Section VIII Safety Valves

[steris]

Hello All,

My company makes jacketed pressure vessels where the jacket and inner chamber are filled with steam. One of our designs takes steam from the jacket and uses that as the steam source for the inner chamber. This is the only source of pressure for the inner chamber. The jacket is rated for 45 PSIG internal and the chamber is rated for 45 PSIG external and 30 PSIG internal. Can we install one safety valve on the jacket and not have one on the inner chamber? The argument is if the jacket safety is set to 30 PSIG and the jacket is the only source of pressure then the chamber is inherently already protected. As always, thank you for the help!

Best,
Steris

 

[vpl]

What Code is being used to manufacture your vessels... and have you reviewed that Code?

Patricia Lougheed

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[davefitz]

you have not defined if the two vessels are in direct communication with each other without intervening valves or flow restrictions, or if there is any exothermic process occurring or means of pressurizing the internal vessle via some other connection.

 

[steris]

We are making them in accordance with Section VIII and there is a valve separating the two vessels. There are no exothermic processes or other possible sources of pressure other than the jacket. I understand that a strict interpretation of the code will require a safety, however can a case be made that only one safety is needed?

 

[Duwe6]

Absolutly not. The vessels can be isolated from each other. thus a PRV is required on each vessel.

 

[davefitz]

Duweg6 is right. The inner vessel could be filled with condensate , isol valve closed, and if the outer jacket heats up the inner vessel then a thermal releif valve , at a minimum , is needed at the inner vessel. You would need to calculate the worst case of thermal relief, and there may be other scenarios possible that may need to be considered.

 

[steris]

Hi all -
I completely understand the arguments and the intention of the code. What I don't understand is that a few of our competitors use only one safety and I'm not sure how they are able to get away with it. Their process flow and piping is roughly the same as ours. I feel like there is an exception somewhere but I don't know where.

Thanks again!

-Steris

 

[steris]

Hi All -

I think I found what I was looking for. There is an exception in Appendix M-5.5 as allowed by UG-125(g) & UG-135(d)(2). I hope that is able to help someone else. Thanks again for all your help!

Best,
Steris