All water supply systems consist of pipes that transmit water from its source to its destination. In this process, pockets of trapped air can form in the pipelines. Pockets of air and wastewater gases are difficult to detect and will reduce the overall efficiency of the piping system. The air pockets may also cause water hammer problems, full stoppage of flow, pipe bursts, system noise and pipe corrosion. Air in the pipeline can also cause unstable operation of control valves, meters and equipment. To secure a safe and efficient operation it is therefore critical to continually remove air and wastewater gases from the water system.
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 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.
Why vacuum occurs in pipelines
Vacuum can occur in pipes due to lack of air in the system causing severe irreversible issues. Negative pressures left unmanaged have been proven to cause:
- Damages of the pipeline causing suction of contaminants, mud and dirt through faulty connections, cracks in pipes and accessories
- Reduced lifetime of the pipeline due to positive and negative pressures acting on the pipe wall. This can result in fatigue loading which will weaken and eventually cause failure of the pipe wall. This is especially important on ageing pipelines where material degradation is likely
- In extreme cases even pipe collapse
Three issues must coexist for contaminant ingress to take place within water distribution systems:
1) The existence of a contaminant source external to the distribution pipe (e.g. contaminated ground water)
2) A pathway providing a route into the system (failing pipe joints or leaks)
3) A driving force (negative pressure within the pipeline)
One solution to prevent contamination is thus to remove one of the three issues: the driving force. Air valves are the most efficient and cost-effective tool to achieve this and by installing and maintaining a system with air valves, negative pressure is managed and removed from the equation, allowing even an ageing network to remain protected from contamination from the soil.
Vacuum in a pressurized pipe system can thus be caused by inefficient air valves not reacting to hydraulic conditions, air valves having failed due to lack of maintenance, and air valves being absent from the pipeline altogether.
Air valves are often associated with the release of air from the system, and therefore commonly known as air release valves. However, this name is misleading as their function of introducing air into the system to avoid vacuum is equally important.
The function of air valves
Air valves are hydromechanical devices with an internal float mechanism designed to release trapped air and wastewater gases during filling and operation of a piping system.
They also ensure air intake during draining to maintain a positive pressure. This is important as negative pressure causes vacuum that may result in pipe collapse and introduction of contaminants to the system.
Therefore, air valves provide protection from transient pressures and entrapped air in pipelines, which are the main causes of bursting, collapsing, and fracturing of pipelines.
With correct use of air valves, you will achieve:
- Reduced energy consumption and operation costs
- Reduced pressure loss
- Reduced delays when filling the pipe system
- Reduced risk of water hammer
- Reduced failure or inaccuracies in flow metering
- Reduced corrosion
- Reduced risk of contamination
- Reduced risk of pipe burst and collapse
- Reduced costs for replacement
- Reduced depreciation due to increased network lifespan
- reduced required pump capacity due to lower head loss
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 continuously release relatively small volumes of air from a pressurized line. Automatic air valves are also known as small orifice air valves and pressure air valves.
- Air and vacuum valves discharge large volumes of air when filling a pipeline. 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 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.
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 providing 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
(Source: 3. American Water Works Association, AWWA M11 "Steel Pipe: A Guide for Design & Installation"; 3rd ed., 1989, pp. 98 to 99.)
- 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 up slope: 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.