PSV design for external fire case 2

[processdream]

Should the design temperature of vessel be considered in the design of PSV for external fire case?

If so then what is the rule of thumb for doing so?

 

[MortenA]

Wet: No
Dry: Yes (check API 520).

Best regards

Morten

 

[djack77494]

Fire or no fire, a PSV protects against over-pressure, but cannot safeguard equipment from the effects of overheating. Under ideal conditions for a liquid containing vessel, the PSV may "buy time" by holding the vessel's contents to the saturation temperature corresponding to the relieving pressure. Then the vessel dries out and it's over.

 

[MortenA]

djack77494

Your observations are correct - but what can you do about it?

The fire case is a sort of "last defence" its the situation where deluge and blow down fails. The vessel is considered lost (anyway) and the main purpose of the valve is what you state: To buy time (for fire fighting) and to prevent escalation while fire fighting/evauation may still be ongoing.

One thing that the standard perhaps does not consider in detail is the risk of the vessel being turned into a missile when its dry, heats up and losses structural integrity, ruptures and then has sufficient internal pressure to actually propel the vessel for quite a distance. I think the phenomena is called blevie.

Best regards

Morten

 

[djack77494]

Morton,
The situation you describe does not fit the definition of bleve, which involves the release of a readily vaporized liquid material. I'm not sure there is a word that fits with the idea of a vessel as a missle in motion. Perhaps you'd like to propose one?

You have identified two of the very few effective options available to provide some level of protection in the event of a fire - deluge systems and blowdown (depressuring). You really cannot practically design a vessel with a high enough design temperature to withstand a fire, so the only other means you have available is fortunately a very effective means. That is firefighting.

 

[JoeWong88]

Morten,

Do you mind to advise the exact clause in API stated that design temperature of vessel be considered in the design of PSV for external fire case for "dry" vessel ?

 

[MortenA]

Joe

I dont think i ever said that and i have never seen it done. It often conuses people when the fire case for the PSV get into the picture: But the design temperature of teh vessel is not that high...

API 521 does however in sectio 5.15.2.2 state that the max temperature considered should be below 1100 deg F for a carbon steel vessel.

Best regards

Morten

 

[JoeWong88]

Morten,

Your response in post #2 is a bit lead reader to that direction...

It is the maximum recommended wall temperature of the material may be considered in the design of PSV for external fire case of "dry vessel", instead of design temperature.

Design temperature in all cases (per my limited experiences) will not be used for PSV sizing for fire case.

 

[MortenA]

I can see what you mean - that was unintended. Your interpretation is correct and i read API 521 the same way.

Best regards

Morten

 

[JoeWong88]

Morten,
Fine. Thanks for your confirmation

JoeWong
Chemical & Process Technology