Pressure Relief Valve and Orifice Calculation

You don't design- unless you are working for valve manufacturer, you select.

Contact your supplier, or Google "pressure relief valves" and select a manufacturer that has catalog on-line and select product relative to your fluids, temperatures, potential back pressure (if piping potential discharge to drain, etc.) according to the catalog guidelines- or contact local rep for the product.

i cannot select from manufactures catalog. it is for a special application. we have to design based on our envelope. Thanks

BS EN ISO 5167-1 can basicly be referred for the calculation of Orifice flanges and/or orifice plates (for restriction). For the thickness of the plate, I would go for the manufacturers catalogues. But as it is stated above, if you are going to purchase it, you do not have to calculate anything, required calculations are to be performed by manufacturers.

I hope this helps.

Is it a liquid or gas that flows. If gas you will get a high flow for a 1000 PSIG difference even for small orifices.

PRVs are available for almost all conceivable applications including blood.

For PRV you can refer to API RP 520 Part 1 & Part 2.

Part 1 - Sizing and Selection

Part 2 - Installation

I have done several PRV orifice calculations using these RP before.

Another good reference is Crosby PRV Engineering Handbook that you can download from internet. Sorry I don't have the address but surely you can search for it.

Hope this helps

My requirement is designing a small size pressure relief valve. Can i find information in API RP 520 Part 1 & Part 2 book? I downloaded "Crosby PRV Engineering Handbook", but the relief valves are of huge size? Is the formula on liquids the same for smaller size design also? please correct me if I am wrong. thanks,

Method of calculation of PRV size is the same regardless of size.

Crosby Engineering Handbook method and API RP 520 is the same so using Crosby method is sufficient.

The smallest standard size available in the market is with orifice 0.074 sq. inch. Normally if the calculated size is lower than the smallest size available, we just install the smallest size. So far I have not seen any problem with that. The smallest size is normally for pressure relief due to liquid thermal expansion.

My piece of advice is don't install anything smaller than 0.074 sq. in unless you system is very-very clean. Otherwise it may subjected to plugging and that defeat the purpose of installing the PRV.

Hope this helps

I work in the hydraulic field. As stated earlier you select from available catalogues.

In hydraulics you do not "orifice" a PRV. it is selected for the flow required. They are also designed to be "fail safe" as they are normally the maximun system pressure and a "fail closed" would result in something else blowing--hose, pipe, primary drive unit, etc.

These units can be run "on relief" for extended periods of time (years) without failure. Flow ratings start at 1gpm, 20 gpm, 40 gpm, 60 gpm 100 gpm etc.

If you are custom designing keep in mind the fail safe properties needed.

I have seen a ball bearing and spring used as a relief valve, but it was not used as a continuous reliefing unit.