Code exemption for vessel containing water?

[barnesed]

A vendor for a water softener package that includes a 300 Gal. vessel containing water (not heated) with a design pressure of 125 psig claims that a relief valve is not required per the ASME code. It appears from the ASME Code Section VIII, U-1 (c)(2)(f), that this is correct since this is "a vessel for containing water under pressure" that has a design pressure under 300 psig and a design temperature under 210 °F. I can't believe this is permitted. Am I missing something? There have been many process "water" vessels in the past that did not exceed these requirements that I did not hesitate for a minute to include a relief valve. I would appreciate any input on this. Thanks.

 

[prex]

The relief valve is not necessary for the safety of the public (the only concern for the code) but might be desirable to preserve the vessel from pressure surges, if they may be present.

prex

http://www.xcalcs.com

Online tools for structural design

 

[EGT01]

I don't have a copy of the most current code but this is what I find the 1998 edition says
"U–1(c)(2) Based on the Committee’s consideration, the following classes of vessels are not included in the scope of this Division; however, any pressure vessel which meets all the applicable requirements of this Division may be stamped with the Code U Symbol:"

"(f) a vessel for containing water under pressure, including
those containing air the compression of which serves only as a cushion, when none of the following limitations are exceeded:
(1) a design pressure of 300 psi;
(2) a design temperature of 210F."

This may be somewhat "picky" about the text but I would say the vendor's vessel "could" fall outside the scope of ASME Sec VIII and therefore would not need to follow the overpressure protection requirements per the code rather than say "a relief valve is not required per the ASME code".

However, the plant I'm working at now requires that any vessel purchased with a design pressure > 15 psig must be built to ASME code. I would ask if the vendor sells his vessel with an ASME stamp, then it seems you would need to follow the code requirements per the last statement in U–1(c)(2).

 

[PVinspector]

A relief valve requirement is based on in-service rules, which are from the jurisdiction, not the construction code. See if the jurisdiction where installed requires it, then the ASME code can help size it, though that is a very confusing area of the code!

ASME: Always-Sometimes-Maybe-Except
PED: Pain Every Day

 

[hacksaw]

if the tank can be blocked in, if it can be pressurized above 15 psig for what ever reason, then common sense dictates having provision for over-pressure relief, if not on the vessel then on the pipe.

relief devices are not that expensive.

That said, most water softener units do not have relief devices. the rational is that the pressure is limited by the mains, and there is no source of heat (as in a hot water heater).