Module I-3 Pressure Instruments

8/13/2019 Module I-3 Pressure Instruments

1/41

TASK DETAILING MANUAL

Module I-3 www.arfanali.webs.com Page 1

-

Practical Demonstration of

PRESSURE INSTRUMENTS

MODULE NO. : I-3

MODULE SUBJ.: Pressure Instruments
http://www.arfanali.webs.com/http://www.arfanali.webs.com/http://www.arfanali.webs.com/


2/41



Tasks:

I-3.1 Demonstrate servicing and calibration of different types of pressure

gauges.

I-3.2 Service and calibrate a high/ low pressure switch.

I-3.3 Service and calibrate a differential pressure switch

I-3.4 Describe the meaning of the terms, zero, span, linearity when

calibrating a pressure transmitter.

I-3.5 Demonstrate servicing and calibrating a pneumatic pressure transmitter.

I-3.6 Demonstrate calibration of a smart transmitter using a hand held

communicator.

I-3.7 Demonstrate servicing and calibrating an electronic transmitter

I-3.8 Demonstrate isolation procedure from process and electrical supply on a

pneumatic/electronic pressure transmitter.

I-3.9 Check function / adjustment / maintenance of pressure regulators for

process lines.

I-3.10 Service and calibrate a pneumatic indicating controller.

MODULE No.: I-3 Pressure Instruments

TASK No.: I-3.1

Demonstrate servicing and calibration of different

types of pressure gauges.


3/41



Reference: OJT Instructor to arrange reference catalogue forpressure gauges relevant to each working area.

Materials: 1. Different types of pressure gauges as models.2. Cleaning rags

Equipment & Tools: 1. Tool Box2. Pressure gauges tool kit

Conditions: None.

Requirements By Trainee:

To study the task and familiarize himself. Be able to identify pressure gauges types. Be able to calibrate different types of pressure gauges. Be able to disassemble and reassemble any pressure gauge. Perform adjustments and service the pressure gauges. Describe an understanding to his trainer. Write observations and procedures in his work book.

TASK No.: I-3.1 Continue

Details:

Pressure Gauges

A pressure gauge is a device, which senses pressure and provides

a visual representation of that pressure. Most pressure gauges have bourdon tube

sensors. Vacuum gauges and low-range gauges often use bellows sensors.

Differential-pressure gauges can use piston or bellows sensors. The sensor type is

usually dictated by the preferred manufacturer and the required range.


4/41



Pressure gauges should be selected so that the expected operating pressure is in

the center third of the gauge range. It is also important that the highest pressure

that will ever be applied to the gauge be below the maximum reading. Usually, the

gauge can be selected so that the gauge maximum is above the set pressure of the

system relief valve and the normal pressure is in the readable range.

Pressure gauges are sometimes liquid filled. This is to protect the gauge dial and

movement from the atmosphere. The liquid fill also provides some pulsation or

vibration dampening. Glycerin is the most common fill liquid.

Pressure gauges lose accuracy when exposed to hot fluids. When the process

temperature is above approximately 180F (82C) a siphon should be installed. If

the process fluid will not condense, at ambient temperature, the siphon can be

filled with a suitable fluid such as ethylene glycol or glycerin.

Differential-pressure gauges are useful when a pressure difference that is small

compared to the static pressure needs to be measured. Differential-pressure gauges

differ from static-pressure gauges in that they have two pressure connections.

Differential gauges must be installed with an equalizing valve so that they will not

be overranged while disconnecting.



5/41



Calibration Check

To check the calibration of a gauge it must be removed from the line or equipment

and installed on a dead-weight tester, or test gauge manifold. Check calibration as

illustrated at 1/3, 1/2, and 2/3 of the dial range. If the readings are outside of the

tolerance limits, the instrument requires recalibration. Remove the front ring, glassfront (lens) and gasket. Then recalibrate as follows:


Recalibration

There are three specific adjustments, these are:A.Zero adjustment,B.Range adjustment, andC.Linearity adjustment.(A) Zero Adjustment

If the gauge shows an equal amount of incorrect reading, either too high or too

low over the entire range, adjustment is made by resetting the indicating pointer.

These are furnished in three types, i.e. micrometer, slotted and plain.

The micrometer type is adjusted by holding the pointer firmly with one hand and

turning the micrometer screw with the other. Follow the same procedure for the

slotted type, except rotate the slotted hub with a screwdriver. For gauges equipped

with plain pointer make minor adjustments by rotation the slotted scale. Where

this adjustment is not sufficient, remove the pointer (using a hand jack ), and reset

in desired position.

(B) Range Adjustment


6/41



If the gauge readings show a progressively increasing (plus or minus) error, from

the lower to the upper end of the range, loosen the two slide screws (1) and

proceed as follows: For plus error, the slide (2) must be moved outward; for minus

error, the slide must be moved inward. After attaining proper adjustment, retighten

the slide screws (1).

(C) Linearity Adjustment

Assuming the gauge reads accurately at high and low points (1/3 and 2/3 of gauge

range) but inaccurate at the center of the scale linearity adjustment is required.

A linearity adjustment, being the most difficult adjustment of the three due to the

geometric function of all components, may in some instances be accomplished in

the field by loosening the two movement mounting screws (3) and rotating the


movement. Without a technical gauge background this would be a trial and error

adjustment.

After each adjustment (which should require no more than a few degrees), the

gauge should be re-tested through the complete range, after setting pointer at

approximately 5% of scale range. Make a zero and/or range adjustment if

required.

Pointer Adjustment, Removal and Reassemble.

In case where pointer adjustment is necessary, the following procedure should be

followed:

For liquid filled gauges drain the liquid from the case.

Loosen the ring using a strap wrench or equivalent.

A slotted screwdriver blade is required so that the screwdrivers blade will

properly engage the slot on the pointer hub and clear the tip of the pinion shaft

protruding from the hub. While holding the tail of the pointer, a clockwise rotation

of the pointers hub will result in a counterclockwise pointer motion and viceversa.

Care must be taken so as not to apply an excessive axial force to the pointer.

If liquid filling is required, hold the gauge in a horizontal position and slowly fill

the gauge with filling fluid.

Should it be necessary to remove the pointer, the standard hand jack set should be

used. When reassembling the pointer on lower connected gauges care must be


7/41



taken to support the pinion shaft. This is accomplished by removing the fill plug in

the back of the case and placing a support against the pinion shaft from the back

side. On back connected gauges, position the driver against the pointer hub and

push until the pinion stops against the socket. Contact will be felt. Hold the driver

in this position and strike it lightly.

In both cases, care must be taken to ensure the pinion shaft is not bent by heavy

blows on the pointer driver. Gauge calibration must be checked after reinstalling

the pointer.

Tighten the ring to the case using a strap wrench.


TASK No.:I-3.2

Service and calibrate a high/ low pressure switch

Reference: OJT Instructor to arrange reference catalogue / servicemanual for pressure switches relevant to each working

area.

Materials: Cleaning rags.

Equipment & Tools: 1. Tool Box2. Digital Maltimeter3. Pneumatic / Hydraulic pressure calibrator.

Conditions: Work permit


To study the task and familiarise himself. To select the proper tools / equipment to perform this task. Draw / Sketch the calibration set-up in his work book.


8/41



Recognises the difference between high & low switch in calibration procedure. Be able to perform routine service of a pressure switch. Discuss an understanding to his trainer.

Write observations and procedure in his work book.


Details:

Pressure Switches

A) Electric Pressure Switches

An electric pressure switch senses pressure and opens

or closes a switch element at a set pressure to signal another electrical device.Electric pressure switches are available in a wide variety of styles.

Most pressure switches trip at a pressure above atmospheric, and are called gauge

pressure or simply pressure switches. Switches can also be manufactured to trip at

a pressure referenced to a complete vacuum and are called absolute pressure

switches. Those set to trip below atmospheric pressure are called vacuum

switches and those which can be set either above or below atmospheric pressure

are called compound switches. Some switches are manufactured so that the trip

point is factory set, while others are field adjustable.

Pressure switches are set to trip at a certain point with rising or falling pressure.

When the pressure is returned to within the acceptable range, the switch does not

reset at exactly the same point that it tripped. The difference in the trip

point and the set point is called dead band or reset. The electricalswitch is usually either single-pole, double-throw or double-pole, double-throw.

Figure 23 shows these types, as well as others less frequently used. The number of

poles determines the number of separate circuits that can be controlled by the

switch, single pole for one circuit and

double-pole for two circuits. The double-throw term means that a common

terminal is connected to either of two other terminals, normally open or normally

closed.

B) Pneumatic Pressure Switches/Pressure Pilots

A pneumatic pressure switch senses pressure and

opens or closes a small valve at a set pressure to supply or vent a pneumatic signal


9/41



to another pneumatic device. Pneumatic pressure switches are commonly known

as pressure pilots. They are frequently used when pneumatic shutdown and control


systems are selected. Often, pressure pilots are used in Division 1 areas, such as

on wellheads even when the primary process control is electronic.

Devices, which are similar to electric pressure switches, are called pneumatic

pressure switches. Pneumatic pressure switches are equipped with a two-way or

three-way valve instead of an electrical switch. The two-way valve is either open

or closed. A three-way valve connects a common port with one of two other ports,

depending on whether the switch is tripped or not.

Devices, which have been designed to be pneumatic, are usually called pressurepilots. The most common types of pilots are the piston-actuated, known as stick

pilots, and the bourdon tube actuated pilots. Stick pilots are more often used on

wellheads and bourdon tube pilots are more often used on process equipment.

Dead band or reset is equally important for pneumatic pressure switches /pressure

pilots as for electric pressure switches. Pneumatic devices tend to have an even

larger dead band than electric devices because more movement is required for

actuation.

Most pressure pilots are equipped with three-way pneumatic valves so that theycan be used either as a high-pressure pilot or a low-pressure pilot depending on

how they are connected.

Pressure Switch Calibration;

Switch operating set point shall be verified at intervals to make sure the

healthiness of the switch and the associated circuit.

For any switch with a tag number prefix ending LL or HH, a maintenance by passis provided on the shut down panel. Care must be taken to operate the correct

switch by an authorised person.

Switch must be isolated, depressurised and drained in order to hook up the test

equipment.


10/41





To calibrate a pressure switch, follows:


11/41



Normally Closed:

1. Clean the switch.2. Connect the switch to Wallace calibrator.3. Adjust the Multimeter to measure Ohm.4. Connect the red and brown leads of the switch to the meter.5. Increase the pressure slowly till you reach the fixed set point, (the meter reads

open circuit).

6. Decrease the pressure slowly and observe the Multimeter indication, record thepressure value when meter read closed circuit.

7. Calculate the dead band value.Normally Open:

1. Clean the switch.2. Connect the switch to Wallace calibrator.3. Select resistance measurement OHM for the multimeter.4. Connect the multimeter to the switch to red and brown leads.5. The meter reads open circuit.6. Increase the pressure more than the set point, till the meter reads zero OHM.7. Decrease the pressure slowly till the meter reads open circuit.8. Check the set point with the name plate set point.


TASK No.: I-3.3

Service and calibrate a differential pressure switch

Reference: OJT Instructor to arrange reference catalogue / servicemanual for diff. Pressure switches relevant to each


12/41



working area.

Materials: Cleaning rags.

Equipment & Tools: 1. Tool Box,2. Digital Multimeter, and3. Pneumatic / Hydraulic pressure calibrator.

Conditions: Work permit


To study the task and familiarise himself. To select the proper tools / equipment to perform this task. Draw / Sketch the calibration set-up in his work book. Recognises the difference between high & low switch in calibration procedure.

Be able to perform routine service of a differential pressure switch. Discuss an understanding to his trainer. Write observations and procedure in his work book.


Details:

Deferential Pressure Switches

Description;

These switches act on the difference between pressures acting on either diaphragm

or bellows. There are two kinds of differential pressure switches:

Single switches; these switches control only one differential pressure to one limit.

Double Switches; these switches control the differential pressure to two limits.


13/41




Differential Pressure Switch


14/41



Calibration Procedure

1. Open the low pressure port to the atmosphere.2. Apply the setting pressure to the high pressure port.3. Adjust the multimeter to (OHM) position.4. Connect the multimeter leads to the terminal strip. (Normally open).5. Use a suitable open-end wrench to turn HEX adjusting nut.6. Adjust the setting point until the multimeter measure the continuity of the

micro switch.

7. Repeat step 6 till you get a precise set point.


TASK No.: I-3.4

Describe the meaning of the terms, zero, span,

linearity when calibrating a pressure transmitter.

Calibration Set-up of Diff. Press. Switch


15/41



Reference:

Materials: None.

Equipment & Tools: None.

Conditions: None.


To study the task and familiarise himself, Be able to define Zero, Span and Linearity of a pressure transmitter, Discuss an understanding with his trainer, and Write the definition of each in his work book.


Details:

Zero Pressure

Is the bottom scale value of measuring range, for example if measuring range of a

press transmitter is from (-2 to 40 psi), zero press. is equal to (-2 psi).

Span


16/41



The difference between the Upper Range Value (URV) and the Lower Range

Value (LRV) of an instrument, on instrument starting at zero, the span equal to the

range, for above example the span = 42 psi.

Linearity

Linearity is the degree to which the calibration curves of matches a straight line,

the linearity error is generally the greatest departure from the best straight line that

can be drawn through the measured calibration points.


TASK No.: I-3.5

Demonstrate servicing and calibrating a pneumatic

pressure transmitter

Reference: OJT Instructor to arrange reference catalogue / Service

manual for pneumatic pressure transmitter modelrelevant to each working area.

Materials: Cleaning rags with solvent.

Equipment & Tools: 1. Tool Box,2. Hydraulic or Pneumatic pressure calibrator, and


17/41



3. Standard output gauge.

Conditions: Work permit.


To study the task and familiarise himself, Be able to describe the operation principle of the pneumatic press. transmitter, Be able to identify the main parts of a pneumatic transmitter, Perform periodic adjustments / calibration of a pneumatic transmitter, Perform periodic and corrective maintenance or replace parts of a pneumatic

transmitters,

Draw / Sketch calibration set-up of a pneumatic press. transmitter, Discuss an understanding with his trainer, and Write observation and procedures in his work book.


Details:

Pressure Transmitters

Pressure transmitters are used when the controller, recorder,

or indicator needs to be located in a control room or panel where it is undesirable

to pipe the process fluid. They are also used when several devices are to be

operated from a single measurement or when elevated zero is required. The output

is usually 4-20, milliamps for electronic transmitters or 3-15 psig (20-100 kPa) for

pneumatic transmitters. Other signals can be used if required by the receiver, but

these are the most common and should be used if possible.

Elevated zero occurs when the base value of the measured variable is not at

atmospheric pressure. Most transmitters have this as an option, while most

controllers do not. Elevated zero is used when the pressure range of

interest is narrow and at a fairly high level.


18/41



Suppressed zero, where the base value of the measured variable is below

atmospheric pressure, is sometimes available. Usually, the zero is as near to

perfect vacuum as possible and the unit is called a absolute pressure transmitter.

The use of pneumatic transmitters is decreasing; however, a number of

manufacturers still make them for the replacement market and some newinstallations are still being made. Pneumatic transmission may be advantageous

when existing equipment is pneumatic with which operating personnel are already

familiar.

The next figure shows the main parts of a pneumatic pressure transmitter, these

are:

1. Bellows Capsule as a sensing element,2. Force Bar,3. Leaf Spring,4. Range Adjusting Bar,5. Booster Relay, and6. Feedback Bellows.TASK No.: I-3.5 Continue


Clibration

Calibration is required if the transmitter has been taken apart for cleaning or for

parts replacement, if a change of range is desired, or if the amount of zero

elevation or suppression is changed substantially.

If the capsule was removed or the flexure cap screw loosened, before calibrating,

make the flexure cap screw adjustment above.

The transmitter to be calibrated to 3 to 15 psi signal pressure range, at which the

receiver signal is used.

Calibration procedures; Detailed bench calibration and in-line calibration

procedures of the pneumatic pressure transmitter are listed in the reference

catalogue or service manual of the applied model. Consult your trainer.


19/41



Maintenance and Servicing

Maintenance and servicing of pneumatic pressure transmitter are limited to clean

or replace its parts, such as:

1. Supply Air Filter blow out at least once a day,2. Replace Screen Filter of the process inlet,3. Clean Nozzle Assembly,4. Clean booster relay Restrictor,5. Replace Booster Relay,6. Change Range Bar,7. Remove / Replace Bellows Capsule, and8. Adjusting Flexure Cap Screw.Disassembly; normal servicing of the transmitter does not require the removal of

any parts other than those already mentioned. Further disassembly is notrecommended because of possible loss of accuracy or damage to the transmitter,

detailed servicing procedures are mentioned in maintenance section of the selected

transmitter model, Consult your trainer.


TASK No.: I-3.6

Demonstrate calibration of a smart transmitter using a

hand held communicator.

Reference: OJT Instructor to arrange reference catalogue / servicemanual for smart transmitter model relevant to each

working area..

Materials: None.

Equipment & Tools: 1. Function generator,2. Digital Multimeter, and3. Pneumatic / Hydraulic pressure calibrator.4. HHC for Smart Transmitter model.

Conditions: None.


20/41




To study the task and familiarise himself, Be able to describe using of HHC keypad,

Be able to define the difference between the electronic press. transmitter andsmart type,

Perform adjustments and calibration of Smart transmitters, Be able to maintain and troubleshoot Smart pressure transmitters, Draw / Sketch calibration set-up, Discuss an understanding to his trainer, and Write observations and procedures in his work book.


Details:Smart Transmitters

Microprocessor-based pressure transmitters, sometimes called

smart transmitters are offered by several companies. These transmitters can be

remotely programmed for any desired range, either by the main process computer

or by a special programmer. These transmitters are usually used in sophisticated

computer control schemes.Smart transmitters uses an integrated circuit sensor to measure the process

variable . This integrated circuit contains microprocessor-based electronics and

digital communication circuitry to add functions over the same two wires used to

transmit power and the measurement signal. The next figure shows electronics

assembly of Foxboro 863DP smart transmitter.


21/41




Smart Press. Transmitter Calibration Set-up;

Calibration equipment should be a minimum of three times more accurate than the

desired accuracy of the transmitter.

Smart Press. Transmitter, Electronic Parts.

4 to 20 mA Calibration Setu


22/41



Calibration is performed by simulating the process pressure. This done by

applying a pressure equal to the transmitter range pressure.

Field Calibrationis performed without disconnecting the process piping.

To perform field calibration, refer to detail procedure in the reference doc.,

Consult your trainer.

Bench Calibrationset-up requires disconnecting the process piping

To calibrate the transmitter , use the detailed procedure of the reference doc.,

consult your trainer.


Fault Analysis;

Fault analysis of Foxboro Smart Press. Transmitter (863 DP) is accomplished


23/41



through the following flow shart.


Fault Analysis Flowchart Continue



24/41



Maintenance;

These transmitters when used in hazardous area, must be deenergised before

removing the covers or performing any maintenance. Maintenance is limited to the

replace electronics modules assembly, sensor and housing assembly, cover O-rings, and optional indicator.

Detailed procedure to replace these parts, refer to the reference catalogue or

maintenance manual of the smart transmitter model, consult your trainer.


TASK No.: I-3.7

Demonstrate servicing and calibrating an electronic

transmitter


25/41



Reference: OJT Instructor to arrange reference catalogue / Servicemanual for electronic pressure transmitter model

relevant to each working area.

Materials: Cleaning rags with solvent.

Equipment & Tools: 1. Tool Box,2. Hydraulic or Pneumatic pressure calibrator, and3. Function Generator and digital Multimeter.



To study the task and familiarise himself, Be able to describe the operation principle of the electronic presser transmitter, Be able to identify the main parts of an electronic transmitter, Perform periodic adjustments / calibration of an electronic transmitter, Perform periodic and corrective maintenance or replace parts of an electronic

transmitters,

Draw / Sketch calibration set-up of the electronic press. transmitter, Discuss an understanding with his trainer, and Write observation and procedures in his work book.


Details:

Electronic Pressure Transmitters

Electronic transmitters with 4-20 milliamp outputs are the most common.

Pressure transmitters are available in a variety of ranges. The ranges available

vary from one manufacturer to another, so it is necessary to consult manufacturer's


26/41



literature before selection. The range and the span are two different parameters.

The span is the actual pressure range to be measured after the transmitter has been

adjusted. The range is the pressure range within which the span can be adjusted.

Most transmitters have two adjustments, zero and span. The zero is adjusted so

that the output is minimum (4 ma, 3 psig, or 20 kPa) at zero pressure, or thebottom of the span if elevated zero is used. The span is adjusted so that the output

is maximum (20 ma, 15 psig, or 100 kPa) when the pressure is at the top of the

span.

But the zero must always be rechecked after the span has been adjusted and the

span checked after the zero adjustment.

Next figure shows the main parts of an electronic pressure transmitter;

Sensing module, and Electronic housing contains; header assembly board, DC ampilfier board, and

calibration board



27/41



Electronic Press. Transmitter, Main Parts.


28/41




Principle of operation;

This electrical block diagram illustrates the operation of the electronic pressure

transmitter.

The process pressure pressure is transmitted through an isolating diaphragmand oil fill fluid to a sensing diaphragm located in the centre of the cell. The

position of the sensing diaphragm is detected by the capacitance plates , the

sensor is driven by an oscillator.

Demodulator consists of a diode bridge, which rectifies the ac signal to dc todrive the oscillator.

Voltage regulator provide a constant voltage of 6.4 V dc for the reference and 7V dc to supply the oscillator.

Current control amplifier drives the current control circuitry to a level thatcauses the current detector to feedback a signal.

The current limit prevent output current from exceeding 30 mA in case ofoverpressure .


29/41




This figure shows the signal terminals which are located in the electrical housing

in a separate compartment. All power to the transmitter is supplied over the signal

wiring and there is no additional wiring required. Signal wiring need to beshielded, but twisted pair should be used for best result. Output current is limited

to 30 mA dc on the 4-20 mA dc unit.

Calibration

Span Adjustment Range; is continuously adjustable to allow calibration any

where between maximum span and 1/6 of maximum span.

Zero Adjustment Range; Transmitters zero is the lowest value of the transmitter

range, at which the output of the electronic pressure transmitter is 4 mA.

The zero and span adjustment potentiometers are accessible externally and are

located behind the name plate of the transmitter.



30/41



The recommended procedure for zero and span adjustment detailed in the

reference catalogue / service manual of the electronic pressure transmitter model,

consult your trainer.

Linearity Adjustment; this is a factory calibration adjusted for optimum

performance over the calibrated range of the instrument and is not normallyadjusted in the field, however only maximise linearity over particular range be

applicable.

Damping Adjustment; the output amplifier boards are designed to permit

damping of rapid pulsations in the pressure source by adjusting the control marked

Damping located on the solder side of the amplifier board.

Calibration procedures for adjusting or changing ranges are outlined in the

calibration section of the transmitter catalogue, consult your trainer to proceed.

Maintenance

Maintenance is a technique for checking out the transmitter components, the

method for disassembly and reassembly, and a trouble shooting guide.

Test Terminal; to assure that their is no leakage current through the diode when a

test reading is resistance of the test connection or meter should not exceed 10

ohms.

Sensing Module Checkout; the sensing module is not a field repairable and mustbe replaced if found to be defective. If no obvious defect, such as a punctured

isolating diaphragm or loss of fill fluid is observed, the sensing module may be

checked. The detail procedure to checkout the sensing module are listed in the

maintenance section of the transmitters catalogue, consult your trainer.

Circuit Boards Checkout; the printed circuit boards Amplifier board and

Calibration board can most easily be checked for a malfunction by substituting

into the circuit


TROUBLESHOOTING

1) SYMPTOM: HIGH OUTPUT


31/41



Potential Source and Corrective Action.

Impulse Piping

Check for leaks or blockage.

Check for liquid in dry lines.

Check for sediment in transmitter process flanges.

Transmitter Electronics Connections

Make sure bayonet connectors are clean and check out the sensor connections.

Check that bayonet pin #8 is properly grounded to the case.

Transmitter Electronics Failure

Determine faulty circuit board by trying spare boards.

Replace faulty circuit board.

Power Supply

Check output of power supply.

2) SYMPTOM: LOW OUTPUT OR NO OUTPUT

Potential Source and Corrective Action

Loop Wiring

Check for shorts and multiple grounds.

Check polarity of connections.

Check loop impedance.

Impulse PipingCheck that pressure connection is correct.

Check for leaks or blockage.

Check for entrapped gas in liquid lines.

Check for sediment in transmitter process flanges.



Check for shorts in sensor lead.

Make sure bayonet connectors are clean and check the sensor connections.

Check that bayonet pin #8 is properly grounded to the case.

Test Diode Failure

Replace test diode or jumper test terminals.


32/41






3) SYMPTOM: ERRATIC OUTPUT

Potential Source and Corrective Action

Loop Wiring

Check for intermittent shorts, open circuits and multiple grounds.

Process Fluid Pulsation

Adjust electronic damping pot (4-20 mA dc only).

Impulse Piping

Check for entrapped gas in liquid lines and for liquid in dry lines.


Check for intermittent shorts or open circuits.

Make sure that bayonet connectors are clean and check the sensor connections.

Check that bayonet pin#8 is properly grounded to the case.





TASK No.: I-3.8

Demonstrate isolation procedure from process and

electrical supply on a pneumatic/electronic pressure

transmitter.

Reference:

Materials: 1. Soap Solution, and2. Cleaning Rags


33/41



Equipment & Tools: 1. Tool Box, and2. Pneumatic or Hydraulic Pressure Source.

Conditions: Work Permit


To study the task and familiarise himself, Be able to demonstrate safe process isolation procedure for the pressure

devices,

Be able to perform safe electrical isolation procedure for the pressure devices, Discuss an understanding to his trainer, and Write observations and isolation procedures in his work book.


Details:

Preparations

1. Fill in work permit.2. Use soap solution and brush to check any gas or air leakage.3. If there is any leakage from fittings, tighten it by a suitable open-end wrench.Flushing the transmitter impulse line; the figure below show typical installationof the pressure transmitter for either liquid or gas services, follow the next

procedure for safe process isolation.

1. Close isolating valve A.2. Slowly open drain valve C.3. Slowly open isolating valve A to flush the impulse line.4. After flushing the impulse line, close valve C, slowly open A, and put the

transmitter on operation.


34/41




TASK No.: I-3.9

Check function / adjustment / maintenance of pressure

regulators for process lines.

Reference: OJT Instructor to arrange reference catalogue / Servicemanual for pressure regulators models in each working

area.

Materials: Cleaning Rags

Equipment & Tools: 1. Tool Box,2. Standard test gauges, and3. Continuous air supply pressure.


Typical Pressure Transmitter Piping


35/41




To study the task and familiarise himself, Be able to identify pressure regulators types, Be able to describe the principle of operation of continuous bleed regulator, State the advantages and disadvantages of continuous and continuous bleed

regulators,

Be able to dismantle, inspect, clean and replace a regulator, Discuss an understanding to his trainer, and Write observations in his work book.


Details:

Pressure Regulators

If a steady constant pressure is required either for instruments supply or process

operations. This is ensured by fitting a pressure regulator in the line.

A standard pressure regulator consists of a body in which is fitted a flexible

diaphragm. The diaphragm is connected to a valve and is top loaded by a spring

which can be adjusted.

System or primary air pressure is led to the underside of the diaphragm. The

diaphragm flexes, opening the valve and allowing air to pass to the outlet side

where it is known as secondary air. A small amount of secondary air is bled to the

top side of the diaphragm which, together with the spring, forces the diaphragm

down and operates the valve. Any change in primary air pressure will cause

movement of the diaphragm opening or closing the valve and stabilising thesecondary air pressure. Permanent changes in secondary air pressure can be made

by adjusting the pressure on the spring with a screw.

There are two types of pressure regulators;

1. Continuous bleed , and2.Non continuous bleed


36/41





37/41




TASK No.: I-3.10

Service and calibrate a pneumatic indicating controller

Reference: OJT Instructor to arrange reference catalogue / Servicemanual for pneumatic controllers models relevant to

each working area.

Materials: 1. Plastic / St.St. Tubing,2. Regulated Air Supply.

Equipment & Tools: 1. Pneumatic calibrator bench or Druck Pump,2. Standard test gauges,3. Service/ Repair Kit, and4. Tool Box.

Conditions: None.


To study the task and familiarise himself, Be able to identify the major components of pneumatic controller, To describe the pricible of operation of pneumatic indicating controller, Be able to perform periodic adjustments / calibration of the controller, To perform previntive maintenance, service, parts replacement of the

controller,

Draw/ Sketch calibration set-up in his work book, Discuss an understanding to his trainer, and

Continuous Bleed Press. Regulator


38/41



Write observations and procedure in his work book.


Details:

Pressure Controllers

A pressure controller is a device, which senses the pressure

in the process and vary an output which controls a device to regulate that pressure.

The control device, or end element, is usually a pneumatic-control valve. The

controller output is usually either a 3-15 or 6-30 psig (20-100 or 40-200 kPa)

pneumatic signal.

Pressure controllers can be categorized either as indicating or blind. The indicating

controller has a mechanism so that the operator can read the process pressuredirectly from the controller. The blind controller has no direct-reading mechanism

and the operator must rely on an adjacent pressure gauge or other device to know

the process pressure. The indicating controller set point is usually marked on the

indicator, thus it is easy to adjust to the desired point.

Adjustment of the blind controller is more of a trial and error process. Indicating

controllers are somewhat more expensive than blind controllers, but the cost

difference is moderate if a pressure gauge can be eliminated.

Pressure controllers must provide an output to control the end element. This can

be an electric or pneumatic signal, but is most often pneumatic for field-mounted

controllers. The pneumatic signal is usually 3-15 psig (20-100 kPa), but it can be

6-30 psig (40-200 kPa) if required to reduce the control valve actuator size. The

control action needed for pressure control is proportional plus integral, or P and I

(integral is also referred to as reset by some manufacturers).

The proportional action varies the output in proportion to the difference between

the measured pressure and the set pressure. The integral or reset action gradually

increases the amount of the correction until the measured pressure is returned tothe set point. A more extensive discussion of control modes and controller tuning

can be found in the controllers catalogue or service manual, consult your trainer.



39/41



A common option for pressure controllers is an auto/manual switch. This is a

valve, which allows the output of a manual regulator to be directed to the end

element (valve actuator) instead of the controller's automatic output. The transfer

can be either bumbles where the outputs are automatically matched to each other

when the auto/manual switch is transferred, or manual balance where the operator

must match the manual regulator output to the automatic output transfer to manualor the set point to the process variable before transfer to automatic.

Pressure controllers are either surface, panel, pipe-stand, or yoke mounted.

Surface-mounted controllers are fastened to a wall or other vertical surface. Panel-

mounted, also called flush-mounted, controllers are mounted in a cutout in a

control panel. Pipe-stand mounting occurs where a vertical or horizontal pipe

support is constructed and the controller is provided with a bracket and U-bolts to

attach it to a two-inch pipe-stand. It is not a good idea to support controllers on

process piping. Yoke mounted controllers are fastened to the valve yoke withspecial brackets. Yoke mounting is convenient when the valve is accessible.


Principle of operation

This figure explains in steps how the pneumatic indicator controller works;


40/41




Clibration

Calibration is required if the controller has been taken apart for cleaning or for

parts replacement, if a change of range is desired, or if the amount of zero is

changed substantially.


41/41

TASK DETAILING MANUALIf the capsule was removed or the flexure cap screw loosened, before calibrating,

make the flexure cap screw adjustment above.

The controller to be calibrated to 3 to 15 psi signal pressure range, at which the

receiver signal is used.

Calibration procedures; Detailed bench calibration and in-line calibration

procedures of the pneumatic indicating controllerer are listed in the reference

catalogue or service manual of the applied model. Consult your trainer.

Maintenance and Servicing

Maintenance and servicing of pneumatic indicating controller are limited to clean

or replace its parts, such as:

1. Supply Air Filter blow out at least once a day,2. Replace Screen Filter,3. Clean Nozzle Assembly,4. Clean booster relay Restrictor,5. Replace Booster Relay,6. Change measuring Range ,7. Remove Measurement Pointer or Set Point Index, and8. Adjusting controllers Alignment.Disassembly; normal servicing of the controller does not require the removal of

any parts other than those already mentioned. Further disassembly is not

recommended because of possible loss of accuracy or damage to the controller,

detailed servicing procedures are mentioned in maintenance section of the selected

pneumatic indicating controller model. Consult your trainer.

Module I-3 Pressure Instruments

Documents

Transcript of Module I-3 Pressure Instruments