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PT Notes

Design Intent for HAZOP 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.

The Hazard and Operability (HAZOP) Study is based on considering deviations from the design intent for a process to identify hazard and operability scenarios. By definition, deviations from design intent are potential problems, for example, lack of flow in a transfer line or overpressuring a vessel. Deviations from design intent are generated by applying guide words to process parameters, at different locations, called nodes, throughout the process. For example, for an inlet line to a vessel, No + Flow = No Flow, or for a vessel, High + Pressure = High Pressure. The process parameters represent aspects of the design intent for the node. The HAZOP Study team brainstorms causes of each deviation within each node and identifies the sequence of events that results for each cause including safeguards that may fail and the consequences. This sequence of events represents a scenario.

Design intent is the set of required or desired behaviors for the process design, as intended by the designer(s). However, there is no universal standard for what should be specified as part of the design intent for a process. Conceptually, the definition of design intent for a process may appear to be straightforward but practically it is challenging. Usually, the design intent for a process is complex with some aspects that are subtle. Of course, not all aspects of design intent need to be addressed in a HAZOP study but a determination needs to be made as to which aspects should be included.

It is essential that HAZOP studies identify and consider all aspects of design intent for which deviations may result in scenarios within the scope and objectives of the study. Scenarios will be missed if the design intent is not defined fully and a complete set of deviations is not considered. However, despite its importance, most texts on the HAZOP Study pay little attention to the definition of design intent.

Unfortunately, HAZOP Study teams often define design intent simply by selecting process parameters from a checklist without full consideration of all key aspects of design intent. This practice likely results in missed scenarios. It is essential that study teams understand the meaning of design intent and consider all important aspects in studies to ensure that scenarios are identified as completely as possible. A preferred approach is to define the design intent for each process node as it is considered and extract parameters from it. This approach encourages a more complete treatment of design intent.

Various other issues affect the consideration and use of design intent for a process during HAZOP studies. They are discussed in the article:

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

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