Nameplate MAWP Less than Design Pressure

[AarVee]

Our customer needs an SA-516-70 pressure vessel that will operate at slightly below 1 atm. The original plan was to rate the vessel for 1 atm, and avoid needing to Code-stamp.

But now we find that the vessel must be pressurized with steam at 2 atm for 48 hours in order to cure the rubber lining that is to be applied. It seems to me that if the vessel is designed for 1.54 atm, there is nothing wrong with pressurizing it to 1.3 x 1.54 = 2.0 atm for 48 hours. Furthermore, it seems that the vessel, although designed for 1.54 atm, can still be tagged as 1 atm MAWP in order to avoid needing to Code-stamp.

Can anyone tell me if there is a flaw in my reasoning for this vessel, which is to be to be installed in the province of Sasketchewan?

 

[TGS4]

Easiest answer is to check with your local jurisdiction here.

I think that your design pressure is 2 atm.

 

[JStephen]

"It seems" is not reasoning, and I don't see any reasoning that justifies either step. You might also consider that if they need to hold 2 atm, you'd probably want your design pressure somewhere above that level, not at exactly the pressure you're planning to operate it at.

 

[steris]

The relief valve for the vessel is required to be set at a relief pressure no more than the stamped MAWP. If the design calculations do not support pressures of at least 2atm, then the code will not allow pressures that high. As an aside, Div 1 of the code does have a large factor of safety but that doesn't mean that you can lower this factor of safety and call it good on your own accord.

 

[racookpe1978]

Is this a one-time pressure run to apply and cure the lining?

Or is it for regular production runs?

 

[SNORGY]

Just because you held the vessel at pressure for 48 hours to cure the lining, it doesn't mean you have to nameplate rate it for the pressure that such activity might otherwise qualify it for. You can nameplate it to anything lower if you so choose.

We deal with non-Code FRP knockout drums all the time, and declare them non-Code and designed to 100 kPag or less, even though the shop involved always hydrostatic leak tests them to 50 psig (345 kPag).

 

[JStephen]

I mis-read the question the first time, and thought you were saying you would routinely operate at 2 atm to cure rubber linings (of other items), and having seen that, agree with Snorgy.
You might check if the difference between 1 and 2 atm really makes much difference in the design or not- I would suspect not.

 

[cloa]

I don't why anyone think this likely occur often- its a lining- they can only line a tank once until it fails and they have to remove remnants and reline the tank.

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[AarVee]

Thank you all for your input.

This is a complex issue to me because it is a tall vessel that must be hydrotested vertically due to logistic considerations. (40 psi static head during hydro on a vessel intended to operate empty at 1 atm.)

I will look into Snorgy's comment, but will also bear in mind that the 48 hours at 2 atm is essentially a pneumatic test. This has its own hazards, and safety is paramount.

 

[SNORGY]

A 6'-0" OD (guessed) x 100'-0" tall (based on 40 psi water head) vessel, pneumatically tested to 29.6 psig, has stored energy approximately equivalent to 8 lbm TNT and a "restricted distance" for testing of about 120 feet. While opinions vary across a broad spectrum regarding the risks associated with pneumatic testing, my thought would be that if the vessel is vertical and has been designed to hold water to its full height, if only for leak testing, then with judicious use of pre-testing UT or radiography, I am not inclined to be too worried about the air test; but that's just me. That said, if this is a shop hydrostatic test, would they really do that in the vertical position? I wouldn't. Is there a requirement for in-situ hydro after erection?

 

[AarVee]

SNORGY,

Without getting into details, this large vessel must be assembled and tested on site. You echo my own thoughts regarding UT and RT, but I was thinking the "restricted distance" should be 500 ft and everyone should be inside cars with the engines running.

Back to my original quandry: It's hard for me to understand why we wouldn't get this vessel (designed and built for 2 atm) stamped to the level of quality proven. The only reason I can see is that the bureaucratic red tape (and fees) are unnecessary. The customer said he only needed a vessel to operate at atmospheric pressure, so it's difficult to argue that he should spend any extra time and money certifying it to 2 atm.

But it sounds like there's nothig wrong with the MAWP stamped on the nameplate being lower than the design pressure.

 

[SNORGY]

AarVee

I agree that the vessel should (indeed must) be designed, built and tested to the most severe condition it will ever see - including the lining cure. After that, you can *nameplate* it to anything less severe.

 

[TGS4]

Quit speculating and ask the jurisdiction! They have final authority.

 

[AarVee]

Excellent suggestion.