API 6D is a quality management system standard for pipeline valves, while API 598 is an inspection and testing standard for all types of valves. In API 6D, the test medium is fresh water, and light oil with a viscosity not exceeding that of water can be used upon negotiation. Compared to API 598, there are significant differences in the selection of test media. API 598 specifies either high-pressure liquid sealing tests or low-pressure gas sealing tests depending on the valve type, diameter, and pressure rating. API 6D, however, does not include low-pressure gas sealing tests in its standard pressure tests; these tests are only performed when specifically requested by the user.
In API 6D, the shell hydrostatic pressure and back seal tests have longer durations. For example, for valves with a nominal diameter of 20 mm or larger, API 6D requires a 30-minute shell hydrostatic pressure test, while API 598 only requires 1-2 minutes. Regarding high-pressure gas testing of the shell, API 6D specifies nitrogen as the test medium and uses a hydrogen leak detector; API 598 uses air as the test medium.
If a customer requests additional seat testing, the API 6D standard requires companies to perform the corresponding test; however, the API 598 standard does not have this requirement. Furthermore, the API 598 standard allows a wider test temperature range than the API 6D standard, and for valves of all sizes, the API 598 standard allows for higher leakage rates than the API 6D standard.
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