CHAPTER 6 DISCUSSION
6.4 PIPE REPLACEMENT MODEL
The following methodology has been developed to create a standardised procedure for the replacement of distribution mains. Five distinct phases have been identified that need to be conducted to ensure that a satisfactory return on investment is obtained.
1. Preliminary Investigation Month 1 to 6 2. Zone Rectification and Pressure Reduction Month 7 to 12 3. Monitoring and Clean Data Collection Month 13 to 24
4. Mains Replacement Month 25 to 36
5. Twelve Month Maintenance Period Month 37 to 48
Table 6.1: Phase 1 – Month 1 to 6 - Preliminary Investigation
10% Of the distribution mains are selected for analysis in Phase 1 based on the areas that have the highest real losses and highest burst frequencies. The replacement programme will at this point be in its infancy and will need reliable data on which to make decisions to establish a proven track record. A replacement rate of 2%, which assumes a pipe life of 50 years, should be initially targeted. As the pipe replacement programme gains momentum and the input data improves, the replacement rates should be based on least cost optimisation. This calculation will
Select zones of interest based on water balance, high real loss value and high main burst frequency
Confirm zone discreteness, reservoir and district meter meter accuracy.
Log all flow meters
Locate all line and scour valves and confirm status and operation; Update GIS with valve data
Perform step testing and active leak detection sweeps
Log minimum and maximum pressures, optimise prv settings and check all prvs working correctly
Decision: If real loss rates and burst frequency are still above threshold levels, proceed to Phase 2
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be performed to compare the cost of replacement (or the cost of bringing the replacement forward in time), versus the cost of maintenance. The replacement rate thereafter will then be based on that replacement rate that returns the best value for the Utility in the long term and not the based short term view of what replacement rates the tariff can support.
Table 6.2: Phase 2 – Month 6 to 12 - Zone Rectification and Pressure Reduction
It is currently planned to roll out pressure reduction throughout the municipal area to lower the AZP to 46m over a four year period. Once this programme has been completed, the AZP will be lowered to 42m over a further five years. Where the minimum AZP has not yet been achieved in each zone where mains replacement is contemplated, then this work will be conducted during the relay investigation. This pressure reduction step can therefore be omitted from the year 2022 onwards.
Table 6.3: Phase 3 – Month 13 to 24 - Monitoring and Clean Data Collection
Ensuring that apparent losses are reduced in each zone that is replaced is another important objective of the programme. Apart from the financial loss, illegal connections and inaccurate meters can skew the performance data of a zone and make the real loss appear higher than it actually is. Based on past experience, the relay Contractor faced considerable resistance from
Design and install additional prv's to minimise AZP.
Commission advanced control and failure warning devices where
appropriate
Create additional DMA's to maximum recommended size of 2000 properties / 10km and install meters
Analyse burst information and investigate causes of bursts and reduce where possible
Perform step testing and active leak detection and calculate the Net Rate in Rise in leakage for the zone
Log minimum and maximum pressures, optimise prv settings and check all prvs working correctly
Decision:
Conduct Cost / Benefit and NPV analysis and if favourable, proceed to Phase 3
Install electronic equipment to monitor pressure and flow data on line and real time. Log minimum and maximum pressures, optimise prv settings and check all prvs working correctly
Continue to monitor
performance data to determine if it will be beneficial to replace mains.
Conduct Infrastructure Condition (ICF) Tests and analyse field samples where appropriate
Ensure that all properties are metered. Verify accuracy of ICI customer meters.
Remove all illegal connections
Perform step tests on all mains above 300mm to check on real loss rates and if replacement is warranted.
Replace connections on these mains as part of investigation if deemed beneficial
Perform step testing and active leak detection Calculate the Net Rate in Rise in leakage
Decision: If real loss rates and burst frequency are still above threshold levels, proceed to Phase 4.
Prioritise the selected areas to tackle the most beneficial areas first
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the communities in some areas when trying to regularise illegal connections and as a result this work was not successful. Macleod (2012) advised that it will be more practical to make this initiative the responsibility of the Department and therefore they must ensure every connection is metered prior to mains replacement being carried out.
Table 6.4: Phase 4 – Month 25 to 36 - Mains Replacement
Larger diameter pipes have a thicker pipe wall, higher structural strength, a lower failure rate and are significantly more expensive to replace. It is therefore advantageous not to assume a blanket replacement policy but to consider each pipe on its merits. The construction contract should allow for testing of any main that is left unchanged to ensure that the new lower loss rates for the zone can be achieved. These rates would be based on the UARL rates in Table 2.7 of 18 litres / km / day / metre pressure and 0.8 litres / connection / day / metre pressure. At the targeted AZP of 42m, this equates to 756 litres / km / day and 33.6 litres / connection / day.
Table 6.5: Phase 5 – Month 37 to 48 - Twelve Month Maintenance Period
Table 6.6 details the advantages of this five phase approach.
Replace all mains 300mm and below and all connection pipes in DMA. Mains above 300mm only to be changed if warranted by assessed real loss rates
After mains replacement, ensure that all meters are located on the property boundaries and are all installed to Manufacturers' recommendations
After mains replacement, check on customer meters in zone and replace all according to meter asset management plan criteria as well as other meters with decreased sales volumes
Log minimum and maximum pressures, optimise prv settings and check all prvs working correctly
Contractor to perform active leak detection during 12 month defect liability period. Minimum levels of leakage and burst
frequency must be achieved by Contractor
Maintenance period commences once Contractor has achieved all
contractually stipulated performance levels
Continue to monitor zone real time on an ongoing basis
Recalculate assumed values relating to reduction in burst frequency and reduction in losses for use in the financial analyses
Perform active leak detection when real losses increase
Log minimum and maximum pressures, optimise prv settings and check all prvs working correctly
Contractor to prove status of all valves at end of maintenance period
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Table 6.6: Advantages and Disadvantages of the Five Phase Approach
OLD RELAY PROGRAMME NEW RELAY PROGRAMME
Illegal connections not addressed Illegal connections to be rectified prior to relay commencing
Sales volume reduced Greater attention paid to water meters and all old meters and damaged meters to be replaced
Increase in burst rates Majority of mains in zone will be new and therefore burst rates will reduce. This will be assisted by the reduction of pressure Insignificant reduction in real losses Contractor responsible for achieving set
targets or penalties to be applied
Savings achieved not sustainable Monitoring of zone and active intervention by Department to ensure that results are sustained
Department not satisfied with programme or results
Better results will be achieved and communicated to all via internal and external communications programme Short term demand spike (4km/day) placed
on Manufacturers and Contractors causing an increase in prices and reduction in quality
Consistent predictable programme to be implemented and consistent production targets to be set and achieved. This is envisaged to be 1km/day