A double check valve or double check assembly (DCA) is a backflow prevention device

Backflow prevention device

A backflow prevention assembly is used to protect water supplies from contamination or pollution.In water supply systems, water is normally maintained at a significant pressure to enable water to flow from the tap, shower etc...

 designed to protect water supplies from contamination

Backflow (plumbing)

Backflow is a term in plumbing for an unwanted flow of water in the reverse direction. It can be a serious health risk for the contamination of potable water supplies with foul water. In the most obvious case, a toilet flush cistern and its water supply must be isolated from the toilet bowl...

. It is also a valve used in air brake systems on heavy trucks.

Usage in Water Supply:
In the case of usage in water supply, it consists of two check valve

Check valve

A check valve, clack valve, non-return valve or one-way valve is a mechanical device, a valve, which normally allows fluid to flow through it in only one direction....

s assembled in series. This employs two operating principles: firstly one check valve will still act, even if the other is jammed wide open. Secondly the closure of one valve reduces the pressure differential across the other, allowing a more reliable seal and avoiding even minor leakage.

Small valves may be so compact as to be barely noticeable, particularly when they are integrated into the bodies of existing taps (faucets). Larger check valves may be installed with ball valve

Ball valve

A ball valve is a valve with a spherical disc, the part of the valve which controls the flow through it. The sphere has a hole, or port, through the middle so that when the port is in line with both ends of the valve, flow will occur. When the valve is closed, the hole is perpendicular to the ends...

s at the ends, for isolation and testing. Often, test cocks (very small ball valves) are in place to attach test equipment for evaluating whether the double check assembly is still functional.

The double check valve assembly is suitable for prevention of back pressure and back siphonage, but is not suitable for high hazard applications. It is commonly used on lawn irrigation

Irrigation

Irrigation may be defined as the science of artificial application of water to the land or soil. It is used to assist in the growing of agricultural crops, maintenance of landscapes, and revegetation of disturbed soils in dry areas and during periods of inadequate rainfall...

, fire sprinkler

Fire sprinkler

A fire sprinkler system is an active fire protection measure, consisting of a water supply system, providing adequate pressure and flowrate to a water distribution piping system, onto which fire sprinklers are connected...

 and combi-boiler systems. If the hazard is higher, even a relatively low hazard such as using antifreeze in the fire sprinkler system, then a more reliable check valve such as a Reduced pressure zone device

Reduced pressure zone device

A Reduced Pressure Zone Device is a type of backflow prevention device used to protect water supplies from contamination...

 may be mandated.

See also

• Air gap
• Hydrostatic loop
  Hydrostatic loop
  A hydrostatic loop, though not often used in plumbing practice, is an arrangement of pipes formed into a vertical loop to prevent backflow of water within the plumbing potable water system. Since a siphon has a maximum height that it can work , a hydrostatic loop is built higher than 33 feet...
  
  
• Pressure vacuum breaker
  Pressure vacuum breaker
  A Pressure Vacuum Breaker is a type of backflow prevention device, used to keep non-potable water from entering the water supply. A PVB is similar to an atmospheric vacuum breaker , except that the PVB contains a spring-loaded poppet. This makes it acceptable for applications that are high...
  
  
• Chemigation valve
  Chemigation valve
  A chemigation valve is an apparatus designed to protect water supplies from agricultural chemicals used during chemigation, the application of chemicals such as fertilizers and pesticides through irrigation water...
  
  
• Reduced Pressure Principle BFP
• Atmospheric vacuum breaker
  Atmospheric vacuum breaker
  An Atmospheric Vacuum Breaker is a backflow prevention device used in plumbing to prevent backflow of non-potable liquids into the drinking water system. It is usually constructed of brass and resembles a 90-degree elbow with a hood on its top to allow air to enter the water system if a siphon...
  
  
• Reduced pressure zone device
  Reduced pressure zone device
  A Reduced Pressure Zone Device is a type of backflow prevention device used to protect water supplies from contamination...
  
  
• Air brake (road vehicle)
  Air brake (road vehicle)
  Air brakes are used in trucks, buses, trailers, and semi-trailers. George Westinghouse first developed air brakes for use in railway service. He patented a safer air brake on March 5, 1872. Originally designed and built for use on railroad train application, air brakes remain the exclusive systems...