rupture disc & PSV combo need same set pressures?

[BenThayer]

an engineering firm is proposing PSD/PSV combination with the rupture disc (PSD) under the relief valve (PSV).

their planning to set the PSD less than MAWP and have the PSV set at MAWP.

i thought it was an ASME issue to have them set at the same pressure but am having trouble finding it and it may just be a practice that i have seen.

note that the PSD & PSV are in series and are not in parallel.

tia,

 

[Latexman]

It is common practice where I work to specify the rupture disk burst pressure and PSV set pressure in a RD/PSV combination to be the same pressure.

Good luck,
Latexman

 

[BenThayer]

been looking more and found an API520 reference in 2.3.2.2.2 that says they should have the same nominal value but can't find a ASME reference.

 

[moltenmetal]

Obvious, but make sure they put an effective telltale and/or venting means between the two, or the disks may never burst, regardless of rating...

 

[lizking]

Out of curiosity, what is the reason to set it lower? If it's because of burst tolerance range, then I would say that that's the same "nominal" value as the relief valve set pressure.

 

[zerosum]

As I remember, Fike technical literature (probably likewise among the other RD suppliers) has a discussion about setpoint issues when a RD is in series with a PRV.

 

[Latexman]

Our company requires (mandatory) a method of monitoring the pressure between the RD and PSV. The preferred way is a pressure transmitter with an alarm. The volume between the RD and PSV is closed and not vented, so if the RD develops a pinhole leak it can be detected. Once the volume is closed in, ambient temperature changes will not affect the pressure more than 3 psi. You do have to design it so process temperatures minimize their affect.

Good luck,
Latexman

 

[pleckner]

The answer is in a little foot note (#52) in ASME Section VIII, Div 1 (2007 edition), UG-127(a)(3)(b) - page 89.

52 "Use of a rupture disk device in combination with a pressure relief valve shall be carefully evaluated to ensure that the media being handled and the valve operational characteristics will result in opening of the valve coincident with the bursting of the rupture disk."

This type of installation is extremely common.

 

[zerosum]

It is important to consult with your RD supplier.

You want the RD and the PSV to actuate at the same time, which not always means identical set points