Turbine Bearing Design Criteria

Introduction:

In a turbine there is a stationary part stator and a rotating part rotor. Rotor has to be supported and hence bearings are used to support the shaft and to bear the load. Bearing used in steam turbine is Journal Bearing, which is made up of low friction material, White Metal is generally used. The hollow cylinder into which the shaft is fitted is provided with some clearance into which oil is introduced under pressure of 3-5 bar. When the shaft rotates at sufficient speed and pressure of oil is also sufficient, the shaft drags the oil, thus making an oil film throughout. Shaft is not in direct contact with the bearing as oil film lies between the shaft and the bearing. This is the principle of Hydrodynamic Lubrication for bearing the load.

If the oil is introduced from the bottom of the bearing, back flow of oil will take place since pressure of oil inside is very high. So oil is introduced from the top of the bearing where pressure is low.

Grooves are cut on the upper half of the white metal so that oil spreads on the bearings properly.

Mechanism of film Lubrication:

At rest, the shaft is resting on the bearing with no or very thin oil film between the shaft and the bearing. As the shaft begins its motion, it climbs on the bearing. After sometime, an oil film of appreciable thickness is formed and a steady speed of rotation exists. When the shaft is rotating, the gap between the bearing and shaft varies from one point to other. Tapering space is formed which is known as hydrodynamic wedge and is essential for hydrodynamic lubrication.

Minimum thickness of oil film is given by t = (r*(1-ε)).
r = Radial clearance
ε = Eccentricity (Distance between the centre of journal to the centre of bearing.
As the load increases ε also increases).

Determination of Co efficient of Friction:

• f - Coefficient of Friction

• P - Load Supported by a bearing (lb), Load on Journal

• Z - Absolute Viscosity of Lubricant (Centipoises)

• N - Speed of Journal (rpm)

• p - Bearing pressure on Projected Bearing Area (psi)

• d - Diameter of Journal ( inch)

• c - Difference between diameter of Bush and diameter of Journal (inch),
Diametric Clearance

• L - Length of bearing (inch)

• K – Factor to correct for end leakage = 0.002 for (L/d) ratio of 0.75 to 2.8

f = ((473/ (1010)) * ((Z*N)/p) * (d/c)) + K

Bearing area = (L*d)

Where p is determined by the formula p= P/ (L*d)

Clearance in bearing should be small enough to provide necessary velocity gradient but also large enough to allow grits and flakes of metal to go through. So a compromise is made between the two.

(L/d) ratio should be large from the viewpoint of the side leakage but due to space requirements and manufacturing tolerances, there is a limit to its size. So a suitable size is selected.

Heat Generated in a Journal Bearing – It is due to fluid friction, it results in power loss. If the bearing temperature increases, viscosity of lubricant decreases and oil squeezes out and the bearing may seize

H1 – It is the heat generated = f*P*V.
V – Sliding Velocity = ( Π*D*(rpm))/12
H2 – Heat Dissipated by the bearing = (C*A) (Tb-Ta)
A – Projected Bearing Area = L*d (inch2)
Tb = Temperature of Bearing Surface, ° F
Ta = Temperature of Surrounding Air, ° F
To = Temperature of Oil Film
C = Heat Dissipation, ft –lb/min/ (inch2 ) of projected bearing area per ° F

Values of C can be determined by Well Ventilated bearing Graph

Also temperature at outer surface of bearing is approximately midway between temperatures of the oil film and ambient temperature i.e.

Tb – Ta = (1/2) * (To-Ta)

Bearing Design:

• Knowing load on bearing, determine length of the bearing

• Suitable Bearing Pressure p= P/ (L*d) (100 – 275 psi)

• (L/d) ratio should be 1-2

• Clearance Ratio c/d ( 0.001)

• Determine operating temperature. It should be between 80 – 140° F with 180° F as a maximum for steam turbine

• If temperature is Low
1. High Viscosity
2. High Power Loss

• If temperature is High
Film breaks down as oxidation of lubricant takes place

• Determine (Z*N)/P value (100) (Z = 2 to 16 )

• Determine coefficient of friction (f)

• Determine the heat generated in Journal Bearing (H1)

• Determine the heat dissipated by the bearing (H2)

After Thermal Equilibrium has been reached, heat will be dissipated at the outer surface of the bearing at the same rate that it is generated in the oil film.

Therefore there should be thermal equilibrium between heat generated and heat dissipated in the journal bearing

If thermal equilibrium is not indicated, Heat to be removed by artificial cooling H=H1-H2

Conclusion:

Hence we found that for steam turbine we should have journal bearings made of white metal which can reduce friction and can take or carry stress to avoid breakage of critical rotating parts by maintaining thermal equilibrium. We also found that the coefficient of friction plays an important role in the performance of bearing.

If any of the above design specifications are not satisfied, then the turbine bearing design criteria would be affected and thus the efficiency of the bearing gets affected and we will not meet our desired requirement.