Quick Overview:
- Air valves are automatic devices that release trapped air and vacuum from pipelines, performing three distinct functions — large-orifice air release on filling, small-orifice automatic air release during operation, and vacuum break on emptying — that together protect pipelines from pressure surge, air binding and structural collapse.
- The three types — air release valves, air/vacuum valves, and combination air valves — serve different purposes and are frequently confused, leading to incorrect specification and system problems ranging from chronic air entrainment to pipeline collapse.
- Placement is as important as selection: air valves must be sited at every high point in the pipeline profile, and their sizing must account for the maximum pipeline filling rate and the maximum drainage rate, not just the steady-state flow.
Air Valves: The Essential Guide to Types, Function and Correct Application
Air in a pipeline is the enemy. A pocket of trapped air at a high point in a water main acts like a partially closed valve — it restricts flow, reduces the effective pipe cross-section, and causes pumping systems to work harder than they should. Large air pockets can completely block flow in smaller diameter pipes. Air bubbles passing through metering equipment cause over-registration of measured volumes. Air trapped in a fire suppression system delays its operation at exactly the moment speed matters. And when a pipeline empties under negative pressure without adequate air inlet, the resulting vacuum can collapse a plastic pipe or cause water hammer severe enough to rupture a metal one.
Air valves solve all of these problems automatically, without any operator intervention. They are among the most underrated components in pipeline engineering, routinely under-specified, incorrectly placed, and inadequately maintained — which is a shame, because a correctly designed air valve installation transforms the reliability and efficiency of a pipeline system.
The Three Types of Air Valve — and Why the Distinction Matters
There are three distinct air valve functions, and the terminology is unfortunately confused in industry, with different manufacturers and standards using different names for the same product. Here is the clear engineering definition of each.
An air release valve (also called a small orifice air release valve or automatic air release valve) has a small orifice — typically 1mm to 10mm diameter — and uses a float mechanism to release accumulated air automatically while the pipeline is under pressure. As air accumulates at the valve, the float drops with the falling water level and opens the orifice to release the air. As water refills, the float rises and seals the orifice. This cycle can repeat thousands of times per day without operator awareness. It does not allow large volumes of air to pass — the orifice is too small — but it continuously purges the dissolved air that comes out of solution under changing pressure conditions during normal operation.
An air/vacuum valve (also called a large orifice valve or kinetic air valve) has a large orifice — sized to allow air to enter and leave at the same rate as the pipeline fills or drains. It is designed for two specific events: rapid air escape when the pipeline fills (preventing the pressurised air slug ahead of the filling water from causing a surge), and large-volume air ingress when the pipeline empties or experiences a vacuum (preventing sub-atmospheric pressures that could collapse a thin-walled pipe). Air/vacuum valves are not designed for continuous operation under pressure; once the pipeline is full and pressurised, the float seals the orifice and the valve remains closed. Any accumulated air during operation stays trapped.
A combination air valve (also called a double orifice valve or triple function valve) combines both mechanisms in a single body — a large orifice for filling and draining events, and a small orifice for continuous air release during pressurised operation. This is the correct valve for most pipeline applications, and the one that should be specified wherever both automatic air release and protection against vacuum are required. Most engineers should default to combination air valves unless there is a specific technical reason to use a single-function valve.
How Air Gets Into a Pipeline
Understanding air entrainment sources helps with valve placement. Air enters pipelines in four ways: dissolved in the liquid (water absorbs approximately 2% of its volume in air at atmospheric pressure — as pressure reduces along the pipeline, this air comes out of solution); entrained mechanically at the pump suction or at partial-open valves where cavitation produces microbubbles; introduced during maintenance activities when pipelines are opened and then refilled; and drawn in from the surrounding medium in negative-pressure sections where pipe joints are not perfectly watertight.
Air accumulates at high points in the pipeline profile because it is less dense than water and rises to the top of the pipe cross-section, migrating along the soffit towards the highest local point. This is why air valve placement must follow the pipeline profile topography, not just the hydraulic grade line.
Sizing Air Valves Correctly
The air/vacuum orifice must be sized to pass air at the maximum pipeline fill rate without creating an air pressure that would create a fill-front water hammer event. The calculation involves determining the maximum water velocity in the pipeline during filling — typically 0.3 to 0.6 m/s for a controlled fill — then calculating the air volume that must be displaced per unit time, and selecting an orifice that passes this volume at less than 0.5 bar differential pressure.
For the vacuum break function, the orifice must allow air in fast enough that sub-atmospheric pressure does not develop. For a sudden pump trip on a rising main, the pipeline may drain at rates equivalent to full flow velocity in reverse — the air inlet must match this rate. Manufacturers publish flow coefficients (Cv values) for each orifice size and differential pressure; apply the appropriate gas flow equation to confirm adequacy.
The small-orifice air release sizing is less critical — any orifice from 1.5mm upward will release accumulated air over time. The criterion is that the valve should release air faster than it accumulates under normal operating conditions, which is almost always satisfied by even a 2mm orifice in a combination valve. Where dissolved air release is a major concern, a larger small-orifice (6–10mm) combination valve is appropriate.
Placement Along the Pipeline Profile
Air valves must be placed at every high point in the pipeline profile — without exception. High points are the locations where air naturally collects, and an air valve at any other location serves no purpose. If the pipeline has a hump profile with five high points, it needs five air valves. Specifying two air valves for a ten-kilometre main with six changes in grade to save cost is a false economy that will result in chronic pump efficiency losses and potential water hammer damage.
Additionally, air valves should be placed at: the discharge of every pump, where air entrainment from the pump suction is highest; downstream of any flow meter, to purge air that would cause over-registration; at the upstream and downstream ends of any section that experiences vacuum during draining; and at long flat runs where dissolved air release over distance can accumulate to a significant pocket size even without a topographic high point.
Materials and Standards
For potable water service, air valves require WRAS approval confirming that all wetted materials are safe for drinking water contact. Ductile iron bodies with epoxy lining are standard for waterworks applications; stainless steel bodies are used in chemical service. Float materials must be compatible with the system fluid — PVC floats for water service, stainless or PTFE floats for chemical applications.
UK and European specifications reference BS EN 1074-4 (valves for water supply — air valves) and BS EN 14450 for sewerage air valves. ISO 2533 provides the atmospheric pressure standard referenced in sizing calculations. AWWA C512 covers air release, air/vacuum and combination air valves for waterworks service in North America.
Conclusion
Air valves are simple devices that perform a critical function. Specifying the correct type — combination air valve in most cases — sizing the orifices correctly for the filling and draining conditions, and placing them at every pipeline high point are the three essentials. Get these right and air in the pipeline becomes a non-problem. Get them wrong and you face years of chronic air binding, unexplained pump efficiency losses, and the occasional dramatic vacuum collapse event that was entirely preventable.