(111b) Using LOPA to Establish SILs for Power Outage Protection

Catastrophic incident root causes often may be correlated with power outages. An unanticipated power outage may be an initiating event that leads to a process upset or even an emergency shutdown of a process unit. If the planning and implementation of the emergency shutdown are managed incorrectly, it may result in an uncontrolled release of hazardous chemicals, a fire, or an explosion.  To prevent a catastrophic incident, it is necessary to employ multiple safeguards to interrupt the cascading chain of events.  A process hazards analysis is the most effective tool for identifying these safeguards and evaluating their effectiveness prior to an incident. Examples of such safeguards are emergency shutdown procedures with operator training, emergency shutdown systems (including safety instrumented systems), and backup power supplies.  The efficacy of these safeguards in functioning as intended can be correlated to their Safety Integrity Level (SIL), which can be investigated through a Layer of Protection Analysis (LOPA) paradigm.  This paper provides guidance on identifying the minimum safeguards and redundancy to meet the overall SIL to protect against a power outage.