Check valves open with forward flow, and automatically return to close against reverse flow. The main function is to protect mechanical equipment, most notably pumps, where back flow can damage the internal components. Check valves may be grouped according to the way the closure member moves onto the seat.
The Swing Check Valve has a flat disc that pivots or swings about a hinge pin. They are low cost, and have low head loss characteristics when fully open. But because of the long stroke and inertia of the disc, the traditional swing check valve may slam in vertical pipe installations. These valves are therefore sometimes outfitted with a lever and weight accessory. They can handle liquids with solids such as wastewater.
The most compact swing check valve is the Double Disc Check Valve (aka dual disc). The body is usually a wafer design (fits between two pipe flanges) and it has a hinge pin about which two opposing D-shaped discs rotate. The closure is assisted by a torsion spring which wraps around the hinge pin. It is not used in wastewater containing solids, as spokes that run across the body can collect debris. Advantages are it is small and light, and it has short travel and quick plate closing reducing slamming and make it more suitable for frequent flow reversals. A drawback is a higher resistance to flow because the plates are always in the flow path. Best for clean liquids.
Another type is the center guided lift Silent Check Valve. These have spring-loaded discs which move along the pipe axis over a short distance to close automatically in a fraction of a second. Because of their fast closure, these check valves rarely slam and hence have earned the name "silent". They are simple, automatic, and cost effective but can result in higher energy costs in the long run. They can be economical to produce and reliable in operation. Silent check valves are commonly used in high-rise buildings and high head applications because of their quiet closure. Best for clean liquids.
Quick closure is the key to water hammer prevention, so it is important to consider the speed the check valve will close. And as illustrated here, in addition to its speed of closure, a center guided silent check valve that is almost closed will only have a small amount of reverse flow, so water hammer is not as likely for that reason as well. Conversely, a single disc or double disc swing check valve's proportional flow rate may be greater than than its percentage open, meaning more reverse flow is present.