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I N D U S T R I A L A N D ENGINEERING CHEMISTRY
Vol. 16, No. 10
Turbine Lu brica tion’9z By N. E. Funk THEPHILADELPHIAELECTRIC Co., 1000 CHBSTNUT ST.,PHILADELPHIA, PA.
I
T IS a o t the intent of the author to attempt to classify lubri-
cating oils according to their chemical properties, the crudes from which they are developed, the methods by which they are refined or any other like characteristics which’ are essentially in the field of the oil chemist; but rather to place before the refining industry the needs of the turbine operator and the difficulties now experienced in applying many lubricating oils that are considered by the refiners to be high-grade products. In presenting this information it, is necessary t o discuss in considerable detail the performance cycles of the lubricating system. PROBLENS O F TURBINE LUBRICATION Perfect lubrication is one of the most essential requirements for the successful operation of steam turbines. The solution of these various problems involves not only the question of the lubricant, but also the methods and points of application, the rate of circulation, the dissipation of heat, and the storageand preservation of the lubricant. All are strictly a matter of design and construction of the steam turbine. Many of the problems, so far as the mechanics of the application of the lubricant is concerned, have been very successfully worked out. The problem of the successful lubricant itself, and of successful preservation of the lubricant has as yet reached a less satisfactory stage. In considering the mechanical design of the tiirbine it is necessary to bear in mind the fact that only one type of lubricant can be used in a single machine and that the design of this machine must re,cognize the three essential and distinct properties required or a lubricating oil. The oil must be (a) a lubricant, ( b ) a fluid medium, and (c) a heat-dissipating agent. PARTS ON WHICHOIL MUSTACT AS A 1,oBRIcm’r-Governor Mechanism. Since the successful operation of the turbine depends absolutely on the correct functions oE the governor mechanism, it is imperative that the essential parts of this sensitive device, operating at a high rate of speed, be abundantly supplied with oil in such a manner as to reduce friction to the minimum and to carry away the heat generated. An oil of high viscosity is desirable, as it u-ill be less readily thrown off by centrifugal force. To conyey the oil from t.hereservoir to the parts to be lubricated some type of forced feed is required. CozipZ,i?zg. On machines of large sizes i t is necessary to install a flexible coupling between various parts or the machine t o allow for shaft deflection and expansion. While this coupling revolves a t a very high rate of speed, the surfaces in contact have a small relative motioii but very close clearances. I t is necessary, however, in most eases to provide some means of introducing a lubricating medium between the surfaces in contact to prevent abrasion of the metals. A low viscosity oil is most desirable (see Clearances). Turbine Shaft and Thrust Bearings. When starting and stopping the turbine unit it is necessary that the turbine oil act as a lubricant. The action of the oil in the turbines a t full or partial speeds will be considered under “Action of the Oil as a Fluid Medium.” An oil high in viscosity is desirable. To prevent abrasion during the period of starting and stopping it is necessary to supply oil under pressure necessitating some type of forced feed system. 1 Presented as a part of the Turbine Symposium before the Division of Petroleum Chemistry a t the 67th Meeting of the American Chemical Society, Washington, D. C., April 21 t o 26, 1924. 2 This paper is a consolidation of the reports of the Subcommittee on Lubrication of the Prime Movers Committee of the National Electric Light Association from 1920 t o date.
These parts are mentioned specifically as examples of those elements of the turbine requiring lubrication during some period in their operation. It must be borne in mind that these are not the only parts requiring lubrication, but are given as the main considerations depicting the service requirements. ACTIONO F THE OIL AS A FLUIDMEDIUM-The Operation O f steam turbines involves the rotation of heavy masses a t high speeds. To make possible the operation of such machines it is necessary to substitute hydraulic friction for metallic friction, by providing and maintaining a liquid film between the surfaces in motion so that the shaft may be floating in the bearings and thus prevent undue wear and seizure. For a better understanding of turbine lubrication it is well to review briefly the conditions met in practice. Unit Pressure. A heavy load per square inch on a bearing tends to bring the surfaces together and to force out the lubricating film. If no other conditions prevailed an oil of high viscosity would be desirable. Rubbang Speed. A high rubbing speed will draw more oil between the surfaces than a low rubbing speed and will maintain the oil film more easily, which permits the use of an oil of low viscosity. Thrust Bearings. The ideal conditions of thrust-bearing lubrication are met when the design and construction of the bearing is such as to adapt itself to the formation and maintenance of a surface film through the generation of an unbroken “oil wedge” or tapered film of oil between the surfaces in contact. The Michell, the Kingsbury, and the Gibbs typps of thrust bearing are well adapted to the oil wedge requirements. Thrust bearings of the multiple runners or collar type, also knowii as marine type, offer more difficulties in the supply, uniform distribution t o all points of contact, and circulation of the oil. The parallelism of the rubbing surfaces does not favor the generation of an oil wedge as does the thrust bearing having pivoted pad% In both cases the unit pressure, rubbing speed, clearances, and temperatures bring the problem of thrust-bearing lubrication within practically the same requirements as to grade of oiY and viscosity as is the case for turbine shaft and bearings. Clearances. It is more difficult to force a n oil film into a small clearance than into a large one. Likewise. a low-viscosity oil will enter and flow in a small clearance more readily than a high-viscosity oil. Bearing clearances vary from 0.001 to 0.002 inch per inch of diameter. This allows some leeway in oil at operating temperatures. OzL Wedge. Owing t o the friction between the lubricating fluid, bearing, and shaft, as well as the internal friction of the fluid, the shaft assumes a n eccentric position in the hearing, its location being determined by the wedging action of the annulus of oil. In its rotation the shaft draws the oil by adhesion, forcing it in the form of a wedge into the clearance between the.shaft and the bearing (Fig. 1). The existence of this oil wedge insures a distribution of the oil film over the entire surface and protects against metallic contact. This is possible only when the two surfaces are not in parallel with each other but have a certain degree o! inclination, allowing the oil film to taper in the direction of motion. At a given temperature, owing to its greater fluidity, a low-viscosity oil will function in a more nearly ideal manner than a high-viscosity oil. The pumping action of the shaft and bearings is not sufficient to draw the oil from the reservoir to the bearing. I t is therefore necessarv t o Durn13 the oil from the reservoir to the bearinn.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
October, 1924
When a shaft comes to rest in its bearings, this condition no longer exists and metallic contact between the bearing and shaft is established a t the bottom of the bearing, the fluid medium being entirely forced out. To prevent wiping of the surfaces when a machine has slowed down to the point where the film will no longer be maintained, it is necessary to force a fluid medium under the shaft until it has come to rest. This condition also maintains on starting. Frictional Heat. Heat generated by metallic friction cannot exist in the turbine bearing, or else both shaft and bearing will be rapidly destroyed.’ The only heat t o be considered, therefore, is that resulting from the friction between the fluid medium and the metal parts, and the friction due to the relative motion of the particles of the fluid medium itself. With the same relative velocity in the fluid medium a greater frictional heat will be generated in high-viscosity mediums than in low-viscosity mediums, owing to the greater resistance of the medium against shearing. Inasmuch as the most important function of turbine lubrication is located in the various bearings, it is evident that the performance of the lubricating system depends principally upon fluid friction. Hence the question of viscosity of the fluid medium is of paramount importance. To transfer the heat from the point a t which it is generated to the location in which it is cooled, it is necessary to circulate the oil a t a fairly high rate. This requirement necessitates the use of a forced-feed system. Hydraulic Functions. In addition to the foregoing characteristics, the fluid medium must perform the function of a powertransmitting agent in producing mechanical operation on some of the automatic turbine control parts. I n the performance of this function, so t h a t time lag may be reduced to permit the machine to respond more readily to the demands made upon it, i t is essential that the fluid medium flow readily. , This requirement is best satisfied by an oil of low viscosity. Obviously, to act as a hydraulic medium transmitting power, it is necessary to supply power to the medium. This requirement necessitates the installation of a forced-feed system.
ACTIONOF THE OIL AS
A
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break down to a more or less serious degree in varying periods of time. DESTRUCTION OF OIL IN OPERATION When a lubricating oil is first introduced in a turbine system it closely approaches a perfect lubricant. This condition remains only for a short time, however, for the oil soon begins to deteriorate. This deterioration takes place in two distinct ways-first, increasing organic acidity, and, second, breaking up the oil into semisolids and emulsions, commonly called “sludge.” TABLE I-REQUIREMENTSOF TURBINE LUBRICANTS ----FUNCTION OF LUBRICANT----VISCOSITY OF As A AS A As A HEAT LUBRICANT LUBRICANT FLUIDMEDIUM DISSIPATOR High
Govqrnor mechanism shaft, etc. (starting and stopping) Coupling
Low
Necessity of forced feed supply
Governor mechanism bearings
High bearing pressures Clearances High rubbing speed Thrust bearings Clearances Shaft (fluid wedge) Frictional heat Hydraulic functions Bearings Governor mechanism
Heat transfer (Low frictional losses)
Heat transfer
.
HEAT-DISSIPATING AGENT
The oil constituting the fluid medium must have the highest possible thermal capacity, more readily to absorb, carry away, and dissipate in the cooler the heat generated in or transmitted to the various parts of the turbine lubricated so as to maintain such parts within operating temperatures. The oil is partly aided in the dissipation of heat by radiation and conduction from bearing housing and piping, although this assistance varies greatly with the atmo:jpheric temperature and other turbine room conditions. For the fulfilment of the requirements of heat transfer an oil of low. viscosity is indicated. The heat transferred by the oil in the lubrication system will balance a t some normally constant temperature, depending upon the frictional losses, heat of steam transferred by conduction through the shaft and casing, location of oil tank, rate of flow of oil, volume in circulat.ion, engine room temperature, radiating capacity, rate of heat exchange, etc. The information in the foregoing paragraphs has been correlated in Table I for greater convenience in considering the properties required of the lubricant. The construction of this table indicates two salient features-first, without any regard to the details of the mechanical design, the fundamental principles on which the turbine operates, as a device requiring lubrication, demand t h e use of a lubricating fluid of low viscosity: second, it is absolutely essential that this lubricating medium be continuously supplied by a forced-feed system. It is possible to obtain lubricants that will satisfy both of these conditions, and if it were not for the deterioration of the lubricant in service, the operation of steam turbines, so far as lubrication is concerned, would not be a problem. Unfortunately, however, this essential condition does not exist, as present lubrication oils
G
D-
a FIG RELATIVE
\
A
POSITION O F SHAFT I N B E A R I N G .
SHAFT
ROTATING
The second indication of oil deterioration, however, is a very serious problem and one that causes the turbine operator great concern. The so-called sludge in the oil deposits in the bearing oil supply and discharge piping, in the small internal bearing openings, on strainers and like parts of the lubricating system, reducing the flow of oil sometimes to a rate impairing lubrication. Many cases are on record where bearings have been seriously damaged by an inadequate oil supply due to clogging from sludge deposits. The sludge also accumulates in small connecting pipes to indicating gages, oil-level gage glasses, and similar controlling devices, which prevents the operator from intelligently and defi-
INDUSTRIAL A N D ENGIN-EERING CHEMISTRY
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nitely ascertaining the conditions of the lubricating system. The sludge settles in large quantities on the surfaces of the oil cooler, cutting down the heat transfer and preventing the proper cooling of the oil, a t the same time covering the surfaces of thermometer wells so that erroneously low readings fail to warn the operator that the oil temperature is unduly high.
f f o u P S RUN F I G . %-EFFECT
TURBINE O I L OF HEATA N D AIR ALONE,A N D AIR, AND WATERIN CONJUNCTION
ON
HEAT,
A survey of about eighty large steam turbines from 20,000 to 60,000 kilowatts in capacity has shown that in the aggregate these units are out of service for about 2 per cent of the time, owing to the necessity For cleaning the sludge deposits from the lubricating system, or in order to repair damage caused by improper lubrication resulting from reduction of the fluid volume by sludge obstruction. The economics Of this problem are serious? for if we consider that in the turbines and their auxiliaries alone there is invested over W5,000,000 for these eighty units, and that this $125,000,000 investment was idle for 175 hours a year as a result of troubles primarily due to the unsatisfactory lubricating medium, we will realize that the present lubricating oils are much further from an ideal product than Some of our friends in the refining industry would have us believe. Since the foregoing conditions are not academic but represent serious practical problems based on known facts, it is necessary for us to determine, if possible, what features in the turbine are contributory to the deterioration of the lubricant, and whether or not these features are inherently basic in the design of this type of machine, or are possible of improvement by deviation from present mechanical methods. A vast volume of information obtained from turbines located in territory spread widely over the United States, supplemented by very careful laboratory experiments and well-established research work, lasting over a considerable period of time, made on variopls types of oil in this country and abroad, in Government and private establishments, seems to indicate that the prime factors causing the deterioration of turbine oils are as follows: heat, light, water, air, dust and dirt, various metals, acids and alkalies. Unfortunately, turbine operation subjects the lubricating oils to the majority of these contaminating factors. Since the characteristic required of the lubricant is that it convey heat from the bearings, it is, of course, absolutely impossible not to subject this lubricant to heat. For many years discussion on hot spot temperatures in bearthere has been ings, some contending that these temperatures rhay be four, five, or even more times the temperature of the discharged oil. While any information relative to this condition is of considerable value, nevertheless, it is a known fact that 212' F. applied continuously will have a very destructive effect on some turbine oils, so that it is not necessary to theorize about the possibility of extremely high temperatures in bearings being the cause of the deterioration of the lubricant. Since the turbine oil in its complete cycle in the turbine is in metal containers, it is evident that the effect of light on the lubricant is practically zero.
Vol. 16, No. 10
Every effort is made in the design of the turbine to arrange the parts in such a manner and to make them of such design t h a t moisture will be excluded from contact with the oil at any place in its cycle. As a practical proposition, however, keeping the oil perfectly free from water is well nigh impossible. The air in the turbine room is always more or less humid. The oil reservoirs are subject to varying temperatures if the turbine is run intermittently. Therefore, a certain amount of breathing will take place in which event moisture will be deposited on the inside t o p of the reservoir. This moisture gradually coalesces in drops and finally falls into the oil. It is surprising what- a comparatively large amount of water may get into the oil in this manner. It is not always possible to shut machines down a t will for the correction of small defects, and in this manner a slight steam leak in a seal or certain sections of the turbine is liable to find its way into the lubricating system. In many cases a leak, unim* portant from an efficiency point of view, exists for some time before it is possible to repair it. It might be contended by the refiner with some degree of logic that this condition should not exist and that it is not fair to charge the oil with the turbine's shortcomings. As against this it must be remembered that the turbine is intended primarily to produce power and not to preserve the oil, and that it is a highly efficient converter, and must, for financial reasons, be kept running the maximum amount of the tirne demanded, and the lubricating medium mustwithstand with degree of satisfaction the small deviations of the turbine from perfect operating conditions. airtight, Since it is impractical to make the turbine is apparent that there will be considerable mixture of air and oil. This occurs in the splash from gearing, where the oil discharges from between the shaft and bearing in a finely divided stream and into the bearing pedestal; it also occurs in passing,through the strainers and similar parts of the turbine. From the presentday knowledge of mechanical arts the reduction of the aeration of the oil has practically reached a point where very little help from improvement of this factor can be expected, unless the vacant spaces are filled with an inert gas, which a t present is quite impractical. The metals having the most Severe catalytic action on lubricating oils are copper and its alloys, while cast iron, steel, and zinc are on the opposite end of the scale, causing the least catalytic
s6 s
4
9
4
b\~
HOORS RUN
FIG.3-EMULSIFICATION
EFFECTS OF IN
VARYING PERCENTAGES OF W A T E X
TURBINE OIL
action. For mechanical reasons zinc is inappropriate. The lubricating system of the turbine has of late years been confined almost entirely to cast iron and steel. The major exceptions to this are the babbitted bearings, some of the governor gearing, and the brass tubing in the oil cooler. Mechanical construction requires that these parts be made of this kind of material, with the exception of the tube bundle of the oil cooler, which so far as the oil surface is concerned could be made of iron. This, however, is not nearly so satisfactory for the water-cooled side: it leads t o considerable corrosion and blocking up of the tubes.
October, 1924
IiVDUSTRIRL A N D ENGINEERING CHEMISTRY
therefore necessitating more or less the use of brass tubing. I t is quite pctasible that more serious conditions result from the wear of the parts, by small particles of the metals or their oxides discharging through the entire body of the oil; both the air and water can fasten themselves to these particles, making the nucleus for sludge formation. This, of course, is not so much a function of the materials from which the parts are built as it is the indication that absolutely theoretical fluid friction has not been established andl there is some contact of parts. The air in the turbine room carries more or less dust and dirt, and i t is possible for some of this material to find its way into thevarious air pockets in the turbine This dust mixes with the oil and acts in a manlier quite similar to small metallic particles. Practical experiencw, however, would lead one to believe t h a t the effect of metals and dust in sludge formation is a minor consideration. Experience has shown t h a t the presence of both organic and mineral acids in the oil accelerates deterioration of the oil by the other factors, producing destructive effects. I n addition, mineral acid should never be present, owing t o its destructive effect on the turbine parts. The presence of alkalies in the oil or in water coming in contact with the oil is more destructive than any of the other agents, since these alkalies form with the water and the oil a soapy emulsion which spreads throughout the oil and which cannot be readily sc parated when reclaiming the oil, thus tremendously increasing its wastage as well as multiplying the troubles in turbine operation due to the increased quantity of broken-down hydrocarbons. The foregoing considerations point to the conclusion that the lubricating oil should withstand the effects of heat, air, water, and catalytic metals without serious deterioration. Mineral acid shourd never be present and it should be possible always to keep the oil free from the contact of alkaline.
serious sludge and emulsion immediately on starting the turbine. This deposit was so severe and rapid that after a comparatively few hours’ operation the bearing supply and drain lines were obstructed and the temperature of the bearings and oil had increased to such an extent that it was necessary to take the turbine out of service. After the system had been cleaned and the oil filtered, it was put back in service and ran quite satisfactorily, although it remained of a soapy nature for about two weeks. After this the experience with the oil was not extremely unsatisfactory, although it required shutting down the turbine frequently for cleaning. “D” oil emulsified immediately a t the beginning of the run, but not severely enough to necessitate shutting down the turbine. This effect decreased for a time, and then the accumulation of sludge and emulsion increased. It is interesting to note that after a few hours’ run the percentage of sludge and emulsion consisted of emulsion alone, and at the end of the run the deposit was a combination of sludge and emulsion, the sludge, however, being a very small percentage. Apparently, those oils which develop high acidity when moisture is absent are the ones that emulsify and sludge the worst when moisture is present.
/
EFFECTS PRODUCED BY DESTRUCTIVE AGENTS Heat, air, and the presence of metals apparently cause organic acidity. These acids may be formed by the combination of all three factors or by any one individually. This acid formation may reach very high values without the production of any sludge, or small quantities of sludge may be produced concurrently with the developme& of acid, although there is some doubt, on account of the necessarily crude method of obtaining the data from the field, as to whether it is absolutely certain that there was no water present during the formation of sludge. The laboratory experiments seem to indicate that, in light oil a t least, no sludge is produced without the presence of water-the oil merely grows more acid (Fig. 2 ) . Both laboratory and field data seem very definitely to establish the fact t h a t the amount of qludge and emulsion formed increases with the percentage of water present. There seems, therefore, to be every indication that one of the most desirable characteristics of turbine oil is the fact that it will not readily emulsify, and if it satisfies this condition one of the troubles, and probably the most serious-that due to the clogging of the passages and reduction of the heat transfer by sludge and emulsion deposits-will be eliminated. Fig. 3 shows the sludge and emulsion produced in a given oil when operating with various percentages of admixture of water. This test was made in a special laboratory device so that conditions could be kept constant and tests duplicated. A very decided change in the amount of emulsion with increase in the quantity of water will be noted. I n Fig. 4 are plotted four commercial turbine oils of supposedly good grade. “C” oil operated for a long period of time with very little attention, very little formation of sludge and emulsion or acid. “A” oil produced high acidity, a large amount of sludge and emulsion, and caused considerable trouble, requiring shutting the machine down a t frequent intervals for complete cleaning of the cooling and general lubricating system. “B” oil developed
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_--HOURS
FIG &-TYPICAL
UU#
PERFORMANCE OF VARIOUS OILS OP
SUMMARY O F
LIKEVISCOSITY
REQUIREMENTS O F TURBINE OIL
The characteristics required of a good turbine oil have been reviewed very minutely in the foregoing paragraphs. These necessary characteristics will be summarized here in a concrete manner. It should be noted t h a t these characteristics are independent of the mechanical design of the turbine and are fundamentally necessary for satisfactory operation. The oil must 1-Act as a lubricant. 2-Act as a fluid medium. 3-Act as a heat dissipating agent. 4-Withstand the effects of heat. 5-Withstand the effects of mixture with air. 6-Withstand the effects of mixture with water. 7-Be uniform in quality.
To satisfy Items 1, 2 , and 3, the oil should have low viscosity. To satisfy Items 4 and 5, the oil must not readily form organic acid.
To satisfy Item 6, the oil must not readily emulsify. From the turbine operator’s point of view the foregoing are the main requirements of the turbine lubricating oil. There are a great many oils that satisfy these conditions satisfactorily, but there are many more t h a t do not. Unfortunately, among these many are some t h a t are good some times and bad a t other times, so that there is always doubt in the operator’s mind as to just what may be exected from a given grade of oil. This is a problem that the refiner should look into very thoroughly, as uniformity of the product is a most essential requirement. To recapitulate these requirements, the oil should have the following properties: (1) low viscosity, ( 2 ) resistance to organic acid formation, ( 3 ) resistance to emulsification.
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INDUSTRIAL A N D ENGINEERING CHEMISTRY CAREOF THE LUBRICANT IN SERVICE
Before it is possible to discuss intelligently the problem of caring for oil in service, it is necessary to know what duties the clarifying or purifying devices are expected to perform. Reference to the preceding paragraphs will indicate that the ideal performance of a clarifying device would cover the complete removal from the oil of (A) water, (B) acid, (C) sludge, and (D) emulsion. REMOVAL O F WATER FROM THE OIL-Water t h a t settles out of the lubricating oil to the bottom of the turbine reservoir may be removed from the system by the installation of an automatic water drain. This drain consists of a IJ-shaped piece of pipe with one end connected to the bottom of the turbine reservoir and the other end free to the air above the oil level in the turbine reservoir. Just below the level of the oil in the turbine reservoir and connected to the evternal pipe is a valve discharging into a funnel. This U-shaped pipe is filled with water until a hydrostatic halance is obtained, and then any water collecting in the base of the turbine reservoir will automatically force water out of the discharge valve without discharging any oil. Water suspended in the oil cannot be removed in this manner. It is therefore necessary to remove this water from the oil by some external means. There are two methods of obtaining this separation. The first is the time-honored scheme of adding a certain percentage of oil to the turbine reservoir and drawing off some of the used oil, passing this used oil through a bag filter with a fairly large precipitation chamber. The oil, in passing slowly through this precipitation chamber, owing to lack of agitation, deposits by gravity the suspended moisture and is fairly dry. The oil then discharges through the filter bags and is returned to the turbine system. The second method is by the centrifuge, which has come into use of late years. The same general scheme is followed of continuously by-passing part of the supply from the turbine base through the separator and returning it to the turbine base. This effectively removes the water. There are many instances where the oil from the turbine base is run into a separate tank in which it is subjected to high temperatures with the intent of driving the water out of the oil. There is no doubt that in fnost cases this accomplishes the purpose desired, but it will be recalled that heat is a n agent promoting the formation of organic acid, and it is quite apparent that this method of treatment, while it may remove the emulsifying agent, on the other hand increases the acidity of the oil and it5 tendency to emulsify or sludge. This boiling process cannot be considered as a desirable method of removing the water. It is merely a subterfuge and should never be used. REMOVAL OF ORGANIC ACID FROM THE OIL-While from a refining point of view this may not be a very difficult problem, nevertheless no apparatus or methods have yet been devised which can be satisfactorily or economically applied to power plant work. There is some promise, however, t h a t there is a possibility of keeping the acidity to a very low point hyintentionally adding water to the oil from time to time. This will cause the formation of sludges and emulsions but reduce the acid by entrainment. The sludges and emulsions can then be removed and the oil reconditloned. This process should never be performed in a turbine base or circulating system. I t may, however, be put into operation in the proper kind of a continuous by-pass system. or in a batch system. These, however, are merely conjectures, although there is little doubt in the writer’s mind of the truth of these statements, since they seem to be amply confirmed by data obtained from turbines in operation. High acidity was current with the absence of water, while low acidity and sludge were concurrent with the presence of water in the same oil. REMOVAL OF SLUDGE AND EMULSION FROM THE OIL-The existence of two distinct types of sludge has been definitely ascer-
Vol. 16, No. 10
tained-one insoluble, and the other soluble in the oil a t operating temperature. Most of the oil-purifying devices available on the market, or even homemade, will remove with more or less efficiency the insoluble sludge. Few, however, will completely remove the soluble sludge. Insoluble sludge, generally of an asphaltic type, has a specific gravity greater than that of the oil and will settle t o the bottom when sufficient time is allowed. The elimination of this sludge is quite easy if it can be passed through the clarifying medium. However, its tendency to collect and adhere to all oil-wetted surfaces makes it a quite difficult problem. The soluble sludge is like a coloring matter; it seems to be in colloidal suspension in the oil a t operating temperatures and becomes insoluble and finally settles with the reduction of temperature. If agitated with the filtered oil a t a temperature sufficiently low to permit its coagulation, it gives a murky or cloudy appearance and returns to the bottom when left to settle. If the temperature of the oil is raised, this insoluble sludge becomes soluble and dissolves in the oil, darkening its color. The physical structure of this soluble sludge is almost identical to that of the insoluble type. Apparently the soluble sludge is the beginning of the formation of the insoluble sludge. No filtration process has yet been developed that can be attached directly to the turbine system and eliminate this soluble sludge a t operating temperature. I t is believed that, if the soluble sludge can be eliminated by some continuously operating process by-passing the turbine reservoir, there will be no difficulty from the deposit of insoluble sludge since, if it is correct that the soluble sludge is the beginning of the insoluble sludge, and this sludge can be eliminated, no insoluble sludge will form. Some experiments with the Hele-Shaw filter indicate t h a t i t has great possibilities in clarifying the oil. This has yet to be proved, however. A method operating on the batch system has been developed, whereby the oil has been drawn from the turbine completely, allowed to settle for 24 or 48 hours in a cooling tank, and then passed through a series filter which is composed of twelve layers of closely compacted Canton flannel, through which the oil must pass in series. After heing discharged from this filter the oil is as bright as new oil. The complete elimination of emulsion from the turbine reservoir is quite difficult, since much of it comes in contact with surfaces cooler than the oil and is deposited on them. It can be removed from these locations only by manual cleaning, which necessitates shutting down the turbine. Since the emulsion consists of a mixture of sludge, oil, and water, it is apparent that the removal of the sludge, both soluble and insoluble, and the removal of water from the oil, will leave the oil clear. It is expected that within six months or a year information will he available as to the general principles underlying an ideal clarifying system, and when this is available it will doubtless be possible to obtain apparatus that will function satisfactorily. One difficulty in the past has been that all devices for oil clarification have been built on more or less rule-of-thumb methods, trying to perform a satisfactory service when the basic fundamental knowledge as to the exact requirements was missing. CONCLUSION The foregoing statements have been based on facts t h a t have been fairly well proved and mechanical knowledge that practice has shown to be sound. Wherever a conjecture of any kind has been made it has been so labeled. If nothing more is accomplished than indicating to the refiner of petroleum products the exacting requirements of turbine lubricating oils, so that they may know definitely what is expected of their products, and its present shortcomings in service, the purpose of this paper will have been accomplished.
