lab.manual.jan2013.air.pressure

EXPERIMENT NO. 1Air Pressure Process Control (AP 922)

OBJECTIVES1. To identify the important components of the air pressure control system and to mark them in

the P&I Diagram.2. To carry out the start-up procedures systematically. 3. To control the pressure in single capacity and two capacity processes using PID Controller.

KEYWORDSAir pressure system, PID controller, Controller tuning

1.0 INTRODUCTIONPressure vessels play 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.

2.0 EXPERIMENTAL EQUIPMENTThe equipment used in this experiment is the air pressure process control training system, Model AP922. This model, AP922, uses AIR to simulate a gas phase pressure process.P&I diagram is supplied at the end of this lab manual.

2.1 AIR PRESSURE 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 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 T91 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 from process vent (VP) to the atmosphere.

2.2 CONTROL SYSTEM AND INSTRUMENTATIONSThe 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:

Pressure control system at pipeline PLI1. Measurement: PT91, gauge pressure transmitter, at vessel T91.2. Controller: PIC91, panel mount PID controller.3. Final control element: PCV91, Control valve, complete with pneumatic position (PP) and a

Current-to-Air converter (I/P Transducer), PCY91.

Pressure control system at pipeline PLII1. Measurement : PT9I, gauge pressure transmitter, at vessel T91.2. Controller : P1C92, panel mount ON/OFF controller.3. Final control element: PCV92, ON/OFF control valve with a solenoid valve, PSV92.

Instrumentation1. Pressure gauges: PG91, PG92, PG93, PG94.2. Temperature gauges:TG91, TG92.3. Flow meters: FI92, Variable area flow meter (Rotameter).

Other accessories1. Pressure recorder, PR91.2. Process air pressure regulator, AR91.3. Instrument Air Regulator: For the control valves PCY91/PP/PCV91 and PCV92.4. Pressure relief valves, PRV9I and PRV92 in T91 and T92, respectively.5. Annunciators, PAD91, Annunciator for a preset deviation band limit. If the pressure at T91

deviates on either side more than the preset deviation band limit, the annunciator PAD91 will be activated.

6. PAL91, Low pressure alarm limit. When the pressure at T91 drops below its preset Low pressure alarm limit, the annunciator PAL91 gets activated.

Experiment 1 Air Pressure Plant Control Process Dynamics & Control Lab

3.0 EXPERIMENTAL PROCEDURES

3.1 IDENTIFICATION OF MAJOR COMPONENTS IN THE PLANT1. Walk around the experimental equipment, IDENTIFY and MARK the following major

components of the system in the P&I Diagram provided at the end the lab manual.

a) T91, T92, Air vessels. PG91, PG92, PG93, PG94, Pressure gaugesb) TG91, TG92, Temperature gaugesc) AR91, Process air supply pressure regulator d) FI92, Rotameter, variable area flow meter e) PT91, Gauge Pressure transmitter f) PIC91, Pressure Controller (PID) g) PIC92, ON/OFF Pressure controller h) PCV91, Control valve with PCY91, I/P Converter and Pneumatic positioner, PPi) PCV92, ON/OFF control valve with Solenoid valve PSV92j) PRV91, PRV92 Pressure relief valves in air vessels T91 and T92, respectively k) V, Vent valve for each tank l) MV91, Manual valve m) MV91B, MVIII, Process air inlet manual valves n) MV92A, MV92B, Discharge manual valveso) PR91, Process recorder (default recorder speed is 500 mm/hr)

3.2 START-UP PROCEDURES1. The following steps constitute the start-up procedures. Go through these steps before starting

any experiment.2. Turn 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 (IAS) regulator to the pressure indicated at the air regulators (this is the air supply to 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 on the upstream and downstream lines 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 by setting

the AR91 to the indicated pressure.8. Check that the vent valve (V) and bottom drain valve (D) of vessel T9I1are 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 I (PLI):Single capacity process:

AR91-MV91-T91-PLI-B92-FI92-PCV91-VP Two-capacity process

AR91-MV91-T91-PLI-MVI-T92-MV92-FI92-PCV91-VP

Pipeline II (PLII):AR91-MV9I-T9I-PLII-MVII-PCV92-MV92A/MV92B-V

14. Open fully the pipeline 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 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 FI92 reads 35 Nm3/hr.

19. Check the pressure and flow rate setting:


a. Shut fully PCV91 from PIC91 in manual (M) mode with by adjusting MV=106.3%. Read the pressure at vessel T91 at the PV reading of PIC91. If the pressure is about 22 to 27 psig, the pressure is correctly set.b. Open fully PCV91 from PIC91 in manual (M) mode with MV= -6.3%. Read the

pressure at vessel T91 at the PV reading of PIC91. If the pressure is not more than 6 psig and the flow rate at FI92 also reads 35 Nm3/hr, the air flowrate is correctly set.

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

3.3 PID CONTROL OF PRESSURE IN SINGLE CAPACITY PROCESS (T91)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 'RCD’ pushbutton at the front of the

recorder PR91.3. 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.

A) Open by-pass valve B92 around T92.B) Shut the manual valves at the inlet, MVI and at the outlet, MV92, of T92.C) Make sure the air flow path T91-PLI-FI92-PCV91-VP is opened through B92.

7. Set the set point of the air pressure for this process at PIC91 as SV = 15 psig.8. Set the PID controller with the first (I) trial values: PB1 = 70%, T11 = 40 s, TD1 = 0 s9. Transfer the controller to Auto (A) mode and wait for the process response to become almost

stable.

DO NOT FORGET TO MARK THE PID VALUES AND THE SET-POINT ON THE CHART PAPER WHENEVER THE V ALUES ARE CHANGED. ALSO MARK ON THE CHART THE BEGINNING OF EACH PART OF THE EXPERIMENT.

10. Switch PIC91 to Manual (M) mode, decrease MV by about (5 to 10) % [do not change the MV to the 5 or 10% value], bring back PIC91 to Auto (A) mode and observe the response for a few minutes.

11. If the response is slow to return to the set point, SV = 15 psig, reduce TI1 value to 20 s and repeat step (10) again.

12. Now set the PID controller with the second (II) trial values: PB1= 45%, TI1 = 30 s, TD1 = 0 s

13. Switch PIC91 to Manual (M) mode, decrease MV by about (5 to 10) % [do not change the MV to 5 or 10% value], bring back to Auto (A) mode and observe the response for a few minutes.

14. If the response is not oscillatory, proceed with the next step. If the response is oscillatory, increase PB1 value to 50% and repeat step (15).

15. Try the third (III) trial values for the PID controller:PB1 = 20%, TI1 = 10 s, TD1 = 2 s

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.

3.4 PID CONTROL OF PRESSURE IN TWO CAPACITY PROCESS (T91 & T92)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.

A) Shut the by-pass valve B92 around T92 B) Open the manual valves at the inlet, MVI, and at the outlet, MV92, of T92. C) Make sure the air flow path T91-PLI-T92-FI92-PCV91-VP is opened through

MVI and MV92.5. Start the chart drive of the recorder by pressing the 'RCD' pushbutton at the front of the

recorder PIC91.6. Switch PIC91 to Manual (M) mode.7. Open control valve PCV91 about 50% from PIC91 by adjusting MV = 50%. 8. Set the set point of the air pressure for this process PIC91 as SV = 18 psig.9. Set the PID controller with the first (I) trial values: PB1 = 70%, T11 = 40 s, TD1 = 0 s10. Transfer the controller to Auto (A) mode and wait for the process response to become almost

stable or continue to oscillate.DO NOT FORGET TO MARK THE PID VALUES AND THE SET-POINT ON THE CHART PAPER WHENEVER THE V ALUES ARE CHANGED. ALSO MARK ON THE CHART THE BEGINNING OF EACH PART OF THE EXPERIMENT.


11. Switch PIC91 to Manual (M) mode, bring MV to 40%, switch back PIC91 to Auto (A) mode and observe the response until it becomes steady or continue to oscillate even after 3 cycles.

12. If the response is slow to return to the set point, SV = 18 psig, reduce TI1 value to 20 s and repeat step (11) again.

13. Now set the PID controller with the second (II) trial values: PB1= 45%, TI1 = 30 s, TD1 = 0 s

14. Switch PIC91 to Manual (M) mode, decrease MV by about (5 to 10) % [do not change the MV to 5 or 10% value], bring back to Auto (A) mode and observe the response for a few minutes.

15. If the response is not oscillatory, proceed with the next step. If the response is oscillatory, increase PB1 value to 50% and repeat step (14).

16. Try the third (III) trial values for the PID controller:PB1 = 20%, TI1 = 10 s, TD1 = 2 s

17. Introduce the disturbance in MV as in step (14) and observe the response until it becomes steady.

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

4.0 SHUT DOWN PROCEDURES1. Stop the recorder chart drive by pressing the 'RCD' button at the front of the recorder. Turn off

the recorder PR91.2. Switch the controller PIC91 to Manual (M) mode with its control output MV= 0%. 3. Switch OFF the main power supply. 4. SHUT the process air supply regulator AR91 and process air supply manual valve MV91. 5. SHUT the instrument air supply.

5.0 RESUL TS1. 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. The report should contain:

a) The P&I Diagram with all major components marked clearly. b) Dynamic Response curves for single capacity and two-capacity processes with

different PID controller values. Include all the details such as setpoint, PID controller values.

c) Discussion on the response characteristics based on different PID and disturbances. (please refer notes).

Figure1.1: An example of process response for PID controller


Figure 1.2: P&I Diagram for Air Pressure Process Control Plant


lab.manual.jan2013.air.pressure

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