2 replies to this topic

[Conchubhar]

Hi all,

I have been asked to size a relief valve to be installed downstream of a bursting disc.
I have a question relating to the design of inlet and outlet piping to the relief valve.

I have been reading the european standard for relief valves (BS EN ISO 4126-1) to brush up on my knowledge.

For discharge piping, the standard states on pg 35 that "the discharge pipe should be at least = to the bore of the PRV outlet, and should be designed to limit the built-up backpressure to 10% of set pressure at certified capacity".

Fair enough.

For inlet piping, the standard states "pressure loss at inlet of PRV should not exceed 3% of set pressure at derated capacity".

The confusion:

There's no mention of a requirement to have an inlet pipe diameter at least the same size as the inlet PRV bore, provided you comply with the 3% rule. This would be handy for me, as on my reactor the relief nozzle is 10". This is followed by a reducing flange to 6", followed by bursting disc.

The question:

I would like to install a 4 x 6" PRV, which would involve reducing down from 6" after the bursting disc. I could then reuse the rest of the existing 6" vent line. Am I in breach of standards/legislation to do this? Can I have a reducing flange or a reducer on an inlet line? I seem to be complying with the ISO 4126 standard.

My thanks in advance for any response.

[herrani]

Hi Conchubhar

I usually use API standards 520 and 521 for the sizing and selection of relief valves, since they are more comprehensive.

Typically, safety valves are installed with a reducer in the inlet line and an expander in the outlet line, because the bore size is smaller than the pipe diameters required to comply with the 10% or 3% pressure drop requirements.

Provided that the pressure drop on your inlet line is lower than 3% of the set pressure, I don't see that you would be breaking any rules. You should remember that the rupture disk, reducers and any other fitting should be counted in this calculation, and that you need to use a 0.9 or smaller derating factor in your actual RV capacity because of the rupture disk.

It would be great if you could share your RV sizing calculations, for more detailed comments.

Good luck!

[Lowflo]

Your proposed design is perfectly OK and in compliance. In your specific case, that rule says that no segment of the inlet pipe can be smaller than 4" (inlet size of the PSV).

You must be thinking that this rule also applies to rupture disks.....it doesn't. The inlet pipe to a PSV can be smaller than the diameter of an upstream disk. In fact, that typically the case. The disk usually has to be a size larger than the PSV inlet in order to satisfy the 3% rule.