CCB 3072 Process Instrumentation & Control Lab

September 2013

Group Project

Title : Pressure Control System

Group : B2

Group Members : Chong Jia Ling 14892

: Izyan Farhana Binti A.Kaher 14736

: Kalisvaran A/L Muniandy 14995

: Kevin Kan Shiu Kwang 16059

: Kuan Chuan Hong 14802

: Muhamad Arief Fikri Bin Muhamad Nasir 15521

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1.0 SUMMARY

In this project, we managed to prove on how a pressure loop can be controlled by using

three basic feedback control modes which are:

1. Proportional(P) controller

2. Proportional and Integrative(PI) controller

3. Proportional, Integral and Derivative(PID) controller

Starting with the simplest mode, Proportional Controller. Proportional Controller attempts

to stabilize the system and avoid fluctuations from occur by responding the magnitude as

well as the direction of the error. When Proportional Controller is used, large gain is needed

in a way to improve the steady state error since stable systems do not have problems when

large gain is used. By other means, Proportional Controller helps in calculating the amount

of error between the measurement and the set point, amplifies it and positions the final

control element to reduce the error. The measurement of Proportional Controller can

completely eliminate offset at only one load condition since the magnitude of its corrective

action is proportional to the error. However, Proportional Controller only can accommodate

one fixed relationship between input and output in order to obtain a zero error if properly

tuned.

Next is Proportional Integral Controller. Integral control has a negative effect on speed of

the response and overall stability of the system. Plus, it is almost never used alone. Rather it

is combined with Proportional Control. Generally, this combination of PI Controller is used

when no amount of offset can be tolerated. That makes PI Controller as a very often used

controllers in industry since speed of the response is not an issue when we are dealing with

PI Controller. When a process upset occurs, the P Controller will register an error and

respond to it. Meanwhile, the Integral Control mode will detect the offset error in the

proportional mode and tries to eliminate the error.

Besides, Proportional, Integral and Derivative(PID) Controller is also to control the

pressure. PID Control have been the dominant control technique for process control for

many decades. A survey has indicated that large scale continuous process typically have

between 500 and 5,000 feedback controllers for individual process variables such as flow

rate and liquid level. Of these controllers, 97% utilize some form of PID control (Desborough

& Miller, 2001). However, its application should be considered carefully because it has

limitations with some processes. It is hard to tune. Hence, the controlled process which is

stabilized using the Derivative Control helps to reduce the oscillation and offset thereby

producing the same speed of response as with proportional action but without offset.

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APPLICATIONS OF PRESSURE CONTROL

There are a bunch of applications in industry which requiring precise pressure control

module in purpose to maximize their production.

1. Steam boiler.

Steam boilers are usually design to work at high pressure in order to reduce their

physical size. Operating them at lower pressure can result in reduced output and

‘carryover’ of boiler water.Plus, reduced pressure will lower the temperature of the

downstream pipework and reduce standing losses. Plus, it might also cause the

amount flash steam generated when condensate from drain taps is discharging into

vented condensate collecting tanks to be reduced. Because steam pressure and

temperature are related, control of pressure is important to control temperature in

some processes.

2. Heat exchanger

For the same heating duty as steam boilers, heat exchanger is designed to operate

on low pressure steam rather than high pressure steam. This is where the pressure

control module is practiced. The low pressure heat exchanger might be less

expensive because of lower design specification.

2.0 OBJECTIVES

The objectives of this experiment are:

i. To study the characteristic of Proportional Only Control.

ii. To study the characteristic of Proportional Band and Integral Action on a pressure

loop control.

iii. To understand the characteristic of Proportional Band, Integral Action and Derivative

Action on a pressure loop control.

iv. To demonstrate the loop tuning procedure on a pressure loop control.

v. To develop a suitable control system to regulate the reaction based on the identified

variables and suggested models by using SIMULINK software

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3.0 METHODOLOGY

Identify the problem statement

Determine the process variables (constant, manipulated, disturbance variables)

Determine the operating conditions (pressure)

Determine the constant parameters

Devise control strategy (feedback, PID controller)

Select control hardware and software (HYSIS,

MATLAB or SIMULINK) and test in simulation

lab

Analyze the results

Achieve set-point?

Background study of process control and design

Formulate control objectives

Discuss the results

Finalize the process control and design

process in the report

Yes

No

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4.0 RESULTS AND DISCUSSIONS

1. Develop a dynamic model of the process and analyze the behavior of the system

using MATLAB/SIMULINK

Focus on the control tank, V-302 to derive a transfer function:

Assumptions:

1. Negligible temperature change,

.

2. Constant volume in control tank ( ),

. Hence, constant in

volumetric flow rate ( ). (Provided in lab manual)

3. Input feed is air. Hence, assume as ideal gas behaviour, .

Since there is no change is volumetric flow rate, hence in order to change the pressure,

there will be change in mass flow rate.

Dividing by volumetric flow rate,

From ideal gas law,

Control tank

V-302

𝑃

𝑤

𝑃

𝑤

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Besides that, from ideal gas law,

Substituting (iii) & (iv) into (ii),

Since volume, temperature and molar mass of air is constant, hence

At steady-state,

,

(vi) – (v), and at derivative form, ,

Applying Laplace transformation to equation (vii),

(

)

Where

,

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Simulink

The transfer function and block diagram are plotted in SIMULINK as shown in figure below.

Figure 1: Block diagram

The result of simulation without controller is as shown in figure below.

Figure 2: SIMULINK Stimulation Results

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Figure 3: Source Block Parameter of Step

From the simulation above, it can be seen that the process have not reached the expected

final value due to the reason that the controller was not installed to control the pressure.

Controller play an important role in making sure that the process reached the desired value

and the set point of the process. For example in a feedback loop of a control system,

feedback loops take the system output into consideration, which enables the system to

adjust its performance to meet a desired output response. Without controller, the desired

process cannot be done and the process will be dangerous as overpressure might cause

explosion.

To test the effectiveness of the process without controller, the process was run using

SIMULINK. As shown in the block diagram in Figure 1, there is no controller (Gc) and also

pressure transmitter (Gm) but the process do have final control element which is valve (Gc).

The process transfer function which is Gp is derived and inserted into the block diagram to

complete the process. After the whole process is completed, source block parameter of step

is adjusted to see the response of the reaction. As shown in Figure 3, the initial value is set

to 0 and the final value is set to 4, after simulation, the graph reached steady state at 2 but

not 4. This is because there is no controller to control and tune the response to desired value

and this caused the ineffectiveness of the process. With the existence of controller at the

tank, the process will be effective and reach desired value. This will be studied at the next

discussion.

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2. Analyze basic instruments of the process control system

From the description of the equipment in the lab manual, a diagram is drawn:

P1=6 bar

1. PID controller (PIC-302)

In this experiment, PID controller is the main tool that is used to achieve the objective. PID is

a abbreviation of the three terms that presence in the controller, that is Proportional (P),

Integral (I) and Derivative (D) term. Figure below shows the block diagram of a PID

Controller.

PID Controller calculates an "error" value as the

difference between a measured process variable

and a desired set-point and it attempts to

minimize the error by adjusting the process

control inputs. A typical Proportional Controller

able to response quickly to upsets however the

measurement of the P-Controller can completely

eliminate only one offset at a load condition.

Receiver

tank

V-301 P2=4 bar

PSV-301

Control tank

V-302 P3=2 bar

PT

PC

PSV-302

PT-302

PIC-302

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PID-controller able to responds to all aspect of process error-direction, magnitude, duration

and rate of change of the process. The only problem of using PID-controller is the tuning

process may be complicated and difficult as it deals with three different types of terms.

2. Pressure transmitter (PT-301)

Pressure is an expression of force that acts on a fluid and prevent it from expending and

measures in terms of force per unit area. Pressure transmitter measures pressure typically

gases or liquids. The transmitter usually acts as a transducer and generates a type of signal

as a function of the pressure imposed.

Pressure transmitter is used to control and monitor in thousands of application in our daily

life such as measuring flow rate, speed, water, level and so on. Types of pressure

transmitter may vary differently in terms of technology, design, performance, application

suitability as well as the cost of the transmitter.

3. Recorder (PR-302)

In the instrument that is used for pressure control

experiment contains a pair of continuous 2 pen

chart recorder. The function of this recorder is used

to record the response of the process instrument of

the input and output through chart representation.

Recorder is needed for us to study the trend in

change of various tuning methods and how the

system response to it. By using recorder, the

change and trend can be obtained and studied

easily.

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4. Control valve (PCV-302)

Control valve is an important tool for any types of process that

requires controlling the amount of flow. All control valves have

inherent flow characteristics that define the relationship between

the valve openings with flow rate under constant pressure

conditions. Different type of valve with various sizes which are

subjected to the same volumetric flow rate and differential

pressure will have exactly the same orifice pass area.

The suitability of the types of control valve to be used varies on the function or unit process

that the control valve to be installed in.

5. Vortex flow meter (FT-301)

Flow meter is used to measure the flow of fluids

such as liquid or gas into the system. One of the

methods to measure the flow is by placing a body

such as shedder bar in the path of the fluid. As the

fluid passes the bar, there will be a disturbance to

the flow forming vortex. The frequency at which

these vortices alternate sides is essentially

proportional to the flow rate of the fluid.

6. Pressure indicator (PI-301, PI-302, PI-303 & PI-304)

Pressure measurements are usually made relative to

ambient air pressure such as absolute, gauge and

differential pressure. In pressure control instrument, dial

gauge pressure indicator is used to measure the pressure

inside the system

Gauge pressure is zero-referenced against ambient air

pressure. The indication on the gauge indicator is equal to

absolute pressure subtract atmospheric pressure and the

negative sign are omitted.

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7. Process tank (V-301 & V-302)

In this experiment, the process tank is used to study the

change in pressure by inserting air into the tank. It is

required to be able to sustain high pressure to avoid the

tank from exploding.

8. Alarm Annunciator (PAL-302 & PAH-302)

An Alarm annunciator, or better known as an annunciator panel, is a system to alert

operators of alarm conditions in the plant. In the case of pressure control, it is used for

controlling the tank’s pressure and indicate the user when the tank pressure is too low or too

high, so that the user is able to make suitable changes to the tank.

9. Pressure Relief Valve (PSV-301 & PSC-302)

Pressure relief valve (PRV) is a type of relief valve, in which its function is to control or limit

the pressure in a system or vessel which can build up by a process upset or instrument

failure. PRV is a mechanically activated device, spring loaded normally closed valve. It can

open and purge air to atmosphere in case of over pressure in tank. It opens when there is

pressure greater than its spring tension.

10. Solenoid valves (HV-301, HV-302, & HV-303)

A solenoid valve is an electromechanically operated valve, in

which it is controlled by an electric current through a solenoid.

It offers fast and safe switching, high reliability, long service

life and good medium compatibility of the materials used. In

pressure control experiment, we use solenoid valves for fault

simulation.

11. Air regulator (PCV-301)

An air regulator regulates the air supply to the process

receiver tank so that it does not exceed the pressure limits.

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12. D/P transmitter (PDT-301)

A differential pressure sensor (D/P transmitter) measures the

difference between two pressures, one connected to each side

of the sensor. In the pressure control experiment, it acts as

differential pressure transmitter for process line, measuring

between the range of 0 – 60 psig.

13. Rotameter (FI-301 & FI-302)

A rotameter is a device that measures the flow rate of liquid or

gas in a closed tube. It belongs to a class of meters called

variable area meters, which measure flow rate by allowing the

cross-sectional area the fluid travels through to vary, making it

measurable. In this experiment, it acts as a variable area flow

meter for purpose line.

14. Hand valve (HV-304 & HV-309)

A hand valve is a kind of isolation valve, which its function is

to stop the flow of process media to a given location, usually

for maintenance or safety purposes. In the experiment, the

hand valves are input/output isolation valves. They determine

the direction of airflow and load changes.

15. Fault simulation switches (HS-301, HS-302, & HS-303)

Fault simulation switches act as a cut-off switch whenever there is leakage at the pressure

control tank. It can be also used during times of loss of instrument air supply, where they will

shut off the outlet to the pressure control tank.

16. Control Panel

A control panel acts as a ‘motherboard’ of pressure control. It mounts the controller, alarm

annunciator, recorder, push button power supply switch and changeover switch between the

distributed control system (DCS) and local control.

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3. Design a feedback control system and analyze its stability using various tuning

techniques

Feedback Pressure Control System

P-only, PI, and PID controller was used to test the stability of the feedback pressure control

system that was designed. A major setback was shown when the design was tested using P-

only controller in the feedback pressure control system. Next, when the design was

experimented using PI controller in the feedback pressure control system, it eliminates the

offset though with one disadvantage where it has more oscillatory. Finally the design was

tested using PID controller in the feedback pressure control system, where it does not only

eliminate the offset but also has less oscillatory hence proven to be superior to PI controller.

This shows that, to tune a feedback pressure control system, it is the best and optimum

selection to use PID controller. The attached diagrams contain the all the results. The

feedback pressure control system is stable using PID and PI controller. All the results are

shown in the attached diagram. The stability of using PID controller in the system using

Bode Diagram is shown stable. The stability of using PI controller in the system using Bode

Diagram is also stable. While only the stability of using P-only controller in the system using

Bode Diagram does not show any stability.

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P controller

Tuning Parameter : P = 22.3 ; I = 9999 ; D = 0

Figure 4: Experiment Result of P controller

Figure 5: Stimulation results from MATLAB/ SIMULINK of P controller

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Figure 6: Bode Diagram of P controller

PI controller

Tuning Parameter: P = 22.3; I = 10.4; D = 0

Figure 7: Experiment Result of PI controller

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Figure 8: Stimulation results from MATLAB/ SIMULINK of PI controller

Figure 9: Bode Diagram of PI controller

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PID controller

Tuning Parameter : P = 22.3 ; I = 10.4 ; D = 6.0

Figure 10: Experiment Result of PID controller

Figure 11: Stimulation results from MATLAB/ SIMULINK of PID controller

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Figure 12: Bode Diagram of PID controller

Discussion

In this project, we compared the outputs of Simulink with the actual results we obtained in

previous experiment for all 3 types of controller P, PI and PID Controller.

For PID controller, Simulink result shows that shorter time is taken to reach the new set point

compared to actual experiment result. Another difference is Simulink graph produced

overshoot whereas the actual experiment did not produce overshoot.

For the PI controller, the result simulated is better compared to the actual results obtained.

Although both the graphs produce overshoot but the graph from Simulink has lower

overshoot compared to the actual graph and also to the PID controller stimulated graph.

However the time taken for the both simulated and actual PI controller graphs takes longer

time compared to both PID graphs.

Lastly for P Controllers, actual experiment has shown that the process would be very

sluggish as it takes a very long time to respond. Simulink produced results where shorter

time is needed to response. Nevertheless, both actual experiment and Simulink graph have

shown that the desired set point could never be achieved by using P controller as it is very

sluggish.

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Transfer function and gain values that we included in running this Simulink software may

contribute to the errors or deviation from the actual experiment results. This may cause the

delay in the system. Besides that, the inaccurate transfer functions and the assumptions of

ideal system might cause the results to be different from the actual results. Besides that, the

order of the transfer function also plays an important role in producing accurate results. The

transfer functions in the stimulation we assumed to be a simpler system whereby the transfer

functions used are of first order. This may not be true as the system in the actual experiment

is more complex and may have transfer functions of other than first order.

5.0 CONCLUSION

Among the entire controller, PID controller is the best controller as it can eliminate

offset and oscillatory response. Furthermore, it does not overshoot too much thus saving

time in becoming stable faster.

For PI controller, it is less efficient and stable as compared to PID controller. This

kind of controller will produce oscillatory response which then affects the desired value.

Besides that, it overshoot from the desired value hence requires time to achieve steady

desired output.

It takes a very long time or near impossible for P controller to achieve desired value

although the set point had been set earlier. Apart from that, the offset cannot be eliminated

as there is no action of both integral and derivative.

From all these analysis, it is found that PID is the best controller to suit the feedback

pressure control system. The time taken for the respond curve for this method has the

shortest time compared to other methods. Besides that, the responds curves obtained are

less sluggish and more stable compared to other methods. This shows that PID is the best

choice for the saving time and most efficient.

6.0 REFERENCES

Hagglund, Tore, PID Controllers: Theory, Design & Tuning.

Rys, R.A. (1984). Advanced Control Methods, Chemical Engineering.49.

Seborg D.E., Edgar T.F. & MelliechampD.A. (1989). Process Dynamics and Control. John

Wiley and Sons: New York.116-118.

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