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Bible Encyclopedias
Steam Turbines
1911 Encyclopedia Britannica
"STEAM TURBINES ( see 25.823 and 842). - The progress of the steam turbine during 1910-21 was very marked both as regards size and efficiency. The pure Curtis type, in which velocity compounding exists at every pressure stage, has been abandoned, except possibly for very small powers, and the design of impulse turbines now follows generally along the lines first laid down by Rateau, and developed principally by Rateau and Zoelly. A single Curtis wheel is frequently used to absorb the velocity due to the expansion of the steam in the first stage, as this practice permits of a greater heat drop in that stage, so that the pressure and superheat are considerably reduced before the steam is admitted to the body of the turbine. Velocity compounding is recognized as less efficient than the abstraction of the energy of the steam by single impulse blading, but the practical advantage of obtaining a large heat drop in the first stage is often considered to outweigh a slight loss of efficiency. The typical impulse turbine of to-day consists of a horizontal shaft carrying a number of disc wheels, each furnished with a single row of blades around its circumference, and running in its own separate compartment. The diaphragms which separate the compartments contain nozzles which are so proportioned that the steam expanding in them from the pressure which exists in one compartment to that in the next acquires just the velocity which can be efficiently absorbed by the wheel in the second compartment. The description later of a modern impulse turbine will make clear its construction and principles of action.
The reaction machine still maintains its position as regards efficiency and, like the impulse machine, is employed for very large powers. In modern machines, although the thermodynamic principles are identical with those of the earlier machines, there has been a considerable change in details of construction. The modern reaction turbine is frequently fitted with a velocity compounded impulse wheel, upon which the steam acts before passing to the reaction blading, the reason for this being the advantage of reducing the temperature and pressure of the steam before it is admitted to the body casing. It is not unusual to design the impulse wheel so that it absorbs about one quarter of the available energy of the steam, with the result that the drum may be materially shortened, the number of rows of reaction blading greatly reduced, and the cost of the turbine lessened. Other features which are typical of modern reaction machines are the great care taken to eliminate causes of distortion in the casing, by avoiding ports and irregularities of the metal. The casing is always made as symmetrical as possible.
The Reaction Steam Turbine. - Enormous progress has been. made with the reaction turbine invented by Sir Charles Parsons, both as regards size and efficiency, and corresponding mechanical developments have taken place in the design. Land turbines of more than about 10,000 I.W. capacity are usually constructed in two or more parts, each part being a complete turbine, but utilizing only a portion of the total pressure drop of the steam. Sometimes the parts are placed side by side, each driving an independent electric generator, but otherwise they are arranged in tandem on a continuation of the same shaft.
This latter arrangement is illustrated in fig. 1, which shows a section through a large modern two-cylinder machine constructed by Messrs. C. A. Parsons & Co. Ltd. The steam passes from left to right through the blading of the high-pressure cylinder, and is then conducted by means of the circular external pipe to the centre of the low-pressure cylinder. Here it divides, flowing axially in each direction through the blading to the exhaust branches whence it passes to the condenser beneath. The low-pressure cylinder is built on the " double flow " principle in order to avoid the excessive length of blades and size of exhaust branch which would otherwise be required. In turbines of the reaction type there is a difference of pressure between the two sides of every row of blades, and there is thus a corresponding tendency for the steam to leak past the row without passing through the blading. This leakage was a source of considerable inefficiency, and to minimize it, the custom was to employ the smallest practicable radial clearance between the tips of the blading and the opposing surface of the drum or casing. These fine clearances were a source of weakness, as in the event of a slight distortion of the parts by straining or uneven heating, or in case of vibration occurring, there was always a chance of contact occurring, and the danger of this resulting in a stripping of the blades. In order to avoid the fine radial clearances with their attendant danger, Messrs. Parsons introduced the system of endtightened blading, which now represents their standard practice for the high-pressure end of the turbine where the density of the steam makes fine clearances essential. This is illustrated in fig. 2.
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MIN It will be noted that the spacing pieces at the roots of each row of blades project above the surfaces of the rotor and cylinder, as the case may be, and form a continuous barrier. There is a thin brass shrouding strip rivetted to the free ends of the blades, which projects over the side of the blades and is almost in contact with the barrier of the next row. The space between the projecting edge of the shrouding strip and the adjacent barrier forms the actual working clearance which can be adjusted to any desired amount while the machine is running. The radial clearances between the shrouding and the surfaces of the rotor and casing are never less than is in., so that contact is out of the question. Considerably greater economy is said to be obtainable with this type of blading.
The modern type of blading is illustrated in fig. 3. The blades, made of drawn brass strip, are assembled in units, complete with spacing pieces and shrouding. The blades and spacing pieces are brazed solidly together at the roots, and circumferential serrations are then cut in the solid part as shown.
Installation. . Date of test. . K.W. Economical | Chicago Nov. 1918 | Carville Nov. 1919 | Lots Road June 1916 | Dunstan Mar. 1921 | rating.. . | 20,000 | 10,000 | 15,000 | 12,000 | K.W. Output. . | 21,227 | 9,991 | 15,047 | 11,967 | Speed, r.p.m.. . | 750 | 2,442 | 1,000 | 2,400 | Gauge pressure at stop valve, lb. . | 202.4 | 251 | 176'5 | 175.7 | Temperature at stop valve, deg. F. . | 54 8 '7 | 687 | 524'3 | 607.1 | Superheat, deg. F. . | 159'8 | 281 | 146.1 | 229'2 | Vacuum, ins. Hg. . | I 13 | 0.96 | 0.97 | 1.02 | Heat drop, B.Th.U. per lb... . | 408 | 459'4 | 402.8 | 421.2 | Steam consump. per | K.W.H... . | 10.84 | 10.04 | II 82 | Io 80 | Efficiency ratio . | 77.4 | 73'98 | 71'80 | 75.20 | Thermodynamic effi- ciency.. . | 25'31 | 25.7 | 23.35 | 24'70 | Length of time tur- bine had been work- ing. .. . | 5 years. | 21 years. | 3 years. | 4 years. The following table gives the results actually obtained with certain large turbo alternators of the Parsons Company: - In all cases the instruments used were calibrated before and after the tests, and the steam consumption was obtained by weighing the condensate, so that a very high degree of accuracy was obtained. The tests go to show, incidentally, that reaction machines do not fall off in efficiency after several years' operation under commercial conditions of service. 1 The Impulse Steam Turbine 2 The Ljungstrom Steam Turbine 3 Speeds of Turbines 4 Governing of Steam Turbines 5 Bearings and Lubrication 6 Mechanical Gearing of Turbines
1 Number of pressure stages in a practicable turbine. 2 Blade height of a practicable turbine in in. 2.1 The Design of Reaction Turbines The Impulse Steam TurbineThe Rateau steam turbine is a typical modern multistage impulse turbine. Fig. 4 shows a longitudinal section through a machine of this type constructed in 1919 by the Metropolitan Vickers Electrical Co. for the Dalmarnock power station, the machine in question having a maximum continuous rating of 18,750 K.W. at a speed of 1,500 revs. per minute. The shaft carries altogether 15 wheels keyed upon it, each wheel running in a separate compartment. The diaphragms dividing the compartments from each other are fitted with nozzles, in which the steam undergoes successive partial expansions in its progress through the turbine, and from which it emerges with a velocity due to the drop in pressure which it has undergone. This velocity is abstracted by the action of the blading which the steam enters after issuing from each set of nozzles, the steam being brought more or less to rest and the energy due to its partial expansion appearing as useful mechanical work on the shaft. In all large machines of this type, especially when they are working with a high vacuum, the volume of the steam at the low-pressure end becomes so great that the length of the turbine blades at this part tends to become excessive. In the machine in question a part of the steam, after having passed through io wheels, being then at a pressure of about 4 lb. abs. is passed out of the casing and used to heat the boiler feed water, the feed heater for this purpose being shown in section in the illustration. This practice diminishes, FlG.2 to a certain extent, the volume of the steam which passes through the remaining wheels, but in the machine illustrated, the makers have employed a special device to permit a reduction of the length of the last row of blades. The steam which enters the last wheel but one, is divided into two parts, that which acts on the outer annulus of the blade ring passing away directly to the condenser, and only that which acts on the inner annulus being afterwards conducted to the final wheel. The blading on the last wheel therefore only deals with about half the weight of steam which passes through the preceding wheel, and it can handle this amount at a very reduced pressure. A rigid coupling is fitted to connect the turbine shaft with the shaft of the alternator, and the turbine shaft is located axially by means of an adjustable thrust block of the Michell type which takes care of any unbalanced end pressure along the shaft. The mean diameter of the blading of this machine is 84 in. and the length of the last row of blades is 24 inches. The mean circumferential velocity of the blading is 550 ft. per second, the tip velocity of the longest blades being 708 ft. per second. The turbine is designed to work with a stop-valve pressure of 250 lb. per sq. in., a temperature of 650° F. and a vacuum of 0.9 in. of mercury, thus having an available heat drop of 455.2 B.Th.U. per lb. of steam. Under these conditions the guaranteed steam consumpion is 10.2 lb. per K.W.H., this figure being the same for both 15,000-K.W. and 18,750-K.W. load. The Ljungstrom Steam TurbineIn the early days of the reaction turbine, a number of machines were built by the Hon. C. A. Parsons in which the steam passed radially outwards between two discs carrying rings of blades projecting axially from their opposed faces, one disc being stationary and the other driving the shaft of an electric generator. Mechanical difficulties were experienced, principally due to the distortion of the discs by uneven heating, and the design was soon completely abandoned in favour of the axial flow type. In the year 1910 Messrs. Birger and Frederic Ljungstrom of Stockholm built an entirely new type of radial flow reaction machine which was conspicuous not only for its mechanical merits but for its great efficiency. The Ljungstrom turbine is now being developed in sizes up to 30,000 K.W. capacity, and is manufactured in Great Britain by the Brush Electrical Engineering Co. and in the United States by the General Electric Company. The steam is admitted between two discs and in its passage from their center to their circumference it passes through concentric blading rings mounted alternately on the faces of the discs. The discs revolve at equal speeds in opposite directions, so that the relative blade speed is twice as great as in an ordinary machine of the same revolutions and diameter, with the consequence that for equal efficiency the number of blade rings is only one quarter as great. Each disc is fastened to the end of a separate alternator shaft, and as the turbine comes up to speed, the alternators come automatically into synchronism and operate in parallel so that they act virtually as a single machine. F I C. 8 The mechanical construction of the Ljungstrom turbine is unique. Fig. 5 shows a section through a machine to develop 5,000 K.W. at 3,000 revs. per minute, the illustration including the two ends of the alternator shafts, upon which the turbine discs are mounted. The construction will be better understood by reference to figs. 610 which show the most important details to a larger scale. The steam enters the turbine through the branched pipe shown in fig. 5 and thence passes to the centre of each disc through the holes marked 2 in figs. 6 and 7, which illustrate the disc alone. It will be seen that the face of the disc contains a number of circumferential grooves. Each groove carries a blade ring, shown to a larger scale in fig. 8, in which i represents the disc; 2 a seating ring; 3 a caulking strip; 4 an expansion ring; 5 and 6 rolling edges; 7 steam packing strips; 8 caulking strips; 9 strengthening ring; io dovetail profile ring; 11 the blade itself. These blade rings are interleaved as they project alternately from the discs, and steam leakage is checked by the thin fins 7. The blades are made from drawn steel strip and are welded solidly into the strengthening rings io. The conical steel expansion ring, 4, is a particularly important feature of the blading system, and similar rings will be seen at 1 in fig. 7, where they serve to connect the three parts of which the disc is composed. The ring of holes shown at 3 in fig. 7 is to admit the extra steam necessary for overload conditions, the inner rings of blading being then short circuited. The pressure of steam in the blading naturally tends to thrust the discs apart. It is therefore balanced by an arrangement of " dummies," or labyrinth discs, as shown in fig. 5. A detail of the labyrinth, to a larger scale, is given in fig. 9. To prevent the high-pressure steam leaking along the shafts, these are fitted with labyrinth packings, a portion of one of these packings being illustrated in fig. io. The whole packing consists of a number of rings keyed alternately to the shaft and to the housing and having deep grooves turned circumferentially in the sides. The rings interleave in the manner shown, the edges of the grooves being bent down so as practically to make contact with the walls of the grooves in the adjacent rings. An extremely effective and compact labyrinth is thus formed. The efficiency of the Ljungstrom turbine is remarkably high for machines of moderate capacity. Independent tests of a 1,500-K.W. machine, after 15 months' service, have shown a steam consumption of 11.95 lb. per K.W.H., with steam at 208 lb. per sq. in. abs. and 569°F. temperature, and a vacuum of 1.29 in. Hg. The no-load consumption of the same machine was only 1340 lb. per hour, or 7.5% of the full-load consumption. The appearance of a complete Brush-Ljungstrom turbo-alternator is shown in fig. i 1. and vacua as high as 29.1 in., with the barometer at 30 inches. No commercial reciprocating engine could work under such steam conditions with anything like the efficiency a turbine would show in similar circumstances. Speeds of TurbinesThe principal use of steam turbines on land being to drive electric generators, the speed at which these can be run controls to a large extent the speeds for which turbines can be designed. Continuous current turbo generators are comparatively small in size and few in numbers, and as these are almost exclusively driven through reduction gearing on account of the diffioilties of commutation at high speeds. their characteristics do not materially affect the design of the turbines. All large land type turbines are directly coupled to alternators and as the frequency of alternation is wry ? '? ' Steam Conditions in Turbines. - The steam consumption of a turbine depends not only upon the excellence of its mechanical design but upon the amount of heat in every pound of steam delivered to the turbine which is available for conversion into work. The available heat may be increased by increasing the pressure and temperature of the entering steam and by lowering the pressure at which it is exhausted. Progress in these directions is limited by constructional difficulties, but nevertheless striking advances have been made. The best practice of the time may be exemplified by the io,000-K.W. machine installed in 1910 at the Carville station of the Newcastle Electric Supply Co., which operated with steam at 190 lb. per sq. in. gauge pressure and a superheat of 150° F. at the stop valve, and a vacuum of one in. of mercury. Under these conditions there was an available heat drop of 407.2 B.Th.U. per lb. of steam. In 1916 a machine of 11,000-K.W. was installed in the same station with a stop-valve pressure of 250 lb. gauge, a superheat of 244°F. and a vacuum of one in. of mercury. This change in steam conditions increased the heat drop to 450.2 B.Th.U. per lb. of steam. In 1921, a machine having an economical rating of 25,000 K.W., installed at Manchester, utilized a stop-valve pressure of 350 lb. gauge, a superheat of 264° F. and a vacuum of 0.9 in. of mercury, thus working with an available heat drop of 484.7 B.Th.U. per lb. of steam. It may be taken that modern practice sanctions steam pressures up to 350 lb. per sq. in., temperatures up to 700 F. NNINNItw? ' ??I 'IM' ????.v... 'Mil e ".? < ?? ?i ? ' /? I?II r........??..? .? ? I? ? ?.?%., ,?? ..±.?. ? ??- :' .. ,?.?I?/`,?? , i ? ? .6// / /h., ' I ' ?. .???...? ? ?. ,,:?, ? !11 ??..???? '/' L... O a ? ??¦??Q 1. NS a-. ? N. ¦? ? ?. ? ?? ? ? .? ?' ? ? ?? ? ?? 4 ' 'r % ,l i ' " .//," %i standardized in Great Britain at 50 and 25 cycles per second, and in the United States and Canada at 60 cycles per second, the speeds of turbines have to be correspondingly standardized. If F denotes the frequency, and N the number of pairs of poles of the alternator, then F N 60 denotes the only possible speed, in revolutions per minute, at which the turbine can be run. In Great Britain the standard turbine speeds are therefore 3,000, 1,500, 1,000 and 750 revs. per minute, while for 60 cycles they are 3,600, 1,800, 1,200 and 900 revs. per minute. It is naturally desirable to build any turbine for the highest speed at which the desired output can be economically obtained. Considerations of stress limit the dimensions for a given speed, and the dimensions limit the volume of steam which can be ?,..,? j/.,,./, efficiently utilized, so that in practice a fairly definite limit of power corresponding to each speed is obtained. Turbo alternators have been satisfactorily built, having a maximum continuous rating of over 6,000 K.W. at 3,600 revs., the limit of economical rating for this speed being at the present time about 5,000 K.W. At 3,000 revs. per minute the maximum continuous rating is about 13 ,750K.W., the economical output being 12,500 K.W., the machine built in 1921 for the Liverpool corporation being of this size. There are several turbines with a maximum continuous rating of 30,000 K.W. running at 1,800 revs. per minute, and at 1,500 revs. per minute, a continuous rating of 35,000 K.W. appears to be about the present limit, both for impulse and reaction machines. Machines of this size and speed were installed in Chicago in 1918, and in Paris in 1921. In machines of 30,000 K.W., and over it is not uncommonly the practice to use two or more generators, the whole unit really consisting of mechanically independent highand low-pressure turbines. Certain units built by the Westinghouse Co. in the United States have a maximum rated output of even 60,000 K.W., but these in fact consist of three independent turbo generators, through which the steam passes in series. This multiplication of cylinders and shafts is of course the usual custom in connexion with marine turbines. The practice of dividing a turbine into two parts, namely a highand a low-pressure cylinder arranged in tandem, was first introduced many years ago and the design has been standardized for the larger machines of the reaction type. It has the advantage that the separate casings are shorter and less liable to distortion than an equivalent single casing, while by making the low-pressure drum of larger diameter and of the double flow type, the requisite area for the enormous volume of the low-pressure steam is conveniently provided for. The importance of this will be realized from the fact that in a modern turbine the ratio of expansion of the steam may be over 800.1. Fig. 1 shows a section through a two-cylinder tandem turbine as constructed by the Parsons Co., and fig. 12 illustrates the appearance of a two-cylinder side by side arrangement as used with gearing for marine purposes. Governing of Steam TurbinesThe speed regulation of turbines is effected by a centrifugal governor driven by worm gearing from the main shaft, which acts in the case of all reaction machines by controlling the pressure at which steam is admitted to the casing. In machines constructed either wholly or partially on the impulse principle, the governor may open up successively extra nozzles or groups of nozzles as the load increases. Loads in excess of the maximum economical load are sometimes provided for by admitting steam to the turbine at some intermediate point, thus raising the pressure there above the normal full load pressure and enabling the turbine to do more work, although at a somewhat reduced efficiency. The by-pass valves for this purpose may be hand operated, but as a rule they are under the control of the governor and are thus automatically opened when the extra steam is required to maintain the speed. In view of the close governing required on turbo generators and of the size and weight of the valves which have to be operated, it is the universal practice to employ a relay arrangement on all but the smallest machines, the governor merely controlling the position of a small balanced piston valve which admits oil under pressure to one side or the other of a piston which does the actual work of operating the valves. The pressure oil is supplied from the lubrication system of the turbine. Bearings and LubricationThe old sleeve bearing, originally devised by Sir Charles Parsons and employed on his earlier machines, has been entirely superseded and turbine bearings are now constructed on ordinary lines, differing only from slow-speed bearings in their proportions and in the provision necessary for their proper lubrication. The bearings are made in two halves, split horizontally, the interior working surfaces being of white metal cast and anchored into the " steps " which are of cast iron or bronze. These are usually fitted with shimplates to provide a fine vertical and lateral adjustment, and are frequently supported in spherical seatings to permit of a certain amount of self-alignment. Safety strips, often of bronze, which normally lie slightly below the surface of the white bearing metal, are usually provided. These are intended to carry the weight of the shaft safely in the event of the white metal being melted out, and thus prevent injury to the blading until the machine can be stopped. In all turbine bearings the important thing is to insure a copious supply of lubricating oil, not so much for lubrication as to carry off the heat generated by friction and to maintain the bearings at a reasonable working temperature. Water-cooled bearings have been used by some makers, but the most approved practice is to rely on the flow of oil through the bearing to keep its temperature down. Oil is usually delivered to the bearings at a pressure of about 15 lb. per sq. in., a gauge being provided on each bearing to indicate whether the pressure is being maintained. On modern turbines an automatic device operated by the oil pressure is fitted, which shuts the machine down in case of any failure of the oil supply. Bearings up to 8-in. diameter are usually bored larger than the shaft to the extent of about 0.004 in. for every in. of shaft diameter. In larger bearings the clearance is proportionately less. This somewhat large clearance enables the heat to be carried away by the continuous wash of fresh cool oil. The shaft, when running, is kept out of metallic contact with the bearing by a thin film of oil continually dragged underneath it by its rotation. It is this film which supports the shaft, and the pressure of the latter on the bearing must therefore not be greater than the film can stand. Theory and experiment both indicate that the greater the surface velocity of the shaft, the more effectively is the film established, and the greater therefore the permissible load on the bearing. But the fact that bearings have to start from rest, when the film is imperfect, imposes a practical limit to the load which can be imposed. A formula connecting permissible pressure with velocity, given by Mr. F. H. Clough, is P =17 1,/ V, in which P denotes the pressure in pounds per sq. in. of projected area, and V = velocity of surface of shaft in ft. per second. This is said to be applicable to bearings of normal design in which the length is from twice to three times the diameter. Many designers, however, use the rule that P X V must not exceed 5,600, a simple rule which gives good results in practice, and probably has a considerable margin of safety when the speeds are high and when there is no vibration. One large manufacturing firm is said to take the permissible pressure per sq. in. of projected area as ranging from 167 to 235 when the velocity ranges from 20 to 73.5 ft. per second. Modern practice is to give P a value not exceeding 150 lb. in bearings where the velocity is not greater than 30 to 35 ft. per second, and the temperature comparatively low, say, moo° to m mo° F. Such conditions would apply to low speed marine turbine bearings. The bearings of land turbines usually work at temperatures from 120° F. to 160° F., but the latter temperature should not be exceeded, as not only is the oil injured, but its viscosity is so low that the supporting film is thinned and the margin of safety becomes low. For the heat generated in a turbine bearing Stoney gives the formula B.Th.U. per hour - 19l 32 v in which l and d are respectively the length and diameter of the bearing expressed in in., v is the velocity of the surface of the shaft in ft. per second, and t is the temperature on the Fahrenheit scale. The same authority quotes the following formula as often used in slow-speed marine practice: B.Th.U. per hour = lXdXv 1 ' 38 . Treating the heat which escapes by radiation and conduction as negligible, these formulae give the heat which has to be carried away by the oil and extracted by the oil cooler. This heat of course is the equivalent of the work lost by friction in the bearing. The increase of temperature of the oil passing through the bearing should not exceed 10° to 20°F., and if the specific heat of oil be taken at 0.31 the minimum quantity of oil required for each bearing may be readily calculated. In practice it is advisable to increase this calculated fig. by from 30 to 50%, to allow a margin for steam heat travelling along the shaft and other contingencies. Mechanical Gearing of TurbinesThe De Laval steam turbine, consisting of a single impulse wheel running at a speed of 30,000 to 1 0,000 revolutions per minute according to the size, has always contained reduction gearing as an integral part of the machine because such speeds are far too high for driving ordinary machinery. Turbines of this type have, however, only been built for powers up to a few hundred horse-power, and although the use of reduction gear may be dated from the introduction of the Laval turbine in 1886, it never became a recognized practice for large powers until it was developed by Sir Charles A. Parsons as the solution of the problem of marine propulsion. De Laval had shown that it was possible to transmit power satisfactorily through mechanical gearing running with a circumferential velocity of over moo ft. per second. The gears he used were of the double helical type with a spiral angle of 45 degrees. The reduction ratio was usually about 10:1, and the pitch of the teeth varied from 0.15 in. to o 26 in., according to the power of the turbine. The De Laval gear embodied all the features which have been found necessary to the successful performance of modern gears transmitting several thousand horse-power through a single pinion. The double helical form of tooth of comparatively fine pitch has been retained, as this design eliminated end thrust and insured silent running by reason of the number of teeth simultaneously in contact. Ample lubrication of the teeth by means of oil jets was also employed by De Laval, who succeeded in producing durable and satisfactory gears which had an efficiency of about 97 per cent. These gears are used up to about 600 H.P. which is the commercial limit of the type of turbine for which they are designed. Steam turbines of any type, designed with due regard to efficiency and cost of manufacture, require to run at a far higher speed of revolution than is practicable for screw propellers, especially when the latter are employed to drive ships of moderate speed. The coupling of a turbine, therefore, directly to a propeller shaft involves a compromise in design, in which the speed is greater than desirable for the propeller yet so low as to require the turbine to be of greater size and weight and of lower efficiency than it would otherwise be. In the case of high-speed vessels direct coupling afforded a commercially acceptable solution of the problem of turbine propulsion, and for vessels of eighteen knots speed and over, such as warships, passenger liners and cross-channel boats, the direct coupled turbine soon became the recognized driving power. But ordinary cargo vessels and tramp steamers, with an average speed of 10 or 12 knots, were outside the practical field of the steam turbine until speed reduction gearing was available to couple a high-speed turbine with a slow moving propeller. It was really the problem of the slow speed ship which brought about the development of marine turbine gearing, and now that the mechanical difficulties have been overcome, the direct coupled marine turbine is likely to be largely displaced by the geared turbine in all classes of vessels. The first example of marine turbine reduction gearing appears to have been in 1897, in connexion with a twin screw launch, in which the Parsons Marine Steam Turbine Co. fitted a to-H.P. turbine driving the two shafts by means of helical gearing having a speed ratio of 14:1. The result appears to have been entirel y satisfactory. Other experiments followed, and in 1909, the "JVespasian," a cargo vessel of 4,350 tons displacement, was fitted with geared turbines driving a single propeller. This vessel had previously been equipped with triple expansion reciprocating engines of the usual type, and before these were removed they were put into perfect order, and very careful tests were made to determine the efficiency and performance of the vessel. The geared turbines drove the same shaft and propeller as the engines had done and were supplied with steam from the same boilers. The power developed was about 1,000 H.P. and the shaft ran at 70 revs. per minute, the gear reduction ratio being 19.9:1. The installation of the turbines resulted in an increase of about one knot in speed for the same coal consumption, and the results of the trials were highly satisfactory in every respect, and convincing as to the advantages of geared turbines over reciprocating engines. After the " Vespasian " had run 18,00o m. in regular service, the pinion was examined and found to be in perfect condition, the wear not exceeding 0.002 inches. (See Trans. I.N.A. 1910 and 1911.) The success of the " Vespasian " led to rapid developments. In 1910 the British Admiralty adopted gearing, the torpedo boats " Badger " and " Beaver " being the first warships to be equipped with geared turbines. In these vessels each L.P. turbine drove its shaft directly, but the H.P. and cruising turbines were geared to a forward extension of the turbine spindles. At full load about 3,000 H.P. were transmitted through each set of gearing. Six years later complete gear drives had become the standard practice for British war vessels of all types and by 1920 some 652 gears, transmitting an aggregate of 7,280,000 shaft H.P., were fitted, or on order for the royal navy (Tostevin, Trans. I.N.A. 1920). The appended particulars of H.M. battle cruiser " Hood," of 144,000 shaft H.P., which was completed in 1920, will indicate the development of gearing for turbines and will at the same time indicate the proportions which have been adopted.
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