Function

In former times air reservoirs had check valves with large rings on the underside. These had to be opened regularly with all air reservoirs to remove water from the system. Today the automatic drainage valves do this automatically with every increase or decrease of pressure.

How it works

The most important component is the in black marked diaphragm in the above drawing, which functions here as a two-way valve. When the pressure rises, it lets air from the boiler flow into the external chamber of the drainage valve. When the maximum pressure is reached, it closes this chamber, both to the boiler as well as to the outside. Only when the boiler pressure decreases (e.g., when braking) does the diaphragm open the way for the air to go out.

In this way, air (plus amount of water on the boiler floor) either flows with each pressure change into the chamber or from the chamber to the outside. The size of the chamber determines the amount of air flowing out, or regulates itself according to the amount of water expected.