An air lock is a pocket of air trapped at a high point of a water pipe that chokes or stops the flow; prevent it by laying the line so that every crest sits below the hydraulic grade line, keeping the box outlet under water, filling the pipe slowly from the top, and fitting a vent at each crest. Clear one by opening the vent slowly at the crest, with your face away, until water comes out in place of air, because a litre of air fills about 2 m of 25 mm pipe at atmospheric pressure and still a metre or more at the pressure on a crest.
Part of Spring Capture
An air lock is a bubble of air that sits in a pipeline’s high point and restricts the water. In a spring line it is the usual reason a supply that worked on the day it was built later runs weak or stops.
Air Rises To The Crest And Stays There
Air is lighter than water and rises to the highest point in the pipe, where it collects until the pocket is big enough to restrict the flow. Air Valves covers the fitting that releases it. This article covers why a spring line collects air, how to tell, and how to prevent and clear it.
Where the air comes from.
- First filling. A new or drained pipe is full of air; filling it too fast traps pockets.
- The box outlet. If the water in the box falls to the outlet pipe, or a vortex forms over it, the pipe draws air in.
- Dissolved air. Water holds dissolved air that comes out where the pressure is lowest, which is at the crests.
- Leaks above the hydraulic grade line. Where the pipe is at lower than atmospheric pressure, any weak joint draws air in.
How Big A Pocket Is Depends On The Pressure
Air compresses with pressure (Boyle’s law: pressure times volume is constant). With absolute pressure p, the pocket volume V2 = V1 × p1 ÷ p2. Atmospheric pressure is about 101 kPa; each metre of water head adds 9.81 kPa.
Worked example: one litre of air at atmospheric pressure, in a 25 mm bore (area π ÷ 4 × 0.025² = 4.91×10⁻⁴ m²):
| Where | Pressure head | Absolute pressure | Volume | Length in the pipe |
|---|---|---|---|---|
| Open to air | 0 m | 101 kPa | 1.0 L | 1.0×10⁻³ ÷ 4.91×10⁻⁴ = 2.0 m |
| Crest with 5 m head | 5 m (49 kPa) | 150 kPa | 101 ÷ 150 = 0.67 L | 1.4 m |
| Low point with 20 m head | 20 m (196 kPa) | 297 kPa | 101 ÷ 297 = 0.34 L | 0.7 m |
The same air is a longer pocket at the crest than it would be lower down, and when it is carried to a lower or open point it grows again, which shows as spitting from a tap.
What a pocket does to the flow. A pocket filling the top half of the pipe leaves half the area for the water, so the speed doubles. Friction loss varies roughly as speed^1.852, so over the length of the pocket it rises by about 2^1.852 = 3.6 times (a crude estimate that ignores the change in shape of the flow). Several pockets, or one long one, can use up the head budget set in Pipe Sizing and stop the flow.
Keep The Crests Below The Hydraulic Grade Line
The hydraulic grade line is the level to which water would rise in a tube tapped into the pipe: the box water level minus the friction lost to that point. At a crest, the pressure head is the height of the grade line above the pipe. Positive means the pipe is under pressure and air stays as a pocket that can be vented; negative means the pipe is under suction and draws in air.
Worked example. The box water surface is at 100.0 m. A crest lies 200 m down the line, where the friction loss for 10 L/min in 25 mm pipe is 2 × 0.72 = 1.44 m (loss per 100 m from Piping from Spring). The grade line there is 100.0 − 1.44 = 98.56 m.
- Crest pipe at 96.0 m: pressure head = 98.56 − 96.0 = +2.56 m. The pipe is under pressure; fit a vent.
- Crest pipe at 99.0 m: pressure head = 98.56 − 99.0 = −0.44 m. The pipe is under suction there and will draw air in at every weak joint. Re-route, bury the pipe deeper to bring the crest down, or add a tank before the crest.
What You Need
- A route survey with heights (Water Tube Level)
- A vent stub with a cap at each crest, or a float air valve (Air Valves)
- A drain at each low point (Drain Points)
- An isolation valve at the box and the tank
- A notebook for the flow readings at the box and the tap
Method: Diagnose, Then Clear
Step 1: Compare the ends. Look at the box. If water is running out of the overflow while the tap at the end gives less than the spring does, something in the line is restricting the flow: an air lock or a blockage.
Step 2: Look for the other sign. If the box level is low and the outlet gurgles, the intake is drawing air; the trouble is at the box (Outlet & Overflow).
Step 3: Find the crests. Walk the line from the box, checking each vent in turn; the one that hisses or spits when opened holds the air.
Step 4: Vent slowly. Open the cap a quarter turn at a time with your face turned away, since a cap on a steep line is pushed with real force. Close it as soon as water, not air, comes out.
Step 5: If the crest has no vent, with the line under pressure, loosen a union or open a fitting close to the crest a little at a time, face away, to let the air out, and tighten it again as soon as water follows. Then fit a permanent vent at that crest.
Step 6: Prevent a repeat. Vent the crest, deepen the pipe there, or re-route.
Common Mistakes
- No vent at a crest. The pocket grows until the flow stops.
- A crest above the grade line. The pipe draws air in and no vent can cure that.
- Filling fast. Fast water traps pockets in descending reaches; fill slowly.
- Forgetting small crests. Every rise collects air.
- Opening a cap at full pressure all at once. It can blow off or spray.
Safety
Caps under pressure, and the force behind them
A cap on a vent at the top of a steep line is pushed with a force of pressure times area. At 60 m of head a 25 mm stub is pushed with about 290 N (Air Valves). Open it a little at a time, face away, and never test a line with compressed air.
What’s Next
- Learn next: Air Valves — the fittings that vent a crest
- Combine with: Pipe Sizing — the head budget that air eats into