Pressure / Safety Relief Valve Sizing

[mgruschow]

I've seen a lot of discussion regarding API and ASME sizing of relief valves; e.g. thread1203-135368. And I agree that the API sizing gives you an "effective" area while the ASME gives you actual area.

What's interesting is that both equations are identical. The only possible variation in the factors is the valve discharge coefficient, Kd (API) and K (ASME) which is manufacturer dependent. Since the API areas are somewhat more conservative than the ASME, you would intuitively expect something in the equations to differentiate between the two.

It would also be interesting to see educated discussion that compared the two approaches along with a tabulation of API vs ASME orifice areas cross-referenced to the Alpha orifice designations.

Any thoughts?

 

[Latexman]

mgruschow,

The difference is history.

The API area came first. It was the actual minimum throat (orifice) area for devices built before the 1960s. At the time it was, and still is, a common practice in the industry. Because of the more stringent flow testing and data treatment required by the ASME Code at that time, pressure relief valve manufacturers actually increased the orifice areas of all of their devices so that they would pass the flow rates claimed by their own literature, BUT according to the ASME test procedures and not their own or APIs. These larger areas, the so-called ASME areas, are the actual areas reported by the National Board.

I think you will find that A_ASME x K_ASME = A_API x K_API, or close enough to each other for the task at hand.

Good luck,
Latexman