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Using Alarms as a Layer of Protection

Alarms and operator response to them are one of the first layers of protection in preventing a plant upset from escalating into a hazardous event. This paper discusses how to evaluate and maximize the risk reduction (or minimize the probability of failure on demand) of this layer when it is considered as part of a layer of protection analysis (LOPA).

The characteristics of a valid layer of protection (Specific, Auditable, Independent and Dependable) will be reviewed to examine how each applies to alarms and operator response. Considerations for how to assign probability of failure on demand (PFD) will be discussed, including the key factors that contribute to it (e.g., operator’s time to respond, training, human factors, and the reliability of the alarm annunciation / system response). The effect of alarm system performance issues (such as nuisance alarms and alarm floods) on operator dependability (and probability of failure on demand) will be reviewed. Key recommendations will be drawn from the ISA-18.2 standard “Management of Alarm Systems for the Process Industries”.

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