Air Release Valves for Water Pipelines: Types, Placement and Sizing

Quick Answer

An air release valve is a float-operated valve that vents air from a water pipeline. Air valves perform three functions. Air release (small-orifice) valves discharge small pockets of air while the line is under pressure; the orifice is small because the float must pull it open against line pressure. Air/vacuum (large-orifice) valves exhaust large volumes of air as the pipeline fills and admit air when pressure falls below atmospheric during draining, a burst or column separation, but stay shut under pressure. Combination (double-orifice) valves do both and are the usual choice at the high points of transmission mains. Fit air valves at high points, abrupt increases in downslope and decreases in upslope, along long ascents, descents and horizontal runs, and at deep-well pump discharges ahead of the check valve. Size the large orifice for the larger of two flows: filling, where air out equals water in, at a fill velocity of no more than about 0.3 m/s (AWWA M51); and draining or a pipe break, where air in equals water out, which usually governs. Anti-slam devices throttle the outflow to limit surges. The main product standards are AWWA C512 and EN 1074-4.

A common pattern on a new transmission main: the pumps deliver less than the design flow, power use is above prediction, and the flowmeter reading jumps about. The pipe and the pumps test sound. The cause is air trapped at high points that were given no air valves, or only small air release valves that cannot vent the volume displaced during filling.

The opposite failure comes when the main is drained for a repair or a pump trips. A long downhill section falls below atmospheric pressure, and thin-walled pipe and joints meet a vacuum they were never checked for. Air valves handle both problems, if they are the right type, in the right place and at the right size.

Why Air in a Pipeline Matters

Air enters with the water and from the system itself:

• Dissolved air: AWWA M51 takes the solubility of air at standard conditions as about 2% by volume for clean water and 5% for wastewater. It comes out of solution where pressure falls or temperature rises, and collects at high points
• Filling: an empty pipeline is full of air that must leave as water enters
• Pumps and leaks: vortices at pump intakes draw air in, and air enters through leaking joints and air valves whenever the line is below atmospheric pressure

Once trapped, air pockets:

• Reduce the flow area, adding head loss and pumping energy; in the extreme, a pocket can air-bind the line
• Magnify surges: compressed air acts as a spring, and pockets released suddenly let water columns accelerate and collide
• Upset equipment: control valves act erratically, pumps wear early, and flowmeters, which assume a full pipe, misread (see our guide to process instrumentation)
• Make tapping the main unsafe

The reverse problem is vacuum. Draining, a burst or the column separation that can follow a pump trip can pull pressure below atmospheric at high points and on steep descents. Large thin-walled pipe can buckle, and leaking joints can draw in groundwater.

The Three Functions, and Two Add-Ons

How the small orifice works

While the body is full of water, the float rides up and seals the orifice. As air collects, the water level in the body falls, the float drops, and its weight, multiplied by a lever, pulls the seal off the orifice against line pressure. Air escapes, water rises and the float reseals.

The force holding the orifice shut is line pressure times orifice area. At 16 bar (232 psi), a 3 mm orifice is held shut with about 11 N; a 12 mm orifice with about 181 N, sixteen times as much, which would need a far heavier float or a much longer lever. That is why air release orifices are a few millimetres across, why makers offer smaller orifices for higher pressure ratings, and why a valve with the wrong orifice for the line pressure stays shut while air builds up beneath it.

A large orifice works the other way: line pressure on its large area holds the float shut, so an air/vacuum valve cannot vent air once the line is pressurised. It reopens only when pressure falls to atmospheric or below.

Where to Place Air Valves

AWWA M51 is the usual design reference. The locations below come from it and from pipe-industry and water-utility guidance:

Interval guidance varies: one US government design note for irrigation pipelines uses about 400 m (¼ mile), and the hydraulic transients literature suggests small-orifice valves every 500–800 m on long uniform sections. Small distribution mains often need none, because hydrants and service connections vent the air. Mount each valve as close to the main as possible, on a full-bore connection at the crown of the pipe.

Sizing

Air release valves

There is no standard method. Makers' tables start from the water flow and the maximum operating pressure; a common assumption is air equal to about 2% of the water flow, in line with M51's dissolved-air figure. Choose the orifice for the maximum operating pressure the valve must open against.

Air/vacuum and combination valves: two duties

Filling. Air out equals water in. M51's rule of thumb is a fill velocity of no more than about 0.3 m/s (1 ft/s) across the full pipe section, and valves are commonly sized to exhaust that flow at a differential of about 0.14 bar (2 psi).

Draining and pipe break. Air in equals water out. M51 gives a maximum draining velocity of 0.3–0.6 m/s for planned draining. The worst case is a full-bore break at the foot of the steepest slope next to the valve, where the water drains by gravity. M51 estimates that flow with a Chezy-type formula; in SI units, Q (m³/s) = 0.217 × C × √(S × D⁵), with D the bore in metres, S the slope, and C about 110 for iron, 120 for concrete, 130 for steel and 190 for PVC. M51's US form is Q (cfm) = 0.0472 × C × √(S × D⁵), with D in inches. The 2016 edition of M51 also covers partial ruptures. Air inflow is commonly sized at a differential of about 0.35 bar (5 psi), or less if the pipe cannot take it.

Vacuum resistance. For a long, unsupported, round steel tube, the classic buckling formula gives the collapse pressure as Pcr = 2E ÷ (1 − ν²) × (t ÷ D)³, so it falls with the cube of the wall-to-diameter ratio; large thin-walled pipe such as spiral welded pipe needs the check most. Valve makers apply a large safety factor (one uses 4). Buried pipe in firm soil gains support from the ground. Plastic pipe such as HDPE has a much lower modulus, and its resistance falls with the duration of the vacuum, so use the pipe maker's rating.

Worked Example: A DN600 Steel Main

A DN600 steel transmission main, 610 mm OD × 6.4 mm wall (597 mm bore), crosses a ridge. The steepest slope next to the summit falls at 2%. Size the summit air valve.

1. Filling. The bore area is 0.280 m². At 0.3 m/s, water enters at 0.084 m³/s (84 l/s, about 300 m³/h), so air must leave at 0.084 m³/s. With a discharge coefficient of 0.6, the value the 2016 edition of M51 assumes, and a 14 kPa (2 psi) differential, the orifice equation gives a required orifice diameter of about 35 mm.

2. Pipe break. With C = 130: Q = 0.217 × 130 × √(0.02 × 0.597⁵) = 1.10 m³/s (about 2,300 cfm), 13 times the filling flow. Taking the air inflow, conservatively, as the same volume of free air, a compressible-flow orifice calculation at 0.35 bar (5 psi) vacuum and the same 0.6 coefficient gives a required orifice diameter of about 110 mm.

3. Can the pipe take 0.35 bar of vacuum? With E = 207 GPa and ν = 0.3, the unsupported collapse pressure of the 610 × 6.4 mm pipe is about 5.4 bar; divided by 4, it is 1.36 bar, more than full vacuum. A DN1200 main with an 8 mm wall (D/t about 150) gives 1.31 bar, or 0.33 bar after the factor of 4: less than 0.35 bar. That main would be designed to a lower vacuum with a larger air inlet, or checked as buried pipe with soil support (AWWA M11 covers steel pipe design).

Result. The break case governs. A valve sized for filling alone would have about a tenth of the orifice area needed. Check the final choice against the maker's tested outflow and inflow curves, and against a surge analysis where the line is pumped.

Slam, Surge and Anti-Slam Valves

A large orifice that vents air freely lets the water column close on the valve at speed. When the water reaches the float, the valve slams shut and the column stops abruptly: a pressure surge. Research reviews link over-sized air valves to severe secondary transients, and one laboratory study of rapid filling recorded peaks of up to 15 times the upstream head. The same happens when a separated column rejoins at a high point after a pump trip.

AWWA C512 suggests two answers:

• Throttling or slow-closing devices for surge suppression: the outflow is restricted once the differential rises, so the last of the air cushions the arriving water
• For water column separation, a vacuum breaker paired with an air release valve, or a combination valve with restricted outflow: air enters freely and leaves slowly

Pump trips and check valve slam are covered in our guide to check valves, and the operating point that sets line velocity in pump curves and system curves.

Wastewater Air Valves

Sewage force mains release more gas (M51 uses 5% solubility for wastewater) and carry solids and grease that foul a clean-water float and seat. Wastewater air valves keep the mechanism away from the liquid, typically with a taller body and larger clearances, and have connections for back-flushing. AWWA C512 brought wastewater into its title and scope in the 2015 edition, adding pressure-testing and cleanout requirements for wastewater valves. Vent the gases where odour and hydrogen sulfide will not be a nuisance or a hazard. See our guides to sewage and wastewater pumps and wastewater treatment equipment.

Standards

• AWWA C512 covers air release, air/vacuum and combination air valves for water and wastewater service. Its current edition, C512-25, which supersedes C512-15 and adds "Metallic Body and Cover" to the title, spans air release valves of 13–150 mm (½–6 in) and air/vacuum and combination valves of 13–500 mm (½–20 in), with grey iron, ductile iron, carbon steel or stainless steel bodies, design pressures up to 2,070 kPa (300 psig) and liquids from above freezing to 52 °C (125 °F)
• EN 1074-4 sets fitness-for-purpose requirements and verification tests for air valves in supply systems for water intended for human consumption, above or below ground, up to DN 300 and PFA 6 to 25 bar
• AWWA M51 is the design manual for air valve location and sizing

Installation

• An isolation valve under every air valve, full-bore and on the crown of the main, so the air valve can be serviced without draining the line
• A pressure class that matches the pipeline
• A chamber or enclosure that cannot flood: a flooded chamber lets the valve draw dirty water into the main when it admits air. Above-ground enclosures, or drained and ventilated chambers, avoid this
• An outlet turned down and screened, with frost protection where needed
• Maintenance: inspect and exercise valves regularly, and flush wastewater valves on a schedule

Mains with pressure reducing stations need the same attention to air; see our guide to pressure reducing valves.

Ordering Checklist

• Profile: chainages with ground and pipe elevations, high points and slope changes marked
• Pipe: material, DN, wall thickness or pressure class, buried or above ground
• Pressures: operating, test and surge
• Flows: filling and draining rates, maximum operating flow
• Fluid: potable, raw or wastewater, and temperature
• Each location: valve type (air release, air/vacuum, combination, air inlet, anti-slam), size, and orifice for the pressure
• Connections: flanged (PN or class) or threaded
• Materials and coating: body, float and seat; potable-water approvals where required
• Accessories: isolation valves, chambers or enclosures, back-flushing kits for wastewater
• Standards and documents: AWWA C512 or EN 1074-4 as specified, with test certificates

Common Mistakes

1. Air release valves only. Small orifices cannot vent the filling volume or admit air on draining, so filling is slow, pockets remain and the line can go to vacuum.

   Prevention: Use combination valves at high points, and keep simple air release valves for intermediate points.

2. Sizing for filling only. The break case usually needs a much larger orifice.

   Prevention: Size the large orifice for the larger of the filling flow and the draining or break flow.

3. Filling too fast. Air is trapped and released in bursts, and columns slam together at high points.

   Prevention: Fill at no more than about 0.3 m/s, with the air valves open and working.

4. Unthrottled large outlets at surge-prone points. The float slams shut on arriving water.

   Prevention: Use anti-slam devices at pump stations and column separation points, and confirm with a surge analysis.

5. The wrong orifice for the pressure. The float cannot pull the seal open, and air builds up.

   Prevention: Select the air release orifice for the maximum operating pressure.

6. No isolation valve. The air valve cannot be serviced without draining the main.

   Prevention: Fit a full-bore isolation valve under each air valve.

7. A flooded chamber. Under vacuum, the valve draws chamber water into a potable main.

   Prevention: Use above-ground enclosures or drained, ventilated chambers.

8. Clean-water valves on sewage. Grease and solids foul the float and seat, and the valve leaks or sticks.

   Prevention: Specify wastewater air valves with back-flushing connections, and flush them on a schedule.

Supply from Kasko Makine

Kasko Makine supplies air release valves for pressurised pipelines. The valve is float-operated: as air collects at a high point and enters the body, the float drops, and a lever that multiplies the float force opens the orifice even under full internal pressure; when water refills the body, the float rises and the valve closes. The product page also lists admission of air when internal pressure drops below atmospheric, protection against vacuum, and reduction of water hammer and pressure surges. A small orifice admits only limited air, so high points that can go to vacuum during draining or a pipe break need an air/vacuum or combination valve; confirm whether the valve you are quoted is single- or double-orifice. The valve is installed at the highest points of pressurised pipelines, for water supply and distribution networks, irrigation systems, pumping stations, industrial process pipelines, and wastewater and treatment facilities.

Engineering support: Send the pipeline profile, pipe material and size, operating pressure, and filling and draining rates. We will review valve types, locations and sizes against your data, and quote the air valves and isolation valves together.

Send your pipeline profile, pipe data and pressures for an air valve layout and quotation — we reply within 24 hours. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading

• Pressure Reducing Valves: Types, Sizing, Cavitation and Installation
• Check Valves: Types, Selection and Water Hammer Prevention
• Pump Curves and System Curves: How to Find the Operating Point
• HDPE Pipes: Properties, Grades, Specifications and Water Infrastructure Applications
• Sewage and Wastewater Pumps: Types, Impellers and Selection Guide
• Spiral Welded Pipe (SSAW): How It Is Made, Standards and Where It Fits

Frequently Asked Questions

Q: What does an air release valve do?
A: An air release valve vents the small pockets of air that collect at the high points of a pressurised water pipeline. A float holds its small orifice shut while the body is full of water; when air collects, the float drops and a lever pulls the seal open against line pressure. Removing the air restores flow area, cuts pumping energy and reduces the risk of surges and meter errors.

Q: What is the difference between an air release valve and an air/vacuum valve?
A: An air release valve has a small orifice and vents small amounts of air while the pipeline is under pressure. An air/vacuum valve has a large orifice: it exhausts large volumes of air while the pipeline fills and admits air when pressure falls below atmospheric, but stays shut once the line is pressurised. A combination valve performs both functions and is the usual choice at high points.

Q: Where should air valves be installed on a pipeline?
A: Install them at high points, at abrupt increases in downslope and abrupt decreases in upslope, along long ascents, descents and horizontal runs, and at the start and end of long horizontal sections. Fit them at deep-well and vertical turbine pump discharges, ahead of the check valve. Interval guidance on long uniform runs varies from about 400 m to 800 m; follow AWWA M51 and the design profile.

Q: How do you size an air/vacuum valve?
A: Size the large orifice for the larger of two flows. For filling, air out equals water in, typically at a fill velocity of no more than 0.3 m/s and a differential of about 0.14 bar. For draining or a pipe break, air in equals water out; gravity flow down the steepest adjacent slope usually governs, at about 0.35 bar of vacuum, or less if the pipe cannot take it.

Q: Why can air valves cause water hammer?
A: A large orifice that vents air freely lets the water column reach the valve at speed. When the float slams shut, the column stops abruptly and a pressure surge follows; the same happens when a separated column rejoins after a pump trip. Anti-slam throttling devices restrict the outflow as the differential rises, and vacuum breakers paired with air release valves let air in freely but out slowly.

Q: Are wastewater air valves different from clean-water air valves?
A: Yes. Sewage releases more gas and carries solids and grease that foul a clean-water float and seat. Wastewater air valves keep the mechanism away from the liquid, typically with taller bodies and larger clearances, and include back-flushing connections. AWWA C512 added wastewater service, with pressure-testing and cleanout requirements, in its 2015 edition. Vent the gases where odour and hydrogen sulfide are not a hazard.

Q: Which standards cover air valves for water pipelines?
A: AWWA C512 covers air release, air/vacuum and combination air valves for water and wastewater. Its current 2025 edition spans air release valves of 13 to 150 mm and air/vacuum and combination valves up to 500 mm, at design pressures up to 2,070 kPa (300 psig). EN 1074-4 sets European fitness-for-purpose requirements for drinking-water air valves up to DN 300. AWWA Manual M51 covers location and sizing.