Vacuum Issues

[Rat1]

Anyone have experience with a vessel (ca 1980) that has a U-1 indicating full vacuum not even being close under the current code? Still has original thickness, looks good, and is sufficiently thick for the pressure rating. BUT, the vacuum is the issue. Evaluating to origianl code is not an option.

Any thoughts??

 

[codeeng]

I don't think the Code has changed in this time regarding vacuum design so it would appear that it was never good for full vacuum in the first instance. Where does it now fail?

 

[Rat1]

Maximum external pressure is ~9.5 psi.

Most likely they do not ever run much vacuum.

Thanks for the input.

 

[jte]

Rat-

My guess is that your vessel was designed for half vacuum. Prior to 1998 (or perhaps the '96 or '97 Addenda) VIII-1 UG-28(f) stated that vessels ...shall be designed for a maximum allowable external working pressure of 15 psi or 25% more than the maximum possible external pressure, whichever is smaller. I designed many vessels to half vac using an external design pressure of 9.375 (that's 1.25*7.5). Somewhere along the way to the U1 form the "half vac" got changed to "full vac."

jt

 

[EdStainless]

You do see a lot of vessels that have had stiffening rings added to them.
Is there no current code condition for partial vacuum?

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[Rat1]

We see many vessels with rings for vacuum support. I think the "code" reqirement is a gray area because it is under the +/-15 psi cutoff. We review the vessel based on the operating conditions, and if the calcs. say X vacuum is acceptable, that is what we allow.