Online Encyclopedia


Online Encyclopedia
Originally appearing in Volume V25, Page 848 of the 1911 Encyclopedia Britannica.
Spread the word: it!
CCCCC CCCCCC_CCC EDDIODBOBInnDODD 1 I 1 material increase in the ratio of steam consumption to output. In tests of a 9000 kilowatt Curtis turbine using steam of about 200 lb pressure and So' C. superheat, with a vacuum of 291 in. the consumption of steam is reported to have been only 13 lb per kilowatt-hour, and this figure remained almost constant for loads ranging from 8000 to 12,000 kilowatts. In a 5000 kilowatt turbine under very similar conditions the consumption is reported to have been 131 lb per kilowatt-hour. In the usual arrangement of the Curtis turbine the shaft is vertical and the wheels lie in horizontal planes, the weight of the revolving parts being taken by a footstep bearing with forced lubrication, and the electric generator is mounted on the top. There are usually in the large sizes four stages of expansion, each stage being separated from the one above it by a diaphragm plate containing the nozzles in which the next step in velocity is acquired. The expansion has been divided into as many as seven stages in a Curtis turbine for marine use, the shaft being then horizontal, and in all except the first stage in that example the pressure drop is so comparatively small as not to require divergent nozzles. 118. Parsons Turbines. In the turbines of De Laval and Curtis the action on the moving blades or buckets is entirely one of impulse. No drop of pressure occurs while the steam is passing the moving blades, and its velocity relative to the blade surface undergoes no change except such as is brought about by friction. In the Parsons turbine, on the other hand, there is a reaction effect. The steam acquires relative velocity and loses pressure as it passes each ring of moving blades: in this respect the action in the moving blades is like the action in the fixed blades. Each pair of fixed and moving rings makes up what is called a " stage " and may be said to constitute a separate turbine: the whole is a series of many such stages. In each stage the drop in pressure and in heat is divided equally between the fixed and moving element, the exit and entrance angles and the form of the blades generally being alike in both. The number of stages depends on what peripheral speed it is convenient to use. Where comparatively high blade speeds are practicable, as in turbines for driving electric generators, the steam is allowed to acquire a fairly high velocity at each ring of blades, and in such cases so few as 45 stages may be suitable. In large marine turbines, on the other hand, where the number of revolutions per minute has to be kept low in the interests of propeller efficiency, the blade speeds cannot be kept high without making the diameters unduly great, and consequently more stages are required: in such turbines the number of stages may be from 100 to 200. The general relation of fixed to moving blades and the characteristic form of both will be seen from fig. 59. Fig. 6o shows a complete Parsons turbine of i000 kilowatts capacity in longitudinal section through the casing. The fixed blades are caulked with separating distance-pieces into grooves turned on the inner surface of the case and project in-wards: the moving blades are similarly secured in grooves which are turned on the surface of the rotating drum. Between drum and case there is an annular space fitted in this way with successive rings of fixed and moving blades. There is considerable longitudinal clearance from ring to ring, but over the tips of the blades the clearance is reduced to the smallest possible amount consistent with safety against contact (generally from 15 to 30 thousandths of an inch in turbines of moderate size). Steam enters at A, expands through all the rings of blades in turn and escapes to the condenser at B. To provide for the increase in its volume the size of the blade passages enlarges progressively from the high to the low pressure end. In the ex-ample shown this is done partly by lengthening the blades and partly by increasing the circumference of the drum, which has the further effect of increasing the blade velocity, so that the expanded steam not only has a larger area of passage open to it but is also allowed to move faster, and consequently each unit of the area is more effective in giving it vent. Instead of attempting to make the change in passage area continuous from ring to ring, as the ideal turbine would require, it is done in a limited number of steps and the several rings in each step are kept of the same size. Thus in the example shown in the figure the first step consists of seven pairs of rings or stages, the next two also of seven each, the next three of four each, the next of two and so on. This is convenient for constructive reasons and gives a sufficiently good approximation to the ideal conditions as regards the relation of steam volume to blade-passage-area and velocity. Fixed Blades ««««« dosing Blades ))))))))))) fixed Blades «\«««< raing Blades ))))))))))) of Parsons Turbine. Ip 111111' ~~ .~RIIIIIIIIIt^iiiiiiiii Mmommunni q Fio. 6o.-Parsons Turbine. 119. Balance of Longitudinal Forces: Dummies.—Since the pressure of the steam falls progressively from left to right there is a resultant longitudinal thrust on the drum forcing it to the right, which is balanced by means of " dummy " rings C' C" C"'. These correspond in diameter with the several portions of the bladed drum and are connected with them by steam passages which secure that each dummy shall have the Jame pressure forcing it to the left as tends on the corresponding part of the drum to force it to the right. No steam-tight fit is practicable at the dummies, but leakage of the steam past them is minimized by the device of furnishing the circumference of each dummy with a series of rings which revolve between a corresponding series of fixed rings projecting inwards from the case. The dummy rings do not touch but the clearance spaces are made as fine as possible and the whole forms a labyrinth which offers great resistance to the escape of steam. Substantially the same device is employed to guard against leakage in the glands DD where the shaft leaves the turbine case. There is a " thrust block " E at one end of the shaft which maintains the exact longitudinal position of the revolving part and allows the fine clearances between fixed and moving dummy rings to be adjusted. ENGINE 122. Drums.—In small turbines the drums carrying the re. volving blades are solid forgings; in large turbines they are also of forged steel but in the form of hollow cylinders turned true inside as well as out. These are supported on the shafts by means of wheel-shaped steel castings near the ends, over which they are shrunk and to which they are. fastened by screws the heads of which are riveted over. The case is of cast iron with a longitudinal joint which allows the upper half to be lifted off. 123. Governing.—The governor regulates the turbine by causing the steam to be admitted in a series of blasts, the duration of which is automatically adjusted to suit the demand for power. When working at full power the admission is practically continuous; at lower powers the steam valve is opened and closed at rapidly recurring intervals. Each revolution of the governor shaft causes a cam, attached to the governor, to open and close a relay valve which admits steam to a cylinder controlling the position of the main steam valve, which accordingly opens and closes in unison with the relay. The position of the governor determines how long the relay will admit steam to the con-trolling cylinder, and consequently how long the main valve will be held open in each period. In turbines driving electric generators the control of the relay-valve is sometimes made to depend on variations of the electric pressure produced instead of variations in the speed. In either case the arrangement secures control in a manner remarkably free from frictional interference, and therefore secures a high degree of uniformity in speed or in electric pressure, as the case may be. To admit of overloading, that is, of workinPP at powers considerably in excess of the full power for which the turbine is designed, provision is often made to allow. steam to enter at the full admission pressure beyond the first G,et of rows of blades: this increases the quantity admitted, and; though .the action is somewhat less efficient, more power is developed. An orifice will be seen in fig. 6o a little to the right of the main steam admission orifice, the purpose of which is to allow steam to enter direct to the second set of blades, missing the first seven stages, so that the turbine may cope with overloads. 124. Absence of Wear.—Owing to its low steam velocities the Parsons turbine enjoys complete immunity from wear of the blades by the action of the steam. A jet of steam, especially when wet, impinging at very high velocity against a metal surface, has considerable cutting effect, but this is absent at velocities such as are found in these turbines, and it is found that even after prolonged use the blades show no signs of wear and the efficiency of the turbine is unimpaired. 125. Blade Velocity.---Experience has shown that the most economical results are obtained when the velocity of the steam through the blades is about twice the velocity of the blades them-selves, and the Parsons turbine is accordingly designed with, as far as possible, a constant velocity ratio of about this value. As already explained, it is convenient in practice to divide the expansion into a comparatively small number of steps (about twelve steps is 'a usual number), giving a constant area of steam passage to the first few rows, a larger area to the next few. and so on. An effect of this is that the velocity ratio varies slightly above and below the value of two to one, but if the steps are not too great this variation is not sufficient materially to affect the efficiency. If the spindle or drum carrying the moving blades were of the same diameter throughout, the blades at the, exhaust end' would have to he exceedingly long in order to give passage to the rarefied steam. By increasing the diameter towards the exhaust end the peripheral velocity is increased, and hence the proper velocity for the steam is also increased. The amount of heat drop per ring is consequently greater towards the low-pressure end: in other words, the number of rings for a given drop is reduced. Taking the turbine as a whole, the number of rings will depend on the blade velocity at each step, the relation being such that InVb2=constant for a given total drop from admission to exhaust, n being the number of rings whose blade velocity is Vb. It appears that a usual value of this constant is about 1,500,0001 for the whole range from an admission pressure which may be nearly zoo lb per sq. in. down to condenser pressure. Speakman, " The Determination of the Principal Dimensions of the Steam Turbine with special reference to Marine Work," Proc. Inst. Engineers & Shipbuilders in Scotland (October 1905). On this subject see also Reed, " The Design of Marine Steam Turbines," Proc. Inst. Civ. Eng. (February 1909). 120. Lubrication.—The main bearings LL are supplied with oil under pressure kept in circulation by a rotary pump F which draws the oil from the tank G. The pump shaft H, which also carries a spring governor to control the speed of the turbine, is driven by a worm on the main shaft. The same oil is circulated over and over again and very little of it is consumed. No oil mixes with the steam, and in this point the turbine has a marked advantage over piston and cylinder engines, which is especially important in marine use. In small fast-running turbines each bearing consists of a bush on which three con-centric sleeves are slipped, fitting loosely over one another with a film of oil between. The whole acts as a cushion which damps out any vibration due to want of balance or alignment. In large turbines this device is dispensed with and a solid brass bearing lined with white metal is employed. 121. Blades.—The blades are generally of drawn brass, but copper is used for the first few rows in turbines intended for use with superheated steam. In the most usual method of construction they are put one by one into the grooves, along with distance pieces which hold them at the proper angle and proper distance apart, and the distance pieces are caulked to fix them. The length of the blades ranges from a fraction of an inch upwards. In the longest blades of the largest marine steam turbines it is as much as 22 in, When over an inch or so long they are strengthened by a ring of stout wire let into a notch near the tip and extending round the whole circumference. Each blade is " laced " to this by a fine copper binding wire, and the lacing is brazed. For long blades two and even three such rings of supporting wire are introduced at various distances between root and tip. The tips are fined down nearly to a knife-edge so that in the event of contact taking place at the tips between the " rotor " or revolving part, and the " stator " or case, they may grind without being stripped off. The possible causes of such contact are wear of bearings and unequal expansion in heating up. With a proper circulation of oil the former should not take place, and the clearances are made large enough to provide for the latter. Various plans have been devised to facilitate the placing and fixing of the blades. In one method they are slung on a wire which passes through holes in the roots and in the distance pieces and are assembled before-hand in a curved chuck so as to form a sector of the required ring, and are brazed together along with the supporting wires before the segment is put in place. In another method the roots are fixed in a brass rod in which cuts have been machined to receive them; in another the rod in which the roots are secured has holes of the right shape formed in it to receive the blades by being cast round a series of steel cores of the same shape as the blades: the cores are then removed and the blades fixed in the holes. The increased diameter at the low-pressure end not only allows the steam velocity to be increased but by enlarging the annulus enables a sufficient area of passage to be provided without unduly lengthening the blades. In the very last stages of the expansion, however, the volume becomes so great that it is not practicable to provide sufficient area by lengthening the blades, and the blades there are accordingly shaped so as to face in a more nearly axial direction and are spaced more widely apart. The area of the steam passage depends on the angle of the blade. If the blades were indefinitely thin it would be equal to the area of the annulus multiplied by the sine of the angle of discharge, and in practice this is subject to a deduction for the thickness of the blade on the discharge side, as well as to a correction for leakage over the tips. Generally the angle of discharge is about 224°; and the effective area for the passage of steam is about one-third of the area of the annulus. Fig. 6t A shows a representative pair of fixed and moving blades of a Parsons turbine, and fig. 61 B the corresponding velocity diagram for the steam, neglecting effects of friction. Vi is the exit velocity from the fixed blades, the delivery edges of which are tangent to the direction of Vi. The blade velocity is u which is 4V1. V2 is consequently the relative velocity with which the steam enters the moving blades. Approximately, the back surface of these blades is parallel to V2, but the blades are so thick near the entrance side that their front faces have a considerably different slope and there is therefore some shock at entrance. In passing through the moving blades the relative velocity of the steam over the blades changes from V2 to V3. Allowing for the velocity u of the blades themselves, this corresponds to an absolute velocity V4, with which the steam enters the next set of fixed blades. In these blades it is again accelerated to Vi and so on. 126. Calculation of Velocity at each Stage.—The acceleration of the steam in each row of blades results from a definite heat drop. Or, if we look at the matter from the point of view of the pressure-volume diagram, the acceleration results from the work done on the steam by itself during a drop by in its pressure. The amount of this work per pound is vbp where v is the actual volume per pound. It is convenient in practice to write this in the form (pv)bp/p, for the product pv changes only slowly as expansion proceeds. In designing a turbine a table of the values of pv throughout the range of pressures from admission to exhaust is prepared, and from these numbers it is easy to calculate the work done at each stage in the expansion, the pressure p and drop in pressure by being known. In the ideal case with no losses we should have +V pip Vie = g(pv) p/p A. Cylinder H B. Rotor C. Balance Piston D. Bearing E. Adjusting Block F. Steam Packed Blondswhere V4 is the velocity before the acceleration due to the drop by and Vi is the velocity after. But under actual conditions the gain of velocity is less than this, owing to blade friction, shock and other sources of loss. The actual velocity depends on the efficiency and on the shape and angles of the blades. It appears that under the conditions which hold in practice in Parsons turbines it is very nearly such that Vie =n(pv)bp/p. In this formula, which serves as a means of estimating approximately the velocity for purposes of design, it is to be understood that in calculating the product pv the volume to be taken is that which is actually reached during expansion. The actual volume is affected both by friction and by leakage and is intermediate in value between the volume in adiabatic expansion and the volume corresponding to saturation. In the case of a turbine of 70 %efficiency the actual wetness of the steam is, according to Mr Parsons's experience, about 55 % of that due to adiabatic expansion in the early stages and 6o %o in the latest stages. In preparing the table of values of pv figures are accordingly to be taken intermediate between those for saturated steam and for steam expanded adiabatically, and from these is found as above the velocity for any given drop in pressure, and also the volume per pound, for which at each stage in the expansion provision has to be made in designing the effective areas of passage. The blade speeds used in Parsons turbines rarely exceed 350 ft. per second and are generally a good deal less. In marine forms, where the number of revolutions per minute is limited by considerations of efficiency in the action of the screw propeller, the blade speeds generally range from about 120 to 150 ft. per second, though speeds as low as 8o ft. per second have been used. 127. Parsons Marine Turbines.—Marine turbines are divided into distinct high and low pressure parts through which the steam passes in series, each in a separate casing and each driving a separate propeller shaft. The most usual arrangement is to have three propeller shafts; the middle is driven by the high pressure portion of the turbine, and the steam which has done duty in this is then equally divided between two precisely similar low pressure turbines, each on one of two wing shafts. The rotor drum of each turbine has a uniform diameter through-out its length, but the casing is stepped to allow the lengths of the blades to increase as the pressure falls. The casing which contains each of the two low pressure turbines contains also a turbine for running astern, so that either or both of the two wing shafts may be reversed. Steam is admitted to the reversing turbine direct from the boiler, the centre shaft being then idle. Each astern-driven turbine consists of a comparatively short series of rings of blades, set for running in the reversed direction, developing enough power for this purpose but making no pretensions to high efficiency. The astern turbine, being connected to the condenser, runs in vacua when the ahead turbine is in use and consequently wastes little or no power. Figs. 62 and 63 are sections of the high pressure and low pressure portions of a typical Parsons marine steam turbine, as designed for the three-shaft arrangement in which the low pressure portion is duplicated. In each figure A is the fixed casing and B is the revolving drum. Steam enters the high pressure turbine (fig. 62) through J and passes out through H. There are 4 " expansions" or steps, with 9 stages or double rows of blades in the first, 9 in the G. Worm for Actuating Governor H. Exhaust to Low Pressure rur61N J. EYean, lntet It. rrrlline Drain L. Oil two M.00 Drain '~'~L~I-G QL - 6III I~ ill ii 4111 II I I Fixed Blades or aimMIMlvli!s ,1 D I rot WI/1V lee MI 1.1 111I 1.1. 1•1 MS WI IOU II OS Upper half : sectional elevation. Lower half : external view.
End of Article: CCCCC

Additional information and Comments

There are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click and select "copy." Paste it into a website, email, or other HTML document.