Safety Relief Valve Sizing Case - What is Adequate Drainage?

[MelMez]

I have not come across this problem before for Fire Case. There are two equations, typically the first is used which suggests that prompt fire fighting and adequate drainage is available. This is the typcical equation that has been used in most Fire Cases.

The question has come up, what constitutes adequate drainage? Does this mean that the spill is contained in the sump? Or do I need to concentrate more on the prompt fire fighting? 5 minutes or 15 minutes?

Your opinions are much appreciated

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When prompt fire-fighting efforts and adequate drainage exist:
Q = 21,000 F (Awet)^ 0.82
When prompt fire-fighting efforts and adequate drainage do not exist:
Q = 34,500 F (Awet)^ 0.82
Where:
Q = Total heat absorption to the wetted surface, BTU per hour.
F = Environmental factor. Reference Table T7-6 on page 7-18.
Awet = Total wetted surface area in square feet. Reference page 7-19.

 

[MJCronin]

Mel....

You are correct in pointing out the fact that the selection of these equations is based on a vague and subjective criteria.

In what part of the world is your installation ?

"Wing Wong" has written a couple of good papers in Chemical Engineering on this specific subject.

Read em !!!!

http://www.scribd.com/doc/29262808/Fire-Vessels-and-the-Pressure-Relief-Valve

In order to find these articles....I used a search engine called "GOOGLE"....its very useful

-MJC

 

[FluidizedBed]

I took a course called "Pressure Relieving Systems" by taught by the API. We talked about these situations.

For the fire case, the following assumptions are made, among others:

-Vessel is blocked in, i.e. no credit for other pressure relief paths
-Overpressure arises as a reult of vapor generation
-Use latent heat at relieving conditions
-No credit for sprinklers or deluge systems
-*Must be adequate source of fuel for fire

and most importantly!: If there is NO flammable inventory, thre is NO exterior fire case!

Here is the caveat about drainage:

You can use the provision of adequate drainage to use the 21,000 factor instead pf 34,500, but it does not eliminate the exterior fire case so to speak.

API Standard 521 covers this case. I will copy verbatim what they say:

API 521 p. 40 (5.15.2.2.1)

"Some measures are necessary to control the spread of major spills from one area to another and to control surface drainage and refinery wastewater. This can be accomplished by the strategic use of sewers and trenches with adequate capacity and/or by using the natural slope of the land. The selection of the appropriate heat flux equation requires determination if there is "adequate drainage." The determination of what constitutes adequate drainage is subjective and left to the user to decide but it should be designed to carry flammable/combustible liquids away from a vessel. The method of removal (e.g. sewers, open trenches, sloping, etc.) should consider not only the flow of flammable or combustible liquids causing the pool fire but also the firewater that is applied by emergency responders. Some examples of drainage criteria are given in API 2510."

Basically, does your drainage have the capacity to handle the flammable liquid and the firewater along with it?

 

[MelMez]

Many thanks for your quick responses. This is what I was looking for