API 521: Sum of fire areas for vessels..Sum duties or areas.. 1

[jamesbanda]

I'd like a quick check on methods for fire area that should be used under API521 to get overall duty..

I believe if we have a system with two vessels the correct way to consider fire load is as below.

method 1: Q (Total) = 43.2 * F* A1 ^0.82 + 43.2 * F* A2 ^0.82
method 2: i believe this is incorrect.. QTotal = 43.2 * F* {A1+A2} ^0.82

for a vessel with area 50m2
Method 1 gives ~2100 KW
Method 2 gives ~1900 KW

So i believe method 1 is correct. thats what i've been using..but i've come across a calc the other way.. so wanted to check..

 

[don1980]

If the environmental factor (f) is different for the two vessels, the method-1 is correct and method-2 is incorrect.

If the F factor is the same, then both metods yield the same answer.

It boils down to whether the two vessels have the same type and same thickness of insulation. That's often not the case, so it's better practice to show method-1 in the documentation. That avoids the possibility of errors.

 

[chicopee]

Mathematically method 2 is incorrect if you are deriving method2 from method 1.

 

[TD2K]

Combining the areas and then taking the 0.82 power gives you a less conservative answer. I don't think I've seen this explicitly discussed before but if they are separate vessels, I think I would have a hard time convincing my colleagues to add up all the areas first and then calculate the heat input. Now, if the areas are extremely large where you are pushing the limits of the fire circle you are using, then you could be justified in using the lower value.

If we add an allowance for piping, it's a % of the vessel area so we are basically using method 2. If however we have multiple vessels, we enter the vessels in our spreadsheet and the individual fire relief loads are calculates and then summed rather than summing the areas and then doing the duty calculation so we use both methods

As Clint Eastwood put it "do you feel lucky?"

 

[Latexman]

I've always been told the equation is empirical, so it must be evaluated as is for each vessel or component in a vessel, then add the heat duties. Therefore, I always use Method 1.

Good luck,
Latexman

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[jamesbanda]

Thanks for your posts.

Agree is is an empirical formula and the adding of pipework area is equally a judgement call some places do this and others don't I've not seen specific guidance on this either.

 

[RaRo]

The ^0.82 in API 521 is there to account for incomplete flame impigement on the vessel wall.
Depending on the layout and geometries, you may have to challenge that ^0.82 (e.g. take ^1 for small cross-section vertical equipments)
So inherently, method 1 is the way to write it down.

 

[jamesbanda]

Thanks RaRo,

I've never seen a factor of 1 used. And, I've looked at calcs for square vessels a factor of A[sup]0.82[/sup] (A^0.82) was used.

Is it common to use a factor of 1 for sqaure vessels and is that documented in API521 - I've reviewed API 521 several times to update project standards and not come over that detail.

I'd appreciate more comments on this

 

[silebi66]

jamesbanda :

I am sorry not see the method 2 equation you provided, but there is two equations common to see for the fire case in API521

1.For equipment with adequate drainage and fire fighting facility
Q = 43,200 ⋅ F ⋅Aws^0.82

2.For equipment without adequate drainage and fire fighting facility
Q = 70,900 ⋅F ⋅Aws^0.82

Adequate drainage facility here can be defined as the facility that shall be able to carry the flammable/combustible liquid and firewater away from the equipment.
Adequate fire fighting facility can be defined as the facility that shall be activated as soon as the fire begins.
The environmental factor F in equations above and hereafter is the credit factor primarily for fire proofing insulation and not for general thermal insulation.

Hope this can help you a little.
Thank you.