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What was once a key element in the VW Beetle advertising campaign of the 1960s is today a problem for many German industrial facilities: the long service life of pumps. It is not unusual for 40- or 50-year-old units to be used to pump process, cooling and feed fluids to their destinations.
With the price of electricity relentlessly on the increase, the potential for making huge savings just keeps on growing. Modern, pump systems controlled as a function of load are in a league of their own when it comes to energy efficiency. Salzgitter Flachstahl GmbH, manufacturer of flat rolled steel, recognised this potential years ago and began to exploit it step by step.
The core of the company’s vast factory premises in Salzgitter is its power plant. Here, process gas from the production of steel is used to generate electrical energy, process steam and district heating. From as far back as the 1960s, the generated district heating has been supplied to a number of nearby companies as well as the neighbouring district of Salzgitter-Lebenstedt. The four KSB pumps originally used to distribute the hot water at temperatures between100 to 140°C to the consumer installations also dated back to this period. Despite their age, the nearly 50-year-old pumps still worked reliably, but their lack of appropriate control systems meant that they were permanently operated at full load.
The actual levels of demand, however, fluctuated considerably. While on the municipal side seasonal conditions were the main reason for this (lower consumption in summer), the heating requirements of industrial customers depended more on economic developments and the growing idea of saving energy.
In the past, the frequent extreme fluctuations were dealt with by throttling the flow using valves. For Frank Adam, production engineer at the power plant, this was an unsatisfactory situation because valuable energy was being wasted. When it was finally time for a general overhaul of the old pumps, the team assessed the benefits of buying new equipment. The goal was to ensure that the power output of pump motors could be flexibly adjusted to the fluctuating levels of demand in an energy-saving manner.
Full power – lots of idle tim
“A detailed flow rate analysis showed us that the installed pumps only ran at the maximum flow rate of 1,600 m3/h for one to two percent of their total operating time per year,” says the energy and heating engineer, explaining the situation at the start of the project. For much of the time the district heating volumes that were used effectively only amounted to a fraction of the operating time. The statistics for the flow rates actually delivered are shown in Diagram 1, and these are already averaged values. Even over the course of individual days, dips and peaks in demand can be observed. Regardless of this, the pumps were permanently running at full load, which meant that a substantial amount of energy was not being used but being wasted.
Demand-based control saves energy
To control such fluctuations in demand, two options are available. The simplest method is the mechanical variant in which the flow rate to the consumer installation is throttled using a valve. From a technical point of view, this is a sheer waste of energy since the pumps utilise the full drive power although they do not deliver the full hydraulic performance. Due to the low investment costs, this method of controlling the flow rate is widel spread ? despite the unsatisfactory energy efficiency aspects. The only ecologically and economically logical choice is therefore demand-based electronic control of the pump output. In this case, the drive power is flexibly adjusted to the flow rates actually required. Frequency inverters are used for controlling such pump systems.
Potential cost savings of 50%
Calculations prior to the investment decision implemented at Salzgitter Flachstahl GmbH indicated that the use of pump systems controlled as a function of demand would result in average savings in electrical drive energy of 50%. On this basis the decision was made to invest in new pumps. Three KSB pumps type Multitec MTC D150/2 were chosen. These pumps have a head of 190 m and a flow rate of up to 580 m³/h. Each pump is equipped with a 390 kW motor controlled as a function of pressure and flow rate by a frequency inverter.
As part of the overall project, the pump specialists from KSB in Frankenthal also took care of the installation and the commissioning of the entire system. Due to the size of the drives, it was not possible to use standard controls such as the PumpDrive variable speed system. This meant that the entire control system, including the frequency inverter, had to be newly designed and integrated in cooperation with a partner company.
The frequency inverters are installed in three control cabinets close to the control station, enabling the current operating conditions to be read directly on the display at all times, for example the rotational speed, power input and pressure. Via manual intervention it is possible to make settings as required. However the system is also digitally connected to the control station and is controlled fully automatically from there. The pumps are installed two floors below on the ground floor.
Trouble-free operation
Since commissioning in 2011, the overhauled system has proven to be stable, providing trouble-free operation. In practice, the theoretically calculated savings potential has even been exceeded since then. Energy consumption has been reduced on average by 55 - 60% which means that the investment will pay for itself relatively quickly. At Salzgitter Flachstahl GmbH the full investment is expected to be recuperated by the end of 2014. From then on, the energy savings will contribute to the company’s profits year upon year.
Further projects awarded
Following this positive experience, Frank Adam is now looking for other areas that may harbour potential savings. The prospects are good, as dozens of pumps are installed in the power plant handling flow rates of several tens of thousands of cubic metres per hour. Currently the production engineer has set his sights on several condensate pumps. Preliminary investigations have shown that here, too, new pumps with an optimal design could slash electricity consumption by around half, equating to around 800 MWh per year.
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Demand-based pump control
Pump control or variable speed systems such as PumpDrive from KSB AG are self-cooling frequency inverters with a modular design. They ensure the smart, continuously variable speed adjustment of asynchronous and synchronous reluctance motors by means of analog standard signals, a field bus or control panel. Applications for pump systems controlled as a function of demand embrace all systems used for transferring water and waste water or other fluids with flow rates subject to large fluctuations.
Control systems can also be retrofitted, subject to certain conditions. The installation can be carried out directly on the motor, on the wall or in a control cabinet. For motor mounting, the maximum fluid temperature is usually limited, as otherwise there is a risk of failures caused by heat convection.
In addition to controlling the drive’s power, systems such as PumpDrive usually offer further protective functions (against overheating, dry running etc.), user-defined rotational speeds and set points, and control over various parameters (fluid level, flow rate, temperature or differential pressure). Practical experience has shown that demand-based control of the motor speed can save up to 60 percent of the consumed energy depending on the particular application.
The digital edition of the magazine is distributed free of charge to readers who meet our qualifying criteria. You can apply to receive your free copy by completing this short registration form.
