Subsea check valves especially on main pipe lines may provide the last line of defense for a platform or installation when there has been a significant failure of the pipe line. They have to meet very stringent design criteria; some elements may be full flow reversal instntatiously, not bang against the seat in low flow conditions and be able to be held open in order to allow pigs through. This last requirement often necessitates some form of gear box which is often complicated by the requirement to allow the valve to close freely in back flow situations.
A check valve, clack valve, non-return valve orone-way valve is a mechanical device, a valve, which normally allows fluid (liquid or gas) to flow through it in only one direction.
There are various types of check valves (wafer,piston, swing, SSIV) used in a wide variety of applications. Check valves are often part of common household items (flapper) up to major gas pipeline subsea check valves (SSIV), which maintain the safety and integrity of refineries and off shore platforms. There for they are available in a wide range of sizes and costs. Check valves work automatically and most are not controlled by a person or any external control; accordingly, most do not have any valve handle or stem. They are often part of a safety system (SSIV) or pump protection to stop reverse flow (swing check).
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