SRV under rupture disk 1

[bragamor]

Do you guys know if it is legal to have a spring pressure relief valve just before a rupture disk. The set pressure of the SRV would be lower than the one of the rupture disk



Thanks for your help on this one

 

[SNORGY]

I suspect this is not ever acceptable, since if the rupture disk is *after* the relief valve, it constitutes an obstruction in the relief path. Further, if the set pressure of the relief valve is lower than the set pressure of the rupture disk, you could have a relief valve with a back pressure of more than 100% of its set pressure.

Usually, if both are used, the rupture disk is below (before) the relief valve.

Regards,

SNORGY.

 

[psafety]

Bragamor,

Are both on the main line -or- is the RD in the PSV discharge ?

As Snorgy said if in the PSV discharge it is WRONG.

But if both are on the main line the PSV could be acting as sentinel valve to take minor upsets to prevent blowing the RD.

 

[bragamor]

they are both in the main line, the PSV is venting to the atmosphere.

Thanks you both for your help

Bragamor

 

[SeanB]

Well....I will have to say that is definitely a peculiar arrangement - one I have never seen before.

 

[Duwe6]

My opinion is its stupid but "Legal". This assumes that both the PSV and the RD are set at or below the MAWP of what is being protected. Also assumes that the PSV discharge volume and velocity have been calculated against the flow restriction that the RD will cause.

Normal useage is to put the RD just below [inboard of] the PSV, to keep the PSV from experiencing corrosive fumes or solids that might accumulate and plus the PSV. In an overpressure event, the RD blows, the PSV relieves; then the RD is replaced and the PSV is cleaned and reset.

 

[don1980]

Rupture disks are sometimes installed in the tailpipe of a PSV to protect the valve from moisture or corrosive vapors.

That's possibly what you're looking at here. I can't think of any other reason for putting a disk on the downstream side of a PSV. In such cases I always suggest a minimally low burst pressure for the disk so that it bursts immediately after the valve opens. However, if there's a reason for setting the PSV at a lower pressure than the disk, you could do so. You just have to make sure that the relief system opens at or below MAWP and you have to account for the open disk in you outlet pressure drop calculation.

 

[MikeClay]

If I understand Bragamor correctly, both the PSV and the rupture disk are connected to the pressure source in parallel and both vent to the atmosphere. In this case the rupture disk would be sized for a massive, improbable relief flow while the PSV could be sized for the more expected cases.

--Mike--

 

[SNORGY]

I should correct my previous post...

Rupture disk downstream of PSV outlet, in the actual PSV tail pipe, is acceptable per UG-127 (a)(3)(c) and footnote 54.

Don't know why one would want to do that in any ordinary instance.

Regards,

SNORGY.

 

[zdas04]

I see the (eventually) described configuration all the time. Usually designed by idiots. It is very common in Oil & Gas onshore (where facilities engineers are rare) to have a PSV and Rupture Disk both set at MAWP. Since both have a tolerance, I often see the RD fail at a lower actual pressure than the PSV--worst of all worlds. When I see this, I always ask "what is the credible scenario that requires both?". Blank Stare, deer in the headlights look and "what is a credible scenario?" followed by "the vessel manufacturer sized them, ask him".

Except for the scenario that MikeClay mentioned (multiple credible scenarios, the PSV is sized for the small one set below 110% of MAWP and the rupture disk is set a bit higher for a larger credible scenario) a PSV and RD in parallel set at the same value is generally an indication of unqualified idiots doing "engineering" with their middle-school education. (A RD under a PSV to protect the PSV from corrosive fluids is quite reasonable, but I don't think that is what we're talking about. I don't agree that a RD in the PSV tail pipe is a great way to keep rain off the PSV, a rain cap is cheeper).

David

 

[twoplates]

If installing the RD below the relief valve, does the pressure setting have to be below the PSV, or just below the MAWP of the vessel being protected?

Or should it be slightly higher to prevent premature rupture and possible contamination/damage to PSV?

Thanks!

 

[Duwe6]

ASME = at, or below MAWP, same as the Relief Valve
Good Engineering Practice = 5 to 10% below the PRV

NEVER above the MAWP

 

[olcrazy1]

I know of an application that does exist where the PD is in the outlet piping of the RV. It is on a ship where the valve vents to the hull of the ship, it is an ocean ship so there is concern of sea water entering the discharge piping so they have installed a monel RD on the outlet to protect the valves discharge piping. Not sure if this is approved by ASME or if ASME has any say so in ship board applications but I know this has been approved by the navy for this application.