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Rather he is obliged to produce a valve which will have a flow rate equal to or greater than that determined by the API formulation.Ģ) API formulations use Kd=0.975 for gas & vapour service, for liquid service they use Kd=0.62 however, as said in point1 the effective areas and API assumed Kds are generally different from actual orifice areas and discharge coefficients that are used to determine certified valve capacities. The manufacturer is not required to produce a valve with bore area equal to effective area. This orifice area is used in API formulations to calculate valve flow rate.

As an example the effective area of a J orifice is 1.287 sq. These standard sizes are characterized by a series of fourteen standard letter orifices ranging from D through T. 1) API developed a series of inlet, orifice, outlet combinations for various flanged valve pressure classes which are utilized throughout the petroleum and hydrocarbon industry.