The general understanding is that the ANSI Standard for Control valve leakage allows for a specific percentage of the rated valve capacity to pass through the seats when closed. The variation in standards is from 0.15 ml per minute (1 bubble) to 0.5% of rated valve capacity from Class VI to Class II, based on specific conditions of test. You also may want to keep in mind the following. If a valve is tested and passes under the standard it is rated to lthat ANSI standard. But if you operate it in the field at out side the exact test conditions the valve will perform differently. But it is still rated to that shutoff class. Example: A valve tested in the factory on Air or Nitrogen at 50 - 125°F at 50 psi, and does not pass more than one bubble in one minute, will pass the ANSI test to meet the Class VI standard. Put that valve in the field, with air at 300 psi at 200°F and the valve may pass more than one bubble per minute. Does that valve meet the Class VI standard? Yes. The point being that when a customer purchases a ball valve with an expected shutoff rating of ANSI Class VI, they must under stand that under service conditions the valve may or may not perform to the standard.

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