Are alarm classes defined in your alarm philosophy document (APD) as required by the ISA-18.2 standard? The use of classes (classification) is a new alarm management concept for many. If your APD was created before June 2009, chances are alarm classes are not defined.
Alarm classification is a method for organizing alarms based on common characteristics and requirements (e.g., testing, training, management of change, reporting). Certainly an alarm that is identified as safeguard in a hazard and operability study (HAZOP), or as an independent protection layer (IPL) in a LOPA will have more stringent requirements for testing and operator training than the “average” process alarm. Classification helps to manage groups of alarms and ensure that their unique requirements are being met.
An ISA-18.2 compliant philosophy will define alarm classes, class descriptions, and associated requirements. Examples include: personnel safety, critical-to-quality, environmental protection, Safeguard, LOPA listed, etc. Alarm classes are assigned during the rationalization process, and can be assigned to one or more classes.
Information on classes can be used during many stages of the alarm management lifecycle. Analyzing alarm system performance by class can be used, for example to estimate initiating frequency for alarms that are used as IPLs. It can also pinpoint if any safety-critical alarms are being suppressed, or behaving as nuisance alarms - both indicators of a dangerous situation.
One of the contributing causes to the Deepwater Horizon accident was that the alarms designed to notify personnel of danger (fire or the presence of explosive / toxic gas) were disabled to prevent a false alarm from waking people at night. Perhaps if these alarms had been classified as personnel safety, then the proper controls would have been in place to prevent them from being disabled.