This is allowing Thames to prioritise repair work to its 24,000 miles (39,000km) of mains pipeline and fix the largest leaks first. Earlier methods of finding underground leaks were slower and therefore expensive. And they gave no indication of seriousness - a key element of any loss reduction programme, since 20 per cent of leaks are responsible for 80 per cent of water losses. In one three-month period, the improved detection methods saved 13 million gallons (60 million litres) of water per day.
The new methods allow the company to work out how much water loss a particular repair would save - and put a value to it. This is vital information for Thames Water, which is spending pounds 20m per year on leak control. If no repairs were done for a year, the company estimates, leakage from the 2.7 billion litres it delivers each day would increase by 100 million litres per day.
Leaks in the system, usually from one of 18 million pipe joints or from hairline cracks, are detected through a series of step tests carried out at night, when demand is low. The flow rate into a particular area is measured, then valves are shut off at various points to isolate part of the pipe network. A drop in the flow rate when a valve is shut indicates a leak in that section.
Previously data collected at night had to be taken away for analysis the next day. Recording the data required cutting the water supply to entire neighbourhoods for several hours, and recharging the mains afterwards. Staff had to make sure there was no air in the system before taking the readings, and the equipment was slow in monitoring changes in the flow rate.
Knowing there is a leak in a section of pipe is not enough. To find out where to start digging, Thames uses a noise correlator that pinpoints the leak by listening electronically and comparing the sound of the water flow at a number of points along the pipe. This is done during the day; unfortunately, the correlator cannot tell the difference between a leak or someone running a bath.
Thames has now developed a Quick Report Leak Spotter in collaboration with Reten Acoustics of Gwent. This instrument is fitted (at night) to the meter at the boundary of the area being surveyed - typically serving 1,500 houses. As the turbine in the meter rotates, it gives off magnetic pulses. The Leak Spotter measures the interval between pulses to an accuracy of one thousandth of a second. A transmitter sends a radio signal to a portable receiver carried by the operator, who could be two to three miles away. The receiver calculates the rate of flow from the number of pulses and the average interval between them.
Valves are then shut in turn, as in the earlier method. The equipment is so sensitive that the valves need only be shut off for seconds, so the system remains full of water. The operator knows exactly what is happening to the system, and can quickly identify where the leak is and the amount of water being lost. Because there is no air in the system, the leak noise correlator can then pinpoint the leak immediately.
Thames also plans to fit pressure control valves which will further reduce losses by moderating pressure at night. It says this is the most cost-effective way of suppressing small leaks that are uneconomic to repair.
Thames estimates that from 1990 to 1993 it cut leaks from the local mains network from 25 per cent of water put into supply to 17.5 per cent. This figure includes losses from customer premises for which Thames is not responsible, but not losses from trunk mains. Detecting leaks in trunk mains continues to be a problem, because they cannot be shut down, and leak noise correlators do not work on pipes with a bore larger than 300mm (12ins).
The economics of reducing leakage has shifted significantly in the past few years because of drought and privatisation, which has allowed water companies to charge more for supply. As a result, they are prepared to spend more to conserve a scarce and costly commodity. They are also looking closely at leak reduction as a means of reducing or postponing capital expenditure on new water sources, or new links in the mains network.