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What are the steps to design a control system for a continuous process?

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A control system is a set of devices and algorithms that monitor and regulate the behavior of a continuous process, such as a chemical reactor, a distillation column, or a heat exchanger. A well-designed control system can improve the safety, efficiency, and quality of the process, as well as reduce the operating costs and environmental impacts. In this article, you will learn the basic steps to design a control system for a continuous process, using the standard PID (proportional-integral-derivative) controller as an example.

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Hafiz Muhammad Zahid Siddiqui

Lead Engineer
Alex Wishart CPEng IntPE(NZ)

Business Development, Professional Engineering and Risk Management. EMBA Candidate
Sachin Kumar Sharma

Sr. Design Engineer

1 Define the objectives

The first step is to define the objectives of the control system, such as what variables you want to control, what are the desired setpoints and ranges, what are the constraints and disturbances, and what are the performance criteria and trade-offs. You should also identify the stakeholders and customers of the process, and their expectations and requirements. This step will help you to formulate the problem statement and the specifications of the control system.

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Hafiz Muhammad Zahid Siddiqui

Lead Engineer
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1- Go through to the P&IDs & PFDs of the Continuous Process 2- Segregate the Local Instrumentation & Smart Field Instrumentation 3- Prepare the Data Sheets of each equipment 4- Select the appropriate Control System with 30% spare I/Os 5-Segerate the I/O type of the Control System 6-Prioritise the control loop (e.g: Cascade & non-cascade) 7- Built casue & effect matrix 8-For continuous process redundancy is the major aspect of the control system 9-The critical components of the Control System like Controllers, Power Supply Units, I/O Cards etc. should be redundant 10-For more details go through the following standards ANSI B11.0 / ANSI B11.26 / ANSI B11.19 - Functional Safety - Control Systems and Risk Assessment for Machinery Package

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Sachin Kumar Sharma

Sr. Design Engineer
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Must fulfill three steps 1. Establishing standards 2. Measuring performance 3. Correct deviation Controls should be appropriate, strategic, acceptable and cost effective.

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2 Select the sensors and actuators

The next step is to select the sensors and actuators that will measure and manipulate the process variables. Sensors are devices that convert physical signals, such as temperature, pressure, or flow, into electrical signals that can be read by the controller. Actuators are devices that convert electrical signals from the controller into physical actions, such as opening or closing a valve, or changing the speed of a pump. You should choose the sensors and actuators that are suitable for the process conditions, the control objectives, and the budget and availability.

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3 Develop the process model

The third step is to develop a mathematical model of the process that describes how the process variables change over time in response to the inputs and disturbances. The process model can be derived from physical laws, empirical data, or a combination of both. The process model can be linear or nonlinear, steady-state or dynamic, and can have different levels of complexity and accuracy. The process model will help you to analyze the process behavior, design the controller, and simulate the control system.

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4 Design the controller

The fourth step is to design the controller that will generate the appropriate outputs to the actuators based on the inputs from the sensors and the setpoints. The controller can be based on different methods and algorithms, such as feedback, feedforward, adaptive, or optimal control. In this article, we will focus on the PID controller, which is one of the most common and simple controllers for continuous processes. The PID controller adjusts the output by adding three terms: the proportional term, which is proportional to the error between the setpoint and the measurement; the integral term, which is proportional to the accumulated error over time; and the derivative term, which is proportional to the rate of change of the error.

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Alex Wishart CPEng IntPE(NZ)

Business Development, Professional Engineering and Risk Management. EMBA Candidate
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A key requirement when considering control is to determine what are my control objectives? Often we will have conflicting control requirements (for example a typical separator has both level and flow considerations and to improve one can degrade the other, -do I want really precise level control (and let the flow fluctuate), or do I want to deliver a stable flow to downstream processes (and let the level fluctuate). Only once you have a clear idea of what your overall objectives are and the relative importance can you really consider your controller design.

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5 Tune the controller parameters

The fifth step is to tune the controller parameters, such as the proportional gain, the integral time, and the derivative time, that determine the magnitude and timing of the three terms of the PID controller. The tuning of the controller parameters can affect the stability, speed, accuracy, and robustness of the control system. The tuning can be done by using different methods and tools, such as trial-and-error, analytical formulas, optimization algorithms, or software packages. The tuning should be based on the process model, the control objectives, and the performance criteria.

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6 Test and validate the control system

The final step is to test and validate the control system by applying it to the actual process or a realistic simulation. The testing and validation should verify that the control system meets the specifications, objectives, and requirements, and that it performs well under different scenarios, such as normal operation, transient conditions, and disturbances. The testing and validation should also identify and correct any errors, faults, or limitations of the control system, and provide feedback for improvement and optimization.

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Vikram Singh

Energy Storage System | EV | Manufacturing I Operations I Project Management I Strategy
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The first step is to design the control flow diagram. Next comes the selection of hardware based on inputs/outputs. Then comes the logic development. After this the validation and back testing need to be done. Then the system migration can be done.

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7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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What are the steps to design a control system for a continuous process?

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1 Define the objectives

2 Select the sensors and actuators

3 Develop the process model

4 Design the controller

5 Tune the controller parameters

6 Test and validate the control system

7 Here’s what else to consider

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