Rupture disk under Pilot operated SV 2

[edanyel]

Is it "common sense", "good practice" or "no objection" for the use of a RD under a POSV?
I'll be thankfull for your comments.
My own feeling is that if a non fragmenting RD is used and
the sense tubing of POSV is under the disk there shouldn't be problem at all, please let me know your opinions.
Thanks in advance
Daniel

 

[don1980]

That's a good question. There might be some rare cases where that design is justified, but in general I strongly advise against installing an RD under a POSV. An RD/POSV design can result in prolonged and undetected leakage through the POSV. Remember, the POSV requires system pressure in order to keep the valve tightly closed. A disk will isolate the POSV from that system pressure, thus resulting in essentially no closing force on the seat of the main valve. If gases leak through a crack or pin-hole in the disk, they will pass through the POSV rather than pressurizing the cavity between the disk and POSV. That condition can go undetected for a very long time.

Would a remote-sensing POSV solve that particular problem? Yes, it would, but it would also undermine the whole reason for installing an RD under the valve. Think about why the RD was considered in the first place. It was probably for one of these two reasons: (1) to isolate the POSV from highly corrosive process chemicals and thus allow the use of a lower metallurgy for the POSV; or (2) to provide a positive seal to prevent fugative emissions. If the reason was #1, then a remote-sensing POSV makes no sense because it brings process fluid around the RD and into the POSV. If the reason was #2, then refer to my comments in the first paragraph.

So, installing a RD under a POSV is generally a bad idea. BTW, your question is the first time I've seen this issue mentioned outside of the company where I work. Since recognizing this hazard several years ago, I've been surprised that I haven't seen this issue discussed anywhere. The risks associated with this design are pretty obvious once you think it through, but honestly I'd never done so until I was involved in an investigation to determine the root cause of a reportable spill.

 

[gerhardl]

There is another reason as well to avoid the rupture disc.

A POSV has as one feature the relatively narrow gap between the max allowed operating process pressure and the set pressure for the POSV. This is smaller than the gap for a spring operated safety valve.

This gap is normally far higher for the rupture disc than for both SV-types,(not true for all constructions), and it will/might not be possible to utilize the process operating pressure up to max.

Please also remember that some POSV's will have the possibillity of a pilot operating fluid separated (through membrane in presssure tank) from the process fluid, taking away the pilot parts permanent exposure to process fluid and process temperature.