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PT Notes

HAZOP Practices to Avoid: Deviation-Centric Studies

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 first 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.

A key aspect of HAZOP studies is the identification of deviations from the design intent of the process being studied. However, some practitioners misunderstand the role of such deviations and perform an incomplete study that misses important hazard scenarios resulting in a process that is not adequately protected against catastrophic risks.

Some practitioners mistakenly believe that deviations are the starting point for studies. They begin by using a checklist of deviations and make a selection for each of the nodes (lines and vessels) that are used to subdivide the process for study. This deviation-centric approach is poor practice. It violates the fundamental objective of the HAZOP method and compromises its ability to provide a thorough identification of scenarios. 

The correct way to perform a HAZOP study is with an approach that focuses on design intent as follows:

  1. Identify the design intent for the overall process. Usually, this is expressed for each of the nodes. It consists of such information as operating limits of process variables, equipment specifications and operating parameters, people who interact with the node and the actions they take, the internal and external environment of the node, etc. The identification of design intent requires careful and critical thought.

  2. For each node, identify aspects of the node intention for which deviations may occur that could result in scenarios. These aspects usually are called parameters and they take the form of process variables such as flow, temperature and level; personnel actions such as changing a set point or replacing a gasket; the corrosivity of the fluid in the node; etc. It is common to define the intention for each individual parameter such as the intended flow rate or action by a person. Of course, these are part of the overall node intention.

    Sometimes practitioners choose parameters from a short list of common entries without considering additional, possibly less common, aspects of the intention for a node. This approach also is poor practice. If important parameters are missed, scenarios will be missed too.

  3. For each parameter, apply HAZOP guidewords (No, More, Less, As Well As, Part Of, Reverse, Other Than) to generate deviations from the intent of the parameter. This should be a brainstorming process rather than just rote selection of deviations from a standard list. Of course, some deviations are common, such as No Flow, High Temperature, and No Level. Reminder checklists are often used to ensure none are missed. However, HAZOP practitioners must use the guidewords creatively to identify deviations that may not be so obvious as those on a simple checklist. Reliance should not be placed on checklists alone.

The process of scenario identification then begins by identifying causes (initiating events) for each deviation.

The key difference between these two different approaches to using deviations in HAZOP studies is that the deviation-centric approach does not employ the creative and critical thinking that should be an integral part of identifying key aspects of design intent and generating deviations from them using guidewords. HAZOP deviations are simply an aid to help in the identification of scenarios. It is essential that deviations be identified as completely as possible to try to achieve completeness of scenario identification. The design intent approach described here does so; the deviation-centric approach does not.

Notably, the design intent approach is the same as that described in the following reference sources:

Guidelines for Hazard Evaluation Procedures, Center for Chemical Process Safety (CCPS), 3rd Edition, 2008.

HAZOP Guide to Best Practice, 3rd Edition, 2015.

IEC 61882, International Standard, Hazard and Operability Studies (HAZOP Studies), Edition 2.0, 2016.

You may find these articles useful:

Design intent for hazard and operability (HAZOP) studies, Process Safety Progress, Volume 35, Issue 1, pages 36–40, March 2016.

Get creative with process safety management, Chemical and Engineering Progress, pages 56 - 60, Vol. 113, No. 4, April, 2017.

A framework for critical thinking in process safety management, Process Safety Progress, Volume 35, Issue 4, pages 337–340, December 2016.

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