Why is it important to install air valves in pipeline systems for water and wastewater applications?

Why does air appear in pipelines?

In fact, dissolved air is present in all liquid transmission systems (including clean water and wastewater). The main sources are:

• Incomplete/full filling, which then leaves an air pocket at the peak of the pipe.
• Dissolved air is released from the liquid as pressure falls and/or temperature rises.
• Air is sucked into the system through exposed parts such as joints, bursts/leaks, and accessories.

If the volume of air pockets and wastewater gas has reached a certain amount, which is especially the case in the upper pipes where air pockets are common, a flow restriction occurs that results in unnecessary pressure loss and risks causing the water flow to stop completely.

Why does a vacuum occur in a pipeline? 

1. Damage to the pipeline causing the sucking of dirt, mud and dust through damaged joints and through cracks in the pipe as well as accessories.
2. Reduced pipeline life due to positive and negative pressures reacting on the pipe walls. This can result in overloading that will weaken-and eventually cause-damage to the pipe wall. This is especially important in older pipelines, where material degradation may occur.
3. In extreme cases, the pipe may burst.

There are three problems that can cause the ingress of dirt into the water distribution system to occur:

1. The presence of a source of dirt outside the distribution pipes (e.g. contaminated groundwater)
2. Pathways that provide a route into the system (damaged or leaking pipe joints)
3. Driving force (negative pressure inside the pipe)

One solution to prevent contamination is to eliminate one of the three problems: driving force. Air valves can be an efficient and cost-effective to address this problem—and by installing and maintaining systems with air valves, negative pressure is maintained and eliminated, so even aging networks can remain protected from soil contamination.
Vacuum in pressurized pipe systems can result from the inefficiency of air valves that do not respond to hydraulic conditions, broken air valves due to lack of maintenance, and air valves not being present in the pipe system at all.

Air valves are often associated with releasing air from the system, which is why they are often referred to as air release valves. However, this title is misleading—their function of introducing air into the system to avoid vacuum is just as important.

What is the function of air valves?

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.