Robust NORD gear units ensure smooth operation in waste water treatment facility in Tunis

The waste water treatment plant in Tunis, which is operated by the Tunisian public enterprise Office National de l'Assainissement (ONAS), supplies approx. 2.5 million people in the metropolitan area with process water. To enable the bacterial decomposition of dirt particles, the waste water needs to be oxygenated by constant churning. Two large geared motors propel the mixers around the clock. In the outdoor facility, temperature can drop to -5 °C in winter, and rise to 45 °C in summer. The present text discusses the need to consider not only performance but also the total cost of ownership.

The facility contains drive technology that is nearly 30 years old and must be replaced step by step. The original suppliers have discontinued the lines, which makes spare parts hard to come by. Also, the originally installed gear units required maintenance to a problematic extent. In particular, the fluctuations in temperature have frequently affected the lubrication.

Getting a grip on heat
Searching a new supplier, ONAS encountered the German manufacturer NORD DRIVESYSTEMS that has been operating a sales office in Tunisia since 2003. In 2009, two combinations of NORD helical shaft gear units and Siemens motors were installed, and have since proven themselves during continuous operation. NORD uses a high-quality synthetic long-life lubricant for the helical shaft gear units. It 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 and tear. Each gear unit requires 210 l of oil which have to be changed after approx. 20,000 hours of operation. Other manufacturers often stipulate shorter maintenance intervals, which makes a considerable difference at a price of 10-18 € per liter. 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 60 °C. Heated oil is then pumped out, cooled by an oil cooler and returned to the gear unit. When the temperature falls below 40 °C the cooling system switches off automatically.

Drive technology in detail
Currently, two helical shaft gear units with a 132 kW performance 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, originally developed by NORD in 1981, which 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.

Tunis1.jpeg: In the Tunis waste water treatment facility, the contents of eight pools are stirred by robust drive units.
Tunis2.jpeg: NORD's low-maintenance helical shaft gear units are operated with a synthetic oil lubricant and thereby resist even high ambient temperatures.

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