The contractor needs to evaluate EVERY credible scenario, and select the worst case/largest relief area required. Defaulting to fire case works a lot, but not all the time.

Good Luck,
Latexman

The contractor should at the very least be considering the causes of overpressure contained in section 4.0 of API-521.

*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

Thanks for quick response, i will communicate with contractor to share details fire case selection.

As a contract company engineer working on just this FG handling unit, you may not have all the details of the system upstream of the FG system. Until you know what process controls, safety devices, process design pressures/temps are installed or specified upstream, it is often not possible to judge if there are process safety concerns with overpressure protection shown on drawings you've been given. Operating Company plant process engineers may disclose such info if requested, but they may not always.

George,
Thanks for your input. I know that operating company engineers will drive the contract company engineers to complete the task as per company operating and control philosophy. anyhow your inputs give me more clarity for further communication with contract company.

We need data.

Does the blocked in case exceed the MAWP of the vessels??

If not then the fire case (also blocked in) becomes that is actually the design case

We don't know as you haven't provided any details.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Sir LittleInch, contractor not yet submitted details, he just sent us BFD and P&ID for our review. I noted that he selected fire case psv while our system have no sources of fire around. Fuel gas supply to cement and power plant. total design capacity is 92 MMSCFD and design pressure is 42 barg. my concern is if there is no fire source then why selected fire case psv.

Quote (MIANCH)

... our system have no sources of fire around... there is no fire source ...

Quote (MIANCH)

... fuel gas system ... includes, KOD, Filter separators and pressure reducing valves....

If there is supposed to be any normal liquid level in a vessel then pool fire case to be considered. See table 4 API 521-2014 for details.
What about a pool fire caused by spilled combustible liquid accumuilated in KOD and separators? Do those have liquid level measurement or control means?

Yes shvet, liquid level measurement included. will check API 521 as you mentioned.

SWAGs since no data:

Can pressure reducing valve fails wide open be a scenario?

If level instrument fails wide open and you get FG blow through, is that a scenario? Perhaps to downstream equipment.

Good Luck,
Latexman