API 6D, BS1868 & ISO15761
The valve that used to prevent backflow in a piping system is known as a check valve. It is also known as a non-return valve or NRV. The pressure of the fluid passing through a pipeline opens the valve, while any reversal of flow will close the valve. It allows full unobstructed flow and automatically shuts as pressure decreases. The exact operation will vary depending on the mechanism of valve.
Check valves (Non-return valve) are used in a piping system to prevent backflow. Discharge line of rotary equipment such as pump and compressor always fitted with a check valve to prevent backflow. The only function of a Non-return valve is to prevent backflow. There is no better alternative. Yes, you can choose the best check valve from the various available type that you have learned but you cannot supplement check valve with another valve
An important concept in check valves is the cracking pressure which is the minimum differential upstream pressure between inlet and outlet at which the valve will operate. Typically the check valve is designed for and can therefore be specified for a specific cracking pressure..
Types of Check Valve
The disc in a swing type valve is unguided as it fully opens or closes. This Valve operates when there is flow in the line and get fully closed when there is no flow. Turbulence and pressure drop in the valve is very low. Disk and seat designs can be of metal to metal or metal to composite. The angle between the seat and the vertical plane is known as seating angle and vary from 0 to 45 degrees. Usually, the seat angles are in the range of 5 to 7 degrees. Larger seat angles reduce the disc travel, resulting in quick closing, thus minimizing the possibility of water hammer. A vertical seat has a 0-degree angle.
Stop Check Valve is a Combination of a lift check valve and a globe valve. It can either be used as a check valve or as an isolation (stop) valve like a globe valve. These valves can be closed with the help of a stem which is not connected to the valve disc during normal operation and make it possible to use these valves as a regular check valve. However, when needed, the stem is used to holds the free-floating disc against the valve seat, just as a globe valve. These valves are available in tee, wye, and angle pattern. Swing type and piston lift type check valves are commonly used as stop check valves..