Measuring the quality of operator alarms
By R Brooks, J Wilson and A Mahoney, Process Plant Computing Limited (PPCL)
Electricity+Control, December 2013 (pages 4 - 9)
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An operator alarm is a request by a process control system for the operator to intervene with human intelligence and judgement in response to symptoms that may indicate an abnormal situation developing in the process.
Over the years, alarm quality has not received the same attention as alarm performance even though it is arguably more important.
An Alarm Quality Index (AQI) is an easily calculated, easily understandable metric. It can be used to quantify alarm quality and track progress in identifying and eliminating fractional annunciation alarm systems. It can also be used to monitor improvement progress after the rationalisation project when alarm performance and improvement have been handed over to process stewardship.
The alarm quality index represents the recognition that no matter how good, automated process control can never anticipate all possible circumstances and it emphasises the strong inter-dependency between control automation and operator alarming. It is expected that the alarm will help the operator in understanding why intervention is required as well as listing the possible operator action. It is generally accepted in recent thinking that an alarm that has no defined action should not be an alarm but a status variable.
Immediate alarm quality issues would be whether the alarm or alarms were appropriate in helping the operator to diagnose the cause of the event in order to understand how the situation arose and whether the alarms gave timely notification.
The information available to the operator for diagnosis is the identity of the alarm(s), its location on the process flow diagram (PFD) and, if more than one alarm is present, the time-order in which they annunciated. Timeliness of event notification matters in that late notification erodes diagnosis time and delays action but also allows the process disturbance more time to gain momentum so that a larger corrective action may become necessary to return to normal. Events that occur without being alarmed would be of the highest concern. Plants in general protect against this by having alarms defined on many more variables than some unknown (and probably unknowable) minimum.
A simple alarm quality measure could be the fraction of variables that account for, say, 90% of all annunciations. For original alarms this was 15 variables out of 60 alarmed variables giving an AQI of 25%. For new alarm sets it was 35 variables out of 60 giving an AQI of 58%. Norms for AQI will appear as this dimensionless measure is collected from larger numbers of plants.
- Operator alarms are inter-related through the process operating envelope.
- The alarm quality index can be used to quantify alarm quality and track progress in identifying and eliminating fractional annunciation alarm systems.
- No matter how good, automated process control can never anticipate all possible circumstances.