Pilot Piping for a pilot operated relief valve downstream of a Bursting disk 1

[sgoel]

Hello everyone

We are installing a pilot operated relief valve on a process line. This relief valve is downstream of a bursting disk (bd). We are debating if the location of the pilot inlet should be downstream of bd or from the main process line.

Request your kind input please.

Thanking you in anticipation please.

Shashank
India

 

[LittleInch]

In my opinion, the location of the bursting disc is irrelevant to the issue. Just pipe the pilot line to the process line you are trying to protect. If the BD ever actually bursts, the relief valve will probably close, but should have been open before the BD pressure.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[don1980]

sgoel - a little more info in needed.

Are you talking about putting the pilot PSV on the outlet line of the BD, or somewhere upstream or downstream of the BD's inlet connection to the pipe?

If it's the former, then installing a pilot PSV isn't generally a good idea. That's because small leakage from a pin-hole or crack in the BD will cause an undetected release to the atm. Pilot PSVs rely on system pressure to remain tightly closed. An integral-sensing pilot PSV won't have any seating force, other than the weight of the piston.Small leakage will flow right through the pilot PSV. A pressure sensor won't detect any noticeable pressure. This condition can exist indefinately before it's discovered.

A remote sense pilot PSV avoids this problem, but such a design begs the question of why you have a disk in the first place. The disk isn't preventing process fluid from entering the pilot valve. So what purpose does the disk serve?

If you're asking about connecting the pilot PSV somewhere upstream or downstream of the BD connection to the pipe, then I agree with littleinch - it makes no difference. However, I would question such a design. Specifically, I'd ask why have a BD and a pilot PSV? The BD is a liability. If the system is compatible with use of a PSV, then get rid of the disk.

 

[LittleInch]

A schematic would be good to confrm what you're talking about - a hand sketch would be fine as I think we're a bit confused as to what we're talking about.

Having a PSV sensing line downstream of the actual disc in the vent line would make no sense at all so I can only assume this is a wording issue and you're referring to where the BD is relative to the PSV.

BD and PSVs together are relatively common where you have two different relief scenarios - a leak / temperature expanision / surge for which you have a PSV and a tube rupture or similar high volume event for which you need a bursting disc.

Pilots need handling correctly, but can reduce your opening pressure accuracy to around 3% compared to 10% for a normal spring. It's all about the relative and absolute pressures, i.e. operating pressure, design pressure, PSV set pressure, BD set pressure.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[SooCS]

BD is very reliable in term of safety; it breaks when it is needed. It is also very unreliable in term of operation; as it breaks when it is not supposed to. It is recommended that the operating pressure should not be higher then 75% of the BD set pressure. PSV is very reliable in term of safety and operation however it is not uncommon for it to leak. In fact API allows certain leakage from the PSV. BD is be installed upstream of the PSV to eliminate the leakage especially if the fluid that is handled is toxic or very expansive.

BD should not be installed downstream of the PSV as the pressure will build up very quickly and may prevent the PSV from opening when needed.

 

[SooCS]

Sorry, I missed read your posting.

I could see the advantage of installing the pilot upstream of the BD. It may solve the 3% delP required for the PSV inlet.