The original API 622 draft was intended to be a test for the packing, whereas ISO 15848 is for the whole gate valve. A standardised test rig is proposed (1" stem, ?" packing), and emissions measurements are based on EPA Method 21, measuring ppm methane, but with a vacuum chamber style bagging of the gland to capture the leakage. It is a block valve type of test with 1500 stem cycles and 3 thermal cycles. There are also corrosion tests and tests for volatile content and lubricants etc.
In terms of the emissions measurements, it is more realistic than the levels given in the ISO 15848. The stem movements can be linear or rotary, and of course certain types of packing sets might only perform well on one kind of movement, so it would be interesting to ensure that any  given packing set is tested both ways, depending on your requirements.

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