An air valve releases air that collects at the high points of a water pipe, because a trapped air pocket blocks flow and causes water hammer. The simple version is a tee with a short vertical pipe and a cap, vented by hand; an open stub only works where the pressure at the crest is nearly zero, since it must rise above the water level, and at 60 m of head the cap on a 25 mm stub is pushed off with about 290 N.
Part of Water Distribution
An air valve or air vent is a fitting at the summit of a pipe run that lets trapped air escape. Air is lighter than water, so it rises to every high point and collects there.
Air Rises To The Crest And Squeezes The Flow
A pocket of air at a crest takes up part of the pipe’s cross-section, so the water has to squeeze past it. Several pockets can block a line entirely, an airlock. The water-distribution page says to install an air release valve at every high point, “or just a tee with a short vertical pipe and cap”, and lists “no air release at high points” as a cause of airlocks. Dissolved air comes out of the water where pressure falls, and pressure is lowest at the crests, so a line that was bled when filled can collect air again.
An Open Stub Only Works When The Pipe Is Almost At Atmospheric Pressure
A capped stub is a manual vent: air gathers in the stub and does not escape until someone opens the cap. A stub left open releases air by itself, but the water in the pipe then rises up it as far as the pressure at the crest allows.
Take a summit 12 m below the tank’s water level, with 3 m of head lost to friction before it while water is flowing. The pressure head there is 12 − 3 = 9 m (0.9 bar), so water would rise 9 m up an open stub, and 12 m once the taps are shut and the friction loss disappears. A 1 m stub would simply spill water.
So an open stub suits only a crest where the pressure head is below the stub height, which in practice means a pipe at almost atmospheric pressure. Anywhere else use a capped stub and vent by hand, or a float-operated air valve, which opens to let air out and closes when water reaches it.
What You Need
- A tee at each crest, with a short vertical stub of the same material
- A cap or a threaded plug that can be removed safely
- A way to find the crest: the survey
- Fine insect mesh if the stub is ever left open, so nothing enters and contaminates the pipe
Method: Find The Crest, Fit The Stub, Vent Slowly
Step 1: Mark every high point on the plan, including small local rises.
Step 2: Fit the tee and capped stub at each.
Step 3: When filling the line, fill slowly from the low end or the top tank and open each cap until water, not air, comes out.
Step 4: Re-vent after any repair, and now and then when flow drops.
Step 5: Keep the pipe below the hydraulic grade line where possible. A summit above that line has less than atmospheric pressure inside, and draws air in through any weak joint.
Worked Example
The force on a cap is pressure times area. A 25 mm stub has an area of π × 0.0125² = 0.000491 m².
- At 9 m of head: pressure = 1,000 × 9.81 × 9 = 88,300 Pa; force = 88,300 × 0.000491 = 43 N
- At 60 m of head: pressure = 1,000 × 9.81 × 60 = 588,600 Pa; force = 588,600 × 0.000491 = 289 N
289 N is the weight of about 29 kg pushing the cap off at the top of a steep line.
Common Mistakes
- No stub at a crest. The pocket sits there and the flow falls.
- Opening a cap at full pressure all at once. It sprays or blows off.
- A stub left open on a pressurised pipe. It floods.
- Forgetting small crests. Every rise, however small, collects air.
Safety
A cap at the top of a steep line is under real force
Open it a quarter turn at a time, with your face away from the opening. Wet fittings can slip. At 60 m of head the cap is pushed with about 290 N, and a pressurised water jet can injure the eyes. An open stub or air valve is also a hole in the pipe: if a drinking-water line passes through ground that can flood, water can flow back in through it. Fit it above flood level.
What’s Next
- Learn next: Pressure Zones — where crests and zone boundaries meet
- Combine with: Drain Points — the matching fitting at the low points