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Handbook of Turbomachinery Second Edition Revised - Ventech!

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Although a light oil does provide a relatively full film between the rotating<br />

shaft journal and the bearing at operating speed, some metal-to-metal<br />

contact occurs between these members during starting, stopping, and slowspeed<br />

turning gear operation. The oil must therefore have adequate film<br />

strength to minimize friction and to protect against wear when journal speed<br />

is low. A properly refined petroleum-based lubricating oil will have a high<br />

film strength value.<br />

Lubrication System Oil Service Life<br />

Turbine generator oil is expected to have a long service life, on the order <strong>of</strong><br />

15–20 years. Certain environmental conditions, however, can result in or<br />

accelerate oil degradation and reduce service life. Primary environmental<br />

conditions that influence oil life are exposure to high temperature in the<br />

presence <strong>of</strong> oxygen, and contamination by particulate materials and water.<br />

The degree <strong>of</strong> exposure to these environmental conditions and<br />

capability <strong>of</strong> the oil (oil quality) to resist them are factors that determine<br />

the exact period <strong>of</strong> service life obtained from the oil. Other important<br />

factors affecting the oil service life are type and design <strong>of</strong> the lubricating<br />

system,condition<strong>of</strong>thesystem afterconstruction, andoilmakeuprate(Fig.<br />

59).<br />

Combinations <strong>of</strong> these factors vary from unit to unit so that service life<br />

<strong>of</strong> the oil is difficult to predict solely from original oil properties.<br />

Each turbine generator lubrication system is unique due to exclusive<br />

conditions that occur during construction and operation <strong>of</strong> the system.<br />

These conditions set the rate at which a new charge <strong>of</strong> fresh oil will lose its<br />

oxidation resistance. A property called turbine severity (B) level has been<br />

established that can be used to take these conditions into consideration<br />

when monitoring the remaining oxidation resistance <strong>of</strong> the oil during its<br />

service life [111]. ‘‘B’’ is defined as the percentage <strong>of</strong> fresh oil oxidation<br />

resistance lost per year due to the oil reactions in the turbine generator<br />

lubrication system.<br />

The severity level for a particular lubrication system should be<br />

determined over a period <strong>of</strong> time beginning with initial operation or<br />

installation <strong>of</strong> fresh oil charge. Accurate record keeping <strong>of</strong> the amount <strong>of</strong><br />

oil makeup is essential, and a rotating bomb oxidation test (RBOT) should<br />

be conducted at three- to six-month intervals for one to two years. By<br />

knowing the oil makeup and degradation <strong>of</strong> the oil with time, the turbine<br />

system with a high severity level requires frequent makeup or completely<br />

new charges, whereas one with a low severity level may have no problems<br />

with routine makeup. Turbine generator units <strong>of</strong> recent design have higher<br />

‘‘B’’ levels than units installed before 1965. Increases in lubrication system<br />

Copyright © 2003 Marcel Dekker, Inc.

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