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Handbook of Turbomachinery Second Edition Revised - Ventech!

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The disadvantages associated with the tilt pad thrust bearing are the<br />

complexity and wear <strong>of</strong> the additional moving parts (linkages and pivots).<br />

Also, the initial cost <strong>of</strong> tilting pad bearings is greater than that <strong>of</strong> the fixedtype<br />

bearings. Furthermore, because <strong>of</strong> their relative complexity, a higher<br />

level <strong>of</strong> skill is required for their maintenance and repair. Therefore, when<br />

designing bearings for large steam turbines, the choice as to which type <strong>of</strong><br />

bearing is most suitable is not clear-cut but depends on the specific<br />

requirements <strong>of</strong> a particular application. Usually, combinations <strong>of</strong> fixedand<br />

tilting pad-types <strong>of</strong> bearings are incorporated in a given turbine<br />

generator unit design [119].<br />

Steam Turbine-Bearing System Problems<br />

Foaming<br />

Foaming <strong>of</strong> the oil may result when air is entrained by the oil and is<br />

thoroughly mixed by agitation and churning. Normally only a thin layer <strong>of</strong><br />

foam will be present on the oil surface due to turbulence in oil return lines,<br />

especially as they discharge into the reservoir. High-quality oil should have<br />

good resistance to excessive foaming, and the air bubbles formed on the<br />

surface should break up quickly. Foaming increases the rate <strong>of</strong> oil<br />

oxidation, since more oil is exposed to the entrapped air than would be in<br />

nonfoaming situations. Oxidation products, in turn, promote the formation<br />

and stabilization <strong>of</strong> the foam. Contaminants in the oil also reduce the<br />

capability <strong>of</strong> the oil to resist excessive foaming. Excessive foam accumulation<br />

can result in loss <strong>of</strong> oil by overflow or seepage. In some instances, foam<br />

weeping from generator bearings has been drawn into the electrical windings<br />

or onto the collector rings to cause insulation damage, short circuits, and<br />

sparking. ASTM Test Method D 892-74, ‘‘Foaming Characteristics <strong>of</strong><br />

Lubricating Oils,’’ can determine the foaming properties <strong>of</strong> oil [120].<br />

Bearing System Contamination<br />

The remaining factor that influences the rate <strong>of</strong> oil oxidation is the amount<br />

<strong>of</strong> contamination present in the oil. Metal particles and water contaminants<br />

promote oil oxidation and contribute to formation <strong>of</strong> foam, deposits, and<br />

sludge.<br />

Particle Contamination<br />

Pulverized coal, fly ash, airborne dirt, and sand can enter the lubrication<br />

system through openings at the bearing housings and at the oil reservoir<br />

Copyright © 2003 Marcel Dekker, Inc.

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