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Oil Sampling and .esting
Oils from turbomachinery should be tested periodically to determine theirsuitability for continued use. however， visual inspection of the oil can be useful in detecting contaminated oil when the appearance and odor is changed by the contaminant.
An oil sample should be withdrawn from the system and analyzed in thelaboratory. The usual tests of the used oil include: (1) viscosity， (2) pH and neutralizationnumber， and (3) precipitation. The test results will indicate changes from the original specificationsand， depending on howextensive these changesare， whether the oil can or cannot be used in the machines.
Oil .ontamination
Oil contamination in a turbomachine is one of the ma.or problems main-tenance crews face. However， while contamination is a continuous problem， the .e.e.. of contamination are what cause the most concern.
The greatest source of contamination is extraneous matter. Atmosphericdirt，for example， is always a serious threat. It can enter the oil systemthroughvents， breathers， and seals. Its primary effect is equipmentwear，but plugging of oil lines and ports， and reduced oxidation stability of the oil are also serious effects.
Metal particles from wear and rust particles from reservoir and oil piping corrosion can lead to premature equipment failure and oil deterioration. It is important to provide suitable filtering equipment to remove these particles from the system.
Water contamination is a constant threat. The sources of water aremany.atmospheric condensation， steamleaks， oilcoolers， and reservoir leaks. Rusting of machine parts and the effects of rust particles in the oilsystem are the ma.or results of water in oil. In addition， water forms anemulsionand， combined with other impurities， such as wear metal and rustparticles， acts as a catalyst to promote oil oxidation.
Contamination from process gas can be a serious problem， particularly during startup. .very effort must be exercised to prevent and detect this type of contamination.
Most hydrocarbon gases are more soluble in cold oil than in hot oil and may lower the viscosity to a dangerous level. The problems of thrust-bearing failures during startup due to low-viscosity oil can be eliminated by equip-ping the reservoir with oil heaters to raise the oil to the normal operating temperatures before starting the machine.
.quipment in HCl and chlorine service must be protected against theexposure of these acid gases to the oil. Obviously， the first line of defense is to eliminate seal failures.However， as asecondary protection， these machines could be lubricated with an alkaline oil. The alkaline additives react to the lowconcentrations of the acidgases， thus eliminating the addition of these acids to the oil molecule.
To remove insoluble contaminants， various types of full-flow filters can be used. Two general types are usually selected: surface filters and depth filters. Both types of filters are effective for the removal of particulate matter.
Surface filters， if manufactured from the correct material， will not be affected by water in the oil. Water-resistant pleated-paper elements have much greater surface areas than the depth-type element and yield a much lower differential pressure when used as replacement elements in filters originally equipped with depth-type elements. Pleated-paper elements are available that will remove particle sizes down to a nominal one-half micron.
The depth-type filter elements are used when the oil is free fromwater， and when particles sizes to be removed are in the five-micron and greater range..enerally， the depth-type element iswater-sensitive， and when oil iscontaminated with moisture， this element type will absorb the water and produce a rapid increase in differential pressure across the filter. The desired maximum differential pressure across a filter with clean elements is five psig at normal operating temperature.
Filter Selection
The filter elements should remove particles of five microns， must be water-resistant， have a high flow rate capability with low pressuredrop， possesshigh dirt-retentioncapacity， and be rupture-resistant. The clean pressure drop should not exceed five psig at 100 .F (3. .C). The elements must have a minimum collapse differential pressure of 50 psig. Pleated-paper elementsare preferred.provided they meet these requirements. Usually， the pleated-paper element will yield the five psig clean drop when used in a filter that was sized to use depth-type elements. This result is due to the greater surface areaof the pleated element， more than twice the area of a conventional stacked disc-type or other depth-type elements.
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燃气涡轮工程手册 Gas Turbine Engineering Handbook 3(2)
