Hazard and Operability Study (HAZOP) The Hazard and Operability Study (HAZOP) is a structured method for identifying hazard scenarios and is well accepted across the chemical industry1. The method begins with identification of “nodes” or a process location where deviations from the design intent will be evaluated. Standard guidewords are used to suggest possible deviations.
HAZOP Study Terms
Node is a location on a process diagram at which process parameters are investigated for deviations.
Node examples:
Pipeline transferring material from one process unit to another
Specific tank or vessel
Design Intent defines how the system is expected to operate at the nodes. It provides a point of reference for developing deviations. Parameter is an aspect of the process that describes it physically, chemically or in terms of what is happening.
Specific parameters: flow, temperature, pressure, etc.
General parameters: addition, reaction, maintenance, relief, etc.
Deviations are discovered by systematically applying the guidewords to each parameter at each node (e.g., more + temperature = higher temperature).
Selecting the correct node is important for an efficient HAZOP. Too small & you will repeat the same discussion with the same deviations. Too large & the team will be confused about what is the actual deviations being discussed or scenarios may be missed. For HAZOP Studies performed as part of a larger Risk Analysis, it is convenient to define a Node as a specific equipment item such as a column, tank or vessel, pipeline, heat exchanger, pump, etc. All material and energy inputs into the equipment item would be included in the Node. This approach facilitates estimating release rate used for determining incident outcomes.
Design Intent is typically summarized as a sentence which includes: • Material or chemical involved include its physical state, quantity, temperature and pressure. • Activity such as heating, reacting, vaporizing, condensing, freezing, transferring (flow rate), etc. • Location: - From tank, vessel, reactor - At tank, vessel, reactor - Through pump, compressor, filter - To reactor, vessel, column
Guidewords are simple words or phrases used to qualify or quantify the design intent and associated parameters in order to discover deviations.
Not all possible deviations are meaningful. It is the HAZOP team’s job to identify meaningful deviations with credible causes.
Additional HAZOP Study Terms
• Causes are the reasons why deviations may occur. Causes can be equipment failure, human error or external events.
• Consequence may be documented as the Loss Event or Impacts resulting from the Loss Event.
• Recommendations are suggested actions to prevent or mitigate the consequence of deviation, or to obtain further information.
Safeguards are the systems in place that reduce the probability of the deviation occurring or mitigate the severity of the consequences. Examples include: o Dikes o Drainage o Instrument systems o Fire protection systems o Documented mechanical integrity programs o Documented sampling or other procedures
Preventive Safeguard is one that intervenes between an Initiating Event and the Loss Event – it prevents the Incident from occurring.
Mitigating Safeguard is one that is designed to reduce the Loss Event impact.
HAZOP Documents The recording process is an important part of the HAZOP Study. The person assigned to scribe the meetings must be able to distill the pertinent results from the myriad of conversations that occur during the meetings. It is impossible to manually record all that is said during the meetings, yet it is very important that all important ideas are preserved.
Each Deviation of a Design Intent is documented with the Possible Causes, Consequences, Safeguards and Recommendations.