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Ancillary Equipment

TB Wood’s Form-Flex couplings help drive world’s deepest dredge pump

When a large underwater sand mine in Florida began to reach a depth where the existing pump dredge was becoming ineffectual, the mine’s operators thought that they would have to abandon their preferred dredging method and commission a clamshell-style dredge to keep the mine operational. The operators were looking to extend the depth of the mine from 21m to 60m, to allow another 40 years of mining.

However, Dredging Supply Co. (DSC), experts in the design and manufacture of dredging technology, believed that they could build the world’s longest suction pump dredge using couplings from TB Wood’s, part of the Altra Couplings group, to form the drive shaft. At the time the deepest pump dredge that DSC were aware of was one that they had built, capable of mining to depths of 36m. However, they were confident that, given the right drive components, a 60m pump was possible.

Bob Wetta, president of DSC, commented: ‘During the design brief it was obvious that clamshell dredging was only being considered due to the depths that will eventually be required of the apparatus. We are always keen to supply our customers with their desired solution so suggested that a 60m dredge pump would be a possibility. We had already built a 36m pump successfully, and we figured that this one is just a little longer.’

Following a year of design work, a 90m floating dredge was proposed that would be capable of operating uninterrupted for the next 40 years. The dredge would excavate, suck up and pump to the surface around 190–230m3 of sand per hour through 40cm (outside diameter) high-density polyethylene piping in a slurry of water flowing at 38,500 litres/min.

The mining process begins by excavating sand from the mine bed using high-pressure water cutting jets. The sand is then lifted under suction by an underwater pump driven by the longest drive shaft ever built in the dredging industry. Sourcing a drive shaft of this length, and which was capable of operating in the harsh underwater conditions, was the major challenge of the project.

Damon Gonzales, director of engineering with DSC, said: ‘Any engineering project of this size involves a number challenges. One of the key issues we had was designing a drive shaft which could transmit the 450kW from the electric motor on the dredge’s hull to the underwater pump on the sea bed. Not only would this be the longest ever drive chain built for a mining dredge, we also had to modify it for operation in the vertical position. In the end we worked closely with TB Wood’s to design a bespoke solution.’

TB Wood’s are one of the industry-leading brands that make up Altra Industrial Motion’s Altra Couplings group. They supplied six 6m long disc couplings which are bolted together to form the drive shaft. The Form-Flex couplings use a hollow tubular shaft – which reduces the overall weight of the solution – with special taper lock mounted hubs that allow for easy field service, including axial adjustment at the pump end.

For this application, TB Wood’s customized the hubs with a unique flange disc pack and tapered bushing mounting to facilitate ease of installation and adjustment to the underwater pump impeller. Because of the abrasive nature of the material being pumped, axial adjustment of the impeller is necessary when it becomes worn down. The couplings are able to handle parallel offset and angular misalignment, which is essential in an application that will be subjected to unavoidable external forces.

Damon Gonzales continued: ‘This project required us to build what is currently the deepest dredge pump in the world; it then had to operate in an abrasive underwater environment until 2049. Any unscheduled maintenance would be very costly in terms of both man hours and downtime, which means that a vital component such as the drive shaft needs to be reliable. Looking back on the project, I am extremely grateful to Altra for providing us with a component that we could trust.’

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