Hydroelectric rotor inspection - how often?

[Skogsgurra]

Hello again.

A little hydro-electric power station with a 1.5 MW synchronous generator has a routine where they take the whole unit apart every year to inspect it. They lift the rotor to see if anything has happened and they also do some cleaning.

I asked them if they ever found anything wrong during these inspections or if the rotor was very dirty? No. Never found anything, never adjusted anything, never changed anything. But they were a bit worried about the bolt-heads. They were getting worn round from these 12 month inspection intervals.

My question: How often would you recommend that the rotor is lifted out of the generator for inspection? Each year? Every fifth year? Never?

Is there any difference between vertical and horizontal machines in this respect? What could happen between inspection intervals?

 

[wbd]

What exactly are they inspecting for? I have 12 hydros and we don't pull the rotors for any inspections. I have both horizontal and vertical units. We do check air gaps and meggar the rotor.
If nothing is ever found and they really enjoy pulling the rotor, why not make it a 5 year event? I'd be more worried about the wear and tear on the bolts, especially if they are torqued to any appreciable load. The threads could be getting stretched and reducing the holding.
On my vertical units, the rotors obviously come out when the runner needs to come out. At that time, we will inspect and clean.

 

[Skogsgurra]

Good question! wbd. I do not think that they know really.

Any other opinions on this? Is environment a factor?

 

[buzzp]

We also never pull anything except sometimes on 5 year intervals depending on testing results and unit history. Sometimes we may pull it to check wedges and such but this is certainly not done on an annual basis (7 units from 6MW to 66MW).

 

[QCE]

I know for a fact that we have a plant with 4 units about that size and we have never pulled the rotors since it has been in service since the late 80's. I don't think it is necessary unless a problem is detected.

It would make more sense to check the turbine and bearings on an annual basis. Don't monkey with the generator.

 

[MachineryWatch]

Seems like the annual inspection is a little overkill. I would recommend that they invest in some on-line condition monitoring instrumentation. I'm sure the cost of it would be more than offset by the reduction in the cost of the inspection. It would still give them much of the information they currently only feel can be garnered by visual inspection. In fact, if the visual inspection is considered indespensable, there are many miniature video inspection systems available that would still allow much visual inspection, without disassembly. Not only that, but the trending of the measured values is often a better indicator of machine health than the visual inspection because it measures the dynamic, rather than viewing the static.

The monitoring system could include shaft displacement, acoustic and accelerometer for cavitation concerns. There is also many suitable pressure transducers available and many on-line generator sensors available to help determine electrical condition in addition to the mechanical and hydraulic concerns.

Skip Hartman

http://www.machinerywatch.com