During each startup the rotor accelerates a little bit above turning gear speed of 6 rpm. The turning gear will then become uncoupled from the rotor. It is only after the rotor speed reaches 0 rpm will the turning gear rengage. The process then repeats itself. In 2006 the turning gear was damaged and required an overhaul but since then, the problem persists. Has anyone seen this issue? What could be driving this?

- What was the cause for the turning gear overhaul?, and what are the impacts to the turning gear?

- what is the set point speed (rpm) of your turning gear? what is the driver of your turning gear? (oil or moto


what i think, it is impossible to have rotor rotates 6 rpm faster than turning gear if they are still coupled, once the governor opens further, rotor speed increases and higher than turning gear and they will be uncoupled. So the rotor will accelerate to its 1st set point and so on, and turning gear will not in service.

so if rotor and turning gear uncoupled during acceleration, it is normal and it will not trigger trip of turbine.

We also have experienced a similar problem during coastdown, the machine would stay at 35-40 rpms.
The problem is most likely to occur when condenser pressure is below 0.60 inHga . This is more prevalent during winter months.
The problem has been blamed on excessive seal steam flow and very low backpressure. Another recommendation is to run the vacuum higher during startup could help draw the unit down to gear speed.

That would be some serious seal steam leakage to drive your unit past the turning gear over-running clutch. The turbine will always spin down over a greater time with vacuum as there is no windage add mass and the rotor will slow (very simple). If it is mandatory to stop the turbine then secure seal steam / break vacuum. If you cannot get the shaft to stop and you have verfied steam shut off to the unit and HRSGs on complete bypass to condensor, no need to worry. This is natural and will take its time to come down, I do not suggest forcing the unit to come down to turning gear speed unless there are issues requiring a stopped shaft.

How much time are you taking to come down to gear w/vacuum? Once in gear is your clutch engaged properly or bumping in and out due to mechanical issues? What make is your turning gear Koenig or Nuttall? Have you tested the gear motor rpm, amps etc..

How do you maintain vacuum, pumps or ejectors? If your D11 is shutting down do you have an aux steam header supply for the seal steam, ejectors? Otherwise you have to break vacuum and the shaft is going to go to gear.