Equipment for Sound Leak Detection

The importance of leak detection can never be overemphasized.  All persons included in this detection have to adopt the methodological approach by learning how to identify different sounds.

Sophisticated and high quality instruments cannot themselves give a response to all problems.

 Sound Detectors - Geophones (Aquaphone)

 Device Preparation for Use

Before starting to operate the geophone, it is necessary to check whether the batteries are recharged enough and whether all accessories (probes, cables, headphones) are in the box with the device.  In addition to that, the operator has to take with him a form in which he shall record daily detection data (form sample attached to document “Procedures for Efficient Data Collection on Measurement and Water Leak Detection with Practical Example in the PZ Morinj – Kotor” – Sachsen Wasser – June 2011), a pen and a hand-held GPS device.

Geophone Operating Principle

The primary problem of water leakage detection by sounding is the sound absorption in the soil.  Depending on the type of soil, sound absorption in the soil ranges around 40 dB/m of soil.  Clayey soils represent bad and sandy soils good medium for sound transmission.  Higher frequencies (including also the frequency produced by the water leaking through the openings and cracks) are unfortunately being more absorbed in the soil than lower frequencies.  

The aforementioned shows that the leak on a 1 m depth can be easily located, while the increase of depth makes locating increasingly more difficult that the depth of 2 m represents the upper limit to which the leakage detection can be performed by sounding.

Successful defect location by sounding shall highly depend on pipe material and profile, type of soil and its compaction, external sounds made by the wind, traffic, construction works, machinery and alike, and it is also necessary that the pipe pressure be minimum 1,5 bars or higher. 

Considering the fact that the pipes in terrains which are the subject of this document are located on deep depths and that the terrain is mostly karst which is generally a good sound conductor and the geophones are perfect devices for water leak detection.

The main parts of Geophone are:

1.Transducer or transmitter with frequency range between 50 and 2000 Hz, sometimes even higher,

2.Contact basis for the transducer (panel, stick, magnet or alike),

3.Amplifier with or without the filter for the reduction of external sound penetration and

4.Headphones which in the recent times have been exclusively stereo

During operation, geophone’s sensitive ground microphone absorbs vibrations caused by water outflow from the pipe, and these frequencies usually range between 100 and 800 Hz.

These vibrations mostly result from turbulence in the liquid during its flow through the pipe crack.  However, it is often the case that the sounds of lower frequencies appear resulting from the movement of the particles in the surrounding soil caused by liquid leak from the pipe.

The sound of water leak under high pressure is similar to deafening noise and can entail occasional clanging and bumping sounds.

Signal registered by the microphone enters the instrument through the sound amplifier which is, just like frequency selection filter, designed to recognize frequencies resulting from leakages and minimize the frequencies caused by steps, traffic and other types of surrounding sounds.  Nevertheless, surrounding sounds are sometimes dominant to such an extent that detection with Aquaphone (Geophone) can be performed only during the night or when it is known that the impact of surrounding sounds shall be minimal.

If certain sounds could be describe in words, then the following could be said if we heard:

1.“whistle” - it is usually a small defect at “good” pressure,

2.“hissing” – it is usually a major defect at “good” pressure,

3.“heads” – the defect is close,

4.Occasional sound represents customer’s consumption,

5.“buzzing”, “screaming” and “bell ringing” are sounds usually produced by different transformers, engines and gas installation,

6.“clicking”  is the sound produced by the water meter.

Control Detection –Prelocation with Geophone

For the pre-location of leaks, Geophones have a special test rod microphone with wide frequency range which, depending on the model, can be between 500 and 3000 Hz.  Pre-location of cracks is performed by sounding at accessible points such as hydrants and valves.  The closer we get to the leak point, the stronger the sound is, and the more we move away from the leak point, the weaker the sound is.  The leak point itself is located where the sound is the strongest.  When pre-location on metal pipes with Geophone and its listening stick is performed, frequency range to cover should be between 500 and 3000 Hz.  In case of other pipes (PVC, PE, PEHD and AC), sound frequency range is between 100 and 700 Hz.

Sound intensity around the valve, hydrant or alike is increasing because the flow resistance in the pipe at these points is higher and comes from the pressure increase inside the pipe which causes vibrations which should be taken into consideration during control detection.

Depending on the pipe material, pipe sounding is performed in the following way:

1.To perform sounding on cast iron, ductile and steel pipes each 120 to 180 m,

2.On AC pipes at each 60 to 80 m maximum,

3.To perform sounding on 150 to 200 mm PVC and PEHD pipes each 30 m and

4.To perform sounding on 250 mm and bigger profile PVC and PEHD pipes each 10 to 15 m.

If during sounding no sound was identified, than it means that at that section of the pipeline there is no defects.

In this manner, control of several km of the pipeline can be performed per day.  

Pinpoint Leak Location

Another way includes precise damage location and it is performed only after the control of the pipeline.  A ground microphone is installed on the geophone.  Precise location is performed between two loudest points detected during pipe control.  Sounding is performed on the ground directly above the pipeline, and in case the pipe is located below a soft terrain, it is necessary to lay down the basis for the microphone.  The point where the sound is the loudest is the point under which the leak is located.

Preparation of the Correlator for Detection

The correlator and each probe run on batteries and before their use it is necessary to recharge them.

Check the accuracy of the device in the following manner:

1.Mount the antennae on the device and the transmitter(s),

2.Mount cables with magnetic microphones on the transmitter(s),

3.Set the transmitters at the longest distance from each other on a steel object such as a radiator, some kind of handle or alike (do not forget to remove the protection from the magnet on the microphone)

4.Switch on the transmitters on the ON/OFF button,

5.Place a correlator on a convenient point somewhere in the middle of the room and switch it on by pressing the ON/OFF button,

6.Take the correlator headset (geophone headset can also be used) and connect it to each of theIf the transmitter is connected to a pipe, the whistling sound should be heard,

7.Connect the headset to the correlator, and on each piece a sound from one transmitter should be heard.

If these sounds can be heard, than it means that the correlator is functioning and is ready to be used.

 Site Work and Customization of the Correlator for Operation

In order to operate the correlator it is necessary to have good knowledge of the network, i.e. it is necessary to possess updated maps of the system with all connections.

If all the aforementioned data is known, probes are placed at the distance of 150 m the most if the pipes are metal, and not over 50 m if the pipes are plastic by bringing the microphone of the probe into contact with the pipeline through existing hydrants or valves, and basic data on pipeline is being entered into the processor.

In all correlators, regardless of their manufacturer, year of production or model, the same following data is being entered:

1.Pipe material,

2.Length of the pipe which is subject to detection. For the measurement of the length it is best to use the measurement wheel. During the measurement at the connection, vertical length of the pipe can beIn case of hydrants, vertical lengths should be added,

3. Outer pipe diameter inIf the diameter of the pipe on which detection is performed is not proposed on the list, then the firs bigger diameter is chosen.

Both probes are activated simultaneously through the central unit and measurement of time in which the sound originating as a consequence of water outflow from the pipe shall be registered by each probe. 

Based on the correlation of these two data, the device detects the location of the defect on the pipeline in the following manner:

D         – represents known distance between the two probes

L          – represents the distance of the between the probe closer to the defect and the defect      itself,

N         – represents the difference in distance between the probe which is more distant from the defect and the probe closer to the defect and the je.

From the aforementioned and the figure the following can be seen:

D = N + L + L

For closer  the following is obtained:

:

L = (D-N)/2

And for a more distant (blue) transducer:

N + L = D - L

The difference in the distance of the two probes from the defect can be represented as a multiplication of the speed of the sound signal (V) and the difference in time (T_d):

N = V x T_d

and when this formula is integrated in the above one, the result is that the red transducer is closer:

L= ((D - (V x T_d))/2

which represents the formula based on which the processor shall process data obtained by two probes.

Correlator is a very expensive device and is highly efficient provided that it is being operated by an adequately trained person with specific experience.

The depth on which the pipeline is located is of no importance for the correlator, and the external sounds are not disturbing it.  Therefore, it can locate several damages on one section at the same time.  

As a result of loss measurements with the correlator, on a central unit we receive the distance between the probes and the damage location, as well as a graphical representation based on which an assessment of the type of the damage can be made.

During measurements with the correlator, each useful water consumption shall be represented as leakage, and therefore, it is essential to be familiarized with the location of each connection, and at the same time it is possible to detect illegal connections or damages on the pipeline which do not necessary always have to be in the form of a pipe break, i.e. an external object such as a stone can be in the pipeline and the correlator can present it as a damage.  Bearing in mind that the stone should not be in the pipeline, such a phenomenon can be regarded as damage.

In the graphical representation, pipeline damage looks like a sharp peak.

In order to be able to detect the damage location with more certainty, the time on the correlator should be set to be as long as possible (3 – 5 min)

in order to avoid mistakes incurring due to not knowing whether the recorded peak is the result of a permanent or occasional leak.

Based on the results obtained during measurements with the correlator, the measurement of the distance between the two probes and additional control at the doubtful micro location with geophone are performed. If the damage cannot be confirmed with the geophone, than before starting the excavations, the damage should be confirmed with at least three analyses with the correlator during three days, but each time during the different time of the day.

Correlators are being produced by different manufacturers, but the operation principle remains always the same.