Hello Everyone,

I have a few questions regarding the above.

1. Is it neccessary to put a bursting disc before an RV. If so why???
2. In our plants we have bursting discs before RV's on some of our vessels. What happens if the bursting disc has a pin hole leak?? Can we put in a pressure switch to warn us of pin hole leaks.

Can anyone fill my yearning for knowledge!!!
Thanks

Part 1 answer is NO. It isn't even common to put a rupture disc upstream of a PSV, though there are some good reasons to do so when the circumstances so dictate. In my mind, one of the primary reasons for doing so is to avoid the use of an expensive alloy for the PSV. Protect it by installing a relatively inexpensive alloy RD upstream, and as long as the RD remains intact, the PSV is protected from the (presumably nasty) process fluids. There are other reasons for this configuration as well; this is just one example.



Yes, a pressure switch or gauge should be installed between the RD and PSV, and it is used to warn you of the pin hole leak.

The rupture disk before the PSV is provided for the cases when the process fluid is corrosive and requires the expensive material of construction for PSV. Also during such cases a TELL TALE assembly is provided downstream of rupture disk which will alert DCS operator regarding bursting of Rupture Disk. Thus, there are two line of safety before loss of containment occurs. Also, in the TELL TALE assembly excess flow valve is provided to take care of pin hole in the rupture disk and to vent out excess pressure because Rupture disk works on the principle of differential pressure across it.

An RD below the PSV will reduce the chance of fugitive leaks through the PSV as long as the RD remains intact. This can be useful if the material is particularly nasty (toxic/corrosive/flammable).

Please note - any residual pressure trapped between the PSV and RD will elevate the bursting pressure of the RD - i.e., if it is assembled at ambient, but process is heated to elevated temp, there will increased pressure from the elevated temp of the static air in the trapped space.

Also be careful of single point failures with pressure switches, it is a blind device. If the location is not remote, I would couple a switch with local pressure indication (guage) or even better, use a pressure transmitter especially if there is to be a shutdown interlock in case the RD/PSV trips.

Dear all,
Good advices and experiences.

Just to add...

RD follow by PSV :
i) Avoid corrosive fluid contacts with PSV to avoid CRA material (as mentioned by Doug)
ii) Avoid material crystallized or polymerized and stick at PSV nozzle - Potentially reduce flow path and cause serious safety concern
iii) Avoid particle stick to PSV disc and seat which potentially increase additional force to open the PSV - It may increase the openning pressure of PSV.
iv) RD is considered a "fast' acting device compare to PSV - In some case like HX tube rupture case, severe surge could occur. A fast acting device is required to tackle the transient surge.
v) PSV passing is normal. An RD upstream can avoid this problem.

Anyone knows if a rupture disk has to be installed at a specific distance below the PSV? We have about 8 meters of piping lenght to chose to install a rupture disk between one end and the PSV end. Does it have to be right under the PSV? How to calculate how far from the PSV should the rupture disk be installed?

Thanks!

Check out API RP 520 PII section 4.6.