5.3 Pressure relationships with leakage, minimum night flow, and burst frequency
5.3.1 Pressure – leakage relationship
The summarised results of computed leakage as well as pressure recordings are provided in Table 5.6, whereas detailed records of pressure are provided in Appendices B5 and B6. Total expected night use (ENU) is obtained from Appendix B3 as computed by SANFLOW model.
Total leakage in this section includes both background and burst leakages as indicated by McKenzie (2001).
Excel analysis
Analysis of the data provided in Table 5.6 was done graphically and results are shown in Fig.
5.2 and 5.3. The results from graphical analysis confirm that the relationship between pressure and leakage follows an exponential relationship as indicated by equation 4.2. In this regard, the excel chart that could be used to make quick decisions as regards to pressure and leakage relationship could be the exponential type. However, a second approach of analysis was also done using the theoretical relationship of equation 4.2 as recommended by Lambert (2001a) and Fanner and Thornton (2005).
Table 5.6 Pressure and leakage figures for Chinyonga and BCA DMAs: 2 to 15 April 2008
CHINYONGA TOWNSHIP DATA BCA TOWNSHIP DATA
Date Measured AZNP
m
ENU m3/h
Total Leakage
m3/h
Measured AZNP
m
ENU m3/h
Total Leakage
m3/h 02/04/08 74.953 1.875 6.135 83.598 0.194 3.956 02/04/08 75.038 1.875 5.685 83.700 0.194 2.566 03/04/08 84.452 0.194 4.586 03/04/08 84.451 0.194 3.886
04/04/08 51.771 1.875 3.985 85.003 0.194 4.986
04/04/08 53.284 1.875 3.525 84.779 0.194 4.906
05/04/08 72.247 1.875 6.340 71.620 0.194 3.376
05/04/08 72.420 1.875 5.925 69.350 0.194 3.235
06/04/08 77.215 1.875 6.428 83.655 0.194 4.581
06/04/08 77.274 1.875 5.925 83.732 0.194 4.486
08/04/08 70.217 1.875 5.320 38.101 0.194 1.891
08/04/08 70.320 1.875 5.125 35.299 0.194 1.726
09/04/08 72.038 1.875 5.100 64.915 0.194 3.856
09/04/08 71.279 1.875 5.013 64.458 0.194 3.706
11/04/08 76.156 1.875 6.810 26.067 0.194 3.822
11/04/08 71.630 1.875 6.645 23.931 0.194 3.046
13/04/08 76.462 1.875 7.735 78.018 0.194 2.727
13/04/08 76.496 1.875 7.665 77.003 0.194 2.614
15/04/08 78.523 1.875 7.010 78.418 0.194 3.276
15/04/08 78.572 1.875 6.531 78.156 0.194 3.226
AZNP is average zone night pressure measured at the same time with minimum night flow measurement; ENU is total expected night use as computed by SANFLOW (Appendix B3), whereas total leakage is computed using equation 4.1b.
Theoretical analysis
The relationship was investigated by carrying out computations using equation 4.3 as well as hour day factor (HDF) model principles, where all the possible combinations of the pressure and leakage data as given in Table 5.6 was used. The results of computed average values of leakage exponents (N1) for Chinyonga and BCA are presented in Table 5.7.
Table 5.7 Determined values of leakage exponents (N1) for BCA and Chinyonga DMAs for the data collected from 01 to 15 April 2008
District Metered Area (DMA)
Average Leakage exponent (N1)
Standard Deviation
Probable N1 Range
Chinyonga 1.6 0.6 1.6 ± 0.6 (n=71)
BCA 1.0 0.5 1.0 ± 0.5 (n=91)
n= number of computations for N1 values from possible combinations of pressure and leakage before the average value was computed. Leakage exponent is coefficient relating pressure and leakage.
The results show that leakage exponent (N1) values for Chinyonga range from 1.0 to 2.2, while for BCA range from 0.5 to 1.5. The difference between the values from the two DMAs is possibly due to the fact that the main distribution pipeline of BCA DMA was renewed 2 years before this study and most of the connections are constructed with galvanised iron (GI) pipes, whereas for Chinyonga DMA most of the connections are constructed with HDPE pipes and the main distribution pipes are over 15 years old. Warren (2005) indicated that most of the times, service connections are the main source of water leakages. As a result, from Warren’s argument and the age factor, it is justified for Chinyonga DMA to have high N1 values as compared to BCA DMA.
y = 1.0544e0.0237x R2 = 0.7622
0.000 1.000 2.000 3.000 4.000 5.000 6.000 7.000 8.000 9.000 10.000
0.000 10.000 20.000 30.000 40.000 50.000 60.000 70.000 80.000 90.000 100.000
Average Zone Night Pressure (m) Measured minimum night flow (MNF) m3/h
Fig. 5.2 Determination of a relationship between pressure and leakage using excel charts – Chinyonga DMA data from 01 to 15 April 2008
In studies that were done by Charalambous (2005) for the water board of Lemesos in Cyprus, N1 values that were found ranged from 0.64 to 2.83, and an average of 1.45 was found.
Similarly, Lambert (2001a) indicated that analysis undertaken in Japan, UK, Brazil, Canada, Malaysia, Australia, New Zealand and USA found that N1 values ranged from 0.5 to 2.5. It can therefore be seen that the determined N1 values from this study are of the same order of magnitude as figures reported by others, which reinforce the use of the leakage – pressure relationship (L1/L0)=(P1/P0)N1. The distribution mains in the Chinyonga and BCA DMAs is a mixture of asbestos cement (AC), high density polyethylene (HDPE), and galvanised iron (GI) pipes.
Walski et al. (2006) established that soil do not have effect on N1 values unless if it is only background leakages being considered, which is not the case for most pipe bursts in the water
distribution system. In this regard, although the results were obtained in one season of the year, they still represent the actual values of N1 in the DMAs since the soil condition would not have notable effects on the leakage.
y = 2.042e0.0074x R2 = 0.2712
0.000 1.000 2.000 3.000 4.000 5.000 6.000
0.000 10.000 20.000 30.000 40.000 50.000 60.000 70.000 80.000 90.000 100.000
Average Zone Night Pressure (m) Measured minimum night flow (MNF) m3/h
Fig. 5.3 Determination of a relationship between pressure and leakage using excel charts – BCA DMA data from 01 to 15 April 2008
The N1 values found could make it possible for a utility to make quick predictions of pressure management opportunities as recommended by Fantozzi et al. (2006) and Tooms and Morrison (2005), and the likely effect on leak flow rates using equation 4.4 or 4.5. For instance, if Chinyonga DMA pressure is reduced by 20%, leakage would be reduced by 30%.
Similarly, if BCA DMA pressure is reduced by 20%, leakage would be reduced by 20%.
Therefore, the calculated results of N1 values could be used for prediction of any leakage reductions resulting from any pressure reductions in the specific study sites. Furthermore, the results found could also be applied in other areas with similar characteristics within the entire water supply area to make predictions for the likely pressure management opportunities. This rapid type of analysis which gives an indication of likely benefits of pressure reductions would be a motivation for most utilities to manage their real losses efficiently.