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Hazard and Operability Study

9 Hazard and Operability Study

A HAZard and OPerability (HAZOP) study is one of the most commonly used Hazard Evaluation techniques in the
chemical industry. The HAZOP technique was initially developed in the 1960s to analyze major chemical process systems
but has since been extended to other areas. It is also used as a basis for reviewing Batch processes and operating
procedures.

The Center for Chemical Process Safety defines HAZOP as a systematic qualitative technique to identify process
hazards and potential operating problems using a series of guide words to study process deviations. A HAZOP is used to
question every part of a process to discover what deviations from the intention of the design can occur and what their
causes and consequences may be. This is done systematically by applying suitable guide words. This is a systematic
detailed review technique, for both batch and continuous plants, which can be applied to new or existing processes to
identify hazards.

9.1 HAZOP Study Terms

As mentioned previously, there are specific term definitions related to HAZOP.

 Node is a location on a process diagram at which process parameters are investigated for deviations. Node
examples:
o A pipeline transferring material from one process unit to another
o A specific tank or vessel
 Design Intent defines how the system is expected to operate at the nodes. It provides the reference point for
developing deviations.
 Parameter or Process Parameter is an aspect of the process that describes it physically, chemically or in terms
of what is happening.
o Specific parameters: flow, temperature, pressure, etc.
o 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).
 Causes are the reasons why deviations may occur. Causes can be equipment failure, human performance or
external events.
 Consequence is the result of a deviation (for example release of a toxic material). Note that Consequence in the
context of HAZOP is essentially Loss Event in the context of Risk Analysis. (Loss Event per the CCPS Glossary
is the point in time in an abnormal situation when an irreversible physical event occurs that has the potential for
loss and harm impacts. Examples include release of a hazardous material, ignition of flammable vapors or ignitable
dust cloud, and over-pressurization rupture of a tank or vessel.)
 Safeguards - Any device, system, or action that interrupts the chain of events following an initiating event or that
mitigates the consequences.
o Examples include:
o Basic Process Control Systems (BPCS)
o Safety Instrumented Systems (SIS)
o Dikes
o Fire protection systems

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