CHEMICAL PROCESS DYNAMICS AND CONTROL

Laboratory Report

RATIO CONTROL

Group A7

NUR MADIHAH BINTI YASER 14490

NOR AMALIA HUSSIN 13291

MUHAMMAD ASHRAF BIN ZAINAL 13205

MARHAINA ISMAIL 13113

Date of submission : 10/04/2012

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TABLE OF CONTENTS

Title Page

1.0 Abstract 1

2.0 Objective 1

3.0 Introduction 1

4.0 Theory 2

5.0 Methodology 3-4

5.1 Experimental Steps

5.1.1 Experiment A: PID Flow Control

5.1.2 Experiment B: PID Control Loop Tuning

5.1.3 Experiment C: Ratio Control

6.0 Results 5-8

6.1 Experiment A: PID Flow Control

6.2 Experiment B: PID Control Loop Tuning

6.3 Experiment C: Ratio Control

6.4 Calculations

7.0 Discussion 9-10

8.0 Conclusion 11

9.0 Error and Recommendations 11

10.0 Appendices 11

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1.0 ABSTRACT In this experiment, Ratio Control, the purpose is to study the characteristics of Proportional Band, Integral Action and Derivative Action on a flow process control loop and also ratio control. The first experiment is about to see the effect on a single loop for PID flow control by changing the derivative time. The derivative time is changed to 1s, 3s and 5s. From this experiment, it is observed that as the derivative action values increase, the process reaches stability faster. It is because the derivative action in a PID controller functions to ensure that the controller output proportionate the rate of change or error. In the second experiment, the Proportional Band values are changed from PB = 1000, 50 and 10 to see the effect on a single loop for PID control loop tuning. The graphs showed that when the values of PB = 1000, the process reaches stability upon some time. At PB value of 50 we observe the graph to be critically stable (sinusoidal waves).whereas at PB value of 10 shows process instability. The optimum value for Proportional Band is between 1000-50. The last experiment is to study the characteristics of ratio control. From there, we observed that the ratio of loop 2 to loop 1 is 2:1 and it shows that by setting the value for one loop will affect the other loop by their ratio.

2.0 INTRODUCTION

Ratio control is used to ensure that two or more process variables such as material flows are kept at the same ratio even if they are changing in value. There are some examples of ratio control such as mixing and blending two liquids, injecting modifiers and pigments into resins before molding or extrusion, adjusting heat input in proportion to material flow and burning air. However, basically, we will use PID controller as a generic controller because it is widely used in industrial control system

3.0 THEORY

There are three separate constant parameters in the PID controller calculation which are the proportional (P), the integral (I), and the derivative values (I). Each parameter has its own roles based on the change of the current rate. Particular operating situation will have particular properties. For each percent change in the variable controlled, the proportional band will give how much corrective signal the controller can produce for particular properties. The amounts of movements that will produce at the control valve can be detected by the controller’s output signal.

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The function of integral action is to continue the proportional controller’s initial corrective signal until there is no difference between the process value (PV) and the desired value (Set point). It is expressed in “Minutes per Repeat”.

Derivative action is the time that the proportional added with the derivative that will take to reach certain amount of output. In advance the proportional action alone would produce the same output. Derivative action is expressed in “Minutes”.

These three terms are summed up to give the output for the PID controller.

4.0 METHODOLOGY

Ratio control is used when the flow rate of two or more streams must be held on proportional or must be kept with the desired/same ratio even if the value is changing. For the first and second experiments, we are about to carry out several evaluation on proportional, integral and derivative effect on a single loop by stimulate the values of D = 1s, 3s and 5s for PID flow control and values of PB = 1000, 500, 200, 100, 50 and 10 for PID control loop tuning. For the third part of this experiment, we are playing with two loops, loop 1 and loop 2 in order to imitate the ratio control effect. The experiment consists of 3 parts:

1. To see the changes or dynamic response when the values of D is changing while other values are kept constant.

D values: 1s, 3s, 5s

2. To see the changes or dynamic response when the values of PB is changing while other values are kept constant.

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PB values: 1000, 500, 200, 100, 50, 10

3. To observe the changes in ratio control when the value of PB for loop 2 (manual mode) is stimulate, and to see the effect on loop 1 (auto mode) when the value is changed.

4.1 Experimental Steps4.1.1 Experiment A: PID Flow Control

1. The steps of start-up procedure is followed before starting the experiment on PID flow control, PID control loop tuning and ratio control.

2. The values of PB = 200, I = 6s and D = 1s are entered.

3. The flow rate set point is set to 50 LPM and the control valve output is adjusted so that the value of set point matches the set point of 50 LPM flow.

4. After setup the values, the recorder is turned on and the control loop is changed from manual to auto mode.

5. In order to stimulate the load changes, HV537 is closed for 3s to disturb the process and then it is returned to its original place.

6. The recorder is turned off once the measurement stabilized and then control loop is put back to manual mode.

7. The recorder is turned on and the control loop is changed back to auto mode.

8. The set point is changed to 75 LPM. Once the measurement is stabilized, the recorder is turned off and control loop is put into manual mode.

9. Steps 2 to 10 are repeated with different values of D = 3s and 5s while PB and I values are maintained.

4.1.2 Experiment B: PID Control Loop Tuning

1. The control loop is put into manual mode and the output is adjusted so that the temperature matches the set point of 50 LPM.

2. The following values of PB = 1000, I = 1000s and D = 0s are entered.

3. The recorder is turned on and the control loop is put into auto mode.

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4. In order to stimulate the load changes, HV537 is closed for 3s to disturb the process and then it is returned to its original place.

5. Steps 2 to 4 are repeated with different values of PB = 1000, 500, 200, 100, 50 and 10.

4.1.3 Experiment C: Ratio Control

1. Control loop is put into manual mode and the values of PB = 200, I = 6s and D = 1s are set.

2. The selector switch is put to (2) to select the ratio control mode.

3. Pump P520 is switched on and FT520 is adjusted to about 40 LPM (this is Loop 2) using HV 533.

4. The recorder is turned on. The control loop 1 is put into auto mode.

5. A change is stimulated by adjusting FT520 to 50 LPM using HV 533.

5.0 RESULT

Experiment A: PID Flow Control

The first experiment was carried out by manipulating the TD values while maintaining the PB and TI values. The following shows the graph of flow responses for derivative time of 50 LPM and 75 LPM set pints. Values that are maintained, PB = 200, I=6s, D= 1s

Figure 1: TD = 1s

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50 LPM

75 LPM

Figure 2: TD = 3s

Figure 3: TD = 5s

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50 LPM

75 LPM

50 LPM

75 LPM

Experiment B: PID control loop tuning This experiment was carried out by manipulating the PB values while maintaining the TD and TI values. TD = 0s and TI = 1000s

Figure 4: PB = 1000, 500 and 200

Figure 5: PB = 100 and 50. The system can be considered as critically stable. At PB = 10, it is unstable

Experiment C: Ratio Control This experiment was carried out by setting the flow rate to 40 LPM. Then, it was stimulated by adjusting the flow rate to 50 LPM.

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PB = 100

PB = 50

PB = 10

Figure 6 : Ratio of 2:1 control system

6.0 CALCULATION

Natural period is calculated using the formula:

Natural period, T = 60 min

Where D = distance in mm between successive crests or valleysSample of calculation: For PB = 50 s, D = 3 mm, Trend Speed = 1200 mm/h (from graph),Natural period, T = 4 mm / 1200 mm/h 60 min/h

= 0.199998 min × 60 s/min = 12 sSteps were repeated for the measurement that oscillates about the set point. From the graph, we can see that graph of PB = 50 and 10 oscillated between the set point. Values for the natural period are shown below:

Proportional band, PB D (mm) Natural period, T (s)50 4 1210 3 9

Table 1: The natural period calculated for PID control loop tuning experiment.

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7.0 DISCUSSION

Basically, a control loop consists of a sensor, transmitter, controller and final control element. A sensor is to measure the measured value. Then, transmitter will pass the signal which is the value measured by sensor to the controller. The controller will decide the specific action to be taken once the value obtained by sensor and transmitted by transmitter is compared with the set point. After that, the controller will then tell the final control element such as the valve to take appropriate actions either to open the valve or vice versa. This mechanism can also be explained by the diagram shown below:

Figure 7: The Mechanism of a Control Loop

In the first experiment, the objective is to determine the effect of the varied Derivative Action on the process stability. Process Stability is reached when the variable tallies with the prior set value. The Proportional Band and Integral Action are set to constant value of 200 and 6s respectively with only varying Derivative Action at 1s, 3s and 5s. The flow rate is originally set to 50 LPM. In order to determine the effect of derivative action on the process stability, a disturbance load is introduced. In this experiment, two forms of disturbances are used, which are closing the valve and also varying the set point. From the observation, the process reaches stability faster at higher Derivative Action values. This is due to the derivative action in a PID controller functions to ensure that the controller output proportionate the rate of change or error. It is useful when sudden

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changes in measured variable occur. This explains why with higher derivative action value, the graph reaches stability faster upon the introduction of disturbances.

In the second experiment, the Integral Action is set to 1000s and Derivative Action is set to 0s. By setting these values, the Proportional Band is activated in the controller. Initially, the Proportional Band is set to 1000, decreases to 500, 200, 100, 50 and 10. Between the values of PB = 1000-100, the process reaches stability upon some time. At PB value of 50, the graph is shown to be critically stable (sinusoidal waves). While, the PB value of 10 shows the process instability. Thus, the optimum value for Proportional Band is between 1000-50. Proportional Band in a PID controller acts to compare the set value with the measured value. By doing this, it will adjust the opening or closing of the valve for the flow rate to reach its set point.

In the third experiment, the objective is to demonstrate the characteristics of ratio control. Ratio Control is controlling two processes simultaneously. In other words, in this type of control, the variable in one loop is determined by the value of the other loop. Firstly, PID controller is set to PB = 200, I = 6s and D = 1s. Then, the flow rate is introduced from another loop. Setting this flow rate to 40 LPM, it is shown that the flow rate of another loop is 20 LPM. At one loop of 50 LPM, the other loop is 25 LPM. Thus, it can be conclude that the ratio of loop 2 to loop 1 is 2:1. From this experiment, it is clear as to how this ratio control system operates. Setting the value for one loop affects the other loop by their ratio.

8.0 CONCLUSION

After completing this experiment,we come to conclusion which are process stability depends on Derivative Action .As the value of Derivative Action increases, the process reaches stability faster. This is because the derivative action in a PID controller is function to ensure that the controller output proportionate the rate of change or error. Besides that,we found that the optimum value for Proportional Band is between 1000-50. Proportional Band in a PID controller acts to compare the set value with the measured value. Last but not least,the ratio of loop 2 to loop 1 is 2:1. In this experiment, we can say that setting the value for one loop affects the other loop by their ratio.

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9.0 ERROR AND RECOMMENDATIONS

1) When closing the valve to stimulate the readings, the person in charge may be accidentally open it back.Recommendation : Make sure the person is always alert.

2) The effect of integral action on the process stability cannot be determined.Recommendation : Develop a way where the students can also observe and learn about the integral part of control.

3) The time taken to close the valve is not the same. There is no stopwatch provided. Students just approximate the three seconds by using their watch.Recommendation : Provide a stopwatch.

10. APPENDICES

1) Seborg D.E., T.F. Edgar and D.A. Mellchamp, ‘Process Dynamics and Control’, John Wiley and Sons, 2nd edition, New York, 2004, pp 116-118.

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