Class 2 design methodology for process control

ICE401: PROCESS INSTRUMENTATION

AND CONTROL

Class 2: Design Methodology for Process Control, Hardware Components

Dr. S. MeenatchisundaramEmail: [email protected]

Process Instrumentation and Control (ICE 401)

Dr. S.Meenatchisundaram, MIT, Manipal, Jan – May 2015

Design Methodology for Process Control:

1. Understand the process: Before attempting to control a processit is necessary to understand how the process works and what itdoes.

2. Identify the operating parameters: Once the process is wellunderstood, operating parameters such as temperatures,pressures, flow rates, and other variables specific to the processmust be identified for its control.

3. Identify the hazardous conditions: In order to maintain a safeand hazard-free facility, variables that may cause safety concernsmust be identified and may require additional control.

4. Identify the measurables: It is important to identify themeasurables that correspond with the operating parameters inorder to control the process.



Measurables for process systems include:

• Temperature

• Pressure

• Flow rate

• pH

• Humidity

• Level

• Concentration

• Viscosity

• Conductivity

• Turbidity

• Redox/potential

• Electrical behavior

• Flammability




5. Identify the points of measurement: Once the measurables areidentified, it is important locate where they will be measured sothat the system can be accurately controlled.

6. Select measurement methods: Selecting the proper type ofmeasurement device specific to the process will ensure that themost accurate, stable, and cost-effective method is chosen. Thereare several different signal types that can detect different things.

These signal types include:

• Electric ● Pneumatic

• Light ● Radiowaves

• Infrared (IR) ● Nuclear





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), andDerivative (D) methods or some combination of those three.

8. Select control system: Choosing between a local or distributedcontrol system that fits well with the process effects both the cost andefficacy of the overall control.

9. Set control limits: Understanding the operating parameters allows theability to define the limits of the measurable parameters in the controlsystem.

10. Define control logic: Choosing between feed-forward, feed-backward, cascade, ratio, or other control logic is a necessarydecision based on the specific design and safety parameters of thesystem. Process Instrumentation and Control (ICE 401)



11. Create a redundancy system: Even the best control system willhave failure points; therefore it is important to design a redundancysystem to avoid catastrophic failures by having back-up controls inplace.

12. Define a fail-safe: Fail-safes allow a system to return to a safe stateafter a breakdown of the control. This fail-safe allows the process toavoid hazardous conditions that may otherwise occur.

13. Set lead/lag criteria: Depending on the control logic used in theprocess, there may be lag times associated with the measurement ofthe operating parameters. Setting lead/lag times compensates for thiseffect and allow for accurate control.




14. Investigate effects of changes before/after: By investigatingchanges made by implementing the control system, unforeseenproblems can be identified and corrected before they createhazardous conditions in the facility.

15. Integrate and test with other systems: The proper integration of anew control system with existing process systems avoids conflictsbetween multiple systems.

References

1. Process Dynamics and Controls - Open Textbook, University ofMichigan - Chemical Engineering.

2. Romagnoli, Jose A. Introduction to Process Control, CRC press,2006.



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.

