Although interior pressures are also quite high in subsea pipelines, it is sometimes the outside pressure from the extreme depths that introduces the most stress on valves and piping. As a result, pipeline valves designed for installation at great depths are often tested in a hyperbaric chamber, where extreme pressure is exerted on the outside of the valve, while the inside is sealed against the external pressure.
All pipeline valves receive seat and shell tests per API 6D or 6DSS, not unlike their downstream counterparts, which are usually tested in accordance with API 598, “Valve Inspection & Testing.” One difference between the two testing documents is that, with API 6D pipeline valves, the holding times for the tests are much longer. For example, a 24-inch valve shell tested per API 598 requires a five-minute duration, while the same size valve tested per API 6D requires a 30-minute duration. These longer holding times for pipeline valve tests are often extended into hours by the supplementary test requirements of many pipeline owners.
While pipelines and pipeline valves lie mostly invisible beneath six feet of earth or under 600 feet of ocean, they are nonetheless highly “visible” when an accident occurs. As a result, pipeline valves are closely scrutinized members of the valve family. They are built to tougher standards and must work every time because they must protect lives and property that lie near their installations. Pipeline valves could borrow the Latin motto of the United States Coast Guard, which is “Semper Paratus,” which means: always ready.
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