Q. Necessity of air valves in pressurized pipelines?

Air valves are broadly categorized into two major types: single air valves and double air valves. Single air valve comprises a small orifice air valve that allows automatic release of a small quantity of accumulated air at the time of normal operation of the pressurized pipeline. Double air valves comprises a small orifice air valve and a large orifice air valve. Large orifice air valve exhausts air automatically during filling and permits admission of air at the time of emptying of the pipeline. Though it can't perform the function of a small orifice air valves. 

Presence of air in the pressurized pipeline is undesirable because of the below reasons:

(i)  Presence of air causes significant impedance to water flow andin the worst case it may even cause complete blockage of system.

(ii) The air persuadessignificant head loss to the system and causes wastage of useful energy.

(iii)It may cause serious damage to meters and even cause inaccurate reading of the meters.

(iv)The presence of air causes water hammer damage to pipeline.

The absence of air (which is during emptying operation of pipeline in routine maintenance) can also produce problems because of the following:

(i)  Suction generated draws in dirt and mud via faulty connections and cracks in pipelines.

(ii) The seals, gaskets and internal accessories would be suctioned inside the pipelines.

(iii)Sometimes, suction forces may be so significant to cause collapse of pipelines.

One may query that if a large orifice air valves can carry out the functions of filling and release of air, why is it essential to add small orifice air valves in pipeline system for release of accumulated air during normal operation? The reason is that air accumulated at the high points of pressurized system would be expelled through the large orifice air valves (in case no small orifice air valves are installed in the system) upon starting of a pump and with this kind of rapid outflow of air through the large valves, high slam pressure can be produced resulting in the damage of pipelines.