A sand mine reached a depth where the existing pump dredge became ineffectual. The solution was a major challenge
When a large underwater sand mine began to reach depths that had previously been inaccessible to pump dredging, the mine's operators thought that they would have to commission a clamshell-style dredge to keep the mine operational.
However, Dredging Supply Company believed that it could build the world's longest suction pump dredge using couplings from TB Wood's of the Altra Couplings group to form the drive shaft.
The sand mine in Florida had reached 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 adopt clamshell-style dredging in order to extend the mine from 21 to 60 metres; allowing for another 40 years of mining.
At the time the deepest pump dredge that DSC was aware of was one that it had built which was capable of mining to depths of 36 metres. However it was confident that, given the right drive components, a 60 metre pump was possible.
Floating dredge
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.
The mining process begins by excavating the 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. Sourcing a drive shaft of this length, which was capable of operating in the harsh underwater conditions, was a major challenge of the project.
Couplings
TB Woods supplied six 6 metre long disc couplings which would bolt 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 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 that is essential in an application that will be subjected to unavoidable external forces.