The API Kd should only be used with the API area – the same with ASME values. Note that one should not mix and match the orifice areas and discharge coefficients. Columns 5 and 6 show the (A * Kd) API values. One also needs to consider differences in valve discharge coefficients.

The fourth column shows that, just based on orifice size, except for the “T” orifice, the ASME orifice flow area is about 16% higher than the API area. The first column is the API letter designation for the orifice, followed by the API and then the ASME orifice areas in the next two columns. The attached table, compares just the differences between orifice designs for two relief valves versus an initial, API design. The final, purchased, relief valve can be greatly oversized. The certified ASME discharge coefficient is derated by a factor of 0.9 resulting in another potential source of over design.When this occurs, the next larger orifice size is chosen resulting in the valve being oversized with just an API orifice Calculated orifice sizes are usually between the standard, letter designated API sizes.Conservatisms in estimating the required relief loads.For most projects, the actual ASME orifice can provide a much greater flowrate. However, the differences in capacity between the initial choice of API orifice and the actual ASME orifice can be significant. They were developed to facilitate choosing a relief valve size early in a project and to ensure that the relief valve finally purchased will have a certified capacity that meets or exceeds the required relief capacity.

According to API-520, Part 1, the API orifice sizes and discharge coefficients are assumed values and are to be used only for the initial selection of the relief valve. Many are not aware of the major differences between the orifice sizes and discharge coefficients suggested by the API and the actual, ASME values used by the relief valve vendors.

Pressure relief valve sizes