Contractors Unlimited

HAZOP - Hazard and Operability Studies

HAZOPs are used to identifying both hazards and operability problems. Although hazard identification is the main focus, operability problems are also identified to the extent that they have the potential to lead to safety or environmental hazards, or have a negative impact on plant profitability.


Hazards are any operation that could possibly cause a release of toxic, flammable or explosive chemicals (including oil and gas) or any action that could result in injury to personnel or harm to the environment.


Operability considers any operation inside the design envelope that would cause a shutdown that could possibly lead to a violation of environmental, health or safety regulations or negatively impact profitability.

The HAZOP Process

The HAZOP team focuses on specific portions of the process called "nodes". Generally these are identified from the P&ID prior to the study by the Hazop chairman. A process parameter is identified, e.g. flow. Then a series of guidewords is combined with the parameter "flow" to create a deviation. For example, the guideword "no" is combined with the parameter flow to give the deviation "no flow". The team then focuses on listing all the credible causes of a "no flow" deviation beginning with the cause that can result in the worst possible consequence the team can think of at the time. Once the causes are recorded the team lists the consequences, safeguards and any recommendations deemed appropriate. The process is repeated for the next deviation and so on until completion of the node. The process is repeated for all other nodes.
HAZOP Study Guidewords
GuidewordDeviationExamples of Potential Problems
FLOWNO/LESSHi Pressure: stagnant lines, instrument tapping blockage, filter blockage.
MOREUndesireable flow, leakages, Hi velocity/erosion, seal/gasket failure, overflows
REVERSEUtility contamination, reverse rotation
TEMPERATUREHIGHThermal expansion, location of instrumentation, fire
LOWHydrates/freezing, flashing liquids, depressurisation
PRESSUREHIGHHydraulic surge, gas breakthrough, tube rupture, thermal expansion
LOWAir ingress, vacuum failure
LEVELHIGHVessel overfill, hi static head
LOWDead legs, cavitation, gas blowby
COMPOSITIONCHANGEContamination, slugging, corrosion products
ERROSIONHIGHLocalised failure, chocked valves
CORROSIONHIGHDissimiliar materials, under lagging, stress corrosion
SERVICESFAILUREPower, heating/cooling medium, instrument air, HVAC
STARTUP / SHUTDOWNPROBLEMS & REQUIREMENTSHuman error, sequential operations, ESD
MAINTENANCE, INSPECTION / TESTPROBLEMS & REQUIREMENTSAccess, layout, isolation, lifting, vessel entry
OTHERAn opportunity for the chair person to prompt team members to raise any other concerns

HAZOP: Guide to Best Practice
HAZOP: Guide to
Best Practice
Hazop & Hazan
HAZOP: Guide to Best Practice
HAZOP: Guide to
Best Practice

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Practical Industrial Safety, Risk Assessment and Shutdown Systems
Practical Industrial Safety,
Risk Assessment and Shutdown Systems

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