This kind of valve is designed to allow the fluid to flow in a given direction but closes to prevent backflow. Types include swing check, tilting disc check and wafer check. Check valves (also called non-return valves) are usually self-acting. 

Wafer Check Valve:
  
Split-Disc Check Valve  
Size: NPS 2"-40" 
Pressure: Class 150-1500 
Material:WCB,LCB,CF8,CF8M,CF3,CF3M,WC6,WC9,C5,C12  
Design & Manufacture Std.: API 6D,API 594,ASME B16.34  
Face to Face Std.: API 594,API 6D  
End Connection Std.: ASME B16.5,ASME B 16.47 
Test Standard:ISO5208,API 6D