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Vent sizing for a wast storage tank

Vent sizing for a wast storage tank

(OP)
Hi everyone,

I have a 5000gal capacity storage tank that is used to hold process waste until our vendor comes in to haul it away. The tank is made of 304SS and is 0.19 inch thick or 0.12 inch thick (drawing says 11GA Shell) and has multiple vents on it. The vendor pulls the liquid out from a 3 inch pipe (with camlok connection) at the bottom of the vessel using vacuum. The waste is mostly water with some caustic. My questions are:

1) Since this tank is designed for ambient pressure, can you use vacuum in this manner to empty it?

2) Can the vents be sized as described in API 2000(different standards more applicable?) or does the fact that vacuum is used to empty it throw some other considerations into the mix? My understanding is that so long as the mass balance works out to be more air in that liquid out, the tank shouldn't implode; if the vents are sized for vacuum drain, then it should be oversized when it comes to thermal expansion/contractions.

Thanks,
Jay

RE: Vent sizing for a wast storage tank

API 650 tanks would be specified with a vacuum design pressure also - API 2000 has some formulas to check the adequacy of the breather vents to keep within this vacuum pressure limit for a given liquid draw rate.

Max liquid draw rate in m3/hr = air rate being drawn in through vents in actual m3/hr.

Using this air draw rate, check what dp must exist between outside atm and tank vapor head space to enable this air rate.

RE: Vent sizing for a wast storage tank

How the liquid is removed from the tank is irrelevant unless there is a significant risk that you're going to expose the inlet and "suck it dry".

All vents allow for both thermal events and also liquid in / out flow rates. Air volumes should match liquid flows in / out in order to keep differential pressures to very low numbers ( usually low inches of water column).

In general, if the vents diameters are equal to or greater then the diameters of the incoming / outgoing pipe then you shouldn't have a big issue.

Your key issue is the potential for high gas flow if the outlet pipe becomes exposed and the "vacuum" truck pulls through large flowrates of air.

At that point your vents might not be big enough if they have only been sized for liquid outflow.

API 2000 is good to use.

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