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Handbook of Turbomachinery Second Edition Revised - Ventech!

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covers and doors. Pipe scale, rust, and other metal particles may enter the<br />

system during construction and remain there even after the postconstruction<br />

lubrication system flush. Fine metal particles act as a catalyst to the oil<br />

oxidation process. All solid contaminants have the potential to cause<br />

excessive wear or scoring <strong>of</strong> the bearings and journals, and damage to other<br />

lubrication system components.<br />

Rusting <strong>of</strong> oil system piping and other carbon steel components can<br />

occur in service on both submerged and exposed locations. Rust can be<br />

produced in both the hematite (red color) form and the magnetite (black<br />

color, magnetic) form. Rust particles can be swept up by the flow <strong>of</strong> oil and<br />

transported throughout the lubrication system. Rust inhibitors are added to<br />

the oil to protect against rusting <strong>of</strong> oil-wetted metal surfaces. Volatile vaporphase<br />

inhibitors (VPI) may also be added to protect the nonoil-wetted<br />

locations. ASTM Test Methods D 665-82 and D 3603-82, ‘‘Rust Preventing<br />

Characteristics <strong>of</strong> Steam Turbine Oil in the Presence <strong>of</strong> Water,’’ are used to<br />

evaluate the corrosion protection <strong>of</strong> the oil [121–123]. However, the most<br />

effective method <strong>of</strong> preventing rust is to keep the lubrication system and the<br />

oil free from water.<br />

Water Contamination<br />

The most common contaminant in a lubrication system is water. Water may<br />

enter the system from leaks in water-cooled heat exchangers, from<br />

condensation <strong>of</strong> humid air, and from leakage <strong>of</strong> the steam turbine shaft<br />

seals. Water in the oil results in promotion <strong>of</strong> rust, formation <strong>of</strong> emulsions,<br />

and accumulation <strong>of</strong> sludge. Consequently, the water-separating ability <strong>of</strong><br />

the oil is an important characteristic and water must separate quickly from<br />

the oil so that it can be collected in and removed from the bottom <strong>of</strong> the oil<br />

reservoir. Large amounts <strong>of</strong> water form persistent emulsions that join with<br />

insoluble oxidation products to form sludge. Accumulation <strong>of</strong> sludge in oil<br />

piping, passages, and coolers impairs circulation <strong>of</strong> the oil and causes higher<br />

bearing and oil temperatures. Higher oil system temperatures, in turn,<br />

increase the oil oxidation rate. Sludge also can cause sluggishness or sticking<br />

<strong>of</strong> oil pressure relays and governor pilot valves if the oil is used as the<br />

hydraulic control fluid.<br />

Water exists in the dissolved state in oil and will precipitate as free<br />

water when it exceeds the saturation (concentration) point. The saturation<br />

point <strong>of</strong> oil varies with its additive package and its temperature. Generally,<br />

more additives in oil increase its solubility limit. Hot oil dissolves more<br />

water than cold oil so that a drop in oil temperature can result in conversion<br />

<strong>of</strong> part <strong>of</strong> the dissolved water to free water that collects in low spots or lowflow<br />

branches <strong>of</strong> the lubrication system. Even periodic excursions in the<br />

Copyright © 2003 Marcel Dekker, Inc.

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