Could anyone help with my query. I work in a refinery and we have some old documents on leakage rates of isolation valves (ball valves). The data sheets specifies design shut off class/leakage rates (Rate A, Rate B through to G) in mm3/s (cubic millimetres per second) as per BS 6755 part 1. However manufacturers and Site have now adopt the ANSI/FCI 70-2 standard that defines 6 shut off classes (Class I through Class VI) in ml/m (milli-litres per minutes).

Does anybody know any documentation that cross references the two standards and any methodology for calculating the leakage rate in ml/m from mm3/s for control valve. I could have used my basic school volume calculation but this is ineluctable given the irregular dimension of the shut off valve. Also which dimension would the calculation take in, the orifice port where the ball/plug seats or flange/pipe size. Many thanks in advance.

The latest version of FCI 70-2 is coordinated with IEC.  This is for control valve leakage.  Control valves leak.  In my area API 598 applies to isolation valves.  At least when new isolation valves are not supposed to leak.

     BS 6755-1 has been superseded by EN 12266-1 (March 2003).
         However, "Rate A" means "no visible leakage" (i.e. no air bubbles or no liquid drops) for the specified test duration... which is more or less the same as API 598 for "resilient seats".
         "Rate B" allows just a little more, i.e. a few ml/min, in a linear function of the valve size.
         The following Rates are multiples of "Rate B"...   

         Leakage Classes defined by IEC 60534-4 or equivalent FCI-70-2, instead, depend on the "valve rated capacity" or on the test pressure, in addition to the valve size. In particular, Class VI depends proportionally on the differential pressure and on the square of valve ND...

         So a comparison is not feasible, unless you fix a valve size and a test pressure (at least).

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