Combination Device with one device not having an ASME certified flow coefficient 1

[PMarinshaw]

I have a vessel with ASME U stamp, and a 15 psig MAWP rating. It is protected by a PSV set at 15 psig. Can I use a rupture disc below the PSV (for corrosion isolation purposes), with a burst pressure of 14 psig (which meets the ASME UG-131 pressure setting criteria), but does not have a UD flow coefficient certification?

 

[Chance17]

In real life the rupture disk should be fully open when the relief valve activates. If the disk partially opens and the differential is not great enough the safety valve may chatter. My recommendation is to set the rupture disk at 15-psig and decrease the safety valve setting to say 13-psig. Also need to be concerned about rupture disk burst tolerances. The best option is to purchase a +0%/-5%tolerence

 

[mk3223]

I believe that the Code vessel is to be protected by a Code stamp safety/relief device with either PSV or rupture disc, which may be specified in the design spec of the facility.

 

[The Obturator]

My take on this;-

The last line of defence here is the PRV, and is presumably certified and UV stamped.

The rupture disc is below 15 PSIG so does not meet ASME and is being used to isolate potential corrosion.

Current API-520 sizing requires a 0.9 derating factor for PRV sizing when used in combination with a rupture disc, specifically where the PRV/rupture disc combination has no certified value.

Therefore, I would review the sizing for the PRV using the manufacturers actual area and coefficient value and apply a 0.9 derating factor.

I would also recommend this solution is passed off by the local inspector on the basis stated emphasising the PRV being the last line of defence.

You will also require a tell tale device between the disc and PRV to meet ASME to detect any rupture disc failure in service.




*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***