Is Operating a Pressure Vessel Above MAWP Safe at Ambient Temperatures?

[Sh1ps]

A pressure vessel is stamped with an MAWP of 150 psi at 450ºF.
We are operating at 120 psi at ambient temperatures around 70ºF.
Is it allowed to operate at the MAWP with a relief valve set to 165 psi if operating temperatures are always at the ambient level.

 

[Q_Goest]

Hi, welcome to the board. I'll assume Section VIII, Div 1 for this discussion, but I believe the other divisions have the same requirements. Please verify which division of the code the vessel is built to.

Take a look at UG-119. That shows what the nameplate should look like. If it has a duel rating (ie: higher pressure rating at a lower temperature) then it should show on the nameplate.

Para. UG-125 provides requirements on setting the valve. In short, you have to set a minimum of one relief valve at the MAWP of the vessel, not higher. The code allows for a second device to be set at a higher pressure, but you need one at the MAWP.

 

[Sh1ps]

Q-Goest,
Thanks for the quick response.

Yes, Section VIII, Div 1 was used according the drawing I have.
There is no nameplate but the stamped info does not show a dual rating - so no luck there.

Am I correct in assuming that using a lower temperature rating would allow for a higher MAWP if we could go for recertification (we would like to run at 150 psi) so really only need to increase the MAWP to 165 or maybe 170 psi.

I have no experience of doing this but could imagine that it may be simpler to buy a new vessel rather than go through the hoops of recertification of a 10 years old vessel.

 

[Q_Goest]

The MAWP wouldn't necessarily be higher for the lower temperature. The rating depends on the material's allowable stress which for most carbon steels, doesn't change between 100 F and 450 F.

I've not heard of pressure vessels being recertified and restamped. It might be possible, but you'd need to have the original manufacturer do that because they're the ones that have all the analysis on it. If I had to guess though, I'd say your chances are slim to none.

There may be one out for you, though I wouldn't recommend it. The code is recognized and legally binding in most states in the US. However, Texas, SC and a few other states don't have a law requireing vessels to be stamped. In those states, it is up to the vessel owner to do what they see fit. In those states, companies generally use the code since it provides them considerable legal protection. However, I've seen companies use uncoded vessels in those states.