Pressure Measurement in Petrochemical Industries

8/13/2019 Pressure Measurement in Petrochemical Industries

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Gowtham Books 2of 90

Volume 1

Pressure measurement

in

Petrochemical Industries

A.Gowthaman ME,MBA

Mail : [email protected]


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INSTRUMENTATION BOOKS SERIES

Volume 1 Pressure and Flow measurement in PetrochemicalIndustries

Volume Temperature and Level measurement in PetrochemicalIndustries

Volume 3 Analytical Instrumentation in Petrochemical Industries

Volume 4 Control Valves Part I

Volume 5 Control Valves Part II

Volume 6 Control Valves Design

Volume 7 Digital Controllers

Volume 8 Distributed Control Systems

Volume 9 Programmable Logic Controller

Volume 10 Supervisory Control and Data Acquisition System

Volume 11 Vibration Systems

Volume 12 Interview Questions

Volume 13 Instrumentation in Process Industry

Volume 14 Logic Distributed Control Systems


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PRESSURE MEASUREMENT

INTRODUCTION

Measurement of pressure, vacuum and differential pressure is very important in Oil

companies, chemical processing and manufacturing industries. In general, pressure gauges will

exceed any other type of instruments. A wide range of pressures, from high vacuum to pressure of40,000 kPa or even greater must be measured and controlled accurately and reliably in process

industries. Because of its wide range, several pressure-measuring elements are required.

In all these pressure measuring elements since the fluid being measured usually fills the

measuring system, care should be taken in the correct installation, selection of the type of

measuring element, pulsation in the fluid pressure, corrosive nature of the fluid etc.

Pressure is defined as, The normal force per unit area exerted on a imaginary or real plane surface

in a fluid or a gas.

Pressure = Force /Area

ATMOSPHERIC PRESSURE : The pressure exerted by the atmosphere is defined as the

atmospheric pressure. This pressure varies with the location. The standard atmospheric pressure is

taken at average sea level and is 101.325 kPa A (14.7 psi).

GAUGE PRESSURE : Gauge pressure is the pressure measured above the atmospheric pressure.

An ordinary pressure gauge measures the difference between the pressure inside and outside the

pressure measuring element.

ABSOLUTE PRESSURE : Absolute pressure is the sum of gauge pressure and atmospheric

pressure. Absolute pressure = Gauge pressure + Atmospheric pressure

DIFFERENTIAL PRESSURE : The differential pressure is the pressure between two pressures. It is

measured by separating the two pressures by a diaphragm and measuring the net force or motion

of the diaphragm, or by observing the height of a column of liquid in a manometer.

VACUUM : Vacuum is defined as the pressure below the atmospheric pressure and is usually

expressed in mm of mercury or mm of water. A full vacuum represents -760mm of Hg or -407.2

inches of H2O or -101.325 kPa.


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GRAPHICAL REPRESENTATION OF COMMON PRESSURE TERMS

Pressure

Pascal ( Absolute Pressure )

( Gauge Pressure )

Absolute Zero Pressure ( Perfect Vacuum )

Atmospheric pressure

( suction )

Pascal

Pascal

Pascal

Pascal

( Absolute Pressure )( Pressure Difference )

( 101. 325 kPa A )Zero Gauge pressure

STANDARD ENGINEERING UNITS AND THEIR INTER-CONVERSIONS

psi(lb/in2) Kg/cm2 kPa Bar In. H2O mm of Hg Atmosphere

1 0.07031 6.895 0.06895 27.70 51.71 0.06804

14.223 1 98.05 0.9805 394.0 735.5 0.9678

0.1450 0.0102 1 0.01 4.016 7.502 0.00987

14.50 1.020 100 1 401.6 750.2 0.987

0.03610 0.002456 0.2490 0.002490 1 1.867 0.002456

0.0193 0.001360 0.1333 0.001333 0.5357 1 0.001316

14.70 1.0333 101.3 1.013 407.2 760 1


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PRESSURE MEASURING DEVICES

SELECTION CRITERIA

Pressure Measuring Devices Selection criteria depends upon the following factors

Range of the pressure to be measured

Application

Corrosive nature of the fluid

Hot and slurry nature of fluids

Based on these factors following devices are used for pressure measurement applications.

MANOMETERS

PRESSURE GAUGES

PRESURE TRANSDUCERS

PRESURE TRANSMITTERS

PRESSURE SWITCHES

PRESURE REGULATORS


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1 MANOMETERS

Monometers are generally used for Low Range Pressure Measurement applications. The

types of Manometers are,

1. U Tube manometer.

2. Single limb manometer.

3. Inclined tube manometer.


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1.1U TUBE MANOMETER

This is used to measure low range of Pressure measurement in Inches of H2O Column and

normally used in the workshop facilities for calibration. This consists of a U- tube closed at one end

or open at both ends. A manometric liquid, such as mercury, glycerine or water is filled to half of

the tube. The liquid is generally coloured by ink or some dye. One of the ends of the U- tube is

connected to the pressure tapping and the other is open to the atmosphere. The height difference

of the liquid will give the pressure or vacuum applied. A scale fitted with the limbs is calibrated in

centimeters or inches.

A glass U-tube is partially filled with liquid, and both ends are initially open to the atmosphere.

When a gage pressure P2 is to be measured, it is applied to the top of one of the columns and the

top of the other column remains open. When the liquid in the tube is mercury, for example, the

indicated pressure h is usually expressed in inches (or millimeters) of mercury.

P1= HIGH PRESSURE

P2= LOW PRESSURE

h = DIFFERENTIAL PRESSURE

To convert to pounds per square inch (or kilograms per square centimeter),

P2= dh

where P2= pressure, psig (kg/cm2)

d = density, lb/in3(kg/cm

3)

h = height, inches (cm)

For mercury, the density is 0.490 lb/in3at 60F (15.6C), and the conversion of inches of mercury to

pounds per square inch becomes P2= 0.490h

The density of water at 60F (15.6C) is 0.0361 lb/in3, and if water is used in a manometer, the

conversion of inches of water to pounds per square inch becomes P2= 0.0361h

The same principles apply when metric units are used. For example, the density of mercury at 15.6

C (60F) may also be expressed as 0.0136 kg/cm3, and the conversion of centimeters of mercury to

kilograms per square centimeters P2= 0.0136h

For measuring differential pressure and for static balance, P2 P1=dh


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1.2SINGLE LIMB MANOMETER

The single limb well-type manometer does not differ much from the U- tube manometer. However

in place of one leg of the manometer, a well is installed which has sufficient capacity to cause the

level to remain practically constant regardless of the height of the liquid column. This arrangement

permits the use of only one glass column and makes it possible to read the pressure directly on the

graduated scale without making any zero adjustment of the scale as is necessary with the U- tube.

The ratio of diameters is important and should be as great as possible to reduce the errors resulting

from the change in level in the large-diameter well.

The pressure difference can be read directly on a single scale. For static balance,

P2 P1 =d (1+

)h

where A1 = area of smaller-diameter leg

A2 = area of well

If the ratio of A1/A2 is small compared with unity, then the error in neglecting this term becomes

negligible, and the static balance relation becomes P2 P1 = dh


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1.3INCLINED TUBE MANOMETER

This is used to measure very low range of pressure measurement in Inches of H2O Column

and normally used in the workshop facilities for calibration. This is identical to Single limb

manometer but is used to measure very small range of pressure or vacuum. This is achieved by

magnifying the level difference. To increase the sensitivity, a less dense liquid may be used. Ranges

of measurement using this type of manometer are usually few millimeters of water column.

This produces a longer scale so that h = L sin


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2 PRESSURE GAUGES

Pressure Gauges are generally used for Medium & High Range Pressure Measurement

applications. The types of Pressure gauges are,

1. Bourdon tube Pressure Gauge

2. Glycerine filled Pressure gauges

3. Bellows Type Pressure Gauge

4. Diaphragms Type Pressure Gauge

5. Capsules Type Pressure Gauge

6. Capillary type Pressure Gauge


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2.1 BOURDON TUBE PRESSURE GAUGE

There are mainly three types of bourdon tubes available namely C-TYPE BOURDON TUBE ,

SPIRAL BOURDON TUBE, HELICAL BOURDON TUBE.

We are normally using stainless steel bourdon tubes although other material of bourdon tubes

are used for specific applications in other industries because of their ruggedness, long life and

corrosive nature of crude oil. Stainless steel bourdon tubes are used to measure pressures from 0-

200 Kg/cm2 to 0-1500 Kg/cm2. Brass bourdon tubes are used to measure pressures from 0-1

Kg/cm2 to 0-75 Kg/cm2.

Bourdon Tube pressure gauges are available with different dial sizes and different connections.

We are using normally 4 and 6 dial sizes with or NPT (M) bottom and back connections.Most of the pressure gauges are fitted with one main isolation valve either ball or gate valve and a

block and bleed valve to safely isolate the pressure gauge from service for calibration or

replacement.


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2.1.1 C- TYPE BOURDON TUBE

The bourdon in a pressure gauge is a C- shaped flattened or oval tube, bent into an arc of

about 250 degrees. One end of the tube is fixed onto a fitting where the pressure to be measured is

admitted and the other end is sealed/brazed. When the applied pressure is increased, the two sides

of the tube are forced apart as a result of increase of the radius of curvature of the tube. The

movement or lift of the closed end of the bourdon tube resulting from the internal pressure change

is converted into rotary motion by means of a sector and pinion arrangement. A pointer attached to

the extension of the pinion moves on a calibrated dial to read the pressure in the desired units.

The material of the tube should be

Hard enough to withstand the pressure

Stable enough to retain its calibration indefinitely.

Immune to corrosion from the fluid.

Easy to fabricate.

The most common material used for construction of the bourdon tubes are

Phosphor bronze, Beryllium Copper, Alloy Steel, Carbon Steel, Stainless Steel and Brass to name a

few.


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2.1.2 SPIRAL AND HELICAL BOURDON TUBES

Spiral bourdon tube is used to measure lower pressure from 0 to 14 bars while helical

bourdon tube is used for higher pressure from 0 to 5600 bars. The main advantage of this type over

conventional C- type bourdon tube is that it eliminates springs, sector and pinion arrangements

thereby increasing the life span of the instrument. This is achieved by increasing the number of

turns in the spiral or helical type bourdon tubes. In this way an enlarged movement of the free end

of the tube is obtained. The movement of the free end of the tube is transmitted to the pen or

pointer through a flexible metal connecting strip, which joins the free end of the tube with the

pointer shaft.


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2.2 GLYCERINE FILLED PRESSURE GAUGES

Liquid-filled pressure gauges provide a number of advantages:

The liquid absorbs vibration and pressure spikes.

The dampening action of the liquid enables the operator to take reading during conditions of rapid

dynamic loading and vibration.

The liquid lubricates all moving elements, dramatically reducing wear in the movement.

Because most liquid-filled gauges are filled with non-aqueous liquid and hermetically sealed, they

perform in corrosive environments and are immune to moisture penetration and icing.


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2.3 BELLOWS TYPE GAUGE

Bellows assembly is often compared to a spring. Available ranges on this type are from 0-5 in.Hg

to 0-3 bar. The material of construction of bellows is 80% Copper- 20% Zinc, Brass, Phosphor

Bronze, Beryllium Copper or Stainless Steel.

In actual construction, a thin- walled tube is taken and formed mainly by special hydraulic

presses onto a corrugated shape. One end of the bellows is completely sealed and the other end

soldered/brazed to a fixture with an opening to apply either pressure or vacuum.


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2.4 DIAPHRAGM TYPE GAUGE

Diaphragm type gauges are used for low pressure measurement, between 25mm water column

and 0.3 bars. Diaphragm seals are used along with C- type bourdon gauges to protect bourdon

tubes against corrosive/clogging fluids.

In construction, it consists of a hardened and tempered stainless steel corrugated

diaphragm of about 65mm diameter held between the two flanges. Pressure is applied to the

underside in the chamber shown, and movement of the center of the diaphragm is transmitted

through the ball-and-socket joint and high magnification link to the pointer as in the bourdon

gauge.

A view facing the bottom of the flange reveals the thin metal isolating diaphragm keeping process

fluid from entering the gauge mechanism. Only inert fill fluid occupies the space between this

diaphragm and the gauges bourdon tube. The following illustration shows how the fill fluid

transfers process fluid pressure to the gauges bourdon tube element while isolating that bourdon

tube from the process fluid (shown here inside a pipe).


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2.5 CAPSULE TYPE GAUGE

This is the most precision type of pressure indicator. This instrument is available in ranges

from 0-30 kPa to 0-10 bar. This has a sensitivity of 0.01% of full range and an accuracy of 0.1% of

full scale. This instrument may be used to measure both differential pressure and gauge pressure.

In case of differential pressure measurement, the higher pressure is applied to the inside of

the capsule and the lower pressure is applied to the pressure-tight case. In case of gauge pressure

measurement, the measured pressure is applied to the capsule and the case is open to

atmosphere. In both cases the meter or pointer movement is similar to diaphragm type gauge

which was discussed earlier.


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2.6 CAPILLAIRE TYPE PRESSURE GAUGE

This is a Diaphragm seal type pressure instrument in which liquid as a pressure transmitting media

is filled between a diaphragm seal parts and bourdon tube as an element. Diaphragm and the lower

flange as wetted parts can be selected according to applications, so these instruments are

appropriate for the measurement of highly corrosive fluid, high viscosity fluid, fluid which contains

solid materials or fluid to be easily solidified.

Because the high-corrosion resistant diaphragm can be used at the pressure receiving portion, this

pressure gauge can be used for the measurement of highly corrosive measuring fluid. For a

pressure gauge in which a diaphragm is attached by welding, the surface of a diaphragm can be

easily cleaned (by loosening the casing bolts.). A zero-adjusting pointer has been applied, so

calibration required due to errors of temperature, elevation, etc. can be easily preformed. With the

application of a welded diaphragm the application for leakage of filled liquid has been decreased.

(diaphragms made of some materials are excluded.)

The only difference between this chemical-seal gauge and a remote-seal gauge is the small

diameter capillary tubing used to connect the gauge to a remote diaphragm. An illustration showing

the internals of a remote seal system appears here.


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3 PRESSURE TRANSMITTERS

Transducer and transmitter are sometimes used interchangeably but there is a difference

between these two.

Block diagram of a transducer

Force

InputOutput

Motion ( Mechanical )

Resistance, Capacitance

Inductance ( Electrical )

Transducer is the part that is in physical contact with the process fluid and produces a

response or a change in motion or force when the measured process value changes. The types of

transducers are

Pneumatic transmitter

Potentiometric Pressure Transducers

Capacitive Pressure Transducers

LVDT Pressure TransducersPiezoelectric Pressure Transducers

Optical Pressure Transducers

Transmitter is a device that responds to a measured process value and produces an output

that becomes the input to a receiver or a controller in a control room.


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Block diagram of a transmitter

Pressure

Input Scaled Output signal

4 to 20 mA ( Electrical )

20 to 100 kPa ( Pneumatic )

Converting pressure to movement

or electrical quantity ( v, A, etc. )

Transducer ( Pressure Element ) Transducer ( LVDT, flapper - nozzle etc. )

Converting movement or electrical quantity

to a scaled output signal


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3.1PNEUMATIC FLAPPER - NOZZLE PRESSURE TRANSMITTER

An increase in the measured pressure will move the force bar. The flapper is attached to the

force bar by means of a flexure strip (A spring). This movement of the force bar will make the

flapper move towards the nozzle. The nozzle backpressure will subsequently increase and thisincreased nozzle backpressure will be amplified by the relay to produce the output signal. The

output signal is also applied to the feedback bellows. As the pressure increases in this bellows, the

bellows will apply a force on the bottom end of the range bar. This force makes the range bar to

move in the oppositedirection to that caused by the force bar. The range bar is also attached to the

flexure strip and the movement of the range bar will cause the flapper to move away from the

nozzle. During the stable condition of the transmitter, that is, when the process pressure is not

changing, the two forces are balanced. Any change in the measured pressure will upset this

balance.


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The sequence of events that will follow such an upset is as follows.

A change in the measured pressure will cause the forces to become unequal.

This will change the flapper-nozzle relationship.

The nozzle backpressure will change.

The changed nozzle backpressure will be amplified by the relay and will be given as the

output and also to the feedback bellow.

The output pressure will now create a new feedback force to counteract the force created

by the force bar.

At the balanced condition the flapper-nozzle relationship is such that the output will

neither increase nor decrease. This specific position of the flapper with respect to the

nozzle is called as the throttle position.

The feedback force is said to be negative because this force is opposite to or opposes the

force that is produced by the diaphragm capsule and the force bar. The amount of the pressure

required in the feedback bellows to generate sufficient force to counteract the force produced by

the feedback bellow would depend upon the following.

The effective area of the bellow (Usually a Constant)

The distance between the bellow and the range wheel (Movable Fulcrum)

It can be seen from the drawing on the next page that as the distance is increased, the mechanical

advantageof the range bar will increase and a lesser pressure is required to balance a given force.

Conversely, when the distance is decreased, a larger pressure is needed to balance a given force.

Changing the mechanical advantage of the feedback mechanism is a convenient means of changingthe gain or the span of the transmitter.

A bias spring (reference adjustment) is provided to preload the feedback

mechanism to obtain a desired output when the pressure measured is zero. This is the zero

adjustment provided by the manufacturer of the device.


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3.2POTENTIOMETRIC PRESSURE TRANSDUCERS

The Potentiometric pressure sensor provides a simple method for obtaining an electronic

output from a mechanical pressure gauge. The device consists of a precision potentiometer, whose

wiper arm is mechanically linked to a Bourdon or bellows element. The movement of the wiper arm

across the potentiometer converts the mechanically detected sensor deflection into a resistancemeasurement, using a Wheatstone bridge circuit.

The mechanical nature of the linkages connecting the wiper arm to the Bourdon tube,

bellows, or diaphragm element introduces unavoidable errors into this type of measurement.

Temperature effects cause additional errors because of the differences in thermal expansion

coefficients of the metallic components of the system. Errors also will develop due to mechanical

wear of the components and of the contacts.

Potentiometric transducers can be made extremely small and installed in very tight quarters, suchas inside the housing of a 4.5-in. dial pressure gauge. They also provide a strong output that can be

read without additional amplification. This permits them to be used in low power applications. They

are also inexpensive. Potentiometric transducers can detect pressures between 5 and 10,000 psig

(35 KPa to 70 MPa). Their accuracy is between 0.5% and 1% of full scale, not including drift and the

effects of temperature.


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3.3 CAPACITANCE PRESSURE TRANSDUCERS

An electronic-type transmitter is shown in the figure above. This particular type utilizes a

two-wire capacitance technique.

Process pressure is transmitted through isolating diaphragms and silicone oil fill fluid to a

sensing diaphragm in the center of the cell. The sensing diaphragm is a stretched spring element

that deflects in response to differential pressure across it. The displacement of the sensing

diaphragm is proportional to the differential pressure. The position of the sensing diaphragm is

detected by capacitor plates on both sides of the sensing diaphragm. The differential capacitance

between the sensing diaphragm and the capacitor plates is converted electronically to a 4-20 mA dc

signal.

Capacitance pressure transducers were originally developed for use in low vacuum research.

This capacitance change results from the movement of a diaphragm element. The diaphragm is

usually metal or metal-coated quartz and is exposed to the process pressure on one side and to the

reference pressure on the other. Depending on the type of pressure, the capacitive transducer can

be either an absolute, gauge, or differential pressure transducer.

Stainless steel is the most common diaphragm material used, but for corrosive service, high-

nickel steel alloys, such as Inconel or Hastelloy, give better performance. Tantalum also is used forhighly corrosive, high temperature applications. As a special case, silver diaphragms can be used to

measure the pressure of chlorine, fluorine, and other halogens in their elemental state. In a

capacitance-type pressure sensor, a high-frequency, high-voltage oscillator is used to charge the

sensing electrode elements. In a two-plate capacitor sensor design, the movement of the

diaphragm between the plates is detected as an indication of the changes in process pressure.


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As shown in Figure the deflection of the diaphragm causes a change in capacitance that is

detected by a bridge circuit. This circuit can be operated in either a balanced or unbalanced mode.

In balanced mode, the output voltage is fed to a null detector and the capacitor arms are varied to

maintain the bridge at null. Therefore, in the balanced mode, the null setting itself is a measure of

process pressure. When operated in unbalanced mode, the process pressure measurement is

related to the ratio between the output voltage and the excitation voltage.

Single-plate capacitor designs are also common. In this design, the plate is located on the

back side of the diaphragm and the variable capacitance is a function of deflection of the

diaphragm. Therefore, the detected capacitance is an indication of the process pressure.

The capacitance is converted into either a direct current or a voltage signal that can be read directly

by panel meters or microprocessor-based input/output boards.

Capacitance pressure transducers are widespread in part because of their wide rangeability, from

high vacuums in the micron range to 10,000 psig (70 MPa). Differential pressures as low as 0.01

inches of water can readily be measured. And, compared with strain gage transducers, they do not

drift much. Better designs are available that are accurate to within 0.1% of reading or 0.01% of full

scale. A typical temperature effect is 0.25% of full scale per 1000 F.

Capacitance-type sensors are often used as secondary standards, especially in low-

differential and low-absolute pressure applications. They also are quite responsive, because the

distance the diaphragm must physically travel is only a few microns. Newer capacitance pressure

transducers are more resistant to corrosion and are less sensitive to stray capacitance and vibration

effects that used to cause "reading jitters" in older designs.

See Flow measurement book for more details.


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3.4LVDT PRESSURE TRANSDUCERS

Figure illustrates the use of a linear variable differential transformer (LVDT) as the working

element of a pressure transmitter. The LVDT operates on the inductance ratio principle. In this

design, three coils are wired onto an insulating tube containing an iron core, which is positionedwithin the tube by the pressure sensor.

Alternating current is applied to the primary coil in the center, and if the core also is

centered, equal voltages will be induced in the secondary coils (#1 and #2). Because the coils are

wired in series, this condition will result in a zero output. As the process pressure changes and the

core moves, the differential in the voltages induced in the secondary coils is proportional to the

pressure causing the movement. This voltage is amplified and rectified by an AC-DC Converter. The

output of this AC-DC converter is a current signal proportional to the input voltage and hence

proportional to the applied pressure. The output is 4-20 mA in most of applications.

LVDT-type pressure transducers are available with 0.5% full scale accuracy and with ranges

from 0-30 psig (0-210 kPa) to 0-10,000 psig (0-70 MPa). They can detect absolute, gauge, or

differential pressures. Their main limitations are susceptibility to mechanical wear and sensitivity to

vibration and magnetic interference.


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Reluctance is the equivalent of resistance in a magnetic circuit. If a change in pressure

changes the gaps in the magnetic flux paths of the two cores, the ratio of inductances L1/L2 will berelated to the change in process pressure (Figure 3-10B). Reluctance-based pressure transducers

have a very high output signal (on the order of 40 mV/volt of excitation), but must be excited by ac

voltage. They are susceptible to stray magnetic fields and to temperature effects of about 2% per

1000 F. Because of their very high output signals, they are often used in applications where high

resolution over a relatively small range is desired. They can cover pressure ranges from 1 in. water

to 10,000 psig (250 Pa to 70 MPa). Typical accuracy is 0.5% full scale.


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3.5PIEZOELECTRIC PRESSURE TRANSDUCERS

When pressure, force or acceleration is applied to a quartz crystal, a charge is developed

across the crystal that is proportional to the force applied. The fundamental difference between

these crystal sensors and static-force devices such as strain gages is that the electric signal

generated by the crystal decays rapidly. This characteristic makes these sensors unsuitable for the

measurement of static forces or pressures but useful for dynamic measurements.

Piezoelectric devices can further be classified according to whether the crystal's

electrostatic charge, its resistivity, or its resonant frequency electrostatic charge is measured.

Depending on which phenomenon is used, the crystal sensor can be called electrostatic,

piezoresistive, or resonant.

When pressure is applied to a crystal, it is elastically deformed. This deformation results in a

flow of electric charge (which lasts for a period of a few seconds). The resulting electric signal can

be measured as an indication of the pressure which was applied to the crystal. These sensors

cannot detect static pressures, but are used to measure rapidly changing pressures resulting from

blasts, explosions, pressure pulsations (in motors, engines, compressors) or other sources of shock

or vibration. Some of these rugged sensors can detect pressure events having "rise times" on the

order of a millionth of a second.


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3.6OPTICAL PRESSURE TRANSDUCERS

Optical pressure transducers detect the effects of minute motions due to changes in process

pressure and generate a corresponding electronic output signal. A light emitting diode (LED) is used

as the light source, and a vane blocks some of the light as it is moved by the diaphragm. As theprocess pressure moves the vane between the source diode and the measuring diode, the amount

of infrared light received changes.

The optical transducer must compensate for aging of the LED light source by means of a

reference diode, which is never blocked by the vane. This reference diode also compensates the

signal for build-up of dirt or other coating materials on the optical surfaces. The optical pressure

transducer is immune to temperature effects, because the source, measurement and reference

diodes are affected equally by changes in temperature. Moreover, because the amount of

movement required to make the measurement is very small (under 0.5 mm), hysteresis and

repeatability errors are nearly zero.

Optical pressure transducers do not require much maintenance. They have excellent

stability and are designed for long-duration measurements. They are available with ranges from 5

psig to 60,000 psig (35 kPa to 413 MPa) and with 0.1% full scale accuracy.


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3.7CALIBRATION OF PRESSURE MEASURING INSTRUMENTS

Pressure instrument calibration is the process of adjusting the instruments output signal to match a

known range of pressures. All instruments tend to drift from their last setting. This is because

springs stretch, electronic components undergo slight changes on the atomic level, and other

working parts sag, bend, or lose their elasticity. Basic calibration procedure includes zero, span, and

linearity adjustments. Proper calibration provides the desired beginning and ending pressures, and

produces an output signal that is proportional to the process pressure. Calibration of the

instrument is carried out by applying to it an air or liquid pressure whose value is accurately known.The method used depends upon the range of the instrument.

Calibration of Low Pressure Gauges

For very low pressures, upto 3 inch of water gauge, an inclined water gauge may be used. For

pressure upto 144 in. of water gauge or 12 in. of mercury gauge, a water manometer may be used,

although at the upper part of this range a mercury manometer becomes more manageable. For

higher pressures upto 72 in. mercury gauge, a mercury manometer may be used. For still higher

pressures, a dead-weight piston tester or hydraulic pumps must be used.

Calibration of Pressure Transmitters


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Typical tools required:

24 VDC power supply

Multimeter digital

Pneumatic hand pump (up to 600 psig) / Hydraulic hand pump (up to 10.000 psig) / Low pressure

hand pump / High precision digital test gauge

HART communicator

Screwdriver toolkit.

Isolate the pressure transmitter and bleed off pressure as per the site Lockout/Tagout Procedure.

Do not perform any adjustments until all "as found" calibration data has been recorded.

Note any deviation from this procedure in the remarks section of the calibration form.

1. Make connections between the pressure source, pressure standard, and the pressure

transmitter sensing line.

2. Connect the digital multimeter to the current output of the transmitter.

3. Turn on the transmitter and allow the internal components to reach normal operating

temperature

4. Using a dead-weight tester or a portable regulated air supply, exercise the transmitter from

zero to full scale and back to zero.

5. Adjust the pressure (vacuum) source to 0, 25, 50, 75,and 100% of the calibration range and

record the following at each data point

a. Digital pressure standard readingb. Current output

6. Adjust the zero to set exactly 4mA output, indicated on a precision meter.

7. Supply 100% pressure to the instrument.

8. Adjust the span to get a reading of exactly 20 mA.

9. Repeat steps 6 to 8 till no adjustment is required.

10.Apply 50% pressure (half-scale input pressure).

11.Adjust the linearity to bring the output signal within the specified tolerance of 12 mA (half-

scale output current).


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In most instruments, adjusting the span may throw off the zero adjustment or vice-versa.

Therefore, it needs to repeat the entire calibration procedure several times to achieve a properly

balanced calibration.

Disconnect the test equipment from the switch under test.

Complete the calibration form and affix the applicable calibration label.

If all checks are within tolerance, restore the pressure switch to operational condition and notify

appropriate personnel of work performed.

Calibration Setup with HART configurator.


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Calibration Record Sample sheet


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3.8Rosemount 3051 Pressure Transmitter

The Rosemount 3051C Coplanar design is offered for Differential Pressure (DP), Gage Pressure

(GP) and Absolute Pressure (AP) measurements. The Rosemount 3051C utilizes Emerson Process

Management capacitance sensor technology for DP and GP measurements. Piezoresistive sensor

technology is utilized in the Rosemount 3051T and 3051CA measurements.

The major components of the Rosemount 3051 are the sensor module and the electronics housing.The sensor module contains the oil filled sensor system (isolating diaphragms, oil fill system, and

sensor) and the sensor electronics. The sensor electronics are installed within the sensor module

and include a temperature sensor (RTD), a memory module, and the capacitance to digital signal

converter (C/D converter). The electrical signals from the sensor module are transmitted to the

output electronics in the electronics housing. The electronics housing contains the output

electronics board, the local zero and span buttons, and the terminal block. The basic block diagram

of the Rosemount 3051CD is illustrated below.

For the Rosemount 3051C design pressure is applied to the isolating diaphragms, the oil deflects

the center diaphragm, which then changes the capacitance. This capacitance signal is then changed

to a digital signal in the C/D converter. The microprocessor then takes the signals from the RTD and

C/D converter calculates the correct output of the transmitter. This signal is then sent to the D/A

converter, which converts the signal back to an analog signal and superimposes the HART signal on

the 4-20 mA output.


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4-20 mA HART wiring


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Rosemount 3051 Pressure Transmitter Exploded View


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Process Connection


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Transmitter selection guide


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3.9Yokogawa Pressure transmitters

Ordering Information


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4 SPECIAL APPLICATION.

4.1 DIAPHRAGM PRESSURE TRANSMITTERS

4.2 CAPILLARY PRESSURE TRANSMITTERS

The following photograph shows a Rosemount model 1151 electronic pressure transmitter

equipped with a remote sealing diaphragm. Here we may see the coiled metal (armor) sheath

protecting the capillary tube from damage.


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A close-up view of the sealing diaphragm shows its corrugated design, allowing the metal to easily

flex and transfer pressure to the fill fluid within the capillary tubing.

Just like the isolating diaphragms of the pressure-sensing capsule, these remote diaphragms need

only transfer process fluid pressure to the fill fluid and (ultimately) to the taut sensing element

inside the instrument. Therefore, these diaphragms perform their function best if they are designed

to easily flex. This allows the sensing diaphragm to provide the vast majority of the opposing force

to the fluid pressure, as though it were the only spring element in the fluid system.

The connection point between the capillary tube and the transmitters sensor capsule is labeled

with a warning never to disassemble, since doing so would allow air to enter the filled system (or fill

fluid to escape from the system) and thereby ruin its accuracy.


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In order for a remote seal system to work, the hydraulic connection between the sealing

diaphragm and the pressure-sensing element must be completely gas-free so there will be a solid

transfer of motion from one end to the other23. For this reason, the capillary system must remain

perfectly sealed at all times! Breaching this seal, even for just a brief moment, will ruin the system.

A protective measure visible in this photograph is the orange compound painted on the screw head

and on the capillary tube connector. This is simply a visual indication that the factory seal has never

been broken, since any motion of the screw or of the tube connector would crack the brittle orange

compound and betray the breach.

A potential problem with using remote diaphragms is the hydrostatic pressure generated by the fill

fluid if the pressure instrument is located far away (vertically) from the process connection point.

For example, a pressure gauge located far below the vessel it connects to will register a greater

pressure than what is actually inside the vessel, because the vessels pressure adds to the

hydrostatic pressure caused by the liquid in the tubing.


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This pressure may be calculated by the formula gh or h where is the mass density of the fill

liquid or is the weight density of the fill liquid. For example, a 12 foot capillary tube height filled

with a fill liquid having a weight density of 58.3 lb/ft3 will generate an elevation pressure of almost

700 lb/ft2, or 4.86 PSI. If the pressure instrument is located below the process connection point,

this 4.86 PSI offset must be incorporated into the instruments calibration range. If we desire this

pressure instrument to accurately measure a process pressure range of 0 to 50 PSI, we would have

to calibrate it for an actual range of 4.86 to 54.86 PSI.

The reverse problem exists where the pressure instrument is located higher than the process

connection: here the instrument will register a lower pressure than what is actually inside the

vessel, offset by the amount predicted by the hydrostatic pressure formulae gh or h. In all

fairness, this problem is not limited to remote seal systems even non-isolated systems where the

tubing is filled with process liquid will exhibit this offset error. However, in filled-capillary systems a

vertical offset is guaranteed to produce a pressure offset because fill fluids are always liquid, and

liquids generate pressure in direct proportion to the vertical height of the liquid column (and to the

density of that liquid).

A similar problem unique to isolated-fill pressure instruments is measurement error caused by

temperature extremes. Suppose the liquid-filled capillary tube of a remote seal pressure instrument

comes too near a hot steam pipe, furnace, or some other source of high temperature. The

expansion of the fill fluid may cause the isolation diaphragm to extend to the point where it begins

to tense and add a pressure to the fill fluid above and beyond that of the process fluid. Cold

temperatures may wreak havoc with filled capillary tubes as well, if the fill fluid congeals or even

freezes such that it no longer flows as it should. Fill fluid expansion/contraction effects may be

mitigated by keeping the volume of the fill fluid to a minimum, which is why capillary (small-

diameter) tubes are used to connect remote seals with instruments.


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Another problem with remote seal pressure instruments is a time delay caused by the viscosity of

the fill fluid as it moves through the small-diameter capillary tubes. This makes the pressure

instrument slow to respond to changes in process fluid pressure. While minimal capillary tube

diameter reduces the effects of temperature changes, it increases the effect of time delay.

Proper mounting of the instrument and proper selection of the fill fluid24 will help to avoid such

problems. All in all, the potential for trouble with remote- and chemical-seal pressure instruments

is greatly offset by their benefits in the right applications.

Some diaphragm-sealed pressure transmitters are equipped with close-coupled seals rather than

remote seals to minimize hydrostatic, temperature, and time delay effects caused by fill fluid inside

a capillary tube. A Rosemount extended-diaphragm pressure transmitter appears in the left-hand

photograph, while a Yokogawa transmitter of the same basic design is shown installed in a working

process in the right-hand photograph:


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4.3 FILLED IMPULSE LINES

An alternate method for isolating a pressure-sensing instrument from direct contact with process

fluid is to fill the impulse lines with a harmless fluid, which in turn directly contacts the process

fluid. Filling impulse tubes with a static fluid works when gravity is able to keep the fill fluid in place,

such as in this example of a pressure transmitter connected to a water pipe by a glycerin-filled

impulse line:

A reason someone might do this is for freeze protection, since glycerin freezes at a lower

temperature than water. If the impulse line were filled with process water, it might freeze solid incold weather conditions (the water in the pipe cannot freeze so long as it is forced to flow). The

greater density of glycerin keeps it placed in the impulse line, below the process water line. A fill

valve is provided near the transmitter so a technician may re-fill the impulse line with glycerin

(using a hand pump) if ever needed.

As with a remote diaphragm, a filled impulse line will generate its own pressure proportional to the

height difference between the point of process connection and the pressure-sensing element. If the

height difference is substantial, the pressure offset resulting from this difference in elevation will

require compensation by means of an intentional zero shift of the pressure instrument when it is

calibrated.

With no isolating diaphragm to separate process fluid from the fill fluid, it is critical that the fill fluid

be compatible25 with the process fluid. Not only does this imply a total lack of chemical reactivity

between the two fluids, but it also means the two fluids should not be readily miscible (capable of

mixing in any proportion).


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An important consideration in filled-line systems is how to refill the impulse line(s) without

damaging the pressure instrument. Hand-operated pumps are commonly used to refill impulse

lines, but such pumps are often capable of generating greater fluid pressures than the instrument

can safely withstand. If we were to connect a glycerin pump to the filled system pictured

previously, it would be advisable to shut the transmitters block valve to ensure we did not

accidently over-pressure the transmitter. This is especially true if the impulse line happens tobecome plugged with debris, and substantial glycerin pressure from the hand pump is required to

dislodge the plug:

In fact, the issue of impulse tube plugging is another reason to consider filled-line connections

between pressure instruments and process lines or vessels. If ever a plug develops in the line,

repumping the lines with fresh fill fluid is a practical way to clear the plug without disassembling

any part of the system. For processes where impulse line plugging is a chronic problem, another

solution exists called purging impulse lines, discussed in the next section.


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4.4 PURGED IMPULSE LINES

Another method for isolating a pressure instrument from direct contact with process fluid,

particularly when the impulse line is prone to plugging, is purging the line with a continuous flow of

clean fluid. Consider this example, where pressure is measured at the bottom of a sedimentation

vessel:

In this system, a continuous flow of clean water enters through a pressure-dropping purge valve

and flows through the impulse line, keeping it clear of sediment while still allowing the pressure

instrument to sense pressure at the bottom of the vessel. A check valve guards against reverse flow

through the purge line, in case process fluid pressure ever exceeds purge supply pressure. The

continuous water purge maintains clean impulse tubing, and ensures the pressure transmitter

never contacts process fluid directly.

A very important element of any purge system is a restriction between the purge supply and the

connection with the process and pressure-sensing device. It is important that the pressure-sensing

instrument senses the pressure of the process fluid and not the (higher) pressure of the purge fluid

supply. In the example shown, the purge valve fulfills the role of this restriction, which is why it

must be left in a partially-open condition, rather than fully-open.

If this purge restriction is not restrictive enough, the purge fluid flow rate will be too great, resulting

in a dynamic pressure drop developed across the length of the impulse line. This pressure drop will

add to the pressure of the process fluid inside the vessel, creating a positive pressure measurementerror at the instrument (i.e. the instrument will register more pressure than there actually is in the

vessel). The purge restriction should be set to allow just enough purge fluid flow to guard against

plugging, and no more.


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Purged systems are very useful, but a few requirements are necessary in order to ensure accurate

and reliable operation:

The purge fluid supply must be reliable: if the flow stops for any reason, the impulse line may

plug.

The purge fluid supply pressure must exceed the process pressure at all times, or else process

fluid will flow backward into the impulse line!

The purge fluid flow must be maintained at a low rate, to avoid pressure measurement errors.

The purge fluid should be introduced into the impulse line as close to the process connection as

possible, to minimize errors due to the purge flow rate through long lengths of tubing.

The purge fluid must not adversely react with the process.

The purge fluid must not contaminate the process.

The purge fluid must be reasonable in cost, since it will be continuously consumed over time.

A useful accessory to include in any purge system is a visual flow indicator such as a rotameter.

Such an indicator is useful for manual adjustment of purge flow rate, and also as a troubleshooting

aid, to indicate if anything happens to halt the purge flow. In the previous example, the purge fluid

was clean water. Many options exist for purge fluids other than water, though. Gases such as air,

nitrogen, or carbon dioxide are often used in purged systems, for both gas and liquid process

applications.


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4.5 Troubleshooting table for 4-20 mA output

Symptom Corrective actions

Transmitter milliamp reading is zero

Verify power is applied to signal terminals

Check power wires for reversed polarity

Verify terminal voltage is 10.5 to 42.4 Vdc

Check for open diode across test terminal

Transmitter Not Communicating with

Field Communicator

Verify the output is between 4 and 20 mA orsaturation levels

Verify terminal voltage is 10.5 to 42.4 Vdc

Verify clean DC Power to transmitter (Max AC noise

0.2 volts peak to peak)

Check loop resistance, 250 W minimum (PS voltage -

transmitter voltage/loop

current)

Have Field Communicator poll for all addresses

Transmitter milliamp reading is low or

high

Verify applied pressure

Verify 4 and 20 mA range points

Verify output is not in alarm condition

Verify if 4 20 mA output trim is required

Transmitter will not respond to

changes in applied pressure

Check test equipment

Check impulse piping or manifold for blockage

Verify the transmitter is not in multidrop mode

Verify applied pressure is between the 4 and 20 mA

set points

Verify output is not in alarm condition

Verify transmitter is not in Loop Test mode

Digital Pressure Variable reading is low

or high

Check test equipment (verify accuracy)

Check impulse piping for blockage or low fill in wetleg

Verify transmitter is calibrated properly

Verify pressure calculations for application

Digital Pressure Variable reading is

erratic

Check application for faulty equipment in pressure

line

Verify transmitter is not reacting directly to

equipment turning on/off

Verify damping is set properly for application

Milliamp reading is erratic

Verify power source to transmitter has adequate

voltage and current

Check for external electrical interference

Verify transmitter is properly grounded

Verify shield for twisted pair is only grounded at one

end


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5 PRESSURE SWITCHES

A pressure switch is an instrument that automatically

senses a change in the measured pressure and opens or closes an

electrical switching element when a predetermined pressure is

reached. In other words, a pressure switch is a digital instrument

as compared to other pressure gauges discussed so far, which areanalog instruments. Pressure switches have pressure-sensing

elements that make a small movement when the measured

pressure varies. Most common sensing elements are

Diaphragms

Bourdon tubes

Bellows

The pressure measuring elements (sensors) produce the necessary movement to actuate

the electrical switching element. These switches are normally snap acting single pole double throw

(SPDT) types. Since they are SPDT switches, they have one normally open (NO) and one normally

closed (NC) and one common (C) terminal to which electric power can be connected. The switch is

said to change over when the common pole changes from NC to NO. By doing this either the

electric power can be connected or disconnected instantly on actuation of the switch. While

connecting the power, check the name plate details of the pressure switch for the contact rating to

prevent burn out of the contacts.


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Applied pressureSet point adj:

Spring support

Spring

Snap actingmicro switch

Sensing diaphragm ( rubber )

Protecting diaphragm ( teflon )

PivotBeam ( lever )

Piston

Switch button

Electrical connections

NO NCCOM

Mounting

screw

F1F2

Threaded base

Pressure multiplied by the area of the diaphragm = F1 Spring tension (varied by turning the set point adj.) = F2

When F1 > F2,

The beam actuates the switch button and switch contacts changes over. NO contact

becomes closed and NC contact becomes open.


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The types of Pressure Switches are,

1. Standard Type Pressure Switches

2. Differential Pressure Switches

3. Diaphragm Type Pressure Switches4. Capillary Type Pressure Switches

5.1STANDARD TYPE PRESSURE SWITCH

5.2DIFFERENTIAL PRESSURE SWITCH


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5.3DIAPHRAGM TYPE PRESSURE SWITCH

5.4CAPILLARY TYPE PRESSURE SWITCH


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Pressure Switch Calibration Procedure

Equipment/ materials needed

Regulated air or nitrogen pressure source

Digital pressure gauge accurate to 0.1% of reading

Multimeter capable of reading a change of state in resistance or voltage

Pressure hoses and fittings capable of withstanding applied pressures

Isolate the pressure switch and bleed off pressure as per the site Lockout/Tagout Procedure.

Do not perform any adjustments until all "as found" calibration data has been recorded.

Note any deviation from this procedure in the remarks section of the calibration form.

Make connections between the pressure source, pressure standard, and the pressure switch

sensing line.

Connect the multimeter leads across the switch contact.

For high pressure switch:

1. Increase pressure until the switch changes state, as indicated by a change in voltage or

resistance reading.

2. Record the pressure switch trip value as indicated on the digital pressure gauge.

3. Slowly decrease the pressure until the switch changes state, as indicated by a change in

voltage or resistance reading.

4. Record the pressure switch reset value as indicated on the digital pressure gauge.

For low pressure switch:

1. Increase pressure until the switch resets, as indicated by a change in voltage or resistance

reading.

2. Record the pressure switch reset value as indicated on the digital pressure gauge

3. Slowly decrease the pressure until the switch changes state, as indicated by a change in

voltage or resistance reading.

4. Record the pressure switch trip value as indicated on the digital pressure gauge.


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If the actual pressure switch trip (and if required, reset) values are greater than 1/2 the specified

tolerance, adjust the switch until values are less than 1/2 the specified tolerance.

If adjustments are made, repeat steps 1 to 4 as applicable and record final values.

Disconnect the test equipment from the switch under test.

Complete the calibration form and affix the applicable calibration label.

If all checks are within tolerance, restore the pressure switch to operational condition and notify

appropriate personnel of work performed.


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TERMINOLOGIES RELATED TO PRESSURE SWITCHES.

SET POINT : The point or the value at which the switch is actuated is called as the actuation point or

the set point. This point (value) is expressed in terms of an appropriate pressure unit (e.g. kPa, bar,

psi etc.).

SWITCH DIFFERENTIAL : Due to practical reasons and constructional limitations the switching

mechanism will not actuate and re - actuate at the same pressure value. Normally there will be a

difference between these two values and this difference is called as the switch differential or dead

band.

ACCURACY : Accuracy is defined as the ability of a pressure switch to repetitively operate at its set

point. For example if a pressure switch is set to actuate at 100 kPa, repeatedly actuates from 99 kPa

to 101 kPa then it is considered to be accurate within 1%.

ADJUSTABLE RANGE : It is defined as the pressure range within which the actuation point of a

pressure switch can be set. For example, the adjustable range of a switch is given as 0.5 kPa (g) to

300 kPa (g) then this switch can be set to actuate at any pressure value between 0.5 to 300 kPa (g).

TOLERANCE : Tolerance is the variation that may happen at the re - actuation point for pressure

switches. For example, three switches with the same specification have a set point of 100 kPa (g).

They all will actuate at the same pressure value of 100 kPa (g). However one pressure switch may re

- actuate at 94 kPa (g), another at 95 kPa (g) and the third one at 96 kPa (g).

PROOF PRESSURE : Proof pressure is the highest pressure (including transients) to which a pressure

switch may be subjected without damage.

CONTACT RATING : Contact rating is defined as the capacity of the contacts designed to pass the

current at the given voltage without burning out the contacts.


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6 AIR PRESSURE REGULATOR

Instrument air enters through the inlet port and then to the drip well. Any dirt or moisture

carried along with air will be collected at the bottom of the drip well and can be drained through

the drain cock (valve).

A supply cum exhaust valve supported by the inlet valve spring controls the air pressure.

The inlet valve spring and the supply cum exhaust valve are housed in spring housing. The valve

sub-assembly comprising of parts are separated by a filter element. Practically clean air is available

at the supply valve port.

The upper section of the filter regulator comprises of an adjusting screw, a lock nut, spring

case, range spring, spring button and a diaphragm sub-assembly. The orifice (exhaust valve seat) at

the center of the diaphragm is in contact with the exhaust valve plug.

When the adjusting screw is turned clockwise it compresses the range spring, which applies

a definite amount of force on the diaphragm. This closes the exhaust port and pushes down the

supply valve to open the supply port admitting the filtered air to pass through the passage in the

filter body and then to the outlet port. The air pressure in the outlet port is also communicated to

the underside of the diaphragm through the aspirator hole to produce the necessary balancing

force to counteract the force generated by the range spring.


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When the downward and the upward forces on the diaphragm are equal, the exhaust valve

is closed and the supply valve is open to supply the set pressure through the outlet port to the

downstream equipment. The outlet pressure is also tapped to a pressure gauge, which is mounted

on the regulator to indicate the set pressure.

Incase of a decrease in air pressurein the outlet port, the force acting on the under side of

the diaphragm will reduce and the spring force will push the supply valve to open the supply port to

admit more air to meet the new requirement and the increased pressure will restore the

equilibrium condition of the diaphragm assembly.

In case of an increase in the output pressure, the force acting on the under side of the

diaphragm will overcome the force generated by the spring. This unbalance in forces will move the

diaphragm upwards to make the exhaust valve seat to lift off from the plug to allow the excess air

to bleed to the atmosphere through the bleed hole (B) in the spring housing. This process will

continue automatically till the forces acting on the diaphragm are in equilibrium. The lock nut on

the adjusting screw prevents it from turning due to vibration and not to cause any changes in the

set point.


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7 PRESSURE INSTRUMENT PROTECTION DEVICES

PRESSURE SNUBBER

This is used as an attachment at the bottom of the pressure gauge to protect the gauge sensor from

damage, excessive wear, pulsation, oscillating deflections etc. Wherever sudden or repetitive

changes of pressures are anticipated, pressure snubber or otherwise called pulsation dampener is

used. Reciprocating pumps and Compressors are typical examples in our PDO applications where

snubber is used.

Two types are available, one with a fixed throttle and the other with adjustable needle valve

throttling. Basically both these types reduce the pulsation thereby eliminating direct impact of the

process medium on the measuring element.

GAUGE PROTECTION AGAINST HOT AND CORROSIVE FLUIDS

Basically there are four types of protection of gauges against hot, slurry and corrosive fluids.

They are

1. Condensing Chamber Type

2. U- Tube Siphon Type

3. Pig-Tail Type

4. Diaphragm Seal Type

The Condensing Chamber type is made out of a 2 pipe welded onto nipples tapped to take the

isolation valve of gauges at the top and a blow-down valve below.

The U-tube Siphon is generally made of a straight or 3/8 pipe or tube itself to help

condensation of hot fluids.


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The Pigtail type is also generally made of a straight or 3/8 pipe or tube. Both these U-tube

Siphon and Pigtail types trap a certain quantity of the condensed fluid always, say water in steam

applications.

The diaphragm seal type is used in such places where impulse line clogging can occur by hot, slurry

and corrosive fluids. The interconnecting piping and the space above the diaphragm is normally

filled with the silicon oil.

A= Condensing Chamber

B= U-Tube syphon

C= Pig Tail

D= Diaphragm Seal


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8 CALIBRATION DEVICES

There are many calibration devices available within PDO or any other industry depending on

the range of the pressure measuring device and the location to be used. Calibration devices are

used mainly to simulate the pressure required.

CALIBRATION ADJUSTMENTS

Generally instruments are provided with a system of linkages, screws, springs etc. in order to

do three basic adjustments namely Zero, Span and Linearity. Hence calibration is required to check

and adjust, if necessary, all these adjustments to maintain the reliability of the reading.

Zero adjustment shifts the entire scale up or down by the same amount.

Span adjustment progressively increases or decreases readings over the range, without

changing the Zero.

Linearity adjustment speeds up or slows down the calibration at either end of the scale to eliminate

intermediate errors.

COMMON CALIBRATION DEVICES

These are the following commonly used devices for calibration of pressure instruments.

1. DEAD WEIGHT TESTER

2. HYDRAULIC OIL PUMP

3. PNEUMATIC HAND PUMP

4. PNEUMATIC VACUUM PUMP

5. PNEUMATIC CALIBRATORS


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8.1DEAD WEIGHT TESTER

Pressure gauges are calibrated on patented hydraulic screw pumps, which transmit the oil pressure

upon an accurately manufactured piston cylinder arrangement. The oil pressure is increased or

decreased by the screw pump and the piston top is loaded with known weights, which can be

conveniently added or subtracted. The weights themselves are manufactured in accordance to the

exact cross sectional area of the piston. The oil pressure is also fed to a suitable pressure gauge

mounting on the tester itself.

In dead weight tester, the weights we place on the weight stand are not actually weighs as

indicated, instated they are calculated based on the piston diameter, hence it is called dead weigh

tester.

A deadweight tester (DWT) is a calibration standard which uses a piston cylinder on which a load isplaced to make an equilibrium with an applied pressure underneath the piston.

The formula to design a DWT is based basically is expressed as follows :

p = F / A [Pa]

where :

p : reference pressure [Pa]

F : force applied on piston [N]

A : effective area PCU [m2]


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It consists of a very accurately machined, bored and finished piston which is inserted into a close-

fitting cylinder. The cross- sectional areas of both the piston and the cylinder are known. At the top

of the piston is provided a platform on which the standard weight, of known accuracy, can be

placed. An oil reservoir with a check valve at its bottom is also provided. The oil from the reservoir

can be sucked by a displacement pump on its upward stroke.

For calibration purposes, first a known (calculated) weight is placed on the platform and the fluid

pressure is applied on the other end of the piston until enough force is developed to lift the piston-

weight combination and the piston floats freely within the cylinder between limit stops. The error

in a dead-weight tester is less than 0.1%.

In order to reduce the friction between the piston and the cylinder, the piston is generally rotated

while a reading is being taken.


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Operation of the tester is controlled by the two valves A and B on the top of the reservoir/high

pressure cylinder block. When initially priming the system valves A and B are opened to fill the

system with oil from the reservoir. Valve B is then closed with valve A left open and the screw

pump operated to provide the lower test pressures. To provide the higher pressures valve A is

closed to seal off the test circuit from the low pressure part of the screw pump and valve B is

opened to allow the liquid in the low pressure part of the screw pump to return to the reservoir as

the pump is operated. This ensures that the pump can be operated without having to put large

forces on the screw pump handwheel. To release the test pressure the screw pump is wound out

and valve A is opened.

Filling the base unit with liquid

Remove filler plug from reservoir by slackening screw and prising plug out. (This plug should

be left out whilst in use).

Open valves A and B.

Wind screw pump handle fully clockwise.

Fill reservoir with appropriate liquid. Use the oil supplied or an approved substitute for oil

systems. Do not use other liquids. Castor based oils, Skydrol, solvents or similar liquids will

attack the seals fitted in the standard tester.

Wind screw pump handle fully anti-clockwise.

Top up reservoir if necessary.


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PROCEDURE

Fit instrument to be tested to gauge stand.

Load the weight carrier/overhang with the weights equivalent to the desired pressure. Each weight

is marked with two pressures, one for the low pressure range and one for the high pressure range.

The equivalent weight of the carrier/overhang must also be taken into account. The piston cylinder

unit has a basic 10 lb/in start.

Close valve B (valve A remaining open).

Wind screw pump handle clockwise until the handle becomes stiff to operate.

Close Valve A and open valve B

Continue to wind screw pump handle clockwise. This will generate pressure up to approximately

2600 bar or 40,000 lb/in2.

When the piston rises and the piston head skirt floats, this indicates it is at its nominal desiredpressure.

When the piston is floating within the blue band this indicates the pressure generated on the low

pressure range (i.e. 1/8in piston).

When the piston is floating within the red band this indicates the pressure generated on the high

pressure range (i.e. 1/160in piston).

The bands are easily visible from a seated position. To engage the high pressure range apply further

pressure with the screw pump until the piston lifts higher and the piston head skirt floats within the

red band. A slight leakage through the vent hole of the piston/cylinder unit is normal.

DURING CALIBRATION

When the tester is correctly set up and there are no leaks the piston should float for many

minutes without it being necessary to touch the screw pump handwheel. On the initial setting up,

however, there may be some air trapped in the base of the piston/cylinder unit. As this leaks past

the piston the weights may fall slightly but it will only be for a matter of a few minutes until the air

has escaped. If the piston continues to fall, check the connections for leaks.

During calibration, the weights should be rotated by hand. It is desirable that the weights should

only be rotated when approximately the correct pressure is obtained. Changing from the low

pressure range to the high pressure range with the weights spinning does no harm. Weights should

not be brought to rest by fully releasing the pressure and allowing the piston head to rotate against

its stop under the full load of the weight pile. Stops come into action if the pressure is too high or

too low and it is essential that the weights should be spinning freely whilst taking readings. At the

lowest pressures the weights will not spin for more than a few seconds unless a very thin oil is used,

but providing the weight is rotated by hand before taking a reading and is obviously floating an

accurate reading will be given.


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COMPLETION

After the test is finished wind screw pump handle anti-clockwise to lower pressure.

Gently open valve A or B to release residual pressure

Ensure that both valves A and B are fully open.

The system is now ready for another test and any residual pressure is relieved.

SAFETY PRECAUTIONS

The following safety precautions are recommended while working with dead weight testers.

1. Wear Personnel Protective Equipment (PPE) namely safety shoes, coveralls and gloves.

2. Pressure to be released slowly and carefully.

3. Weights to be handled properly.

4. Use Teflon tapes with pressure gauges for sealing against leaks.


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Troubleshooting.

Fault Possible cause Remedy

Equipment does notprovide any output

pressure.

No liquid in tester. Check that tester is filled with liquid. Fill theequipment with fluid as necessary.

Valve B is open. Close valve B and try again.Component being tested has a largevolume. Pre-fill component with liquid before test.Missing or damaged liquid seals shown by

signs of unexplained liquid leaks.Examine seals on equipment to ensure they are

fitted correctly and are undamaged. Replace asnecessary.

Valve B handwheel disconnected fromspindle. Examine valve B. Tighten up nut securinghandwheel to spindle as necessary.Valve B assembly or valve seat damaged. Examine condition of valve B and valve seat.

Replace valve assembly or return tester toManufacturer for overhaul as necessary.

If unable to locate a cause. Return tester to Manufacturer for investigation.

Equipment providespressure but pressuredecays to zero

Incorrect operating procedure being used. Ensure that correct operating procedure is beingfollowed

Missing or damaged liquid seals shown bysigns of unexplained liquid leaks. Examine seals on equipment to ensure they arefitted correctly and are undamaged. Replace as

necessary.Valve A or Valve B valve assembly or valveseat damaged. Examine condition of valves A and B and valveseat. Replace valve assembly or return tester to

Manufacturer for overhaul as necessary.If unable to locate a cause. Return tester to Manufacturer for investigation.Equipment providespressure but pressuredecays when valves Aand B are operated.

Incorrect operating procedure being used. Ensure that correct operating procedure is beingfollowed

If unable to locate a cause. Return tester to Manufacturer for investigation.

Equipment providespressure but pressuredecays to lower valuethenremains steady.

Insufficient liquid in tester. Check liquid level in reservoir. Fill reservoir withcorrect liquid as necessary.

Air in the system Prefill component under test with appropriateliquid. If necessary re-fill tester with appropriateliquid.

If unable to locate a cause. Return tester to Manufacturer for investigation.Internal damage Return tester to Manufacture for investigation.Incorrect operating procedure being used. Ensure that correct operating procedure is being

followedIf unable to locate a cause. Return tester to Manufacture for investigation.

Tester screw pressbecomes very stiff tooperate when tester isbeing used in rangebelow 140 bar(2 000 lb/in

2)

Internal damage Return tester to Manufacture for investigation.

Tester screw pressbecomes very stiff tooperate when tester isbeing used in rangeabove 140 bar(2 000 lb/in

2)

Incorrect operating procedure being used. Ensure that correct operating procedure is beingfollowed.

If unable to locate a cause. Return tester to Manufacture for investigation.


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8.2HYDRAULIC OIL PUMP

The hydraulic Quick Test Hydraulic Pump is a portable

source of hydraulic pressure (up to 14000 kPa) for

field calibration. Internal parts are brass, aluminium

and stainless steel, compatible with a variety of

hydraulic fluids including petroleum-based oils and

water.

A transparent fluid reservoir permits a quick visual

check of fluid level. The bleed-off valve allows a slow

bleed-off of pressure and fluid back to the reservoir.

Included with the pump is a rugged Test Gauge. Gauges are mounted in the same swivel fitting and

are easily removable. Gauges available include 0-1000, 1500, 2000 psi (0.5% full-scale accuracy).

Also available: 0-70 and 0-140 bar. The pressure probe is connected to the hose with a standard

flare connection facilitating replacement with a variety of fittings to meet the requirements.

QUICK TEST HYDRAULIC PUMP

SAFETY PRECAUTIONS

The following safety precautions are recommended while working with Hydraulic Oil Pumps.



3. Select the correct range of test gauge.

4. Use teflon tapes with pressure gauges for sealing against leaks.


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8.3PNEUMATIC AIR PUMP OR QUICK TEST AIR PUMP

The Quick Test Air Pump is a portable, hand operated source of air pressure (up to 200 psi)

for the use in field calibration of pressure instruments.

Included with the pump is a rugged Test Gauge. Gauges are mounted in a quick-change swivelfitting which is easily removable from the pump assembly. This allows you to exchange gauges and

match the gauge range with calibration range. Gauges in the following ranges are available: 0-5, 15,

30, 60, 100, 160 and 200 psi (0.5% full-scale accuracy). Also available: 0-1, 2, 4, 7 and 14 bar.

The pressure probe is connected to the hose with a

standard flare connection facilitating replacement with a

variety of fittings to meet the requirements. A bleed-off

valve permits a slow bleed-off of pressure to

atmosphere for downside calibration.

SAFETY PRECAUTIONS

The following safety precautions are recommended while working with Pneumatic Air Pumps.






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8.4 PNEUMATIC VACUUM PUMP OR QUICK TEST VACUUM PUMP

The Quick Test Vacuum Pump is a portable lightweight pump, which will generate approximately

23 Hg vacuum. It includes 0.5% full scale accuracy, 0-30 Hg test gauge which makes it a

convenient method to field calibrate vacuum instruments.

It is complete with hose, bleed-off and pressure probe. Probe is identical to that discussed with Air

and Hydraulic pumps and fits all Quick Test Fittings.

QUICK TEST VACUUM PUMP

SAFETY PRECAUTIONS

The following safety precautions are recommended while working with Pneumatic Vacuum Pumps.






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8.5 PNEUMATIC CALIBRATOR

This calibrator is a highly accurate, portable instrument, designed primarily for the field checking of

pneumatic instruments using non-corrosive gases, mainly compressed air. It is available in several

ranges, from as low as 0 to 300 mbar, up to 0 to 7 bars. Basically the Calibrator is a shock-mounted,

precision, dial manometer in a robust, suitcase type carrying case.

The main component of this Calibrator is the dial manometer. Its pressure-measuring element is a

precision C type Capsule, specially formed and heat-treated to minimize any change from aging.

When connected to a source of air pressure, the Calibrator can apply and measure accurately two

different pressures using two different regulators through ports P1 and P2. Port P3 is used to

measure any pressure within the range, for example, output of a pressure transmitter. The

Calibrator can also be used to measure the difference between two pneumatic signals where

neither signal exceeds the Calibrator range.


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SAFETY PRECAUTIONS

The following safety precautions are recommended while working with Pneumatic Calibrator.



3. Use clean, dry air of maximum 7 bars.



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Rosemount 3051 HART menus


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IMPULSE PIPING

The piping between the process and the transmitter must accurately transfer the pressure to obtain

accurate measurements. There are five possible sources of error: pressure transfer, leaks, friction

loss (particularly if purging is used), trapped gas in a liquid line, liquid in a gas line, and density

variations. The best location for the transmitter in relation to the process pipe is dependent on the

process.

Before connecting the transmitter to the process, study the transmitter installation location, theprocess piping layout, and the characteristics of the process fluid (corrosiveness, toxicity,

flammability, etc.), in order to make appropriate changes and additions to the connection

configurations.

If the impulse line is long, bracing or supports should be provided to prevent vibration.

The impulse piping material used must be compatible with the process pressure, temperature, and

other conditions.

A variety of process pressure tap valves (main valves) are available according to the type of

connection (flanged, screwed, welded), construction (globe, gate, or ball valve), temperature andpressure. Select the type of valve most appropriate for the application.

Use the following guidelines to determine transmitter location and placement of impulse

piping:

Keep impulse piping as short as possible.

For liquid service, slope the impulse piping at least 1 in./foot (8 cm/m) upward from the

transmitter toward the process connection.

For gas service, slope the impulse piping at least 1 in./foot (8 cm/m) downward from the

transmitter toward the process connection.

Avoid high points in liquid lines and low points in gas lines.

Make sure both impulse legs are the same temperature.

Use impulse piping large enough to avoid friction effects and blockage.

Vent all gas from liquid piping legs.

When using a sealing fluid, fill both piping legs to the same level.

When purging, make the purge connection close to the process taps and purge through

equal lengths of the same size pipe. Avoid purging through the transmitter.

Keep corrosive or hot (above 250 F [121 C]) process material out of direct contact with the

sensor module and flanges.

Prevent sediment deposits in the impulse piping.

Maintain equal leg of head pressure on both legs of the impulse piping.

Avoid conditions that might allow process fluid to freeze within the process flange.


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MOUNTING REQUIREMENTS

Impulse piping configurations depend on specific measurement conditions.

Liquid flow measurement

Place taps to the side of the line to prevent sediment deposits on the transmitters

process isolators.

Mount the transmitter beside or below the taps so gases can vent into the process line.

Mount drain/vent valve upward to allow gases to vent.

Gas flow measurement

Place taps in the top or side of the l

Pressure Measurement in Petrochemical Industries

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