EXPERIMENT 8: TEMPERATURE CONTROL
8.1 OBJECTIVE OBJECTIVE OF THE EXPERIMENT EXPERIMENT
(i) To demonstra demonstrate te the charact characteris eristic tic of Proport Proportional ional Only Only Control Control.. (ii) To demonstrate demonstrate the characteris characteristic tic of Proportional Proportional Band Band and Integral Integral Action Action on a temperature temperature process control loop. (iii (iii))
To demon demonst stra rate te the charac characte teri rist stic ic of Prop Propor orti tion onal al Band, Band, Integ Integra rall Acti Action on and Deri Deriva vati tive ve Action on a temperature temperature process control loop.
(iv) (iv)
To demo demonst nstrat rate e the the loop loop tuni tuning ng proce procedur dure e on a tempe temperat rature ure proces process s contr control ol loop loop..
8.2 INTRODUCTION The Air Temperature Control has been designed on how a temperature loop for an exchanger can be controlled using a microprocessor based controller. The control panel is connected to a Distr Distribu ibuted ted Contro Controll System System (DCS) (DCS),, which which can remote remotely ly contro controll the proces process s plant plant using using supervisory control mode (SCADA) or direct digital control mode (DDC). A selector with located at the control panel is used to select between SCADA or DDC mode. In SCADA mode the DCS can monitor and control the process through the process controller and in DDC mode; the DCS can directly control the plant through the Field Control Station.
The Air Temperature Control Module is an air process where 6 bar(g) compressed air is charged into the air receiver tank V-102 and regulated to about 4 bar(g) by the air regulator PCV-102. Air from V-102 flows through the process line into the air heater K-101 where it is heated up to 150 0C and is then discharged to the atmosphere.
8.3 EXPERIMENTAL EQUIPMENT Variou Various s types types of instru instrumen mentat tation ions s are instal installed led in the proces process s line. line. An RTD RTD Temper Temperatu ature re Transmitter, TT-102 monitors the product temperature and feeds the signal to a PID loop TIC102-1 in the process controller. Another thermocouple temperature transmitter TT-101 monitors the surface temperature of the air heater and also feeds the signal to another PID loop TIC-102-2 in the process controller. The output from both PID loops is sent to a signal selector and the selected output is then used to regulate the energy in the air heater via a thryristor controller, TYTY102. This high/low high/low signal signal selection selection is often used in industry industry to protect protect equipment. equipment. Once the
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product temperature TT=102 reaches steady state, hand valve HV 116 is manipulated to simulate load changes for the purpose of observation of the effectiveness of the controller in use.
There is another temperature controller TIC-101 that is used to cut off the electrical supply to the air heater in the event of over-temperature, which is set at 250 0C. This is also to protect heater burnout. An airflow switch FSL-101 also cuts off the electrical supply to the air heater in the event of no airflow. Pressure Relief Valves PSV-101 and PSV-102 are installed to prevent over pressure during the course of the experiments.
Solenoid valves have been installed for the purpose of fault simulation in various sections of the process line. Fault simulation switches have been installed to simulate these faults, which will create errors in the process line.
Table 8.1 Instrument function and capabilities No 1 2
Instrument PID controller
Tag No TIC-102
Temperature
TIC-101
Controller 3 4
Recorder RTD Transmitter
TR-102 TT-102
5
T/C Transmitter
TT-101
6 7
Rotameter Tyyristor
FI-101 TY-102
8
Pressure
PI-101 PI-102 PI-103 TI-101 TI-102
Indicator 9
Temperature
10
Indicator Process Tank
11
Alarm Annunciator
12
Pressure Relief Valve
13
Solenoid Valves
V-102 F-101 FAL-101 TAH-102 PSV-101 PSV-102
HV-101 HV-102
Description Microprocessor based PID controller, heater, temp On/Off controller, turns off heater when heater surface temperature exceeds preset limit Continuous 2 pen chart recorder Signal type PT 100, for product line, 4-20 mA Temperature transmitter for heater element, 4 to 20 mA Air flow rate control and load Controls amount of energy input to the heater Dial gauge pressure indicator for local pressure indication For local temperature indication Receiver tank Air heater Process line detecting alarm low Control tabk temperature alarm high Mechanically activated device, spring loaded normally closed valve. Opens and purges air to atmosphere in case of over pressure in tank Solenoid valves for fault simulation
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Range 0-3000C
0-2000C 0-4000C 0-50 SCMH 0-25 A 0-7 bar 0-10 bar 0-7 bar 0-1000C 0-2000C 250 L 1780C
150 PSI 150 PSI
-
14
Hand Valves
HV-103
HV-109 15
Fault Simulation
No 16
Instrument Air regulator
HS-101 HS-102 HS-103 Tag No PCV-101
17 18
Air Flow Switch Control Panel
FSL-101 -
Switches
Input/Output isolation valve. Determine the direction of airflow and load changes Loss of process air Leakage at heater tank Heater burn out Description Regulates the air supply to the process receiver tank (V102) No flow sensor Mounting/installation of controller, alarm annunciator, recorder, push button, power supply switch and changeover switch between DCS and local control
-
-
Range -
-
8.3.1 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 which will maintain the desired balance. Tuning the controller adjusts it so it can achieve that balance as quickly as possible. This is done when instrument is first put in service and later on a periodic basis as part of preventive maintenance. When tuning remember that each controller is part of a closed loop. All the parts of the loop are interactive, behaviour of other devices in that loop. The controller response must be matched to that of the process. There are several procedures for doing this, some mathematical most using trial and error.
A simple three step method for tuning most three mode controllers follows. Batch contollers and one through processes are special cases discussed after the three mode and two mode controllers) . This three steps procedure is based on a simple test to determine the nature period of oscillation of the process.
Step 1 : Set the integral time of the controller at its maximum and the derivative time at its minimum, thereby providing proportional only control. Then reduce the proportional band until oscillation begins. Measure the period of this oscillation (also called the natural period) as the time between two successive crests or valleys (Figure 8.1).
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Figure 8.1 Period of oscillation with proportional only controller after first tuning step
Step 2 : Set the derivative time at 0.15 times the natural period and the integral time at 0.4 times the natural period. Observe the new period of oscillation there should be a 25 percent decrease(Figure 8.2). If the new period of oscillation is shorter than this reduce the derivative time, if period is longer, increase the integral time.
Figure 8.2 Period of oscillation for correctly tuned PID controller after second tuning step
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Step 3 : Finally the proportional band to achieve the desired degree of damping (the amount of correction to a process upset which when too much or too little shows up as either overshoot or sluggishness respectively)
When adjusting a two mode PI controller a slightly different method should be used since integral mode introduces phase lag that is not counteracted by derivative. The procedure follows:
Step 1 : Set the integral time of the two mode controller at its maximum and the derivative time at its minimum, providing proportional only control just as with the three mode controller. Then reduce the proportional band until oscillation begins and measure this period.
Step 2 : Set the integral time to the natural period. The period of oscillation should increase about 40 percent (ideally 43%). If the period is longer than this, increase the integral time (Figure 8.3)
Figure 8.3 Period of oscillation for correctly tuned PI controller after second tuning step
Step 3 : Finally adjust the desired degree of damping is achieved. Adding integral will always increase the proportional band required for stable control.
Some consideratio must be given to processes with variable dynamic characterisitics. Once through processes such as tubular heat exchangers exhibit a natural period that varies inversely with flow. In such situations. One combination of controller settings cannot be ideal for all flow rates. Integral time should be set according to the lowest anticipated flow rate and the derivative of time accordingly to the highest.
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Some batch controllers because of their mechanical arrangement will become unstable if equal values of integral and derivative time are used. Always keep their integral time at least twice the derivative time.
8.4 PROCEDURE Table 8.1 Start up procedure STEP
ACTION
REMARKS
Start compressor and wait for sufficient air pressure to build up in receiver tank, V-102, recommended air supply pressure 1
at 6 bar
2
Turn on the instrument main supply
3
Check recorder is working and pens have ink
4
5
Use selector switch to select TT101,TT102 or auto selector control.
Set controller TIC102-1 to Manual Mode with setpoint 1000C
Check and make sure hand valves positions are as follows:Close HV115 Close HV 117 6
Open HV 111
Hand valves to be
Open HV 112
Open/Closed Fully.
Open HV 113 Open HV 114 Leave alone HV 116 For the control studies we will only experiment with TIC 102-1 that control the product temperature TT 102. TIC 102-2 controls the heater surface temperature is only used for the high/low auto selector controls. Air temperature can be measured using RTD and thermocouple
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as per given experiment. This requires the observation of the temperature readings from TT 101 and TT 102 during all the experiment runs following the start up.
Table 8.2 Closed Loop Proportional Control STEP
ACTION
REMARKS
Allow pressure to build in V-102 to 6 bar(g) then set FI-101 to 1
35 m3/nr by regulating hand valve HV 116
2
Set TIC-102-1 to Manual Mode with setpoint at 100 0C
Enter the following values: 3
The values will set the
PB = 100
controller to Proportional
I = 1000
Only Control mode
D=0
4
5
6
7
Gradually adjust the output so that the product temperature TT 102 matches the setpoint.
Put the control loop into Auto Mode
Simulate a load changes by opening HV 116 so that FI 101 reads for 50m3/hr for approximately 60 seconds
Restore HV 116 to its original position and observe the measurement for about 5 minutes
8
Change the setpoint to about 1500C and observe the
9
response of the system for another 5 minutes. Repeat steps 3 through 8 for the following PB values. Retain the previous I and D values.
PB = 10 and PB = 2
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Take note the offset values for each of the PB values. Table 8.3 Closed Loop PI control STEP
ACTION
REMARKS
Allow pressure to build in V-102 to 6 bar(g) then set FI-101 to 1
35 m3/nr by regulating hand valve HV 116
2
Set TIC-102-1 to Manual Mode with setpoint at 100 0C
Enter the following values: 3
The values will set the
PB = 50
controller to Proportional
I = 180
+ Integral Control mode
D=0
4
5
6
7
8
Gradually adjust the output so that the product temperature TT 102 matches the setpoint.
Put the control loop into Auto Mode
Simulate a load changes by opening HV 116 so that FI 101 reads for 50m3/hr for approximately 60 seconds
Restore HV 116 to its original position and observe the measurement for about 5 minutes
Change the setpoint to about 1500C and observe the response of the system for another 5 minutes. Repeat steps 3 through 8 for the following I values. Retain the previous P and D values.
9 I = 60 s and I = 10 s
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Table 8.4 Proportional and Integral and Derivative Control STEP
ACTION
REMARKS
Allow pressure to build in V-102 to 6 bar(g) then set FI-101 to 1
35 m3/nr by regulating hand valve HV 116
2
Set TIC-102-1 to Manual Mode with setpoint at 100 0C
Enter the following values: 3
PB = 100
The values will set the
I = 180
controller to PID mode
D = 60
4
5
6
7
8
Gradually adjust the output so that the product temperature TT 102 matches the setpoint.
Put the control loop into Auto Mode
Simulate a load changes by opening HV 116 so that FI 101 reads for 50m3/hr for approximately 60 seconds
Restore HV 116 to its original position and observe the measurement for about 5 minutes
Change the setpoint to about 1500C and observe the response of the system for another 5 minutes. Repeat steps 3 through 8 for the following D values. Retain the previous PB and I values.
9 D = 30 and D = 10
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Table 8.7 PID loop Tuning STEP
ACTION
REMARKS
Allow pressure to build in V-102 to 6 bar(g) then set FI-101 to 1
35 m3/nr by regulating hand valve HV 116
2
Set TIC-102-1 to Manual Mode with setpoint at 100 0C
Enter the following values: 3
The values will set the
PB = 100
controller to Proportional
I = 1000
Only Control mode
D=0
4
5
6
7
Gradually adjust the output so that the product temperature TT 102 matches the setpoint.
Put the control loop into Auto Mode
Retain the I and D values. Slowly decrease PB until the measurement PV oscillate about the setpoint.
Repeat steps 4 through 6 for the following PB values. PB = 50 ,PB = 10 and PB = 2
Determine the natural period using the following method Natural
period , T
8
=
D Trend Speed
×
60 min
where D = distance in mm between successive crests or valleys Set the integral time to natural period, T that was calculated 9
and repeat step 4 to 7. The period of oscillation should decrease by 40 %. If the period is longer than this increase the integral time.
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10
Finally adjust the PB until the desired degree of damping is achieved.
During the experiment, various faults can be simulated by the unit, to create errors similar to those that can be experienced in the industry. This can be done with switches that have been installed for this purpose. It is required to detect the errors, its location and solve the error.
Switch HS-101 : Loss of instrument Air supply Switch HS-102 : Leakage at Heater Tank (K-101) Switch HS-103 : Heater Burnout
8.5 REFERENCES Seborg D.E., T.F. Edgar and D.A. Melliechamp, ‘Process Dynamics and Control’, John Wiley and Sons, New York, 1989, pp 116-118.
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