2022 December 2nd Week VISTA Technical Knowledge: How to Define the Thrust Bearing and Radial Bearing for Steam Turbine?

Turbine thrust bearing and radial bearing and common problems of turbine bearing in power plant.

1. Turbine thrust bearing and radial bearing

The thrust bearing of the steam turbine mainly bears the axial thrust of the rotor and determines the axial position of the rotor in the cylinder.

When the rotor rotates, the radial bearing generates an oil film, which mainly bears the mass of the rotor and the centrifugal force caused by the unbalance of the rotor mass, and determines the radial position of the rotor.

Thrust bearing is a special bearing used to bear axial force, which is the bearing with shaft parallel direction force, also known as thrust bearing.

Thrust ball bearing is a kind of separated bearing. The shaft ring and race can be separated from the components of cage and steel ball. The shaft ring is a ring that fits with the shaft, and the race is a ring that fits with the bearing seat hole, and there is a gap between the race and the shaft. The thrust ball bearing can only bear the axial load, the one-way thrust ball bearing can only bear the axial load in one direction, and the two-way thrust ball bearing can bear the axial load in two directions; The thrust ball bearing cannot limit the radial displacement of the shaft, and the limiting speed is very low. The one-way thrust ball bearing can limit the axial displacement of the shaft and the housing in one direction, and the two-way bearing can limit the axial displacement in two directions.

The thrust roller bearing is used to bear the combined axial and radial loads dominated by axial load, but the radial load shall not exceed 55% of the axial load. Compared with other thrust roller bearings, this kind of bearing has lower friction coefficient, higher rotating speed and self-aligning performance. The roller of 29000 type bearing is an asymmetric spherical roller, which can reduce the relative sliding of the roller and raceway during operation. The roller is long, large in diameter, large in number, and high in load capacity. It is usually lubricated with oil, and can be lubricated with grease in some low speed cases. Priority shall be given to selection in design; 80000 type thrust cylindrical roller bearing, 90000 type thrust tapered roller bearing and AXK type thrust needle roller bearing can withstand unidirectional axial load, which is much greater than the axial load capacity of thrust ball bearing, and has large rigidity and small axial space occupation. Thrust cylindrical roller bearing and thrust needle roller bearing are suitable for the occasion with low rotating speed. The rotating speed of thrust tapered roller bearing is slightly higher than that of thrust cylindrical roller bearing.

The radial bearing is the bearing that mainly bears the radial force.

The part of the shaft supported by the bearing is called the journal, the part matching the journal is called the bearing bush, and the whole cylindrical bearing bush is called the bearing sleeve. The part of bearing bush assembly is generally called shell, the upper part is called bearing cover, and the lower part is called bearing seat. The cover and the seat are connected by studs. The joint surface of the two is positioned by a seam or a pin. Shims with different thicknesses can be placed to adjust the bearing clearance. The joint surface of most bearings is horizontal and inclined to adapt to the requirement that the load direction is close to perpendicular to the joint surface. In order to facilitate lubricating oil to enter between friction surfaces, the bearing cover is provided with an oil injection hole, and the bearing bush is provided with an axial oil groove for distributing lubricating oil. Bearing covers and seats are mostly made of cast iron, and those bearing heavy loads are of cast steel or steel plate welded structure.

2. Common problems of turbine bearing in power plant

2.1 Common problems of thrust bearing

When the thrust bearing is damaged, the axial displacement data will increase, and the tungsten temperature of the thrust pad will increase, leading to an increase in oil temperature. In serious cases, the thrust pad will smoke. The main reasons are as follows:

(1) During operation, instantaneous load rejection of the unit or incorrect closing of the intermediate pressure valve may lead to burning of the thrust bearing.

(2) The steam quality is not high, and scaling occurs to the blades. This may also occur when cleaning the steam blades due to too little cleaning effort.

(3) In the process of turbine operation, excessive load or water hammer occurs, and incorrect treatment methods for steam temperature reduction will lead to burning of thrust bearing.

(4) When using the oil system, the impurities in the oil system were not cleaned timely due to inadequate management, which led to the destruction of the oil film of the thrust pad.

(5) The clearance of diaphragm steam seal is too large, which increases the axial thrust.

(6) When the generator rotor is not properly grounded, the axis current will cause electric corrosion of the thrust pad.

(7) When the unit is reconstructed, there is a relatively large deviation between the theoretical calculated value and the theoretical value of the actual axis thrust, which leads to the expansion of the size of the oil return and oil inlet pipelines not meeting the standard and destroys the thrust bearing.

(8) The thrust pad, journal lift and thrust disc parallelism are not up to standard.

2.2 Air bearing excitation

In general, the characteristics of air bearing excitation are as follows:

(1) The steam density at the inlet of the supercritical steam turbine is too high, and the steam pressure is too high. Because the energy provided by the interference ability of the steam vortex is directly related to the steam density and the excitation point on the trajectory, the higher the steam density, the easier the gas flow excitation occurs.

(2) The rotor of the unit is long, with large capacity, large rotor deflection, reduced rigidity, and low natural vibration frequency. When the excitation frequency is close, there will be vibration.

(3) When the load of the unit increases to a certain value, steam self-oscillation will occur. If it is not handled in time, the vibration will disappear after the load decreases to a certain number.

(4) Air flow concussion usually occurs on rotors with large capacity? Air flow excitation is the most serious.

2.3 Bearing vibration

Bearing vibration is a common problem of steam turbine in power plant. If it is not handled in time, it will lead to tripping accident of steam turbine. Generally, the main reasons for bearing vibration are as follows:

(1) Because the inlet steam pressure of supercritical steam turbine is too high, the steam density is high. The density of steam and the excitation point on the track directly affect the interference ability of steam vortex, so the higher the steam density is, the more serious the air flow excitation will be generated.

(2) The rotor installation is unqualified, and there is deviation in the center of the dynamic and static parts. There is a large gap between the choke ring and the rotor compound ring on the circumference. When the steam passes through these small positions, the circumferential pressure will lose balance, increasing the probability of steam exciting the rotor.

(3) Because the rotor of the unit is relatively long and has large capacity, the rotor has relatively large deflection, poor rigidity, and the natural vibration frequency of the rotor will also decrease. When the frequency of steam acting on the rotor is close, vibration will occur.

(4) When the unit load increases to a certain value, steam induced vibration will occur. If the treatment is not timely, the vibration will gradually disappear when the load decreases to this value.

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