Check Valves

Check valves are self-activating safety valves that permit gases and liquids to flow in only one direction, preventing process flow from reversing. They are classified as one-way directional valves. 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 valves 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.

Types of Check Valves

Check valves use a variety of technologies to allow and stop the flow of liquids and gases. A single-disc swing check valve is designed with the closure element attached to the top of the cap. A double-disc or wafer check valve consists of two half-circle disks hinged together that folds together upon positive flow and retract to a full-circle to close against reverse flow. These should not be confused with a double check valve, which is an assembly that contains two distinct check valves.  Lift-check valves feature a guided disc. Spring-loaded devices can be mounted vertically or horizontally. Silent or center guide valves are similar to lift check valves, with a center guide extending from inlet to outlet ports. The valve stopper is spring and bushing actuated to keep the movement "quiet." A ball check valve uses a free-floating or spring loaded ball resting in a seat ring as the closure element. Cone check valves use a free-floating or spring loaded cone resting in the seat ring as the closure element.  Duckbill check valves are flow-sensitive, variable-area, valves.  They get their name from their shape, which consists of two flaps shaped like a duck's bill. In zero flow conditions, the valve remains closed. As the flow increases, the pressure on the flaps increases and the valve opens. Umbrella check valves are elastomeric self-actuating devices.  These valves simply press into a hole and can be designed to function within a specified pressure range.  The name umbrella comes from the general shape of the device. Back flow preventers are assemblies of two check valves and two shut-off valves. They may also include test cocks for each chamber within the assembly.

Check Valve Performance Specifications

Selecting check valves 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 valves 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 check valves) , polyvinylidene fluoride (PVDF), rubber, steel, or stainless steel.

Connections for check valves can use threads, compression fittings, tube fittings, bolt flanges, clamp flanges, butt welds, socket welds, and union connections. In terms of media, check valves 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.