The Dredge That Refused to Work on Monday Morning

One of the first commissioning jobs I had to do for my previous employer, was on the dredge ‘Sylvia’, as she refused to work on Monday morning. And no one knew why. She was purchased by the TIWAG, as they recognised that power dams are a blocking the natural sediment transport and could potentially damage the turbines in the power dam1. To prevent anything serious happening, the power dams were fitted with ‘silt traps’. TIWAG ordered a dredge to clean up the silt trap and make some money on the excavated sand, classified by a dewatering bucket wheel on shore2.

At a power dam you would expect, that electric power wouldn’t be a problem. Well, those power dams generate power, but they don’t like to let you plug in your dredge. So, ‘Sylvia’ had to have her own power generation with a diesel engine. As power dams are usually high in the mountains, the dredge pump should preferably be placed on the ladder. The natural setup would then be an electric drive. On ‘Sylvia’ the classic and reliable electric shaft principle was chosen. The diesel engine speed is transferred to the dredge pump by the variable frequency. Depending on the generator speed, the field coil is excitated according to a controller, that can act as some sort of electrical clutch.

One thing to be aware of with an electric shaft, is the starting current of the electric motor. The electrical engineer that designed the system just worked with the nominal operating point. I wasn’t involved in the choices, but nowadays I will warn people to about this misconception.

If the pump is started on an empty pipe, even with a moderate speed, the dredge pump receives no resistance and the capacity through the pump increases enormously. The power requirements increase to a level above the nominal operating point! After several blown fuses and an incinerated generator the system was slightly modified. Also, to restrict the power surge, a discharge valve was installed to ramp up against. At zero capacity, the pump requires virtually no power.

The difficulty with ‘Sylvia’, was, that there was no possibility to hook up the discharge valve to the hydraulic system. Instead, the valve was connected to the working air compressor. It primarily served as an air source for the bubble point to measure the operating depth of the dredge wheel.

After installation, this worked fine. No blown fuses anymore. Should be fine. Until next Monday morning. Fuses tripping at every start-up. Only after several hours she would run normally. It turned out, the gate did not close completely under pressure. It required the full pneumatic operating pressure to close all the way down.

Aha! As the compressed air vessel was leaking air through the bubble point on the ladder over the weekend, there was not enough pressure to close the gate valve completely. As the mixture only needs a small opening to already draw a lot of capacity, the fuses tripped every Monday morning. Once the compressor was able to top off the vessel later in the morning, the gate valve did close completely and the dredge pump could start. A simple ball valve to close the bubble point remedied the leak and no more problems on that system again.

References

1. Kraftwerk Langkampfen, TIWAG
2. Several million tons of gravel extraction with a suction dredger, Stichweh

See also

A Reservoir of Dredging Opportunities