Leakage reduction

The results of pressure management when the excess pressures and surges have been reduced. Source: Fantozzi & Lambert 2007

It is now well known that reducing pressure in a DMA also reduces leakage. Leakage can be analysed more closely thus:

Background leakage  - Many small leaks uneconomic to repair individually but aggregating to large continuous losses.

Unreported leakage - This gradually accumulates at an average rate of rise. Once the leakage reaches a certain level, a team will go into the DMA and detect and fix all the unreported leaks. The optimal time to intervene is once the value of the unreported leakage under the triangle equals the cost of intervention. In this example, this happens once every year prior to pressure management.

Reported leaks and bursts - Normally fixed within reasonable time.

The above figure shows how a reduction in pressure has affected each type of leakage:

• The background leakage, which is very sensitive to pressure, is significantly reduced.

• The number of new leaks and bursts is reduced, lowering the rate of rise of unreported leaks.   Intervention now occurs less frequently than before.

• Both frequency and flow rate of new reported leaks and bursts is reduced.

The average level of total leakage (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 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 anything between 5% and 15%, which in an area of water shortage or high leakage could be significant. After the i2O system has been installed, it can automatically calculate the value of N1 for the DMA. However, it may often be necessary to forecast leakage reduction before the system has been installed in which case N1 can be estimated as follows:

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 1.0. This value could be used if there was no other information available on the pipe materials or the type of leakage.