Process Hazard Analysis (PHA) – Introduction to HAZOP and HAZID
Process Hazard Analyses (PHAs) are commonly used for analyzing industrial processes and hazards to improve the ability to take steps towards increased safety. A PHA can be defined as follows:
“A systematic effort designed to identify and analyze hazards associated with the processing or handling of highly hazardous materials” (OSHA, 1992). The key objective of the PHA is to identify the inherent hazard potential, identify barriers included in design or asset operation, and evaluate further risk-reducing measures to achieve acceptable risk.
Two common PHA tools and hence systematic ways of identifying and analyzing potential hazards are the Hazard and Operability (HAZOP) and Hazard Identification (HAZID) studies. Apart from the acronyms being quite similar, both analyses are often conducted through multidisciplinary workshops with participants from relevant disciplines. This approach is chosen to stimulate creative thinking and imaginative examination of the design under review through interaction between relevant disciplines, as well as using the experience and knowledge of the participants to identify risks and recommend areas of improvement. The HAZOP and HAZID methodologies also follow a similar structure. So how are they different, and when is one more suitable than the other?
Similarities between HAZID and HAZOP
First, let us look at similarities with regard to methodology. For both HAZOP and HAZID studies, the following outline applies:
The system under review is divided into smaller parts called nodes (one example of a node being a separator, including inlets and outlets).
For each node, the systematic application of guidewords is performed to stimulate creative thinking of potential causes and consequences for hazards related to the guidewords.
The consequences are assessed assuming all safeguards fail, typically focusing on the impact on personnel, environment, finances, and reputation/community.
Identification of safeguards either preventing or mitigating the defined scenario. For operating assets, the focus is on existing safeguards. A safeguard is a device, system, or action that will prevent a hazardous event from reaching its potential. For projects, the focus is on safeguards already implemented in the design.
In case it is deemed necessary, actions and recommendations are raised in the HAZID and HAZOP. These are raised when the safeguards are unlikely to prevent or mitigate a consequence or there is a shortfall in compliance with a regulation or company standard.
Differences between HAZID and HAZOP – when is one more useful than the other?
Even though the methodologies and approaches are similar, there will be practical differences between the two analyses. Here is a quick overview of the key differences:
A Hazard Identification study focuses on identifying potential hazards. A HAZID can be viewed as a more general risk analysis tool designed to identify the inherent hazards at a plant, due to e.g., its location, the planned activities (loading, offloading, processing, etc.) the amounts of substances and fluids present, and their inherent (hazardous) properties, as well as other external events (fires, floods, lightning, etc.) that may affect the plant (i.e., the identification itself being in focus). The HAZID is normally the starting point for all other assessments of inherent risks and should normally be done as early as possible.
For a HAZOP study however, the focus is on identifying potential hazards related to how the fluids at the plant are processed, and how the plant is intended to be operated (e.g. the intended temperatures, pressures, flows, etc.), Hence, the main goal is to identify potential deviations from design intention, and how the process is designed to keep operating conditions within safe limits and prevent excursion of these limits to be realized as hazardous consequences Since the focus is on process-related hazards, piping and instrumentation diagrams (P&IDs) are an essential part of conducting a HAZOP. Hence, the design must have matured to a point where P&IDs are available.
A HAZOP node is defined as part of a process system, e.g. from a pump discharge to a separator. A HAZID node is normally defined as an area, e.g. a module or deck level.
Parameters/guidewords for a HAZOP are standardized for process systems, e.g. Pressure (More/less), Temperature (More/less), while guidewords for HAZID should cover a wider range of hazards (Loss of containment, Collision, Dropped objects, Extreme weather);
The output from a HAZOP is used to further develop a process system and add safeguards. Output from a HAZID can be used for design improvements, input to barrier management, safety strategies, and the basis for a hazard register.
To summarize, the methods are similar but serve different purposes, are possible to do at different stages of the project, and focus on different aspects. A HAZID is recommended at an early stage, as input to later phases and design developments, while a HAZOP is performed when the process design is matured.
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