Why does rotor eccentricity increase when the turning gear is engaged on a steam turbine?
- Thread starter hohoim2
- Start date
@hohoim2,
Would you please provide some before and after eccentricity readings when the turning gear is started and engaged--both when the steam turbine rotor is hot and when it's cold?
Does the eccentrity get better over time when the shaft is on turning gear and cooling after shutdown?
I can think of at least a couple of reasons for this, including the force being applied to the rotor by the turning gear being in a slightly different vector (direction) AND speed than the normal running speed which probably means the oil wedge formed in the bearing is not exactly the same as when the machine is running at rated speed. Also the weight of a steam turbine rotor is usually pretty substantial and the temperature of the steam usually puts the rotor metal in its plastic range which means it doesn't take long for the rotor to begin to flex and sag under its own weight due to the heat and slow speed that doesn't occur when the machine is at operating speed and the centrifugal forces are high and keeping the shaft straight. Even when the shaft is cold it can deform under its own wieght when its at rest for a long time--the difference is that the shaft is only in its plastic range, which is why putting the machine on turning gear prior to starting will remove most (but not ALL) of the eccentricity.
In my personal experience it's not the intent of turning gear speed/operation to keep the rotor from sagging, rather its purpose is to prevent the sag which would occur were it not slowly rotating the turbine shaft especially when the shaft is hot/warm.
Would you please provide some before and after eccentricity readings when the turning gear is started and engaged--both when the steam turbine rotor is hot and when it's cold?
Does the eccentrity get better over time when the shaft is on turning gear and cooling after shutdown?
I can think of at least a couple of reasons for this, including the force being applied to the rotor by the turning gear being in a slightly different vector (direction) AND speed than the normal running speed which probably means the oil wedge formed in the bearing is not exactly the same as when the machine is running at rated speed. Also the weight of a steam turbine rotor is usually pretty substantial and the temperature of the steam usually puts the rotor metal in its plastic range which means it doesn't take long for the rotor to begin to flex and sag under its own weight due to the heat and slow speed that doesn't occur when the machine is at operating speed and the centrifugal forces are high and keeping the shaft straight. Even when the shaft is cold it can deform under its own wieght when its at rest for a long time--the difference is that the shaft is only in its plastic range, which is why putting the machine on turning gear prior to starting will remove most (but not ALL) of the eccentricity.
In my personal experience it's not the intent of turning gear speed/operation to keep the rotor from sagging, rather its purpose is to prevent the sag which would occur were it not slowly rotating the turbine shaft especially when the shaft is hot/warm.
Thank you for the explanation. I understand now that the rpm drops causing reduced centrifugal force, which leads to increased eccentricity readings. The reason I asked this question is because I heard that if the eccentricity reading is high after engaging the turning gear, they cannot start up the turbine until the reading stabilizes. If the eccentricity reading is above 2 mils, the steam turbine will not start. Thank you for the response!
