Australia’s ongoing drought has increased the importance of water-saving measures, including pressure management, says Wide Bay Water Network Modelling Engineer Eric Skowron. Pressure could be reduced considerably when demand is at its minimum at night, but without a pressure management strategy, pressures tend to rise at times when demand is low, he says. High pressures bring undesirable results, including mains breaks, increased leakage and greater stresses on the infrastructure.
Implementing pressure management keeps the system pressures as low as possible while ensuring that they remain adequate to meet customer demands. Pressure management brings a number of benefits, says Mr Skowron. It extends the asset life, reduces pipe failures and conserves water. Cost savings come both from the reductions in wasted water and the need for fewer mains repairs. Pressure management also improves customer service as there are fewer interruptions to supply.
InfoWorks has helped Wide Bay Water to delineate district metered areas (DMAs) within some of its supply networks for better management of pressures across the network. Wide variations in topography meant that some low-lying areas had been experiencing exceptionally high pressures in order for those in higher elevations to receive adequate pressure.
'One of the first steps in implementing pressure management strategies is to determine the potential for water savings and cost savings,' said Mr Skowron. There may not be many gains from implementing pressure management strategies in networks that have been set up with lower pressures and are not subject to great fluctuations in pressure. 'But in most cases implementing pressure management strategies will have significant benefits.'
Taking the first steps
Delivery standards must also to be considered; in particular the required pressures and flow rates to serve customers and meet fire flow demands. It is also important to identify any critical customer requirements, such as local industry and hospitals.
A calibrated model of the existing network is the starting point for using InfoWorks to develop a pressure management strategy. The next step is to divide the network into distinct district metered areas explains Mr Skowron. 'Once you delineate those, you can verify through modeling that you will be able to achieve the reduced pressures.'
Topography is an important consideration he says. The critical pressure points might be at the highest points within a zone, or at the places that have suffered head loss by being furthest away. 'Typically it’s a little bit of both,' he says.
Choosing the right valves
One of the places where Wide Bay Water has used InfoWorks to introduce pressure management is a small community in a mountainous area. Providing adequate pressures to customers in higher elevations resulted in very high pressures in the valley. The InfoWorks model was used to split the area into two DMAs, each with its own pressure zone. Use of the model enabled optimization of the equipment that would be installed, including flow meters and pressure reducing valves.
Wide Bay Water chose flow-modulated pressure reducing valves (PRVs) after considering fixed set-point and time-control valves. Time-control PRVs could have problems in the event of a night fire, says Mr. Skowron, as reduced pressure at night would likely be inadequate for a fire demand . Fixed set-point PRVs are often easiest to implement, as no telemetry is required. “But we are finding that flow-modulated PRVs make the most sense for a lot of reasons, especially for supply of fire flow demands.'
A flow modulated PRV will adjust downstream pressure in accordance with fluctuations in system demand. The flow-modulated PRV opens up according to a predefined control profile as the demand increases. It is important to review the control profile, stresses Mr Skowron, to ensure that adequate pressures are being maintained at the critical pressure point within the network.
“The controllers for some flow-modulated PRVs allow input of only a minimum and a maximum value, with a linear interpolation between the two, but we often use a nine-point curve for demand versus pressure. There is more control over how the DMA is managed if you select a controller that allows you to enter multiple points,' he says.
The InfoWorks model is used to refine the boundary locations and produce the final proposed delineation for implementing pressure management. This involves splitting the network into discrete sectors and carrying out a detailed evaluation of flows, pressures and network performance.
Implementing pressure management
Wide Bay Water has carried out several case studies in the Gold Coast region. One of these is for the Guineas Creek Road DMA, which has approximately 1,500 connections and 1,600 properties. The average pressure within the DMA was reduced from 95m to 59m (312’ to 194’), and the night flow was reduced from 4.87 l/sec to 2.67 l/sec. 'This resulted in savings of more than 0.25 Ml per day (66,000 gpd),' says Mr Skowron.
Dropping the pressures in the mains when demand is low at night reduces leakage. Another benefit of the lower pressures is a reduction in mains bursts, further extending the asset life, he adds. The number of bursts has reduced by 60% to about four a month since the implementation of pressure management.
The flow-modulated PRVs have also been shown to cope successfully with the fire-fighting demands, following a major blaze in a commercial district that had recently implemented pressure management strategies. Fire crews began fighting the fire at about 4am, greatly increasing demand. The PRV duly opened up, enabling the flow rate to increase and the pressures to rise.
A recent case study has involved a small mountainous community in northern New South Wales where part of the district had been experiencing exceptionally high pressures. 'We are in the process of implementing pressure management there,” says Mr Skowron. “We used our InfoWorks model to verify how we should delineate the district metered areas.'
The network has been split into a high-level zone and a low-level zone as the central area was at a much lower elevation than those to either side. Providing adequate pressures to customer points in the hills meant that pressures at some places in the valley had been exceeding 90m (295’).
The InfoWorks model results were studied, looking at the resulting pressures to ensure that the low level zone was as extensive as possible. The topography dictates that new zones largely follow the contour lines. Extending the low-level boundary zone further out would mean including customer points that had a much higher elevation. “We wouldn’t be able to meet the minimum customer service requirement of 20m (66’) pressure,' says Mr Skowron. 'We took a look on the grid view output at what the minimum and maximum pressures were at all the hydrants. We typically looked at the hydrants because those are locations where we can put loggers in the field for verification.'
Creating separate low and high level DMAs only requires relatively minor infrastructure changes, according to Mr. Skowron. “We found that there were some duplications of mains within the town centre so we were able to isolate some of those mains to select a dedicated feed that went off to the higher customer points. We could then drop the pressures significantly in the lower area.' Achieving this involved the closure of eight existing boundary valves and the replacement of two cross fittings with standard couplings to isolate the dedicated main to serve the uphill areas. A 100mm (4”) flow meter and 100mm (4”) PRV were also installed.
InfoWorks has been used to calculate values at the PRV location for the low level zone before and after pressure management. With the new arrangements, the average pressure is now about 27m (89’). The value at the maximum pressure point had been about 90m (295’). 'We are able to drop that down to just below 57m (187’),' says Mr Skowron. Analysis in InfoWorks also showed the values for the critical pressure point, where the original pressure had been almost 65m (213’). According to the model, 'we are able to drop that down to just over 22m (72’)'.
Publicity plays an important role in briefing the public about the project’s purpose and the resulting drops in pressure, says Mr Skowron. 'We tend to bring it down incrementally - we don’t want to reduce pressure from say 60m to 27m (197’ to 89’) suddenly.' When implementing Pressure Management strategies, pressures are typically reduced incrementally and gradually to minimize impacts to customers.
This article is based on a presentation given by Eric Skowron of Wide Bay Water Corporation at the InfoWorks WS Australian user group meeting held in Sydney in November 2007.