Operator Guidance - FCS BMIC Operator's Manual

Bmic Operator Guidance:
Background information for effective acoustic leak
detection
All acoustic leak detection methods are based on the premise that normal water passage
through pipelines takes place noiselessly. When the water passage is disturbed a noise is
created. Causes can include partial pipe blockages, sudden changes in pipe diameter,
abrupt changes in pipe direction, pumps or meters installed in the pipeline, consumer
usage or pipeline damage. Pipeline damage can include holes, cracks or splits, complete
pipeline rupture, leaking joints or leak-ing valves.
Careful application of leak detection techniques will enable the operator to eliminate
detected noises generated by poor pipeline design or consumer usage and to identify
leakage due to pipe system damage.
The strength and clarity of noise generated by leaks will be affected by the water
pressure, the size and shape of the orifice allowing leakage, the type of ground material
around the pipeline, the type of ground cover over the pipe, the diameter, wall thick-
ness and material of the pipeline and the quantity of water leaking.
A small orifice or hole and high water pressure generally produce a higher frequency
noise. Often the noise level increases around valves, pipe elbows, T connections, pipe
ends etc. since the partial obstruction increases pressure and creates some further
disturbance in the water path.
Leak noise is transmitted along the pipeline both through the water and the pipe wall as
well as into the ground around the pipe. The noise travels much better through "hard"
materials so that the noise travels much further along metallic pipes than asbestos
cement pipes which themselves are better than plastic pipes. Ground material generally
provides a poorer travel path than the pipeline itself. Soft sandy ground provides a worse
travel path than well compacted ground with a hard paved surface covering.
The leak noise can change in strength and pitch as it travels along the pipe or through the
ground. The deeper the pipe is buried and the softer the ground the more the noise will be
dampened. The harder the pipe material, the higher the frequencies detected will be, and
the further from the leak the noise will be detected, this makes it harder to accurately find
the exact point of maximum noise.
Background noise can interfere with leak detection. Traffic and machinery noises can
travel for considerable distances through both air and ground material and often occur in
the same frequency bands as leak noise. Sometimes it is necessary to use leak detection
techniques at night when interfering noises are less.
It is very important to adopt a methodical approach when using any instrument for
acoustic leak detection.
It is necessary to practice good technique in order to distinguish between different sounds,
recognising background or interfering noises, so that they can be eliminated. It is also
essential that other (non-leakage) noise sources such as consumer draw-off or partially
closed valves are eliminated by logical site inspection practices prior to any excavation
taking place.
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