Before deciding on inspection, there are some factors that would definitely warrant opening the unit:
1. Was the coast down time normal?
2. Did the turning gear engage and the unit cooled down nomally? Abnormal vibrations during coastdown?
3. Is the shaft free to turn with no binding or strange noises?
4. Any debris found in the oil?
5. Any debris found in the exhaust?
6. Any opportunity to inspect the last stages without opening?
7. Any opportunity to borescope?
Depending on the criticality and downtime cost i would first exhaust all alternatives to do an educated assessment.
If the manufacturer's recommendations are that after such event the unit shall be opened and inspected then there is no room for argument. But if the instructions are silent or it is at the operator's discretion, I would try to avoid fixing something that is not broken.
10MW is much smaller than the machines my experience is on, but my gut feel is 'no, I wouldn't roll it without inspection'. The manufacturer is the best one to make the call. My guess is that they would play safe and require inspection.
To add to Tick's list, were there any concerning vibrations during the overspeed event? Did it spend any time in a resonance range? How long was the duration of the overspeed?
----------------------------------
If we learn from our mistakes I'm getting a great education!
Sometimes the decision is already made, the folk in charge just want someone to point the finger at if things go wrong. Under those circumstances I think you absolutely did the right thing by refusing to lend credence to what sounds like a pretty reckless plan.
----------------------------------
If we learn from our mistakes I'm getting a great education!
Before restarting the unit, it would seem sensible to:
- Figure out what caused the overspeed event in the first place.
- Figure out what automatic controls failed to stop the unit before it got much above 100pct, and fix them.
I suggest you write down a 'contemporaneous account' of whatever information exchanges you had about the overspeed event, and keep it for a long time. It smells like the kind of thing that eventually, inevitably, ends up in court, because of the amount of money involved.
E.g.: For the Diesels that my outfit sells, a single overspeed event ends your warranty. Period. Not negotiable. End of discussion.
Perhaps the STG manufacturer has a similar policy about overspeed events, in which case the perpetrators have career incentives to sweep such events under the rug, and to find someone else to throw under the bus when it arrives.
Most of this conversation has been about the STG but I'd also be concerned about the generator especially if it is a 2 pole machine with retaining rings. At that speed the forces on the retaining rings and rotor slot wedges would have been tremendous. Better to have a look.
Thnks rmw. the generator would be my major concern too. 2pole air cooled w/rings
I think you might gather my question for recomendations following event was not for the excellent confirmations of items to inspected, but as a self check for me since it seemed I was the only one concerned. Since I've declined to assist, Im pretty sure I will not hear from them what they do one way or the other
Sometimes a man has to do what a man has to do. You gotta live with yourself. If one of those retaining rings comes unglued and shreds the generator, at least it won't be on your watch. It looks like they had their minds made up but needed a scapegoat in case they were wrong.
150% overspeed! Wow! Didn't the machine have a trip governor to interrupt the steam supply at the turbine? I've never seen even very small steam turbines without one.
I agree with previously stated concerns that the generator is a greater issue--air-cooled generators can catch fire quite readily.
You certainly were wise to stay clear of that obvious set-up. Any short-term compensation would almost certainly have been quickly overshadowed by less than pleasant events.
