3,

whereas that in the gauge at, say, 2700C mean for a 0.25 m (10 in) water column

averages 0.77 g/CM3. The manometer will then balance with the gauge indication

showing about 0.146 m (5 3/4" in) lower than the actual level in the drum, after

allowance is made for the additional weight of the steam accommodated in the

difference (the steam density of 0.137 g/CM3 is significant at 358OC).



The visual gauge system is, further impeded by referactive index changes at high

temperatures and pressures.

Simple manometers with a condensate column used as a reference,also suffer urther

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PAGE 85



from density changes resulting from different temperature gradients within and between

the two columns causing increased errors.

Density errors can be reduced by using a steam-jacketed manometric gauge but they are

still substantial.

Thus if an analogue signal is obtained from a manometric system then density errors

must be compensated for by deriving an analogue signal from drum pressure (or

temperature).

The Hydrastep vessel also uses a side-arm method of attachment to the drum, and

carries a number of separate electrodes spaced vertically at invtervals, usually of 2550

mm (1~2 in), each of which is associated with a separate channel of the electronic

indicating system. The design of the vessel, however, gives a very much reduced

density error. The conventional visual gauge body has a small cross-sectional area and

a small bore, with only a small flow of condensate. A by-pass tube is often fitted so

that condensate from the steam, pipework is diverted from the gauge. Because of the

small cross section, the heat flow into the gauge body occurs more or less equally from

both the steam and the water, and becausg of the slow flow, the temperature gradient of

the water column is large.

The Hydrastep vessel has a metallic cross~section some four times that of the visual

gauge, and a bore cross-section of about ten times. The reduced thermal resistance

vertically permits a substantial quantity of the heat required by the lower half of the

vessel to be supplied from the steam space, which is of course maintained at saturation

temperature. In addition to providing a larger surface area for heat exchange purposes

in the steam space, the large bore reduces the turbulence of the high condensate flow

and encourages the formation of a significant boundary layer on the inside of the vessel

wall below the water/steam interface.- and this layer acts as a partial thermal insulator.

Instead of the mean water column temperature for a half-full gaugebeingabout

90"Cbelow saturation temperature, as in the visual gauge, the Hydrastep vessel exhibits

only about 80C mean drop, which results in a density error of only one-sixth of that of

the visual gauge.

Basic principle of the Hydrastep system



The principle upon which the Hydrastep is based is that of the differing electrical

resistivities of water and steam. Accepting for the moment that this difference exists,

the vertical arrangement of electrodes in the vessel, each of which is connected to a

separate detector vessel, each of which is connected to a separate detector circuit,

enables the level to be determined at which the transition occurs between water and

steam values.

Each electrode, with its associated portion of the vessel, forms a cell in which the

resistance measured is a function of the contents of the cell. Except at the interface,

each cell is filled either with steam (high resistance) or with water (low resistance); at

the interface for a continually falling water level (gradually increasing cell resistance)

the corresponding channel of the detector follows the curve shown. As the output

voltage reaches about +4.SV DC, the corresponding logic circuit changes to indicate

PAGE 86



PAGE 89

steam. A small amount of hysteresis is built in so that in the reverse direction the

change back to water occurs at about +3.5V DC, to avoid excessive operation due to

insignificant level fluctuations.

At the time that the use of a direct electrical measurement was first considered for the

determination of drum level, little was known of the resistivities of water and steam at

elevated temperatures as exhibited in a dynamic system with continuous condensate

flow through a side-arm vessel from the boiler drum. A series of measurement were

made, therefore, using the vessels designed for the field trial equipments, on boilers in

the 1 1 0 and 183 bar pressure groups. These measurements are presented and show



that up to 360'C, the water resistivity is always less than 106 ohm. cm and the steam

resistivity always greater than 5xl 07 ohm. cm (and almost always greater than lffl ohm.

cm i.e. a differential of two orders of magnitude from cold to 360OC).

Other work shows that an adequate differential for Hydrastep exists between the

resistivities of the water and the steam in a side-arm gauge at boiler pressure up to

about 216.5 bar (3140 lb. f/in2, Tsat 372.8OC).

This switching band for the Hydrastep electronic circuits is also shown superimposed at

approximately midway between the water and steam resistivities. The anomaly shown

at 140-1800C concerned the related water and steam readings taken during a severe

steam valve leak which resulted in steam entrainment in the vessel water column and

water droplets in the steam space.

To ensure absolute safety to personnel the maximum voltage which appears at an

electronic terminal is 1OV rms, and its maximum short circuit current is 1OuA, 50 Hz.

Each electrode circuit therefore meets the requirements for intrinsically safe apparatus

with a margin of safety of five orders of magnitude in respect of current. At 10 uA, the

maximum electrode current is only one-fortieth of the 0.5% human perception current

at 50 Hz. The mechanical design of the electrode is such that its centre be ejected from

the vessel in the event of failure of the ceramic insulation, and a guard is fitted to

deflect any steam jet which may arise from a faulty electrode or seal.

The potential on each electrode is applied to its own individual discriminatorchannel to

control an output electromagnetic relay carrying six sets of changeover contacts.

One set of contacts is used for display purposes in the control room either for water or

steam, as appropriate, for each channel of the electronics, each display module being

arranged in the order corresponding to the disposition of the associated electrodes in the

vessel. Two further sets of contacts on each relay are used in a logic matrix to raise an

alarm should a fault occur such that any channel is "out-of-step", i.e. that it gives an

indication which is a physical impossibility such as "water above steam" or "steam

below water".

(Note: This logic matrix can be connected to a recognised alarm system, such as the

station annunciator, for hydrastep to be accepted as a legal alternative to the visual

gauge if required).

The three remaining contact sets are available for high or low level alarm purposes



or for additional logic configurations to provide validated control signals, alarms and/or

emergency tripping of the generator and its auxiliaries.

Further "Fail-operative" safeguards are provided by the connection of alternate channels

and electrodes on any one vessel to electrically separate power supplies, providing an

interleaved system. The loss of one power supply will still allow even a single

Hydrastep to operate within the terms of the Factory Inspectors' Certificate of

Approval, and the instrument can be repaired with the generator still on load.

Failure Characteristics

The appearance of the display under normal conditions is shown in the left hand

column of the drawing. A colour change principle is used for each display module to

avoid the ambiguity possible between a true fault and a burnt out lamp where a simple

on-off arrangement is used.

A power supply failure appears as in either column 2 or 3, and the failure of a single

channel as in the columns 4 to 1 1. In no case of an electronic fault does the indication

error exceed one step. It is a simple matter to include an automatic comparison

between adjacent steps, on the premise that water cannot exist above steam in the vessel

in sufficient quantity to cause such an indication (columns 4 and 8). This comparison

may be performed quite easily by means of additional contacts on the relays controlling

the display lamps. The usual station annunciator operates when a connection is made

between the alarm bus-bars. In the logic matrix, wherever the water/steam interface

may be, all channels above it should show steam with their contacts in the upper

position and all below should show water. If a fault should indicate water more than

one channel above the interface - for example, as shown dotted on channel 1 1 - the

bars are shorted through 8 and 7, causing an alarm. Other logic systems, and

techniques other than relay circuitry, could be used, including the station computer if

spare capacity is available.

Although electrically separate, the odd and even logic drive circuits are physically

adjacent for ease of interconnection and are mounted with the power supplies close to

the display unit in the control room.



A design of colour change module using sub-miniature long-life fflament lamps has

enabled a small graphic display to be used, which could be mounted directly into the

control console.

Two-gauge Hydrastep

In the simplest multi-gauge instrument vessle A carries the odd numbered electrodes

and vessel B the even, when the whole of the electronic system may be identical to that

already described for the single Hydrastep. Separate pipework for the two vessels is

essential so that pressure variations caused by a fault on one vessel will not affect the

performance of the other, and so that either may be shut off independently. Since the

water steam interface'within the drum is not a plane surface under operating conditions,

the inner ends of the waterside pipes must terminate at substantially the

PAGE 91

same point in the drum so that the same head of drum water is applied to both

manometers. This prediction is not required for steam connections.

It is worth noting that a leak or blockage on either the steam or the wat r i of a

vesseloritspipe-workwillresultinafractionalpressuredropinthevesselconcemed, the

manometer will then rebalance the vessel showing a higher level than for the nonfaulty

one. This means that the faulty half of a two-gauge Hydrastep arrangement may be

identified and switched outso that the gauge still remains operative using the sound

vessel. The only exception to this condition occurs with a leak of such proportions that

water cannot remain in the v@sel. Such a leak would normally have developed

comparatively slowly from a minor leak which should already have been recognised;

but in any case, the operation of a low level alarm with one half of the Hydrastep

showing a level within normal tolerance will identify the fault.

Twin Hydrastep

The standard twin Hydrastep gives additional security by the provision of identical

Hydrastep units, A and B operating from both ends of the drum.

Not only does this extra redundancy permit the shut down of a complete end (e.g. to

exchange a faulty electrode or valve packing), with the generator on load, but under

normal conditions the adjacent arrangement of the two electrically separate displays

from the drum ends gives the operator valuable information concerning end-to-end



level variations either cyclic or static, which can occur under certain plant running

conditions.

Four-gauge Hydrastep (Twin Two-gauge)

All Hydrastep systems are self validating by the continuous comparison between

adjacent channels and the "water-above-steam" logic matrix alarm, which gives an

immediate alert to the operator should a gauge fault cause a reading error in excess of +

1 step (normally ± 50 mm). Considerably higher security can be provided, even against

double coincident Hydrastep faults, where a Hydrastep installation is to be used for

control purposes, and in particular in those installations which are required to provide

an automatic emergency trip for the complete generator set, in the event that a

minimum water level is reached at either end of the drum. It should be appreciated that

on a typical 500 MW unit at full load, approximately 14 @6inds are available for shut

down to be initiated after the minimum level is reached before internal pipework is

subjected to damaging conditions. Because of the possible endto-end level difference,

each drum end must be separately protected and reliance cannot be placed upon cross-

validation between Hydrasteps at opposite ends of the drum. Furthermore, because of

the very close timing sequence for the start up of standby feed pumps or the opening of

alternative feedwater valves, it is necessary that a trip must not be initiated before the

minimum level is reached. This is basically an economic consideration: if a trip should

be initiated while the situation could still be saved by the standby plant, a loss of

revenue could result from the need to use lower efficiency plant to meet demand during

that period (.i.e. the penalty for an "unwanted action" occurring). On the other hand if a

trip is not initiated at the

PAGE92

required minimum level widespread damage could be caused to the boiler, furnace,

turbine and generator (i.e. the penalty for failure of a 'wanted action"). It is essential

in this application that the security of the Hydrastep must be maintained and that the ±

one step tolerance must be eliminated even in the presence of a gauge fault, since

there is no permissible deviation from the "wanted" trip level. Such characteristics

can be achieved using a four vessel Hydrastep in which the water side pipework for

both vessels at either drum end terminate at substantially the same point (about 100

nun apart) within the drum.



The relay logic matrix is connected to give a 'three-out-of-four' system, that is three

out of the four gauges have to indicate a low level before a trip is initiated.

Hydrastep - Display Unit

Instead of a sliding box as in the rest of the equipment, the display unit uses a hinged

door form of construction, with a removable rear cover. Sufficient clear space around

the unit must be left to allow access. The philosophy of continuous comparison

between adjacent channels by the logic matrix ensures that the indication presented to

the operator has been fully verified. The channels at either end of one vessel (i.e.

channels 1 and 12 of both drum end indicators in the case of the standard Twin

Hydrastep) can each be verified on one side only, since channels 0 and 13 do not exist.

If, for example, the case of a steady falling water level is considered in conjunction

with a fault on channel 1 such that water is permanently indicated, it would appear to

the operator that some water still existed in the gauge although, in fact, the fall in level

had continued past this point. Had a channel 0 existed below channel 1, as the level

continued to fall, a "water abovesteam" condition would haveappeared and the fault

on channel 1 would have been recognised. Without channel 0, therefore, channel 1

cannot be fully verified and is not presented to the operator since it could be n-

dsleading; similarly channel 12 could be suspect without a channel 13 for verifica-

tion. To ensure that no misinterpretation can occur, therefore, only channels 2 to 11

inclusive are displayed in normal operation channels 1 and 12 being covered by the

hinged outer panel. This outer panel, however, may be opened by the Instrument

Engineer to gain access to the remote test switches and telephone socket, when the

indication from these channels can also be observed.

Because of the level differences between drum ends which can occur in normal

operation, it is impractical in a Twin Hydrastep to cross-validate any channel on one

vessel and the corresponding channel on the other, and the restriction in the previous

paragraph must apply to both columns of the dual display. However, in the special

case of the four-gauge system, whether or not the automatic tripping facility is

connected, crossvalidation may be incorporated at all the available corresponding

levels between the two vessels at the same end of the drum and all indications

including the extremes may then be presented to the operator.

3.5.5.2 CAPACITANCE METHODS (Fig. 3.21)



A capacitor is a device for storing electrical energy. In its simplest form it consists of

two plates of area, A, separated by a distance, d. The air between the plates is called the

dieletric.

PAGE 93

PAGE 94

When a voltage is applied across the plates an electrical charge is stored proportional to

the applied voltage.

Q = CV where



Q = Charge (coulombs)

v = Applied voltage (volts)

c = Capacitance (farads)

The constant of proportionality, C, is called the Capacitance of the device.

The capacitance of a system is detern-dned by the physical parameters of the system and

is given by the equation,

Capacitance, C = Eo,Er A/d (when the dieletric is other than air)

Where

Eo = Perinitivity of free space

Er = Relative permitivity of the medium between the conductors (the dielectric

A = Area of conductors facing or influencing each other

d = Distance between the conductors

A capacitance transducer makes use of the fact that if E,, A or d are varied then the

capacitance of the device will change. The capacitance transducer can be connected

into an electronic circuit such that change in value of capacitance will affect the circuit

to produce a change in its output signal.

Capacitance level measurement involves the use of an electrode which extends the full

length of the tank and form a capacitance between itself and earth where earth may be

the vessel, the contents or a concentric cylinder around the electrode, depending on the

type of electrode involved.

A variation of capacitance will occur when the depth of the medium in the vessel alters

therefore the capacitance change will be proportional to level.

By this method the level of liquids, powders or granular solids may be measured.

Conducting Mediums

When the medium is a good conductor of electricity then the system works as a variable

area capacitance transducer. The electrode is one'plate' of the capacitorand is insulated

with a material that is compatible with the medium, the insulation forming the dieletric.

The medium in the vessel from the other'Plate'of the capacitor. Thus, as the level

changes, the area of the capacitor 'plates' varies. If level rises then area increases and

capacitance increases, if level falls then area decreases and capacitance decreases.



Non-conducting Mediums

When the medium is non-conducting the electrode is not insulated and the system

works as a variable-dieletric capacitance transducer. The dielectric is made up of,

say, liquid in the tank and the air or gas in the space above the liquid, thus the

dieletric constants will be different (normally those of liquids are much greater than

gases). As the liquid level varies then the overall capacitance will change due to

change in dielectric. A rise in level will increase capacitance and fall in level will

decrease capacitance.

3.5.5.2.1 TUNED OSCILLATOR SYSTEM (Fig. 3.22)

This system works on the principle that the frequency of oscillation of a tuned

circuit is determined by the equation,

fr = 1

2nFLc

Where fr = Resonant frequency

L = Inductance

C Capacitance

Thus, if the capacitance electrode is used as the capacitorina tuned drcuit with a fixed

inductance then, as level varies, the value of capacitance of the electrode varies

therefore frequency of oscillation varies. This change in frequency can be converted to

a change in voltage using a frequency discriminator circuit.

The vessel walls and plate A form the two electrical points of reference, such that

changes in liquid contents will change the capacitance between these two points.

This system is not often used as a method of continual indication of level but is useful

for level alarm or on/off control purposes.

A 'high level' alarm system where the medium does not come into contact with the

electrode can, be obtained by mounting the electrode horizontally in the vessel above

the high alarm datum point.

3.5.5.2.2 ROTATING PADDLE SYSTEMS

This system, which consists of a paddle rotated by a motor, is used for detecting levels

in vessels containing solids. It does not give continuous level indication.



When solid material covers the paddle, which is driven by a robust low speed

synchronous electric motor, rotation is prevented, and the motor stalls. The torque-,

reaction of the motor frarlie rotates it slightly, to actuate a micro-switch, signalling the

presence of solid material, and the motor remains stalled and operating the switch,

PAGE 96

PAGE97

provided that power supply is maintained, until the solid level falls sufficiently to allow

the paddle to rotate again.

3.5.5.2.3 VIBRATING TUNING FORK LEVEL SWITCHES (Fig. 3.23)

This system has proved very reliable for solid particle level alarm measurement and

could be used in Power Stations to detect pulverised fuel & ash levels in the precipitator

hoppers etc. Rotating paddle switches and capacitance controllers have been found to



be high maintenance items and u nreliable for this application. The self cleaning, self

checking vibrating level switches have proved to be the solution.

The system consists of a Vibrating Fork Transducer which has incorporated in it two

piezoelectric crystals and a Signal Processing Unit.

The property of piezoelectricity means that if electrical oscillation is hnpressed across

the long axis of the crystal, corresponding mechanical vibrations will be set up across

the flat axis and vice versa. One of the transducer crystals is used as a transmitter

excited by an 80 Hz source. The tails of the tuning fork are arranged to pick up this

vibration a,.id, in turn, vibrate at the same frequency with an amplitude of 0.5 mm.

The second crystal acts as a detector and senses the mechanical vibration of the fork

and converts it to an electrical signal. It transmits a current to an amplifier and alarm

relay in the signal processing unit.

In this condition the alarm senses that the fork is in air. When the dust level rises

within the hopper to immerse the fork, vibrations are damped out and the detector

crystal no longer detects a signal. Thus the current signal to the processing unit is

reduced such that the alarm relay contacts indicate the 'hopper fuly condition.

When the hopper level drops, the probe becomes free of material and the vibration of

the tails is restarted by the transmitter crystal.

An in-built test circuit detects an open or short circuit fault in the transducer to signal

processing connections and gives L.E.D. indication of this on the signal processing unit

front panel.

These transducers can be inserted to lengths of up to 4 metres and will operate at

temperatures up to 150"C. The signal processing unit has an operating temperature of -

2011 to +600C.

3.5.6 ULTRASONIC METHODS

Ultrasonic

Ultrasonic beams are a form of energy transmitted by means of mechanical vibrations

and carried through the transmitting medium in a series of waves. They are produced

by a generator converting one type of energy into mechanical vibrations which are

received by a device which detects the ultrasonic beams con @verting them into a more



PAGE99

readily usable form of energy. Above a certain frequency (20 kHz) it is known as

ultrasound or ultrasonic sound. For level switching a range between about 36 and 40

kHZ is used. Ambient noises or their harnionics are ineffectual in this range.

3.5.6.1THE ULTRASONIC LEVEL SWITCH (Fig. 3.24a and 3.24b)

This system detects ultrasonic sound at a diaphragm of the receiver and converts it into

electrical energy; this, in turn, is fed into the input of a high gain amplifier which feeds

it to the, transmitting sensor. Here the electrical energy is converted into ultrasonic



energy once again and radiated from the diaphragm of the transmitter. If the transmitter

and receiver diaphragms are facing each other, and there is no solid body in their path,

then energy will be picked up by the receiver, again amplified and fed to the transmitter

forming a closed loop. Once established, if an object appears in the beam, losses are

introduced into the system and the signal will cease. Hence rise of liquid or solid level

up to the beam can initiate an alarm or a switch. The device cannot be used for

continuous measurements.

Principle of Operation of the Sensors

When certain materials, mainly nickel, iron and cobalt, are placed within a magnetic

field, their wave lengths will vary by an amount dependent on the strength of the

magnetic field.

The fundamental generatoris a nickel tube which carries the coil and biasmagnet. The

current through the coil either weakens or strengthens the field, depending on the

direction of the current. Application of an alternating current causes the length of the

tube to increase and decrease at the supply frequency. Owing to the mechanical

properties of the tube it will tend to oscillate longitudinally as a half-wave resonator.

Similarly with the receiver, a sound wave impinging on the diaphragm will cause a

relatively large amount of movement in the nickel tube, if within the band path's

frequency, virtually none if outside. Changing the length of the tube will cause a

change in the magnetic strength of the bias magnet, thereby generating an e.m.f. within

the coil. Hence the same unit can't be used as either a transmitter or a receiver.

The system is unaffected by dirt, vapour, moisture etc. The sensors are

temperaturesensitive; the resonant frequency falls as the temperature rises, but there is

no effect if both sensors are at the same temperature. Very little maintenance is

required.

3.5.6.2 LIQUID PHASE TYPE (Fig. 3.25)

Pulses of ultrasonic energy are directed upwards, through the liquid, to the surface of

the liquid. These pulses rebound from the surface back to a receiver which may be a

component part of the pulse transmitter. Level, variations are very accurately measured

by detecting the time interval taken for the waves to travel to the surface and back again

to the receiver. The longer the time interval the further away is the liquid surface,

which means that the greater the depth of liquid the greater the time interval.



The ideal applications are large storage tanks, oils, inflammable liquids, chemicals,

free flowing and semi: solids also as level measuring methods in aircraft and marine

equipment.

3.5.6.3VAPOUR SPACE TYPE

The units are similar to those used in the liquid phase type except that the ultrasonic

sound waves are transmitted from the transmitter through the "vapourspace' (gas or air)

above the surface to the free surface of the liquid where it again rebounds. As before

the time interval changes with depth of liquid, but now an increase in depth decreases

the time interval, as now column has lower density.

It should be stated that these sonic systems have been described and illustrated very

simply and briefly. Some are highly complicated in design, application and operation,

and are also very expensive.

3.5.7 NUCLEONIC METHODS

Nucleonic

Since the advent of nuclearre actors and there a dy availability of radio active materials,

nuclear techniques have been employed for the extension of some of the more

conventional methods of level measurement, as well as the invention of new methods.

The special advantage of nuclear gauges is that they can operate entirely from outside

the containing vessel. They may be designed to provide on/off control at a fixed level

in the vessel, or to provide continuous indication of level over a given range

The nucleonic type level instruments involve a radioactive source, a radiation detector

and electronic measuring circuits.

As detectors for converting nuclear radiation into electrical energy are related to level

change, two methods are commonly used:

a) With aid of a Geiger counter.

b) With the use of a specially designed gas ionisation cell.

With Geiger counters two basic relationship are garding intensity of radiation

received

are applied.

1) Intensity varies in proportion to the thickness of any material interposed between

the radiation source and the detector.



2) Intensity varies in inverse proportion to the square of the distance between the

radiation source and the detector.

PAGE103

3.5.7.1 FIXED SOURCE SYSTEM (Fig. 3.26)

In this method the radiation source is fixed. The geiger countermeasures the intensity

of radiation. It gives out a series of electrical pulses. An increase in radiation gives an

increase in the number of pulses generated in a given time. These pulses are fed to an

electronic circuit which gives out an electrical current proportional to the number of

pulses being fed in. This current is used to drive an indicator or recorder. As the level

of the liquid or solid changes it can be seen that the thickness of the material between

the source and the detector changes.

From the first law as stated, it can be seen that changes in level will cause a change in

the intensity of radiation reaching the geiger counter. As the level increases the

radiation will drop in intensity and as the level decreases the radiation will increase in

intensity. Since the geiger counter and associated electronic equipment convert

radiation intensity into an electric current, the indicator can be calibrated into units of

level.

This system is only suitable for level measurement 'over a very restricted range due to

the response/level characteristic being non-linear (logrithmic). Poor accuracy will be

experienced over the higher level part of the range. Changes in density of the liquid

will also affect the accuracy of the displayed level reading.



A. FIXED RADIATION SOURCE

B. GEIGER COUNTER

C. CONVERTOR 70 CONVERT PULSES TO CURRENT

D. INDICATOR

FIG. 3.26 LEVEL MEASUREMENT LUCLCONIC METHOD FIXED SOURCES SYSTEM

PAGE104

3.5.7.2 LINE SOURCE SYSTEM (Fig. 3.27)

This system gives an approximately linear output and is almost independent of medium

density. In involves using a strip of gamma radiation down the full length of the vessel.

To a first approximation the transmission of radiation through the liquid may be ignored

and only the transmission through the air above-the liquid level considered. Change of

level then reduces the length of the strip source, as seen by the detector and there is

therefore an approximately linear relationship between the detector output and the level

of the liquid (measured from the top of the tank). In some cases both a long strip

radioactive source and a long tubular detector are employed. However, long tubular

detectors are not a normal commercial item and if they have to be specially

manufactured, then such installations can prove expensive.

FIG.3.27 LINESOURCESYSTEMLEVELMEASUREMENT-NUCLEONICMEMOD

3.5.7.3 GAMMA SOURCES



It should be recognized that all radio active substances are characterized by a' half-life'

which is the time required for its activity to fall to half of its initial value. Hence, the

meter calibration becomes inaccurate as the source decays, unless the electronic

apparatus is adjusted periodically to compensate for this decay. The difficulty can be

avoided to some extent by using a source of long half-life. A cobalt-60 radioactive

PAGE 105

source, emitting gamma rays of 1.17 and 1.33 MeV and with a half-life of 5.3 years,

decays by 1 % in about 24 days. On the other hand caesium 137, emitting 0.661 MeV

gamma-rays and with a half-life of 37 years, decays by 1 % in about 6 months.

3.5.7.4 NEUTRON DETECTION METHODS

This method depends on the use of neutrons in place of gamma rays. The principle,

however, is quite different and depends on the fact that liquids containing hydrogen or

other low atomic number materials produce an appreciable number of slowed down

(low energy) neutrons when bombarded with fast (high energy) neutrons. Moreover,

the flux of slowed down neutron is a measure of the hydrogen content of the material

under examination. This is, in fact, the basis of a number of methods of measuring

moisture content of soils and other materials. However, for level gauges detection of

the hydrogen only is involved. Hence, we place a 'ee', i.e. one emitting high energy

neutron on one side of the tank, and a slow neutron detector, i.e. one responding to

slowed down neutrons on the other side and by moving the assembly vertically the

position of the liquid level can be accurately determined. Less shielding is required

than for the gamma ray methods and the advantages e.g. only external measuring

equipment, are retained. However, this method will only work with hydrogenous

materials.

PAGE 109

FLOW MEASUREMENT 4.1 INTRODUCTION

Fluid flow in industrial undertakings occur in two general forms: either as a flow in a

pipe or conduit or, in the case of liquids only, as a flow in an open channel. In both

cases, the rate of flow is of primary importance, and, in a large number of plants, the

totalized flow over a specified period is required in addition. The rate of flow

measuring instrument will be examined first.



4.2 RATE OF FLOW MEASURING INSTRUMENTS:

This class may be broadly subdivided into:

a) Differential pressure flowmeters -

i) Orifice pattern

ii) Venturi and nozzle pattern

ii) Venturi and nozzle pattern

iii) Pitot tube pattern

iv) Dall tube pattern

b) Variable area flowmeters

c) Displacement and inferential flowmeters

d) Electromagnetic flowmeters

e) Ultrasonic flowmeters

4.2.1 DIFFERENTIAL PRESSURE FLOWMETERS:

Before describing any particular model of differential pressure flowmeters, it will be

necessary to consider some aspects of Bernouilli's Theorem.

Bemouilli's Theorem - Incompressible Fluids.

Bemouilli's Theorem will be stated here, but not derived, since we are concerned only

with the final result. It must be pointed out that it applies to ideal fluids under

streamlined flow conditions. The full treatment may be found in standard textbooks

dealing with fluid flow. For a stream of ideal fluid, in steady flow' , with no frictional

force involved but acting under gravitational forces only, the sum of the pressure

energy, kinetic energy and potential energy is a constant. Expressed mathematically.

PAGE 110

pg + pV2 +gpz k..... (1)

where p = the pressure

p = the density

v = the velocity

g = gravitational acceleration

z = height above an arbitrary datum plane

k = a constant

The first terirn on the left hand side of equation (1) represents the pressure energy, the

second the kinetic energy and the third the potential energy.



Now consider Fig. 4.1, a fluid stream constricted to flow from a section area AI at AB

to a smaller area A2 at Cd. The fluid, at first, will be taken to be incompressible, so that

the density is unchanged from AB to CD. Continuity of flow demands that the quantity

of fluid Q flowing through A 2 per second is the same as tha t through AI' Since Q is

the product of the area and velocity at each section.

Q A1v1 = A2V2 ..... (2)

where V1 = the velocity at AB

V2 = the velocity at CD

This means that the kinetic energies obtaining at AB and CD are pv12 and pv2

2

2 2

respectively.

The potential energy at AB is gpZ1 and that at CD, gpZ2.

A pressure change is involved from AB to CD. The reason for this will be found in the

various treatments of Bernouilli's Theorem. It can be deduced by considering the work done

in transferring a small element of fluid from AB to CD. Let us designate the pressure energy

at AB P,g and that at CD P2g. The total energy at AB is the same



as that at CD since no heat transfer, etc., is considered involved. Then, the following relation

holds:

p1g + pV12 + gPZ1 = P2 g + PV2

2 + gPZ2 ............ (3)

2 2

Rearranging (3).

p (V22 - v1

2') = g(P1 - P2) + gP (Z1-Z2) ........ (4)

2

V22 - v1

2 = 2g (p1,-p2) + 2g (Z1-Z2) ........ (5)

p

We have seen that

Q = A1v1 = A2V2

Ifm= A2

Al

and (5) becomes

V2 2 (1-m2) 1/2= 2g (P1 -P2 ) + 2gp (Z1 -Z2 )

............ (6)

giving

v2 = 1/(1-m2) 1/2 (2g [(P1 -P2 ) + 2gp (Z1 -Z2 )] / p)1/2 ..............(7)

But since Q = A2V2 (7) can be written

Q = A2 / (1-m2) 1/2 (2g [(P1 -P2 ) + 2gp (Z1 -Z2 )] / p)1/2 `............ (8)

A further step yields

Q =t.A2 (2g [(P1 -P2 ) + 2gp (Z1 -Z2 )] / p)1/2 `............ (9)

where E = 1/ (1-m2) 1/2 and is known as the velocity of approach factor.

PAGE 112

Equation (2) is of the utmost importance as it infers that by constricting a fluid stream to a

smaller area, a pressure differential may be set up which is a measure of the flow rate Q of the

fluid.

The difference (Z1 -Z2 )can be written as H. Then ........... (10)



Q = E.A2 (2g [(P1 -P2 ) + Hp] ) / p) 1/2

If the flow is through a horizontal pipe Z1 -Z2 and (10) simplifies to

Q = E.A2(2g [(P1 -P2 ) / p) 1/2 ........... (11)

or

Q = E.A (2gpd/p) 1/2 ..............(12)

where pd = (Pl-P2)

4.2.1.1 COMPRESSIBLE FLUIDS

With compressible fluids such as gases and vapours, the density does not remain

constant when the pressure changes from p, to P2' An adiabatic gas expansion is

considered to take place, that is, no heat flows from or to the fluid, and no external

work is done on or by the fluid. Applying certain thermodynamic laws, we find 'as a

final result that the original equation (12) must be modified by a constant known as the

expansibility factor, denoted by E. The actual value of the factor is a function of the

ratio of the upstream and downstream pressures, the gas specific heat ratio, and a re

ratios. Curves for, deducing the appropriate value are available in B.S.1042. Since the

density alters with pressure, it must be specified at one - articular pressure value.

This is normally P1

Equation (12) now becomes

Q = E.A2 2gPd / p1 ...................(13)

where p, is the density at pressure p1

Accepting equation (12) or (13) as a basis for measurement, the alteration of area

shall be made. The transition may take place abruptly or gradually. We shall

consider the abrupt change first.

PAGE 113

4.2.2 CONCENTRIC ORIFICE PLATES

A universally used method of making an abrupt change in the cross-sectional area of

fluid stream flowing in a pipe is the concentric orifice plate. This involves a circular

metal plate with a central hole or orifice concentric with the circumference of the plate.

It is fixed between the pipe flanges and is located by the flange bolts. The orifice is

then concentric with the internal bore of the pipe. (see Fig. 4.2).



It will be convenient before describing particulars to see what occurs when an orifice

plate is inserted in a fluid stream in a pipe, and a liquid flow is considered. Fig. 4.3

illustrates the action in a simplified manner.

FIG. 4.3 ILLUSTRATING THE VARIATION OF STATIC PRESSURE UPSTREAM

AND DOWNSTREAM OF THE ORIFICE

PAGE 114

Suppose that tubes are inserted through the pipe wall at the position shown in the

diagram, the pipe liquid will rise in these until the pressure due to the column of liquid

in each tube is equal to the static pressure at that position. The column heights are



then a measure of the pressures, and from observing the different values we may trace

the pattern of the pressure changes as we proceed along the pipe. At positions 1 and 2

there is no pressure change worth specifying.

At 3 and 4, just before the orifice, we find a slight increase in pressure. The stream is

then constrained to flow through the smaller area of the orifice, from which it issues as

a jet. At positions 5 and 6 there are lower pressures than at the upstream positions due

to the change in the stream sectional area. Since this is smaller, the velocity has

increased, and the pressure has fallen. The stream orjet cross-secti(,n decreases in

area after leaving the orifice until it reaches a point, indicated as 7 in the diagram,

where it is minimum and the velocity maximum. This is mainly due to the liquid

being directed inward as it approaches the orifice, and, through inertia effects,

persisting in this direction for a distance after it leaves the orifice. The static pressure

also reaches its minimum value at this position, which is known as the vena contracta.

The distance from the orifice varies with the ratio of orifice diameter to pipe diameter,

but an average value will be about one half the pipe diametf?r. From the vena

contracta, the stream section expands until it reaches the pipe diameter at 8. Two facts

emerge from a study of Fig. 4.3. One is that the downstream static pressure never

recovers its upstream value. This would appear to be caused by the velocity changes

being accompanied by considerable turbulence with resulting dissipation of energy

involving a pressure loss. Taking a typical value of 0.6 for orifice to pipe diameter

ratio, the percentage loss works out at 65 per cent of the differential pressure. Where

pressure loss is important this factor should be borne in mind.

The second point which emerges is that there appears to be a -variety of positions at

which to take pressure tapping or connections for obtaining the differential pressure.

The following are the main tapping positions (shown diagrammatically in Fig. 4.4).

4.2.2.1 D AND D/2 TAPS (RADIUS OR THROAT TAPS)

The upstream pressure tapping is taken at one pipe diameter D, upstream from the face

of the orifice and the downstream pressure tapping is taken at one half pipe diameter,

D/2, downstream from the orifice face, approximately the vena contract a position. The

tappings correspond roughly to positions 2 and 7 in Fig. 4.3.

4.2.2.2 CORNER TAPS



Comer tappings are taken via holes cut obliquely through a flange or bossont epipe,

bringing the inside openings of the holes adjacent to the orifice positions 4 and 5 in Fig.

4.3 correspond.

4.2.2.3 PLATE TAPS

In this variety connecting holes are actually bored in the orifice plate itself each hole

communicating with one face.

PAGE 115

FIG. 4.4 THE POSITION OF VARIOUS PKESSURE TAPPINGS RELATIVE TO AN

ORIFICE

4.2.2.4 FLANGE TAPS

These are situated 1 in. from the upstream and 1 in. from the downstream face of the

orifice plate, with the tappings bored through the flanges.

4.2.2.5 VENA CONTRACTA TAPS

The upstream tappingis I pipe diameter from the upstream face, and the downstream

tapping is determined from the curve, relating the required dimension to the ratio of

orifice to pipe diameter. These are very similar to the D and D/2 taps.

4.2.2.6 PIPE TAPS

These may be 2 1/2 pipe diameters upstream and 8 diaameters downstream from the

upstream face of the orifice plate.

4.2.2.7 CARRIER RING

Where it is not desirable to drill or tap actual pipes, bosses, or flanges, a self-contained

orifice assembly may be inserted between pipe flanges. It consists of a metal ring



holding the orifice plate, with tappings drilled through the ring to communicate with the

upstream and downstream sides of the orifice. Fig. 4.5 shows diagrammatically a

carrier ring assembly. One advantage of this type is that all drillings etc. are carried out

at the manufacturers works and errors due to site operations are eliminated.

Having established the possibilities of a definite constructive device for fluid flow

measurement under ideal conditions, we must now examine what modifications are

necessary in practice.

PAGE 116

4.2.2.8TURBULENT FLOW

In practice in all cases of flow in pipes for industrial purposes the flow is turbulent, that

is, the particles of the fluid do not follow paths parallel to the direction of flow. Some,

if not all, of the particles have a transverse motion as well as longitudinal one and form

little eddies or swirls giving rise to turbulence. Stream line or laminar flow formulae

will not apply here without modification and a new set of equations must be derived.

4.2.2.9DISCHARGE COEFFICIENT

Due to friction and velocity distribution, the practical flow figures do not agree to the

the oritical ones. Observe that the stream area contracts after leaving the orifice to the



vena contract a position (Fig. 4.3). The cross-sectional area there may only be about 0.6

of that of the orifice, and since it is the stream area from which equations (12) and (13)

have been evolved, for determining the measurement, the equation m- ust include a

correction factor for the friction effects and another for the contraction effect. It is

customary to incorporate these two into one constant,, nd it by name Discharge

Coefficient and letter C.

4.2.2.10 REYNOLDS NUMBER

All liquids are viscousin nature and the viscosity enters in to the determination of flow

constants. particularly the discharge coefficient. In fact, it may be shown that the

latter is @a function of vdp where v is the mean velocity through the orifice, d is the

orifice diameter, p is the fluid density, and n its absolute viscosity. The term v@p is

known as

PAGE 117

the Reynolds number. This factor is dimensionless, and a useful criterion by which

to compareflowsingeometricallysimilarinstallationsbutwithdifferingflowconditions.

It also furnishes a means of indicating the conditions where stream line flow ceases

and turbulent flow begins, and the Reynolds number for the transition region is about

2000-2200. In any of the official publications covering flow practice, e.g., "Fluid

Meters" by the American Society of Mechanical Engineers, British Standard

Specification No. 1042. and the German V.D.I. publication on' Rules for normal

flow nozzles and Orifices" will be found, discharge coefficient prepared for varying

Reynolds numbers and orifice/pipe diameter ratios so that calculation is rendered

easier.

4.2.2.11 GENERAL RULES FOR ORIFICE DESIGN

Some general rules relating to concentric orifices may now be quoted. In view of the,

comprehensive nature of B.S. 1042, part 1, it is only possible to quote a few general

provisions.

1) The thickness of the orifice plate should be sufficient to prevent distortion by the

differential pressure across it. This, however, does not normally call or

excessive thickness. The thickness should not exceed 0.1 d for d/D < 0.2 and

.02D for d/ D > 0.2. If the thickness exceeds this, the downstream edge is

bevelled.



2) The upstream edge of the orifice must be quite sharp except for conical entrance

and quarter circle types and the bore should form a right angle with the face of

the plate. Any alteration in this will effect the discharge coefficient.

3) Coefficients are published for vanous types of orificesinpipelines. The

minimum orifice diameter varies between 6.4 mm and 0. 707D for pipe dia <

50 mm and less than 0.837D for pipes > 50 mm dia.

4) The orifice plate should possess an identification tongue as indicated in Fig.

5.2.

5) The orifice plate must have a small hole drilled in it. In the case of liquid flow,

the hole is situated above the orifice opening to allow passage of entrained air

or gases and so prevent a gas or air pocket building up. In the case of air, gas,

or vapour flow, the hole is placed below the orifice, and nearly flush with the

pipe bottom, to allow condensed moisture to drain through. The hole must be

at least 901 displaced from the pressure tapping.

6) Orifice (either plate or carrier type) positioning is effected by the ring of bolts

clamping the flanges together. The outside diameter of the orifice plate must

be made so that it fits accurately' Within the bolt ring.

4.2.2.12GENERALFACTORSAFFECTINGTHEPERFORMANCEOFORIFICEMETE

RS

1) In paragraph 2 of General Rule s for orifice Design reference was made to alteration to

the sharp edge of an orifice. It may suffer from'wear by abrasion, so modifying its

effective diameter. At the same time, the internal surface of the

PAGE 118

pipe may undergo corrosion or erosion. From either aspect the ratio of the orifice

diameter tpipe diameter can be affected seriously enough to introduce unacceptable

errors. Calibration can also be affected by a difference between the actual diameter of a

new pipe and its quoted value.

2) Certain precautions must be observed in the location of an orifice. A pipe bend or

control valve if sufficiently close to the orifice can produce abnormal flow conditions,

such as swirls or eddies, influencing accuracy of measurement. It is usual to specify

minimum straight lengths of unimpeded pipe upstream and downstream from the

orifices.



In some cases, straightening devices are recommended upstream of an orifice to correct

the distortion. Typical units are indicated in Fig. 4.6 as a set of vanes or a nest of pipes.

The nature of the disturbance should be known, as the use of straighteners cannot be

made indiscriminately.

FIG. 4.6 FLOW STRAIGHTENING DEVICES IN A PIPE

3) The location of the measuring instrument relative to the orifice (or indeed other

differential pressure elements) together with the nature of the fluid being metered give rise

to certain requirements regarding piping and the use of seals and other devices.

PRACTICAL FORMS OF EQUATIONS

Equation 12 now takes the following practical forms:

W = 0.01252CZ Ed2 (hp)1/2 ..... (14)

Q = 0.0125CZED2 (h/p)1/2 ..... (15)

PAGE 119

Where W = the flow rate in Kg/hour.

Q = the flow rate in M3 /hour.

C = the discharge coefficient.

d = the orifice diameter in mm.

E = the velocity of approach factor.

h = the pressure differential across the orifice inmm of water column.

p = the liquid density in kg /M3 at the up-stream pressure va ue.

Z = a compound factor. In the case of fluid flow it is the product of

the correction for Reynolds number and the pipe size constant.



E = expansibility factor. For liquids this may be taken as 1.

4.2.2.13 SEGMENT (OR CHORD) AND ECCENTRIC ORIFICES

Whilst the concentric orifice is satisfactory for most fluids, but where suspended solid

is encountered it is preferable to use a segment type orifice or an eccentric one (Fig.4.7

and 4. 8). In the case of the segment or chord orifice, the solid segment is at the top

part of the orifice plate, and the open part has its circumference coincedent with he pipe

so that passage of solid material is not interferred with, and there is no building up of

solid matteragainst the upstream face of the orifice. The eccentric orifice follows a

similar course with the lower part of its orifice opening flush with the lower part of the

pipe. There are one or two other aspects of these types of orifices which may favour

their use in place of the concentric type. With a large m ratio, e.g. 0.7 or above,

arranging the concentricity of the orifice opening relative to the pipe bore may be

matter of difficulty. With either of the above versions possible errors due to

misalignment are eliminated. Again some gas flow mains can be of extremely large

diameter, perhaps 1-1.5 mt. The use of a segment orifice with a turned flange on it

enables the orifice plate to be bolted to the pipe surface and dispenses with the necessity

of having a pipe flange for installation.

4.2.2.14 ORIFICE MATERIALS

Materials used for orifice plates include mild steel, stainless r teel, monel, phosphor

bronze, gunmetal, depending on the application. A rough classification would be:

Water Metering : Gunmetal, bronze, stainless steel

Air Metering : Gunmetal, monel, mild steel

Steam metering : Stainless steel, monel

Sweage, Fuel Oils, Coal Gas : Stainless Steel

4.2.3.1VENTURITUBE

We have seen the effect of inserting an orifice plate in a fluid stream, causing an abrupt

change in stream area to produce a differential pressure. The operation can be

accompanied by a fairly high permanent pressure loss, and where pressure loss is

PAGE 120



important, it is necessary to turn to other methods of producing differential pressure.

Let us consider devices with a gradual change in area. The first of these is the Venturi

tube.

The principle was first explained by Venturi, the Italian Scientist, as far back as 1797,

but practical form was really given to it by Herschel in 1887. The basic design is

relatively simple. There are virtually three sections to the tube; the inlet or upstream



cone, the throat, and the outlet or downstream cone. Fig. 4.9 indicates the general

layout of the long or standard pattern. The inlet cone tapers down from the pipe area

to the throat section of smaller area to produce the necessary velocity and pressure

change. The outlet cone expands from the throat to the pipe area. Pressure tappings

are taken at the inlet entrance to the cone, and at the throat. The tappings take the

shape of annular chambers, the inside surfaces assuming the form, as a rule, of

smoothly machined liners with holes pierced at regular intervals round the

circumference. This enables the pressure to be average before transmission to the

measuring instrument. B.S. 1042 specifies conical angles between 51 and 15' for the

outlet cone, the design of which has an influence on efficiency in terms of pressure

loss. As an example, take a throat/pipe diameter ratio of 0.060 giving m=0.36.

Expressed in the conventional manner, with a 50-71 expansion, the net pressure loss is

about 9 percent of the differential pressure between inlet and throat, and

corresponding loss with a 140-150 expansion would be 14 percent. Compare these

figures with the 65 percent loss for an orifice/pipe diameter ratio of 0.60.

FIG. 4.9 THE ELEMENTS OF A VENTURI TUBE

The effect of the inlet cone is not so important, and if a little greater loss of pressure

can be accepted, another pattern of Venturi tube may be used with considerable saving

in weight and space. This type is known as the short pattern and the inlet area change



is made in several ways. In one form it takes the profile of a nozzle; in another it is

almost as abrupt as an orifice.

4.2.3.2 DISCHARGE COEFFICIENTS

The discharge coefficient of a standard type of Venturi tube is about 0.99 and remains

substantially constant for all values of throat/pipe diameter ratios between 0.25

PAGE 122

and 0.75 (m=0.05 to 0.55). At 1ow values of Reynolds number, the Rd coefficient may

fall to 0.95 and so modify the flow. In practical the approach to the throat is often given

a curved profile by means of its linear, to maintain a constant discharge coe

For the short Venturi tube, the discharge coefficient ranges between 0.98 and 0.92.

The throat diameter must be between 0.224 and 0.742 of the entrance pipe diameter and

must not be less than 19.3 mm.

The effect of a high discharge coefficient is apparent if any of the equations (14) and

(15) are examined (these apply equally to orifice plates, Venturi tubes or nozzles). For

the same flow in a given pipe, and with the throat diameter the same as that of an

orifice, a much smaller differential is needed, resulting in further decrease in pressure

loss. Alternativelyj with the same differential and throat diameter a considerably large

flow is achieved than with an orifice. For a simple and rough comparison let us

consider an example.

Suppose the m value of a Venturi tube be 0.40. its discharge coefficient 0.99, and

orifice of the same in ratio the discharge coefficient will be differential approxirnatefy

0.60. Let the differential be h2 With the flow remaining the same in

both cases:

0.99 (h1)1/2 = 0.60 (h2) 1/2 .........(17)

(h1) = (0.60/0.99) (h2)1/2 .........(18)

h1 = ((0.60)2 /0.99)h2 ..........(19)

h1 = 0.36h 2 (approx) ..........(20)

The differential required for the Venturi case is less than half that required for the orifice.

if the differential remains the same and the throat and orifice diameters are equal and

if Q1and Q2 are the prospective flows:

Q1/0.99 = Q2/0.60 ...........(21)



Q1 = 0-99 Q2/0.60 ...........(22)

Q1 = 1.66Q2 ...........(23)

PAGE 123

In other words, the flow has increased by more than half in the case of the Venturi tube

for the same conditions, and the pressure loss, as we have seen above, is considerably

less.

At first sight, the tube would appear to have immense advantages over the orifice plate.

From a purely measurement point of view this is probably correct, but its use is only

justified when the orifice plate cannot be used, e.g. where the orifice/pipe diameter ratio

is in the region where the coefficients are uncertain, and where the pressure recovery is

most important. Its cost compared with the orifice plate is extremely high, and the

latter may be manufactured relatively quickly and easily in a normal workshop. Again,

the dimensions of a Venturi can be very large. In a 250 mm diameter pipe, for a throat

diameter of 175 mm the overall length for a standard Venturi tube would be about 2.5

mt. Compare this with an orifice plate of thickness 1.6 mm, which may even be

installed on an existing plant by springing the pipe flanges apart a sufficient distance to

allow for insertion. Careful consideration is always given to a flow measurement

problem, before a Venturi tube is specified.

4.2.3.3 CONSTRUCTION FEATURES

To some extent, the construction of the Venturi tube depends on the application. For

normal uses, the sections would be of gun-metal, cast iron, or meehanite, and smoothly

machined liners of gun-metal or stainless steel inserted at the inlet and throat pressure

tappings. The use of gun-metal or stainless steel reduces the risk of corrosion. To

facilitate construction work a victaulic joint is sometimes inserted in the downstream

cone. The extreme ends of the cast sections are flanged to mate with the pipe flanges,

and with the adjacent section, and pressure tappings are arranged for screw-in orflanged

connections depending up on the particular installation conditions.

For high pressure hot water flow, as in boiler feed water in a power station, the above

design is used, and a gun-metal lining is inserted. The lining is made in three sections:

inlet cone, throat, and outlet cone, profiled as for a standard Venturi. This design is

suitable for pressures up to 90 bar. Another pattem has a maximum working pressure

of 135 bar.



The Venturi tube possesses a big advantage over the orifice in that its section need not

be circular. Square orrect angular shapes have be enused for measuring large volumes

of fluid flow. The non-circular sec ion lends itself to constructional materials other than

metal, and concrete has even been used for one or two very large flows. Note that the

design renders the tube useful for fluids containing suspended matter because of its

gradual area changes.

4.2.3.4FORMULAE

Equations (14), (15) and (1 6) are siutable for Venturi tube calculations, but the orifice

diameter now becomes the throat diameter and m is the throat/pipe area ratio for

determining E.

PAGE 124

4.2.5 CHARACTERISTICS OF ORIFICE PLATES, NOZZLES, AND VENTURI

TUBES

Nozzles and Venturi tubes, which are not so easy to instal as orifice plates but which

cause less pressure loss as shown in Fig. 4.1 OA. This dagram refers to percentage loss

in relation to the 'M' ratio of the pressure difference element; 'M' being defined on the

area of the Pipe. For an' M' ratio of 0.5 the orifice causes a pressure loss equal to 50%

of the differential pressure where as a 150 taper venturi tubes causes a loss of only 10%

of the differential pressure.

4.2.4 NOZZLES

The nozzle falls between the Venturi tube and the orifice plate as a means of flow

measurement. It approximates to a Venturi tube with the curved form of approach,

giving a gradual change of sectional area and has the same order of discharge

coefficient. But the absence of a downstream expansion cone brings the pressure loss

into the same region as that for an orifice plate. It is cheaper than a Venturi tube, and at

high velocity flows its use in place of an orifice plate may be necessary. There is some

divergence on nozzle design. B.S. 1042 specifies the I.S.A. nozzle and gives details of

its designand coefficients. On the other hand, manufacturers have their own models,

for which the I.S.A. nozzle coefficient do not apply. Fig. 4.10 indicates a nozzle with

corner pressure tappings.



FIG. 4.10 A TYPICAL NOZZLE UNIT

A feature of the nozzle is that its graduated profile renders it useful where fluids with

suspended matter are encountered. Again, as in the case of the Venturi tube, the nozzle

possesses some advantages over the orifice plate, but its cost, whilst less than that of a

Venturi, is still more than an orifice, so that its use requires consideration if cost is

paramount.

PAGE 125



FIG.4.10-A NET PRESSURE LOSS FOR DIFFERNT FLOW SENSING ELEMENTS (

EXTRACTS FROM B.S)

PAGE 126

4.2.5 PITOT TUBE

Let us study the effect of placing a blunt object in a fluid stream as an obstruction to the

flow (Fig. 4.1 1). As the fluid approaches the object, the velocity will decrease until it

reaches zero at the point where it impinges on it. From the above it may be inferred

that a declaration should mean an increase In pressure. This would follow from Be

mouilli's Theorem.

From Equation (3) this is:

p1 + v12 = p2 + V2

2 .........(24)

p 2g p 2g

If p1 and v1 are respectively pressure and velocity upstream from the object, and p1 and

v1 the respective pressure and velocity in the neighborhood of the object, at the point of

impact, v2 is zero. In other words the kinetic energy has become converted to potential

energy and the result is reflected in the value of p2 at the impact point. This now

consists of the normal static pressure plus another pressure produced as a result of

energy conversion. There is another way of explaining the same operation. It will

be noticed that: P1' P2' V12' V2

2

p p 2g 2g

have dimension of a length or a head p, and p2 are therefore denoted by the term

'pressure head' and V12 and V1

2 by the term 'velocity head'. We can say that

2g 2g

the velocity head V22 has become converted to a pressure head ps and the sum

2g p

of these two then gives us P2

P

px + PS = P2 ...........(25)

p p p

If v2 = 0 Equation (24) can be written



P1 + V12 = P2 .................(26)

p 2g p

Rewriting:

v12 = 2g (P2 - P1)

p

V1 = 2g (P2 - P1) 1/2

p ..................(27)

PAGE 127

FIG. 4.11 THE EFFECT OF IMPOSING A BLUNT FIG. 4.12 SEPARATE STATIC

AND IMPACT PRESSURE TUBES

OBJECT IN THE FLOW STREAM

We now replace the blunt object with a tube having a small opening facing the direction

of fluid flow. Next consider that the tube is joined to one connection of a pressure

measuring instrument of the diaphragm type suitable for measuring relatively sn-tall

differential pressures. There is no flow through the tube and the point of impact or zero

velocity can be considered to be at the impact hole. This produces p 2 in Equation (27)

and if a static pressure tapping is taken upstream, a little way from the tube, this gives

us Pl. Both pressures are applied to the differential pressure instrument, and a means of

measuring the velocity of the fluid is obtained since both g and p will be known. We

have, then. the Pitot tube in its simplest form (Fig. 4.12).



It is very convenient to beable to measure the static pressure in the close neighbourhood

of the tube. Both designs consist basically of inner and outer tubes. The inner one

leads from the impact hole to one connection of a differential measuring instrument.

The outer tube, referred to, sometimes, as the static tube, has a series of holes bored into

it so that its interior connects to the outside surface to be in contact with the static

pressure. This tube is joined to the second connection of the measuring instrument

(Fig. 4.13).

The Pitot tube can only measure velocity at one position in the cross-section of a pipe.

Now the velocity of a fluid in a pipe, taken across the section, is not uniform, varying

from zero at the pipe surface to a maximum at some point (not necessarily the centre)

along a diameter. To find the mean velocity it is necessary to make a traverse of the

pipe with the tube, taking the differential pressure at certain specified positions. An

ideal distribution curve is shown Fig. 4.14. For Reynolds numbers above 100000, the

ratio of average velocity to velocity at the centre of the pipe is frequently specified as

0.82 or 0.83. Whereas this value would apply for ideal cases for a curve of the type in

Fig. 5.14, the actual curve may be different. The desirability of carrying out a

transverse, therefore, is obvious. Once having determined the ratio value, the Pitot

PAGE 128

FIG. 4.13 STANDARD PATTERN PILOT TUBES FIG. 4.14 AN IDEAL TRAVERSE OF A PIPE

tube may be placed at the pipe centre and the instrument calibrated in terms of average

velocity.

Another theoretically possible means of determining the average velocity is to select a

position where the velocity corresponds to the average value. This has some practical



drawbacks. The location may be near the wall of the pipe, a very approximate value

being 0.25 of the radius in from the wall. It could be at a point where the velocity curve

slope is fairly steep as any misplacement could lead to significant errors in velocity

determination. At the centre of the pipe, by comparison, the curve is normally flatter

and errors in location are not so serious.

4.2.6.1FORMULAE

Equation (27) is not quite in a practical form. It is necessary to modify this by a

velocity coefficient C, in some types. For some patterns (like BSNPL), however, the

value of C is taken as l.'Those for the single tube type may be lower depending on the

design.

A formula commonly used for calculation purposes is:

v = 4.43 (h/p)1/2 ............. 28

where v = the velocity of the fluid in m/sec.

h = the differential produced in mm of water at 60 F.

p = the density of the fluid in kg/@.

Compressibility for industrial purposes does not introduce any serious errors by being

neglected.

Two other types of Pitot tube deserve mention. One is the double, tip pattern shown

in Fig. 4.15 in which there are two holes, one facing upstream and the other down

stream, the former measuring the impact head and the latter the suction head.

PAGE 129

The differential pressure obtained is greater than with the standard types, but is not

double the value. Actually, the increase is between 35 per cent and 40 per cent

depending upon the position of the tube in the pipe. The other type is the PitotVenturi.

It is a combination of two concentric Venturi tubes, the outlet cone of the inner one

terminating in the throat of the outer. The throat pressure of the inner tube and the

impact pressure on an impact hole in the supporting tube give 7-10 times the

differential produced with the normal types under the same conditions.



"YAW"

It can be seen with all types of tubes there is a possibility of the axis of the head not

being in alignment with the direction of flow. "Yaw" having been said to have taken

place. The effect on the standard B.S. pattems is very small for any normal mis

alignment. At about 200 yaw, the error in velocity determination reaches 2 percent.

For the double tip type the error is about 1 per cent for a 50 yaw.

4.2.6.2ADVANTAGES

1 The pressure loss caused by the insertion of a Pi tot tube in a pipe or duct is very

small unless its dimensions are large compared with the pipe diameter.

2. It is extremely useful for determining actual velocit.y profiles.

3. Its cost is low compared with that of a nozzle or Venturi tube. It is also cheaper

than an orifice, although this may depend upon the pipe diameter.

4. It can be inserted through a comparatively small hole into the main without the

necessity for shutting down the main.

PAGE 130

4.2.6.3DISADVANTAGES

1. The fluid must normally be moving at a relatively high velocity to produce a

measurable differential pressure. About 15 m/sec of air produces a differential

of 2.5 mm water column and about 1.5 m/sec of water produces 250 mm water

column differential. A manometer sensitive to pressure changes of about 5xl 0-3



mm w.g. will be required to measure the velocity head corresponding to a speed

of 3m/sec with an accuracy of + 1 %.

2. The small opening may become blocked if used with a fluid carrying solid

particles.

4.2.6.4THE ANNUBAR

For the more permanent type of Pitot-tube form of installation an Annubar may be used

Fig. 4.15(a) shows a typical Annubar installation. The pressure sides are located in

such a way that they measure the representative dynamic pressure of equal annulii. The

dynamic pressure obtained at the four sides facing into the stream is then averaged by

interpolating inner tube which is connected to the high pressure side of the manometer.

The low pressure side of the manometer is connected to the downstream element which

measures the static pressure less the suction pressure. In this way a differential pressure

representing the mean velocity along the tube is obtained which is a measure of the

flow through the pipe/duct. The accuracy for the tube of tubes is within +0.55 to +1.5%

of actual flow on SOmm to 60Omm sizes. Accuracy for 25mm to 4Omm and 65Omm

to 4.6 meter sizes is +1.1 to +2.3%.

4.2.7 DALL TUBE

The principle features of the Dall tube are indicated in Fig. 4.16. It involves two

truncated cones separated by a narrow throat. The throat length is between 0.03d and

0.ld where d is the throat diameter. The inlet cone has an included angle between 401

and 501, the outlet cone between 1211 and 170. The mouth diameter Dm, the inlet pipe

diameter D and the throat diameter d are connected by the following relation

Dm4-d4 = k (D4 - d4)

where k = 0.5 to 0.75.

Observe the diameter of the inlet cone is less than that of the pipe, resulting in a sharp

step. This creates an impact pressure which is additional to the static pressure existing

at the step. The high or upstream pressure connection is made just in front of the step.

The other connection is made at the throat where the relatively abrupt change in area

results in a marked static pressure depression. The original patent specification claims

a pressure loss expressed as 5% or 6% of the differential pressure. This compares with

a loss of between 2 and 3 times this value with a normal Venturi tube. In addition, the

Dall tube has the advantage of being considerably shorter than the normal Venturi.



FIG. 4.15 A

PAGE 132



4.2.8 SQUARE LAW EFFECT

The nature of the relation between rate of flow and differential pressure influences the

design of measuring instruments for orifices, nozzles, Venturi and Dall tubes.

Taking one of the flow equation (14), at maximum flow

L1 = 0.01252 CZd 2.E (hl)2 ..........(30)

At any other flow L2,

L2 = 0.012520.z.d2.E (h2)2 ...........(31)

Where L is flow rate and P is liquid density in kg/M3

Dividing (30) by (31)

L2/ L1 = [ (h2)/ (h1)]2 .........(32)

L22/ L1

2 = [ (h2)/ (h1)]

h2 = L22/ L1

2 (h1)]

Assign some figures to equations (30) and (31).

If h1 = 50 mm. of water for full range,

PAGE 133

and L1 = 200 000 kg per hour,

at half the maximum flow,

L2= 100 000 kg per hour,

but h =[ L22/ L1

2 (h1)]

h = (100 000)2 / (200 000)2.50

h = 12.5 mm of water.

The square root effect is now apparent since for half the maximum flow we have only one-

quarter the maximum differential. At still lower flows, e.g. 1/5 maximum, or 40 000 kg per

hour, the differential is only 2 mm.

Suppose we wish to use any of the normal differential pressure instruments. Here, the

movement or deflection of the measuring element has a substantially linear relation to the

applied differential pressure. If a normal linear magnifying mechanism is used, the pen or

pointer movement will be directly proportional to the di fferential pressure. In other words, if

E)1 is the maximum pointer travel in degrees over an indicator scale,

62 at any other value is

Θ2 = L22/ L1

2 .......... (35)

Taking a typical circular indicator scale span as 3001 and keeping to the previous figures,



Θ2 = L22/ L1

2

Θ2 = (100 000)2/(200 000)2.300

Θ2 = 750

The pointer, therefore, only moves a quarter of the way round the dial for half maximum flow

value. Most companies introduce a square law compensating device into their flow indicating

or recording meters.

PAGE 134

4.2.9 VARIABLE AREA METERS

Let us revert to one of the fundamental equations for a turbulent fluid flow through an orifice.

Here:

= C.E.A2 (2gpd ) / p ................ (36)

Where 0 = the rate of fluid flow

c = the discharge coefficient

E = the velocity of approach factor

A2 = the orifice area

g = the gravitational acceleration

Pd = the pressure differential across the orifice (PI-P2)

P1 = the fluid density.

We have proceeded, hitherto, on the basis that the orifice area A 2 'S fixed, and the

pressure differential Pd varies with the flow rate Q. Equation (36), however, suggests

a possible alternative by keeping the Pd constant and adjusting the orifice area A 2 in

proportion to Q. Equation (36), again, lays no restriction as regards the shape of the

orifice. For example, we could have an annular pattern formed by the space between a

solid disc and the inside of a pipe. Equation (36) would still apply, the values of C

and E, of course, being different from the concentric orifice case. Developing the idea

still further, consider a vertical tube of conical shape, the area gradually expanding

from the bottom to the top. In the tube a fluid is flowing in an upward direction, and

in it is placed a disc, free to move, so that it acts as a sort of float in the fluid. .(Ref.

Fig. 4.17). An orifice is set up between the disc and the inside surface of the tube, and

a pressure drop exists across it. Certain forces act on the disc and these are in

equilibrium when it is at rest. A change in flow will affect the pressure drop, altering

the relation between inlet and outlet pressure thus upsetting the equilibrium of the



forces acting on the disc. The disc will then move up or down the tube adjusting the

area of the orifice (due to the conical shape of the tube) until the pressure drop is at the

original value, when the forces are again in equilibrium. The position of the float in

the tube is then a measure of the rate of flow. (Ref. Fig. 4.18)..An' elementary theory

of the variable area meter follows. It should be understood that this is for explanation

purposes only, as a very simple equation expressing the operation of the meter is not

possible. What are the forces acting on the float in the vertical column of liquid?

They are:

1 The effective weight X of the float in a downwards direction.

X Vf (P2 -P1) ........... (37)

PAGE 135



where Vf = the float volume

P2 = the float material density

P1 = the fluid density

2) The total pressure, Y, acting in a downwards direction on the upper surface of the float.

Y = P2Af .............(38)

Where P2 = the pressure per unit area on the upper surface Af of the float.

3) The total pressure, Z, acting upwards on the lower surface of the float

Z = p1Af .............(39)

Viscous effects are neglected for the time being.

For equilibrium, the upwards acting forces mustbalance those acting downwards,

and

Z=X+Y ..............(40)

P1Af = Vf (p2- p1) + P2Af ...............(41)

Imagine that the flow increeases, and that the float does not move immediately. An mcreased

differential (PI-P2) results, and the ratio of PI/P2 is increased. This means that the force P1Af

is now greater than Vf (P2-Pl)+P2Af. Since the float is free, however, it will be moved in the

direction in which pi.Af is acting, i.e. upwards. As it moves upwards it increases the orifice

area due to the expanding sectional area of the tube and the pressure differential falls

proportionately. The operation continues until (p, P2) reaches its original value, when the

forces, as indicated in equation (40), are in equilibrium again. The new float position must

now be a measure of the increased flow value. The operation is reversed on decrease in rate of

flow.

From (41)

Vf (p2- p1) + P2Af . ...........(42)

Substituting in (36)

n = C.E.A 2 [ 2g (Vf) / Af)( (p2- p1) / p1 )] 1/2 ..........(43)

PAGE 137

If the tube is a conical one,

Dt Di + 2x tan /2 ............(44)

where Dt = the tube diameter at a distance x from the inlet of the tube.

Dt = the inlet diameter



X = distance or height of float from inlet

2 = the cone angle.

Letting Df the float diameter equal Di, and neglecting valuesX2 tan 2 E)/2 as a first

approximation,

Q = K.C.E.x [2g (Vf) / Af) ((p2- p1) / p1 ) ]1/2 ............(45)

where K is a dimensional constant involving Di tan E) etc.

The velocity of approach factor E is without significance so that (45) can be reduced

to

Q = K.C.X [2g (Vf) / Af) ((p2- p1) / p1 ) ]1/2 .......... (46)

If it is desired to measure weight flow (e.g. Kg./hour) instead of volume flow, since

W = Qpl

W = K.C.x [2g (Vf) / Af) ((p2- p1) p1 ) ]1/2 .......... (47)

These two equations are produced for a particular purpose.

4.2.9.1 DENSITY VARIATION AND COMPENSATION

The density of a fluid can vary, e.g. with temperature changes, and errors will be

introduced into, the flow measurement. The force X will be affected since.

x = Vf (p2- p1) ............(48)

This means that the equilibrium of the float will be upset and it will change to a new

position corresponding to the new value of X. Errors will thus be introduced into the

flow measurement. We require, therefore, that the change of rate of flow with density

PAGE 138

be zero. Expressed mathematically,

dQ / dp1 or dW / dp1 = 0 ..........(49)

Differentiating (46) with respect to p,, and equating the result to zero, produces the

condition that for complete density change immunity, P2 must be infinite. Practically,

of course, this cannot be achieved, but the density of the float may be made very large,

i.e. many times that of the fluid. This results in reducing the density errors to

reasonable proportions.

Differentiating (47) with respect to p, and equating result to zero, produces the

following condition:



P2 = 2p, (50)

For liquids, this condition may be achieved practically by making the body of the float

either hollow or of solid plastic material. 10 per cent density variations from a mean

value do not introduce any significant errors into the flow measurement.

Fig. 4.17 shows the essential features of a glass tube variable area flow meter with a

normal float. Such flow meters are used in flow measurements for water streams in

water treatment between the 10m exchange columns.

The establishment of two different density compensation conditions may seem a little

surprising at first, but it will be apparent that the fluid density, p,, enters into flow

measurement in a different manner for volumetric and weight scales, if equations (46)

and (47) are examined. p, appears as a divisor in (46) and as a multiplier in (47). If

follows, therefore, that the conditions of compensation will be unlike for the two cases.

4.2.9.2 VISCOUS FLOW

At slow flow rates, laminar or viscous flow conditions may exist. The force of viscous

drag must now be taken into account since this is a function of length Lf along the

direction of the flow and viscosity n of the fluid. Highly complex theoretical equations

have been worked out to explain the action under conditions of viscous flow. For

reference purposes these are given below:

Q = Vfl.g. (P2 - Pl) (Dt - Df)3 (Dt + Df) / 24 (Dt2+ Df

2)Lf n ......... (5 1)

or

W = Vfl.g.p1 (P2 - Pl) (Dt - Df)3 (Dt + Df) / 24 (Dt2+ Df

2)Lf n ..........(52)

PAGE 139

From an examination of either equation the flow is now dependent on the value of n

the viscosity of the fluid under measurement in addition to the density p,. We have

seen how to compensate for density variation, and it now becomes equally important

to render the instrument immune from viscosity changes. In short, we must reduce the

effect of viscous drag force D to a minimum. Since this is dependent on the effective

length Lf of the float, the reduction of this length should result in a lessening of the

viscosity effect. Practically, therefore, the floats take the form shown in Fig. 4.18.

The effective orifice part of the float has been reduced to a sharv edged disc, i.e. with

the minimum length along the direction of flow. In yet a third design of float, the



"body" is removed to a position above the float out of main flow stream, a thin conical

shell acting as the float itself.

4.2.9.3TYPES OF VARIABLE AREA METERS

Three typical types are described below.

4.2.9.3.1 GLASS TYPE

The basic feature of this type of meter is the conical section glass tube. For accuracy.

the diameter of this must be maintained at very close limits. Clear borosilicate glass is

used which is highly resistant to thermal shock and chemical action, and the method of

its manufacture enables tolerances of 1/400 of mm, to be observed. The use of glass

introduces the question of a safe working pressure for the fluid being measured. At

present this is about 4 kg/CM2 and to the smaller diameter tubes. For larger sizes the

safe working pressure falls from this figure. (The normal diameters range from 2mm to

6Omm depending on the flow to be measured). The tube is normally clamped in a

metal frame, the inlet and outlet being sealed into connections as required, e.g. flanged

or screwed. Where danger may occur from flying glass resulting from a fracture of the

tube, 'Armour Plate" glass protection windows encase the instrument.

The standard float shape is indicated in Fig. 4.17 and is perfectly free. Viscosity

immune floats, however, may demand a guide, as the float disturbs the equilibrium of

the liquid. In one pattern, the guide is a central rod around which the float is made to

rotate, so that visual evidence that the float is moving freely is obtained.

The glass tube type measures from 2 cc/min up to 3000 litres/min of gas, and 0.5 cc to

2@5 litres/mi'n of liquid.

The pressure drop will depend on the type of float being used and the nature of the

fluid, but varies between about 0.2 cm w.g. for small gas flows and 3.5 cm w.g. for

liquid flows.

4.2.9.3.2 METAL TUBE TYPES

For larger flows than the glass type tubes can accommodate, a conical metal tube

pattern is introduced. Here, the metal body is of gunmetal, cast iron or stainless steel

with a stainless steel float. The latter is carried on a rod which moves between two

guides, one at the lower end and the other at the upper end of the tube. The guide rod

PAGE 140



passes through the upper part of the tube into a compartment with a glass scale, the

end of the rod acting as an indicator (Fig. 4.18). This type of meter has typic ally

ranges from 250 to 120 000 litres/min of gas flow and 20 to 7000 litres/min of liquid.

The maximum fluid working pressure is 30 kg/cm'. When used with, opaque liquids,

a compressed air supply may be connected to the top of the scale. unit and the level of

liquid depressed, so that a clear view of the indicator is obtained. Opaque liquids may

also be metered by the high pressure version.

4.2.10 ELECTROMAGNETIC FLOWMETERS

The principle of the electromagnetic flow meter may be understood better if we first

consider a very thin disc of an electrically conducting liquid moving with a velocity V

along a pipe of internal diameter d. An external magnet system directs a magnetic field

of strength H across the section of the pipe so that it acts at right angles to the direction

of motion of the disc. Now, by Faraday' slaw of induction, when an electrical

conductor of length L moves through a magnetic field of strength H at a velocity V in a

direction at right angles both to the magnetic field and its length, an e.m.f. is generated

of value.

E=KHLV ..... (53)

where K a constant

Our disc liquid is a conducto @ obeying the g eneral requirements of Faraday's law,

and it can be seen without much difficulty that L in equation (53) is replaced by d, the

diameter of the disc. If, now, there is an infinite number of such moving discs

contiguous to one another, we have. the equivalent of a conductin liquid strea m

flowing continuously through the pipe (see Fig. 4.19).



FIG. 4.19 DIRECTIONS OF VELOCITY V, MAGNETIC FIELD H

AND ELECRROMOTIVE FORCE E

The stream will satisfy the following equation

E=KHVD ...........(54)

In (54) d is constant and if H remains constant

E KV

PAGE 141

where K = a general constant ...........(55)

Alternatively, since Q = VA where A is the area of the pipe,

E=CQ ............(56)

C being a general constant.

Thus, provided we can physically measure E, a very simple means of determining the

flow rate of liquids in pipes is available.

4.2.10.1 PRACTICAL DESIGN

1. In general, the electromagnetic flow meter takes the form of a metering tube

of non-magnetic material. This ensure that the magnetic flux does not go

into the tube wall and by-pass the flowing liquid.

2. The tube, if made of conducting material, must have an insulating lining to prevent

short circuiting the e.m.f. Although this need only apply in the neighbourhood of the

electrodes, the insulation is usually extended the whole length of the tube. Non-

metallic tubes will not require this lining unless for reasons of corrosion, etc.

3. The electrodes are usually of point form normally made from stainless steel. Platinum

is occasionally used where the liquid handled is severely corrosive. The faces of the

electrodes are flush with the lining of tube surface. There should be no lining in the

tube where the electrodes are placed.

4. The magnet coils are energized from an A.C. supply. The advantages and

disadvantages of A.C. over D.C. energized magnet are discussed in a paper devoted to

this pattern of flowmete@ and need n t concern us here.

5. The system acts as a source of e.m.f. with a finite resistance dependent on the

conductivity of the liquid and the size of the meter. Precautions must be taken in the

measuring circuit so thust only an extremely minute current drains from the e.m.f.

source.



The measuring circuit is normally a null balance one, a typical one being shown in,Fig.

4.20. (Null balance circuits will be described in a later chapter).

4.2.10.2 ADVANTAGES OF THE ELECTROMAGNETIC FLOWMETER

1. Linear relation between flow rate and measuring signal as compared with the

square law relation of differential pressure devices. This results in a range ability

of differential pressure devices. This results in a range ability of the order of 100/1.

2. The measuring instrument can be arranged with a centre zero for measuring

PAGE 142

FIG. 4.20 SIMPLE BLOCK DIAGRAM OF ELECTROMAGNETIC

FLOWMETER\ANDMEASURING CIRCUITS (FOXBORO-YOXALL LTD.)

flow in either direction. Alternatively the electrode leads may be changed over to

measure a reverse flow.

3. The only pressure loss is that due to the length of tube, forming the metter. But a

pressure loss would be present with the same length of ordinary pipe so that the

introduction of the meter cannot be said to involve significant additional pressure

losses.

4. There is no obstruction to flow which renders the meter suitable for liquids containing

suspended matter. Abrasion may be avoided by choosing a suitable lining material.



Wood pulp and paper mills stocks, cement slurries, sewage, food pulp are but a few

difficult fluids which may be metered.

5. The design lends itself to the rnetering of corrosive liquids since parts in contact with

the fluids may be made of corrosion-proof materials.

6. It is not affected by velocity profiles, since the e.m.f. is at all points proportional to

the velocity of flow across the diameter.

4.2.10.3 DISADVANTAGES OF THE ELECTROMAGNETIC FLOWMETER

1. It is not suitable for measuring gas or vapour flows.

2. The normal design is not suitable for hazardous areas.

3. Liquids to be metered must be conductors of electricity.

4. There is a minimum value of conductivity which is related to the lengths of cable

leads to amplifiers and the size of the meter. The readings are unaffected by

increases in conductivity above the minimum value, but decreases cause the meter to

read low.

PAGE 143

5. If a concentric build up of deposit of much different conductivity to that of the

liquid takes place, significant errors may be introduced. Note that it is possible for

non-conductive deposits to insulate the electrodes. Where the concentric deposit is

of the same conductivity as the metered liquid, the meter continues to read

correctly.

A meter has been constructed of 2.5 mm diameter with a flow range .00076

M3 to .076M3 /min. In contrast, a typical large diameter meter has been 1000 mm

covering a range 190M3 to 1900M3 /min.

4.2.11 ULTRASONIC FLOWMETER

1. Consider Fig. 4.21 in which a fluid is flowing at a velocity V. A transducer Ti

transmits a beam of sound to receiving transducer T 2situated at a distance d

downstream. If C is the speed of sound through still fluid, t the time for sound to travel

Ti to T2 is

t = d / C+V ...............(57)

with no flow,

t0 = d / C ................(58)

The difference between t and t,, At is given by



At = Vd / C(C+V) .................(59)

C for most fluid$ is of the order of 1500 metres/sec whilst V for most industrial applications

would be a few metres/sec. Equation (59) then reduces to

Δt = Vd / C2 ................. (60)

This suggests that At could provide a measurement of V. But it involves a knowledge of t. not

readily measurable, and C, which varies with temperature and pressure. To eliminate t., the

differential arrangement shown in Fig. 4.22 may be used.

2. Two sets of transducers, T1 and T2 and T3 and T4 are installed in the pipe, the distance

between T1 and T2 and T3 and T4 being d. A beam of sound is transmitted from T1 to T2

downstream and from T3 to T4 upstream, both being of the same frequency. The time for the

beam to travel from T1 to T2 is

T1 = d / C+V ..................(61)

PAGE 144

Where C= the velocity of sound under the temperature and pressure conditions existing in the

pipe, and from T3 to T4

t1 = d / C-V .................(62)

The difference between T1 and T2 is

t3 - t4 = Δ t = 2 Vd / C2 - V2 .................(63)

If V is small compared with C; (63) can be reduced to

Δ t = 2Vd / C2 .................(64)

3. The measurement of At now involves some problems. It may be solved by pulse

techniques or a continuous wave beam may be used. In the latter case, the transmitting

transducers are driven from a common source and the phase difference between the two

received signals measured. The phase difference AO is given by

Δt = 2WVd / C2 .................(65)

Where W = the angular frequency.

4. Observe that in all the methods considered, C the velocity of soun d is present. This can

be eliminated if the method of Fig. 4.23 is adopted. A short pulse is emitted from

transducer T1 and is received by T2. The arrival of the pulse from T2 triggers another one

from T1.The time between pulses is

T1 = d / C+V ..................(66)

The pulse repetition frequency is f, and since



f1 = d / C+V ..................(67)

PAGE 145

FIG. 4.21 SIMPLE ARRANGEMENT FOR ULTRASONIC FLOWMETER WITH ONE

TRANSMITTING AND ONE RECEIVING TRANSDUCER. THE DIRECTION OF FLOW

IS FROM LEFT TO RIGHT.



FIG. 4.22 DIFFERNTIAL ARRANGEMNT FOR ULTRASONIC FLOWMETER. TE

DIRECTION OF FLOW IS FROM LEFT TO RIGHT.

FIG. 4.23 PLUSE SCHEME FOR ELIMINATING THE EFFECT OF C THE VELOCITY OF

SOUND. THE DIRECTION OF FLOW IS FROM LEFT TO RIGHT.

PAGE 146

A similar pulse is tr1ansmitted from T toT and calling the repetition frequency

here f2.

f1 = C-V / d ........... (68)

f1 - f2 = Δf = 2 V/d .............(69)

Equation (69) is independent of C.

5. A further technique u ed has been a differential arrangement across the pipe. It can be

shown that a beam of sound can be deflected in the downstream direction in traversing a

pipe from one side to the other. The.deflection x is approximately given by



X Vd / C ...........(70)

Which of the methods outlined is most suitable for industrial applications? There are.

several factors t6 consider.

1. In the phase difference method the phase difference is given by

ΔǾ = 2WVd / C ............(71)

AO proportional to the operating frequency W and is suggested that W should be as high

as possible.

Measurements of AO above 2 n are not desirable, from the point of view o interpretation,

but AO should be as large as possible below this limit. But the higher the frequency the

greater the attenuation since it is a function of the square of the frequency. Thus, already

we have two conflicting factors. There is yet a third effect: that of the beam width. This

is a function of the velocity of sound in the liquid, the radiating. area of the transducer

and the operating frequency. There may have to be a compromise between all the factor

sin volved.

2. In the frequency difference method, care must be taken to avoid coupling between

neighbouring circuits carrying frequencies relatively close to one. another. The

frequency difference Af is dependent on the flow rate V and may be of extremely low

value, e.g. 10 c/s or 20 c/s unless the flow rate is relatively high.

3. The be am deflectionme thod suffers from the fact that the deflection is proportional to

flow rate and, at low flow rates, may not be sufficient for accurate measurement.

PAGE 149

TEMPERATURE MEASUREMENTS - VARIOUS

TYPE OF INSTRUMENTS 5.1 The most fundamental and important parameter in process industry and thermal power

plant is temperature and its measurement is required to work out the energy balances

and energy transfers in all thermal processes, and to take care of the material safety.

Though several methods are available to measure the temperature, an appropriate

method is to be selected for any particular measurement. The selection of the type of

measurements is based on the following considerations.

i) The accuracy required



ii) The range of temperature

iii) The corrosive action of the process media on the sensing element

iv) The catalytic phenomena of the sensing element on measuring media

v) The layout conditions and restrictions

vi) Facilities available for the calibration of the instrument

5.2 theory of temperature measurement

Temperature rise in a substance is due to the resultant increase in molecular activity of

the substance on application of heat which increases the internal energy of the material.

Therefore there exist some observable properties of the substance which change with its

energy content. The temperature measurement is based on this very fact. The changes

may be observed in the substance itself or in a subsidiary system in thermodynamic

equilibrium with it, and it is called the testing body while the system itself is called the

hot body.

The various methods and instruments used for temperature measurement are tabulated

in the chart attached herewith as Annexure-1.

5.3 THERMAL EXPANSION OF TESTING BODIES

On application of heat, testing bodies either in the form of solids liquids or gases

expand ahnost proportional to the rise of temperature and this principle is utilised in

various thermometers.

5.3.1 EXPANSION OF SOLIDS

The expansion of solids is utilised by means of bimetallic strip to measure tempera-

PAGE 150

tures. Two or more layers of metallic alloys having different coefficients of thermal y

expansions are coiled in the form of (a) spiral (b) helical or (c) multiple helical

depending upon the range of temperature. One end of the coil is fixed on to the bulb

to be used as a test bod and the other and free to move carrying the pointer over y

a scale calibrated in degrees.

A simple bimetallic strip composed of a layer of brass (high expanding.material) and a

layer of invar (low expanding material) will deflect when sbjected to a change of

temperature and if the stripis coiled, it sangula rnotion will be given by CTL/t where

C=strip constant/'C, T=temperature change in degree C=Length of strip in CMS and

t=thic"ess in CMs Fig. 5.1 shows the -typical bimetallic gauge.



5.3.2 EXPANSION OF LIQUIDS (Fig. 5.2)

Changes in volume of a liquid by the application of heat enclosed in a test body is

utilised to, measure the quantity of temperature. The liquids normally used are,

mercury and hydrocarbons such as ethyl alcohol, for low temperature, metaxylene for

medium range temperature, tetrahydro naphthalene (tetralene) for higher temperature.

The test bodies (bulb) are either glass or of steel material.

5.3.2.1Class Thermometers

Aglassthermometerconsistsofaglassbulbjoinedtoalengthofglasstubinghav'mg a capillary

bore, the bulb being filled with a liquid, usually mercury although this may be spirit

where the thermometer is to be used at temperature below the freezing point of mercury

(= -400C = -40'F) or for measuring oil temperature in transformers because mercury

would cause winding short circuits if the thermometer accidentally breakes.

The coefficient of expansion of mercury is approximately eight times that of glass,

hence if the bulb is heated, mercury will expand more relative to the glass, and the

length of mercury column will be related to the temperature applied.

During the manufacturer of glass, white enamel is introduced at the billet stage and it is

this that forms the background which helps in reading the scale. The blowing and

drawing of glass tubing from the billet is entirely by hand, and skill is required to

comply with three essential characteristics, quality of mix, outside diameter and bore of

hole.

All industrial and scientific thermometers have round tubes with oval shaped bore, and

this latter effect also assists in reading the mercury column clearly. Spirit thermometers

usually have round tubes whilst clinical thermometers have what is called a 'nosed' tube

to magnify the width of mercury column. Four types of glass used for thermometers

are given in next page:



Name Remarks and temperature ranges

1. Lead Class : For ordinary Industrial work used between -200 and 11OOC.

2. 'Normal grade : The Gerinan Jena e.g. jena-16 used between 1001 and 3000C.

At higher temperature the grade of glass is subjected to

'secular' change i.e. dearrangement of molecular structure

and zero shift needs frequent checking at the higher tem-

perature end.

3. Borosilicate : Less subjected to "secular change" in the range 200-500'C

and is used for more accurate work. If properly annealed has

little zero shift but needs checking near higher temperature

end.

4. Supermax : This is a special glass made for use at 500-6000C.

The range of thermometer is governed by the capacity of the bulb in relation to that of the bore

of the tube; the larger the bulb the smaller the range.

Mercury boils at 358'C but thermometers are made for use above this temperature by the

introduction of nitrogen under pressure, which will increase in pressure as the mercury

expands, so raising the boiling point of mercury. Other liquids used with

PAGE 153

addition to mercury for thermometric work are given below:



Liquid Temp. range 'C

1 Mercury -20 to + 600

2. Alcohol -80 to + 70

3. Tolvene -80 to +100

4. Pentane -200 to +30

5. Creosote -5 to + 200'C

When reading a thermometer care should be taken to avoid a parallax error and to allow

sufficient time for the mercury column to stop changing its length before taking the

reading.

Mercury~in-Class Thermometers are available in three grades.

1. Class-A: Special thermometers tested by National Physical Laboratory.

Accuracy depends on temperature range, scale graduations and readability of

subdivisions.

2. Class-B: Thermometers manufactured to same grade as Class-A, but not tested

by the National Physical Laboratory. Accuracy is specified by-the manufacturer whose

calibration are accepted.

3. Class-C: Commercial grade thermometers for which errors may be +1% or more,

but can be calibrated in station laboratory.

5.3.2.2Liquid is Metal Bulb Thermometers:

The liquid filled system consists of an element sensitive to temp . erature change (i.e.

bulb), an element sensitive to volume change (bourden, bellow or diaphragm), means of

connecting these two and a device for measuring and indicating.

The liquid is filled in a bulb from which a capillary is drawn & which ends in a bourden

or bellow or a diaphragm. The entire system is filled completely with the liquid at OIC

at high pressure of the order of 70 kg/CM2. When the temperature rises, the volume of

the liquid increases thereby tending to enlarge the enclosure. As a result a mechanical

motion is achieved which is transmitted to the dial indication by lever arrangement or

rack and plm'on arra 1ngement. Instead of capillary connection, a short solid stem is

also used. There is also a possibility that the metall enclosed (bulb & capillary) also

may increase in volume due to thermal expansion which will add to the error of the

system. To remove this error, a compensation means is provided.

5.3.2.1COMPENSATING LINK (Fig. 5.3)



This method uses two metals with different coefficients of expansions, inserted in the

capillary as a link chamber. The chamber contains a core of Invar having negligible

PAGE 154

PAGE 155

CO-efficient of expansion. The wall of the chamber is made of the steel material. The

space between the core and the wall is filled with the system liquid. If the size of the

chamber and volume of the Invar material are carefully proportioned, then on any

change in ambient temperature, the volume of the angular space, due to the expansion

of the outer wall is sufficient to accommodate any variation in volume of the liquid in

the capillary and so prevent it exerting an effect on the bourdon tube.

5.3.2.2DOUBLE CAPILLARY

The second method used a second capillary of the same diameter as the first one fined

with the same liquid under the same condition. This second capillary and is sealed off

without the bulb and run along the first capillary and connected to a second bourdon.

This bourdon is made to act on the instrument points in an opposite sense to that of the

main bourdon.



Since both capillaries and bourdon tubes are subjected to the same conditions it can be

seen that the ambient temperature effect in the main system is counteracted by that of

the second system.

Fig. 5.3(i) & 5.3(ii) explain the above two principles.

5.3.3 RESPONSE CHARACTERISTICS

Any thermometer bulb attains a given percentage of the total change in a given time,

irrespective of the magnitude of the change in bath temperature. The bulb that responds

to 95% of 1000C temperature change in one minute will also respond to 95% to 5000C

change in one minute. Practically it has been determined that the lag coefficient is the

tin-ie taken by the bulb to attain 53.2% of the change of temperature.

5.3.4 BULB DESIGN

All manufacturers keep the change of bourdon volume for all ranges in their production

a constant for commercial reasons. This leads to varying bulb sizes for various ranges

as would be seen from the following relation i.e. AV = VBR(D= constant where

Vb=bulb volume, in C.C. /(D is the differential expansivity between bulb and liquid in

cc/cc/oc AV = bourdon volume change C.C., R=range in oc.

Therefore for higher range smaller bulb volume is required. The volume change AV

lies between 0.05 and 0.15 cc depending on the shape of Bourdon, tube, bearing in

mind that there often have 'Helical Configuration' for operating the instrument

mechanisms and since F for most suitable organic liquids is about 1 03 /'C, the bulb

volume is about 100/R. For 100'C range, the bulb would be ICC for organic liquids and

F about 10 times, if mercury filled because. F for mercury/steel is about one-tenth its

value for organic liquids.

5.3.5 EXPANSION OF CASES

Here the changes in pressure of the gases filled in the test bodies (bulbs) at constant

PAGE 156

volume on changes of temperature is utilised as. means of measurement of the

temperature. The gas used normally is nitrogen. The system works on the gas law PV

= RT. Therefore the pressure of the gas is proportional to the temperature. The bulb

is evacuated and filled by the gas at a required pressure and then the system is sealed.

Rest of the system is the same as the liquid filled system and here the bourdon



becomes sensitive to pressure changes. The ambient temperature effect on capillary

and bourdon are corrected by similar systems discussed on 5.3.2.1 and 5.3.2.2.

5.3.6 EXPANSION OF VAPOUR

This works on the basis principle that all enclosed liquids at a given temperature will

create a definite vapour pressure if the liquid is only partially filled. This vapour

pressure will increase with temperature and this property is utilised for measurement of

temperature. A sensing bulb containing liquid such as alcohol or toulene is connected

to the capillary and bourdon tube. When heated, the liquid vapourises, the vapour

pressure being transmitted to the bourdon tube which operates in the same manner as

the bourdon tube pressure gauge, thus causing a deflection calibrated in terms of

temperature. This indication will be completely independent of the volurie of the

system and therefore independent of expansion due to ambient temperature changes but

since the capillary is liquid filled precautions to avoid head errors should be taken and

correction applied. The rate of increase of pressure increases with temperature resulting

in a scale in which the size, of the divisions increases with temperature, a property

which can be of advantage in reading the higher temperature more accurately. The

systems discussed in 5.3.2,3.5 and 3.6 can be categorised as filled system thermometry

which are used for local indication of temperature. The advantages of the filled system

are:

i) Simple and self contained system.

ii) Sensitivity, response tirne and accuracy are comparable with other methods, of

temperature measurements.

iii) No auxiliary power needed for the operation of these instruments.

The system has the following shortcomings too:

i) Limited up to certain temperature say up to 5000C.

ii) Bulb size is too large to be accommodated in smaller space for example to

measure the bearing temperature winding temperatures etc.

iii) In case of failure the entire system is to be changed

iv) Remote indication and other telemetering are impractic6:)

5.4 THERMO ELECTRICITY



The principle of thermoelectricity discovered by Seeback in 1821 states that when

conductors of two different metals are joined at one end and this junction is heated

PAGE 157

to a higher temperature with respect to free end, a voltage is developed at free ends and

if a mfilivoltmeter, or potentiometer, is connected to the free ends, the e.m.L is

measured which corresponds to the temperature at the junction. The two metal wires

are called thermo couples.

The e.m.f developed as discussed above is ascribed to two phenomena.

i) PELTIER Effect

This governs the e.m.f. resulting solely from the contact of two disshnilar metals and its

magnitude varies with the temperature at the point of contact.

ii) Thomson Effect

The e.m.f. resulting from this effect is due to the temperature gradient along a single

wire and this is less prominent.

In commercial instruments by suitably selecting the materials, the e.m.f due to

Thomson effect is made negligible and the total e.m.f. becomes the sum of the e.m.L at

the two junctions due to the peltier effect only. If one junction is kept at constant

temperature or its e.m.f. generation is compensated, then the e.m.f. of the thermocouple

becomes the measure of the temperature of the hot junction.

There are three laws of thermo electricity which govern the entire theory and practice of

thermocouples.

i) Law of Homogeneous Circuit

This law states that an electric current cannot be sustained in a circuit of a single

homogeneous metal however varying in section by the application of heat alone.

ii) Law of Intermediate Metals

The algebraic sum of thermo e.m.f. in a circuit composed of any number of disshnfiar

metal is zero if all the circuit is at a uniform temperature. This law enables the use of

additional metallic wires to connect the secondary instruments.

iii) Law of Successive or Intermediate Temperature

This law states that if two dissin-dlar homogeneous metal produce a thermal

e.m.£e2when the junctions are at temperature TI and T2and the therTnale.m.£e2 when



the junctions are at T2, T3, the e.m.f. generated when the junctions are at T, and T3wffi

be e, + e2.

This law enable to measure the temperature of hot unctions when the cold junction

tempera ture could not be maintained at a constant value.

PAGE 158

5.4.1 THERMOCOUPLES

Thermocouple consists of two wires of suitable materials which are joined together at

one end by twisting together and then joining the tips by brazing or welding. The wires

selected should have the following characteristics.

i) They must physically withstand the temperature for which they are selected, apid

changes in temperature and the effect of corrosive atmospheres.

ii) Their composition should not change at these temperature range.

iii) They should possess reasonably linear temperature e.m.E relationships through-

out the range.

iv) They should develop an e.m.f. per degree change of temperature that is

detectable with standard measuring equipment.

v) They should not change its characteristics by physical fatigue caused by some

materials.

Very few combination of wires have been developed so far satisfying the above

conditions. These are:

a) Base Alloys

Iron-Constantan Type J; range upto 760'C Composition of constantan, has got bright

appearance and non-magnetic. Iron-constantan has the.highest e.m.f. for a given

temperature.

b) Chromel Alumel Type K, range upto 1260OC; composition:

Chromel: Ni; 89% Cr. 9.8% Fe 1% Cobalt 0.2% Dull appearance, non-magnetic

Alumel: Ni 94.5% AI 2% Mn 2.5% S 1.0%.

Glossy surface and slightly magnetic. The chief disadvantage of this couple is

that both wires are prone to damage by sulphurous gases which are likely in the

vicinity of boiler plant.

c) Copper Constantan

Type T; range-180'C to + 370'C



Cu 60%. Ni-40%. It is a stable couple resistant to both oxidising and reducing

atmosphere but needs protection from acidic vapour.

d) Chromel Copel Type E; range 0-8700C

e) Rare Alloys

Platinum-R Hodium-Platinum

Range 0-14800C with 10% Rhodium type S and with 13% Rhodium type R

Thermo electric e.m.f. characteristics are illustrated in Fig. 5.4.

PAGE 159

5.4.2 SELECTION OF THERMOCOUPLE WIRE SIZE AND LENGTH

Though there is no general rule for the selection of wire size, but it is suggested to

select a smaller gauge wire where sensitivity is desired and heavier size wire is

preferred for longer life and higher temperature applications.

The length also should be sufficient to minimize the effect of conduction. Insufficient

insertion causes low readings. Though there is no rule for the length, a conventional

methods has been devised according to which the length should be 4 times the outer dia

of the protecting tube.



5.4.3 PROTECTING TUBE

Maximum accuracy and sensitivity are obtained if we use bare thermocouple wires.

But the possibilities of corrosive action and mechanical injuries call for a protecting

tube; for thermocouples both metallic and ceramic tubes are used.

5.4.4 EXTENSION WIRE

Since it becomes extremely costly to take the thermocouple wires upto the measuring

instrument located at far of places some substitute wires are used to connect the

thermocouples to the instruments. These wires possess the same characteristics as

PAGE 160

that of the thermocouple wires but upto a lesser temperature. These wires are tenned as

compensating leads.

Also ordinary copper wires are used as extension wires after compensating the e.m.E at

the thermocouple terminals the difference in temperature of the tern-dnals and the cold

junction using a bridge circuit utilising the law of intermediate temperatures discussed

at (iii) of 4.

Cold junction compensation circuit is a wheat stone bridge consisting of three arms

with constant resistance of mangan due to the temperature changes and the fourth arm

with copper wire wound resistance which is sensitive to temperature. This bridge is

supplied with 4 volt D.C. and is balanced at the temperature of reference junction (say

at 27OC).

When the temperature change unbalance occurs due to the change in resistance copper

wire across diagonal which adds or substracts the thermocouple e.m.L accordingly.

Other method of providing cold Junction compensation are by means of constant

temperature oven or icebox in which the reference junctions are kept inside a constant

temperature oven.

5.4.5 MEASURING INSTRUMENTS

Two methods are used to measure the thermo e.m.f. and are calibrated in terms of

temperature.

5.4.5.1 PYROMETRIC INDICATORS (P.l.) (Fig. 5.5)

This is a moving coil milli voltmeter of D' arsonwal galvanometer type. For many

applications, the sensitivity, accuracy and automatic control feature of this instrument

are qusuitable.



This instrument consists of a moving coil supported with help of pivots and jewel

bearings in a permanent magnetic field. In series with this coil, is a thermistor shunted

by the copper coil to correct the resistance variation due to temperature change and a

series calibrating resistance. An indicating pointer is attached to this circuits through

a line resistance.

Electrical current from the thermocouple passes through the coil and sets up an

opposing magnetic field. This causes the coil to turn and the pointer to move across

the scale. The restoring torque is obtained though spiral hairspr'mgs. These

hairsprings also act as leads to the current in and out of the coil. This type of

pyrometers can be used to measure the temperature of number of points in conjunction

with a manual selector switch.

5.4.5.2 POTENTIOMETER TYPE PYROMETERS (Fig. 5.6)

Here the e.m.L developed by the thermocouple is compared with a standard voltage

source in a potentiometer and the unbalance is converted into an A.C. signal which is

amplified and fed to control winding of a servometer which balances by moving the

slider over a slide wire of the potentiometer circuit.



FIG. 5.6 SIMPLIFIED AUTOMATIC MULL-BALANCE ARRANGEMENT FOR

THERMOCOUPLES

The advantages of the potentiometers over the moving coil instruments are:

i) Higher speed of operation, greater sensitivity, and accuracy of measurement.

ii) Better reproducibility.

iii) Since it adopts null balance method, no current flows at balance conditions

and the resistance of extension wires has little effect on the performance.

iv) Wider scale is possible since the deflection need not depend upon the e.M.f.

v) Use of galvanometer is dispensed with electronic circuits.

PAGE 162

5.5 CHANGE OF ELECTRICAL RESISTANCE

Another effect of increase molecular activity caused bv heat is the change of electrical

resistance of a wire with temperature changes. Resistance thermometers utilise this

property and the temperature measurements are conveniently made by the change of

resistance of suitable metals of known characteristics.

The materials selected are having two types of characteristics (1) having positive

temperature coefficient of resistance i.e. increase of resistance with temperature; called

resistance thermometers or resistance temperature detectors (R.T.D.) (2) Negative



temperature coefficient i.e. decrease of resistance with increase of temperature, called

The rmistors. The resistance change is given by the formula

Rt = R0 (1 + at)

Where Rt = the resistance of the element at t0C in ohms

R0 = The resistance of the element at OOC ohms

a = The temperature coefficient of resistance in ohm/ohm/C.

5.5.1 RESISTANCE THERMOMETERS (Fig. 5.7)

The materials selected for resistance thermometers should have the following prop-

erties:

i) Stable temperature - resistance relationship.

ii) The specific resistance should be within the limit for easy construction

iii) Little change in the resistance due to non-temperature methods such as strains

etc.

iv) Change in resistance w.r.t. temperature should be large.

v) Commercially available with consistent quality.

PAGE 163

Three types of resistance thermometers are available having the above properties.

They are Nickel, Copper and Platinum.

5.5.1.1 NICKEL RESISTANCE THERMOMETERS

Its characteristics are not linear throughout the range but is frequently used due to its

specific resistance and less cost. The specific resistance is 6.38 n-ticro ohm-cm,

temperature coefficient .0066 ohm/ohm (OC).

5.5.1.2 COPPER.RESISTANCE THERMOMETERS



It has got a linear characteristic; specific resistance of copper is very less of the order of

1.56 micro ohm-cm. Temperature coefficient of copper resistance thermometer is

0.00425 ohm/ohm (IC).

5.5.1.3PLATINUM RESISTANCE

Though costly, platinum is more suitable than either copper or nickel. It's usage is

restricted to jobs that cannot be properly handled by the other two types of

thermometers.

Specific resistance 9.38 micro ohm-cm

Temperature coefficient 0.00385 ohm/ohm OC

5.5.2 CONSTRUCTION DETAILS

The different types of wires are employed in different ways depending upon the ranges

and the measuring media normally wire size 0.05 to 0.07 mm dia is used.

i) Platinum wire element wound on mica strip and protected by the mica strip.

ii) Coiled wires mounted on ceramic mendrals and casted in ceramic..

iii) Copper or Nickel element wound on an ebonite plate or on metal mendral.

iv) The coiled element wound on a mica cross.

The sensing elements wound as above are provided with porcelain beads, and inserted

inside a protecting sheath and the terminals brought to the porcelain blocks. The sheath

is provided with a head and cover where the block is kept.

5.5.3 MEASURING CIRCUITS

The change in resistance of the temperature sensitive resistance element can be

measured by:

Cross coil indicators (CI) called ratiometet

ii) Wheat stone bridge

PAGE 164

a) Null balance method,

b) Deflection galvanometer type

5.5.3.1RATIOMETER (Fig. 5.8)

The ratiometer consists of two crossed moving coils placed in the field of permanent

magnet at an angle of 200 approximately and connected as detector of a wheat stone

bridge with the resistance thermometer constituting its one artn.



The current flowing through the crossed coils create deflecting and restoring moments.

The change in resistance of thermometer with temperature causes change in current

through the crossed coils. Since the crossed coils are connected in cross opposition the

difference of current flowing through two crossed coil produces deflecting moment to

the pointers in such a way that the deflection is a ftmction of ratio of the two ci-rrents

and measures the temperature on the scale.

To lead in and out the currents through the coils and to bring the pointer to zero in off

conditions - two hair springs are connected.

6.5.3.2WHEAT STONEBRIDGE (Fig. 5.9)

a. Null Balance Method

In the null balance instruments, the unbalance voltage across the bridge which is

proportional to the change in resistance, is fed to a phase sensitive amplifier and after

amplification feeds to the control winding of the servomotor which operates and adjust

the slider to balance the bridge.

At balance the pointer coupled to the servomotor indicates the temperature value.

b) Deflection Galvanometer Type

Here the unbalance is directly fed to a galvanometer whose deflection is proportional to

the change of temperature and calibrated in terms of temperature.

5.5.3.3Compensation for Lead Wire Resistance

Since the resistance element is generally far away from the secondary instrument, the

resistance of the lead wire also will be included alongwith the element resistance to the

arm. Since the lead wire will be of copper which has high temperature coefficient, it

will cause an appreciable error in the readings. In order to eliminate the error, 3 wire or

4 wire systems are employed which include the lead wire resistance in the opposite

arms and thus cancels the effects.

5.5.3.4ADVANTAGEOFRESISTANCETHERMOMETERSOVERTHERMOCOUPLES

i) Thermocouples require that the reference junctions temperature should be



maintained constant or a suitable compensation is to be applied. It is very difficult to

maintain the reference junction temperature constant since it is in the instruments which

have significant heat dissipation. Also compensation cannot be practically provided

perfectly.

But in resistance thermometer, the measurement of temperature is absolute

measurement, and no reference is required.

ii) Resistance thermometers have greater sensitivity because the change of

resistance per OC is much larger hence is more easily measured than the

microscopic change of voltage per OC in thermocouples.

5.5.3.5Thermistors

Thermistors are semi conductor material having resistance values which vary by a ratio

of 1 0,000,000 from ~1 000 to +450'C. Thermistors due to their thermoresistive

characteris tics, stability, and high sensitivity have bec ome more versatile to ol for

temperature measurement. Their temperature coefficient varies from 1 to 5 ohm/ohm

per OC the semi conducting materials of which the thermistors made are metal oxides

and their mixtures like' oxides of cobalt copper, iron etc.

5.6 INTENSITY OF TOTAL RADIATION

This method employs the Steffan Boltzmann Law of radiant energy which states that

the intensity of radiant energy emitted from the surface of a body increase

proportionately to the fourth power of the absolute temperature of the body.

5.6.1 OPTICAL PYROMETERS

The radiant energy is measured by photometric comparison of the relative brightness of

the object of unknown temperature with a source of standard brightness such as the

tungsten filament of an electric lamp

5.6.2 RADIATION PYROMETER

A common form of pyrometer is the disappearing filament pattern, as shown in Fig.

5.10(i)

It measures the intensity of a monochromatic beam of the visible light radiated by a hot

body.

In one type, the radiation source is viewed through a telescopic system consisting of

objective lens A and eyepiece B. Inside the telescope is a small lamp C heated by



battery D. The current through C is adjustable by resistance R and a milliammeter is

connected in the heating circuit. A red optical filter is interposed between eye and

lamp. On looking through the eyepiece, the source is seen as a bright circle, square,

rectangle or other shape, and in the centre of it is the image of the filament of the lamp.

The resistance R is adjusted until the brightness of the filament is equal to that of the

radiation. When this occurs, the filament image appears to merge into the radiation

PAGE 167

image and present a uniform picture to the eye. This is indicated in Fig. 5.10(ii)b. If the

filament is not as bright as the source image, it appears dark against a lighter'

background (Fig. 5.' 10(ii)a). If, on the other hand, the filament is brighter than the

source it appears as a light band against a dark background (Fig. 5.10(ii)c). The

transition points between a, b, and c are definite, and different observers can record the

same values within small limits of personal error.



The radiation pyrometer is ideally suited for -

i) When very high temperatures are involved, temperature beyond the practical range for

thermocouple measurement.

ii) Where furnace atmosphere is detrimental to thermocouples and cause erratic

measurement and short life.

iii) Where for other reasons, it is impractical to contact the material whose temperature is to

be measured.

PAGE 168

5.7 CHANGE OF STATE OF TESTING BODIES

For pure chemical elements or compounds change of state viz. from solid to liquid to

gaseous etc. takes place at a fixed temperature and this property thus gives a method to

measure the temperature.

5.7.1 FUSION METHOD

Fusion of different metals takes place at different temperature. Pyrometric cones are

made for different temperature and are placed inside the furnace which will indicate the

temperature when the rated fusion temperature is attained.

5.7.2 VAPOURISATION METHOD

Vapourisation temperature of different volatile liquid are different. This property is

utilized to measure the temperature.

PAGE 169

Annexure-I

TEMPERATURE MEASUREMENT

Thermal Expansion of Thermal Electricity

Test Bodies Thermo Couples

Expansion Expansion Expansion Expansion Type Type Type Type Type

of solids of liquids of gases of vapour E I K S R

Chromel Iron- Chromel Plati- P187%

Copel Const- Alumel num Rhol3%

8700C anatam 1260"C 14800C Rohdium

760'C 10%

Platinum

14800C



Bimetallic Nitrogen

Thermometers filled

-650C to 1300C to

540'C 6700C

Mercury Hydro- Methyl

Filled carbon Chloride,

thermo- filled Ether,

meter Butane,

Hexane,

Toluene etc.

-390C + -85'0C to -850c to

6000C 2800C + 3400C

Change of Electric Intensity of Other Types

Resistance Total Radiation

Resistance Thermistors Radiation Optical Fusible Pyrometric Colour

Thermometers Pyrometers Pyrometer Thermometers Cones Change

Indicator

500OC to -400C 600'C- 1000C

6000OC -1300OC 2000OC 800OC

Platinum Copper Nickel 0-300OC Lence type Photocell

-1800C -2000C -700C 0-300OC 500-OC Pyrometer

to 630OC to 150OC to 150Oc 2000OC 150-2000OC

PAGE 173

6. PNEUMATIC INSTRUMENTS

6.1 INTRODUCIRION

For many years pneumatic instrument systems were the main method of monitoring

controlling and industrial plant. Electrical instrument systems, with fast response times

and ease of installation, have already overtaken pneumatic systems and are now used



for most applications previously considered to be the duty of their pneumatic

counterpart.

The slow response and costly installation problems of a pneumatic system are,

however, accepted when thep7, ailing conditions make electrical system sun acceptable.

Pneumatic instruments also find service in the smaller on off control system, where

transmission lags are small due to the size of the loop.

6.2 FLAPPER/NOZZLE (Fig. 6.1)

Pneumatic instruments rely on the accurate conversion of mechanical movement to a

proportional pneumatic signal. In most cases this conversion is achieved with the use )f

a transducer known as a flapper/nozzle.

Air is supplied at a pressure of 1.5 bar. Due to the fact that the nozzle orifice is three

times larger than that the restrictor orifice air can, in fact, exhaust faster than it can pass

through the restrictor. This will result in gauge reading zero.

If the flapper is now positioned so as to seal off the nozzle, the pressure will build up to

the supply pressure and be indicated on the gauge. In actual practice the flapper would

be connected through some fortn of linkage to the measuring element and it would be

the movement of the measuring element that moved the flapper. It follows that

movement of the measuring element changes the flapper relative to the nozzle and will,

therefore, change the air output pressure in a similar manner to that shown in the graph

(Fig. 6.2).

The flapper movement required to change the output from maximum to minimum is

very small, the actual movement/pressure change ratio win depend upon nozzle and

restrictor sizes but is usually about 0.02 mm.

Provided the flapper nozzle output is restricted to the straight line portion of the graph

we can say that output will vary proportionately to measuring element movement.



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Whilst in principle the single flapper/nozzle is an effective transducer it does have

sortie serious drawbacks, for instance any change in supply pressure would affect the

output pressure and also since the amount of flapper movement is so small even the

slightest amount of wear on pivots o@ linkages would render the system useless. The

difficulties may be overcome by the use of negative feed-back bellows. The feed-back

can be used to oppose the measuring element force (force balance) or it can be used to

change the position of the flapper relative to the nozzle (position balance).

6.3 POSITION BALANCE - PRINCIPLE (Fig. 6.3)



The flapper nozzle can float and can be moved by the feed-back bellows as well as the

measuring element. Assume the measuring element moves the flapper towards the

nozzle, the output pressure will increase and the feed-back bellows will expand. The

upper end of the flapper will, therefore, be moved away from the nozzle and the

effective movement of the flapper about the nozzle is reduced. This increases the

amount of measuring element movement needed to give the complete range of output

pressure and gives a proportional relationship between measuring element movement

and the corresponding output pressure. Small changes in supply pressure will not effect

the output. If the measuring element is in the position where the output pressure should

be 0.6 bar for example, and the air supply suddenly increased, the output pressure

would tend to increase, but the increase in pressure would expand the bellows, pushing

the flapper away from the nozzle until 0.6 bar is again obtained. This technique is used

extensively in pneumatic proportional control.

6.4 FORCE BALANCE PRINCIPLE (Fig. 6.4)

The force balance principle also uses negative feed-back but not, as in the position

balance, to move the flapper. The force created by the feed-back bellows is used to

oppose the force of the measuring element. Consider a change in the measured variable

that causes the flapper to move closer the nozzle, this would result in an increased

output. The increase in output would also cause the feed-back bellows to expand until

the force created by it balances the force of the measuring element, when at such time

there will be no further increase in pressure and hence no further increase in output.

When the movement of the measuring element cause the flapper to move away from the

nozzle the reading drop in pressure cause the feed-back bellows to reduce the force in

opposition to the measuring element force, until equilibrium is again established.

If we consider the force balance principle as shown in Figure 6.4.

where F = Force of measuring element

p = Pressure of air in bellows

A = Area of bellows

a = distance of F from pivot

b = distance of bellows from pivot

PAGE 176



Since for equilibrium

Clockwise moments = anticlockwise moments

then:

F x a = (PA) b

Fa = PAb

P = Fa

Ab

We can see that if distances a or b are made adjustable then the range over which the

system will operate can be varied by adjusting a or b. This technique is used extensively

in pneumatic telemetry.

RELAY VALVES OR AMPLIFIERS (BOOSTER RELAYS)

All flapper and nozzle systems are usually operated together with a relay valve, this is

intended to reduce any lag or sensitivity which the introduction of the feed-back

bellows may produce, and it is also necessary where a large volume of air is involved in

the connecting pipework to the secondary element.

If the nozzle alone carries out the transmitting operation, all the air supply must come

from it and the inflation and deflation of the various volumes (receiving elements and

pipe works) may take some time, causing delay in the signal getting to the receiver,

therefore, again creating a loss of sensitivity.



A relay valve is, therefore, used to improve the response factor and it is also a

pneumatic amplifier of volume as well as sometimes a pressure amplifier i.e. the

variation in output pressure and quantity of air may be greater but proportional to

flapper and nozzle movement.

In general use there are two basic forms of relays.

PAGE 178

a) continuous bleed type

b) non-bleed type

6.5.1 CONTINUOUS BLEED RELAY (REVERSE ACTING) (Fig. 6.5)

The nozzle pressure enters the diaphragm chamber and adjusts the position of the valve

in relation to the valve seats of the valve chamber. The air continuously escapes via the

vent and the rate of leakage determines the back pressure in the output chamber and

thus the output pressure will increase as the nozzle pressure decreases.

6.5.2 CONTINUOUS BLEED RELAY (DIRECT ACTING) (Fig. 6.6)

The nozzle pressure enters the diaphragm chamber and adjusts the position of the

double seat valve in relation to the valve seats of the valve chamber. If the nozzle

pressure increases the force on the diaphragm moves the double seat valve to the left.

This results in the output pressure increasing proportionately. A drop in nozzle

pressure would cause the double seat valve to move to the right, this seals off the supply

and causes a drop in the output, excess pressure vents through the vent hole. It,

therefore, follows that any increase in nozzle pressure results in an increased output.

Since, with both the reverse and direct acting continuous bleed relay, output pressure is

maintained by venting excess air to atmosphere there is a continuous consumption of

air. Typically this will be about 0.5 cubic feet/minute and can be overcome by the use

of a non-bleed type relay.

6.6 ELECTRICAL/PNEUMATIC CONVERSION

Because of the modem trend towards electronic control and display equipment it is

frequently necessary to convert pneumatic signals to a proportional electrical signal or

to convert an electrical signal to a proportional pneumatic signal.

This can be achieved by the use of a pneumatic/electrical converter or and electro/

pneumatic converter, a typical application is shown in Fig. 7.8.



Electro pneumatic converters are also used where transmission signals cover great

distances or to improve response times of existing pneumatic equipment.

Essentially an E/P converter is an electricity to movement transducer with a flapper

nozzle system converting the mechanical movement to a pneumatic signal. The

mechanical movement is normally achieved by electro magnetic means, i.e. the

attraction or repulsion of a permanent magnet suspended within or, at close proximity

to a coil.

Because the E/P converter is essentially a flapper nozzle device it is important (as

previously discussed) to ensure a clean dry air supply at the correct pressure i.e.

filtration and regulation.

PAGE 179



PAGE 181

6.6.1 THE FIELDEN E/P CONVERTER (Fig. 6.9)

This Fielden E/P converter is a force balance device without feed-back. Because of the

lack of feed-back the setting up of the nozzle is critical. The device is supplied with air



at 1.5 bar and has restrictor and nozzle size ratios shnilar to a conventional flapper

nozzle system i.e.3:1.

The beam is pivoted at one end whilst the other end is attached to a permanent magnet,

the plug of the primary valve is also connected to the bream. Zero adjustment is

achieved by varying the spring tension and positioning the primary valve plug relative

to its seat (nozzle). Current is applied to the coil and a magnetic field is set up, (the

strength of which depends upon the value of current) the permanent magnet is forced

down which brings the primary valve closer to its seat, pressure builds up and forces the

diaphragm down which seals off the exhaust valve and opens the secondary valve,

resulting in an increase in output pressure. If the value of current falls the permanent

magnet will rise relieving the pressure on top of the diaphragm which closes the

secondary valve in. Excess pressure is vented through the exhaust, resulting in a drop

in pressure. Oil damping is provided on the magnet to give smooth operation.

FIG. 6.9 FIELPIEN EIP CONVERTOR

6.7 DIAPHRAGM AIR MOTORS

The Figure 6.10 shows one of the more common types of control valve. It consists of a

motor unit which contains a flexible diaphragm. The diaphragm virtually seals the



chamber into two parts, the upper sections receiving the pneumatic signals from the

controller via the air input. The input signals deflect the diaphragm which is fixed to

the thrust plate, the spindle attached to the thrust plate, extends downwards into the

body of the valve. The deflection is opposed by the range spring whose, rating

determines the extent of travel of the spindle for a given pressure range and effective

diaphragm area. Since the spindle is connected to the valve plugs we have the means of



automatic ary adjusting the orifice area in response to action from the controller and

thereby altering the flow of the medium through the valve.

6.8 PNEUMATIC CYLINDER MOTORS

Although position control of pneumatic cylinders is possible, in practice it is seldom

done without using some form of feed-back. A theoretical type of cylinder positioning,

without feed-back, is shown in Fig. 6.1 1.

The output pressure of the regulator is fixed at say 3 bar. The air operated pressure

regulator will accept inputs between 0.2 and 1 bar, 1 bar input resulting in 3 bar output.

The range of output would be dependent upon piston specification and condition and

would, therefore, be set up in situ. It follows therefore, that by varying the pressure on

top of the piston we can effectively position the piston rod anywhere within its length of

travel.

PAGE 183

A control valve must be capable of responding smoothly and rapidly to small changes

in the controller output signal. The quality of control will be impaired if any force, for

example, that due to friction of working parts, opposes the movement of the spindle



and the valve plug. This can be overcome by employing mechanical feed-back in the

form of a positioner.

6.9 THE VALVE POSITIONER

The primary function of a valve positioner is to ensure that the control valve plug

position is always directly proportional to the value of the controller output pressure,

regardless of gland friction, actuator hysteresis, off-balance of forces on the valve plug

etc. This is usually achieved by incorporating a feed-back lever that acts in opposition

to the movement to the input bellows. This is shown in block diagram in Fig. 6.12.

The system can be either a position or force balance system but in practice force

balance systems are more common. Positioners can be incorporated into diaphragm, or

cylinder type actuators, a typical actuator positioner in shown in Fig. 6.13.

The controller output signal does not directly actuate the valve stem but is fed to a

bellows unit. Assume that the system is in equilibrium and then the controller output

increases slightly. The flapper is moved towards the nozzle and the relay output

pressure begins to increase.

This output pressure continues to increase until the valve spindle moves, mechanical

feed-back then restores the equilibrium. Thus the force applied to move the valve

spindle is sufficient to overcome the effect of all forces, no matter what the origin,

which tend to oppose the spindle movement. Without the positioner the slight change

in controller output signal may have been too small to initiate any corrective action.

The matching of input signal range to valve travel range is achieved by changing the

ratio of bellows/nozzle distance to feed-back arm/nozzle distance.

PAGE 184



Positioners incorporated into pneumatic cylinders generally operate on a pilot valve

principle of which two are shown in Fig. 6.14 and 6.15.

6.9.1 KENT MARK IV (Fig. 6.14)

The controller output acts onto the bellows, the pilot valve spool is attached to the

bellows via a connecting link. Assuming an increase in the controller out put the

bellows will expand, unbalancing, the spool of the pilot valve. Air is then admitted to

the top of the piston and it begins to move down. In doing so it takes the cam with it, as

the cam moves down the bell crank lever turns about its pivot and through the spring

opposes the movement of the bellows and restores the spool of the pilot valve to its

original position. The system is back in equilibrium. An equalizing valve is included to

enable manual positioning of the piston.

6.9.2 BAILEY CONTROL DRIVE (Fig. 6.15)



The controller output acts on the bellows, the spool of the pilot valve is attached to

the pilot beam at one end whilst the other end is anchored and pivoted. Assuming an

increase in the controller output the bellows will expand and push the pilot beam up

against the restraining force of the spring. This unbalances the pilot valve and causes

air to be admitted to the top of the piston. The piston, therefore, begins to move down,

this results in the positioner drive arm turning the cam which puts more tension on the

spring and so restores the pilot beam to its original position.

6.10 PNEUMATIC SEQUENCE CONTROL

Sequence control is essentially the carrying out of a series of events in aa logical

progressive manner.

The actual event is usually the carrying out of some physcal work utilizing the

movement of an actuator. In pneumatic sequence control compressed air can provide

power through either linear motion or rotary motion or rotary motion i.e. diaphragm

valves, pneumatic cylinders or air motors. Since the pneumatic cylinder is by far the

simplest form of actuator it shall be used as the basis for the following notes, although

the principle can be applied to any actuator.

The controlled sequence is usually achieved by the use of various types of three and

five port valves.

6.10.1 TYPES OF CYLINDER

The simplest type is the single acting cylinder (Figure 6.16). With this type air is used

to make the unit out stroke or extent (+). Once the pressure has been removed, the

return or in stroke (-) is achieved by mechanical means, in this case a spring. The signal

can be air to extend (application of a signal will push the piston out) or air to retract (

application of a signal will push the piston in).

In the double acting cylinder, if air is applied to P1 ( with P2 open to exhaust) the piston



PAGE 187

Will outstroke (+); and if air is applied to P2 ( with P1 open to exhaust) the piston will

in-stroke (-).Ref. Fig.6.17.



The symbols + and - are often used as a shortened notation to indicate movement of a

cylinder, particularly when describing the sequence of operation of a circuit. For

example, there may be three cylinders A, B and C which operate in the sequence

A+,B+,B-,A-,C+,C-.

The circuit symbols for pneumatic cylinder actuators are shown in Figure 6.18.

6.10.2 PNEUMATIC CYLINDER CUSHIONING

On high pressure systems, piton speeds can be in the order of 450 mm / sec and impact

forces at the ends of the stroke can be great. In order that damage may not be caused by

sudden contact between the fast moving piston and the cylinder end housing, some

form of buffer or cushioning can be used. This does not limit the piston

PAGE 188

Travel but allows gradual deceleration in the last 25 mm or so of travel, this is

achieved as shown in Fig.. 6.19.



As the cylinder outstrokes under the action of applied pressure P air is displaced from

the other side of the piston to atmosphere through the main part and needle valve. When

the cushioning seal, the main port is blocked off, air can, therefore, only escape through

the needle valve at a much slower rate, thereby causing the piston to slow down for the

premium period of travel.

This results in the cushioning effect as shown in Fig. 6.20.

The circuit symbols for cushioned cylinders are as shown in Fig. 6.21.

6.11 SEQUENEC CONTROL VALVES

An ordinary on off valve has one inlet and one outlet. However, for sequence control

PAGE 189



FIG. 6.21. CIRCUIT SYMBOLS FOR CUSHIONED CYLINDERS

Applications facility must be made to exhaust any unwanted signal. The two most

common types of valve used in sequence control are three port and five port types.

6.12. THREE PORT RELAYS

The basic three port valve consists of a two lobe spool running in a surface ground

cylinder, compressed air can be switched to the outlet by the application of a force to the spool

as shown in Fig. 6.22A.

The force can be removed and the outlet will remain connected to the air supply.

The application of a second force will return the spool to its original condition, main air

will then be isolated and the device connected to the outlet will exhaust from port 3 as shown

in Fig. 6.22B

The application of the force can be applied in many ways and will be covered later in

this section.



6.15 FIVE PORT RELAYS

The basic construction of the five port relay is the same as the three port relay, the only

difference being the use of a three lobe spool. Compressed air can now be routed through the

valve whilst at the same time a signal can be exhausted through it. The direction of force will

determine the routing of supply and exhaust as shown in Fig. 6.23.



Control & Inst_ V-I

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