By Dr. Steve Minett

Summary: Zagreb’s new high-rise buildings required a high-pressure water supply. This meant higher energy consumption and a strain on municipal piping, which lead to major leakages. The solution proved to be a unique booster pump station equipped with static frequency inverters that allow them to vary pump speed based on water usage or demand. They’ve reduced power consumption by 30-to-60 percent and if leakages are reduced by only 3 percent investment costs will be paid back within four years.

The city of Zagreb, in the Central European country of Croatia, had a problem. To supply all of the new high-rise buildings in this capital city with an adequate water supply, the municipal water system was required to run the water at high pressures. Running the supply system at high pressures required that pumps and generators consume large amounts of expensive electricity. The high pressures also put a strain on the municipal piping system, leading to a substantial loss of water through leakage. The water authorities were faced with a conundrum—how to supply high rise structures with the pressure needed without continuing to tax the economics and mechanics of the municipal water supply system.

City water supply
For any municipal water system to work efficiently, adequate water pressure has to be provided at the consumer’s location. This can be efficiently influenced by providing the proper adjustments in the supply network to reduce pressure, thus economizing distribution expenses. According to current European standards (e.g., DIN 1988), it’s sufficient if the pressure at the most unfavorable location outlet remains at 1 bar—14.7 pounds per square inch (psi) or one atmosphere. This means that at the supply point of a six-story building, the pressure should be 3.5-to-4 bar (51-to-59 psi), a value usually found in public water supply systems.

Too much pressure
The existing water supply system in Zagreb, which has a population of roughly one million, is split into three zones. The first or lowest zone is where the majority of the consumers and all of the urban pumping stations are located, and where the pressure problems exist.

Zagreb provides its consumer connections with a pressure of 5-to-7.5 bar (74-to-110 psi), which means a discharge pressure of 9 bar (132.3 psi) at the pumping stations. The undersize of the supply network and the erection of high-rise apartment and office buildings are responsible for this high discharge pressure.

In spite of the high discharge pressure, the supply situation in high-rise buildings having more than 15 stories was not satisfactory. The reasons for this are insufficient and/or oscillating incoming pressure.

In studying a way to find the optimal solution for the water supply system, the municipal authorities aimed to reduce the pressure in the system, which would result in great savings of electrical energy and reduce the leaks in the network. However, to reach a favorable result, an economical solution was needed for the supply of the high rise buildings in Zagreb.

Booster pumps
The need to reduce water pressure at the pumping stations necessitated installing all high-rise structures with “booster stations.” About 120 high rise buildings in Zagreb are already equipped with pump units from ITT Vogel, which has headquarters in Vienna, Austria. Commonly known as booster pumps, the installation of these units was required to increase the incoming pressure by 4 bars (58.8 psi) to supply sufficient water pressure to the most unfavorable outlets (generally the highest) in the building.

These state-of-the-art booster stations are equipped with static frequency inverters—the “Hydrovar Concept”—which allows them to exactly follow the demand in the building by the variation of the pump speed. Emissions of noise and vibrations from these booster stations are also far below what’s required by regulations. Direct connection of the booster stations onto the incoming water mains also allows usage of incoming pressure, thus reducing power consumption of the driving motors.

Economic efficiencies
The basic advantage of the installation of booster stations comes from the reduction of the pressure in the supply system. Upon supplying the high-rise buildings in the first zone with the booster pumps, the discharge pressure at the supply pumping stations can be reduced by at least 2.5 bar (37 psi). At some of the stations, pressure may be decreased from 9 bar (132 psi) to 6.5 bar (96 psi). The power consumption at these stations will be reduced proportionally. The savings on the quantity of water pumped unnecessarily amounts to 130 million cubic meters (m3), or 34 billion gallons, estimated due to water loss from leakage annually.

Energy savings
Based on these data, the estimate for the total power consumption for municipal water pumping prior to the booster station installations came to 60 million kilowatt hours (kWh); and, out of that, 45 million kWh for the first zone only.

After all necessary improvements and reconstruction have been made and the discharge pressure has been reduced by 2.5 bar (37 psi), the power consumption of the first zone may come down to 33 million kWh, or 26.6 percent less. Calculating with the average cost of energy—based on the Croatian currency, the Kuna (Kn)—of 0.504 Kn/kWh (0.14 deutsche marks or DEM/kWh) the savings will be: 12,000,000 x 0.5043 = 6,048,000 Kn (about DEM 1,680,000). This converts to approximately US$0.0717/kWh or US$860,567 in total savings.

Vogel’s Paul Zinniel notes, “This is not a unique problem. A lot of cities in the former Eastern block countries face this problem but Zagreb was one of those cities which, at the end, took the challenge and invested in an economic solution for the long term.”

“One of the major reasons that the Hydrovar pump unit was selected,” he added, “was that it is the most flexible and economic solution from the long-term point of view.”

Water, maintenance savings
The new booster units will be directly placed between the incoming pipe from the main and the existing installation in the building.

The power consumption of the new boosters will be about 30-to-60 percent lower than that of previous booster units, because they’ll only create (due to speed control) the actual required pressure. The lower water pressure in the system will reduce the number of breakdowns of taps (or faucets), valves, pipes and fittings, thus reducing service costs.

The water loss reduction by cutting system pressure will be very important as well. Although very difficult to evaluate economically, these losses are now estimated at about 25-to-30 percent of total water pumped through the system. If the reductions were only three percent, resulting savings could exceed those of the power savings.

Overall water consumption in the first zone comes to 110 million m3 (29 billion gallons) a year. A three percent reduction means 3 million m3 (792 million gallons) per year. Based on a cost of 1 DEM/m3 (US$0.5122/m3), the savings will reach 3 million DEM (US$1,536,728) yearly.

The city’s investment cost in this project will be paid back within four years by reducing the power consumption and the water losses by only three percent.

Of the 445 booster units supplied, about 40 percent will be installed in existing high-rise buildings for technical reasons discussed here and to safeguard the water supply. The remaining units are to be installed in those buildings where, due to the general reduction of the water pressure in the public mains, the incoming pressure is not sufficient to supply those buildings properly. And that does not take into account new buildings still under construction now.

The author would like to thank Paul Zinniel, international sales, ITT Vogel in Stockerau, Austria, and Milan Lackovic, the company’s sales representative in Varazdin, Croatia, for their input into this article.

About the author
Dr. Steve Minett heads Minett Media of Cambridge, England. He holds a bachelor’s degree from the University of Sussex in the United Kingdom and a post-graduate diploma in social science as well as master’s and doctorate degrees from Stockholm University. He writes about a wide variety of water quality and supply issues around the globe, and his work has appeared in WC&P before. Minett can be reached at +44 1954 230-250, +44 1954 232-019 (fax) or email: [email protected]


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