Helical shaft gear units are more reliable in extreme conditions

In water works and waste water treatment facilities, pumps often have to process large amounts of liquid. High-performance drives are therefore necessary for smooth operation. In such applications, drive units are often subject to extreme environmental conditions, and must be designed accordingly. But, in addition to climatic conditions, the total cost of ownership needs to be taken into account.

The waster water treatment plant in Tunis, which is operated by public enterprise ONAS (Office National de l'Assainissement), has eight pools in which mixers powered by two large geared motors ensure steady circulation. This oxygenates the waste water, enabling the decomposition of dirt particles through bacteria. The plant supplies Tunis and the surrounding area with process water, covering the needs of approx. 2.5million people. It features drive technology that is nearly 30 years old and must be replaced step by step. A robust design is a must for new drives, since the units are installed outdoors and operate non-stop while the temperature fluctuates considerably. In winter, the temperature can drop to -5degC, while up to 45degC is possible in summer. Up to now this has frequently affected the lubrication of the originally installed gear units, requiring maintenance to a problematic extent. The goal was therefore to implement lower-maintenance drives that are better suited to the extreme environmental conditions.

The original equipment in the Tunis waste water treatment plant was supplied by manufacturers from South and West Europe, which traditionally have closer ties to Tunisia than German manufacturers. However, the units that need to be replaced now have been discontinued, which means that spare parts are becoming increasingly scarce and lead times are becoming longer. This fact contributed to the decision to find a new supplier, which is where drive systems expert Nord Drivesystems from Germany came into play. The company has been operating a sales office in Tunisia since 2003, which had already successfully implemented some smaller-scale projects. Drive technology from Nord, which is slightly more cost-intensive than similar products from competitors, requires considerably less maintenance, thereby minimising operating costs and optimising the availability of the plant. ONAS wished to benefit from these advantages. During the course of 2009, the drive system expert delivered two combinations of Nord helical shaft gear units and Siemens motors, which have since proven themselves during continuous operation.

Synthetic lubricant

Nord uses a high-quality synthetic long-life lubricant for the helical shaft gear units. This withstands the ambient temperatures considerably better than mineral oil - with the latter, the oil film would break down at high temperatures, resulting in increased wear. Each gear unit requires 210 litres of oil, which has to be changed after approximately 20,000 hours of operation. Other manufacturers often stipulate shorter maintenance intervals, which makes a considerable difference at a price of EUR10-18 per litre. An oil cooling system additionally increases operational safety in the Nord gear units: a temperature sensor constantly monitors the temperature of the lubricant and activates the cooling system as soon as a configurable limit is exceeded. In this application, the limit is 60degC. Heated oil is then pumped out, cooled by an oil cooler and returned to the gear unit. When the temperature falls below 40degC, the cooling system switches off automatically.

Currently, two 132kW helical shaft gear units from Nord's SK 12382VZ series are installed at the waste water treatment plant. They are combined with Siemens motors via IEC adapters. Nord's helical shaft gear units are available as two- or three-stage basic types with hollow or solid shafts, in foot-mounted, flange-mounted, or face-mounted versions - each with the same housing. Suitable for integration into limited installation spaces, they can be combined with many components from the modular Nord system. ATEX models and models with IP55 or IP66 protection are available on request for all types. All gear units are manufactured according to the Unicase concept that enables maximum performance for small gear dimensions and ensures operational safety, high efficiency, high output torques, high resistance to axial and radial loads, low-noise operation, low maintenance and a long lifetime.

Contact Nord for more information about SK 12382VZ series helical shaft gear units.