When modeling an air valve, it must conform to one of the four available types: (selected from the "Air Valve Type" attribute) Double Acting, Triple Acting, Vacuum Breaker and Slow Closing. Industry terminology is sometimes not consistent with HAMMER's definition of these types, so it is important to understand their behavior and assumptions. Below describes each air valve type and when it should be used.

Double Acting - This type of air valve has two actions:

1. Air inflow through an inflow orifice diameter
2. Air outflow through an outflow orifice diameter

The diameters of these orifices don't change during the transient simulation. This type of air valve should be used when air enters the valve through a specific size opening, and leaves the system through another specific size opening, without any transition. The opening that allows air outflow is typically smaller, in order to control air release. Here are some examples of when the Double Acting air valve type would be used:

• An air valve with an "anti-slam", spring loaded disc with perforations, which opens under vacuum conditions. When pressure returns, the spring closes the disc and air is forced to exit through the small perforations. The air inflow orifice would be the size of the opening through which air flows when the disc rises off the seat. The air outflow orifice would be the equivalent orifice size of the perforations in the disc.
• An air valve with a spring loaded orifice that admits air on vacuum conditions and a separate, smaller opening that expels air. The spring loaded orifice would be the air inflow orifice and the smaller opening would be the air outflow orifice.

Triple Acting - This type of air valve has three actions:

1. Air Inflow
2. Air Outflow through a large orifice
3. Air Outflow through a small orifice

Air inflow passes through an opening with a fixed size. Air outflow first passes through a large-sized opening, which switches to a smaller sized opening just before all of the air has escaped. This cushions the air pocket collapse and subsequent collision of the water columns. This type of air valve should be used when the opening through which air is expelled changes based on some condition. The condition to trigger the reduction in size of the outflow orifice can either be based on a pressure differential or an air volume. Typically a float is used to decrease the opening size, but not always.

Here are some examples of when the Triple Acting air valve type would be used:

• An air valve similar to the one seen in the above diagram, consisting of two openings and a float. When the volume of air in the system becomes less than the "transition volume", the float rises, which partially closes the outlet opening. The air inflow orifice would be the size of the "inlet" opening. The "large air outflow orifice" would be the full size of the outlet opening. The "small air outflow orifice" would be the size of the outlet opening after the float has risen.
• An air valve with a float that closes off the outlet opening completely, forcing air out of a separate, smaller opening. The "large air outflow orifice" would be a diameter equivalent to the size of the main outlet opening plus the small opening. The "small air outflow orifice" would be the size of the separate, smaller opening alone.
• An "anti-slam" air valve with a disc or float that first allows air outflow to freely pass out of a large opening. As air velocity increases, the float is "blown" into position by the pressure differential it creates, forcing air out of a smaller opening. The "large air outflow orifice" would be the large size opening (before the float rises) and the "small air outflow orifice" would be the smaller sized opening (after the float rises). "Transition Pressure" would be selected as the outflow orifice trigger type.

Vacuum Breaker - This type of air valve has only one operation: air inflow. During subatmospheric pressure, air enters through the air inflow orifice diameter. The outflow orifice diameter is assumed to be very small (effectively zero) so it doesn't let air out. When looking at the detailed report, you may notice the air volume change as the air pocket is compressed, but the mass of air in the pipe doesn't reduce. There are probably a limited number of applications for this type valve, but it may be used for a draining pipeline.

Slow Closing - This type of air valve has two actions:

• Free air inflow upon subatmospheric pressure
• Linear closure of the air outflow orifice when air begins to exit

Although similar to the other air valve types, the slow-closing air valve only has a single orifice involved; for the expulsion of air and liquid. An air inflow orifice is not required because HAMMER assumes that air will be freely allowed into the system (no throttling) when the head drops below the air valve elevation. The valve starts to close linearly with respect to area only when air begins to exit from the pipeline (after the head begins to rise).

It is possible for liquid to be discharged through this valve for a period after the air has been expelled, unlike the other air valve types, which closes when all the air has been evacuated from the pipeline. Typically you will want the valve to be fully closed after all air has been expelled, but before too much water has been expelled.