Selection of Pressure Relief Valve

"3. The materials of construction must be compatible with the contained fluid."

How did you know it's application is a fluid? HAHA, Some of these questions are just too funny!

Not to speak for Tornado, but you may be confusing a liquid with a fluid.

I believe that if you look at the definition of a fluid you will realize that fluids can be liquids, gasses, plasmas and a few other special cases.

Not sure valves will work with anything other than fluids.

Contact a pressure relief valve sales rep and tell him you know absolutely nothing about PRV's and ask him to select the proper valve for you. That way, you'll hardly have to do any work.

If you do it Tornado's way, you'll have to learn a bunch of stuff. But, then again, someday you might be able to select a PRV based on your own observations and your personal knowledge of PRV selection, without having to ask someone else to think for you.

Good luck.

If you are dealing with something that has potential pressure spikes (ie O2 + H > 5%) you might be better off with a rupture disk.

The two prime parameters are pressure and flow and the third is seated or unseated. Consult your local supplier.

Oliver Dunthorne

Tornado's is a good answer I would just like to expand on Point 2.

There are many reasons that over pressure can occur in a vessel / piping. One of the most important job of the Engineer is to determine the load caused in these upset cases. Other cases that should be considered:

Failure of control valves so that fluid is not allowed out of a vessel or high pressure fluid is allowed into a vessel at too high a rate.

Failure of cooling systems eg overhead coolers on distillation columns

Failure of the INSTRUMENT AIR / POWER SYSTEMS.

Burst tube in an exchanger

Unexpected / runaway reaction

Irrespective of the area in which you work API 520 / 521 give clear advice on how to calculate these loads and how having calculated the load (mass flowrate) how you then calculate the orifice size of the relief valve.

Relief valves come in a series of standard orifice sizes.

In my opinion you only go to a vendor when you know the design case for the valve the set point for the valve and the orifice size (having down the load calculation) - a good vendor will check your orifice sizing calculation but they will not be able to check the load calculation.

The inlet and outlet flanges on the valve will also be set based on set pressure and operating temperature.

One further complication is the allowable back pressure. Typically dangerous materials go to a flare and there is a back pressure from the flare on the valve which can reduce the capacity of the valve.

Back Pressure less than 10% set pressure - Conventional valve

Back Pressure less than 33% of set pressure - Balanced Bellows

Otherwise use Pilot Operated PRV

The back pressure can be reduced by making the flare header bigger but for low pressure vessels there is often no way round using a Balanced Bellows valve.