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Acoustic Emission Technique the optimum solution for leakage detection and location on water pipelines Marco Fantozzi ASM Brescia S.p.A., Via Lamarmora 230, 25124 Brescia, Italy. Emilio Fontana consultant Via Buschi 3, 20100 Milano, Italy. Contact

ABSTRACT

Leaks in water pipelines cause unnecessary waste of limited resources, thus the necessity of leakage prevention and detection.
The experience of water distribution companies shows that the reduction of leakage and the preservation of a low leakage level can be achieved with a strategy that requires a loss analysis followed by leak detection and location survey.
Effective techniques of leak detection by acoustic emission have been developed and tested and this paper describes the experience and results obtained with the application of these techniques in the last fifteen years in several water systems including but not limited to those managed by ASM BRESCIA S.p.A. in Italy.

ASM introduction

Since 1908, ASM is the Municipal Services Board of Brescia, which is a town of 200,000 inhabitants situated in the North of Italy.
ASM, which is largely owned by the Municipality of Brescia, is in charge of several services, the main of which being: production and distribution of electricity, district heating, street lighting and traffic lights, distribution of natural gas, collection, treatment and distribution of drinking water, sewage treatment, urban transport, parking management, telematic services, collection and disposal of urban solid waste (including separate waste collection, landfill management and incineration of the rest with combined production of energy and heat).

Active approach to leak detection

The water systems managed by ASM, whose extension is around 2,200 km are constructed mainly of ductile iron and cast iron pipes. Over 120 boreholes and 30 spring sources supply the networks delivering to users a total of 47 million mc a year.
Since 1988, ASM BRESCIA S.p.A. has been engaged in an active programme of leakage reduction.
Various methods of leakage monitoring and detection have been employed by ASM. They include:
• District metering technique and step testing (using quadrina insertion flow meters and data loggers)
• Leak detection and location using leak noise correlators
• Area surveys using acoustic loggers (Aqualogs)
• Analysis of the results by the Company's Maintenance Database
ASM's commitment to leakage reduction is demonstrated by the reduced level of leakage achieved in many of the managed water networks.

In order to reduce pumping and hence operating costs to a minimum, an intensive leakage location program was initiated in the late 1980's, with the aim of systematically checking the network at regular intervals using correlators. As successful as this approach proved to be in locating the leaks, it provided no guarantee that the reduction in leakage would be sustained. What was needed was a permanent leakage control system.

District metering technique

ASM decided to divide the network into a number of small zones called districts that has proved by experience in different parts of the world, to be the most efficient method of controlling leakage. Then, permanently closing the boundary valves and installing flow meters on the few supplying mains can continuously monitor the level of leakage. If an increase is registered in the night consumption, a team is sent in to locate the leaks. In this way, leakage is under permanent control, but intervention occurs only at the optimum moment.

Leak detection and location

The modern leak noise correlator is now the most effective and widely used system for leak detection and location.
For this reason the leak inspection on water pipelines using the cross-correlation method were standardised in 1991 by a work group of the CNR (Italian National Research Council).
The code of practice highlights those elements necessary for carrying out the leak detection survey in order to improve the quality and standardize the activity. This document can be used by the Water Distribution Companies as well as by Service Companies as a useful reference.

Object and target
The method is based on the analysis of casual continuous acoustic noise generated by the water escaping from the pipeline and carried out by using the cross-correlation technique.
The target is to standardize the method of leak detection by using the cross-correlation technique in order to detect and locate the leaks themselves yet not their amount.
The method of testing requires the use of sensing devices placed on existing pipelines fittings as well as conditioning, acquisition and signal analysis instrumentation in order to detect and locate the leaks.
The method described applies to the control of underground supply and distribution water pipelines of steel, ductile iron, cast iron, asbestos cement, polyethylene and PVC.
Cast iron, steel or asbestos cement pipe sections of a maximum length of 250 meters can be controlled by using non-intrusive sensing devices (accelerometers) and up to 600 meters by intrusive sensing devices (hydrophones).
The maximum controllable length of plastic pipes such as PVC or (high and low density) polyethylene is 50 metres only, when accelerometers are used, and 120 meters when hydrophones are used.

Method of testing
The method of testing requires the use of non intrusive sensing devices (accelerometers) or intrusive devices (hydrophones) placed on existing pipeline fittings as well as conditioning, acquisition and signal analysis instrumentation in order to detect and locate leaks.

The location of the leaking point in the pipe is obtained knowing: the distance between the sensors that span the leak, the propagation velocity of the leak sound in the pipeline and the time delay, measured by the cross-correlation function (see figure 2), that the leak sound takes to reach the two sensors.

Fig 1: Method of leak location



• X = distance of the point of leak from the reference sensing device;
  
• D = distance between the two sensing devices;
  
• V = propagation wave speed;
  
• Dt = time delay obtained from the peak position of the cross-correlation function.

The figure 1 shows how the position of the leaking point may be obtained and the figure 2 shows an example of the cross-correlation function.

Fig 2: Cross-correlation function plot.

Fig 3: Coherence function plot.

The diagram in figure 2 shows that the position of the leak, in relation to the two sensing devices, is determined by detecting the maximum of the cross-correlation function related to the time delay of the signals.

The coherence function shown in figure 3 allows establishing the reliability rating of the measure carried out. It expresses the dependence of the signals, detected at the two measurement points A and B, from a common leak noise source. The Coherence is normally represented between zero and one, therefore, the nearer the coherence is to one the closer is the link between the two detected signals.

On site inspection results
The results obtained over a sample of 4820 km of water distribution network in different Italian cities that have been surveyed using the cross-correlation technique in the last ten years are now outlined.
During the systematic survey concerning the above mentioned networks - about half consisting of cast and ductile iron pipes and the other half of steel pipes and asbestos cement pipes (only 33 km of plastic pipes have been inspected) - a total of 3450 water leakages have been detected.
Out of the detected leaks, 3312 (96%) have been located exactly and have undergone repair. Some of the remaining 174 leaks have been located during the repair excavation at distances greater than 3-4 meters. The location errors are essentially due to the uncertainty of the used distance between sensors.

Area surveys using acoustic loggers

In the last few years other acoustic techniques have been developed to optimise water leakage management in identifying leakage areas prior to directing leak detection operators to pinpoint the leak.
Thus have been developed systems for acoustic noise monitoring and recording that can be permanently or time limited installed at hydrants, valves or house connections. These "noise loggers" record typical noises in the network during low consumption hours at night and identify areas of potential leakage for further investigation. The ultimate advance consists in transmission of leak presence from the noise loggers to a receiver module, which may be hand carried or vehicle-mounted.

Noise Logger
This logger is installed at fittings via a simple magnetic coupling, and is battery powered with no maintenance requirement, and no problems for being immersed in water.
The separation distance between loggers depend mainly on the pipe material, with plastic pipes requiring closer spacing than metallic.
Each unit is intelligent and adapts itself to the environment. If no leak is present, a radio signal is transmitted to indicate normal background conditions. However, as soon as a leak is detected, the unit enters an alarm state and transmits a radio signal to indicate a "leak condition". Signals are received by a module that can be mounted in a patrolling vehicle, or can be easily hand-held. This receiving module analyses and "homes in" on signals to identify the location of units indicating a "leak condition", and thus the approximate position of a likely leak.

The reading of an area meter could easily include the monitoring of the loggers within it, so that new leaks are localised at exactly the same time as increases in the night flow are noticed.
This should mean a prescribed leakage level can be easily maintained, because the detection time is greatly reduced.
This innovative technology offers the possibility of continuous, permanent monitoring for leakage for the entire distribution system or just for those parts that are known problem areas.

THE FUTURE

The next step will probably be the automatic cross-correlation analysis between permanently installed loggers.
The noise logger will be enabled to correlate a leak position with an adjacent logger and transmit the exact position by interfacing through SCADA with a GIS system. This process would enhance the leakage control process significantly.

CONCLUSIONS

In the last fifteen years the use of acoustic emission techniques has shown that leaks can be accurately identified and localised much faster than with any conventional method.
These experiences in leakage detection and location have proved that the application of acoustic techniques gives Water industry the most effective tools of conserving precious water resources.
In particular, the use of cross-correlation to detect and locate the leaks on underground pipelines has gained larger and larger approval within the water industry, because it offers a more accurate location of the leak, less dependence from operator interpretation and it can be used in very noisy conditions.
The obtainable benefits due to the application of the considered technique are dependent on the care and manner in which it is applied and the results are as good as the operators strictly observe the guideline.
With the application of the "noise loggers" which record typical noises in the network during low consumption hours at night is now possible the permanent acoustic monitoring of the distribution network. This new technology will help to achieve further leakage reduction without increasing the costs for water leak detection.

REFERENCES

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