Acoustic leak detection sensor technologies rely on the fact that liquid and gas leaks are noisy. However, it’s not necessarily a noise that the human ear can detect, it can be in the ultrasonic region. Leaky water pipes are a significant environmental challenge — UN Sustainable Development Goal No. 6 states that clean (drinking) water is a scarce source that needs to be protected; aging infrastructure adds to the challenges. Leaking gas pipes can be dangerous as well as costly. In either case, acoustic leak detection can provide a solution.

When a fluid or gas exits a pressurized pipe, acoustic noise is generated. Gases tend to generate more high-pitched noises, up into the ultrasonic range, compared with liquids that have acoustic signatures up to about 2 kHz.

Leaky water pipes
Many factors influence the noise generated by leaky water pipes including diameter of the pipe, size of the leak and the water pressure. In addition, the pipe material and the ground surrounding the pipe have strong impacts on how the noise is propagated. For example, water pipes act as low pass filters for acoustic energy and plastic pipes in particular introduce greater levels of damping compared with metal pipes.

The acoustic energy generated by a leak is transmitted through the pipe wall, through the soil surrounding the pipe, and through the water in the pipe. Those sounds generally travel greater distances in rocky soil and dry soils. In many instances, the formation of a pool of water surrounding the area of the leak will dampen the sound to a level that’s too quiet to hear, even with highly sensitive equipment. As noted earlier, gases tend to produce higher frequency sounds, and those sounds can carry farther compared with low frequency sounds typical of water leaks. In cases where the noise caused by the water leak is too faint to detect, a tracer gas can be injected into the pipe to facilitate the location of the leak using a simple acoustic listening stick (Figure 1).

Many of today’s water distribution networks have permanently installed listening devices at strategic locations. Using frequency analysis and cross-correlation methodologies, permanent listening devices can help to detect leaks as they occur. Frequency analysis measures and filters out ambient noise that interferes with leak identification.

Cross correlation can be used to identify the exact location of a leak. It uses two sensors, and the leak must be located between the sensors. In cross correlation, the time delay of the leak noise between the two sensors is combined with the speed of sound in the specific type and size of pipe to determine where the leak is between the sensors. Unfortunately, local factors such as soil condition can vary along the length of a pipe making it challenging to determine the exact speed of sound. Placing sensors closer together throughout the distribution pipeline is one way to improve the performance of leak detection systems.

In a growing number of cases, acoustic noise sensors are being integrated into water meters. Meters are found at numerous and strategic locations and provide good coverage for leak detection. The meter is an integral part of the water distribution system and has a good mechanical coupling needed to listen for leaks. And smart meters have an integral wireless communications link that can deliver information about potential leaks in addition to information about water usage.

The primary challenge when integrating leak detection into smart meters is low power consumption. Smart water meter batteries are expected to last at least 15 years. To address the issue of power consumption, very low-power acoustic sensors are used, and a single acoustic noise signature at the meter is transmitted only once per day.

New pipe materials bring new challenges
The turbulence at the leak location caused when the inside and outside pressures are trying to equalize is the source of the noise and is dependent on the pipe material as well as other factors. Leaks in copper, steel, and cast-iron pipes typically generate sounds in the range of 500 to 1,500 Hz. The range of sounds generated by leaks in PVC pipes is typically in the range of 70 o 850 Hz.

In addition, PVC pipes have a stronger coupling with the water compared with metal pipes which results in significant attenuation of the noise signal, and leak noises do not propagate as far in PVC pipes as they do in metal pipes. As a result, the use of two leak detectors together with acoustic correlation to locate leaks is not as simple or practical since many more detectors are needed and they need to be closer together in the case of PVC pipes.

The relatively high rates of attenuation of leak noise propagating in PVC pipes (the “wave speed”) and the variability in the speed at which it propagates both affect correlator performance. Accurate estimates of wave speed are especially important when looking for leaks in PVC pipes. In most installations, the wave speed is estimated from historical databases and determined from calculations made using assumed material properties and pipe geometry. The use of historical databases greatly limits the applicability of this correlating technique. More recently, a finite element method (FEM) has been developed using multiphysics software to calculate the wave speed and wave attenuation values used in the correlator (Figure 2). The results of the FEM calculations were compared for two cases, in the first instance, the out-of-bracket excitation mechanism was a loudspeaker; in the second instance, the out-of-bracket excitation was a leak the predictions using both out-of-bracket excitation mechanisms were in close agreement with actual systems.

Ultrasonic sensors detect gas leaks
Leaks in pressurized gas lines emit ultrasonic noise in the 25 to 100 kHz range. Traditional gas leak detectors measure accumulated gas and have a lag time before they can react. Ultrasonic leak detectors can identify a leak immediately as it occurs, triggering a much quicker warning.

For example, one stationary pole-mounted gas leak detector uses four ultrasound acoustic sensors to monitor wide areas for gas leaks (Figure 3). It can be used indoors or in outdoor environments and can withstand precipitation and wind and can identify leaks regardless of their location or gas stratification or dilution. Features of this system include:

• Instantaneous response to leaks of toxic, combustible, or inert gases over a range of 2 to 40 meters (7 to 130 feet)
• Sensors have no moving parts, do not age or drift, can operate without calibration, and include automated self-test for failsafe operation
• Operating temperature range of -40 to 85 °C

A handheld instrument for identifying leaks in industrial compressed air systems uses a matrix of 64 microphones as acoustic sensors arranged in a specific pattern plus a visible camera in the middle of the matric to provide an image of the area being scanned (Figure 4). Depending on the relative position of the sound source and the instrument, the sound is received at slightly different times by the 64 microphones. The inter-microphone time differences are used to locate the sound source which is then superimposed on the image being taken by the camera to show the operator the exact location of the leak.

Compressed air leaking from a pipe creates broadband noise across the audible and ultrasonic frequency ranges to 40 kHz and higher. Many industrial compress air leak sensor systems use narrowband ultrasonic sensors centered at about 40 kHz. The distance and angle between the sensor and the leak can impact the effectiveness of ultrasonic sensors. The use of narrowband ultrasonic sensors has limitations, including:

• Ultrasonic frequencies are strongly attenuated by atmospheric absorption.
• The measurement angle has a strong impact on the sound pressure level produced by a compressed air leak.
• Noisy environments that are common in industrial settings degrade the performance of narrowband ultrasonic sensors.

Replacing narrowband ultrasonic sensors with broadband sensors that can be used in both the audible and ultrasonic ranges can compensate for the limitations of narrowband sensors. The expanded frequency range increases the quality and accuracy of a leak detection system. In the case of compressed air leaks, the audible frequency range has the largest sound pressure level while the sound pressure level in the ultrasonic ranges in significantly lower, making detection using narrowband ultrasonic sensors challenging.

Summary
As shown, accurate location of water and gas leaks is possible using various acoustic leak detection technologies. The turbulence at the leak location caused when the inside and outside pressures are trying to equalize is the source of the noises and is dependent on numerous factors. Water leaks tend to emit sounds below a few kilohertz and can be effectively detected using narrowband acoustic sensors while gas leaks produce broader frequency spectrums extending into the ultrasonic range and can benefit from using broadband sensors for detection.

References

About Ultrasonic Gas Leak Detectors, Emerson
How Acoustic Measurements are used to Locate Water Leaks, Kamstrup
Leak Rate Quantification (LRQ) Method for Acoustic Imaging Cameras, Fluke
On the role of vibro-acoustics in leak detection for plastic water distribution pipes, International Conference on Structural Dynamics