Hazards of unplanned power outages: Implementing appropriate safeguards

Abstract

Accident root cause investigations often identifyunplanned power outages as the cause of a cata-strophic accident. Unplanned power outages can bean initiating event that leads to an emergency shut-down of a process unit. If the emergency shutdown ismanaged incorrectly, it can result in an uncontrolledrelease of hazardous chemicals.To prevent a catastrophic accident, it is neces-sary to employ one or more safeguards to interruptthe chain of accident events. A process hazardsanalysis is the most effective tool for identifyingthese safeguards and evaluating their effectivenessprior to an incident. Examples of such safeguardsare emergency shutdown procedures with operatortraining, emergency shutdown systems (includingsafety instrumented systems), and backup powersupplies.This article explores the ways in which hazardanalysis and emergency planning can be utilized toensure a safe transition to a secondary powersource during an interruption of primary power. Acase study of an accidental release of hydrochloricacid vapor is presented in which a safeguard failedto function during a power outage. An interestingfeature of this incident was the excessive testing ofthe emergency power system that led to its prema-ture and undetected failure. Thus, when com-manded to operate during a power outage, thebackup system failed. We illustrate how a layer ofprotection analysis, coupled with the development ofbest practices from industrial safety standards,would have suggested a better inspection and testprotocol for the emergency power system. 2011American Institute of Chemical Engineers ProcessSaf Prog 30: 99–103, 2011Keywords: process hazard analysis, layer of protec-tion analysis, power outage, emergency power systems