Tank Liquid Overfill Rate

[Bill3752]

I have a 30' tank, feeding 1300 gpm liquid. There is a 8" open vent on top of the tank as well as a 24" emergency vent.
To calculate the the maximum flow rate through the 8" open vent, should I use the design pressure of the tank or the tank elevation?

 

[LittleInch]

An open vent is not intended to be used as an overflow - two separate things.

The tank is likely to have virtually no design pressure (literally inches of water column). All you will ever have is the inches of water column above the vent.

Your tank with liquid really should have suitable independent shutdown systems.

But vents are for gas, overflow is for liquid.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[LittleInch]

A good ROT is to make the overflow at least as big as the pipe coming in

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[don1980]

I assume this is an API tank (either API 650 or API 620). In both cases, these tanks are not mechanically designed for liquid in the roof section of the tank (above the tank cylinder). In the rare case that a tank contains a liquid such as cool water, which is safe to overflow, then an overflow line can be connected at the top of the cylinder (near the roof seam). Overflow lines were relatively common many decades ago, but not today. That's because, for almost all tank contents, the risks associated with overflow are not tolerable.

If an overflow line is in fact safe in this case, then connect it to the tank cylinder near the top of the tank, and size the overflow line to be self-venting. If that critically important sizing detail is overlooked, an overflow situation can easily result in the tank failing due to vacuum.

 

[MJCronin]

A very good document covering the rules of tank venting is API-2000

https://global.ihs.com/doc_detail.cfm?document_name=API STD 2000&item_s_key=00010485

Sizing and configuring atmospheric Tank vents has been discussed many times in the "Storage Tank Engineering" forum

Another important point .... IMHO

The comments and cautions about tank overflow sizing (by Don1980, above) are very significant

Sizing piping for "self venting" overflows can get surprisingly large.

It would not be unusual for the size of a self-venting overflow pipe to be one to two sizes larger than the tank liquid inlet pipe

This has been discussed repeatedly in the past on eng-tips




MJCronin
Sr. Process Engineer