Advanced Pressure Control in a City Water System
Non-revenue water accounts for roughly 4 million gallons (151 million liters) per day, Mortier said. This includes leakage and other problems such as meter inaccuracy.
Starting in 2012, the city water district began a program to investigate using advanced pressure control to reduce leakage, as well as reduce the cost to repair broken lines and minimize the energy required for pumping.
A METHODICAL APPROACH TO LOSS CONTROL
The American Water Works Association (AWWA) offers guidance in pursuing loss control in the M36 Water Audits and Loss Control Programs. Will Jernigan, director of water efficiency at Cavanaugh & Associates (www.cavanaughsolutions.com/) described the process steps, which show a cyclical process starting with an annual water balance audit, which repeats in order to refine the results (see table).
The question is how to cost effectively manage loss within a water utility. Loss is not just leakage. It may be billing-related or due to meters that are under-registering.
The M36 program covers four factors:
- Pressure management
- Active leakage control
- Maintenance, rehab and repair
- Speed and quality of repairs
All these are important, Jernigan said. The focus of the webinar was pressure management, “a pretty powerful tool in the toolbox to address loss.”
PRESSURE REDUCING VALVES
A key component in advanced pressure management is the pressure reducing valve (PRV), as described by Mark Magda, Singer global sales manager, Mueller Water Products. The PRV ensures that adjusted pressure downstream remains constant and does not fluctuate with upstream pressure changes. If high pressure drops are an issue, he said, users need to look into anti-cavitation PRVs to handle those situations.
With one pilot and one pressure setting, a manually adjusted valve can reliably do the job, he said. Alternatively, electronic controls options integrate with the district’s SCADA system.
BENEFITS OF PRESSURE MANAGEMENT
“Pressure does affect water flow rates,” Jernigan said, showing a graphic of a leak under low, medium and high pressure. “This is important in understanding what modulating the pressure will do to the flow rates within the system.”
Control involves average and maximum pressures in a system. If these pressures are reduced, the following benefits can result:
• Conservation: Reduced flow rates, resulting in
- lower consumption
- reduced leakage rates
• Water utility and customer: Reduced frequency of bursts and leaks, resulting in
- reduced repair costs
- extended asset life
- reduced cost of active leakage control
- fewer customer complaints
- fewer problems on customer plumbing and appliances
A CASE STUDY IN ASHEVILLE: GRAVITY WEST ZONE
The Gravity West Zone, one of three zones the Ashville district studied, is part of the area gravity-fed from the main reservoir for the system, Jernigan said. It includes 68 miles of water main and 4,792 connections. It is one of the older parts of the system, developed about 100 years ago during the boom time of the 1920s. It was chosen because it had a relatively high existing leakage rate and a relatively high frequency of breaks for both mains and services (customer connections).
The first question to be answered: How isolatable is the zone?
Preliminary modeling showed the zone could be isolated and controlled with four supply-point PRVs. The installation costs are a function of how many control valves or PRVs need to be installed.
The next step was to confirm the zone boundary. For the process to work, flow into and out of the zone needed to be monitored and controlled. About 30 valves had to be verified in the field, including exercising them to confirm they functioned correctly. This was a lengthy process, Jernigan said, and “when you go out closing valves, it may stir things up that require extra flushing.”
Next, temporary insertion meters were installed at the four supply points to collect flow and pressure data. The focus of the analysis was that there was excessive overall pressure in the zone.
Meanwhile, the leak team made a couple of sweeps through the zone. On the first, they found 12 leaks on mains and services, and eight more on the second sweep. Sometimes if two leaks are near each other, Jernigan explained, the acoustic method used can find only the larger leak, as the smaller one can’t be detected, so a second sweep is performed.
ANALYZING PRESSURE DATA
Critical pressure is the lowest pressure in the system, typically at the highest elevation. The pressure data showed even at the lowest critical pressure (around 80 psig (552 kpag)) it was in excess of that needed to provide customer service. “Here there would be easily 30 to 40 [psi] (207 to 276 kpa) of potential reduction, even at that critical point,” Jernigan said. “We also saw nighttime increases—when demand goes down, pressures tend to go up.” The nighttime represents another 10 or 20 psi (69 or 138 kpa) of reduction potential if it could be modulated through advanced pressure control.
Additional testing and analysis, including fire hydrant flow tests, were completed. Various roadblocks cropped up, including some test results that indicated the water system map did not match the actual system. This is a common problem in such old systems, Jernigan said.
The last step was to look at consumption. In addition to collecting overall usage data, the water district used its radio meter-reading capability to do sweeps at 2:00 and 4:00 a.m. Readings then, along with adjustment for actual usage during that time, showed baseline leakage rates.
THE BUSINESS CASE FOR ADVANCED PRESSURE CONTROL
Analysis of the data allowed an estimate of possible savings. With an average reduction of 60 psi (414 kpa) in the zone, from 130 to 70 psig (896 to 483 kpag), annual leakage was projected to be reduced from 189 to 80 million gallons (715 to 302 million liters). Annual breaks would likely go from 65 to 32.
The payback period relates the savings to the cost of installing the PRVs, about $500,000. Taking into account both operating and capital savings, the payback period was between two and three years, Jernigan said. Even just counting the operational savings, payback would be between three and four years.
PLANS GOING FORWARD
The water district is looking at future opportunities for pressure optimization. A large zone, which has a single point of control, is a likely prospect. So far, the likely pressure reduction might be only 15 or 20 psi (103 or 138 kpa), but since it covers a large area (about three-quarters of the network) the benefits may be substantial.
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