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Understanding Three Layers of Integrated Control and Safety Systems


In this post, I want to share with you about three layers of integrated control and safety systems. The layers consist of Basic Process Control System (BPCS), alarm, and Safety Instrumented System (SIS). I heard about these terms of BPCS and SIS quite late in my career as process engineer. I hope those do not happen to you

Three layers of integrated control and safety systems
Three layers of integrated control and safety systems

Basic Process Control System (BPCS) is a system used to ensure that the process runs smoothly in accordance with specifications. The BPCS will work to make sure that the product is on-spec, and not off-spec by driving the process smoothly.

What will happen if the process run out of control? The alarm is raised. The alarm will alert the operator to take stronger action to correct the situation.

When the process abnormality is still moving fast toward dangerous level, then the Safety Instrumented System (SIS) will take control of the unit. The action of SIS is drastic. The goal of SIS is to protect people, asset, and environment. SIS does not care about process smoothness.

What if SIS still fails to bring process under control? We will move to final level, which is relief system. It is a mechanical system to protect the plant, environment, and operators, and other personnel.

Collectively, BPCS, alarm, and SIS are combined and called Integrated Control and Safety System (ICSS). Figure below illustrates three layers of integrated control and safety systems.

Three layers of integrated control and safety systems
Three layers of integrated control and safety systems

This figure illustrates how ICSS works in a plant.

Process control layers
Process control layers

For example, we want to control the operating temperature of a particular equipment at 100oC. However, the operating condition is not constant, and temperature will fluctuate. It is very normal in a plant, due to change of environment temperature, feed temperature, or change of feed composition. That is the reason why we need to specify a range around the desired operating temperature of 100oC for the BPCS to operate, for example from 95oC to 105oC. For the purpose of maintaining that temperature, we should install a BPCS. The function is to regulate the temperature for process smoothness, by adjusting several parameters.

If a situation arises because the BPCS unable to control the temperature, therefore the temperature increases or falls to a level outside the specified range, an alarm will be activated. The function of alarm is to alert operators to the danger and to give indication to take corrective action.

If the operator cannot correct the situation, the control system will go to the next control level. This is where SIS takes over to protect equipment, environment, and personnel. The action of SIS is on-off action, such as opening or closing switching valve of starting and stopping a pump.

If none of these three layers of protection work successfully, the last defense is a mechanical relief system.

That’s all about three layers of integrated control and safety systems. I hope you find this simple post useful.

Reference:

Toghraei, Moe. 2022. A Practical Approach to Chemical Engineering for Non-Chemical Engineers. Elsevier

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