OBJECTIVE

The objective of the experiment Flow Ratio Plant Control was:

To identify the major components of the flow ratio process control system

To perform start up procedures systematically To study single loop flow control using PID controller To study flow ratio control using linear PID controller

SUMMARY

The purpose of the experiment Flow Ratio Plant Control was to identify the major

components of the flow ratio process control system, to start up the process systematically, to

compare flow measurements by three different flow meters, to study single loop flow control

using a PID controller and to study ratio flow control using linear PID controller. The

experiment was start with the involved valves were opened and closed beforehand. Then, the

main switch on the control panel was switched on. Next, the PID Single Loop Flow Control

(Normal Operation) was tested on, followed by the Set Point Step Test (Normal Operation).

In addition to that, the PID Controller Tuning (Normal Operation) and the Flow Ratio

Control, Linear PID (Normal Operation) were then carried out. Throughout the experiment,

when two set point trials conducted which were the first trial set point was set with (PB =

100% and TI1 = 5 secs) while the second trial set point with (PB = 150% and TI1 = 10 secs),

it can deduce that the larger the proportional band, the more stable the control, less oscillation

but the greater the offset and vice versa. Therefore, the second trials have satisfactory

response than the first trial second because of the increases of PB and TI percentage value in

the second trial. Meanwhile, when in the cascade mode, the flow was maintained at a specific

value .Next, for PID Controller tuning test, the Plant ratio of the system when the instrument

ratio was equal to 1 was 0.9626 m3 where the plant ratio was equivalent to the equipment

ratio. The test which was involved with one WF pump and two WF pump showed that the

system was successful in controlling the both one and two WF pump as the plant ratio was

equivalent to the equipment ratio so the PV variable was equal to SV variable for both one

and two WF pump . Lastly, when the instrument ratio, R=1.8 were conducted for one WF

pump, the plant ratio was equivalent to the equipment ratio while when involving two WF

pump, the plant ratio was 1.0165 which was lower than the equipment ratio. The PV variable

was equal to SV at one WF pump while for two WF pump, the PV was not equal to SV.

There were few possible errors that can occur during the experiment. Firstly, the instrument

might have a leakage at the pipelines which affect the readings of the flow ratio. Besides, the

configuration of the valves has been wrong that leads to error results. The valve also might

have error that leads to different results recorded. Next, the parameters was not stabilized

when the readings were recorded which could lead to an abnormal trend of results. Other than

that, the instrument was not at the optimum performances. Thus, the ideal expected results

could not be achieved.

INTRODUCTION

Basically, this experiment was about the flow ratio plant control by using equipment model of WF922. For this type of model, it uses water to stimulate a liquid phase flow process. In terms of flow measuring principles, it uses three different types which are differential pressure measurement across by an orifice, measurement by variable area flow meter and measurement by von Karman vortex shedding principles. Next, the flow control then controlled by single loop PID controller at different set points. A flow ratio then is applied where the wild stream flow rate is measured and the other stream is controlled so as to maintain a constant ratio of their flow rates.

This experiment also had been divided into some parts which were first, the experiment that used PID single loop flow control in normal operation, secondly was the set point step test also in normal operation, PID controller tuning (normal operation) and flow ratio control, linear PID (normal operation). For flow ratio control, linear PID, there were four tests had been investigated which were for test 1, which was by using one WF pump with instrument ratio, R=1, test 2 which was by using two pumps with instrument ratio, R=1, test 3 which was by using one WF pump with instrument ratio, R= 1.8 and lastly test 4 which was by using two WF pumps with instrument ratio, R=1.8[5].

THEORY

The theory involved the experiment was Ratio Plant Control and Cascade Mode. The

simplest cascade control scheme involves two control loops that use two measurement signals

to control one primary variable. In such a control system, the output of the primary controller

determines the set point for the secondary controller. The output of the secondary controller

is used to adjust the control variable. Generally, the secondary controller changes quickly

while the primary controller changes slowly. Once cascade control is implemented,

disturbances from rapid changes of the secondary controller will not affect the primary

controller. Cascade control gives a much better performance because the disturbance in the

flow is quickly corrected. [4]

Usually, the process design and operations often were done by keeping a certain ratio two or more flow rates. One of the flows in a ratio- control scenario, sometimes called the master flow or wild flow, is set according to an external objective like production rate. The ratio controller manipulates the other flow to maintain the desired ratio between the two flows [1]. Upon this experiment, PID controller had been used to study the single loop flow control as well as flow ratio control. The flow controlled by the ratio controller then is called the controlled flow [2]. When ratio control is applied, one process input, the dependent input, is proportioned to the other process input, known as the independent input. The independent input may be a process measurement or its set point. The proportion that is to be maintained between the inputs is known as the ratio. For example, a ratio of 1:1 would specify that the two inputs are to be maintained in the same proportion. As the value of the independent input changes, through ratio control the other process input is changed to maintain the proportion of the inputs specified by the ratio set point. In nearly all ratio control applications, the ratio controller sets the set points of the flow controllers rather than the valve positions, as illustrated below,

Figure 1

Generally, this process plant had been ran by using some equipment such as stainless steel tank (T21), centrifugal pumps (P20, P21, P22A/P22B), vortex flow meter (FT22), variable area flow meter, rotameter (F122), orifice plate (FE21), flow transmitter (FT21), panel mount PID controller (FIC21), flow control valve (FCV21), pressure gauges ( PG21, PG22), temperature gauge (TG21), and recorder (FR21). Each of the equipment actually had applying their role in conducting this experiment. Firstly, the main input which is water is recirculated by pumps P21 and P22AB around tank T21 and the pump P20 also uses same pipeline as P21. Next, the process plant uses three flow meters, which were vortex flow meter (FT22), rotameter (F122), and an orifice plate (FE21). Apart from that, two pressure gauges were installed at the inlet and outlet of the flow control valve FCV21 to measure pressure drop, which is related to flow rate. FT21 and its pipeline also had been selected to be either the controlled flow (CF) or the wild flow (WF). Hence, it is then possible to study any effect of different flow meters on the PID tuning of a flow loop [3].

In current industries, the flow 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. In industrial control, examples of ratio control that come to mind are burner air/fuel ratio, mixing and blending two liquids, injecting modifiers and pigments into resins before

molding or extrusion, and adjusting heat input in proportion to material flow.

RESULTS

Table 1: The PID Single loop flow control (Normal operation)

Parameter ObservationSet point (SV) 1.8 m3 /hrPID Values PB 100%PID Values TI 5 secsPID Values TD 0 secs

Table 2: Set point step test when PB was 100%, TI1 was 5 secs and TD1 was 0 secs.

Parameter 2.6 m3 2.8 m3 3.2 m3 3.8 m3 4.2 m3

PV 2.4 2.8 3.2 3.8 4.18SV 2.4 2.8 3.2 3.8 4.20MV 64.4 70.4 79.4 88.4 99.7

Table 3: Dynamic Response Test for Set point step test when PB was 100%, TI1 was 5 secs and TD1 was 0 secs observation

SV (m³ /hr ) Observation

1.8 The flow rate is stable

2.4 The flow rate is increases than 1.8 m3 and the reading is stable

2.8 The flow rate is increases than 2.4 m3 and showed minor oscillatory

response

3.2 The flow rate is increases than 2.8 m3 and showed unstable and

response sinusoidal

3.8 The flow rate is increases than 3.2 m3 and showed unstable, minor

oscillatory response

4.2 The flow rate is increases than 3.8 m3 and become stable

Table 4: Set point step test when PB was 150%, TI1 was 10 secs and TD1 was 0 secs.

Parameter 2.6 m3 2.8 m3 3.2 m3 3.8 m3 4.2 m3

PV 2.60 2.80 3.20 3.80 4.10SV 2.60 2.80 3.20 3.80 4.20MV 64.4 68.4 70.4 74.9 89.4

Table 5: Dynamic Response Test for Set point step test when PB was 150%, TI1 was 10 secs and TD1 was 0 secs observation

SV (m³ /hr ) Observation

1.8 The flow rate is stable

2.4 The flow rate is increases than 1.8 m3 and the reading showed good

response

2.8 The flow rate is increases than 2.4 m3 and the reading showed good

response

3.2 The flow rate is increases than 2.8 m3 and the reading showed

unstable at beginning but stable afterwards

3.8 The flow rate is increases than 3.2 m3 and the reading showed

minor oscillation

4.2 The flow rates is stable

Table 6: PID Controller tuning test for maximum flow rate of Controlled Flow and Wild Flow

Controlled Flow (CF) Wild Flow (WF)Maximum Flow rate 4.12 m3 4.28 m3

PR = 4.12 m3 /4.28 m3

= 0.9626 m3

Table 7: Test 1 and 2 by using one WF pump and two WF pump with instrument ratio, R=1.

Parameter / Pump One WF pump Two WF pumpInstrument Ratio, R 1 1Red (FT21) 2.24 4.10Green (FT22) 2.24 4.25PV 2.26 4.28SV 2.26 4.28Plant Ratio = ( FT21/FT22) 1 0.9647Is PV = SV? Yes Yes

Table 8: Test 3 and 4 by using one WF pump and two WF pump with instrument ratio R=1.8.

Parameter / Pump One WF pump Two WF pumpRatio 1.8 1.8Red (FT21) 4.11 4.30Green (FT22) 2.26 4.23PV 4.03 4.26SV 4.03 6.38PR ( FT21/FT22) 1.8186 1.0165Is PV = SV? Yes No

P&I Diagram for WF922 Flow Ratio Plant Control

DISCUSSIONS

The purpose of the experiment was to identify the major components of the flow ratio

process control system, to start up the process systematically, to compare flow measurements

by three different flow meters, to study single loop flow control using a PID controller and to

study ratio flow control using linear PID controller.

The theory of simplest cascade control scheme involves two control loops that use

two measurement signals to control one primary variable. The output of the primary

controller determines the set point for the secondary controller. The output of the secondary

controller is used to adjust the control variable. Generally, the secondary controller changes

quickly while the primary controller changes slowly. Once cascade control is implemented,

disturbances will not affect the primary controller.

The experiment was began with the involved valves were opened and closed

beforehand. Then, the main switch on the control panel was switched on. Next, the PID

Single Loop Flow Control (Normal Operation) was tested on, followed by the Set Point Step

Test (Normal Operation). In addition to that, the PID Controller Tuning (Normal Operation)

and the Flow Ratio Control, Linear PID (Normal Operation) were then carried out.

Based on figure 2, table 2 and table 4, the results recorded when PB was 100%, TI1

was 5 secs and TD1 was 0 secs and when PB was 150%, TI1 was 10 secs and TD1 was 0 secs

showed in increasing in MV as the SV increases and PV increases. Meanwhile, based on

table 3, the dynamic response test for set point test when PB was 100%, TI1 was 5 secs and

TD1 was 0 secs showed that at first the flow rate was stable, however it becomes more

oscillatory and unstable as the set point increases. But when the set point reached 4.2 m³ /hr,

the manipulated valve was 100%, the flow rate become more stable. Besides that, the flow

rate from set point 1.8 m³ /hr increases when set point increases to 4.2 m³ /hr. Next, based on

table 5, the dynamic response test for set point test when PB was 150%, TI1 was 10 secs and

TD1 was 0 secs showed that at first the flow rate was stable, however it becomes unstable as

the set point increases. But when the set point reached 4.2 m³ /hr, the manipulated valve was

100%, the flow rate become more stable. Besides that, the flow rate from set point 1.8 m³ /hr

increases when set point increases to 4.2 m³ /hr. However when it was compared based on the

PID setting, the oscillatory of the dynamic response of each graph of table 3 (when PB was

100%, TI1 was 5 secs and TD1 was 0 secs) was much more oscillatory and unstable then the

dynamic response of each graph of table 5.

Two trials conducted with different value of proportional band showed different

observation. The first trial was set with PB = 100% while the second trial set point with PB =

150%. From the graph obtained, it can concluded that the larger the proportional band, the

more stable the control, less oscillation but the greater the offset. In fact, the narrower the

proportional band, the less stable the process but the smaller the offset. As the proportional

band is reduced, the controller response to any change in measurement becomes greater and

greater. At some point depending upon the characteristic of each particular process, the

response in the controller will be large enough to drive the measurement back in the opposite

direction so far as to cause constant cycling the measurement. This proportional band valued

is a limit on the adjustment of the controller in that loop. On the other hand, if too wide a

proportional band is used, the controller response to any change in measurement is too small

and the measurement is not controlled as tightly as possible. However, as the set point

increases, the MV increases and there were deviation of response in each changes of set

point. In both of the set point trial, it can observed that the second (PB = 150% and TI1 = 10

secs) trial have satisfactory response than the first trial second (PB = 100% and TI1 = 5 secs).

This is because of the increases of PB and TI percentage value in the second trial. Based on

the theory, it was stated that increases in PB improved the damping and the response will be

more stable. For first trial, it may control the lower flow rate and for higher flow rate, the

response may be too oscillatory. To damp out the oscillatory, the PB and TI1 should be

increases. Furthermore, by using the PID controller, it eliminates the offset remaining error.

Integral actions give a steadily increase of the corrective action as long as an error will

continue to exist. It can compared when the TD was remains constant and the value of TI was

changed from 5s to 10s, the adjustable parameter for the integral is termed’ repeats per

minute” which the number of times per minute that the integral action output changes by the

proportional output.

The mode of the equipment was changed to the cascade mode caused the flow to be

maintained at a specific value. The cascade control accounts for the disturbances in the

primary variable more quickly. By the definition, ratio control used to maintain the

relationship between two variables to control a third variable. In contrast it is to maintain the

flow rate of one stream in a process. In the ratio control, the output changed was the ratio

factor. The CF and WF value was manipulated by disturbing the process using the whether

one WF pump or two WF pump.

For PID Controller tuning test (table 6 and figure 3), the maximum flow rate of

Controlled Flow (CF) and Wild Flow (WF) were 4.12 m3 and 4.28 m3. Thus, the Plant ratio of

the system when the instrument ratio was equal to 1 was 0.9626 m3. This showed that the

plant ratio was equivalent to the equipment ratio hence the system was successful in

controlling the whole process. Based on table 7, there were 2 test was conducted which

involved with instrument ratio, R=1. Both test which was involved with one WF pump and

two WF pump showed that the plant ratio was equivalent to the equipment ratio hence the

system was successful in controlling the both one and two WF pump. The PV variable was

equal to SV variable for both one and two WF pump. For test 1, the PV was equal to SV

showed that there were no error and the process was successfully controlled by using the PID

controller. For test 2, the PV was equal to SV proved that the system managed to eliminate

the error when the disturbance was applied by successfully change the MV to cause the

changes in PV to have the same value as SV. Meanwhile based on table 8, there were 2 test

was conducted which involved with instrument ratio, R=1.8. Test 3 which was involved with

one WF pump showed that the plant ratio was equivalent to the equipment ratio. However,

for test 4 which involved with two WF pump, the plant ratio was 1.0165 which was lower

than the equipment ratio. The PV variable for test 3 was equal to SV but the PV for test 4 was

not equal to SV. This is because for test 3, the PV was equal to SV showed that there were no

error and the process was successfully controlled by using the PID controller. Meanwhile for

test 4, the PV was not equal to SV because the system did not manage to eliminate the error

when the disturbance was applied. This was because of unsuccessful changes of the MV that

cause the changes in PV did not achieve the value as SV. Hence, it can be concluded that

Cascade control gives a much better performance because the disturbance in the flow is

quickly corrected as three out of four test was successful in manage the PV to be equal to SV

[6].

There were few possible errors that can occur during the experiment. Firstly, the

instrument might have a leakage at the pipelines which affect the readings of the flow ratio.

Besides, the configuration of the valves has been wrong that leads to error results. The valve

also might have error that leads to different results recorded. Next, the parameters was not

stabilized when the readings were recorded which could lead to an abnormal trend of results.

Other than that, the instrument was not at the optimum performances. Thus, the ideal

expected results could not be achieved.

CONCLUSION

As the conclusion, all the objectives of the experiment were successfully achieved.

Based on the results obtained when two set point trials conducted, it can deduce that the

larger the proportional band, the more stable the control, less oscillation but the greater the

offset and vice versa. This is because when the proportional band is reduced, the controller

response to any change in measurement becomes greater. Therefore, it can conclude that the

second trial have satisfactory response than the first trial second because of the increases of

PB and TI percentage value in the second trial. Meanwhile, when the experiment was

changed into cascade mode, the flow was maintained at a specific value as the cascade

control accounts for the disturbances in the primary variable more quickly. Next, for PID

Controller tuning test, the Plant ratio of the system when the instrument ratio was equal to 1

was 0.9626 m3 where the plant ratio was equivalent to the equipment ratio as the system was

successful controlling the whole process. The test which was involved with one WF pump

and two WF pump showed that the system was successful in controlling the both one and two

WF pump as the plant ratio was equivalent to the equipment ratio so the PV variable was

equal to SV variable for both one and two WF pump as there was no error and the system

managed to eliminate the error when the disturbance was applied respectively. Lastly, when

the instrument ratio, R=1.8 were conducted for one WF pump, the plant ratio was equivalent

to the equipment ratio but when involving two WF pump, the plant ratio was lower than the

equipment ratio. The PV was not equal to SV as the system did not manage to eliminate the

error when the disturbance was applied when involving two WF pump. Overall, it can deduce

that Cascade control gives a much better performance because the disturbance in the flow is

quickly corrected. There were few possible errors that can occur during the experiment.

Firstly, the instrument might have a leakage at the pipelines which affect the readings of the

flow ratio. Besides, the configuration of the valves has been wrong that leads to error results.

The valve also might have error that leads to different results recorded. Next, the parameters

was not stabilized when the readings were recorded which could lead to an abnormal trend of

results. Other than that, the instrument was not at the optimum performances. Thus, the ideal

expected results could not be achieved.

RECOMMENDATIONS

There was recommendation to overcome the possible errors. Firstly, the instrument

should be maintained and checked before the experiment begins as there might have a

leakage at the pipelines which affect the readings of the flow ratio. Besides, the configuration

of the valves should be checked so that there would nothing has been wrong that leads to

error results. The valve opening should be also checked in order to overcome error that leads

to different results recorded. Next, the parameters should be wait until it was stabilized when

the readings were recorded which could prevent the abnormal trend of results. Other than

that, the instrument should be monitor whether it was at the optimum performances. Thus, the

ideal expected results could be achieved.

REFERENCES

[1] Ratio Control. (2015) Modeling and Control. [Online].[Accessed 24 March, 2015].

Available from World Wide Web:

http://modelingandcontrol.com/2011/02/ratio_control/

[2] Process Control Fundamentals. (2015) Pa Control. [Online].[Accessed 24 March, 2015].

Available from World Wide Web:

http://www.pacontrol.com/download/ProcessControlFundamentals.pdf

[3] Tutorial Ratio Control. (2015). [Online].[Accessed 24 March, 2015]. Available from

World Wide Web: http://www.dcnz.com/resources/tutorials/ratio_control.pdf

[4] Cascade Control. (2015). [Online] [Accessed 24 March, 2015]. Available from World

Wide Web: https://controls.engin.umich.edu/wiki/index.php/CascadeControl

[5] Lab Manual Process Dynamic and Control Lab, Experiment 4 Flow Ratio Plant Control

[6] I.J. Nagrath, Control System Engineering, Ashan Ltd, 2008, p477