SRI RAMAKRISHNA ENGINEERING COLLEGE [Educational Service : SNR Sons Charitable Trust] [Autonomous Institution, Accredited by NAAC with ‘A’ Grade] [Approved by AICTE and Permanently Affiliated to Anna University, Chennai] [ISO 9001:2015 Certified and all eligible programmes Accredited by NBA] VATTAMALAIPALAYAM, N.G.G.O. COLONY POST, COIMBATORE – 641 022. DEPARTMENT OF ELECTRONICS AND INSTRUMENTATION ENGINEERING 12EI2602 - Process Control (Answer Key) Part – A 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. b) e-TS b) Mercury in glass thermometer kept in boiling water. a) kc is reduced a) Kc (1+(1+Ts)) b) ISE c) Frequency c) Pneumatic c) 3 b) faster a) Feedforward Part –B 11. Two tank in series, non-interactive system 12. Problem: 13. . 14. Compare Pneumatic and Electric Actuators. 15. Part –C 16. i) Transfer Function ii) Mass balance Equation. 17. a) i) Explain the working of PI controller and Summarize its Characteristics. The PI controller combines the behaviour of the I controller and P controller. This allows the advantages of both controller types to be combined: fast reaction and compensation of remaining system deviation. For this reason, the PI controller can be used for a large number of controlled systems. In addition to proportional gain, the PI controller has a further characteristic value that indicates the behaviour of the I component: the reset time (integral-action time). The controller output is given by where is the error or deviation of actual measured value (PV) from the setpoint (SP). A PI controller can be modelled easily in software such as Simulink or Xcos using a "flow chart" box involving Laplace operators: Where Setting a value for is often a trade-off between decreasing overshoot and increasing settling time. The lack of derivative action may make the system more steady in the steady state in the case of noisy data. This is because derivative action is more sensitive to higher-frequency terms in the inputs. Without derivative action, a PIcontrolled system is less responsive to real (non-noise) and relatively fast alterations in state and so the system will be slower to reach setpoint and slower to respond to perturbations than a well-tuned PID system may be. 17. a) ii) 17. b) i) ii) 18. a) i) Compare feed forward controller with feedback controller with an example. Also bring out its merits and demerits Sr.no Point Difference 1 of Feedback control system Feed Forward Control system Definition Systems in which corrective action is taken after disturbances affect the output Systems in which corrective action is taken before disturbances affect the output 2 Necessary requirement Not required Measurable Disturbance or noise 3 Corrective action Corrective action taken after disturbance occurs on the output. 4 Block Diagram 5 6 the Corrective action taken before the actual disturbance occurs on the output. Control Variable adjustment Variables are adjusted depending on errors. Variables are adjusted based on prior knowledge and predictions. Example Use of roll sensor as feedback element in ship stabilization system. Use of flowmeter as feed forward block in temperature control systems. ii) Discuss briefly the control valve sizing b) i) With an example explain the various design procedures to tune a controller using Ziegler Nichols tuning. Discuss the design criteria considered for controller settlings. Ziegler-Nichols Controller Tuning Example The Ziegler-Nichols method uses a closed controller loop & requires the following steps: • Bring system to steady state operation. • Put on P control. Introduce a set point change and vary gain until system oscillates continuously. This frequency is CO and M is the amplitude ratio. • Compute the following: The original Z-N tuning settings are given in the following table. These controller settings were developed to give a 1/4 decay ratio. However, other settings have been recommended that are closer to critically damped control (so that oscillations do not propagate downstream). These PID controller settings are shown in the following table. General 3rd Order Overdamped System Example As an example, let's assume we have a 3rd order overdamped system with negligible dynamics in the final control & measurement elements. Then: ii) Draw the diagram of electronic proportional controller and explain how gain of the system can be changed. 19 a) i) List and explain the procedure of control valve sizing. The style of control valve is usually determined by the user’s requirements, past experiences or plant preference. Valve selection can be a tricky process, but sizing the valve can be even more difficult. Valves are often incorrectly specified at the time of installation. The most important variables to consider when sizing a valve include: Medium will the valve control Effects will specific gravity and viscosity have on the valve size The inlet pressure be under maximum load demand. The inlet temperature Pressure drop (differential) will exist across the valve under maximum load demand Maximum capacity should the valve handle The maximum pressure differential for closing the valve ii) Define Cavitation and explain the procedure to prevent them. To understand how to prevent pump cavitation, it’s important to have a good understanding of what the problem is and how it arises. There are several types of cavitation which we’ll discuss below, but the process is similar. Cavitation Defined: Cavitation is the formation and accumulation of bubbles around a pump impeller. This tends to form in liquids of any viscosity as they are being transported through and around a pump system. When each of these tiny bubbles collapses or bursts, it creates a high energy shock wave inside the liquid. Imagine throwing a stone into a pond. The circular ripples which are created in this process are similar to cavitation bubbles exploding. The difference here is that due to the sheer number of bubbles creating these shock waves, the impeller and other pump components can be eroded over time. How to prevent cavitation due to vaporization 1. Lower the temperature. 2. Raise the liquid level in the suction vessel. 3. Change the pump. 4. Reduce motor RPM if possible. 5. Increase the diameter of the eye of the impeller. 6. Use an impeller inducer. 7. Use two lower capacity pumps in parallel b) i) Define cascade control and explain how it is implemented in heat exchanger process? Cascade control design considers the likely disturbances and tailors the control system to the disturbance(s) that strongly degrades performance. Cascade control uses an additional, "secondary" measured process input variable that has the important characteristic that it indicates the occurrence of the key disturbance. For the stirred-tank heat exchanger, all measured variables are shown in Figure 14.1. The secondary variable is selected to be the heating oil flow, because it responds in a predictable way to the disturbances in the oil pressure, which is not measured in this case. The control objective (tight control of the outlet temperature) and the final element are unchanged. The manner in which the additional measurement is used is shown in Figure 14.2. The control system employs two feedback controllers, both of which can use the standard PID controller algorithm. The important feature in the cascade structure is the way in which the controllers are connected. The output of the exit temperature controller adjusts the set point of the flow controller in the cascade structure; that is, the secondary controller set point is equal to the primary controller output. Thus, the secondary flow control loop is essentially the manipulated variable for the primary temperature controller. The net feedback effect is the same for single-loop or cascade control; in either case, the heating oil valve is adjusted ultimately by the feedback. Therefore, the ability to control the exit temperature has not been changed with cascade. A few important features of the cascade structure should be emphasized. First, the flow controller is much faster than the temperature controller. The improvement results from the much shorter dead time in the secondary loop than in the original single-loop system; ii) Draw the block diagram of model reference adaptive control and explain. When the process parameters are unknown or change in time, in order to achieve and to maintain the desired performance, an adaptive control scheme known as Model Reference Adaptive Control (MRAC) is implemented. Here, a reference model which is the realization of the process with desired performances. This scheme is based on the observation that the difference between the output of the process and the output of the reference model (called as the process-model error) is a measure of the difference between the real and the desired performance. TheAdjustment Mechanism directly adjusts the parameters of the controller in real time in order to force the process-model error to zero. MRAC is also known as Direct Adaptive Control because of the direct adjustment of the controller parameters using the model output. block diagram of model reference adaptive control (MRAC) 20 a) Draw the distillation equipment and briefly describe its components. In the distillation process, a feed mixture is separated by volatility or boiling point into 2 or sometimes more component streams. Energy in the form of heat is introduced at the bottom of the column in the reboiler and removed at the top of the column in the overhead condenser. Also explain any two control schemes employed in distillation column. Reboiler and Steam Control Condenser and Pressure Control Analyser Control Temperature Control Feed Preheat Control Reboiler and Steam Control Condenser and Pressure Control Analyser Control b) What is the use of distillation column in process industries? Explain driefly the various control methods used in distillation column. Distillation is a natural process using temperature variation to separate a liquid and one or more other ingredients that are mixed together. Distillation is widely used in industries that refine oil, desalinate water, create liquor, beer and wine, and produce many chemical products used in homes and factories Control of Distillation Columns Degrees of Freedom Analysis to determine the number of control degrees of freedom in a distillation column. There are two equivalent procedures based on the equation C.D.F. = Total No. of Streams - No. of Phases Present + 1 All we have to do is count all the streams in the process. Separately count the total number of extra phases i.e. add up all occurrences of phases greater than one in all units. The number of control degrees of freedom is the difference between these two numbers. Controlling Pressure in Distillation In a distillation column it is usually necessary to regulate the pressure in some way. Below there are five different methods described for doing this. Vent to Atmosphere Cooling Water Flooded Condenser - 1 Flooded Condenser - 2 Partial Condenser Controlling Tops Composition in Distillation As well as pressure, the other parameter most likely to be controlled is the composition of the tops product. The reason is that the final product will most probably come from the top of the column and it is important to know its composition. Again, as with pressure, there are many different ways of controlling the tops composition. Three methods are described below. Reflux Rate Reflux Ratio Distillate Rate