Couplings help to drive the world's deepest dredge pumpInnovative couplings have proved vital in the design of the drive shaft for the world's longest suction pump dredge, operating with absolute reliability in some of the harshest conditions imaginable.
When a large, underwater sand mine in Florida reached a depth where the existing pump dredge was becoming ineffectual due to the depth that the sand deposit reached, the mine's operators thought that they would have to abandon their preferred dredging method and adopt clamshell-style dredging in order to extend the mine from 21 to 60 metres, and so allow another 40 years of mining. They approached DSC, one of world's foremost experts in the design and build of dredging technology to supply the equipment.
The company approached Dredging Supply (DSC) - the industry leader in dredging technology - with its brief, which got DSC to wondering if there might not be a pump dredge solution after all. DSC president Bob Wetta comments: "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 we suggested that a 60 metre dredge pump would be a possibility. We had already built a 36 metre pump successfully; we figured that this one's just a little longer."
Following a year of design, a 90 metre floating dredge was proposed that would be capable of operating uninterrupted for the next 40 years. The dredge would excavate, suction and pump to the surface 190 to 230 cubic metres of sand per hour carried through 40cm OD high density, polyethylene piping in a slurry of water flowing at 38,500 litres per minute.
Harsh underwater conditions
The mining process begins with the excavation of sand at the seabed using high pressure water cutting jets. The sand is then suctioned by an underwater pump which is driven by the longest drive shaft ever built in the dredging industry. For the new dredge pump, sourcing a drive shaft of this length, which was capable of operating in the harsh underwater conditions, was a major challenge of the project. DSC director of engineering Damon Gonzalas says: "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."
The answer was to work closely with TB Woods - a brand within Altra Industrial Motion's couplings group - to design a bespoke solution. TB Woods supplied six 6m long disc couplings which could be bolted together to form the drive shaft. The Form-Flex couplings use a hollow tubular shaft - which reduced the overall weight of the solution - with specialised taper lock mounted hubs which allow for easy field service, including axial adjustment at the pump end.
For this application TB Woods customised 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 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.
Gonzales concludes: "This project required us to build what is currently the deepest dredge pump in the world, which would then have 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. Working with the Altra Couplings group has provided us with a component that we can trust."
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