Class 2 design methodology for process control
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This document outlines the design methodology for process control in 15 steps: 1) understand the process, 2) identify operating parameters, 3) identify hazardous conditions, 4) identify measurables, 5) identify measurement points, 6) select measurement methods, 7) select control methods, 8) select control systems, 9) set control limits, 10) define control logic, 11) create redundancy, 12) define fail-safes, 13) set lead/lag criteria, 14) investigate effects of changes, and 15) integrate and test systems. It also lists the typical components of a control system and provides an example of a surge tank level control system.
Class 2 design methodology for process control
- 1. ICE401: PROCESS INSTRUMENTATION AND CONTROL Class 2: Design Methodology for Process Control, Hardware Components Dr. S. Meenatchisundaram Email: meenasundar@gmail.com Process Instrumentation and Control (ICE 401) Dr. S.Meenatchisundaram, MIT, Manipal, Jan – May 2015
- 2. Design Methodology for Process Control: 1. Understand the process: Before attempting to control a process it is necessary to understand how the process works and what it does. 2. Identify the operating parameters: Once the process is well understood, operating parameters such as temperatures, pressures, flow rates, and other variables specific to the process must be identified for its control. 3. Identify the hazardous conditions: In order to maintain a safe and hazard-free facility, variables that may cause safety concerns must be identified and may require additional control. 4. Identify the measurables: It is important to identify the measurables that correspond with the operating parameters in order to control the process. Process Instrumentation and Control (ICE 401) Dr. S.Meenatchisundaram, MIT, Manipal, Jan – May 2015
- 3. Measurables for process systems include: • Temperature • Pressure • Flow rate • pH • Humidity • Level • Concentration • Viscosity • Conductivity • Turbidity • Redox/potential • Electrical behavior • Flammability Design Methodology for Process Control: Process Instrumentation and Control (ICE 401) Dr. S.Meenatchisundaram, MIT, Manipal, Jan – May 2015
- 4. 5. Identify the points of measurement: Once the measurables are identified, it is important locate where they will be measured so that the system can be accurately controlled. 6. Select measurement methods: Selecting the proper type of measurement device specific to the process will ensure that the most accurate, stable, and cost-effective method is chosen. There are several different signal types that can detect different things. These signal types include: • Electric ● Pneumatic • Light ● Radiowaves • Infrared (IR) ● Nuclear Design Methodology for Process Control: Process Instrumentation and Control (ICE 401) Dr. S.Meenatchisundaram, MIT, Manipal, Jan – May 2015
- 5. Design Methodology for Process Control: 7. Select control method: In order to control the operating parameters, the proper control method is vital to control the process effectively. On/off is one control method and the other is continuous control. Continuous control involves Proportional (P), Integral (I), and Derivative (D) methods or some combination of those three. 8. Select control system: Choosing between a local or distributed control system that fits well with the process effects both the cost and efficacy of the overall control. 9. Set control limits: Understanding the operating parameters allows the ability to define the limits of the measurable parameters in the control system. 10. Define control logic: Choosing between feed-forward, feed- backward, cascade, ratio, or other control logic is a necessary decision based on the specific design and safety parameters of the system. Process Instrumentation and Control (ICE 401) Dr. S.Meenatchisundaram, MIT, Manipal, Jan – May 2015
- 6. Design Methodology for Process Control: 11. Create a redundancy system: Even the best control system will have failure points; therefore it is important to design a redundancy system to avoid catastrophic failures by having back-up controls in place. 12. Define a fail-safe: Fail-safes allow a system to return to a safe state after a breakdown of the control. This fail-safe allows the process to avoid hazardous conditions that may otherwise occur. 13. Set lead/lag criteria: Depending on the control logic used in the process, there may be lag times associated with the measurement of the operating parameters. Setting lead/lag times compensates for this effect and allow for accurate control. Process Instrumentation and Control (ICE 401) Dr. S.Meenatchisundaram, MIT, Manipal, Jan – May 2015
- 7. Design Methodology for Process Control: 14. Investigate effects of changes before/after: By investigating changes made by implementing the control system, unforeseen problems can be identified and corrected before they create hazardous conditions in the facility. 15. Integrate and test with other systems: The proper integration of a new control system with existing process systems avoids conflicts between multiple systems. References 1. Process Dynamics and Controls - Open Textbook, University of Michigan - Chemical Engineering. 2. Romagnoli, Jose A. Introduction to Process Control, CRC press, 2006. Process Instrumentation and Control (ICE 401) Dr. S.Meenatchisundaram, MIT, Manipal, Jan – May 2015
- 8. Control System Components: A control system is comprised of the following components: 1. Primary elements (or sensors/transmitters) 2. Controllers 3. Final control elements (usually control valves) 4. Processes Look at the surge tank level control as given in Figure 2.1. Process Instrumentation and Control (ICE 401) Dr. S.Meenatchisundaram, MIT, Manipal, Jan – May 2015