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TRUNK MAIN LEAKAGE


Leaks in trunk mains can go unreported for considerable lengths of time, it can be assumed that if a large diameter water main is leaking it would be possible to see this water at the surface, this is not always the case as it depends on the sub-surface material and location of the leak in the pipe. If a leak is not detected and eventually bursts it can become a catastrophic failure with the pipe completely rupturing. These failures can start as a simple weep.


There are 4 parts to the life of a leak:


Weep-Leak-Burst-Catastrophic failure

Weep/small loss       A small amount of water leaving a pipe from a small failure <0.27m3/hr

Leak/medium loss    An amount of water that leaves a pipe through an orifice 0.27-11m3/hr

Burst/large loss        An amount of water that leaves a pipe from an orifice 11m3/hr – 27m3/hr

Catastrophic failure  A complete rupture of the pipeline >27m3/hr

(figures from ‘Leak Detection’ IWA Publishing, chapter 8, 8.4 Life of a Leak. Authors Stuart Hamilton and Bambos Charalambous)

In a recent study 2080 leaks were categorised:

Weep/small loss         143
Leak/medium loss      1278
Burst/large loss           659
Catastrophic failure     0

The greatest number of leaks were found in the ‘Leak’ category but the ‘Burst’ category had the greatest volumetric loss per hour.  The ‘Weep’ category could be the most expensive to repair if looking at the cost of water saved compared to the repair cost so could be considered as ‘beyond economic repair and become equivalent to background leakage’.  These ‘Weeps’ still need future investigation as they will become ‘Leaks’.

(figures from ‘Leak Detection’ IWA Publishing, chapter 8, 8.6 Conclusions. Authors Stuart Hamilton and Bambos Charalambous)

The typical method employed to detect leaks on trunk mains would be to conduct ‘trunk walks’ where a Technician would walk the length of the trunk main listening on each fitting, fire hydrants, wash outs, air valves and valves with a manual listening stick.  This method has some success but as the noise frequency created by leaks in large diameter mains can be below the threshold of human hearing as the distance from the leak increases leaks can go undetected.  Available fittings on trunk mains can also be quite scarce compared to district metered areas, where available they may be at distances of 1km or more adding to the limitations of ‘trunk walks’.

With time technology improves and today there are a number of other survey methods including:

Real time computer based digital correlators using acoustic sensors or hydrophones
Acoustic sensors or hydrophones are deployed on suitable fittings on/in the water main to detect the noise created by a leak.  This signal is transmitted by radio to a receiver connected to a computer where the data is processed using mathematical algorithms and advanced filtering techniques.  Entering pipe details into the analytical software is required to give the distance to the leak position.

Multi-point correlating loggers utilising high sensitivity hydrophones
Multiple hydrophones are deployed on suitable fittings in the water main with the signals being recorded by connected loggers.  This data is then read into the analytical software where it is processed using mathematical algorithms and advanced filtering techniques.  Entering pipe details between each logger into the analytical software is required to give a distance to the leak position.

Tethered systems incorporating CCTV, ultra high sensitivity sensor and sonde
Tethered systems are deployed into the water main through suitable fittings with or against the flow direction.  As the sensor passes through the pipe data is transferred by the umbilical cable to a processing unit where it can be analysed by the user.  Once a leak is detected the sensor can then be located giving an accurate indication of the leak location.

Free swimming systems with integral sensors
Free swimming systems are inserted into the water main and travel along with the water flow.  A capture device is used at the end of the inspected water main to retrieve the sensor.  Data is recorded during the sensors travel through the pipe and processed once removed identifying any leaks that were passed.

Gas injection
Hydrogen or helium is injected into the water main where the leak is suspected.  This gas will escape through the leak with a gas detector on the surface detecting the presence of this gas giving the leak position.

Ground penetrating radar
Signals are transmitted from the surface and reflected back to a receiver with the strength and time required for the response measured.  The data from these signals are then processed with an image of the sub-surface materials displayed on a screen for the user to analyse.

These are all proven technologies and have their advantages and disadvantages depending on site conditions, available access points and cost to complete a successful trunk main survey.

The technology is only one part in completing a successful trunk main survey, the other part is the user of that technology.  The data obtained from each technology needs to be analysed and it is the expertise of the user to understand this data to obtain the best results.

Services:

- Trunk main correlation surveys

- Distribution network, leak detection   and location

- Step testing

- Commercial leak detection and           location

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Phone Number: +44 (0)7860 868428

Email: andrew.mackenzie@wlls.co.uk