An excellent design guide for Understanding Gravity Flow Water Systems
More factsheets from the same source:
Gravity-pressured flow is prone to partial or total blockage by trapped air pockets.
These can be erratic; several hours or days of system use before blockage occurs.
Source of Air. Air may enter a gravity-pressured pipe by:
• first operation of a pipeline after winter draining – the pipe will be full of air and
how easily it can escape will determine the ease of getting a flow of water started
• entry at the inlet of the pipe – ensure inlet is submersed sufficiently to prevent a
vortex from forming that will draw in air
• entry along the pipe – high points in an undulating pipeline where the water
pressure is low enough to draw in air through loose fittings or small holes
• dissolved air coming out of solution due to temperature increase (e.g., black PE
pipe laid on the ground) or to pressure decrease (see negative pressures, above)
• entry into the pipe at a stock trough that is installed as a flow-through trough
– these troughs are often used as a “pressure break” in a gravity feed system but
can be points of air entry into the line exiting the trough (as at the main inlet)
How Air Locks Form. Air locks can form either during static conditions (no
water flow) or dynamic conditions (water flow). Air that enters the pipeline when
water is flowing must be carried downstream to an outlet to ensure continued water
flow. If the system is static, the air must be able to rise and exit at the water inlet
point (which, except for siphons, is the highest point in a gravity feed system). If the
air becomes trapped in any high point along the pipe an air lock forms.
Total Air Locks. A total air lock can form in a pipe which will completely block
the flow of water. Figure 3, below, illustrates a simple, single undulation pipe
example where a gravity pipe supplies a float-controlled trough.
When the trough float valve is closed, any air in the pipe that can’t escape up and out
the water supply pipe end can accumulate in the pipe undulation. In this static
condition, the pressures at A and B are the same and equal to the height of water HW.
In the dynamic condition, the trough float valve is opened, and the trapped air bubble
will move down the pipe. Because air has negligible weight compared with water, the
pressures at A1 and B1 are the same and are equal to the height of water H. Therefore
A1 will support the height of water H behind it and B1 will support the same height of
water H in front of it. If the outlet is higher than H no water will flow.
Solutions, on the next pages, include having a flow rate sufficient to move the air
with the water, or using an air release valve at the high point C to automatically let
out air before it accumulates to form an air lock (see air release valve picture page 9).
Breaking Line Pressure & Releasing Air.
Where appropriate, a trough can be used to release air, but the line pressure is reduced to zero, returning to atmospheric pressure. This is sometimes used in gravity systems with large elevation falls creating excess line pressure and where air release is required.