De-Rating Vessel; How do I handle Vacuum rating?

[ehbadger]

I've got a ASME SectVIII DivI pressure vessel with some CUI. Overall wall thickness inspections have determined we need to derate from 50psi MAWP to 40psi MAWP. Generally for a derate we just change the rupture discs and relief valve settings to the new lower pressure.

However for this vessel it was originally full-vacuum rated. Calcs now show we are only good for ~6psi of vacuum rather than full vac. Obviously a relief valve is not going to protect against vacuum so it isn't as simple as changing that out.

Do I need to do anything per code to address the vacuum risk? Our integrity "code/license" guy said all the de-rate requires is the relief valve adjustment and expressed concern only of the MAWP part of it and didn't seem to care or think we needed to do anything on the MAEP side of things.

So can we handle the vacuum risk purely with process controls (i.e. alarms, pressure monitoring, interlocks, etc)? Or are we required to install some vacuum breaker system? I realize this may be specific to the jurisdiction/state (in Kentucky btw). I just don't have experience derating anything and don't know what the requirements are. And our pressure vessel SME is absent on Spring Break

Thanks

 

[TGS4]

How do you handle other vessels that cannot withstand full vacuum? Do that. You are very wise to notice the vacuum derate - good catch!

 

[Duwe6]

Yes; if process controls - DCS interlocks, etc. - are not guaranteed to keep a vacuum from forming, you will need a vacuum relief valve. And three years ago, we could not find a "VR" rated vac relief valve. It was API rated.