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
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) Proportional(P) controller 2. Proportional and Integrative(PI) Integrative(PI) controller controller 3. Proportional, Integral and Derivative(PID) Derivative(PID) controller 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 steady state error since stable systems do not have 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 offset can be tolerated. That makes PI 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. 1|Page
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 experiment are: i.
To study the characteristic characteristic of Proportional Proportional Only Control.
ii.
To study the characteristic character istic of Proportional Band and Integral Action on a pressure loop control.
iii.
To understand the characteristic characteristic of Proportional Band, Integral Action and Derivative Action on a pressure pressure loop control. control.
iv.
To demonstrate demonstrat e 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
Background study of process control and design
Formulate control objectives
Identify the problem statement
Determine the operating conditions (pressure)
Determine the process variables (constant, manipulated, disturbance variables)
Determine the constant parameters
Devise control strategy (feedback, PID controller)
Select control hardware and software (HYSIS, MATLAB or SIMULINK) and test in simulation No
Analyze the results
Achieve set-point? Yes
Discuss the results
Finalize the process control and design roce rocess ss in the re ort ort
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4.0 RESULTS AND DISCUSSIONS DISCUSSI ONS 1. Develop Develop a dynamic mod el of the process and analyze analyze the behavior of the system using MATLA B/SIMULIN B/SIMULINK K
Focus on the control tank, V-302 to derive a transfer function:
Control tank V-302
Assumptions: 1. Negligible Negligible temperature change,
.
2. Constant volume in control tank (
), . Hence, constant in
). (Provided in lab manual) volumetric flow rate ( 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,
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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) – (vi) – (v), (v), and at derivative form, , Applying Laplace Laplace transformation transformation to equation (vii),
( ) , Where 5|Page
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 6|Page
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 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 instrum ents of the proc ess con trol system
From the description of the equipment in the lab manual, a diagram is drawn:
P1=6 bar
Receiver tank V-301
P2=4 bar
PSV-301
Control tank V-302 PT-302
PT
PIC-302
PC
P3=2 bar -
1. PID controller (PIC-302) In this experiment, PID controller is t he 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. 8|Page
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 tthrough hrough chart representation. 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-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. exploding.
8. Alarm Annunciator (PAL-302 & PAH-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) 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 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 – 0 – 60 60 psig.
13. Rotameter (FI-301 & FI-302) A rotameter is a device that measures the flow rate 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 tthe he pressure control tank.
16. Control Panel A control panel acts as a ‘motherboard’ ‘motherboard’ of pressure control. It mounts the controller, alarm annunciator, recorder, 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 feedback con trol system and analyze analyze its stability using various tunin g 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 Ponly 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 14 | P a g e
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 17 | P a g e
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 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 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 might cause the results to be different different from the actual results. 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 t ransfer functions used are of first order. This may not be t rue 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 point had been set earlier. Apart from that, the offset cannot be eliminated 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 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. .49.
Seborg D.E., Edgar T.F. & MelliechampD.A. (1989). Process Dynamics and Control. Control. John Wiley and Sons: New York.116-118. 19 | P a g e
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