Alarm Philosophy

[TJT]

Hello,
I am looking for a standard or a document that I can refer to when determining where a control loop should alarm. For example; a level with a range of 0 - 100%, what should L1/L2/H1/H2 be set at? What affect does the SP have on this example if it's a control loop? I realize that most often there is some type of alarm rationalization required, but is there some rule of thumb out there?
Thanks in advance.

 

[controlnovice]

ISA 18.2 document may help you in creating an Alarm Management philosophy.

But it is the owner's responsibility to determine where the alarms shall be set. This is done through a Process Hazard Analysis, or similar process. In this analysis, each measured variable that the operator sees should be evaluated to determine where the alarm points are to be set. The owner needs to be present in this analysis, but they can always hire another company familiar with this process to facilitate.

I do vaguely remember some US Gov't regulation on tank levels (may have been an EPA regulation) where the HH interlock or alarm had to no more than 95% of tank volume....but that been over 20yrs ago...I can't remember that far back.

For a standard measured variable (no control), there will be Low Low, Low, High and High High (LL, L, H and HH). You have to determine the value of each, and how the operator will response. For example, you may use the Low Level of a tank to turn on a valve to start filling the tank. You don't want an alarm every time there is a low level. But you would want an alarm at Low Low level to warn the operator that the tank is leaking, or not filling when needed. (This is a very generic example)

For a controlled variable (Temperature control, level control, pressure control, etc...) it is the same thing with the addition of Deviation Alarms. You still can have LL, L, H and HH, but there should also be a Deviation Low and Deviation High alarms. These are deviations around Setpoint. If you have a temperature setpoint of 120C, and the process variable starts to increase from 120C, you may want to alert the operator with an alarm before the temperature hits the H or HH level. A deviation is typically in Engineering Units or % around SP. So, in this example, the SP is 120 C with deviation alarms at 115C and 125C (5 degrees around SP). But, if the SP changes to 90C, the new deviation alarms are 85C and 95C. The LL, L, H and HH alarms stay the same no matter the change in SP.

Ultimately, it is up to the owner to determine the actual values.

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