it is called the Air Valve which determines the air - water balance in the system. During the filling of the pipeline; The air in the line evacuates the air in the system quickly. Due to various reasons, it allows small quantities of air to accumulate in the pipeline during operation, under pressure. 
During the evacuation of the pipeline, air is sucked into the pipe to prevent vacuum formation, and cavitation hazards are prevented by balancing the system pressure with the atmospheric pressure. 

Plastic Air Valves are two types; 

1-Single Effect (Kinetic) PlasticAirValve ½" - ¾" - 1" and 2"
2-(Automatic) Plastic Air Valve ½" - ¾" - 1" and 2" 
Connections; ½" - ¾" - 1" and 2" threaded

Plastic Air Valves Usage Locations; 

1-    in agricultural irrigation, (every 400-500 mt on straight lines on the main line, at the beginning of the slope, 400-500 mt at the upward inclines at the peak points, before the beginning of the deflection and before the end of the line and before the irrigation valv    e (At the points indicated in the figure)
2-    in filtration systems, (Disc Filter, Hydrocyclone, Gravel Tank, Automatic Horizontal Filters, ete.).
3-    Factory instaliations in industrial areas, in treatment systems and so on.

Models 

1 Single Ball / Single Function Air Valves

2 Single Ball / Single Function Air Valves

3 Double Ball / Triple Function Air Valves