Class 900 PSV in a class 600 system? 1

[ivarfar]

We have a case where the system piping was determined to be class 600 and later the PSVs were found to be class 900.

The stainless steel PSVs have been dimensioned to size "L". Set pressure is 7100 kPa abs and relief temp. is 38°C. This results in a class 900 (inlet) PSV according to API 526.

Is there a gap/mismatch between ASME code and API 526 definitions of pressure class? Is this intentional?

The suggested solution is to introduce a spec. break upstream of the PSV because the set pressure cannot be changed for various reasons. Is this good engineering practice?

 

[EGT01]

This is just a guess but I suspect there is some limitation with the way a full nozzle fits into the body of the relief valve and becomes part of the mating surface for the connecting pipe flange. If you notice, the maximum set pressure for pilot operated relief valves seem to follow the pressure ratings of the flange class, that's probably because they have a semi-nozzle design and the relief valve flange directly mates to the pipe flange.

Depending on the responses you get from this forum, you may want to try one of the other forums too, like the Piping & fluid mechanics engineering Forum (forum378) or better yet, give a call to your favorite relief valve supplier, I'm sure they will be more than happy to assist.

As far as using a relief valve with a 900# class on a 600# class system, I don't think there is any real concern with using a higher rated material in a lower rating service as long as you recognize the limiting component in your system. Certainly you should add the spec break on your P&IDs to make it clear what you have installed. If nothing else, this will alert everyone to the fact that you will need to have a 900# class flange fitted to the end of your pipe to allow proper connection to the 900# class flange on your relief valve.

 

[sshep]

I don't know why there is a difference, but I know that there sometimes is and EGT01's advice about the spec break is good. I have done several projects where we had to rework flanges on piping already fabricated or installed. I think this is common experience.

Tough part is: the preliminary sizing of the valve doesn't catch this. It invariably comes up later when the valve is selected. By then the project is working on approved p&ids and the late change can easily slip through despite good people, change order systems, etc.

I see flange rating step-ups on 150# and 300# systems as well. Sorry I can't add anything useful about why the difference comes up.

best wishes,
sshep