Check valves or non-return valve

The check valve, also known as the non-return valve, allows fluid to flow in one direction only. The word ‘fluid’ is used rather than ‘liquid’ because check valves are designed to allow uni-directional flow of both liquids and gases. These valves are used in different fields and their construction features are carefully studied depending on their final use.
The most common check valve features a stem disc and spring construction. Flow is guaranteed in one direction only. When pressure follows the direction of the arrow on the valve, flow is constant; when pressure is balanced in the opposite direction, the spring flexes the disc back to the closed position.

The check valve is installed to prevent fluid from flowing against the intended direction.