Why use air valves in water and wastewater applications

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.

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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.

Water always contains dissolved air, this air will appear in the form of bubbles when there is an increase in temperature and/or a decrease in pressure.

There is also air ingress in the pumping due to the vortex that forms at the suction inlet, and due to faulty seals in the pump body and suction pipes.

Also dissolved air is released due to turbulence in the pump and suction.

Purpose of Air Valves

Excess air in a pipe can cause slower flow, pipe churning damage, cause turbulence, and increased corrosion rate.

• Air valves allow excess air to escape from the pipeline while containing pipeline fluids within the pipeline during operation. They also allow air to escape from the pipes during filling.

• The valves can inject air into the pipes while they are being emptied, preventing a vacuum, which would cause the pipe to collapse.

Applications of Air Valve

All fluid transport systems

• Potable
• Sewage
• Residual
• Raw water
• Saltwater (reverse osmosis plants)
• Acids in solution (mining).

Air valves are helpful under following operations.

Pipe Filling

When starting the filling of the pipe, the air inside it needs to be evacuated in a controlled manner. This is carried out with air valves located at high points in the system.

The required diameter depends on the filling rate of the pipe.

Pipe Emptying

When emptying the pipe of water, air must enter to avoid the appearance of negative pressures that can damage the pipe. This is carried out with air valves located at high points in the system.

The required diameter depends on the diameter of the drain or drain valve and the difference in height between the air valve and the drain valve.

Air Bleed

The water contains dissolved air at all times, as the water is transported through the pipe, the pressure drops due to the effect of friction on the walls.

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When the pressure drops, the water reduces its capacity to contain dissolved air and this manifests itself in the form of small bubbles that are dragged by the water and accumulate in the high points of the system.

If this accumulated air is not removed, the effective water circulation area is reduced and localized pressure drops are generated. In extreme cases, the pipe can be completely clogged.

Types of Air Valves

We discuss mainly four types of air valves

1. Air release valve
2. Air and vacuum valve
3. Combination air valve
4. Kinetic Air release valve

Air Release Valve

This type of valve employs a light weighted float that senses the presence of air. A mechanism linked to the float opens the valve orifice and releases the air.

These types of valves work best when the water pipe is at full pressure, as it has a limited ability to release and inject air.

Air and Vacuum Valve

Air and vacuum valves, also called large orifice valves, injectors release air in large quantities while the pipeline is being filled or emptied.

These valves are also sealed by a ball. As the water fills the pipe, the ball stays below the hole and the valve is sealed.

When the pipe is being emptied, the ball falls with the water level and allows air to enter the pipe.

Combination Air Valve

The combination air valves are nothing but combined air and vacuum valves.

The automatic air release function releases accumulated air from the system while it gets pressurized.

The air and vacuum function discharges take-in large volumes of air in the course of filling or draining pipelines.

The combination air valve will open to relieve negative pressures whenever water column separation occurs.

Kinetic Air Valve

It evacuates air during pipe filling. Allows large volume air intake in case of the pipe drainage network. The kinetic air valve provides excellent protection against vacuum formation with its advanced aerodynamics design and kinetic orifice.

The air valve provides improved sealing under low-pressure conditions.

Incomplete discharge of air during filling:

When the pipe is filled with water, the air it contains must be released to the outside, if the measures taken to make this happen are insufficient, part of the air will remain trapped inside the pipe.

Disadvantages of Air Valves in the pipes lines

The control of the presence of air – inside the pipe can cause severe problems:

1. The impedance of the flow in pipes or reduction of the effective section:

It results in an increase in energy losses. Also the inadequate supply of water to the different areas of the system due to the obstruction of the flow and the accumulation of localized head losses.

2. Inaccurate readings on gauges and hydrometers.

3. Serious damage to internal rotating parts of gauges.

4. Inadequate water supply due to incorrect meter and hydrometer readings.

5. Corrosion and Cavitation problems.

Disadvantages due to lack of Air in the pipeline

The problem of air in the pipes is not only due to its presence of air but in some cases, it is due to its absence, such as the

1. Lack of air damage pipelines is a consequence of negative pressures or depression.

2. The collapse is due to transient pressures or water hammers.

3. And also by the suction of mud and dirt in the connections, suction of seals, gaskets, and other internal parts.

4. Sucking of chemicals injected into the system.

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