Seven Columns, Seven Questions: The Essence of HAZOPs | AIChE

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Seven Columns, Seven Questions: The Essence of HAZOPs


Conduct a hazard and operability study (HAZOP) by answering a series of seven questions.

Before a process risk assessment can be performed, the process hazards (e.g, fires, explosions, toxic exposures) must be identified via a process hazard analysis (PHA). PHAs are required for processes covered by the U.S. Occupational Safety and Health Administration’s (OSHA’s) Process Safety Management (PSM) standard. Processes covered by the standard contain more than 10,000 lb of a flammable gas or liquid, or they contain one or more of 137 toxic chemicals in quantities above specific thresholds. For processes not covered by the PSM standard, PHAs are a best practice.

The PSM standard (29 CFR 1910.119) specifies six methods for conducting PHAs that are appropriate for various processes. For example, checklists are ideal for commonly used off-the-shelf equipment like boilers or chillers. Fault tree analysis, on the other hand, is suitable for estimating the probability of a hazardous event, but only after the hazard has already been identified by some other method; fault tree analysis is rarely, if ever, appropriate for identifying hazards.

The most common and versatile method for conducting a PHA is the hazard and operability study (HAZOP). Facilitators conduct HAZOPs using a variety of approaches that all consider deviations in a node. Each node is evaluated in its own worksheet that lists deviations, causes, consequences, safeguards, recommendations, likelihood, consequence severity, and risk...

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