Intro Theory Level Flow.docx 5p1i17

3.0 INTRODUCTION AND THEORY

Theoretically, this experiment was about the flow ratio plant control by using equipment model of WLF992. The objective of this experiment was to identify the important components of the level and flow control system, to carry out the start-up procedures systematically, to study level control system using PID controller and to study Level-Flow cascade control. This model uses water to simulate liquid phase physical processes. Level and flow can be controlled with PID controllers. When the system suffers from fluctuating inflow and a more precise control of level is required, the single-loop PID controllers are not sufficient. Cascade controller must be employed to smoothen the fluctuating inflow by using a secondary or a slave loop. The level controller becomes the master or primary controller and its control output remotely sets which is cascades, the set point of the slave or secondary controller. There is only one control valve and it is installed at the inflow. Basically, this experiment involved by gravity flow, in which case the level process in T31 is a self-regulating process and by pump P31 in which case the level process in T31 is a non-self-regulating process. The level of tank T31 is required to be controlled at its set point despite variations in inflow into T31. This is done by using a PID Level control system (LIC31) cascading into PID Flow control system (FIC31) at the inflow. LIC31 and FIC31 can also be studied independently one at a time. T31 was operated as OPEN TANK and CLOSED TANK when start up the procedure. Moreover, after that, the experiment will involved level control system by using PID controller and the level-flow cascade control. Mostly, proportional-integral-derivative (PID) controllers are used for liquid level control in most applications and can be applied to many industrial processes and mechanical systems. PID controllers proven to be a perfect controller for simple and linear processes, but when it comes to controlling of non-linear and multivariable processes, the controller parameters have to be continuously adjusted. In process control systems, nonlinearity is the rule rather than the exception. Most control loops such as pressure, temperature, composition, etc., are significantly nonlinear. This may be because of nonlinearity due to control valves, or on of variations in process gain, time constant, and dead time, as discussed in. Therefore, the study of control system has contributed to huge impact positively to our modern day development. Cascade control is one of the most successful methods for enhancing single-loop control performance. It can dramatically improve the performance of control strategies,

reducing both the maximum deviation and the integral error for disturbance responses. Cascade uses an additional measurement of a process variable to assist in the control system. The selection of this extra measurement, which is based on information about the most common disturbances and about the process dynamic responses, is critical to the success of the cascade controller. Therefore, insight into the process operation and dynamics is essential for proper cascade control design. Cascade control can improve control system performance over singleloop control whenever either disturbance affect a measurable intermediate or secondary process output that directly affects the primary process output that we wish to control; or the gain of the secondary process, including the actuator, is nonlinear. (Stephanopoulos,1984).

THEORY In years back level control has been a major issue in the industrial processes. The controlling of liquid level is essential in most industrial processes such as: food processing, nuclear power plants, water purification systems, industrial chemical processing, boilers etc. Although, most industrial problems such as: controlling the speed of motor, or fluid level in a tank, or temperature of the furnace are due to the installation of control process when the control concepts had not been properly understood. However, the ingenuity of control engineer can often overcome these challenges by producing a well-behaved piece of equipment. (Altmann, 2005). Design of Cascade Control

Figure 3.0.1: Cascade control block diagram Consider the block diagram of a cascade system shown in Figure 3.0.1. To simplify the presentation we assume that the transfer functions of the measuring devices is one. The dynamics of the secondary loop are:

Figure 3.0.2 shows a simplified form of the general block diagram where the secondary loop has been considered as a dynamic element. The overall transfer function for the primary loop is

The stability of the primary loop is determined by the characteristic equation,

Let see how the design can be carried out for the following example:

and

Note that from looking at the time constants that the secondary process is faster than the primary.

Figure 3.0.2: Simplified cascade control block diagram Cascade Control Consider a cascade control system similar to that of Fig. 3.0.1. The open loop transfer function for the secondary loop is given by Eq. 1. Assuming a simple proportional controller yield:

There is no cross over frequency for the secondary control loop. Large values for the gain KcII can be used that yield fast closed loop responses. Once KcII is selected for the secondary loop, the cross over frequency for the overall process can be obtained as before. Then

KcI

can

be

selected

method.(Mellichamp, 1989.)

for

the

primary

controller

using

Ziegler-Nichols

7.0 REFERENCE

1. Marlin, T., Process Control: Deg Processes and Control Systems for Dynamic 2. Performance, McGraw Hill, New York, 1995. 3. Seborg, D., Edgar, T., and Mellichamp, D., Process Dynamics and Control, Wiley &sons, New York, 1989. 4. Stephanopoulos, G., Chemical Process Control: An Introduction to Theory and 5. Practice, Prentice Hall, 1984. 6. Smith, C. and Corripio, A., Principles and Practice of Automatic Process Control, 7. Wiley & sons, New York, 1997. 8. Shinsky, F., Process Control Systems, McGraw Hill, New York, 1988. 9. Singh, S. K. (2010).Process control: concepts, dynamics and applications. New Delhi: PHI Learning. 10. Altmann, W. (2005).Practical Process Control for Engineers and Technicians. Amsterdam, The Nertherland: Elsevier.