Although polyethylene pipelines have the reputation of being 100% leakproof, as borne out by over 40 years of experience in the field of gas distribution, the use of mechanical unions on polyethylene pipelines used to carry water has given rise to leaks – So how can they be detected?
Several techniques are available for finding leaks in pipelines, but we are going to concentrate here on acoustic correlation.
The principal:
A leak in a pipeline will generate an acoustic wave that propagates at a specific constant speed through the conduit material.
The amplitude of the acoustic wave becomes attenuated as it propagates and eventually becomes indistinguishable from the background noise.
The correlation technique involves positioning sensors at 2 pipeline access points (if possible either side of the leak) and looking for similarities between the noises they pick up.
When the noise of a leak is identified, its position can be calculated from:
– the distance between the two sensors,
– the propagation speed of the noise in the material or water,
– the time difference between the 2 sensors picking up the noise generated by the leak (the noise reaching the sensor farthest away from the leak will be attenuated and delayed).
Implementation:
Acoustic correlation requires the following equipment:
– mobile sensors for when the leak has been narrowed down to a section,
– fixed sensors installed to provide day-to-day surveillance of the pipeline.
The type of sensor used (accelerometer or hydrophone) depends mainly on:
– the pipe material – hydrophones work better on plastic pipes.
– valve boxes or tapping points that are close enough together to fit the sensors to carry out the correlation.
The sensors can be read by RF, GSM, Wi-Fi or wire link.
To prevent correlation errors, the readings are taken when the ambient sound and consumption levels are at their lowest.
The operation must be prepared using a detailed plan of the pipeline showing what the pipe is made of and all the valve boxes and tapping points on which the sensors can be placed.
Precise correlation calls for a good understanding of the technique so that the correlator can be fine-tuned (frequency bands, sound propagation speed, distance between the sensors, and propagation environment). For the technique to be effective, the correlation parameters such as distance between the sensors, materials, etc. must be known and the two sensors must be synchronised properly (to within less than a millisecond).
Source: Astee
