Air valves are installed at local high points to allow air to come into the system during periods when the head drops below the pipe elevation and expels air from the system when fluid columns begin to rejoin. The presence of air in the line limits subatmospheric pressures in the vicinity of the valve and for some distance to either side, as seen in profiles. Air can also reduce high transient pressures if it is compressed enough to slow the fluid columns prior to impact.

There are essentially two ways in which an active air valve can behave during the transient simulation:

1. Pressure below atmospheric - air valve is open and acts to maintain pressure to 0 on the upstream end and maintains the same flow on the upstream and downstream side.
2. Pressure above atmospheric - air valve is closed and acts as any junction node.

If an air valve becomes open during the initial conditions calculation (steady state or EPS), the hydraulic grade on the downstream side may be less than the pipe elevation. This can be displayed as the hydraulic grade line drawn below the pipe. This should be interpreted as a pressure pipe that is not flowing full. Full flow resumes at the point where the hydraulic grade line crosses back above the pipe.

Because air valves have the possibility to switch status during a steady state or EPS, they can lead to instability in the model especially if there are many air valves in the system. To improve the stability of the model, it is desirable to force some of the valves closed. This can be done by setting the property "Treat air valve as junction" to True for those valves that are expected to be closed anyway.

If all of the pumps upstream of an air valve are off during a steady state or EPS, the pressure subnetwork is disconnected in that area and the model will issue warning messages for all nodes in that vicinity indicating that they are disconnected.

The following HAMMER attributes describe the air valve behavior:

Slow Closing Air Valve Type

• Time to Close: For an air valve, adiabatic compression (i.e., gas law exponent = 1.4) is assumed.The valve starts to close linearly with respect to area only when air begins to exit from the pipe. If air subsequently re-enters, then the valve opens fully again. It is possible for liquid to be discharged through this valve for a period after the air has been expelled.
• Diameter (Air Outflow Orifice): Diameter of the air outflow orifice (the orifice through which air is expelled from the pipeline).

Triple Acting Air Valve Type

• Air Volume (Initial): Volume of air near the valve at the start of the simulation. The default is zero. If volume is nonzero, the pressure must be zero.
• Trigger to Switch Outflow Orifice Size: Select whether the transient solver switches from the large air outflow orifice to the small air outflow orifice based on Transition Volume or Transition Pressure.
• Transition Pressure: The local internal system air pressure at the air valve above which the transient solver switches from using the large air orifice to the small air orifice (in order to minimize transients.
• Transition Volume: The local volume of air at the air valve below which the transient solver switches from using the large air orifice to the small air orifice (in order to minimize transients). This volume often corresponds to the volume of the body of the air valve.
• Diameter (Small Air Outflow Orifice): ): Diameter of the air outflow orifice (the orifice through which air is expelled from the pipeline) when the local air volume is less than the transition volume (TV), or the air pressure is greater than the transition pressure (TP) (depending on which trigger is used to switch the outflow orifice size). This diameter is typically small enough for the injected air to be compressed, which can help prevent severe transient pressures. Generally air flows out the large air outflow orifice for some time before switching to the small air outflow orifice for the final stages of air release.
• Diameter (Large Air Outflow Orifice): Refers to the discharge of air when the local air volume is greater than or equal to the transition volume (TV), or the air pressure is less than or equal to the transition pressure (TP) (depending on which trigger is used to switch the outflow orifice size). This diameter is typically large enough that there is little or no restriction to air outflow. Generally air flows out the large air outflow orifice for some time before switching to the small air outflow orifice for the final stages or air release.
• Diameter (Air Inflow Orifice): Diameter of the air inflow orifice (the orifice through which air enters the pipeline when the pipe internal pressure is less than atmospheric pressure). This diameter should be large enough to allow the free entry of air into the pipeline. By default, this diameter is considered infinite (i.e. there is no restriction to air inflow).

Vacuum Breaker Air Valve Type

• Diameter (Air Inflow Orifice): Diameter of the air inflow orifice (the orifice through which air enters the pipeline when the pipe internal pressure is less than atmospheric pressure). This diameter should be large enough to allow the free entry of air into the pipeline. By default, this diameter is considered infinite (i.e. there is no restriction to air inflow).