Three main types of air valves
There are three types of air valves installed in water systems: automatic air valves, air and vacuum valves, and combination air valves.
Automatic air valves
Automatic air valves continuously release relatively small volumes of air from a pressurized line. They are also known as small orifice air valves and pressure air valves.
Air and vacuum valves
Air and vacuum valves discharge large volumes of air when filling a pipeline. The Air and vacuum valves make it possible to admit large volumes of air when lines are drained or when the pressure suddenly drops below atmospheric pressure. Air and vacuum valves are also known as kinetic air valves, large orifice air valves, vacuum breakers, low-pressure air valves, air relief valves, and single-acting air valves.
Combination air valves
Combination air valves combine the function of automatic air valves and air and vacuum valves. The automatic air release function releases accumulated air from the system while it is under pressure, and the air and vacuum function discharges and admits large volumes of air during the filling or draining of pipelines. Combination air valves are also known as double-orifice air valves, double-acting air valves, and dual-orifice air valves.
Why air appears in pipelines
Dissolved air exists in all fluid transmission systems. The principal sources are:
- Incomplete filling of the line which leaves air pockets at pipeline peaks
- Dissolved air released from the fluid when pressure drops and/or temperature rises
- Air is sucked into the system through openings at e.g. joints, breaks/bursts and accessories
If the volume of the air and wastewater gas pocket reaches a certain size, which especially occurs at pipeline peaks where air pockets naturally collect, a restriction of the flow occurs that produces unnecessary headloss risking complete stop of the water flow.
Sizing and location of air valves
Air valves are dimensioned for a specific installation with considerations of flow, location and air valve performance. Thus, the DN of the air valve is not equal to the DN of the pipe, on which it is installed. The size and location of air valves for a specific project are defined by means of advanced calculation programs. That provids analyses such as fill rate analysis, drainage analysis, burst analysis, water column separation analysis, and energy-saving analysis.
The primary function of an air valve is to release unwanted air pockets from a pipeline, but it only functions effectively when correctly installed. The proper location of the air valve can improve water flow performance significantly. Therefore, air valves should be installed at those points where the pipeline is most susceptible to vacuum conditions. In general, the optimal installation of an air valve is at the pipeline peak, and the valve must always be installed vertically.
Before installation of an air valve, the location must be considered so that the valve is protected against freezing, pollution and flooding. The air valve should be easily accessible to allow maintenance, operation and inspection.
Recommended placement of air valves
- High points: combination air valve
- Long horizontal runs: air release or combination valves at 380-meter to 760-meter intervals
- Long descents: combination air valves at 380 meters to 760 meters intervals
- Long ascents: air/vacuum valves at 380 meters to 760 meters intervals
- Decrease in an upslope: air/vacuum valve
- Increase in a down slope: combination air valve
On very long horizontal runs, air release and combination air valves should be used alternately along the pipeline. Note that combination air valves can be used at any location instead of air release or air/vacuum valves to provide added air release capacity on the pipeline.
