PetroStreet

When does it necessary to undertake HAZOP study?

if you are referring to PHA, then answer would be very simple. Any change in process, either mechanical or operational (SOPs) mandate PHA, especially additions of new facilities. But if you are asking about "HAZOP" specifically, then we have to dig further.

What I have understood is that HAZOP works under PHA as a tool.
So in which cases you use this tool?

octane, let me put some light on PHA methodologies, which are;
Qualitative Hazard / Risk Assessment

• Job Safety Analysis (JSA)
• Logic diagrams
• What-if/Checklist
• Failure Modes and Effects Analysis (FMEA)
• Hazard and Operability Study (HAZOP)

Quantitative Hazard / Risk Assessment

• Fault Tree Analysis (FTA)
• In-process energy modeling
• Event probabilities
• Risk/cost trade-off

Every method has its own limitations including pros n cons. For example FMEA method is frequently used to asses the hazards and risk with in any logic or control loops. And HAZOP technique is used for huge and complex processes, due to its systematical approach. Whereas What-if / Checklist is a very detailed and usually recommended of simple processes due to lack of in-scope/out-scope features.

Anyhow I think you got the flavor of mostly used techniques. For details on HAZOP, do refer to this post.
http://www.thepetrostreet.com/forums/vi ... f=15&t=528