Kinds of Alarm

26 Oct 2019 at 20:06 // under Simple Saturdays Process Control

This post explains different types of alarm that can be used on a control system.

Process High or Low

These are the simplest type of alarm. The control system measures something such as pressure, temperature, level or flow, and compares it with the alarm value. If the measured number is above a high alarm point or below a low point, the alarm is triggered.

Examples could be:

• High pressure of 20 barg on the reactor inlet
• Low flow of 20m³/hr cooling water
• High liquid level of 80% in the flash drum

It is sometimes possible to put two alarms on the same measurement, such as a high alarm (PVHI) and a high-high alarm (PVHH).

The first one can be used to warn the operator "this doesn't look right, the control system is having trouble here, can you please help?", where as the second is used to say "No seriously, this is getting dangerous, you need to act NOW".

Bad Reading

This alarm is triggered when the value being measured stops giving a reading. This could be caused by:

• The real value is so high or low that the measurement can't actually read it.
• A damaged instrument or broken cable.
• Some other fault with the control system.

In any case, the system does not know what the process is actually doing.

Why do you want to know? Why bother the operator?

Generally there are two occasions the operator would need to know that the transmitter has stopped reading:

• There is another alarm on that tag (such as a high temperature alarm) and that you now can't tell if the other alarm should be active.
• The measurement is used as part of a control loop which will no longer work.

In ether case, the operator will need to look at all the other information available to them before deciding what to do.

Output High or Low

This alarm is occasionally set up on the output of a control loop. It warns the operator that the output is higher or lower than expected. Usually this can earn the operator that something is going wrong before the system actually loses control.

As an example, let's assume that we have a system that uses cooling water to keep a different flow at the correct temperature. Most of the time the valve sits between 20 and 50%. For some reason the valve has had to open up more. Perhaps the cooling water supply has dropped off, or the hot process had gotten hotter. If there is a high output alarm set at 70%, this would warn the operator while the control loop was still managing to control the temperature at the setpoint, but was at the point it was beginning to struggle. This could give the operator enough time to work out what the problem is and do something about it.

Deviation Alarm

This is put on a control loop and is used to warn when the measurement and the setpoint have deviated from each other. This suggests that the control loop is no longer working and the operator needs to assist.

This can be useful when the control of something is quite critical to the process, but that the setpoint could vary significantly depending on the rest of the process, such as the reflux flow.

Rate of change

A rate of change alarm can be set to warn when the procese changes too quickly.

This can be useful for startup and shutdown conditions to avoid damaging plant with restricted heating or cooling rates.

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