ASME code pressure exclusion requirements 2

[Arnieg]

We have a 15,000 gallon liquid nitrogen storage tank that is operated at 14 psig. The tank is enclosed in a vacuum casing under a full vacuum, therefore the outer pressure on the liquid storage tank is 0 psia. Under these conditions the net pressure on the liquid storage tank is 28.7 psi, (14 + atmospheric pressure of 14.7). Does this tank meet the code pressure exclusion requirement because it is operated at 14 psig or does it fall under the code because it is being operated at 28.7 psi.

 

[chicopee]

Suppose that you were able to place a pressure guage in the interstitial space and read it under normal operating condition, what would the pressure read? That should answer your question.

 

[metengr]

Does this tank meet the code pressure exclusion requirement because it is operated at 14 psig or does it fall under the code because it is being operated at 28.7 psi.

First off, you need to review the rules or regulations where the vessel will be installed. If in the US, there is a division of boiler/pressure vessel safety that establishes rules for regulating boilers and pressure vessels.

If this vessel will be installed in any of the Jurisdictions in the US, which use the ASME Boiler and PV Code, the answer to your question is most likely no.

You can still design and fabricate to Section VIII, Div 1 requirements (which I would recommend). The only difference is that you do not need to have it stamped.

 

[jte]

Arnieg-

I'll put myself in an unusual position by disagreeing with metengr. I would argue that this vessel is within the scope of VIII-1; jurisdictional requirements will vary.

You do not get the "not more than 15 psi" excemption from the scope due to:

(2) combination units having an internal or external pressure in each chamber not exceeding 15 psi (100 kPa) and differential pressure on the common elements not exceeding 15 psi (100 kPa)...

Note that the wording of this part has changed, I believe first published in A06.

The shell should have been designed to handle the differential pressure.

...For this condition and for test conditions, the maximum difference in pressure between the inside and outside of a vessel, or between any two chambers of a combination unit, shall be considered...

Was the shell designed for 29+ psi differential pressure?

design pressure: the pressure used in the design of a vessel component together with the coincident design metal temperature, for the purpose of determining the minimum permissible thickness or physical characteristics of the different zones of the vessel. When applicable, static head shall be added to the design pressure to determine the thickness of any specific zone of the vessel (see UG-21).

Finally, Appendix 9 places you in its scope based on combined pressure:

9-1(c) Where the internal design pressure is 15 psi (100 kPa) or less, and any combination of pressures and vacuum in the vessel and jacket will produce a total external pressure greater than 15 psi (100 kPa) on the inner vessel wall, then the entire jacket shall be interpreted as within the scope of this part.

Based on the above, I'd say that this vessel is within the scope of Section VIII Div. 1.

jt

 

[JoeTank]

jte,
You place yourself in great peril by choosing to disagree with metengr. But, I believe you have picked your times wisely. I concur with your disagreement. The design pressure for the inner vessel is about 29 psig.

Joe Tank

 

[metengr]

Well, you win some and you loose some among your peers. I suppose one could question if this is truly a "combination unit" in Code language because the casing around the vessel to me can be be added or removed w/o affecting the functionality of the liquid nitrogen storage tank (as it is designed to hold a fluid).

Having an external casing (could be sheet metal or fiberglass) to me in this application provides a vacuum necessary for zero heat transfer (no heat gain to warm the liquid nitrogen). Without further information as to the actual function of the casing and it's design, I agree with the above.