The SFPE Handbook and various vendors advise that subsurface injection of foam is not recommended for internal, floating roof tanks. Are there any studies or incidents that support this position?
I had a reply to this question outside of the Exchange. I have pasted the comments below of those interested in this topic. Any additional comments are appreciated. Thanks
NFPA 11 Standard for Low, Medium and High Expansion Foam (2016)
Note to Table 184.108.40.206.8
(3) The most viscous fuel that has been extinguished by subsurface injection where stored at ambient conditions [15.6°C (60°F)] had a viscosity of 2000 SSU (440 centistokes) and a pour point of −9.4°C (15°F).
Subsurface injection of foam generally is not recommended for fuels that have a viscosity greater than 440 centistokes (2000 SSU) at their minimum anticipated storage temperature
220.127.116.11 Subsurface and Semi-subsurface Injection. Subsurface and semi-subsurface injection shall not be used for protection of open-top or covered floating roof tanks because of the possibility of improper distribution of foam at the fuel surface.
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sorry have to spell it out
Just put it together
The reason is that the floating roof will prevent foam distribution on fuel surface according to NFPA 11 2016Ed, 18.104.22.168 and 22.214.171.124.4. Even if you select a full surface fire as a design basis for this type of tank, improper foam distribution will be envisaged due to the sunken or distorted floating roof.
In my opinion, the rim seal area protection which is designed and installed per NFPA 11 would be the best option to protect internal floating roof tanks.
You can consult with WILLIAMS when a client determines the full surface fire on large crude storage tanks exceeding 60 m of diameter.
In my opinion providing a rim seal protection in internal floating roof tank is of no benifit. A rimseal area fire is least possible in an IFR tank.Moreover such tank will be under nitrogen purging above the floting roof area which further limits the chance of any fire.Fixed or semifixed foam system is enough for fire protection
Nitrogen blanketing of large atmospheric tanks is rare; at least in the North American oil industry
It depends on the process design of the storage tanks. If stored liquid contained toxic substances like hydrogen sulfide or PO, the nitrogen blanketing would be necessary to prevent unwanted dispersion.
However, the large storages tank storing Diesel or Jet A1 are designed as an atmospheric tank and have internal floating roofs to minimize vaporization in the middle east, instead of blanketing. That way, they are less likely to be involved a serious fire as you pinpointed when the floating roofs are designed and installed as per NFPA 30 2018Ed, 22.2.2.
For either fixed or semifixed system you applied, you need to select and install applicable discharge outlets on the tanks such as Type II, above seal or below seal, or subsurface or semi-subsurface based on anticipated or designed fire scenario.
Feel free to contact me if you need further clarification.
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