Introduction

Combination air valves are normally installed at high points where air would naturally rise ahead of the incoming liquid during filling and collect during operation and/or where negative pressure first occurs during draining.
The large (air & vacuum) and the small (air release) orifices are both open during filling with the majority of the air is passes through the large orifice.
The rate at which air is discharged is a function of the differential pressure across the air & vacuum valve.
During filling, incoming water compresses the air in the “empty” pipe until the pressure differential across the air valve is sufficient that air is discharged at the same volumetric rate as water is entering the pipe.
After all the air has been purged, liquid enters the valve and lifts the floats and closes both orifices.
While the large orifice remains closed, the small orifice reopens as necessary to release any air that collects in the valve after it has closed. Should the internal pressure fall below atmospheric, both orifices open to allow air to flow through the valve into the system at the same volumetric rate at which the liquid is draining thus avoiding critical vacuum and damage to the pipe.