Eskom broken generator R3bn setback
It will cost Eskom R3bn to replace the turbine generator that spun out of control and burst during a maintenance test near eMalahleni (Witbank).
But almost worse than the monetary damage is the loss of 600MW in generating capacity for more than a year.
It would take more than a year to replace the unit, said Hillary Joffe, Eskom’s head of communications. 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 overspeed test on the unit was conducted on the evening of February 9. 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 has a bolt that starts to cut the rotational speed of the turbine as soon as it goes 10% over the design speed of 3 000 revolutions a minute. This time the safety mechanism failed for some other reason. (see below to find out what actually happened at Duhva)
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. Pieces of shrapnel made hundreds of holes in the remainder of the 30-metre-high roof. - Sake24
Amazing how pics like these can get out even after Eskom put a blanket of secrecy around the whole incident ... Check out the holes in the roof - it's like a bomb went off - and that boken driveshaft lying on the floor is +-350mm diameter.
ESKOM blows up a turbine - R3Billion later "So I found out what happened at Duhva.
They were doing a test of the turbine overspeed protection system, and in short, the protection did not kick in. Conventional wisdom tells me that there should be a better way to test a protection system than to try and destroy the turbine and see if it feels like protecting itself, but that’s basically what they did.
The turbine has a governor valve which controls the amount of steam coming into the turbine in order to keep it running at the right speed (3000 rpm for our grid frequency) and then it has a main isolation valve to shut the steam off completely. The protections systems (of which there are 3 independent systems, and a dude with his finger on the emergency button) are supposed to close this main isolation valve in a fraction of a second when the turbine overspeeds.
So they get ready for the test, they dump a helluva lot of steam onto the turbine, speed starts going crazy, it went from 3000 RPM to 4500 RMP in ten seconds (they are generally only designed for 10 to 15% overspeed, all three protection systems should have kicked in by the time you get to 110%). Anyway, all three systems failed, and the dude with his finger on the manual trip button wasn’t at his post. So the result was a big bang, some fire and a lot of steam going where it shouldn’t go.
Scary thing is Duvha has a shared turbine hall. All six units are placed in one long straight stripe, with no missile shield between them. And if you look at the third last pic you can see how big that shaft is, if that landed on another turbine it would have destroyed that too. They are very lucky they didn’t lose the entire station.
So anyway, what gets reported in the news? “Unforeseen maintenance” at one of the units at Duvha requires it to remain shut down for 18 months. Understatement of the century in my book. But you shut off the containment ventilation system at Koeberg for one hour and a radiation alarm goes off, then it’s a front page news national crisis. I give up."
Source: http://www.4x4community.co.za/forum/showthread.php?t=79395
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