The Difference Between API and ASME Pressure Safety Valve (PSV)

In the vendor data sheet, calculation sheet, and or catalogue of the PSV, it was stated two different terms which is API and ASME PSV that usually makes confusion to a young engineer. Some times it is stated as API area, ASME area, actual area, etc. So actually what is the difference between this API and ASME pressure safety valve (PSV)?

To answer this question, first of all we need to clearly understand that only API standard has a specific formula to calculate the required area of the PSV, while ASME standard doesn’t have such formula. But in world wide oil & gas application, the PSV usually required to be stamped with UV mark which is an ASME certification for the PSV. There is a PSV certification based on ASME because this standard has a certain criteria to certify the PSV. So based on this understanding, API standard scope is to calculate while ASME standard scope is to certify.

The Difference

In ASME BPVC VIII Section I it was mentioned that the coefficient of discharge (Kd) on API formula shall be multiplied by 0.9. The required orifice area in the PSV calculation formula is inversely proportional to the coefficient of discharge. It means that smaller the coefficient of discharge, larger the required orifice area. So by multiply the coefficient of discharge on API formula by 0.9, it will make the coefficient of discharge become smaller than before. This smaller coefficient makes the required orifice area larger than the PSV that calculated using an API Kd. Thus the PSV that required having a UV stamp on it (ASME certification) always has larger orifice area than the required orifice area calculated by API formula.

List of API orifice size and its ASME area and API area

Orifice	ASME in2	API in2
D	0.134	0.110
E	0.273	0.196
F	0.373	0.307
G	0.589	0.503
H	0.881	0.785
J	1.457	1.287
K	2.097	1.838
L	3.284	2.853
M	4.093	3.600
N	4.987	4.340
P	7.215	6.380
Q	12.910	11.050
R	17.810	16.000
T	28.870	26.000