Air Valves in Pipelines (Part I – Air in Water!)
Air in water transmission pipelines
There is always some air in water pipelines. Air in water pipelines is usually more than 2 percent as dissolved air or gas. Primary sources of air in a pipeline are, pump startup, rise in temperature or a drop in pressure and through equipment such as pumps, fittings, and valves when vacuum conditions occur. The air may be absorbed in free surfaces, or entered in turbulent flow at the entrance to the line.
Fig 1- Air entry to water pipeline 
The trapped air in a pipeline can have serious effects on system operation:
1- Air in pipeline leads to increasing pressure and reducing flow rate periodically.
2- Air trapped in pipeline increase corrosion and provide a place for bacterial activities on pipeline coating
3- Air pockets in pipelines sometimes move along pipeline that results in transmission pressures in pipeline and affects pump efficiency
4- Air Aggregation in pipeline connections increase the possibility of pipeline failures.
5- Due to a very lower density of air rather that water, air rapidly exits from gaps and air valves. This leads to increasing Acceleration of water column behind air pockets and accruing high pressures.
6- Air trapped in pipeline increase transmission pressures while shutting down and up pumps.
However, the air in the pipeline is required in cases such as vacuum enhanced problems and down-surges and consequent up-surges. The absence of an air cushion in such situations can increase the damages of surge and slam phenomena.
 Pipeline Design for Water Engineers, David Stephenson