Lesson 6 - Process Control Mechanisms
This lesson looks more closely at the mechanisms of process control. It identifies and explores the different ways in which variables can be controlled, including the types of process control used for this. It also investigates how process controls work. and will briefly touch upon control configurations and sequence control.
Sequence Control
Sequence control is an important part of batch control.
We do not need to cover sequences in great detail here. Sequence control consists of several steps that must be carried out in a sequence. Each step has actions associated with it, setpoints required, as well as conditions to be met before going to the next step. Let us look at a typical process, such as transferring beer from one tank to another.
Steps will include the following:
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Check routing
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Open outlet valve
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Open inlet valve
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Start pump
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Control flow at a set volume per hour (hL/hr) with flow control valve using PID controller
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Wait for empty signal from tank
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Stop pump
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Close valves
This may look simple, as it describes the ‘positive’ path in which everything goes according to plan. However, each of these steps will have built-in safeties called interlocks and alarms. These exist in the event that we end up on the ‘negative’ path and something goes wrong.
What is an interlock? It is a section of code written to prevent a device from operating unless the correct conditions are met. In this example, the pump will be interlocked with the outlet valve of the tank. If this closes, the pump will stop. Some interlocks can be overridden by an operator, but others, which are linked to safety, cannot.
The code can also raise alarms if something is not within the correct range, without necessarily stopping the process.
This is just a brief view of sequence control. The principles of sequence control remain the same whether you are making a cup of tea or operating a nuclear power plant!