The test medium for API 6D is air or nitrogen

API 598 covers the inspection, examination, and testing requirements for resilient-seated, non-metallic-seated, and metal-to-metal-seated gate, glove, plug, ball, check, and butterfly valves.

Methods of checking the Critical valves

The valve seat tests procedure involves placing a valve under pressure, then measuring the amount of leakage on the other side of the valve

The maximum permissible leakage rate shall be 0.18 cubic inch (3 cubic centimeters) per minute per inch of nominal pipe size

Valve seat leakage tests are not required for pressure relief valves

Backseat tests are required for valves with a backseat element, including gate and globe valves.

Closure test is required for

No leakage is permissible in the backseat test