Section VIII Operating Pressure Clarification

[greg87]

In ASME Section VIII (one of ) the criteria that determines is a vessel is covered in the code is if the “operating pressure” is 15 psig or higher. There are some cases where the codes definition of “operating pressure” is not clear to me. Maybe best using a hypothetical: Let’s presume we have a tank with a short open vent, with no valve. There is 100 psig nitrogen fed to the tank with a regulator set at 5 psig and there is no relief valve on the regulator. The pressure in the tank is normally 1 psig or less. (Let’s also presume that any other overpressure scenarios have relieving flows far smaller than the nitrogen.) For the following cases, would the tank require an ASME code stamp, ie. is the pressure that it is operated at above 15 psig?

1] The size of tank’s vent has been sized by rigorous calcs (& documented) to ensure that it can handle the regulator failure flow with the tank well below 15 psig.
2] same as #1, but with a breather valve on the vent, normally maintaining 1 psig in the tank. It is sized to handle the full regulator failure flow.
3] The vent is not sized; and let’s presume that the tank would be pressurized to well over 15 psig if the regulator failed.
4] same as #4 but there is a relief valve set at 10 psig on the regulator line, sized to handle the full flow of the regulator, but not an ASME approved relief valve.

Please note that I am not looking for input on design, but only on what the code requires (my actual designs would fall well on the conservative side.) But I want to know what the code is saying, and be able to tell a client, why I am including the features that I am including.
Thanks for the help.

 

[metengr]

greg87;
In reference to the ASME B&PV code, Section VIII, Div 1 the design not operating pressure is what is critical and is used for stamping purposes. The definition of design pressure can be found in UG-21. Based on statement 3 above (critical statement), and using the definition stated in UG-21 .... vessels shall be designed for at least the most severe condition of pressure and temperature.... this would imply to me that you must have the vessel supplied with a U stamp.

You would have to re-size the vent to assure that the vessel would never exceed 15 psi design pressure in any worst case scenario. If this is done, the vessel then would be exempt from Section VIII, Div 1 stamping requirements.

As a second option, you can contact the Jurisdiction where the vessel will be installed to determine if they have any additional requirements above and beyond ASME Section VIII, Div 1. In some Jurisdictions, pressure vessels are NOT regulated so having a U-stamp falls back on the owner of the vessel and the insurance company (if the equipment is insured). If the owner is self insured, they have the final say.

 

[prex]

Don't agree with metengr: if you read the whole first sentence of UG-21, you'll see that the a vessel shall be designed for at least the most severe condition of coincident pressure and temperature expected in normal operation.
Now the failure of a regulator is an abnormal condition (unless it could occur at plus or minus anticipated intervals of time), so all of the cases illustrated by greg87 do not require a code stamp.
Of course this doesn't relieve the designer from inspecting the consequences of any abnormal conditions whose probability of occurrence is not negligible, and taking appropriate measures: the simplest and normally adopted solution for the regulator failure would of course be the appropriate sizing of the vent.

prex

http://www.xcalcs.com

Online tools for structural design

 

[metengr]

Prex;
In my earlier email I probably did not make myself clear and statements 1,2,4 scenarios would not require a U stamp, we are in agreement. Scenario 3 however is possible even under normal operation, because we have had regulators fail in service. I still believe in a more conservative design approach and as such the vent sizing is critical to meet no stamping requirements.

 

[deanc]

All

Please review Definitions in Appendix 3.

Be safe.

 

[prex]

Now we are in agreement, metengr.
I agree also that case 3 is a bit more complex: I simply passed over it, because it is quite unrealistic: why should someone not size the vent for an anticipated failure?
And even if this was a possibility, the tank would not necessarily need be stamped, unless the regulator failure was to be somehow considered as a normal condition (it will likely happen at least once in the life of the equipment?).
Of course the designer might decide anyway to do so, but again: who would realistically decide to stamp a tank only to avoid sizing a vent from 1" to 3" (or any other diameters)?

prex

http://www.xcalcs.com

Online tools for structural design