Air Pressure Process Control Lab Manual

UNIVERSITI KUALA LUMPUR MALAYSIAN INSTITUTE OF CHEMICAL & BIOENGINEERING TECHNOLOGY

EXPERIMENT 5 Air Pressure Process Control PROCESS CONTROL (DIPLOMA PROGRAM)

LECTURER NAME ____________________ SUBJECT CODE ____________________ SEMESTER ____________________

Experiment 5: Air Pressure Process Control

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OBJECTIVE 

To identify the important components of the air pressure control system and to mark them in the P&I Diagram.



To carry out the start-up procedures systematically.



To control the pressure in single capacity and two capacity processes using PID controller.



To tune the PID controller using Ziegler Nichols closed-loop method.

1. INTRODUCTION Pressure vessels play an important role in chemical industries as storage vessels for gas phase components, reaction vessels to carry out a gas phase reaction, etc. Precise control of pressure in the pressure vessels is necessary in order to (i) maintain a constant concentration of components in the vessel, (ii) maintain the number of moles in the vessel, and (iii) protect the vessel from being damaged due to very high pressures. 2. EXPERIMENTAL EQUIPMENT The equipment used in this experiment is the air pressure process control training system, Model AP922 (Figure 6.1). This model, AP922, uses AIR to simulate a gas phase pressure process. 2.1 PROCESS PLANT, AP922 The process plant consists of two air vessels T91 and T92 with the associated piping, valves and fittings... Each vessel generally has a single/multiple air inlets and multiple outlets and is to be 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). Process air from an external compressed air supply is manually regulated into vessel T9I via the pressure regulator (AR91) and the manual valve MV91. This plant can either be operated as a single capacity system or as a two-capacity system. For a single capacity system operation, tank T91 is used while the tank T92 is by-passed. Air is discharged vi the process vent (VP) to the atmosphere.


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2.2 CONTROL SYSTEMS AND INSTRUMENTATION The control system consists of a microprocessor based PID controller with an inbuilt ON/OFF controller. It is equipped with the necessary instrumentation and signal transmission system. A brief description of the control system and the major instrumentation is given below: 1. Pressure control system at pipeline PLI Measurement :

PT91, gauge pressure transmitter, at vessel T91

Controller

PIC91, panel mount PID controller

:

Final control element: PCV91, Control valve, complete with pneumatic positi zr (PP) and a Current-to-Air converter (I/P). PCY91 2. Pressure control system at pipeline PLII Measurement :

PT9I, gauge pressure transmitter, at vessel T91

Controller :

P1C92, panel mount ON/OFF controller

Final control element: PCV92, ON/OFF Control valve with a solenoid valve PSV92 3. Instrumentation Pressure gauges

PG91, PG92, PG93, PG94

Temperature gauges :TG91, TG92 Flow meter :

F192, Variable area flow meter (Rotameter)

4. Other accessories: Pressure recorder PR91 Process air pressure regulator: AR91 Instrument air Regulator:

For the control valves PCY9 I /PP/PCV91 and PCV92

Pressure relief valves : PRV9I and PRV92 in T91 and T92, respectively.


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5. Annunciators PAD91 : Annunciator for a preset deviation band limit. If the pressure at T9I deviates on either side more than the preset deviation band limit, the annunciator PAD9I will be activated. PAL91 : Low pressure alarm limit. When the pressure at T91 drops below its preset Low pressure alarm limit, the annunciator PAL9I gets activated.

3. EXPERIMENTAL PROCEDURES 3.1 IDENTIFICATION OF MAJOR COMPONENTS IN THE PLANT 1. Walk around the experimental equipment, identify and mark the following major components of the system in the P&I block diagram provided. (Figure 6.2)                

T91, T92 = Air vessels PG91, PG92, PG93, PG94 = Pressure gauges TG91, TG92 = Temperature gauges AR91 = Process air supply pressure regulator FI92 = Rotameter, variable area flow meter PT9I = Gauge Pressure t ransmitter PIC91 = Pressure Controller (PID) PIC92 = ON/OFF Pressure controller PCV9 1 = Control valve with PCY91 UP converter and pneumatic positioner PP PCV92 = ON/OFF control valve with Solenoid valve PSV92 PRV91, PRV92 = Pressure relief valves in air vessels T91 and T92, respectively. V = Vent valve for each tank M V 9 1 = M a n u a l v al v e MV91B, MVIII = Process air inlet manual valves MV92A, MV92B = Discharge manual vale P R 9 I = R e corde r


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3.2 START-UP PROCEDURES 1. The following steps constitute the start-up procedure. Go through these steps before

starting any experiment. 2. Switch the "PANEL, SCADA/DDC" selector switch at the front of the cubicle to the

"PANEL, SCADA" position. Switch ON the main power supply at the front of the cubicle. All the panel instruments will lit up. 3. If the recorder chart drive is running, STOP it by pressing the 'RCD' pushbutton. If

any annunciator gets activated, press the ACKNOWLEDGE button to silence the buzzer. 4. Set the instrument air supply regulator to the pressure indicated at the air regulators at

the pneumatic control valves PCV91 and PCV92. Purge if there is any condensed water in the air regulators by opening the bottom drain valves 5. Make sure that the by-pass valves around the control valves PCV91 and PCV92

are shut fully and the pair of manual valves upstream/downstream of these control valves are opened. 6. Check that the control valve positioner (PP) of PCV91 is connected (ON) (i.e., not

by-passed). 7. Make sure the compressed air is available at the pressure inlet air regulator AR91. 8. Check that the vent valve (V) and bottom drain valve (D) of vessel T9I are shut. 9. SHUT fully the manual valves MV91B and MVIII. 10. SHUT fully the process inlet manual valve MV91. 11. Place the panel controller PIC91 in manual (M) mode and adjust its MV = - 6.3% to

open the control valve PCV9I fully. 12. Open fully the final discharge manual valve (VP). This valve is at downstream of

pressure control valve PCV91. 13. From the process air supply, trace the two pipe lines PLI and PLII as follows:

Pipeline PLI: Single capacity process: AR91-MV91-T91-PLI-B92- F192 -P CV9 1 -VP Two-capacity process AR9 1 -MV9 1 -T91 -PLI-MVI-T92-MV92-F192 -PCV 9 1 -VP Pipeline II (PLII): AR9 1 -MV9 I -T9I-PLII-MVII-PCV92-MV92A/MV92B-V


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14. Open fully the pipelines PLI (Single capacity process) and PLII at their various

manual valves. 15. Make sure the manual by-pass valve B92 around vessel T92 is opened and the manual

valves at the inlet MVI and outlet MV92 of T92 are shut. 16. The pipeline PLII has two discharge manual valves, MV92A (globe valve) and

MV92B (gate valve). SHUT both the valves MV92A and MV92B. 17. Check and adjust the pressure at AR91 is approximately at the pressure indicated at

AR91. Check MV91 is fully shut. 18. Slowly open MV91 and regulate it until the flow rate at the variable area flow

meter F192 reads 35 Nm3/Hr. 19. Check the pressure and flow rate setting: 

Shut fully PCV9I from PIC91 in manual (M) mode with MV=106.3%. Read the pressure at vessel T91 at the PV reading of PIC91. If the pressure is about 22 — 27 psig, the pressure is correctly set.



Open fully PCV91 from PIC91 in manual (M) mode with MV=-6.3%. Read the pressure at vessel T9I at the PV reading of PIC91. If the pressure is not more than 8 psig and the flow rate at FI92 also reads 35 Nm 3/Hr, the air flow rate is correctly set.



Now the process air supply pressure and flow rate are set. DO NOT readjust AR91 and MV91 hereafter.

20. The recorder PR91 chart speed is default by 500 mm/Hr (fast speed).

3.3 ON/OFF PRESSURE CONTROL It uses the ON/OFF pressure controller PIC92 to control at the High pressure limit of PIC91. 1. Make sure that you have gone through the START-UP procedures very carefully. 2. Start the chart drive of the recorder by pressing the 'RClir pushbutton at the front of

the recorder. 3. Shut fully PCV91 from PIC91 in manual (M) mode with MV=106.3%. Read PV at

PIC91, the pressure at vessel T91. The pressure should be in the range of 22-27 psig. 4. To open PLll: Open only the manual discharge globe valve MV92A at the common

vent of PLII. Keep the gate valve MV92B at the common outlet of PLI1 closed.


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5. Note the response of PT91 at the recorder PR9 I .

PCV92 opens when PT91 exceeds 21 psig and closes when PT91 drops below a fixed Dead Band (0.5 psig), i.e., 20.5 psig.

6. Try to repeat the ON/OFF action by opening and closing PC 91. This is done from

PIC9I in manual (M) mode by reducing the MV such that pressure PV drops below 20 psig. Then shut PCV91 fully again with MV = 106.3%. 7. Stop the record chart drive and retrieve the relevant portions of the chart paper. 3.4

3.4 PID CONTROL OF PRESSURE IN SINGLE CAPACITY PROCESS 1. Make sure that you have gone through the START-UP procedures very carefully. 2. Start the chart drive of the recorder by pressing the 'RCDT pushbutton at the front of

the recorder. 3. Make sure that pipelines PLI and PLII are opened throughout . 4. Open only the manual discharge globe valve MV92A at the common vent of PLII. 5. Close the gate valve MV92B at the common outlet of PLII. 6. For single capacity operation, only vessel T91 is used. Vessel T92 is by-passed.



Open by-pass valve B92 around T92.



Shut the manual valves at the inlet, MVI, and at the outlet, MV92, of T92.



Make sure the air flow path T91-PLI-F192-PCV91-VP is opened through B92.

7. Set the set point of PIC91 as SV = 15 psig. 8. Set the PID controller with the first (I) trial values:

PB1 = 70%, T11 = 40 secs, TD1 = 0 sec. 9. Transfer the controller to Auto (A) mode and wait for the response to become

almost stable. Switch PIC9I to Manual (M) mode, decrease MV by about 5 to 10%, switch back to Auto (A) mode and observe the response for a few minutes. DO NOT FORGET TO MARK THE PID VALUES AND THE SETPOINT ON THE CHART PAPER WHENEVER THE VALUES ARE CHANGED. ALSO MARK ON THE CHART THE BEGINNING OF EACH PART OF THE EXPERIMENT.


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10. If the response is slow to return to the set point SV = 15 psig, reduce TI I to 20 secs.

and repeat the step (10) again. 11. Now set the PID controller with the second (II) trial values:

PB I = 45%, T11 = 30 secs, TD1 = 0 sec. 12. Switch P1C91 to Manual (M) mode, decrease MV by about 5 to 10%, switch back to

Auto (A) mode and observe the response for a few minutes. 13. If the response is not oscillatory, proceed to step (16). If the response is oscillatory,

go to the next step, step (15). 14. Increase PB1 by 5% and repeat the step (13). 15. Try the third (111) trial values for the PID controller

PB 1 = 20%, TI 1 = 10 secs, TD I = 2 sec. 16. Introduce the disturbance in MV as in step (13) and observe the response. 17. STOP the chart drive and retrieve the relevant portions of the chart paper. Attach the

chart paper along with the results. 3.5 PID CONTROL OF PRESSURE IN TWO CAPACITY PROCESS

1. Make sure that pipelines PLI and PLII are opened throughout 2. Open only the manual discharge globe valve MV92A at the common vent of PLII. 3. Close the gate valve MV92B at the common outlet of PLII 4. For two- capacity operation, vessels T91 and T92 are used. 

Shut the by-pass valve B92 around T92



Open the manual valves at the inlet, MVI, and at the outlet, MV92, of T92.



Make sure the air flow path T91-PLI-T92-F192-PCV91-VP is opened through MVI and MV92.

5. Switch P1C91 to Manual (M) mode. 6. Open PCV9I about 50% from PIC91 with MV = 50%. 7. Set the set point of P1C91 as SV = 18 psig. 8. Set the PID controller with the first (I) trial values:

PB1 = 70%, TIl = 40 secs, TD1 = 0 sec. 9. Start the chart drive of the recorder by pressing the 'RCD' pushbutton at the front of

the recorder.


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10. Transfer the controller to Auto (A) mode and wait for the response to become almost

steady or continues to oscillate even after 3 cycles. 11. Switch PIC91 to Manual (M) mode, decrease its MV by about 5 to 10%, switch back

to Auto (A) mode and observe the response till it becomes almost steady or continues to oscillate even after 3 cycles. DO NOT FORGET TO MARK THE PID VALUES AND THE SETPOINT ON THE CHART PAPER WHENEVER THE VALUES ARE CHANGED. ALSO MARK ON THE CHART THE BEGINNING OF EACH PART OF THE EXPERIMENT. 12. If the response is slow to return to the set point SV = 18 psig, reduce TI1 to 20 secs.

and repeat the step (10) again. 13. Now set the PID controller with the second (II) trial values:

PB1 = 45%,.,. T11 = 30 secs, 'TD1 = 0 sec. 14.

Switch PIC91 to Manual (M) mode, decrease MV by about 5 to 10%, switch back to Auto (A) mode and observe the response for a few minutes.

15.

If the response is not oscillatory, proceed to step (16). If the response is oscillatory, go to the next step, step (15).

16.

Increase PB1 by 5% and repeat the step (13).

17.

Try the third (III) trial values for the PID controller: PB I = 20%, TI1 = 30 secs, TD 1 = 2 sec.

18.

Introduce the disturbance in MV as in step (13) and observe the response.

19.

Switch PIC91 to Manual (M) mode, decrease set -point to 9 psig, switch back to Auto (A) mode and observe the response till it becomes steady.

20.

STOP the chart drive and retrieve the relevant portions of the chart paper. Attach the chart paper along with the results.


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4. RESULTS 1. The recorder PR91 records the results of the experiments. Suitable portions of the recorder chart paper should be submitted as RESULTS of the experiment 2. T h e r e p o r t s h o ul d co n t a i n 3 . The P&I diagram with all major components marked clearl y. 4. A detailed report o f the start-up procedure. 5. The response curves of the ON/OFF controller with all particulars such as Setpoint and Dead Band. 6. Response curves for single capacity and two-capacity processes with different PID controller values. Include all the details such as setpoint, PID controller values. 1.

A discussion on the response characteristics.


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P & I Diagram Air Pressure Process Control System


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Air Pressure Process Control Lab Manual

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