Air valve is a valve mounted in “TEE” configuration on a pipeline to discharge or admit air into or out of the pipeline.

Why should the air in the pipeline be controlled ?
The existence of trapped air in a pipeline under pressure can cause negative effects on the operation and efficiency of the system. Air pockets accumulating at high points, of changing slope sign, reduce the effective cross-section of the pipeline in the accumulating location, which in turn, causes a decrease in the flow rate, and the energy needed to pressurize the waterful is increased as a result. The overall system efficiency is then reduced. Air pockets beyond some critical quantity in the system may even restrict the whole pipeline from flowing –“locking” – the pipeline.