expt. 4 - pressure control

7/27/2019 Expt. 4 - Pressure Control

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PRESSURE CONTROL Laboratory Manual (Last Update: 18 Feb 2009)

_______________________________________________________________________________Chemical Engineering Department

1

302265 ChE381Process Lab Projects

Pressure Control: Close Loop PID Temperature Control Loop Tuning (Zieglerand Nicholas Method)

_________________________________________

Laboratory Manual__________________________________________

This manual is constructed by the teaching team of ChE381Process Lab Projects in the Department of

Chemical Engineering of Curtin University of Technology according to the manufacturers instruction

of the operation unit. Experiments are selected by the unit coordinator based on the suitability and time

required for the project.


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OBJECTIVES

i) To demonstrate the characteristic of PB (Proportional Band) + I (Integral

Action) + D (Derivative Action) on a pressure process control loop.

ii) To demonstrate the loop tuning procedures on a pressure process

control loop

BACKGROUND AND EQUIPMENT

The SOLTEQ Pressure Control Trainer (Model: SE401) is used in this experiment. It

is made of industrial graded instrumentation and designed to exhibit a realistic working

environment of standard industrial pressure control loop.

It is used to demonstrate as well as give the student a hands on experience on how a

pressure loop can be controlled using a microprocessor based controller.

PID Control

In operation, the proportional controller calculates the amount of error between

the measurement and set point, amplifies it, and positions the final control

element to reduce the error. Integral action avoids the offset created in

proportional only control by bringing the output back to the set point.

Proportional-plus-integral (PI) control is generally used on processes where no

amount of offset can be tolerated. However, some processes are so difficult to

control or so critical to maintain at set point, that the use of all three modes will be

helpful in achieving desired control. Derivative control responds to the rate of

range of error. Hence, combined PID control responds to all aspects of process

errordirection, magnitude, duration, and rate of change. The output of a PID

controller is a linear combination of P, I, and D modes of control.

Processes that benefit most from PID control have rapid and large disturbances in

which derivative action can respond to the rapidity of the changes, and integral

action can respond to the duration of them.

Figure 1: System response to a process upset with different modes of analog Control


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Loop Tuning

The closed-loop control system attempts to achieve a balance between supply

and demand by comparing the controlled variable to the set point and regulating

the supply to an amount that will maintain the desired balance. Tuning the

controller adjusts the supply so that balance is achieved as quickly as possible.The performance criteria for a PID controlled can be evaluated by measuring

the damped period. Optimum tuning generally results with a QAD-period that is

approximately equal to the natural period. The damped period will be referred to

as PID, and is equal to o.

Recommendations for response setting are:

Method: Ziegler and Nichols for closed loop

PB = 2(PB)*

I= 0 /1.2

D = 0/8

where (PB)* is the proportional-band setting that produces constant amplitude cycling.

GENERAL OPERATING PROCEDURES:

General Start-up Procedures

1. Start compressor and wait for sufficient air pressure to build up;

recommended air supply pressure is 6 bar. Close HV1, set pressure

regulators RG1 to 4 Bar or 4 kg/cm2 and do not adjust RG2.2. Remove both caps of the red and green recorder pens.

3. Switch on instrument power supply.

4. Check recorder is working and the pens have ink. Then stop the

recorder.

5. Set controller to manual with 0% output.

6. Check and make sure hand valves position are as follows:

Close HV3,HV4

Open HV1, HV2

7. Open the isolation valve HV1. Air will start to charge into the receivertankTN1 until pressure reaches 4 bar. Adjust RG1 if the pressure drop

below 4 bar.

Note:1. For Yokogawa controller and recorder operations please refer to

Appendix A.

2. Yokogawa recorder chart speed has been preset at 3600mm/hr.

General Shut Down Procedures

1. Close HV1.


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2. Put controller in "Manual" mode, and then set controller output to 50%.

Release compressed air in the trainer system by opening globe valve

HV2 to set the air flow rate at FI1 to 40 LPM. Wait until both pressure

gauges PI1 and PI2 read 0 bar.

3. Shut off main compressed air supply.

4.

Switch off the main power. Then, replace the caps of the recorder pens.

Note: ALWAYS put on the caps to the pen after end of experiments, as the

ink will dry rapidly with the tip exposed.

Equipment Maintenance

1. ALWAYS put on the caps to the pen after end of experiments, as the ink

will dry rapidly with the tip exposed.

2. Release the air completely from both tanks when the unit is not in use.

3. Wipe off any spillage on the unit immediately.

4. Always check that all indicators are functioning.

5. Always check that the pressure gauges are working properly.

6. Clean the equipment with damp cloth every month.

7. Replace the chart paper and the felt pens of the chart recorder whenever

necessary.

Safety Precautions

1. The unit must be operated under the supervision of trained personnel.

2. Do not adjust the pressure regulator RG2 unless the setting pressure drops

below the required pressure as written in the specification plate.

3. Check the relief valve every half year to make sure that it is in a good

condition.4. Open valve V3 and V4 to drain any condensate collected every week.

5. All operating manuals supplied with the unit must be completely read and

understand before conducting any experiment on the unit.

REFERENCES

1. Perrys Chemical Engineers Handbook, 7th Edition, (Ed: R.H. Perry,

Green, and J.O. Maloney), McGraw-Hill, New York, 1997

2. Principles and Practice of Automatic Process Control, 2nd Edition, Carlos

A. Smith and Armando B. Corripio, Wiley, 19973. CE Refresher: Process Automation, McGraw-Hill, Inc. New York, 1985

EXPERIMENTAL PROCEDURES

Experiment 1: Close Loop P plus I & Derivative Control

Objective: To demonstrate the characteristic of PB (Proportional Band) + I

(Integral Action ) + D (Derivative Action) on a pressureprocess control loop


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Procedures:To start the experiment follows the start up procedures.

1. Put the control loop into "Manual Mode". Then adjust the output to

50%. Let the pressure build up in TN2. Manipulate HV2 to adjust FI1

to 20 L/min.

2.

Enter a PB value of 200 %, I value of 3 sec and D value of 0 sec.3. Adjust the output gradually so that the pressure of the tank matches the

set point of 50%.

Note:

We recommend 50% as the initial output. From there, the output can be

increased or decreased as appropriate. If the measurement is too far

from set point, use HV2 to expedite the process. Closing HV2 will

increase measurement input, and vice versa.

4. Start the recorder. Then, put the control loop into "Auto mode".

5. Simulate a load change by opening HV2 so that FI1 reads 40 L/min for

5 seconds; restore HV2 to its initial position.

6.

Wait until the measurement exhibit a uniform pattern, and then stop therecorder.

7. Put the control loop into "Manual mode". Adjust the output gradually

so that the measurement matches the set point.

8. Start the recorder, and then put the control loop into "Auto mode"

Increase the set point to 75%, wail until the measurement exhibit a

uniform pattern, then stop the recorder.

9. Maintain the PB and I values. Repeat Steps 2 to 8 by using the

following D values.


Appendix A.


Experiment 2: Loop tuning procedures on a pressure process control loop

1. Enter a PB value of 100, I value of 9999 seconds and D value of 0

second (setting I to its maximum and D to its minimum provides

proportional-only control).

2. Put the control loop into Manual mode. Adjust set point to 50 and

slowly adjust the output so that the measurement matches the set point.

(Approximate output value is 50, adjust as necessary).

3. Make a step change in the controllers set point up to 60% and observe the

resulting measurement cycle.

4. Remain the value of D and I. Slowly reduces PB further if the

measurement cycle damps out to a steady-state value, and, flowing this,

make another change in the controllers set point.

D (second)

05

20


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Suggested values are as follows:

5. Repeat steps 3 and 4 until the measurement cycles at a constant amplitude

and period as shown in Figure 2.

Note:Be certain that the final actuator is not oscillating between its limits

because such oscillation will yield erroneous results.

6. Based on the chart, determine the natural period using the following method

Natural Period, sXTrendSpeed

DT 3600=

Where D = Distance in mm between successive crests or valleys

7. Find the P, I and D values by using Ziegler and Nichols method.

PB = 2(PB)*

I= 0 /1.2

D = 0/8

where (PB)* is the proportional-band setting that produces constant amplitude

cycling.


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8. Then substitute the optimized P, I and D values into the loop controller.

9. Put the control loop into Manual mode. Adjust set point to 50 and slowly adjust

the output so that the measurement matches the set point. (Approximate output

value is 50, adjust as necessary).10. Simulate a load change and a set point change with this PID setting. Observe the

response from chart recorder.


Appendix A.



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Flow diagram of the unit

PRESSURE CONTROL TRAINER (MODEL: SE 401)

HV1 RG1

COMPRESSED

AIR

HV 3

DRAIN

PI1

HV4

DRAIN

PI2

VENT

ATM

PSV 2

HV 2

PT1

I/P1

RG2

AIRPIC1PR1

CV1

CONTROL TANK

TN 2

RECEIVER TANK

TN 1

FI 1

ATM

PSV 1

Instruments Functions

1 Air Regulator(RG1)

- Supply 4 Bar or 4 kg/cm2 air to Receiver Tank (TN1).

2 Air Regulator(RG2)

Supply 2.8 bar instrument air to I/P positioner. Do notchange it unless air pressure drop below 2.8 bar.

3 Hand Valves(HV1-HV4)

- Input / Output isolation valves.HV2 is used to regulate the desired airflow and also act asa load change valve for testing purposes.

4 Pressure gauge(PI1, PI2)

- Direct pressure measurement sensor, indicates pressure inthe receiver and control tank.

5 Relief Valve(PSV1, PSV2)

- Mechanical activated device, the spring loaded normallyclosed valve, open prior to pressure greater than its springtension. In case of over pressure in the tank this relief willopen and purge the air to the atmosphere. For safetyreason, do not adjust this valve. PSV1 and PSV2 arefactory set at 5.0 Bar and 2.5 Bar, respectively.

6 Air Receiver Tank(TN1)

- The tank is used as a buffer tank to stabilize the air supplypressure. Release the air completely when finished using


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the trainer.

7 Control Tank(TN2)

- The pressure in this tank will be controlled by a completeclosed loop control system. Release the air completelywhen not in used.

8 Rotameter (FI1) - It is a direct airflow instrument used to indicate flow rate ofthe air. The process airflow rate is indicated by a floatposition to the scale of the tube.

9 PressureTransmitter (PT1)

- A transducer is used to convert process variable (Pressurein the control tank) to 4-20 mA analog signal that feeds tothe controller.

10 PressureController (PIC1)

- Final control element, the valve reacts proportionally to thecontroller output to control the pressure in the tank.

Microprocessor based controller. This controller receives4-20 mA signal from PT1 transmitter and also output 4-20mA to the I/P positioner on the valve actuator

11 Control Valve(CV1)

- Final control element, the valve reacts proportionally to thecontroller output to control the pressure in the tank.

12 PressureRecorder (PR1)

- The pressure recorder is used to record the processmeasurement PT1 and controller (PIC1) set-point value. Itis very useful for studying the dynamics of the processcontrol loop and for tuning purposes.


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AppendixInstrument Operation Guide


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Appendix A .1: YOKOGAWA Loop Control ler Operation

i. Selection of Operation Panel Group1. When power is turned on, initialization of the controller is carried out. Then, the controller shows the

operation panel group.

2. Press the key while holding down the key, the LCD display panel group is selectedone after another in the following sequence:- Operation Panel Group- Tuning Panel Group- Engineering Panel Group

The controller shows the operation panel group again by pressing the key while holding down the

key.

ii . Selection Operation of the Operation Panel1. Press the key while holding down the key to select the Operation Panel Group, and

an operation panel will be displayed.

2. The operation panel changes from current to next one every time you press the button as shownbelow:

Loop 1 & Loop 2 Trend 1 & Trend 2 Alarm


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iii. Au to /Manual Mode Selection1. Select the Loop 1 panel (as described in Section ii).

2. Press / mode key to select the desired control operation mode. The LED indicator in thekey which corresponds to the selected operation mode key is lit.

: A Mode KeySelects the A (Automatic) control operation mode

: M Mode KeySelects the M (Manual) control operation mode

iv . Change of Output/Manipulated Value (MV)1. Select the (Manual) control mode (as described in Section iii), use the and

keys to operate the output / Manipulated Value (MV) manually.

2. An increase or decrease of the MV value can be accelerated by holding down the key, thenpressing the MV operation key.

: MV Operation Increasing KeyIncreases the MV value

: MV Operation Decreasing KeyDecreases the MV value

v. Change of Set Point/Set Value (SV)1. Use the / key to change the set value (SV) of the control loop. This function is available in

the A (Automatic) and M (Manual) control operation mode.

: SV Operation Increasing KeyIncreases the SV value

: SV Operation Decreasing Key

Decreases the SV value


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vi . Change of PB, I and D Value1. Press the key while holding down the key to display the Tuning Panel Group.

2. Press the push button to observe PID 1 pull-down menu. The PID menus provide detailedinformation regarding the PID function blocks.

3. The and keys (adjacent to the & keys) allow you to step upward ordownward through the menu.

4. The and keys permit changes to the selected line.

5. To change the PID value, first toggle the key until PB1 (Proportional Band Value) is highlighted.PB1 is shown at the top of the display below the PID1 title. The PB1 value can be changed by holding

down the or keys until the desired value is set. Then toggle the key to select TI1and TD1 to change the value of the Integral Action and Derivative Action, respectively.

6. Pressing the button transfers the display to the Tuning Menu Group menu.

7. Finally, press the key while holding down the key again to get back to the OperationPanel.


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vii. Au to Tun ing/Sel f-Tuning Control (STC)1. First, set the control operation mode to (Manual Mode).

2. Press the key while holding down the key to display the Tuning Panel Group.

3. Press the push button to observe STC 1 pull-down menu. The STC menus provide detailedinformation regarding the PID function blocks.

4. Toggle the key until MI 1 (Signal Defection applied to MV Value) is highlighted. MI1 is shown at

the top of the display below the STC 1 title. Change the MI 1 to 20 % by holding down the or

keys. The MI 1 value designated will shift the process variable by about 20 % during the autotuning process.

5. Toggle the key until the STC is highlighted. Set the STC mode to ATSTUP by holding down the

key for 3 seconds.

6. Pressing the button transfers the display to the Tuning Menu Group menu.

7. Then, press the key while holding down the key again to get back to the Operation

Panel. Set the control operation mode to the (Automatic Mode) and now, the automatic tuningbegins.ATSTUP appears on the control status display position of the loop panel. With the automatic tuning, PIDcontrol has not been started yet. After 30 seconds, the controller applies the step variation (MI %)automatically to the manipulated output.

8. When the process variable has been stabilized, the controller will automatically set the manipulatedoutput back to the previous setting.

9. When all setting items are prepared, the mode is automatically set to STC=ON, and the PID control is

started. At the same time, the control status display of the loop panel changes to STC-ON.10. To turn off the STC mode, go to the STC1 pull down menu and set the STC mode to OFF.11. Another way to turn off the STC mode is to change the PB, TI and TD value to any desired value.


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Appendix A .2: YOKOGAWA Char t Recorder Operati on

i. Changing t he Chart Speed1. Hold down theMENU key for 3 seconds to enter the Setting Mode.

2. Press the key to showChart and then press the key.

3. Press the key to select the desired speed. Then press to enter the set value.4. When the Setting com plete screen appears, the new chart speed is applied.5. Press the ESC key to change the chart speed again or hold down the MENU key for 3 seconds to return

to Operation Mode.

ii . Starting/Stopping the Recorder1. Turn on the Power Switch of the recorder.2. Check that the recorder pens and chart paper have been installed properly.3. Press the RCD key to start recording. The status display shows the word RECORD.4. While the recording is in progress, press the RCD key to stop recording. The word RECORD on the

status display clears.


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Appendi x A.3: Temperature Control ler Operation

iii. Changing the Set Point1. Press button to select the set point setting mode.

2. / button to set the desired temperature. The set point is indicated on the SV display(yellow digit LED display)

3. Press button again to register the setting value.

expt. 4 - pressure control

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