Shuttle valve:

• A shuttle vale also known as a double check valve/ OR gate, allows pressure in a line to be obtained from alternative sources. It is primarily a pneumatic device and is rarely found in hydraulic circuits.
• Construction is very simple and consists of a ball inside a cylinder. If pressure is applied to port X, the ball is blown to the right blocking port Y and linking ports X and A. Similarly, pressure to port Y alone connects port Y and A blocks port X