Fire Case - Tank Area 1

[Bill3752]

For a horizontal tank that is more than 1/2 full (say 2/3), what area should be assumed to be exposed to a fire from below? Only the bottom half? Thanks, bill

 

[Chance17]

The area exposed to Fire corresponds to the liquid level in the tank.
If the level is 25%, the wetted area corresponds to 25%.
One has to ajust for liquid swell.
If the level is 25% AT AMBIENT, the level will expand at actual relief temperature.
Something like hexane can expand alot and can cause problems.
If one starts with 85% level, the swell may expand to 95-100%. Two-phase flow calculations would be triggered.

 

[Bill3752]

Thanks, so the bottom line is that I should assume the fire will hit the top of the tank also.

To bring this to a conclusion then, if I have a vertically mounted cylinder that is completely full, I should count the very top of the tank in the area calculation? For the sake of discussion, forget about swelling and two phase flow.

 

[Chance17]

Yes and also include the top head if liquid resides inside the head.

There is a concept of flame height associated with wetted area.
The exact flame height depends on standard being used.
I believe API uses 25-ft for low hazard process areas and 50-ft for high hazard process areas.
For tank farms 30-ft flame height is used.
Eurpoean folks use 10-meters.
NFPA has something different depending on the situation.

If the bottom of a 10-foot verticle tank was 23-ft above grade, the wetted surface would correspond to 2-ft assuming the 25-ft flame height standard

 

[stookeyfpe]

You should also review API 2000 and NFPA 30. Wetted area is the internal area of the tank that can be wetted by the stored liquid. I would not be comfortable with a design based on filling a tank to a given percent volume - you should instead base your vent flow rate on the available volume of the tank.

 

[sheiko]

Dear Yitbos,

You said "Eurpoean folks use 10-meters". COuld you please give us your source?

Thanks

"We don't believe things because they are true, things are true because we believe them."

 

[skearse]

Ditto on the 50 foot for API "high hazard process areas." The only API 520/521 flame height I'm aware of is 25 ft above a surface that could conceivably contain a pool of flammable material (thus supporting a pool fire).

Bill,
If you are a subscriber at Che.com [or receive Chemical Engineering Magazine] there is a great article for calculating wetted areas for partially filled horizontal vessels.
Doane, R.C. "Accurate Wetted Areas for Partially Filled Vessels." Chemical Engineering Magazine, December 2007. pp 56-7.