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Keywords:

  • process hazard analysis;
  • layer of protection analysis;
  • power outage;
  • emergency power systems

Abstract

Accident root cause investigations often identify unplanned power outages as the cause of a catastrophic accident. Unplanned power outages can be an initiating event that leads to an emergency shutdown of a process unit. If the emergency shutdown is managed incorrectly, it can result in an uncontrolled release of hazardous chemicals.

To prevent a catastrophic accident, it is necessary to employ one or more safeguards to interrupt the chain of accident 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.

This article explores the ways in which hazard analysis and emergency planning can be utilized to ensure a safe transition to a secondary power source during an interruption of primary power. A case study of an accidental release of hydrochloric acid vapor is presented in which a safeguard failed to function during a power outage. An interesting feature of this incident was the excessive testing of the emergency power system that led to its premature and undetected failure. Thus, when commanded to operate during a power outage, the backup system failed. We illustrate how a layer of protection analysis, coupled with the development of best practices from industrial safety standards, would have suggested a better inspection and test protocol for the emergency power system. © 2011 American Institute of Chemical Engineers Process Saf Prog, 2011