Sucking mud in the Nile river

Electrical switching & drive systems & components

Fourth Quarter 2005 Electrical switching & drive systems & components

Frans van der Merwe, customer support manager

Replacing a 600 kW drive becomes the adventure of a lifetime...

The call for assistance came from Vacon, Benelux, in Holland. The task? To remove a 600 kW Vacon variable speed drive on a dredger that had failed after becoming very wet - and then to fit a replacement drive.

A quick reaction time was necessary as the dredger keeps open the channel that feeds the Ressoirs Hydro Electric Station on the White Nile in the Sudan. With the onset of the rainy season it was essential to open the channel before the water became too deep.

The nature of the challenges this job held was not only technical. The stringent visa requirements of the Sudan had to be met and flights booked - at a time when SAA staff were on strike, affecting every available flight into Central Africa and onward. Arrival in Khartoum was only half the battle won. What followed was a 10 hour trip by 'road' (sic) to the Hydro Power Station. Added to this was a day temperature of 45 to 50°C. (It is so hot that when a dog chases a cat, they both walk.)

(At that stage I wondered what on earth made me agree to this madness.)

At the hydro power station examination of the failed drive showed that water had entered the drive via an aperture punched through the roof of the engine room for control cables leading to the bridge of the dredger, whence the dredger is operated.

The airconditioner for cooling the air on the bridge was the culprit. The drain hole through which condensation from the airconditioner was supposed to flow was blocked and the condensate instead flowed down the cable entry hole onto the drive. Water and electricity do not mix.

Removing the failed drive was a challenge on its own, with the confined space and the 400 kg mass of the drive ruled out any chance of simply picking it up and walking out with it. Rollers were made up on site and the drive, comprising two units, master and slave, rolled out. The replacement units had to be lowered by crane from the dam wall onto a transport barge in the harbour area. This had then to draw up alongside the dredger to offload the units.

Strong water currents made the transfer a tricky job. I thought to myself: "All we need at this point is for the replacement drive to go for a swim." Thankfully, the barge master did a wonderful job. Repositioning the new drive was not too much of a problem and once the drive had been mounted all that was left was merely to programme the correct parameters into the drive and then start up. Or so I thought.

During installation we discovered control wiring not reflected on the 'as built' drawing and that the programmable logic controller (plc) would not reset without interrupting the power to it. After some serious head scratching (due not only to the problem but mainly to a gratuitous gift from a previous passenger on the airline that had condescended to provide a seat on the flight), the control wiring was sorted out and the drive was able to run.

The initial tests were performed at low pump speeds with relatively clean water, as the dredger had not operated for a long period. Sediment in the outflow pipe had to be cleared before the speed could be cranked up, but purging the outflow pipe was really a soft job for the Vacon drive. Once the outflow pipe had been cleaned it was connected to the overflow pipe that takes the dredged material 35 m over the dam wall. With a pipe diameter of 450 mm, this represents a significant column of water that requires moving.

With everything going well a 24 hour test at full load was done - but not without more drama. The dredger anchor snapped and had to be repaired to prevent the dredger ending up in the fast-flowing centre of the White Nile. A brave diver risked life and limb in the dark, murky, muddy waters to fix the steel anchor rope. The rest was plain sailing.

The return journey is best forgotten - and I was tempted to kiss the earth when we landed in this beautiful and much-blessed country, South Africa.

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Electrical switching & drive systems & components

Sucking mud in the Nile river

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