Articles & Case Studies

Competitive advantage: efficient watering of vegetable crops

Posted: Friday 22nd July 2011

In agriculture efficiency is of the essence: the careful use of soil, water and energy follows ecological as well as economical dictates. In many countries the exploitation of natural resources is also subject to legal regulations. The Barleben agricultural co-operative in the Federal State of Saxony-Anhalt, Germany is optimising local water resources for vegetable production irrigation using a new pumping station designed by KSB AG.

Central to the irrigation system are six Etanorm E 080/250 G10 pumps controlled by a PumpDrive system with frequency inverters that ensure optimum control of the varying, demand-based pump output. Output ranges between 50 and 900m³/h and it is understood that the Barleben pumping station currently provides the highest frequency inverter-controlled output in the world.

Located to the north of the Hartz mountains, the agricultural area around Barleban does not get sufficient rainfall to support vegetable crop production without irrigation. Using arrtifical methods, the members of the Barleban co-operative grow apprpoximately 10,000 tonnes of vegetables annually, but the success of the crops and financial security are wholy dependent on an energy-efficient and reliable irrigtation infrastructure. By law, only vegetable crops may be irrigated in Saxony-Anhalt and the Berleben co-operative cultivates vegetables on 600 hectares of land using a 5 km network of undergound pipes and above-ground mobile steel pipes fed by a 4,500m³ water holding tank.

Between 2006 and 2008 the Barleben vegetable growers took a critical look at the condition of their 35 year-old irrigation system, and decided it was time to invest in a better priced and more energy-efficient water supply system. The old pumps were designed to handle a far larger irrigation area and their power consumption was unnecessarily high. Coupled with this, the antiquated control system could only be operated manually and during periods of extremely dry weather the pumps would deliver far higher volumes of water than permitted by law.

Based on their specific requirements the brief demanded an energy-efficient pumping system would have to supply the maximum flow rate of 900 m³/h required by the extensive irrigation systems. Also the pumps would have to operate energy-efficiently at near zero minimum output. Further requirements included operator-free functionality, self-priming and the ability for the system to be capable of keeping the water pressure constant automatically and independently of the downstream irrigators. This would enable the growers to withdraw water without any additional equipment even at times when the system was not in operation.

Having evaluated presentations from several pump manufacturers, a design from KSB was selected by the Barleben vegetable growers on the grounds that it met their requirements both technically and economically. The design consisted of six single-stage pumps, dry-installed above the basin – similar to the pumps being replaced. However, the pumps differ from their predecessors in that they meet the demand for efficiency even at minimum pump output, i.e. they run near the point of best power consumption even when there is less land to be watered.

”We often have only one of our two hose-type irrigation systems in operation at a time,” explains Hans-Ulrich Zander, master electrician for the Barleben co-operative. “It supplies our fields with water at a rate of 75m³/h. The KSB solution is ideal for this typical requirement, just as it is for discharging volumes ten times this figure. Compared with the old solution, the energy saved per volume unit of water pumped amounts to more than 50%. At approximately 1.5 kW/hour electrical power per cubic meter of water pumped, which is the average power consumption we calculated for the years from 2005 to 2008, our power consumption dropped to 0.73 kWh in the first year of operation.”

The solution:

KSB’s solution centres around six wear-resistant cast iron Etanorm E 080/250 G10 pumps with bronze impellers. With a power output of 45 kW each, the maximum flow rate is 900m3/h, typically at 7 bar. The pumps’ speed is controlled by a PumpDrive unit employing frequency inverters. Based on the operational parameters making up the pump characteristic curve, the frequency inverters ensure that the motor generates no more than the electrical input needed to supply the demand-based pump output. The motor always runs in the most energy-efficient range. The variable speed range realised for the application is from 41- 50 Hertz, and at pressures between 6.5 and 7 bar, this corresponds to a capacity of between 50 m3/h and 900 m3/h, i.e. almost twenty times the lower figure.

Although multistage pumps with their steeper characteristic curves are generally easier to control, KSB decided in favour of single-stage pumps for this application. The reason for this was because single-stage pumps have a larger impeller passage than multistage pumps. Considering the condition of the slightly contaminated water being pumped, single-stage pumps were less likely to be affected and be more reliable. If the power input rises from a lower to a higher value whilst the pumps are running at their maximum output, the drop in water pressure will cause the next pump to be switched on. PumpDrive makes sure that the speed, and hence the output, of all pumps in operation is the same. As output demand reaches the top limit, all six pumps will run at full capacity.

The system also included new 500lt pressure vessel connected in parallel to the main pipeline for the purpose of making up for any minor leakage that could occur. The vessel is filled with water to1/3 of its capacity, the rest being air. Pressure surges caused by the changes in the pump output volume are for the most part compensated by the Etanorm pumps themselves, but the volume of the vessel is large enough to make up for the pressure losses caused by any leakage.

The water runs from the tank into the pumps through a direct pipe connection. KSB deliberately designed the system without suction power-reducing valves, swing check valves or other types of resistance within the fluid flow. To prevent air pockets forming on suction and discharge sides, in particular during pump standstill, each of the six Etanorm E 080/250 G10 pumps is connected to an evacuation system. This consists of a liquid-ring vacuum pump with a collecting line and separate connections upstream and downstream from the impeller which are connected in parallel. Although the evacuation loop is a closed loop, it makes up for any leakage losses occurring. The Etanorm pumps are completely filled with water, ready for instant start-up and are only emptied at the end of the irrigation season.

A further efficiency and availability enhancing feature is the presence of a control valve fitted to the suction pipe of each pump. This makes it possible to partially open the valve when an additional pump is being started, drawing in the water even at a relatively low pressure. In contrast, the usual valves, which typically are either ‘open’ or ‘closed’", would require such a high pressure during pump start-up that the system would come to harm.

Experience and findings

From the day the pump system was commissioned, the Barleben operators have felt sure their decision was right. “The amount of energy saved during the first growing season alone covered the cost of the system, so theoretically it as already paid for itself.paid for”, reports Jürgen Hermann, Manager of the co-operative. “Our solution is proving to be a success both at minimum irrigation duty and when running at maximum capacity and if there is a problem, KSB’s service specialists put things right.”

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