Manual Aqua M300D
page 31
16. Information for Use in the Field
16.1. Acoustic Leak Detection
16.1.1. How Sound is Created
When there is a leak in a high-pressure pipe system, water that leaks
out generates sound that is caused by the friction that occurs when the
water escapes through the hole at high speed. This also leads to
oscillations in the pipe itself. The sound travels along the pipe and can
be picked up at contact points such as valves, hydrants, fittings, etc.
which can actually be quite far from the actual leak itself and can be
transformed into audible sound by structure-borne sound microphones.
16.1.1.1. Ground-borne Sound
When water that is escaping from a leak comes into contact with the
ground, this causes the parts of the ground that it comes in contact with
to oscillate. The sound spreads out from the leak in circles and can be
picked up by a ground microphone near the actual leak. The frequency
of these signals lies between 30 and 700Hz.
Frequencies whose wave lengths are smaller than the depth that the
pipes have been laid in are strongly absorbed because of the low-pass
effect of the surrounding soil. This means that only the low frequencies
normally reach the surface of the ground.
If the water has been escaping from the leak for a relatively long period
of time, a water blister sometimes forms and continues to contain the
water that is leaking from the pipe. The sound that is transmitted through
the ground is therefore barely audible and is almost impossible to pick
up with a ground microphone.
The sound caused by the leak can be made audible again if
compressed air can be fed into the pipe system that is being examined.
In such a case, the compressed air is fed into the pipe system with a
minimal amount of overpressure via a hydrant or house connecting
point. This combination of water and air generates a sound at the leak
which is clearly audible.
The diagram below shows the influence of the ground formation on the
distance that the ground-borne sound waves can travel with reference to
the leak frequency in metres. Low-frequency sounds spread out further
than high-frequency sounds, and compact ground conducts the sound to
the surface better than ground which is not as compact.
16.1.1.2. Structure-borne Sound
Structure-borne sound oscillations occur when water escapes from a
leak with high pressure and at a correspondingly high speed which
causes the pipe in question to oscillate.
The sound that is generated at the point where the water is escaping
spreads out to all sides of the pipe. Small-diameter or thin steel pipes
will oscillate strongly and the sound the leak is causing can be picked
up at a considerable distance from the leak itself. Thick pipes or
especially pipes made of plastic material, on the other hand, do not
oscillate as strongly and the sound generated by the leak does not
spread out nearly as far.
The frequency and the material of the pipes play a major role as far
as the distance the sound travels is concerned. As is the case with
ground-borne sound, low-frequency sounds travel further distances,
and softer materials like PVC or PE pipes absorb the energy caused
by the leak more strongly than metallic pipes.
16.1.1,3 Current-induced Sound
Current-borne sound is generated at narrow points or bottlenecks, for
example at valves that are only half or partly open, at household
connecting points where pipes can have different diameters or
dimensions or when corrosion has formed and pipes are crusted in rust
on the inside. These factors can cause turbulent currents which can
generate frequencies of over 4,000Hz.
16.1.1.4 Interference Factors
Sounds from surrounding sources that have been absorbed and filtered
by the ground have a frequency spectrum similar to the sound that a
leak generates. The interference this causes can be likened to stop-and-
go traffic on the roads, but with the difference that such a traffic situation
is far more disruptive with regard to traffic-flow when it happens on a
country road than somewhere in the city centre. The higher the pressure
in the pipe on which an inspection is to be carried out, the higher the
amount of energy that forms at the leak. This means that leaks become
less audible if the pressure in the pipe is lower than 3 bar. If the
pressure is lower than 1.5 bar, a leak cannot be heard even when it is in
the close vicinity.
16.1.2. Schematic Leak Detection
In order to keep costs down, it often makes sense to adopt a
systematic approach to carrying out leak detection. This is especially
true when water pipes are concerned, for example. The first thing you
need to know is the course of the pipe. You will also need to
differentiate clearly between the
preliminary leak detection stage
and
the
stage which involves pinpointing the actual leak
. If this first stage is
not performed, the whole length of the pipe will have to be inspected in
order to determine the exact location of the leak.
16.1.2.1 Narrowing Down the Leak by Using a Stick Microphone
You can narrow down the position of a leak by inspecting the parts of
the pipe system which you are able to access with the probe tip of the
stick microphone. Particular attention must be paid to the type of sound
which is recorded: leaks generate a dull, muffled sound and valves
produce a brighter, sharper sound. Both sounds are very helpful when it
comes to narrowing down the position of a leak, but it is important to
remember that similar sounds – like that of water flowing through a pipe
– can be generated when water is taken from the pipe via a tap for
example. When narrowing down the leak, it is important to ensure that
no value exceeds the range on the display, as otherwise you will not be
able to identify the actual maximum value. The fact that the measuring
values can be saved into the internal memory is an added benefit, as
they can then be “carried over“ to the place where measuring is to be
carried out next. This method allows you to determine the particular pipe
section with the highest sound level without changing the control
settings. The next leak detection stage is then to be carried out on this
particular pipe section over ground.