Relief Valve Fire Sizing - Pressure Vessel 4

[butelja]

In evaluating the pressure relieving requirements for a pressure vessel in the fire sizing case (UG-133(b)), the pressure is allowed to 21% above the vessel MAWP. There is no mention of vessel temperature, although Appendix M-14(a) suggests it should be considered.

If a high boiling point liquid is in the vessel, the case may arrise where the design temperature is exceeded well before the MAWP is reached. In this case, which of the following is correct?

1.) The vessel must stay below the design temperature and below 121% of the MAWP when relieving.

2.) The vessel must stay below 121% of the MAWP, and temperature is not considered.

3.) The vessel MAWP is derated for temperature, and the pressure must stay below 121% of the derated MAWP at the elevated temperature?

 

[CHD01]

Lets get back to the original question on relief temperature due to fire exposure being greater than the design temperature for the vessel at the allowable overpressure of 21%

This is a favorite subject of mine, the situation arises when design pressures and temperatures for vessels are specified without a complete review of the relief systems to be installed early-on in the development of the scope of work. I think we should try to design vessels for expected relieving temperatures even if not sustained for long periods of time - but lots of engineers will disagree and their reasoning is valid as well. Bottom line is to build a safe plant within a reasonable cost; this aught to be possible.

As a design engineer, you must protect the plant from failure and as others in this thread have suggested - other measures beyond the relief valve need to be taken to prevent failure.

Technically, the ASME Code does not require the vessel relieving temperature to be less than the vessel design temperature. But you still need to look for other means of protection. If not yet built, perhaps the MAWP and temperature for the vessel can be increased without severe cost. Otherwise, look to interlocks, deluge systems, depressurization, etc, in addition to the relief valve. Also, don't forget to avaluate both the wetted and un-wetted walls of the vessel for over-heating and allowable stress reduction.

Charlie D.

The more you learn, the less you are certain of.

 

[MarkkraM]

One persons favorite subject is anothers biggest headache.

I've posted two discussion threads on essentially the same issue - preventing vessel failure through exceeding design temperature (my cases however were not external fire)

Unfortunately almost all regulatory standards only address failure due to overpressurisation, not excessive temperature.

Varunpant posted a good summary of risk minimisation relating to external fire. For vessels that have a risk of runaway reactions (causing uncontrolled temperature excursions) often an automated system for vapor depressurising is the only reliable protection to excessive temperatures. Unfortunately regulatory standards give the impression that instrumented controls are not suitable for protecting vessels against failure. In any event, as Varunpant has already mentioned, it is our duty of care to ensure the safety of our plant and surroundings so measures should still be taken even if they are not defined in regulatory standards.

P.S. If anyone is interested, API RP521 addresses automated Vapor Depressuring in Section 3.19 as a guard to temperature excursions.

 

[isteinb]

My message is for Mr. Rusman and TD2K,

I really need help for calculating the flow rate and P-T evolution in a pressure vessel during depressuring. So I would thank each of you if you can let me have a copy of your spreadsheet and a copy of Grote's paper. Any other help and advice is welcome.

saitzaj@hotmail.com

Thanks a lot