Check Valves 
Function:
Non-return Check Valve or one-way Check Valve is a mechanical device, a valve, which normally allows fluid (liquid or gas) to flow through it in one direction only.
Operation:
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The main mission of a Check Valve is to avoid back pressure in the process (avoid that the flow is switching direction)
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A Check Valve can also be used for isolation purpose.
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There will normally be regulations telling how many and in which location Check Valves are to be installed in the process.
Design:
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As for most valves there are a lot of different designs of Check Valves. The basic design is a Duo Check Valve with two discs mounted on an hinge pin in the center of the Check Valve. The Check Valve has a standby closed position with springs pushing the discs towards the seats. These valves are designed for installation on a horizontal line where the hinge pin is placed in a vertical position.
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To reduce space and weight these Check Valves are often delivered as wafer valves.
Other types of Check Valves:
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Swinging Check Valve
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Tilting Disc Check Valve
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Piston Check Valve
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Non slam Check Valve
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Needle Check Valve
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Nozzle Check Valve