Have a look at this incident. I don't like to name facilities, but I am familiar with this one. It's a General Electric 4x2 power station - Two power blocks, each consisting of 2 gas turbines and boilers creating steam for a single steam turbine.
The news is pretty vague on the cause of the fire and return to service date, but I have some insight.
Steam turbines have a nasty tendency to overspeed if the load is suddenly removed. That's because the generator is the only resistance that keeps them from over-revving to destruction. Even a minor trickle of steam into an unloaded steam turbine will cause it to wind up and overspeed. This happens because the steam turbine is under vacuum, and so there is no braking action, other than a generator that is electrically synchronized with the grid.
When the steam turbine generator breaker trips, the steam valves allowing steam into the turbine also trip, in a scheme known as cross-tripping.
Because steam turbines are so prone to overspeeding, there are many safety features engineered in to prevent an overspeed event from occurring. There are six speed pickups, each with its own output signal. Three of the speed pickups are used to control turbine speed and load (and back-up overspeed protection), and three are for primary overspeed protection.
The three primary speed probes each go to a different computer module. The three modules constantly vote on whether they should activate a steam turbine trip. When 2 of the 3 protective modules decide that turbine speed is too high, a trip signal is sent to the trip relays, and the steam turbine trips. This is the electronic portion of the overspeed trip mechanism.
How this is accomplished mechanically is described below.
A modern steam turbine will have several steam inlet valves. The steam turbines in the above image would have Two High Pressure (HP), Two Reheat/Intermediate Pressure, and a single Low Pressure steam inlet valve. It gets more complex yet, because each of these is actually a pair of valves - a control valve and a stop valve. So there would be a Left HP control and stop valve, a Right HP control and stop valve, a Left Reheat Intercept and Stop valve, a Right Reheat Intercept and Stop valve, and a LP control and Stop valve.
Below: The inside of a steam turbine control valve. The control valve and seat are at the bottom. Steam pressure helps to seat the valve shut.
Below: Steam turbine stop valves, with hard-had for size reference. The valve seats are at the far end.
The control valves are intended to throttle the steam flow, while the stop valves are intended to positively seal steam from flowing into the turbine. Each of these valves has an enormously powerful spring forcing them shut. They are opened against this spring pressure by a dedicated hydraulic system that operates at a pressure of 1600 psi. The hydraulic oil forces a piston upwards against spring pressure. This piston is attached to the steam valve, and opens it. To trip the steam turbine, hydraulic oil is rapidly dumped back to the tank, and spring pressure will slam each of the steam valves rapidly shut.
The above power plant apparently had a steam turbine hydraulic leak. The hydraulic oil likely sprayed out at 1600 psi, which caused it to become a mist. This oil then sprayed onto a very hot steam turbine and ignited. It's an unusual event, because nearly all of the hydraulic lines are metal, so failure would almost have to be a manufacturing defect, or vibration induced. Not anyone outside the company will likely ever know the root cause.
On the bright side, I wouldn't expect that an incident like this would cause too much damage or keep the unit offline for too long. Some electrical cables, instrumentation wiring and the hydraulic skid probably had some damage, but the turbine itself should be fine. They will probably have to flush and re-commission the hydraulic system, but that's not a show-stopper.
In short, it looks a lot worse than it is. That's just a hundred gallons or so of hydraulic oil burning.
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