Johannesburg, South Africa — ESI-AFRICA.COM — 22 March 2011 – It will cost South African national power utility Eskom Holdings R3 billion to replace the turbine generator that spun out of control and burst during a maintenance test near Witbank.
Revealing this here, Eskom emphasised that almost worse than the monetary damage was the loss of 600MW in generating capacity for more than a year.
It would take this long to replace the unit, said Eskom head of communications Hillary Joffe. Unofficial sources reckoned it could take 18 months before a new unit “’ which would have to be ordered from and specially manufactured by French suppliers “’ could be put into operation.
Photographs taken for the purposes of investigation of the damage shortly after the accident have since been widely published on the internet and show that the gigantic unit at Duvha, one of Eskom’s biggest power stations, was irreparably damaged by the explosion.
According to Joffe, an over-speed test on the unit was conducted on the evening of 9 February. The turbine generator unit normally spins at 3 000 revolutions a minute when it is linked to the national power grid and the network operated at its normal 50 Hz voltage level.
But during the test the unit is disconnected from the national network. A valve controlling an enormous flow of steam to drive the turbine is then gradually opened to raise the pressure of the steam on the turbine. The speed at which the turbine rotates increases as the steam pressure rises.
Joffe said the system had a bolt that started to cut the rotational speed of the turbine as soon as it went 10% over the design speed of 3 000 revolutions a minute. This time the safety mechanism had failed for some or other reason.
As a result the turbine spun faster and faster. The rotational speed increased too quickly for anything to be done before the enormous machine burst with a tremendous explosion, with debris scattering in all directions.
The explosion ripped off several steel plates in the roof of the turbine hall, and pieces of shrapnel made hundreds of holes in the remainder of the 30-metre-high roof.