A non-return valve allows fluid to flow through them in one direction only, so they are commonly assigned to one-way valves. NRVs are also a type of two-port valve as they feature two openings in the body, one for fluids to enter and the other for them to leave. Non-return valves frequently work automatically, meaning they do not require to be regulated manually by a person or external control. As such, most NRVs do not feature handles or stems.
Non-return valves are used to prevent fluid backflow, which could damage equipment or upset the process. Such valves are especially useful in protecting a pump in liquid applications or a compressor in gas applications from backflow when the pump or compressor is shut down. Nonreturn valves are also used in process systems with fluctuating pressures, which must be kept separate.
Check valves are generally installed in pipelines to restrict backflow. A check valve is a one-way valve in which the flow can run freely one way, but if the flow turns, the valve will close to protect the piping, other valves, pumps, etc. If the flow turns and no check valve is installed, a water hammer can occur. Water hammer often happens with extreme force and will instantaneously damage a pipeline or components.
A check valve relies on a pressure differential to work. Check valve requires a higher pressure on the valve's input side than the output side to open the valve. When the pressure is higher on the outlet side, the valve will close. Depending on the valve type, the closure mechanism is different. Unlike other valves, they do not need a handle, lever, actuator, or human to work correctly.
Fundamentally – check valves (CVs) and non-return valves (NRVs) are different fittings and serve other purposes. The check valve has a specific purpose as a device legally permitted to prevent backflow in defined circumstances. The non-return valve, while useful for preventing reverse flow in pipes in many situations, is not a recognized backflow prevention device. There are different performance specifications for each type of device, and non-return valves have to meet less demanding standards than check valves. There’s industry-wide confusion surrounding NRVs and CVs' naming, with manufacturers’ and suppliers’ websites using either name or sometimes both, for the same type of device. It led to problems where alterations have been done to comply with an enforcement notice issued by the water supplier requiring additional check valves. In several cases, installers have unknowingly purchased NRVs, only to replace them with the correct check valves when the water supplier re-inspects the work.