What is an alarm flood?
ISA 18.2 defines an alarm flood as follows:
- “condition during which alarm rate is greater than the operator can effectively manage (i.e. more than 10 alarms per 10 minutes)”
Alarm floods are determined by using the number of alarms annunciated to the operator in each 10-minute interval:
- Alarm flood begins when the alarm count in any 10-minute interval is more than 10
- Alarm flood ends when the alarm count in any 10-minute interval is less than 5
Alarm floods can be described using four different metrics:
- Alarm flood count – the number of alarm floods occurring in a specified interval of time
- Duration – the length of the flood condition in minutes (or hours)
- Alarm count – the total number of alarms that occurred during the flood period
- Peak alarm rate – the highest number of alarms that occurred in a single 10-minute interval
Overall, the percentage of time any alarm system should be in a flood condition should be less than ~1%.
A typical example of an alarm flood may be:
- Compressor Lube Oil pump trips (1 alarm)
- Low & Low Low Lube Oil Pressure (6 alarm for triplicated instrumentation)
- Auxiliary Lube Oil pump is started (1 alarms)
- Aux pump fails (1 alarm)
- Compressor trips on Low Lube Oil pressure (5 – 10 alarms)
- Compressor Low Suction pressure alarms, Compressor Low Discharge pressure alarms, Compressor Low flow alarms, more trip conditions met, auxiliary system alarms (seal gas, buffer gas, etc.), KO drum level alarms, … (>50 alarms)
- Alarms generated by process conditions upstream and downstream of the compressor (20 alarms)
When do alarm floods occur?
When first starting an Alarm Management initiative, many facilities are running in what can be considered a “flood” condition at all times, i.e. the normal (or steady state) rate of alarms that are presented to the operator in the control room exceed 10 alarms in 10 minutes. During times of upset, the alarm rates exceed even flood conditions, rendering the alarm system useless.
Why is it important to eliminate alarm floods?
The biggest danger associated with alarm floods is the possibility of missing a critical alarm. The likelihood of missing important alarms is increased with the number of alarms occurring in the flood. When tens or hundreds of alarms are occurring in rapid succession, the operator may not even see a particular alarm before it scrolls off his alarm summary display. At times like this, the operator tends to ignore the alarm summary as it becomes more of a hindrance than a helpful tool.
Dynamic Alarm Management