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 increase 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.