Liquid Seal Vacuum Formation in Tank Gravity Draining Liquid Outlet

[ChemEngSquirrel]

I’m installing a drain tank which will normally drain diesel fuel. The tank can occasionally also receive hydrocarbon gas. This is vented to a safe area via the tanks vent, but hydrocarbon gas must not be allowed to follow the liquid outlet line i.e. gas blow-by must not occur. To prevent gas blow-by, the liquid outlet line (which simply gravity drains to another tank) has a simple 'U' tube configuration to act as a liquid seal.

How can I determine if my liquid seal (U tube arrangement) requires a vacuum breaker?

My concern is that the liquid outlet could form a vacuum in the gravity draining section (downstream of ‘U’ tube arrangement) and siphon tank liquid content, thereby allowing gas blow-by. If I concluded that vacuum formation was possible, I would simply install a vent line downstream of the U tube arrangement.

Thanks in advance.

 

[katmar]

The criterion for determining whether you will form a siphon or not is the Froude number. If Fr > 1.0 you will quickly flush the vapor out of the line and initiate a siphon. For Fr < 0.31 the vapor will remain in the pipe and no siphon will form. Between 0.31 and 1.0 the vapor will be gradually moved along the pipe and you may get some instability. To be sure that you do not siphon your U leg seal dry you should either put a vent downstream of the U leg, or design for Fr < 0.31.

There was a detailed discussion of the problem in thread378-289211

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[unclesyd]

You might want to check out the offerings by Armstrong to see if they are applicable to your operations.

http://www.armstronginternational.com/air-gas-vents