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PT Notes

HAZOP Practices to Avoid: Consequence-Before-Cause Approach

PT Notes is a series of topical technical notes on process safety provided periodically by Primatech for your benefit. Please feel free to provide feedback.

This PT Note is the fourth in a series to help you ensure your HAZOP studies follow best practices. Poorly conducted studies result in omitting hazard scenarios which may cost people’s lives and result in catastrophic property damage. It exposes companies to regulatory and legal liabilities that can cost millions of dollars.

Some practitioners jump to consequences before considering causes of a deviation in a HAZOP study with the intent of speeding up the study. They believe that if they cannot identify any consequences within the scope of the study there is no need to spend time identifying causes. This approach is dangerous because some consequences may not be obvious until all credible causes have been identified.

For example, the deviation “No Flow” might be under consideration. In considering the possible consequences of this deviation, the study team may focus on the inability of material to be charged to a reactor and conclude that only an operability scenario is possible. However, if causes of the deviation are considered before consequences, the study team might identify a closed valve as a cause of “No Flow” with the consequence of no charge to the vessel but also a line failure as a cause of “No Flow” that results in a release of toxic material to the reactor room and exposure of operators. Thus, a critical scenario would have been missed had the consequence-before-cause approach been used.

In another example, consider the node “Reactor, R1” and the deviation “Higher Pressure”. The reactor is protected with a pressure safety valve (PSV) so if the study team jumps immediately to consideration of consequences they may identify the consequence, “Potential for activation of Reactor, R1, PSV with release directed to flare and inefficient operations due to loss of product”. However, if the team first considers causes, they may identify the cause, “Operator adds excess catalyst”. This has the much more serious consequence, “Potential for runaway reaction in Reactor R-1 leading to vessel failure and release of its contents with an explosion”. Again, a critical scenario would have been missed had the consequence-before-cause approach been used.

Clearly, causes should always be addressed before consequences in HAZOP studies to avoid missing critical scenarios.

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