Check valve are self-activating safety valve that permit gases and liquids to flow in only one direction, preventing process flow from reversing. They are classified as one-way directional valve. Fluid flow in the desired direction opens the valve, while backflow forces the valve closed. The mechanics of check valve operation are relatively simple. Most check valve contain a ball that sits freely above the seat, which has only one through-hole. The ball has a slightly larger diameter than that of the through-hole. When the pressure behind the seat exceeds that above the ball, liquid is allowed to flow through the valve; however, once the pressure above the ball exceeds the pressure below the seat, the ball returns to rest in the seat, forming a seal that prevents backflow.

Check valve use a variety of technologies to allow and stop the flow of liquids and gases. Single-disc swing valve are designed with the closure element attached to the top of the cap. Double-disc or wafer check-valve consist of two half-circle disks hinged together that fold together upon positive flow and retract to a full-circle to close against reverse flow. Lift-check valve feature a guided disc. Spring-loaded devices can be mounted vertically or horizontally. Silent or center guide valve are similar to lift check valve, with a center guide extending from inlet to outlet ports. The valve stopper is spring and bushing actuated to keep the movement "quiet." Ball check valve use a free-floating or spring loaded ball resting in a seat ring as the closure element. Cone check valve use a free-floating or spring loaded cone resting in the seat ring as the closure element.

Selecting check valve requires an analysis of performance specifications, materials of construction, connection types, and media. Performance specifications include valve size, pressure rating, media temperature, and valve flow coefficient. Most check valve are made of acetal polymers, aluminum, brass, bronze, cast iron, chlorinated polyvinyl chloride (CPC), ductile iron, copper, polyethylene (PE), polypropylene (PP), polytetrafluoroethylene (PTFE), polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), rubber, steel, or stainless steel. Connections for check valve can use threads, compression fittings, tube fittings, bolt flanges, clamp flanges, butt welds, socket welds, and union connections. In terms of media, check valve are rated for air, compressed air, water, hot water, cold water, potable water, salt water, wastewater, gasoline, diesel fuel, high-purity gas, liquefied petroleum gas (LPG), natural gas, sour gas, corrosive gas, abrasive material, acids, corrosive chemicals, adhesives, concrete, grout, coolants, hazardous materials, high-viscosity fluids, hydraulic fluids, liquid metal, lubricants, oil, powders, solids, rendering wastes, sludge, slurry, ash slurry, or steam.