According to Engineering Toolbox, API 609 covers: Butterfly Valves: Double Flanged, Lug- and Wafer-Type. The standard covers design, materials, face-to-face dimensions, pressure-temperature ratings, and examination, inspection, and test requirements for gray iron, ductile iron, bronze, steel, nickel-base alloy, or special alloy butterfly valves that provide tight shutoff in the closed position and are suitable for flow regulation.
API 598 covers: Valve Inspection and Testing. The standard covers inspection, supplementary examination, and pressure test requirements for both resilient-seated and metal-to-metal seated gate, globe, plug, ball, check, and butterfly valves. Pertains to inspection by the purchaser and to any supplementary examinations the purchaser may require at the valve manufacturer's plant.
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I believe that Who writes the valves' specification should define the maximum allowable seat leakage (and test procedure) based on the specific service conditions and actual plant needs: this can be done either by using customized criteria or, most likely, by choosing among the leakage rates or classes defined by API 598, EN 12266-1, ISO 5208, ANSI/FCI 70-2 and/or IEC 60534-4 standards, just to mention the most common ones.
I'm not aware of any leakage test standard defining acceptance criteria based on valve type only, and I believe this in not by chance, because “the required leak tightness shall be related to the function of the object under consideration” (as also EN 1779 standard says, for instance).
