ECNG 3032 CONTROL AND INSTRUMENTATION Introduction To Process Control

Transcription

1 ECNG 3032 CONTROL AND INSTRUMENTATION 1 1. Introduction To Process Control The Aim of this module is to provide students of Electrical and Computer Engineering with a useful and practical introduction to process control. It is a powerful tool for those students who have had little or no contact or experience with real chemical engineering processes. 1

2 Key Concepts Design, Is the procedure by which an engineer arrives at a complete control system specification that satisfies all performance objectives in a particular process. 2

3 During the design the engineer must: (a) clearly state the objectives: The definition of the objective is vital for the analysis of the others procedure steps. (b) determine needed process modifications: Making minor changes such adding a bypass, selecting and alternative manipulated variable, or changing the capacity of some equipment. (Process Eng. Task). (c) specify instrumentation performance and alterations: Selection of measured variables and instrumentation (i.e. sensors, final elements). (d) define control structure (loop pairing: How to connect the controlled variables and the manipulated variables). (e) select algorithms: Selecting the proper algorithm for feedback and feedforward control. (f) define special tuning requirements. 3

4 Flash process flow diagrams Figure Example flash process Note that the equipment may be incomplete and contain errors and a complete error-free design will be develop in class. 4

5 (a) clearly state the objectives The objective is usually stated to be the reduction of variability in the operation of a process plant. 1. Safety: The safety of the people in the plant and in the surrounding is the vital care. (b) determine needed process modifications (c) specify instrumentation performance and alterations P 1 2. Environmental protection: The process must have the capacity to convert potentially toxic components to benign material. 3. Equipment protection: Much of the equipment in plant is expensive and difficult to replace, therefore operation condition must be maintained within bounds to prevent damage. 4. Smooth operation:the smooth operation of the process is desirable, because it results in few disturbances to all integrated units. 5. Product quality control: The final product from the plant must meet the demanding specifications set by purchases. 6. Profit: The typical goal of the plant is to return profit. 7. Monitoring and Diagnosis: Complex chemical plants require monitoring and diagnosis by people as well as excellent automation. Environment is protected by containing the material within the process equipment. L 1 P 1, L 1, F 1 (move upstream V3) A 1, T 6 (inferential variable), relocated in the vapor space as feedfoward variable F 2, F 3 (move V 2 upstream) F 3 heat of vaporization released to the process, that will impact on the product composition, also it has and economic impact. F 1, F 2, F 4, F 5, T 1, T 2, T 3, T 5 T 2 and T 5 are redundant, remove T 5 5

6 Flash process Figure Example flash process (after specifying instrumentation performance and alterations) 6

7 (d) define control structure (loop pairing) (e) select algorithms (f) define special tuning requirements 1. Safety: The safety of the people in the plant and in the surrounding is the vital care. P 1 -V 5 FC1 PI control 2. Environmental protection: The process must have the capacity to convert potentially toxic components to benign material. 3. Equipment protection: Much of the equipment in plant is expensive and difficult to replace, therefore operation condition must be maintained within bounds to prevent damage. L 1 -V 4 LC1 P only control L1 (30 70 %) 4. Smooth operation:the smooth operation of the process is desirable, because it results in few disturbances to all integrated units. F 1 -V 3 FC1, FC2 (SP from remote LC (?)) 5. Product quality control: The final product from the plant must meet the demanding specifications set by purchases. TC6 FC3 cascade control temperature to flow PI or PID 6. Profit: The typical goal of the plant is to return profit. F 3 -V 2 FC3 PI Control 7. Monitoring and Diagnosis: Complex chemical plants require monitoring and diagnosis by people as well as excellent automation. 7

8 Figure Example flash process (complete error-free design) 8

9 Five important features of control system design 1. Rich definition of the objectives or performance that the design is to satisfy. The objective is usually stated to be the reduction of variability in the operation of a process plant. 2. Large number of decisions that can be considered When a plant is being designed initially, the engineers can make essentially any design changes 3. iterative nature of the decision-making process Engineers must be ready to rethink previous decisions 4. ambiguity in determining the conclusion of the design procedure judgments must be used in deciding when the design is good enough 5. concurrent application of process engineering and automatic control technologies There is not a single, structured procedure for control design because design involves an element of creativity in adding process or control equipment, altering objectives and specifying control structures. 9

10 1.1 Levels of Process Control Design: developing a process flowsheet and using design parameters that produce an easily controllable plant. Structure: determining what to control, what to manipulate, and how to match one controlled variable with one manipulated variable (called pairing ). Algorithm: deciding what type of controller to use (P, PI, PID, multivariable, model predictive, etc.). Tuning: determining the values of controller tuning constants that give best control. 10

11 1.2. Process Control Laws First Law: The best control system is the simplest one that will do the job. Complex and elegant control systems look good on paper but soon end up on manual (taken out of service) in an industrial environment. Bigger is definitely not better in control system design. Second Law: You must understand the process before you can control it. No degree of sophistication in the control system (from adaptive control, to expert systems, to Kalman filters, to nonlinear model predictive control) will work if you do not know how your process works. Many people have tried to use complex controllers to overcome ignorance about the process fundamentals, and they have failed! Learn how the process works before you start designing its control system. Third Law: Liquid levels must always be controlled. The structure of the control systems must guarantee that the liquid levels in tanks, column base, reflux drums, etc. are maintained between their maximum and minimum values. The only exception to this law occurs in recycle systems, where the level In a recycle surge drum is typically not controlled, but floats up and down with recycle circulation rate. 11

12 1.3 Process Control: General Concepts and Terminology 12

13 1.4 Process Diagrams Process flow diagrams (PFDs) Figure 1.4 Process flow diagrams 13

14 Process and Instrument drawings (P&IDs), sometimes called piping and instrumentation drawings. Figure 1.5: P&ID. Typical chemical plant and control system 14