Two distinct terms have been indicated, namely API and ASME PSV, which generally confuses young engineers. It is sometimes referred to as an API region, ASME area, real area, etc. So what’s the distinction between this API and the safety valve (PSV) for ASME pressure?
First of all, to answer this question, we need to realize obviously that only the API standard has a particular formula for calculating the necessary PSV region, whereas the ASME standard has no such formula.
But in the worldwide application of oil & gas, the PSV usually had to be stamped with a UV mark that is a PSV ASME certification. There is an ASME-based PSV certification because this standard has some PSV certification requirements. Therefore, based on this knowledge, the normal range of the API should be calculated while the standard scope of the ASME should be certified.
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 implies that the lower the discharge coefficient, the bigger the orifice region needed. This will create the discharge coefficient smaller than before by multiplying the discharge coefficient on the API formula by 0.9.
This lower coefficient increases the necessary orifice region compared to the PSV calculated using an API Kd. Therefore, the PSV requiring a UV tag (ASME certification) always has a bigger orifice region than the necessary orifice region calculated by the formula of the API.