1 reply to this topic

[asade abiodun]

Hi all,

i need more light in the design of a pressure relief valve based on external fire exposure scenario.

Section 3.15.3.1 of API RP 521, says that i need to calculate the total rate of heat absorption and divide it by latent heat of the liquid to obtain the relief capacity.

Questions:

1. if the liquid in the vessel is hydrocarbon liquid (crude oil), can i use the heavier carbon component latent heat with the assumtion that it will cater for the lighter components when the vessel is exposed to fire? or is there any method to go about it?

2. I need broad explanation of paragraph 6-8 of Section 3.15.3.1

3. Any useful information in sizing for mixed phase fluid.

I would appreciate your responce

Regards

[pleckner]

As stated in API RP521, Section 3.15.3.1, you may very well have to perform a time-dependent model analysis. You would have to calculate the relief valve size at each step in the boil-off process until you reach a maximum PSV size. For crude oil and the like, you can't just assume the lights or the heavies by themselves because you can't forsee if you fall into a critical properties situation unless you calculate for it. Also, in some oils, you will probably reach a state of decomposition rather than vaporization and this will create its own vapor rate and vapor properties. This is why I feel that API's statement: "When no accurate latent-heat value is available for these hydrocarbons near the critical point, a minimum value of 50 British thermal units per pound is sometimes acceptable as an approximation." is dangerous. Yes, they say "sometimes acceptable". But how can you determine when it is acceptable?

To answer your last question, see API RP520, January 2000.