External fire on equipment: standard practice re: associated pipe 2

[colemanstoops]

API 521, §5.15.1.1 is not definitive as to whether or not associated piping is included in the calculation of the area for impingement of a pool fire. In fact, it essentially says that it's at the discretion of the user.

I'm trying to find out what standard / common practice is; that is, does such practice include or exclude the piping? My sense is that such piping is EXCLUDED, since including it would open subjective questions as to where to draw the envelope. I'd like opinions/experience, please. Thanks.

 

[greenche]

In the refining industry I have seen it both excluded and added as a percentage of the equipment area (e.g. an additional 10-20%) but more often excluded. This can be interrelated with how the company handles calculation of the heat of vaporization and the confidence level in the description of the fluid composition.

 

[Chance17]

I can not explane why -- but it is common practice to add wetted surface for Reboiler pipe on Distillation Units.
I have not seen pipe wetted surface commonly used for other applications.
It is part of engineering judgement.

 

[colemanstoops]

I can rationalize including reboiler piping on a distillation unit: seems to me the thinking is that a pool fire would necessarily involve both the column and reboiler, and that such piping would be entirely within the "equipment" envelope.

That said, it also seems to me that it's reasonable to exclude piping that clearly exits the equipment envelope since that provides a consistent criterion as to where to draw that envelope (e.g., at the flanges where such piping connects to the equipment in question).

I'm still interested, by the way, in other opinions.

 

[Armen75]

The question you should ask is can the surface of the pipe be in the same pool fire as the vessel?
If the line is from the top of a tall tower or in a pipe rack, than it might not be necfessary to include that surface area since it will not really be affected by the fire;
however the bottoms line from the column (going to the reboiler) would surely be in the middle of the fire and therefore should be added.
One might also include the other vessel trims (drains, purge, level gauges and stand-pipes...) - and to simplify we usually lump all that in the 10%...

it all depends on the piping arrangement you are looking at.

 

[MortenA]

For most wetted vessel cases pipeing would not count. For some dry it might. Remember that max height of fire is (hmm i think it may by 25 ft. - its around 7 m from my memory). So some piping e.g. on bridges may be elevated above height of the pool fire?

Best regards

Morten

 

[jamesbanda]

I think you have to take a view on what your trying to do with a relief valve. You not trying to protect every event - its your very last line of defence and stops major loss of vessel integrity which would cause a major accident hazard (eg. BELEVE.. or tank rupture..) as much as pipe ruptures are bad and very undersirable it is not possible to protect small pipes.. there is enough conservativism in api to cover pipework around vessels without adding extra area..

 

[hniaboujemaa]

I have done calculation for such case many time and the best practice as per my experience is to add a 10% in the wetted area to cover the piping around the vessel exposed to fire.

Regards

 

[MortenA]

hniaboujemaa - best practice how? Have you ever had your design "tested" in an actual fire? You can always add loads. Sometimes it will result in a bigger valve, this will increase the flare load etc. Dont just add margins on margins. Choose a value that makes sense.

That said: In may apps. your assumption is OK with me.

Best regards

Morten