1.1.5: DESIGN METHODOLOGY FOR PROCESS CONTROL SYSTEM
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 controlled variables (process outputs or measured variables): It is
important to identify the controlled variables (measured variables) that correspond with
the operating parameters in order to control the process.
Controlled variables (or Measured variables) for process control systems include:
• Temperature
• Pressure
• Flow rate
• Liquid level
• pH
• Humidity
• Concentration
• Viscosity
• Conductivity
• Turbidity
• Electrical behaviour (Current, voltage, frequency)
5. Identify the points of measurement: Once the measured variables are identified, it is
important locate where they will be measured so that the system can be accurately
controlled.
6. Identify the process inputs (manipulated variable and disturbance variables):
a. Manipulated variable is Process input that can be manipulated to control the
process output at set point value.
b. Disturbance variables are the process inputs that are not manipulated but their
changes upset the process output.
7. Design process control system or control methods: Choosing between feed-back, feedforward, cascade, ratio, or split range control is a necessary decision based on the
specific design and safety parameters of the system.
8. Design feedback or feedforward controller: In order to control the operating
parameters using feedback control system, On/off is one control method and the other
is PID based continuous control. Continuous control involves Proportional (P), Integral
(I), and Derivative (D) methods or some combination like PI or PID controllers.
9. Select industrial 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.
10. Set control limits: Understanding the operating parameters allows the ability to
define the limits of the measurable parameters in the control system.
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. 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.
14. Integrate and test with other systems: The proper integration of a new control system
with existing process systems avoids conflicts between multiple systems.
------------------------------------------------------------------------------------------------------------------------Example: Identify the Controlled variable (Measured variable or Process output),
Manipulated variable, and Disturbance variables in following feedback control based liquidlevel control system.