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The results of pressure management when the excess pressures and surges have been reduced. Source: Fantozzi & Lambert 2007
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It is now well known that reducing pressure in a DMA also reduces leakage. In order to analyse this effect more closely, it is useful to break the leakage down into the following three components:
Background leakage - This is made up of small undetectable leaks, often at joints and fittings, running continuously and aggregating to significant annual volumes.
Unreported leakage - These are larger, detectable, but not visible leaks which gradually accumulate over a period of time. The increasing leakage is monitored using the District Meter, and once the leakage reaches a certain threshold, teams will go into the DMA and detect and fix all the unreported leaks. The optimal economic time to intervene occurs when the value of the unreported leakage under the triangle equals the cost of intervention. In the example above, this happens once every year, prior to pressure management.
Reported leaks and bursts - These are large and visible leaks which are reported by the customer and should be fixed within a short time of them being reported.
The diagram above shows how a reduction in pressure affects each different type of leakage:
• The background leakage which is very sensitive to pressure is significantly reduced.
• The number of new unreported leaks and bursts may be reduced lowering the rate of rise of unreported leaks. Intervention now occurs less frequently than before.
• The flow rate of new reported leaks and bursts is reduced and the frequency of new leaks and bursts and bursts may also be reduced.
- The average level of total leakage as shown by the dashed line is significantly reduced.
Pressure / leakage relationship
The value of leakage reduction from a given level of pressure reduction can be calculated using the
FAVAD equation. FAVAD stands for Fixed and Variable Area Discharges and is so named because the areas of many leaks do not remain constant but change with pressure.
An example of this would be a longitudinal split in a plastic pipe. As the pressure increases the split will open out further.
The FAVAD equation (Ref. 1) is as follows:
If the average pressure is reduced from P0 to P1, flow rates through existing leaks change from L0 to L1, and the extent of the change depends on the ratio of average pressures and the exponent N1.
The significance of this relationship is that it may still be economic to pressure manage a DMA where the pressure is already low. For example, reducing the Average Zone Pressure (AZP) by only 3m (10%) from 30m to 27m could give an immediate reduction in leak flow rates of 10% (NI=I) and up to 20% for a predominantly flexible pipe network, which in an area of water shortage or high leakage could be very significant (Ref. 2)
Tests carried out in Japan, the UK, Brazil and other countries since 1977 show that, on average, for large systems with mixed pipe materials, the value of N1 is close to 1.15. i2O use a conservative value of 1.0 where no other information is available
References
1) May, J. Pressure Dependent Leakage. World Water and Environmental Engineering, Oct 1994
2) Thornton J and Lambert A. "Progress in practical prediction of pressure: leakage, pressure: burst frequency and pressure: consumption relationships." Proceedings of IWA Special Conference 'Leakage 2005', Halifax, Canada, September 2005