When air is admitted into the pipe, an in “Air Pocket” is created in the local high points where the Air / Vacuum valve is located. The returning flow re-fills the “pocket”. Too-high velocity of the approaching water column may generate a pressure surge when it reaches the valve.      

Air venting The Surge Arrestor addition of “DAV-P” valves limits the air outflow, when the escaping air velocity exceeds a threshold value. This optional addition creates a temporary, slow closing “Air Cushion” that decelerates the water velocity, preventing water hammer effect.

Vacuum Breaking (Air Intake) Decrease or the pressure in the system to negative value and the simultaneous drainage of the valve chamber, forces the floats down, allowing the admittance of air into the pipe. The SA disc moves back and allowing unrestricted air flow into the system.