Power Plant O&M engineers may be able to visualize this phenomena.
Dear Vijay,
Uneven cooling of the rotor, particularly after shutdown, also causes the rotor to contact stationary parts. After a unit shuts down, the relatively high-temperature rotor may bend solely due to the mass of the rotor and the distance between bearing supports, if left in a stationary position to cool. This situation can cause permanent shaft bending. This situation commonly occurs when the high-mass rotor at operating speed comes in contact with a stationary surface, typically caused by a too-small clearance between the labyrinth or diaphragm gland seals and the rotor. Secondarily, there may be a localized temperature increase at the point of contact, causing increased metal temperatures at the point of contact due to friction.
Following reason for shaft seizes.,
1. Uneven cooling/heating of the shaft rotor.
2. Localize Temperature increased.
3. Rubbing of rotating parts with stationary parts.
4. Uneven shaft warming caused by rubbing between rotating and stationary parts.
5. Insufficient clearance of labyrinths or rotating /stationary parts.
6. Shaft located off centre.
7. The forces produced by the impact of the large rotating rotor mass with the poorly functioning stationary seals often impress a layer of metal on the surface of the rotor.
Solution-
1. The allowed bending in 3,000-rpm turbines is up to 0.02–0.03 mm in each section. When on turning gearing, the limit is 0.05 mm.
2. You may open Thrust bearing/journal bearing and check rubbing mark and realignment the parts as per rubbing.
3. Realignment of the shaft.
4. Remove foreign material from parts.
Regards,
Prem Baboo
Article CAUSE AND PREVENTION FOR STEAM TURBINE BLADE SCALING & FOULING
SEIZURE of turbine rotor arises from following causes as stipulated in your question
1. Failure of lubricating oil system
2.Jamming of turbine gland seals
3. Flooding of condenser hot well
4.Failure of barring gear mechanism
Both of you have provided some general logical answers for common or probable causes for jamming of rotor. but my question was specific one. Turbine was coming on to turning gear after hot tripping but, every time when people thought that everything is alright, its rotor used to get jammed sometime between 2 to12Hrs after running on turning gear. There was no repeatability of behaviour and each jamming appeared to be a unique case and even time duration in all the cases were different. But in this particular case also the mechanism for jamming would always be the same old and established ones i.e. the jamming of rotor due to deformation of rotor itself or casing or combination of the two. Deformation occurs when the thermal stresses within the rotor/ casing are exceeding the maximum permissible stresses limits. This particular case became elusive as it was the combination of the two.
Dear Vijay,
What types of jamming were occurred in previous days? please specify. The barring(tuning gear) was running after that jammed. This is typical case, each jamming was differ from previous one.
Regards,
Prem Baboo
I have specified it clearly above. Please go through my explanation again. Nothing more than that. Type of jamming were for similar reasons in all the jammings.
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