An Air Valve is the model element used to represent air relief valves, air release valves, vacuum breaker valves and combination air valves. The underlying behavior of the valves is to be closed when then hydraulic grade is above the valve elevation and to open to maintain atmospheric pressure when the hydraulic grade would otherwise have dropped below the valve.

In the implicit and explicit solvers, air valve elements are treated essentially as manholes with a bolted cover which can let air in and out but do not allow release of water. They do not accurately model air flow through orifices and as such cannot be used for sizing orifices in the air valves.

While they are useful in normal operation, they are especially important in transient analysis. This is described in "Air Valves in HAMMER". They are also used in filling and draining of pipelines which can be addressed in the implicit and explicit solvers in Bentley StormCAD .

While air valves are often placed on pumps to remove air, their primary use is at high points in pressure piping systems which are the first locations which can experience negative pressure in pipeline systems and are the most likely places where air and other gases can accumulate.

In sewer force mains, irrigation systems and raw water transmission systems, pressure can drop when pumps turn off or head loss becomes excessive and they can often be in the open position and pipes can be partly full, immediately downstream of the high point. Drinking water distribution systems are designed not to have negative pressures.

If there is no air valve placed at a high point and the pressure drops below zero, the valve will behave as a siphon. This is generally not recommended as flexible pipes may collapse.

The behavior of air valves can be best viewed using a profile view. With an air valve in place, the valve would prevent the negative pressure by opening to atmosphere. There may be partially full flow downstream of the high point (where the hydraulic grade line is below the pipe). The location where the hydraulic grade line rises over the pipe is the location where full pipe flow is restored.

In most cases, when the pump is operating, the hydraulic grade line will remain above the pipe and the air valve will be closed.

When the pump or other source on the upstream side of the high point is shut off or closed, the pipe generally remains full. The display when the pump is off will look like this. In this example, the fact that the pipe downstream of the high point is still draining at this time is reflected in the hydraulic grade above the pipe in some places.