Marco referencial

The following techniques can be used to indicate the general location of the leak, or to pinpoint it prior to excavation for repair. These techniques are either limited to, or are most effective, on pipes less than 150 mm in diameter.

Pin-pointing currently involves work teams inspecting the length of the pipe and recording where any suspected leak locations are for repair. Newer technologies such as correlating data loggers can cut down the number of teams needed to pin-point a leak thanks to the ability to monitor the pipes for a prolonged period of time and communicate automatically.

Acoustic and electronic listening sticks

One of the most basic and widespread techniques used in the water industry to locate and pinpoint leaks is listening for the noise produced by the leak. The most basic tool is the listening stick which consists of an ear piece mounted onto a metal or wooden rod. One end of the rod is placed on the ground or on part of the water pipework and the operator listens to the other end. The rod transmits the sound to the operator‟s ear. More expensive battery powered electronic devices are also available. These use a microphone, amplifier and headphones to magnify the amplitude of the signal to increase sensitivity (Hunaidi, Chu, Wang, & Guan, 2000). These tools require a degree of experience from the operator to determine whether a noise is due to a leak or to some other cause. Some newer systems do not require the human ear, but will automatically sense the presence of a leak at the push of a button.

Photograph 5.2 Acoustic listening sticks

Source: (Charalambous, B, 2010)

Listing sticks are inexpensive and widely available but they require experience to use and are vulnerable to interference. One of their greatest advantages is that they can be used through the ground without requiring direct access to the water pipework. For larger leaks under high pressure they are highly cost effective but quieter leaks are harder to detect over background noise (Hunaidi, Chu, Wang, & Guan, 2000).

will be carried along the pipe better than through the ground. The noise level at adjacent fittings can be compared to give an indication of the leak location relative to the fittings.

Electronic listening sticks have the theoretical advantage of being able to give a clearer signal. However, they are less robust and more expensive than traditional listening sticks and may face opposition amongst more conservative members of a leak detection team.

Ground microphones are a more substantial form of electronic listening stick specifically designed to detect leak noise through the ground; they can include a display of the amplitude and frequency of the leakage noise. The operator traces the pipe and records the amplitude of noise as they progress. The position where the noise is loudest is likely to correspond to the actual leak position. Like all noise based detection systems, ground microphones are less effective on soft ground (Dray, Loveday, Tod, & Tooms, 2010).

Photograph 5.3 Ground microphone in operation

Source: (Sewerin, 2014)

Leak Noise Correlator

The most popular device for leak location in recent years is the leak noise correlator, which is used to detect the position of a leak between two sensors placed on a length of pipe. A schematic of this type of device in use is shown in Figure 5.1. A sensor is placed at each end of the pipe section, usually at existing fittings such as hydrants or valves. The sensors are connected to a signal processing unit that takes the signal from each sensor, amplifies it and then compares the signal shape detected at each end. The principle is to look for the time delay of the noise from the leak reaching each sensor – the closer the leak is to the sensor the earlier the noise arrives in comparison to the other sensor. Knowing the length and material of the pipe and the speed of sound in that size and material of pipe, the approximate

location of the leak relative to the two ends can be determined. Typically, a listening stick is then used to confirm the exact location of the leak from above ground. The poor transmission of sound along plastic pipes makes the use of correlators on plastic less successful. Recent developments in correlator technology have improved the lengths of plastic pipe over which leaks can be located.

Figure 5.1 Schematic of leak noise correlator

Source: (Pipefix, 2014)

Photograph 5.4 Leak noise correlator

Source: (HMW, 2011)

Two different types of sensor can be used with the leak noise correlator – accelerometers or hydrophones. Accelerometer type sensors are designed to detect the vibration induced into the pipe walls by a leak. They have the advantage that they can be attached to the outside of

metallic pipes between 50 and 300 mm. Larger pipes see a drop off in effectiveness due to rapid attenuation of the vibrations along the pipe wall for larger pipes.

Hydrophone type sensors are placed directly in contact with the water allowing the device to measure vibrations in the water as opposed to measuring vibrations along the pipe wall, and hence are less affected by pipe material. Typically they are used at hydrants where gaining direct access to the water is relatively easy.

Pre-location noise loggers

Noise loggers are a combination of an acoustic sensor and data logger. They are attached, via magnetic fittings, to the outside of an accessible pipe fitting such as a valve stem or hydrant, where they will detect and record the acoustic noise in the surrounding network. The data collected is processed and analysed to look for changes that might indicate the presence of a leak.

Loggers can either be long term monitors permanently attached to the pipe or used in temporary deployments to detect a leak. Permanent deployment tends to take place in cities or other areas where routine survey work would be difficult. Battery life is typically 5 years. Some of the more advanced models are self-learning, capable of detecting anomalies from normal flow and flagging them up for passing technicians. Most modern designs are designed to transmit their data directly to the water company‟s local office.

The distance between noise loggers depends heavily on the type of pipe they are attached to. Noise loggers are more effective on metallic pipes less than 150 mm diameter. In such a situation the loggers can be deployed up to 250 m apart. Noise loggers are less effective on larger pipes and pipes made of plastic (due to poor noise transmission along the pipes) and therefore require closer positioning, often less than 50 m.

It is the density of noise loggers required and the associated cost that has prevented them being used more often as permanent installations. However, the cost of noise loggers is steadily reducing creating more opportunities for them to be permanently installed.

Noise loggers are non-intrusive, and do not require extensive training as the associated software does the analytical work. They are less affected by ambient noise compared to traditional noise detection having been buried beneath the ground and in close proximity the pipe. Noise loggers can be more cost effective than acoustic sounding at reducing leakage levels within a short period of time.

However, like more traditional sounding techniques, noise loggers have difficulty with detecting small leaks (typically below 80l/hr). Noise loggers are considered as leak localisation devices rather than pin pointing devices. Precise planning is needed to gain the best results (Dray, Loveday, Tod, & Tooms, 2010).

Correlating noise loggers

The correlating noise logger is a more sophisticated version of the noise logger. It pinpoints a leak by having two or more detectors placed on a pipe and measuring the noise. The correlating noise loggers feed the data through a cross-correlation algorithm and calculate the distance along the pipe to the leak from the detectors. The two readings can be cross- referenced to determine the location of the leak.

Unlike conventional noise loggers, they are used almost exclusively for temporary deployments to aid in leak pin-pointing. They should not be confused as a replacement for noise loggers as their function is different. They are used on smaller, metallic pipes and suffer the same limitations to sensitivity as leak noise correlators when deployed on larger pipes and plastic pipes (Hunaidi, Detecting Leaks in Water-Distribution Pipes, 2000).

They are attached to pipe fittings using a magnet in a similar manner to a traditional noise logger and can be accessed by direct inspection, use of a passing patrol vehicle with a receiver or via telemetry depending on the type.

Correlating noise loggers can be resilient to background noise and are effective over long distances in the right conditions. They do not require signals to be sent between units, reducing the sound interface and detection error. However, they cannot be used for live in situ correlations, instead requiring the data gathered to be downloaded for analysis. They also require a greater degree of training compared to traditional noise loggers (Dray, Loveday, Tod, & Tooms, 2010).

Magic carpet

The magic carpet is a form of acoustic sensor used in pin pointing water leaks. The device consists of a plastic mat, 1.5 m long, with evenly spaced acoustic sensors. The magic carpet is placed on the ground where a leak is expected. The acoustic sensors inside the mat are used to triangulate the location of the leak.

Magic carpets are accurate to 0.3 m in ideal conditions and are more resistant to background noise interference compared to other acoustic devices. Several water companies have started to adopt the technology as part of their leak detection toolkit.

Photograph 5.5 Magic carpet device

Source: (Stest, 2014)

Gas Tracing

Gas tracing may be used for leak detection on small pipes (less than 4”) but it is not in common use in the UK. At larger diameters, the cost of the gas is prohibitive, and is likely only to be used as a last resort. A gas (either hydrogen mixed with nitrogen or sulphur hexafluoride) is fed into an isolated and drained pipe, or bubbled into a pipe under pressure. The gas will escape through any leaks and infiltrate its way through the soil to the surface. This gas is then picked up by sensors moved along the line of the pipe by the operative. The gases are chosen to be non-toxic, non-polluting and non-flammable (at the concentrations used).

Photograph 5.6 Gas tracing equipment

Source: (Edenbros, 2014)

The biggest advantage of this method for pin-pointing a leak is that it will work on plastic pipe networks. It is also highly sensitive, as it can diffuse out of leaks only a few thousandths of a millimetre across (Pregeli, Drab, & Mozetic, 1997).

The ability of this method to pin point a leak depends heavily upon the ground surface. The gases will percolate through loose soil, block and flag pavement and gravel but will have difficulty penetrating dense clay, concrete and tarmac. Inaccuracies can occur as the gas takes the easiest path to the surface and the technique is poor for large pipes due to the time it takes for the pipe to fill with the gas (Dray, Loveday, Tod, & Tooms, 2010).

Ferret

Ferret is a relatively new technology that uses pressure monitoring to locate the source of a leak. It is highly accurate in smooth bore pipes (including PE and PVC pipes). The Ferret system consists of a head which is inserted into the pipe via a stop cock, meter or fire hydrant connection (and sized for a specific diameter of pipe) and a flow sensor that can be used to determine the scale of a leak.

The Ferret head is inflated and is pushed along the pipe. The pressure between the entry point and the ferret is then raised and monitored. A pressure drop indicates there is a leak in the section being checked. The Ferret is then gradually withdrawn until the pressure can be maintained. At this point the Ferret will have been withdrawn past the leak location.

Figure 5.2 Operation of a ferret

Source: (Ferret, 2012)

The Ferret can detect small leakage rates (0.03 litres per hour) and is most suitable for pipes between 10 mm and 40 mm in diameter. Whilst it is only available for service connections and small bore mains at present, larger diameter versions are being considered.

Photograph 5.7 Ferret equipment

Source: (Ferret, 2012)