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Foam Backpressure Calculation for Tank Base Injection 1

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CSGWB

Mechanical
Oct 9, 2003
47
I am currently trying to size the line from a high backpressure foam generator to a base injection unit on a fuel storage tank and I am having trouble finding general mechanical properties of expanded foam to determine the likely pressure drop/backpressure.

The Foam is 3% AFFF.

Are there any rules of thumb to cover this?

Regards

CSGWB

 
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You are pumping the foam water concentrate using subsurface injection. The water/foam solution behaves the same as water alone. Follow the Hazen-Williams formula and the requirements of NFPA 11.
 
Thanks Stookeyfpe.

When calculating downstream of the foam generator, I assume calculations are then based on the aspirated foam velocity/density to generate friction, rather than the non aspirated upstream rate with everything else based on Water and C = 120 for Galvanised Pipe? (ie I will need a proportionately larger line size downstream)

CSGWB
 
You are correct. The friction loss will be based on the required discharge density and pipe sizes should be adjusted accordingly.

 
I understand that NFPA 11 provides a velocity for foam injection into the tank (approx 10ft/sec) for the last 15 pipe diameters, to permit proper mixing. Is there a maximum velocity for the aspirated foam in the pipework leading up to the injection point? My concern is that there may be a point at which the velocity of the foam may cause it to separate as it travells down the line? IS 30ft/sec too high?

CSGWB
 
Based on an API document concerning management of large AST fires (maybe API RP 2021, I'm not sure) I would keep the velocities to a maximum 10 feet/second. I have concerns that the foam can become separated from the water and the 10 foot/second value is reliable. Your pipe and installation costs may increase a little but it is not worth it to me, especially given the loss potential of fires in field erected ASTs.
 
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