Rupture Disk & PSV combination 1

[someguy79]

In sizing for critical flow of a relief device according to API 520 Part I section 3.6.2.1.1 the effective coefficient of discharge is described as follows:
"Kd= effective coefficient of discahrche. For preliminary sizing, use the following values:

=0.975 when a pressure relief valve is installed with or wihtout a rupture disk in combination.

=0.62 when a pressure relief valve is not installed and sizing is for a rupture disk in accordance with 3.11.1.2."

My question is this: Shall Kd be taken as 0.62 if a relief valve and a rupture disk are to be installed in paralell? Given the arrangements shown for disks and valves in API520 & 521, it seems as if Kd=0.975 if the disk and valve are in series, but not necessarily when they're in paralell.

A very strict reading of the quote would lead one to use 0.975 in paralell and series cases so long as there isn't a significant flow resistance between them.

Comments?

 

[MikeClay]

When a relief valve and a rupture disk are installed in series, the flow rate/required area is derated by the combination correcdtion factor, Kc. This factor accunts for the fact that the opened rupture disk is not a pure pipe segment. However, since the rupture disk diameter will be much larger than the relief valve orifice diameter, a simple correction factor is used instead of a rigorous hydraulic analysis.

When the relief valve and the rupture disk are in paralell, the flows are two separate paths and the appropriate design equations need to be used.

It should be noted that API 520 section 3.11.1.2 is only applicable under very specific conditions. The rupture disk must discharge directly to the atmospher, not a common header. In addition, there are limits to inlet and outlet piping lenght. Unless these conditions are met, you will be doing a detailed hydraulic analysis.

--Mike--

 

[someguy79]

Thanks Mike!

 

[JimCasey]

one concept you have not addressed is that rupture discs have "operating-to-burst ratio". A reverse-buckling disc which is suitable for mounting UNDER an SRV because of its nonfragmentation properties has operating/burst ratio of 90%.

If your rupture disc and the SRV are set at the same pressure they >should< both burst at the same time. IF the SRV goes and the RD does not, then the next time the pressure gets high,the RD will blow early.

Forward-bursting discs are even worse-O/B ratio of about 0.7. You might exceed 70% of burst a time or 2, but ultimately the disc will burst early when it goes.