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Tank External Pressure

Tank External Pressure

Tank External Pressure

Is there any regulation that enforce a minimum external design pressure for a storage tank (gasoline storage tank)?
Let me explain why i’m asking this,
I can understand that there should be a minimum internal design pressure for a storage tank since it’s affected by many things (of course not atmospheric tank) like temperature, vapor pressure, storage tank operating conditions, AHJ regulations, etc...
API 650 tank is designed normally to 1” vacuum (Appendix V is excluded) but in which cases shall we consider a higher external design pressure?
Is there any scenario that I cannot seem to see?
If the tank is equipped with a well designed PV vent that works with a 1” design external pressure then why would i need a higher design vacuum?
I know that temperature changes, condensation normal inbreathing and out-breathing affect the external pressure but if the PV is designed and sized big enough to work with a 1” vacuum to the specified CFM then why do i need a higher external design pressure?
Can anyone please explain to me.

Detailing is a hobby,

RE: Tank External Pressure

Usually, I see a data sheet with no explanation of why a particular number is used.
One case, the customer was having trouble with valves leaking when designed for very low set points.
Occasionally, a flood condition, which does not give a uniform external pressure, but still requires Annex V.

RE: Tank External Pressure

Another case is the client operates the tank with with a vacuum type scrubber system. This could introduce a constant external operating pressure

RE: Tank External Pressure


Tanks from plastics materials are very vulnerable to external pressure - due to low Modulus.

Certainly connection of a scrubber under -ve pressure to the vent (without a by-pass)
has caused the failure of the cone cover.

Or, connection of vent (ditto) to local extraction system (in one case, same tank, twice!)

Then there is wind pressure from extreme gusts.

Waste water treatment plants are often located on flood planes, and yes a 1.5 m flood
level did cause failure of a tank bund.

And I can recall, hydrogen over sodium hypochlorite, and a faulty motor did really
partially 'blow the cone off'.

Ed Clymer
Resinfab & Associates

RE: Tank External Pressure

One thing to watch is any requirement for steam cleaning or sudden cold rain on any warm vapour can lead to very high inrush of air if the steam or vapour condenses.

More than one tank has collapsed due to that.

Any tank which is gas blanketed also can suffer if outflow for what ever reason exceeds incoming blanket gas supply.

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