Drain valve

An automatic drain valve for draining liquid from a reservoir for pressurized gas, comprising:

• a housing having an inlet opening, an outlet opening, and means for communicating the inlet opening with the reservoir;
• a movable piston positioned to divide the housing into a main chamber and a sensing chamber, each of said inlet opening and said outlet opening communicating with the main chamber;
• a fixed valve seat carried by the housing to sealingly engage the piston and close communication between the main chamber and the sensing chamber;
• a valve element for closing the outlet opening;
• biasing means for biasing the valve element into a closed position and the piston into sealing engagement with said valve seat;
• and
• actuating means for moving the valve element into an open position in response to movement of the piston toward the main chamber;
• said piston being positioned to move toward the sensing chamber and out of engagement with said valve seat, to open communication between the main chamber and the sensing chamber, in response to higher pressure in the reservoir and the main chamber than in the sensing chamber;
• and to move toward the main chamber and out of engagement with said valve seat, to open communication between the main chamber and the sensing chamber and to move the valve element into said open position, in response to a drop in pressure in the reservoir and the main chamber.