A waste water treatment plant is using mixers, powered by helical shaft gear units combined with motors, to oxygenate the water for thedecomposition of dirt particles

At the Tunis waste water treatment plant, which covers the needs of around 2.5 million people, mixers powered by large geared motors are used within the eight pools to oxygenate the water and enable the decomposition of dirt particles through bacteria. The drive technology used, however, was over 30 years old, discontinued, and needed to be replaced.

The new solution needed to withstand a number of requirements – it had to be robust, as the units are installed outdoors and operate non-stop – in winter, the temperature can drop to -5°C, while up to 45°C is possible in summer. In the past this has affected the lubrication of the gear units, so the aim was to implement lower-maintenance drive solutions which would be well-suited to extreme environmental conditions.

To meet the requirements, ONAS (Office National de l’Assainissement), which operates the plant, turned to NORD Drivesystems.

During the course of 2009, the company delivered two combinations of NORD helical shaft gear units and Siemens motors. As a benefit, a high-quality synthetic long-life lubricant is used for the helical shaft gear units which is capable of withstanding 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 litres of oil which would have to be changed after approximately 20,000 hours of operation.

Furthermore, an oil cooling system 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.

Helical shaft units

There are currently two helical shaft gear units with a 132 kW performance from NORD’s SK 12382VZ series installed at the plant. These are combined with Siemens motors via IEC adapters.

The 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 components from the modular NORD system. ATEX models and models with IP55 or IP66 protection are available on request for all types.

According to the company, all gear units are manufactured according to the UNICASE concept, 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.

Nord Gear

T: 01235 534404