The tracing and location of leaks has become a major area of operation for water companies and others that utilise pipelines for fluid transport, including gas.
There are in general three main options that are available to locate leaks in pipes:
Acoustic – listening to the noise produced by a leak which produces a sound wave that propagates along the pipe material and through the fluid itself. This is known as noise correlation.
Pressure – detecting the change in pressure within the fluid column after it passes a leak point.
Tracer gas – this will also leak through any damage into the surrounding ground and can be detected from the surface over the leak point.
Alongside sophisticated modern technologies it is not uncommon to see water engineers out at all hours of the day, but more especially at night, with a metal rod and listening bud on the end probing water mains at access points or meter locations to listen for that all-telling noise that highlights a leak somewhere nearby.
Leak noise detection and correlation from surface
Noise correlation from surface outside of the pipe works by matching sound waves against each other. This is done by using sensitive microphones attached at strategic points on a pipeline network. By monitoring the sound waves travelling through the pipeline and noting the difference in arrival time at the microphones of matched sound signals, and knowing the approximate travel speed of the sound waves through the pipe material and fluid, a measure of the distance of the sound source from the microphones can be estimated. This identifies the location of the possible leak. Monitoring from a number of locations can help pinpoint this sound source more accurately. Portable systems or static monitoring systems can be used for this type of location operation.
Portable systems tend to be used where leaks (often visible at surface) have been reported and the precise location of the leak needs to be established prior to undertaking a pipeline repair. Network monitoring systems are used to ensure that leaks that may be less obvious are located before significant damage is done or before they are sufficiently large to appear at surface, thereby minimising fluid loss to the network owner.
In the case of gas pipes another method of leak location is the boring of small probe holes from surface along what is believed to be the pipeline route and monitoring the gas level in the substrate. In principle the higher the concentration the closer the probe hole is to the leak.
Leak detection from inside the pipe
Where access to the pipeline is limited, for example long lengths of pipeline with few access points, it may be necessary to locate any leaks from within the pipeline. This is still achieved using acoustic methods to locate leak noise.
In essence, instead of the microphone being located on the outside of the pipe it is placed inside the pipe and rodded or winched through, or the fluid flow is utilised to carry the device along the length of the pipe being investigated. As the microphone passes leak locations it picks up the increased noise level. The device is monitored by its distance into the pipe thus identifying the location of the leak. It should be noted that it may be necessary to complete a full utility survey prior to undertaking the leak detection work to correctly establish the route of the pipeline as changes in direction and depth may mean that accessing the pipeline may occur in the wrong place if the pipe is not on the expected line.
Another option for leak detection from inside the pipe is to monitor the pressure loss within the fluid after it passes a leak point. A specially designed pressure sensor is passed through the pipeline from a known access point on a pipeline. The pressure in the fluid is used to drive the sensor forward just sufficiently to move it. When the sensor passes a leak the pressure drops so that it is no longer sufficient to drive the sensor forward. The distance from the access point to the stop point indicates where the leak is located. Again it may be necessary to map the pipeline length fully prior to using this system or it may be possible add a locating sonde (similar to the walk-over directional control systems used in HDD) to the pressure sensor, which can be detected from surface to pinpoint the leak location.
Electric Leak Monitoring
Electric Leak Monitoring, also known as Focused Electrode Leak Location (FELL), is used in working pipelines. Essentially, a probe is passed through the pipeline within the fluid and an electric current is applied to the probe head. This is connected to an electrode driven into the ground close to the pipeline. Where no leaks are found the electricity is retained within the pipe. Where the probe passes a leak, electricity leaks out into the surrounding ground. This completes the circuit to the ground electrode and the voltage can be measured to provide a measure of the size of the leak. The location of the leak is indicated by the distance the probe has passed into the pipeline as with other systems described above.
Permanently installed leak detectors
The modern requirement to manage leakage effectively over time has led to a significant increase in the installation of permanently installed leakage monitoring systems. These systems may be installed to monitor a single length of pipe or with strategically placed microphones they may monitor whole networks within an area. The data produced can be either collected by personnel visiting the sites with Wi-Fi connectivity to a data storage system or may be connected directly to the main office using internet or Wi-Fi connection. Manufacturers provide specific software to analyse the data and highlight any anomalies that may be present that would indicate the presence of a leak and its location within the network. Various manufacturers offer such systems.