The Horizontal Steam Turbine For Stationary Plants 1920

sometimes occurs. This condition should become normal after a few hours’ operation and will not have any detrimental effect on the service of the circulating oil. Continued excessive foaming is evidence that air is being drawn into the suction pipe by the oil pump. Air leaks should not be allowed to exist. DEPOSITS As has been shown the oil used in hori- zontal steam turbines is subjected to severe strain. The general belief among engineers in the past has been that petroleum lubri- cating oils are indestructible. This is very nearly true in practice, where the older methods of lubrication are employed,. as in the case of drop feed oilers for reciprocating engines. However, where the turbine is lubricated by means of an oil circulating system, in which the oil is foræd to the bearings under pressure, then collected, filtered, cooled and repeatedly returned to undergo the same severe service, deposits may form, due to the breaking down of some portions of the oil from the following causes: Air and heat Solid impurities Water Broken down o il Foreign substances Adding new o il Air and Heat The circulating oil always contains more or less air, and when the temperature of the oil is above normal—say more than 140 degrees F.—the air has a strong tendency to oxidize the oil. This will be realized when it is remembered that the oil film in the bearings is very thin and that the air is present in very fine bubbles, which are inti- mately mixed with the oil. The result is that the oil darkens consid- erably in color. In extreme cases, a very dangerous black carbon deposit develops, which may choke the oil inlets to the bear- ings and cause the oil-worked piston in the governor gear to stick. Solid Impurities Due to the high temperature at which the oil passes through the circulating system, the oxidizing effect of impurities—such as iron oxides, dust and dirt, etc.—is very con- siderable, particularly where ordinary oils are in use. The effects produced are the quick dark- ening of the color of the oil, a considerable increase in viscosity, the production of a large percentage of petroleum acids and the breaking down of the oil from oxidation. The oil in this condition smells bumt and throws down a slimy deposit which lodges in the system, particularly in the oil cooler. In addition, if there is even a slight leak- age of water into the system, the oil will emulsify badly, owing to its al ready weak- ened condition from the oxidizing effect of the air and solid impurities. This will explain why, when starting up a new turbine for the first time, emulsification of the oil may occur as a result of the com- bined effects of the water, air and dirt pres- ent, even though the oil may be of high quality. Water Water has an emulsifying effect on the oil, particularly if the water contains impurities. Where considerable quantities of water leak into the system and emulsification takes place, the mixture becomes yellow or brown- ish-yellow in color. If a sample is taken out and heated, it will separate into clean oil at the top (Fig. 11), more or less milky water at the bottom and a spongy sludge separ- ating the two. The clean oil will be founcl darker in color and heavier in body than the original and will have a strong characteristic odor. When subjected to rapid circulation, espe- cially in small systems, the oil and water forms an objectionable mixture, which clogs the screens, forms a slime on the cooling coils, and accumulates in the clearance spaces throughout the system. This mixture will generally become hard, and in some instances cause partial stoppage in the oil pipes. In such cases, the increasing temperature of the bearings indicates insuffi- cient supply of oil; and this in turn demands an immediate shutdown of the turbine to thoroughly cleanse the system. 14