CHE373 Exp1 Pressure

7/24/2019 CHE373 Exp1 Pressure

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Universiti Teknologi MARA

CHE373

Exp1/AP 1

EXPERIMENT 1: AIR PRESSURE CONTROL: MODEL AP922

(A)

OVERVIEW

Study Model AP922 carefully.Walk around the plant and identify the various equipment, processes and the instrumentation andcontrol systems. Also refer to the Process and Instrument (P and I) drawing.

a) The process plant consists of three air vessels T91, T92A and T92 with the associatedpiping, valves and fittings. The air vessels are made from larger diameter standard pipesections. Each vessel generally has a single/multiple air inlets and multiple outlets and is tobe operated with continuous air inflow and outflow. Each vessel has its own vent (V),bottom drain (D), pressure relief valve (PRV) and pressure gauge (PG).

b) i. An external air supply from the compressor air system is connected to the Model plantat the process inlet at the pressure regulator, AR91, into the vessel T91.

This inlet pressure (Barg, psig) is preset according to the pressure indicated at AR91,with its downstream manual valve MV91 shut.

ii. The maximum inflow rate can be varied by regulating the manual valve MV91immediately downstream of AR91.

c) The instrument air supply regulators (IAS) should be set according to the pressure indicatedat the respective air regulators.

d) The process air inlet flows into T91 and then along pipeline PLI-T92A-T92-FI92-PCV91-discharge via the process vent valve VP. The process air also flows from T92 along pipelinePLII-PCV90-discharge via either manual globe valve MV92A/orifice OR manual gatevalve MV92B.i. The air pressure in T91, T92A and T92 are monitored by their respective pressure

gauges PG91, PG92A and PG92. Excessive pressures will be relieved by their respectivepressure relief valves (PRV91, PRV92A and PRV92).

ii. The pressure in T91 is also measured by a gauge pressure transmitter PT91 andcontrolled by PID pressure controller PIC91, which throttles control valve PCV91 viathe Current (4-20mA) to Air (3-15 psig) converter, PCY91 and the pneumatic Positioner

(PP). Control valve PCV91 is ATC (Air-to-Close) which means it will stay open if thereis no air signal to its actuator such as during instrument air failure or during shut-down.The process air finally discharges to atmosphere via the process vent VP throughPCV91 and the variable area flowmeter FI92.


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Exp1/AP 2


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Exp1/AP 3

iii. The pressure measured by the gauge pressure transmitter PT92 is controlled ON/OFF

by the ON/OFF pressure controller PIC90. PIC90 will switch to close the normally-open (NO) solenoid valve PSV90, when the preset high pressure limit is exceeded. TheON/OFF control valve PCV90 is ATC (Air-to-Close).

Thus when the pressure in T92 or its discharge pipeline is above the preset high limit,PIC90 switches to close PSV90, cutting off the instrument air supply to the actuator ofPCV90. PCV90 then opens to vent (V) air to the atmosphere. The rate of discharge canbe manually set at the manual valves (MV92A, MV92B) at the common vent line (V)downstream of PCV90.

Note that both the control valves PCV91 and PCV90 are Air-to-Close (ATC) so that

when there is instrument air failure, they stay open to vent out any pressure inside theplant.

iv. Furthermore, PID Air Pressure Control systemcan be studied:

Process: 3-Capacities in series, vessels T91+T92A+T92 and itsdischarge Pipeline.Process inflow into T91 is from pipeline PLI.

Controlled Variable: PT91, Pressure in T91.

Manipulated Variable:

PID Controller:

At pressure control valve PCV91, outflow of T92.

PIC91 (PID1)

Control Loop: Single loop: PT91-PIC91-PCY91/PP-PCV91

e) OutletThe final process air discharge from Model AP922 is to atmosphere via its process vent VPfrom T92/T92A/T91 and via its common vent V downstream of PCV90/MV92A orMV92B.

f) In general, T91, T92A and T92 have their own individual vent valves which should fully

shut during normal operation. They also have their own individual bottom drain valves which should be purged

occasionally of any condensed water vapour. Otherwise, these should remain fully shut

Trace the various discharge piping from the Model plant. The final discharge toatmosphere is outside the building.


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Exp1/AP 4

(B) CONTROL SYSTEM AND INSTRUMENTATION

Identify the following summary list of instruments and control systems. Read Each row fromleft to right as a control loop. The items that are asterisked * are field (plant) mounted.

Sensor/Transmitter Controller Final Control Element

PT91* PIC91 (PID1) PCY91*/PP*/PCV91*NOTE:PIC91 (PID1) is configured inside the single unit of microprocessor-based PID panelcontroller (Programmable).

Field-Mount Instruments : Sensors, Transmitters, Control Valves, Variable Area Flowmeter,Gauges etc

The instruments mounted at the field (plant) are asterisked (*) in the previous summary listand further detailed below. For more details, please refer to a separate manual for theirspecifications etc.

PT91 : Gauge Pressure TransmitterMeasures air pressure in T91Calibrated Range: 0-25 psig

PT92 : Gauge Pressure TransmitterMeasures air pressure in T92 OR at its discharge Pipeline (i.e. one ofthe tapping point must be manually shut always).

Calibrated Range: 0 25 psig

PCV91 : Pressure control valve . Equal % characteristics, Air-to-Close(ATC). Regulated by PID controller PIC91 or PIC92

PP : Pneumatic Positioner, with By-pass.PCY91 : Current-to-Air Converter (I/P) : 4-20mA to 3-15 psig

PSV90 : Solenoid valve, Normally-Open (NO)PCV90 : ON/OFF pneumatic control valve, Air-to-Close (ATC).

PCV90 is opened by ON/OFF controller PIC90 via PSV90, whenthe pressure measured by PT92 is above its preset high pressurelimit.

PG91, PG92,P92A, PG93, PG94

: Pressure Gauges

TG91, TG92 : Temperature Gauges

FI92 : Variable area flowmeter (Rotameter). Metal construction safety type.

PRV91, PRV92A,PRV92

: Pressure Relief Valves


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Panel-Mount InstrumentsThe panel-mount instruments are listed below.

Switch the PANEL, SCADA/DDC selector switch at the front cubicle to the PANEL,SCADA position for Panel operation.

PIC91 : microprocessor-based Panel Controller (Programmable) configured forpressure controls-

PIC91 : (PID1), PID control of PT91, pressure in T91.

PIC90 : ON/OFF control for PT92 using the High alarm limits setat P01 and the Dead Band set at P02, at PT Register.

PR91 : Chart Recorder analog inputs with pen/bar graph and selective displayin engineering units.

Red pen (Channel 1) : Pressure PT91, Calibrated range 0 - 25 psig.Green pen (Channel 2): Pressure PT92, Calibrated range 0 - 25 psig.

: Analog display in the form of horizontal coloured bar or pen-chartpaper are to be read as follows:-Analog display in % x Maximum (calibrated range), engineering units =Actual readings, engineering units

AnnunciatorsThe Annunciators are:-

PAD91 : The pressure at T91, measured by PT91, exceeds its preset Deviation fromthe setpoint, whether above or below. The Deviation alarm limit is set atDL1 at the PID1 page or panel.

PAH92 : The pressure, measured by PT92, rises above its preset High alarm limit. TheHigh alarm limit is set at P01 at the PT Register page or panel.

A buzzer will come on and the respective alarm windows (PAD91, PAH92) will lit up whenthe above abnormal or alarm conditions occur. Pressing the Acknowledge button will silencethe buzzer sound. The dedicated alarm window remains lit as long as its process variable is stillin the alarm condition. The alarm window light will go off when the process variable isrestored to normal.

The Test button is to test if the Annunciator alarm window lights are working.


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(C) EXPERIMENT PROCEDURES

AIM : To study single loop gas pressure control

CONTROLSYSTEM

: Single Loop Gas Pressure ControlPT91-PIC91-PCY91/PP/PCV91

SPECIALREMARKS

: The panel-mount 1-2 Position selector switch for the pressurecontroller PIC91/PIC92/PIC90 should be in Position 1,operatingPIC91 (PID1, Loop1) only.Select the PIC91 controller main faceplate display.

KEY PROCESS

VARIABLE

: Gauge pressure in the vessel T91, measured by PT91.

PROCEDURES1. Quickly check the following :-

Switch the PANEL, SCADA/DDC selector switch at the front cubicle to the PANEL,SCADA position.

Switch ON the main power supply at the front of the cubicle. Power will be distributedautomatically to the control systems and instrumentation. The panel instruments will litup.

Also open the swing front cover of the recorder PR91 and press the RCD push buttonto STOP its chart drive, if it is running. Start the recorder chart drive only during theexperiment proper.

WHENEVER ANY ANNUNCIATOR IS ACTIVATED, PRESS THEACKNOWLEDGE (RED) BUTTON TO SILENCE THE BUZZER SOUND BUTTHE LIGHT REMAINS LIT. CHECK AND RATIONALISE THE CAUSE OFTHE ALARM CONDITION.

. Check that the instrument air supply are set accordingly at their air supply regulators(IAS). Frequent and exact pressure adjustment of the IAS is not necessary. Purgeregularly all the regulators to remove any condensed water, using their bottom drain

valves.

The valve Positioner (PP) at PCV91 is ON and connected i.e. USE THEPOSITIONER

With PIC91 in Manual (M)mode, stroke the control valve PCV91 manually with MV= 100%, 50% and 0% and note that the corresponding valve stem position at its stemindicator scale is 0%, 50% and 100% opened, only if the Positioner (PP) isconnected.

Check that the by-pass valves around the two control valves PCV91 and PCV90 arealways shut but the adjacent pair of manual valves upstream/downstream of these

control valves are opened.


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Check that the individual vent valves and bottom drain valves of T91, T92A and T92are shut. Occasionally purge the bottom drain valves to remove any condensed water

vapour.

Check that the final discharge manual valve VP downstream of PCV91 is fully opened,for air final discharge.Both pipelines PLI and PLII must remain opened throughout unless otherwiseinstructed. Check that the manual discharge globe valve MV92A at the common vent(V) of pipeline II, downstream of PCV90 is opened but the gate valve MV92B is shut,before proceeding further. If ever PCV90 is opened and shut during the course of thefollowing experiments, let the pressure response stabilise by itself. OTHERWISE, shutMV92A as well.

Operate the pressure process using the 3 vessels T91, T92A and T92 for air to flow inseries from one vessel to the next. Check that the By-pass valves B92A and B92 aroundT92A and T92 are shut, but all the direct interconnecting manual valves between T91-T92A-T92 are fully opened.

2. Model AP922 runs with compressed air as the process medium.The process air is from the air compressor via the pressure regulator AR91.

The maximum Pressure and Flow rate can be set as follows :-a) Shut fully the manual valves MV91B and MVIII at the process air pipeline coming from

any other interconnected Model Air plant. Check and make sure that the manual

discharge valve MV901 from Model AFT922 is fully SHUT.

b) Set PressureWith the process flow totally shut at the process inlet manual valve MV91, check thatAR91 is approximately at the pressure indicated at AR91 (26-28 psig). Open MV91 byabout turn and then shut it again. Reconfirm the pressure at AR91 is 26-28 psig.Otherwise, adjust AR91 to the pressure indicated and then recheck again with MV91open and then shut to read the pressure. Leave MV91 fully shut, after having confirmthat the pressure at AR91 is correctly set.

c) Shut pipeline PLII by shutting both its manual discharge valves MV92A and MV92B.

d) Check that pipeline PL1 with the control valve PCV91 and the final discharge manualvent valve (VP) downstream of FI92/PCV91 are fully opened. PCV91 is opened fromthe panel controller PIC91 in Manual (M) mode with MV = -0%, e.g. 6.3%.

e) Set FlowrateOpen the process inlet manual valve MV91 by about 1/2 turn (180) and regulate MV91till the process air flowrate at the variable area flowmeter FI92 read 20Nm/Hr MAXand NOT more.. Observe the flowrate till it remains steady for about 1 minute.


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f) Check Pressure/ Flowrate are correctly set

To check that the correct supply inlet pressure/flowrate are set, check (i) and (ii) asfollows before proceeding.(i) From the panel controller PIC91, shut PCV91 fully with its MV=106.3%. The

pressure at tank T91 indicated at the PV of PIC91 must exceed 22 psig but notexceed 27 psig.See OTHERWISEbelow

(ii) From PIC91, open PCV91 fully with its MV= - 6.3%. The residue pressure at tankT91 indicated at the PV of PIC91 must not exceed 5 to 6 psig, or else, the flowrateset at MV91 is too high. FI92 must be less than the flowrate stated above.See OTHERWISEbelow

- OTHERWISE, refer back to the previous procedures and start again setting thepressure at AR91 (procedure b) and flowrate (procedure e) at the manual valveMV91.

g) The instructions are summarised in the Table below.

TABLE : SUMMARY INSTRUCTIONS TO SET THE PRESSURE AND FLOWRATEAT MODEL AP922

Set Pressure Set AR91 at 26-28 psig

Set Flowrate If FI92 is calibrated at 2 Barg, 200C,Adjust MV91 till FI92 is 20Nm/Hr max and NOT more.DO NOT exceed the above stated flowrate.

Check Pressure With PCV91 fully shut, PV1 at PIC91 (i.e. PT91) should be 22-27 psig.OTHERWISE, reset AR91.

Check Flowrate With PCV91 fully open, PV1 at PIC91 (i.e. PT91) should be less than 5-6psig and FI92 is not more than the recommended flowrate.OTHERWISE, readjust MV91 for maximum Flowrate.

3. Locate the Gauge Pressure Transmitter PT92 and trace its impulse lines to two pressuretapping points:- TP-T at T92- TP-P at the discharge pipeline of T92Open TP-T and Shut TP-P

4. With PIC91 in the Manual (M)mode, open the control valve PCV91 fully with MV= -6.3%. For confirmation, check the control valve stem position at PCV91. Take note of theminimum pressure PV1 (in PIC91).


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Exp1/AP 9

5. Concentrate on the Pressure PID panel controller PIC91/PIC92/PIC90and the recorderPR91.

Within the Pressure PID panel controller PIC91/PIC92/PIC90, note the followingcontrols:PIC91:

1-2 Position:SelectorSwitch

PID controller for PT91 throttling the control valve PCV91 to controlthe pressure PT91 at T91, in a 3-Capacity pressure process comprisingT91/T92A/T92/discharge Pipeline T92 to PCV91.

Switch to Position 1, to select PIC91 only, for Single loop control.

Start the recorder PR91 by pressing the RCD pushbutton at the front of the recorder withits swing front cover opened. Note thatPT91: Red pen (Channel 1), Input Range: % of 0-25 psig.

Channel 1 is in psigThe recorder chart drive is preset for fast speed (500mm/Hr).

6. Check that vessels T91, T92A and T92 are connected for air to flow in series from onevessel to the next. Check their respective pressure at the pressure gauges PG91, PG92Aand PG92. The manual By-pass valves B92A and B92 around T92A and T92 should befully shut. The final manual discharge valve (VP) downstream of the pressure control valvePCV91 should be fully opened.

7. a) Open PCV91 fully from PIC91 in Manual (M)mode, with MV = -6.3%. Make sure theby-pass valve around PCV91 is shut but its two adjacent manual valves are opened.

b) Access the PID values in the pressure PID controller PIC91, at the PID1 page or panel.Set the following first (I)trial PID values.PB1 = 70 %, TI1 = 40 secs, TD1 = 0

Adjust the setpoint SV to 18 psig(72%) at PIC91.Make sure the chart drive of the recorder PR91 is running. Otherwise, press theRCD button at the front of the recorder with its swing front cover opened.

Transfer PIC91 toAuto (A) mode and watch the response at the recorder PR91 tillit is steady at its setpoint SV to within 0.1 psig or continues to oscillate even after3 cycles.

.


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Exp1/AP 10

PIC91 PID Pressure Control of T91/92A/T92

ProcedureReference

PIDValues

Setpoint,psig

Test

7 3-Capacity pressure process, T91/T92A/T92. With Positioner.Controller: PIC91

c.d.

e.

f.

g.

(i).

(ii).

(i).(ii).(iii).

(i).

(ii).

(iii).

Same PID, different SV.First (I): 70/40/0First (I): 70/40/0

Same PID, different SV.Second (II): 45/30/0

Second (II): 45/30/0

Same PID, different SV.Third (III): 35/30/0Third (III): 35/30/0Third (III): 35/30/0

Different PID, same SV.Third (III): 35/30/0

Second (II): 45/30/0

First (I): 70/40/0

1818-9

9

9-18

1818-1515-9

15

15

15

Load step testSetpoint step test

Load step test

Setpoint step test

Load step testSetpoint step testSetpoint step test

Setpoint step testLoad step testLoad step test

Load step test


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c) Load step test

With the setpoint SV=18 psig (72%) and the first (I) trial PID values of 70%/ 40secs/ 0 sec, apply the load step testas follows:

When the response is steady, switch PIC91 to Manual (M)mode and decrease its MVby 5-10%. Quickly switch PIC91 back toAuto (A)and observe the pressure response atthe recorder PR91 till it is almost steady to within 0.1 psi, or continue to showoscillatory response even after 3 cycles.

THE CHART RECORDING CONSTITUTES THE RESULTS OF YOUREXPERIMENT. NOTE DOWN ON THE CHART THE PB1, TI1, TD1, SVVALUES. WATCH CLOSELY FOR OVERDAMPED, UNDERDAMPED

RESPONSE AND THE POSSIBILITY OF ANY INLET LOAD DISTURBANCE.CAREFUL CONSIDERATION MUST BE TAKEN THEN TO PROVIDEEFFECTIVE CONTROL.

d) Setpoint Step disturbanceWith PIC91 inAuto (A)mode, apply a step change in setpoint(SV) from 18 to 9 psig(72% to 36%). Observe the chart response similarly and note down the SV and PID1values on the chart. (Note: When the 1-2 Selector Switch is in Position 1, PV1, SV1becomes PV, SV respectively).

Check for process inlet load disturbance- If the response tends to oscillate outside the desired control band of 0.1 psig i.e.

below 8.9 and above 9.1 psig for SV = 9 psig OR below 17.9 and above 18.1 psig forSV = 18 psig, switch PIC91 to Manual (M)mode and observe if the pressure is stilloscillating i.e. IF THE PROCESS AIR INLET SUPPLY PRESSURE ISOSCILLATING, WHICH IS A LOAD DISTURBANCE AT THE PROCESSINLET.If it is, then the PID values (70% / 40 sec/ 0 sec) may have slowed down thecontroller action, till it is slower than the process inlet disturbance.

- Try a faster TI i.e. set TI1 = 10 to 20 secs instead of 40 secs, for faster setpointrecovery.Observe similarly for any improvement.

e) Testing at different setpoints(i) With the setpoint SV at 9 psig (36%), use the second (II)trial PID shown below:

PB1 = 45% TI1 = 30 secs TD1 = 0 secRepeat the previous (c) using the same load step testand observe similarly.

(ii) With PIC91 inAuto (A)mode, apply a setpoint step test by changing the setpointfrom 9 to 18 psig(36% to 72%) and observe.

- If the response oscillates outside the control band of SV 0.1 psig, and stays too longaway from the setpoint SV, try a faster TI secs and observe similarly, but note that astronger Intergral action (smaller TI secs) is more oscillatory.

PB1 = 45% TI1 = 20 then 10 secs TD1 = 0 sec


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Exp1/AP 12

f) (i) With the setpoint at 18 psig, use the third (III)trial PID values as follows:

PB1 = 35% TI1 = 30 secs TD1 = 0 secRepeat the previous (c) using the same load step test and observe similarly. If it isoscillatory, observe for 10 cycles to see whether the oscillation can be damped out. Itis very likely that the controller gain is too high at PB%=35%, at setpoint SV= 18psig

(ii) With PIC91 inAuto (A) mode, apply asetpoint step testby changing the setpointfrom 18to 15 psig (72% to 60%). Observe the response patiently for a while. If it isoscillating continuously, observe for about 10 peaks.

(iii) With PIC91 in Auto (A)mode, apply a setpoint step testby changing the setpoint

SV from 15to 9 psig (60% to 36%). Observe the response patiently for a while. If itis oscillating continuously, observe for about 10 peaks.

However, if the pressure control loop appears more or less oscillatory at differentoperating setpoints (SV), then the pressure control loop in this application is Non-Linear. i.e. the loop has variable gain. Discuss the likely cause of this Non-Linearity.

g) Different PID comparison, at the same Setpoint SVFor the same setpoint SV, different PB% and TI secs will produce different controlresponses, as generalised in the table below.

TABLE : INCREASING/DECREASING PB%, TI SECS

DIFFERENTPB%, TI secs

EFFECT OF PB%, TI secs ON CONTROL RESPONSES.

Larger PB% Lower controller gain : Stabilising.Slower, more damped, less oscillatory.

Smaller PB% Higher controller gain : Destablising.Faster, less damped, more oscillatory.

Larger TI secs Less powerful integral action : Stabilising.Slower recovery to setpoint, less oscillatory, less overshoot/undershoot

Smaller TI secs More powerful integral action : Destabilising

Faster recovery to setpoint, more oscillatory, higherovershoot/undershoot.


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Exp1/AP 13

Try to verify the above using the following tests :

(i) With PIC91 still in Auto (A) mode, change the pressure setpoint SV to 15 psig(60%) and observe similarly.

Set the third (III) trial PID values as shown in the next TABLE: TESTINGWITH DIFFERENT PID AT THE SAME SETPOINT.

Switch controller PIC91 to Manual (M)mode and decrease MV by about 5 to10%.

Quickly switch PIC91 back toAuto (A)mode.This is a load step disturbanceof about 5 to10%. Watch the response at the

recorder PR91 until it stabilises to within 0.1 psi of its setpoint or showsoscillatory response even after 3 cycles.

(ii) and (iii). Repeat the above at the same SV = 15 psig, using instead the (ii) second (II)and (iii) first (I) PID values as shown below. Apply the same load stepdisturbanceafter each change of PID values.Compare the three responses at the three different PID values at SV = 15psig, bearing in mind that the objective is to control the ressure of tank T91 atits setpoint SV to within 0.1 psig.

TABLE : TESTING WITH DIFFERENT PID AT THE SAME SETPOINT

PROCESS : Three (3) Capacity Pressure ProcessPID

ParametersFirst (I) trialPID values

Second (II) trialPID values

Third (III) trialPID values

PB1TI1TD1

70 %40 secs0 sec

45 %30 secs0 sec

35 %30 secs0 sec

Setpoint SV 15 psig 15 psig 15 psigResponses Damped, slow recovery.

Can control be improvedby using a faster TI (i.e.TI1= 10 to 20 secs)?

Is it between trial PID(I) and (III) responses?

Is it faster?Is it oscillatory?

8. To shut-down AP922,- Shut MV91 but do not adjust AR91.

Purge any water collected at the individual air supply regulator (IAS) but do NOTadjust the top knob, which has been preset to the required pressure.

- Switch PIC91 to Manual (M) mode with its MV = 0%- At the Recorder, press the RCD pushbutton to stop the chart drive.- Switch off the power supply at the front of the panel.- Shut fully the isolation manual valve at the main air supply line to Model AP922.


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Exp1/AP 14

RESULTS/DISCUSSION

The recorder PR91 records the results of the experiments.

RELEVANT PORTIONS OF THE RECORDER CHART RESPONSES SHOULD BESUBMITTED AS RESULTS OF THE EXPERIMENT, WITH THE FOLLOWINGPERTINENT DATAS MARKED ON THE CORRESPONDING CHART RECORDING:-

CONTROLLER INAUTO (A), MANUAL (M)MODE.

SETPOINT (SV), PB, TI, TD.

ANY ANNUNCIATOR ACTIVATED.


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Exp1/AP 15

TYPICAL RESULT SHEET, EXPERIMENT 1, PROCEDURE 7

GAS PRESSURE PID CONTROL: PIC91 Enclose a few chart responses using different sets of PID values for PIC91.

Use the Positioner (PP) at PCV91.

PIC91 Setpoint(SV1): __ psigProcess for Single loop : T91+T92A+T92 and its discharge Pipeline.PIC91: PB1: ________ %, TI1: ________ seconds, TD1: ________ secondsTest disturbance: Setpoint step change/Load step change (Tick which one)Note any significant Annunciators activated :

PIC91 Setpoint(SV1): __ psigProcess for Single loop : T91+T92A+T92 and its discharge Pipeline.PIC91: PB1: ________ %, TI1: ________ seconds, TD1: ________ secondsTest disturbance: Setpoint step change/Load step change (Tick which one)Note any significant Annunciators activated :

CHE373 Exp1 Pressure

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