What is the difference in application between check valve, double check valve, and back-flow preventer?

Check Valve as we normally use in plumbing, chilled or firefighting system is normally a swing type valve which can be specified for its application which is not for any sort of contamination or rugged used.

Double check valve is same but having normally two spring loaded valves in same assembly used for water supplies back flow prevention for contamination. It is just a safety device even if one valve is wide open other can perform the function also one valve reduces the pressure for other valve. Means double safety. Can be used in hot fluid.

BOTH OF ABOVE ARE NORMALLY USED IN PRESSURIZED LINES.

Back flow preventer can be in drainage main line in case of back pressure it will not allow water to reverse in the pipes. OR where we have siphon phenomenon or water is coming from hill or for some cases potable can be mixed with non potable water (cross connections) or when the water is freezed in the pipes or pipe is burst so there is a chance of contamination. In these situations back flow preventer is used.

ALL OF ABOVE PERFORM SAME FUNCTIONS BUT WITH DIFFERENT SCENARIO.

A check valve, double check valve, and backflow preventer that normally allows fluid to flow through it in only one direction.

Check valve normally we use in pumps like chilled water pump, fire fighting pumps, water supply pumps, check valve is an automatic valve, its main function is to prevent the fluid backflow, prevent pump and drive motor reverse.

A double check valve, it consists of two check valve assembled in series, this employ 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. Protect water supplies from contamination. It is also a valve used in air back systems.

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

Check valves are two-port valves, meaning they have two openings in the body, one for fluid to enter and the other for fluid to leave. There are various types of check valves used in a wide variety of applications. Check valves are often part of common household items. Although they are available in a wide range of sizes and costs, check valves generally are very small, simple, or inexpensive. Check valves work automatically and most are not controlled by a person or any external control; accordingly, most do not have any valve handle or stem. The bodies (external shells) of most check valves are made of plastic or metal.

A double check valve or double check assembly (DCA) is a backflow prevention device designed to protect water supplies from contamination. It is also a valve used in air brake systems on heavy trucks. It consists of two check valve 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.

A backflow prevention device is used to protect potable water from contamination or pollution due to back flow. In water supply systems, water is normally maintained at a significant pressure to enable water to flow from the tap, shower etc. When pressure fails or is reduced, as may happen if a water main bursts, pipes freeze or there is unexpectedly high demand on the water system, then such reduced pressure in the pipe may allow contaminated water from the ground, from storage or from other sources to be drawn up into the system.