Air valves are placed at high points in piping systems, to bleed air during filling, release air that accumulates over time and allow inflow of air to prevent negative pressure and possible pipe collapse.

In typical wastewater collection system piping, pressures are positive between the pump station and a high point along the force main when the pumps are running. When the pump turns off, the force main that was pressurized remains full while any downward sloping pipes drain. The behavior of air valves becomes very important in force mains with multiple high points.

No loads can be assigned to air valves. Air valve elements can only be attached to pressure pipes, not on gravity conduits. They should be attached to two pipes. If they are attached to more than two, an error will be issued. If they are placed at the end of a dead end pipe, they will not do anything.

In the Bentley storm and sanitary sewer products, the behavior of air valve elements depends on the active solver.

GVF-Convex Solver

The GVF-convex solver uses a true pressure solver where the pipes are generally treated as full. When the hydraulic grade is above the valve elevation, the valve will remain closed. When the hydraulic grade line drops to the valve elevation, the air valve prevents the hydraulic grade from dropping below the valve and the pipe acting as a siphon. The GVF solver enables the user the ability to see which pipes flow partly full as those pipes will have a hydraulic grade line below the pipe on the downhill side of a high point.

A user can force an air valve to be closed in a model run by setting the "Treat Air Valves as Junction" property to True. The default setting in sewer models is False. However, only those air valves than may reasonably open during a simulation need to have that property set to False. This will make the simulations run faster as fewer checks will need to be made on valve status.

If an air valve at a high point is closed or treated as a junction and the pressure becomes negative (hydraulic grade drops below valve), the pipes will behave as a siphon. This is usually undesirable because the pipe may collapse under the negative pressure or gases may come out of solution and cause excess head loss at the high point. The maximum that a high point of a pipe can be above the hydraulic grade is 32 ft (9.8 m) or a siphon cannot be formed.

Gravity Solvers -- Implicit (Dynamic Wave), Explicit (SWMM) and GVF rational Solvers

These solvers do not have the unique air valve behavior as in the GVF convex solver since none of these are true pressure solvers. An air valve, assigned to a model with these solvers, is treated as a manhole with a bolted cover which is essentially the same as an active air valve. It is not possible to solve for a high point as a siphon with these solvers.