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1.0 TITLE Multivariable Control System (Temperature-Flow Process Control) 2.0 INTRODUCTION

2.1 Background Multivariable control system is a system in which the variable interacts strongly. This kind of system must have more than one input and one output. A disturbance in any input causes a change of response from at least one output. Besides that, this kind of system have as many inputs and outputs as needed to control the process. A system with an equal number of inputs and outputs is said to be square. A disturbance in any variable can cause a change in response in any output in its signal path, depends on the system design, these paths can be direct or indirect (Denizen, 2017). In most control system, a particular input disturbance causes one output to respond by a larger percentage than the other outputs. The response of the other output is called interaction. Interaction is often a result of system design and cannot be avoided. However the system must either correct or compensate for interaction (Denizen, 2017).

Figure 1: Block Diagram of Multivariable Control System

The liquid temperature and flow process control (cascade control) is a control algorithm in which the output of one control loop provides the target for another loop (Delta, 2018). In singleloop control, the controller’s set point is set by an operator and its output drives a final control element. If there are two or more controllers of which one controller’s output drives the set point of another controller. For instance, multivariable control system. The controller driving the set point is called the primary, outer or master controller while the controller receiving the set point is called the secondary, inner or slave controller (Jacques, 2016).

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The shell and tube heat exchanger of Figure 2 is a typical candidate for cascade control. The primary process output is the temperature of the tube side effluent stream. The re are two possible secondary variables, the flow rate of steam into the exchanger and the steam pressure in the exchanger (VanDoren, 2014). The steam flow rate affects the effluent temperature through its effect on the steam pressure in the exchanger. The steam pressure in the exchanger affects the effluent temperature by its effect on the condensation temperature of the steam. Thus, the steam flow rate or steam pressure in the exchanger can be used as the secondary output in a cascade control system. The disturbances that affect the effluent temperature determine the choices of which to use.

Figure 2: Shell and Tube Heat Exchanger

One of the advantages of cascade control is improving the performance of control over single-loop control whenever there are disturbances affect a measurable intermediate or secondary process output that directly affects the primary process output that need to be controlled. In the first case, a cascade control system can limit the effect of the disturbances entering the secondary variable on the primary output. In the second case, a cascade control system can limit the effect of actuator secondary process gain variations on the control system performance (Delta, 2018). Such gain variations usually arise from changes in operating point due to the set point changes or sustained disturbances. Moreover, cascade control allows inner loop to handle non-linear valve and other final control element problems. According to Delta (2018), cascade control allows operator to directly control inner loop during certain modes of operation and controller to respond quickly to the faster inner loop.

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2.2 Problem Statement The controller tuning problem gives an effect on closed-loop stability and overall process control. It difficult to find the simple and easy implement table approach for tune the parameters. It also difficult to achieved the optimum parameter in controller tuning with method to minimize the largest error and settling time. To develop a good performance controller tuning method is also hard. 2.3 Objective The purpose of this experiment is: i. To interpret the Piping & Instrument Diagram (P&ID). ii. To understand the liquid temperature and flow process control (cascade control). iii. To understand the behavior of liquid temperature and flow cascade process and plant operation. iv. To understand and demonstrate the effect of load disturbance and set point disturbance on liquid __

___temperature and flow cascade process.

2.4 Scope of Experiment To achieve the objective of this research, there are several scopes that have been identified: i.

Identify the Piping & Instrument Diagram (P&ID).

ii.

The requirements of the liquid temperature and flow process control (cascade control): •

Inner loop system dynamics must be significantly faster (such as four times faster) than the outer loop system dynamics. If the inner loop is not faster than the outer loop, then the cascade will not offer any significant improvement in the system control.

iii.

•

Inner loop must have influence over the outer loop.

•

Inner loop must be measured and controllable.

The behavior of the plant operation by using liquid temperature and flow process control (cascade control). Once both loops of the liquid temperature and flow process control (cascade control) have been set up and tuned, the system can be controlled by sending motion commands to the outer loop axis. There are a few items to consider: •

Control Order The inner loop must always be in closed-loop control when the outer loop is controlling the system. When manually controlling the inner loop, the outer loop should be in open loop control.

•

Starting Up the System 3

Every time the system starts up, send the commands will be sent to the inner loop axis. Then, the outer loop axis can be controlled. 3.0 MATERIALS AND METHODS

3.1 Materials

Figure 3: Multivariable Control System (Model: MULTIVARIABLE PPT-03-C07-PH)

a) TK-701 Preheated Water Tank b) TK-702 Cold Water Storage Tank c) HE-704 Shell And Tube Heat Exchange d) TK-703 Level Tank / Pressure Vessel e) 704 CV 001 Proportional Control Valve f) C-706 Condensing Unit g) 701 HTR 001 Electrical Heater h) 701 P01 / 702 P01 Centrifugal Pump i)

30L302 303 TT Temperature Sensor RTD

j)

301FT001 Vortex Flowmeter 4

3.2 Methods

3.2.1

Variables Involves In The Temperature-Flow Process Control System

A cascade control system reacts to physical phenomena shown in blue and the process data shown in green.

Figure 4: The Block Diagram of Temperature-Flow Process Control (Cascade Control) System

Figure 5: Temperature-Flow Process Control (Cascade Control) Block Diagram

Manipulated Variable (MV) is one that can be adjusted by the control system used to maintain the controlled variable at its desired value. In Cascade Controller: Flow rate that are controlled by pneumatic valve (L/s)

Controlled Variable (CV) is the variable that quantify the performance or quality of the final product, _ which also called output variables (set point). In Cascade Controller: Temperature of the outlet stream (℃) Disturbance Variable (DV) is an input variable that can cause the controlled variables to deviate from their respective set point. However, DV cannot be manipulated. In Cascade Controller: Feed flow rate (L/min) 5

3.2.2

Procedures of The Experiment

(A) Start-up Procedure

The control panel was turned on.

The water level in Tank TK-701 and TK 702 was ensured at least 80% full or at the overflow line.

The valve positions was set according to Table 1.

At the HMI, "LEVEL" was selected and Level Manual Valve 704CV-002 was changed to 100% open.

A flash drive was inserted into the available USB Port.

START button was pressed on the HOME screen.

At the HMI, the "CASCADE TEMP-FLOW" was selected.

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Hand Valve

Position

Check List (√ )

701 HV-001 (WS)

Close

√

701 HV-002 (R)

Half-open

√

701 HV-003 (D)

Close

√

702 HV-001 (WS)

Close

√

702 HV-002 (R)

Half-open

√

702 HV-003 (D)

Close

√

703HV-001

Close

√

703HV-002

Open

√

703HV-003

Open

√

703HV-004 (D)

Open

√

703HV-005

Open

√

703HV-006

Close

√

704HV-001(D)

Close

√

704HV-002

Open

√

704HV-003

Open

√

704HV-004

Close

√

Indication: WS: Water Supply | D: Drain | R: Recirculation Line Table 1: Hand Valve Position for Multivariable Control System

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(B) Understanding the Process Behavior (Temperature Flow Process Control

The process plant was set up with power, air and water like in ACTIVITY C-6B.

At the HMI, the "CASCADE TEMP-FLOW" was selected.

The Chiller, 705CH-001 was turned on. Chiller was set to below 19℃.

In the trending graph screen, the data logging START icon was clicked to download the data to the drive.

The 704CV-002 was set to 100% open. The Heater, 701HTR-001 was turned on and was set to 55℃.

The graph button was clicked. The "AUTO" button was clicked to put the controller (701TIC-001) into auto mode.

The set point (SP) was set up to 31℃. The time was recorded during the experiment.

The SLAVE PID was set according to the Table 2. (Note: Optimum slave PID value could be achieved through experiment in Liquid Flow Single Loop)

Parameter

Value

Gain, Kc

0.66

Reset, 𝝉i

1

Rate, 𝝉D Table 2: Slave PID Parameter 8

The pumps 701P-01 and 702P-01were turned on.

In the trending graph screen, the data logging STOP icon was clicked to download the data to the drive. The value of PV for 60seconds was noted and written in Table 4.

Step 10 was repeated by entering the MASTER PID value and different Set Point (SP) value according to Table 3.

Parameter

P Control

PI Control

PID Control

Trial 1

Trial 2

Trial 1

Trial 2

Trial 1

Trial 2

Kc

4.3

6.3

6.3

6.3

6.3

6.3

Tt

0

0

5.0

11.0

11.0

11.0

tD

0

0

0

0

1.1

1.1

Table 3: PID Values for Master Controller

The steps above for each setting in the table were repeated for three trials by using P, PI and PID Control.

The results were recorded and the graphs were ploted.

Based on each graph, the results were discussed based on: i. The process response pattern. ii. The effect of process controllability in each control.

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(C) Shut-down Procedure

The heater 701HTR-001 was switched off.

Pump 701P-01 and 702P-01 were switched off.

The controller 704TIC-001 was put to manual mode.

The chiller 705CH-001 was turned off.

The air supply was turned off.

The panel and plant switch were turned off.

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Temperature (°C)

4.1 Result

Graph of Temperature against Time

Time (s)

Graph 1: Process Respond Curve for Slave PID

11:19:49 11:19:52 11:19:55 11:19:58 11:20:01 11:20:04 11:20:07 11:20:10 11:20:13 11:20:16 11:20:19 11:20:22 11:20:25 11:20:28 11:20:31 11:20:34 11:20:37 11:20:40 11:20:43 11:20:46

4.0 RESULT AND DISCUSSION

28

27

26

25

24

23

22

21 11:19:40 11:19:43 11:19:46

Process Variable

Set Point

11

Temperature(°C) 32

31

30

29

28

27

26

25

24

Graph of Temperature againts Time

Time (s)

Graph 2: Process Respond Curve for Master P Controller (First Trial)

11:21:40 11:21:58 11:22:16 11:22:34 11:22:52 11:23:10 11:23:28 11:23:46 11:24:04 11:24:22 11:24:40 11:24:58 11:25:16 11:25:34 11:25:52 11:26:10 11:26:28 11:26:46 11:27:04 11:27:22 11:27:40 11:27:58 11:28:16 11:28:34 11:28:52 11:29:10 11:29:28

process variable

Set Point

12

Temperature (°C) 36

35

34

33

32

31

30

29

28

Graph of Temperature againt Time

Time (s)

Graph 3: Process Respond Curve for Master P Controller (Second Trial)

11:30:10 11:30:36 11:31:03 11:31:29 11:31:55 11:32:21 11:32:46 11:33:13 11:33:38 11:34:05 11:34:31 11:34:57 11:35:23 11:35:49 11:36:15 11:36:41 11:37:07 11:37:33 11:37:59 11:38:25 11:38:51 11:39:16 11:39:43 11:40:08 11:40:35 11:41:01 11:41:27 11:41:53

Process Variable

Set Point

13

Temperature (°C) 40

39

38

37

36

35

34

33 11:43:36 11:43:39 11:43:42 11:43:45 11:43:48 11:43:51

Graph of Temperature against Time

Time (s)

Graph 4: Process Respond Curve for Master PI Controller (First Trial)

11:43:15 11:43:18 11:43:21 11:43:24 11:43:27 11:43:30 11:43:33

11:43:54 11:43:57 11:44:00 11:44:03 11:44:06 11:44:09 11:44:12 11:44:15 11:44:18 11:44:21

Process Variable

Set Point

14

Temperature (°C) 45

44

43

42

41

40

39

38

37

36

Graph of Temperature againts Time

Time (s)

Graph 5: Process Respond Curve for Master PI Controller (Second Trial)

11:44:50 11:45:01 11:45:12 11:45:23 11:45:34 11:45:45 11:45:56 11:46:07 11:46:18 11:46:29 11:46:40 11:46:51 11:47:02 11:47:13 11:47:24 11:47:35 11:47:46 11:47:57 11:48:08 11:48:19 11:48:30 11:48:41 11:48:52 11:49:03 11:49:14 11:49:25 11:49:36 11:49:47

Process Variable

Set Point

15

Temperature (°C) 49

48

47

46

45

44

43

42

41

Graph of Temperature against Time

Time (s)

Graph 6: Process Respond Curve for Master PID Controller (First Trial)

11:50:00 11:50:13 11:50:26 11:50:39 11:50:52 11:51:05 11:51:18 11:51:31 11:51:44 11:51:57 11:52:10 11:52:23 11:52:36 11:52:49 11:53:02 11:53:15 11:53:28 11:53:41 11:53:54 11:54:07 11:54:20 11:54:33 11:54:46 11:54:59 11:55:12 11:55:25 11:55:38 11:55:51

Process Variable

Set Point

16

Temperature (°C) 53

52

51

50

49

48

47

46

Graph of Temperature against Time

Time (s)

Graph 7: Process Respond Curve for Master PID Controller (Second Trial)

11:56:05 11:56:24 11:56:43 11:57:02 11:57:21 11:57:40 11:57:59 11:58:18 11:58:37 11:58:56 11:59:15 11:59:34 11:59:53 12:00:12 12:00:31 12:00:50 12:01:09 12:01:28 12:01:47 12:02:06 12:02:26 12:02:45 12:03:04 12:03:23 12:03:42 12:04:01 12:04:20 12:04:39

Process Variable

Set Point

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4.2 Discussion

This experiment consists of two parts and involves two types of controller. The primary controller (master) and the secondary controller (slave). In this experiment, the primary controller is the temperature controller which measures the water temperature in the tank and asks the secondary controller for more or less heat. The secondary controller is the flow controller which measures and maintains steam flow rate directly. The first part of this experiment involves the process behaviour for slave PID where the PV value on the 704 TT 002 was observed. The optimum slave PID value could be achieved through the experiment in the liquid flow single loop. The parameter involves in this part were gain, 𝐾𝑐 , reset 𝜏𝐼 and data, 𝜏𝐷 where the values for each parameters are 0.66, 1, and 0 respectively. This experiment starts at 11:19:00 AM with setpoint temperature of 23.0℃. The value of PV for 60 seconds was observed. For this part, every interval of 10 seconds, the temperature change was recorded. Based on Graph 1, we can see that for the first 10 seconds, the PV increases from the set point to 24.08℃ and the value kept increasing until the first 30 seconds with a value of 24.16 ℃. However, by the 40 seconds to 60 seconds, the PV decrease to 24.11 ℃. In addition, from the graph, we can see that the set point which is the temperature desired in the water tank remain constant for the first 8 seconds and started to increases to 27 ℃ and remains unchanged . However, the PV curve still could not achieve the set point even though there are several fluctuation in the PV values. This is due to only the slave controller involves in this part, because slave controller job is only to control the short term flow fluctuation Second part of this experiment involves the master and the slave controller where a primary or a master controller will generates a control effort which serves as the set points for a secondary or a slave controller. The controller then uses the actuator in order to apply its control effort directly to the second process. The valve used by the steam flow controller to maintain the steam flows will acts as the actuator serves directly on the secondary process and indirectly on the primary process. The second part in this experiment involves 3 types of controller which is P,PI and PID controller . In this part, two trials were conducted for each controller and for each set point change,

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the temperature was increased by 4℃ and the value for Slave controller was fixed to 0.66 for gain, 𝐾𝑐 , and 1 and 0 for reset, 𝜏𝑡 and data, 𝜏𝐷 respectively. In addition, when activating the 1 component for master and slave, controllers, the system have a tendency to oscillate. Against this background, master controller uses the PID structure whereas the slave uses the PD structure. A control deviation will always arise for slave controller. Meanwhile, the master controller will ensure the adjustment of the actual value. In the 1st trial for the P controller, the parameters value was set to 4.3 for gain, 𝐾𝑐 , and 0 for both reset, 𝜏𝐼 and data, 𝜏𝐷 . The 1st trial for P controller started at 11:21:40 AM with a set point of 31.00 ℃. Based on Graph 2, with a starting temperature of 27℃, we can see that before trying to achieve the set point temperature, there is a bit of fluctuation in the PV curve between 11:21:40 AM and 11:22:08 AM. In the first 14 seconds, it is observed that the PV curve shows an increase from 27 ℃ to 27.102 ℃. Then, the PV curve declined and the temperature dropped to 26.79℃ . However the PV curve started to increase aga...