Boosting energy efficiency in drinking water supply

By Bryan Orchard

The Ruhstorfer Group water supply company in Germany was looking to reduce power consumption of the pump motors in their main pumping station. With the replacement of the original pumping system in 2016 and the commissioning of a new facility, energy consumption dropped by about 20 per cent.

The Hart machine house is the centrepiece of the facilities of the Ruhstorfer Group. It employs pumps driven by synchronous reluctance motors capable of achieving optimum efficiencies even in part load operation

The vertically installed in-line pumps from KSB’s Omega series can be connected to the DIN 300 piping system in a straight line — enabling a flow-optimised connection. Variable speed systems match the flow rate to the actual demand

The Ruhstorfer Group supplies drinking water to about 12,000 households around the spa towns of Bad Füssing and Bad Griesbach. The drinking water source for the region is from the two wells around Osterholzen and Aigener Forst, and the central element to the drinking water pump station is the Hart machine house, commissioned in 1975. The Hart machine house balances the supply pressure and the fill level in two distant water towers — each with a capacity of around 2000m³. Annually, the Ruhstorfer Group delivers between 2.5 and 2.8 cubic metres of drinking water into the 75km long supply network, of which 80 per cent of this amount is supplied by the pumping station in the Hart machine house.

The number of households supplied to has also increased since 1975 along with water consumption, which has also rose at the same rate; suggesting that consumers have become more economical due to more efficient household appliances.

Energy-saving goals

Although a complete overhaul of pumping equipment was not considered necessary in 2016, the operators had their eyes on the future.

“After a long operating period, the system as a whole was starting to show its age, and we wanted to expand our focus to include energy efficiency as well as a reliable water supply,” explained Armin Bauer, plant manager of Ruhstorfer Group.

As part of an energy study, the Bavarian water supply company checked its network for possible leaks and analysed its facilities to determine their energy efficiency, of which 50 per cent of the study was financed by the German government.

Reissnecker and Eberhart, the Regensburg-based engineering company contracted to perform the study, developed a set of energy saving measures.

“The equipment in the Hart machine house was in need of modernisation in the medium term due to evidence of cavitation damage caused to the pumps and the age of the piping, which dates back to the system’s early days,” reported Bauer.

More projects from the energy study were also implemented alongside these measures, enabling the Ruhstorfer Group to obtain the DIN EN ISO 50001 certification as it continued its endeavour to achieve energy savings.

“As well as securing a reliable water supply, the principle planning criteria was energy efficiency and potential reductions in electricity costs,” said Michael Reissnecker of Reissnecker and Eberhart.

Hydraulically optimised piping geometry with matching in-line pumps The first stage was extensive planning work. The existing pumps’ cavitation damage occurred as the connecting pipework had to include bends — leading to turbulent flow. Hence, the modernisation plan included the renewal of piping system that would allow for hydraulic optimisation.

Working together with the operators, the engineering company had the entire piping system of the pump station located on one level so that pipe bends leading from floor to floor could be avoided.

The design required high-efficiency in-line pumps with discharge and suction nozzles lying in a straight line, which was met by KSB Omega pumps with SuPremE® motors and the PumpDrive variable speed system. The pump casings were installed vertically to ensure optimal hydraulic properties.

“Pumps must be capable of flexible operation” On average, the pump system comprising of three vertical high pressure pumps deliver 75 l/s into the supply network — the flow rate varies between 75 to 115 l/s. In order to determine the pump sizing, consultant engineers worked with the water supply company’s technical staff to measure the as-is volume flow rates.

“Precise data for pump selection can only be obtained if the system’s actual operation is analysed.This allows new pumps matching the optimum operating point to be selected. A central criterion in planning the system was that the new pumps could be operated extremely flexibly and achieve good hydraulic efficiency levels across their entire speed range,” explained Reissnecker.

The KSB PumpDrive variable speed system could continuously adjust the output power of the 110 kW pump motors to match the operating point, depending on demand.

“This demand is determined by the fill level in the water towers and the current consumption levels in the network. The system’s previous configuration with smaller pumps and limited speeds did not allow for demand-driven operation – during most operating conditions, two pumps provided too much output, and one pump too little,” said Alexander Hutterer, water manager for Systems Technology and Energy Management office of Ruhstorfer Group.

Pump motors offer high efficiency levels, even in part load operation

PumpMeter, the digital display and analysis unit, shows the pumps’ operating data at a glance

The tender specified IE4-class drives, which was met by the KSB SuPremE® motor range with gold-coloured housing that are conspicuous in the Hart machine house.

“Their ability to achieve high efficiency levels in part load operation makes these motors particularly suitable for applications with fluctuating demand. These pump motors use synchronous reluctance motor technology which offers a clear advantage in terms of efficiency in part load operation when compared with conventional asynchronous motors,” explained Miriam Nagler.

The IE4 requirements only refer to efficiencies achieved in full load operation, but in practice this rarely occurs — especially in applications with fluctuating demand.

“In terms of supply reliability, it was important for us that one pump alone could cover the water quantities required at the two lowest demand levels, but at the same time that the pumps could be run in part load operation without their efficiency being compromised,” explained Hutterer.

The solution implemented represents the result of detailed planning work by the engineering company.

“Contractor and pump manufacturer entered into a contractual agreement that the efficiency level promised is actually achieved in operation,” explained Reissnecker.

In order to ensure that the efficiency level offered did not turn out to be an empty promise, the customer requested KSB to simulate pump operation with the original pumps including motors and frequency inverters. This was undertaken in the certified test facility at KSB’s Halle factory using the measured hydraulic system data.

Modernisation while in operation

The installation of the new pumps in autumn 2016 was accompanied by the renewal of the 40-year-old piping system. The straight pipework (DN 300) leading to the pump connection facilitated swift modification – a task that was starting to look challenging for the water supply company.

“Pump operation could only be interrupted for a maximum of six hours. That is how long it takes before the water towers are empty and at least one pump has to begin operation,” explained Hutterer.

The engineering company developed a plan whereby the installation work would be performed in stages.

“Thanks to this pre-emptive planning, only a few brief interruptions to the station’s operation were necessary during the modification work. It was thus possible to complete the modernisation in only two months without interrupting the region’s water supply,” reported Reissnecker.

Energy savings exceed expectations

In December 2016, operators officially begun operation of the new pump equipment and piping system. Project staff anticipated savings in pump power consumption of around 100,000 kWh/year. However, annual consumption projections based on performance measurements following the modernisation indicate even larger savings, namely that consumption had dropped by over 20 per cent which raised a few eyebrows in the waterworks.

“When the power consumption was initially recorded in the first month after commissioning, administrative staff enquired as to whether consumption levels this low might not be some kind of mistake,” recalled Hutterer.

The reductions achieved in power consumption totalled 21 per cent, of which 18 per cent is attributed to the pump motors and three per cent to the hydraulically optimised piping.

“As a future-oriented water supply company, we now consider power consumption alongside our focus on ensuring reliability of supply. In terms of energy efficiency, the modernisation also featured the installation of a photovoltaic system supplying 60.9 kWp for the facility’s use. Our upgraded pumping station can now most certainly be considered a great example of a modern system,” said Bauer.

*All photos are courtesy of KSB.

Bryan Orchard is a freelance technical journalist specialising in pump and valve technologies.