Redesigned drive gets sewage works into gear

Aerator gearbox breakdowns kept a sewage treatment works well below its planned capacity until Brevini designed a reliable replacement drive system.

Yorkshire Water's Rawcliffe Sewage Treatment Works processes waste water for the equivalent of 45,000 homes, treating up to a maximum flow rate of 23,600m2 and adhering to strict standards for safe disposal.

The problem Dave Milburn, Rawcliffe's Site Manager faced was, up until recently, this treatment capacity was an "ideal" rather than a reality.

Due to gearboxes breaking down on the site's ten large aerators, all ten were never working at the same time, until Brevini designed a reliable replacement drive system employing Wimes spec EFF1 efficiency motors, inverter drives and high-toque Brevini planetary reduction gears.

Jon Snaith, Senior Brevini Engineer in charge of the project, said: "We faced a number of challenges on this project, including having to design a retrofit solution that would fit in place of what was already there with little or no additional engineering".

"We first looked at the failure mode of the existing gearboxes, then designed and engineered a system to solve these issues".

Two aspects were key; the first was the physical weight of the aerator paddles, up to half a tonne each with the majority of the weight sitting on the gearbox end bearings.

The second was vibration, caused by the fact that the paddles were not perfectly balanced to start with and compounded by the effects of subsequent uneven corrosion and weathering that is typical of these items.

Said Snaith: "Vibration is the bane of all mechanical systems, and in extreme cases such as this it can weaken mountings, wear out gears and cause bearings to collapse".

"We started from the mounting upwards, selecting special anti-vibration washers and nylon-sleeve locking bolts to reduce the effects of vibration, balancing the paddles and fitting large taper roller bearings to the final output stage of the gearboxes that could take far more punishment, up to ten tonnes thrust load in this case".

All ten of the aerator drives have now been replaced, with an estimated payback period of just two years for the entire project.

The wear life even in constant operation is a predicted 100,000 hours with just simple and inexpensive oil changes required periodically.

Dave Milburn Yorkshire Water Site Manager comments: "We knew there had to be a better solution for these drives and with increasing demand on the system, we were at a stage where we could not afford to have one or two aerators always out of action for one or two weeks at a time".

"Since the refit we walk the site every day and there is noticeably less vibration from the units, which are all ramped up and down to 100% operating speed many times a day".

"The commissioning was completed within three weeks and caused no downtime thanks to it being a pre-engineered system that dropped into place of the original units, an essential requirement due to the continuous nature of this site".

The other major consideration Brevini addressed was the service factor on the gearboxes themselves, the previous ones Brevini estimated to have been specified with a service factor of 1.7, which in this tough operating environment and with very high usage was totally inadequate.

Brevini works on a service factor of 7 on this type of application, the company's experience and accurate calculations based on whole-life-costs to ensure the whole aerator drive reached its predicted wear life before requiring any maintenance or eventually replacement".

The Brevini gearboxes used were designed specifically for arduous, continuous applications such as this, the planetary design having inherent advantages over conventional helical gearbox designs because there is an equilibrium of forces on individual gears so the planet carrier is in charge of transmitting torque only.

Conventional Helical gearboxes employ a system of bearing supported gear carrying shafts where forces are concentrated.

Planetary gearboxes have a complete gear system that is in-line, so there are no high torsional loads on the bearings and gear elements - the result in an extremely long wear-life.

Sensors that monitor the oxygen content of the wastewater trigger the aerators.

The 5.5kW motors supplied are inverter driven for maximum efficiency, the drives also allow smooth ramp-up and ramp-down and an optimised 1440rev/min motor speed.

The motors drive two planetary stages at 20:1 reduction ratio to achieve 70rev/min at the output shaft, rated for a continuous output torque of 13,000Nm and a 20,000Nm peak intermittent load.

Keith Donaldson of Alfred McAlpine Business Services, which was responsible for the project management of this scheme, said: "Reliability was a paramount concern for us and we were impressed with Brevini's track record in providing this type of solution to other water authorities such as Thames Water".

"The efficiency of the system was also excellent and the fact that it could be engineered as a package and fitted directly to the existing mounting with little or no fabrication on site was crucial for us".

The savings are not just limited to those made on repair and replacement cost, Brevini planetary gear reduction stages are 98% efficient and a five-stage reduction can be powered by a relatively small electric motor working at maximum efficiency.

Using less power the package delivers an overall efficiency of over 87.5% providing substantial energy savings in the long term.

Snaith adds: "Brevini UK are expanding their solutions within the utility sector and offer a series of planetary reduction stage gearbox designs that are ideal for use in arduous applications in the water industry such as scrapers, thickeners, aerators and pumping installations".

"The company has a long-standing history of supplying OEM manufacturers of new water treatment equipment worldwide because the majority of applications lend themselves to the low-speed, high-torque characteristics that make planetary solutions ideal".

"More recently Brevini have been supporting more UK water authorities with 'engineered solutions' to troublesome and obsolete gearbox drives".

Back to top