How does a Air Release Valve work? TWS valve will tell you. 

Composite high speed air release valve is composed of two parts:
The high pressure diaphragm automatic air release valve and the low pressure intake air release valve.
The high pressure diaphragm automatically releases a small amount of air accumulated inside the pipe under pressure.
Low pressure air valve can discharge the air in the pipe when the empty pipe is filled with water, and automatically open and air inlet into pipe to eliminate vacuum when the pipe is drained or vacuum or under the condition of water column separation.