Liquid filled vessel emergency depressuring

[tkdhwjd]

Dear Collegues,

For vapor filled vessels/reactors, API standards have some guidelines regarding how fast it should empty 50% of equipment inventory (15 minutes to get rid of 50% or something like that). For liquid filled vessels/reactors, I have not been able to find any depressuring standards. Can anyone tell me if there are any depressuring guidelines for liquid filled equipment? Thanks a bunch!

 

[pleckner]

I haven't seen any in over 10-years. However, check out this article,

"Consider Bottom Venting for Reactive Liquids", Kenneth L. Walter and Victor H. Edwards, CEP, June 2001.

I think it will give all some interesting insight on the topic.

 

[Guidoo]

For vapor filled vessels, you want to depressurize in case of an external fire. There is no boiling liquid inside the vessel that provides cooling, and therefore a relief valve will not do the job. The API guideline (depressurize to half design pressure or 7 barg whichever is lower in 15 minutes) is there so that pressure decrease will be faster than decrease of vessel wall strength (as a result of temperature increase of the steel).

Note that there are reports (especially made by Norwegian companies) that state that the API guideline may not be safe enough (see article in Chemical Engineering Progress by Per Salater et al, Norsk Hydro, "Size depressurisation and relief devices for pressurised segments exposed to fire", September 2002, pages 38-45. This article has been extracted from Norsk Hydro's Best Practice on Depressurisation and Fire Relief Design.) They argue that for every segment a calculation should be done based on material properties, wall thickness and type of fire expected.

In case your vessel contains liquids you may not want to depressurize since the fire relief valve and boiling liquid provide sufficient protection against loss of containment (together with active and passive fire protection).

 

[JLSeagull]

API RP 14G (Offshore fire prevention) mentions blowdown and depressuring with references to API RP 520, RP 521, and RP 14E as guides in designing blowdown systems.