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What are Check Valves?
Check Valves 
A check valve allows fluid to flow through it in only one direction. Check valves are two-port valves, meaning they have two openings in the body, one for fluid to enter and the other for fluid to leave. There are various types of check valves used in a wide variety of applications. Although they are available in a wide range of sizes and costs, check valves generally are very small, simple, and/or cheap. 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.
Design Features
Seat
For carbon steel check valve, the seat material is usually forged steel. The sealing surface of the seat is spray welded with hard alloy specified by the customer. Renewable threaded seat is used for NPS≤10 check valves, and welded-on seat is optional. Welded-on seat is used for NPS≥12 carbon steel gate valves. For stainless steel check valve, integral seat is usually applied, or to weld hard alloy directly integrally. Threaded-in or welded-on seat is also optional for stainless steel check valve if required.Body-Cover Gasket
Stainless steel + flexible graphite wounded gasket is used for Class150 and Class 300 check valve; stainless steel flexible graphite wounded gasket is used for Class 600 check valve, ring type joint gasket is used for Class 900 check valve; pressure seal design is used for Class1500��Class 2500 check valves.Body-Cover Connection
The body and bonnet of Class150 to Class 900 check valves are usually with studs and nuts. And the body and bonnet of Class 1500 to Class 2500 check valves are usually of pressure seal design.
Cast Steel Swing Check A swing check valve consists of a flap or disc which hangs down in the flow path. With flow in the forwards direction, the pressure of the fluid forces the disc to hinge upwards, allowing flow through the valve. Reverse flow will cause the disc to shut against the seat and stop the fluid going back down the pipe. In the absence of flow, the weight of the flap is responsible for the closure of the valve.
Normative reference
API 6D Pipeline Valves
BS 1868 Specification for Steel Check Valves (Flanged and Butt-Welding Ends) For The Petroleum, Petrochemical and Allied Industries
ASME B16.34 Valves Flanged, Threaded and Welding Ends
ASME B16.5 Pipe Flanges and Flanged Fittings: NPS 1/2 through 24
ASME B16.47 Series A Large Diameter Steel Flanges: NPS 26 Through NPS 60 Metric/Inch Standard
ASME B16.25 Buttwelding Ends
ASME B16.10 Face to Face and End to End Dimensions of Valves
MSS SP-25 Standard Marking System for Valves, Fittings, Flanges and UnionsSize available
NPS 2 to NPS 24 (DN50 to DN600)
Pressure rating available
Class 150 to Class 2500
Forged Steel Swing Check
A swing check valve consists of a flap or disc which hangs down in the flow path. With flow in the forwards direction, the pressure of the fluid forces the disc to hinge upwards, allowing flow through the valve. Reverse flow will cause the disc to shut against the seat and stop the fluid going back down the pipe. In the absence of flow, the weight of the flap is responsible for the closure of the valve.
Normative reference
API 602 Steel Gate, Globe and Check Valves for Sizes DN 100 and Smaller for the Petroleum and Natural Gas Industries
BS 5352 Specification for Steel Wedge Gate, Globe and Check Valves 50 mm and Smaller for the Petroleum, Petrochemical and Allied Industries
ASME B16.34 Valves Flanged, Threaded and Welding Ends
ASME B16.5 Pipe Flanges and Flanged Fittings: NPS 1/2 through 24
ASME B16.25 Buttwelding Ends ASME B16.10 Face to Face and End to End Dimensions of Valves
ASME B1.10.1 Pipe Threads, General Purpose (inch)
ASME B16.11 Forged Steel Fittings, Socket-Welding and Threaded
MSS SP-25 Standard Marking System for Valves, Fittings, Flanges and UnionsSize available
NPS 1/2 to NPS 4 (DN15 to DN100)
Pressure rating available
Class 150 to Class 2500
Wafer Check Valve Wafer check valves, also know as dual-plate check valves, has two spring loaded half-moon shaped discs that rotate about their shaft. The disc plates are kept shut by the torsion spring. When fluid flows in the forwards direction, the pressure of the fluid causes the disc plates to hinge open, allowing flow. The check valve is closed by the spring as soon as flow ceases, before any reverse flow can occur.
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