Air in water pipes: What is an air valve and how does it work?

Air valves are an essential part of water supply networks. Whether it be a water distribution line or sewage main, you can be sure to find an air valve there. So, what are air valves? What function do air valves perform on critical pipeline infrastructure? This article, will answer these questions and give you more technical insight into air valves.

What causes air in water pipes?

Air and vacuum formed in water mains may lead to serious operating problems and even some dramatic consequences. Air can enter piping systems in several ways:

• Empty Pipelines
  Pipelines not in operations are occupied with air. Most of it is evacuated during startup, but some air pockets can remain in the system.
  
• Air in The Fluid
  Depending on the temperature and pressure, liquids like water can contain trapped or dissolved air. During fluid flow, it separates from the liquid and can become trapped at the system’s high points. Also, in pipelines conveying sewage, the liquid waste can undergo chemical reactions and evolve into gases that can get trapped in the wastewater system.
  
• Mechanical Equipment
  Air can also get into the pipeline through mechanical systems like pumps, pipe joints, valves, etc. Leaks or faulty seals in these components can lead to air infiltrating the piping system

How does a vacuum form in a pipe?

Vacuums in pipelines occur when the pressure in the pipeline drops below the atmospheric pressure. Vacuums can appear due to sudden changes in the velocity of the flowing fluid. For example:

• When the pumps are switched off too fast
• When the valves are suddenly closed
• Emptying of line sections of the pipeline in a wrong way

What are the consequences of air and vacuums in pipelines?

Air and vacuums formed in the pipeline can have serious effects on the operation of the pipeline. Some of these consequences include:

• Reduced Pumping Efficiency: Air in pipelines can reduce the efficiency of the pumping system. The air trapped at the high points of the system blocks flow, increasing the pressure head and thus the energy required for flow to occur.
  
• Pipeline Corrosion: Depending on the pipeline's temperature, oxygen in the trapped air can be a powerful corrosive agent. The oxygen oxidizes the metal gradually, which leads to rusting, blockage, and structural failure of the pipe.
  
• Faulty Metering and Instrumentation Devices: Air pockets and vacuums can cause problems for flow measuring and control devices. They affect the ability of the devices to accurately measure and control the flow.
  
• Air Hammer: When a trapped air pocket is present in a pipe, pressure builds up around the blockage. The pressure of the water swirling around this blockage sends vibrations throughout the pipe. This vibration, known as an air hammer, can potentially damage important fixtures.
  
• Pipe Failure: Vacuums can cause catastrophic failures in pipelines. If there is a substantial vacuum, the pipe can collapse inwards due to the pressure difference on both sides of the pipe's walls.

What is an air valve?

How can one prevent the formation of air and/or vacuum in water supply systems? A solution would be an air valve, a special hydromechanical flow control device that allows the metered flow of fluid in one or both directions. Its function on a pipeline is to release accumulated gases or admit air into the pipeline during the filling, draining, or operation of the liquid pipeline systems.

Where to install air valves?

Choosing the right location for the installation of an air valve is very important. To obtain maximum efficiency, the air valves are placed at strategic locations on the pipeline’s run. For proper aeration and ventilation of water and wastewater systems, air valves are required at the following points:

Maximum High Points

Air bubbles accumulate and form air pockets that get stuck at the highest points in the system. Placing air valves here helps ventilate the air pockets out to the atmosphere.
Valve Required: Air release valve / combination valve.

Temporary High Points

Local high points can also serve as an accumulation spot for air bubbles. An air valve here vents the bubbles out to the atmosphere.
Valve Required: Air release valve/combination valve.

Long Rising or Falling Pipe Sections

Dislodged air pockets can flow downstream and form larger air pockets at the top section of the pipe. Also, rapid flow in downward sloping pipe sections can form a vacuum. As a rule of thumb, air release valves and air vacuum valves should be installed every 800m on long pipe runs to properly vent and draw air into the system.

Functions of a automatic air valve

Long Rising or Falling Pipe Sections

Dislodged air pockets can flow downstream and form larger air pockets at the top section of the pipe. Also, rapid flow in downward sloping pipe sections can form a vacuum. As a rule of thumb, air release valves and air vacuum valves should be installed every 800m on long pipe runs to properly vent and draw air into the system.

After Fast Closing Valves

When valves are suddenly shut off, vacuums can form downstream from the valve due to the momentum of the flow. For proper functioning of the pipe, the negative pressure needs to be relieved by admitting air into the pipe.

After Flow Throttling / Boosting Devices

After flow throttling devices like turbines or control valves, the pressure and velocity differences can lead to the formation of a vacuum. Air vacuum valves are needed close to these devices to draw air into the system Flow boosting devices like pumps and reduced pipe sections also draw air bubbles into the flow. Air release valves need to be installed after these devices to vent these air pockets.

Finally, always install your air valves at accessible, well-ventilated locations. This ensures they always have enough air to draw into the systems, and the air expelled has a route out. Also, it makes it easier to reach the valves for maintenance.