STEAM TURBINE JOURNAL BEARING AND IT’S FUNCTION
Steam turbines have journal bearings and thrust bearings. Journal bearings are at each end of each rotor to support the weight of the rotor. One thrust bearing typically is provided for the entire steam turbine to maintain the axial position of the rotor.
The journal bearings are constructed of two halves that enclose the shaft.
Journal bearings is a cylinder which surrounds the shaft and is filled with lubrication oil. it consists of a split outer shell of hard metal and a soft metal at the inner cylindrical part . In this journal bearings a shaft or journal rotates inside the bearing over a layer of lubricating oil, separating the shaft and bearing due to fluid dynamic principal. The two types of journal bearings commonly used are shell and tilting pad. The shell bearing is used in heavily loaded areas. The tilting pad bearing is used in lightly loaded areas where rotor instability (oil whip) could occur. The tilting pad bearing is made of a series of individual babbitted segments (usually four) or shoes upon which the shaft rides. During normal operation, the pads tilt in the direction of rotation.
This lubricating oil layer support the shaft preventing metal to metal contact. Oil is pumped in to the journal bearings through oil pump. When rotor rotates, lubrication oil is drawn up around the journal due to hydrodynamic effect of lubrication. When lubrication is introduced between two surfaces of rolling contact, it creates a large increase in pressure. As viscosity is exponentially related to pressure, a large increase in viscosity occur between journal and bearing creating a thin oil film preventing contact. It creates an oil wedge that supports the shaft and relocates it within the bearing clearance. This oil film wedge lifts and supports the shaft. Other than support the shaft.
Oil also carries out the heat that is generated in the bearing due to friction and heat of the shaft due to conduction. This oil is the cool in a oil cooler and supplied again to the bearing. Inner surface of this bearing is coated with a soft metal called as white metal or Babbitt. This is a tin or lead based alloy having thickness from 1 to 100 mills depending of the bearing diameter. Babbitt alloy large amount of tin with a smaller amount of antimony, copper, and lead. Babbitt lining is a sacrificial layer and provides a surface which is not damage the shaft If contact is made accidentally.
Some of the failure mechanism of Babbitt are :-
minor Babbitt imperfections can be repaired by scrapping or lapping. Major repair can be done by locally pudding or re Babbitting journal bearing also facilitate damping. More viscous and thicker lubrication oil provides higher damping properties. a suitable bearing design holds the rotor at a fixed position during transient moments such as startup and shutdown and load changes . Damping property also limits vibration of rotor.
Rotor supported by journal bearing moves relative to the bearing housing during transient. Vibration probes are used to measure this vibration/relative motion. Non contact type eddy current or proximity pickup sensor is used to measure this vibration. These sensors are mounted on bearing pedestal. To measure bearing temperature, RTD’S are embedded in the bearing and its lead is take out from the bearing pedestal for further termination.
Lube oil supply and return pipe lines are connected to the bearing pedestal. Sealing ring is provided to avoided oil leakage from the point where rotor penetrates the bearing. This oil seal ring , seal oil flow, in the same principal as labyrinth rings seal steam at gland area. Front bearing pedestal is free to expand at both the axial direction to facilitate expansion of turbine casing.
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