Since Sir Charles A. Parsons produced in 1884 his first practicable steam turbine, enormous advances have been made with this form of prime mover. The generation of electricity on a rapidly growing scale all over the world during recent years has led to the almost universal use of steam turbines for the driving of electric generators where water power is unobtainable.
Other fields of their application are numerous and cover a range of sizes from the little 5 horse-power turbine running at 30,000 revolutions a minute, to the 200,000 horse-power turbine. On land and sea, fast-running pumps, fans and the like are now driven by steam turbines. Turbine-driven merchant ships afloat aggregate over 9,000,000 tons and the leading navies of the world are almost entirely propelled by steam turbines. Enormous concentration of power, combined with the highest economy, has placed the steam turbine in this outstanding position.
This power and economy could not be obtained in the old reciprocating steam engine, for, in the first place, in this type the piston has to reverse its movement at the end of each stroke. In the turbine the turning force is applied in one direction continuously. Secondly, in the reciprocating engine, due to the use of a crank and connecting rod, the force turning the shaft changes every moment and this irregularity is aggravated by the varying pressure in the cylinder.
In the turbine the turning force is always constant and the symmetrical shape of the rotating parts, moreover, enables an almost perfect balance to be obtained. The reciprocating engine has many sliding parts in close contact, causing friction and wear and needing continual lubrication. In the turbine the moving parts do not touch the stationary ones and only the shaft bearings need to be oiled. From this it follows that no oil is carried over into the condenser, an important advantage, particularly in marine work. Again, a steam turbine can use a higher condenser vacuum, than the reciprocating engine—a fact which enables more of the heat in the steam to be turned into work.
The turbine, however, is not without disabilities. It is not reversible in the same way as the reciprocating engine. This does not matter in, say, a turbogenerator in a power station, but at sea it means that separate turbines are necessary for going ahead and for going astern. Again, the turbine must run at a high speed; sometimes, as at sea, too high for direct utilization, and so a speed-reducing gear is necessary. In the steam turbine steam at high velocity is caused to impinge on to a number of small curved blades mounted on a wheel, or a drum, so that a rotary motion is imparted to the shaft.
Not all turbines are the same. There are two principles on which turbines may work—impulse and reaction. The impulse turbine is best illustrated by the simple type which was invented by a Swede, De Laval, at about the same time as Parsons was working on the reaction turbine. Inclined to the bladed wheel are nozzles through which steam is admitted. These nozzles are tapered inside in such a way that as the steam passes through them its pressure falls but its velocity increases. Part of this velocity is imparted to the blades and so the wheel spins round. The pressure is the same on both sides of the blades, and on leaving them it is thus generally in a fit condition to do some more work.
The steam may therefore be made to pass through a second set of nozzles, in which the pressure drops as before, and then to impinge on another set of blades. This process may be carried out in a number of stages so that the steam is expanded as far as it usefully can be in a manner analogous to the expansion of steam in the cylinder of a reciprocating engine. This type of machine is called the pressure-compounded turbine, but it is not a true De Laval turbine.
As soon as Parsons had shown the possibilities of the turbine as a motive power, other inventors got to work. Professor Rateau, in France, developed the pressure-compounded turbine in 1898, and the Swiss engineer Zoelly invented an impulse turbine of the same class.
The American Curtis also brought forward an impulse machine with multi-pressure stages, though he combined it with an effect known as velocity compounding. The simple De Laval machine is not suitable for large powers because the high speed at which it must work prevents the use of large parts, but the other impulse machines do not suffer from this disability and some powerful sets have been built.
Although the wheels in a typical multiple-stage impulse turbine are all fixed to the same shaft, each runs in a chamber of its own. These chambers are formed by partitions, called diaphragms, attached to the outer casing, and either the diaphragms may be perforated near their circumference with a ring of nozzle holes, or the nozzles may be replaced by a ring of guide blades which act in a similar manner. The system used depends on the views of the maker.
A reaction turbine at first sight does not appear to be much different, but in principle and construction the Parsons reaction turbine is widely different. There are no diaphragms in this type of turbine. The wheel or rotor is generally in the form of a drum which is provided with a number of blades fixed in grooves running circumferentially. These rings of blades alternate with exactly similar blades fixed to the casing but projecting inwards. The steam expands, moreover, in the fixed blades as well as in the moving blades, so that the pressure is falling constantly as the steam moves from the inlet end of the turbine to the outlet end and not in stages as in the impulse turbine.
In both main types of turbine the steam in passing through the turbine, in a zigzag fashion but generally in a direction parallel to the shaft, causes the bladed wheels to turn by changes in pressure or velocity or both combined. There is one form of turbine, of the reaction type, in which the steam does not move parallel to the shaft but in a radial direction. This is the Ljungström turbine, a Swedish invention of 1913.
The large multistage turbine is not necessarily a single piece of machinery. When high pressures are used, it is customary to divide the turbine. Thus there may be two cylinders, as they are called, a high-pressure and a low-pressure one. In large marine installations it is not uncommon to find an intermediate-pressure cylinder as well. In an impulse turbine the rotor is generally made up of a shaft and wheels. The shaft forging is made from high-grade steel, and is of ample dimensions to ensure safety under the maximum loads which may be applied when working. It has to be rigid enough to withstand high speeds of rotation.
A hole is bored right through the centre of the shaft from end to end, so that the metal can be carefully examined throughout its length for any flaws which might exist at the centre. This inspection can be done by means of an instrument called a "Borescope," which lights up the interior of the borehole and enables a minute scrutiny to be made of every spot on its surface.
In the rotors of smaller impulse turbines the wheels carrying the running blades may bo forged solid with the shaft, thereby ensuring the most rigid construction, especially as the smaller machines run at higher speeds. In the large rotors, and particularly those for the low-pressure end, this cannot be done, and the wheels are made separate.
The shaft is turned in a lathe to the various sizes along its length to accommodate the wheels, the main bearing journals and other parts connected with it. Keyways are sometimes cut along the length of the portion occupied by the wheels, so as to secure these firmly in place and to transmit the power from the wheels to the shaft.
The size and form of the running wheels depend on the size and type of turbine. They are, however, all made from solid forgings of high-grade steel or from special alloy steel subject to stringent specification, tests and inspection, and of solid disk form. Thick at the boss centre, and tapering to a minimum thickness at the outer edge, sufficient to take the root of the blades, the section of the metal throughout is carefully calculated to combine the best distribution of material for equality of stress, with the maximum stiffness to resist axial vibrations.
Perfection of Balance
The hole bored through the boss of the wheel is finished to a size slightly smaller in diameter than that through the shaft. The wheel is then heated over a gas-fired furnace until it is sufficiently hot to expand the boss bore to just perceptibly larger than the shaft diameter. The wheel is then quickly slipped over the shaft to the position it will finally take, and is allowed to cool again, so that it will have shrunk and have obtained an extremely tight grip.
The wheels are most accurately machined all over their surface, with a fine polished finish, so that the slightest flaw is detectable. The outer edge is turned with a groove to the particular form of dovetailing which the maker considers most satisfactory, so as to obtain a rigid grip of the blades attached to them. The wheels are dynamically balanced at low speed in a balancing machine, and the perfection of balance is finally proved by running the complete rotor with the wheels on it at 15 per cent above the normal running speed. A failure of these wheels rotating at the maximum speed would, be as disastrous as a serious explosion ; so the overspeed test running of the rotor is generally carried out in a massive reinforced concrete safety chamber.
To build a machine which will stand up to the high centrifugal and other forces involved and at the same time be highly efficient, reliable and easily controlled, is an engineering problem which demands the highest skill and constructive ability.
As a single example of these forces, if the rotor of a steam turbine were out of balance to the extent of only 1 lb. (the rotor of a large turbine weighs several tons) at a radius of, say, 3 feet from the centre of the shaft, the periodic force, when running at 2,000 revolutions a minute and tending to produce vibration, would amount to nearly two tons. Even large machines now run at 3,600 revolutions a minute, at which speed the force in question, which varies as the square of the speed, would be almost six tons.
In the Parsons reaction turbine the rotor generally consists of a shaft which is forged with large drums, or parts of larger diameter, where the blades are situated. The same scrupulous care is taken with internal and external inspection and balancing as with the plainer impulse shafts, but as there are no wheels the number of manufacturing operations is reduced. The blading is attached directly to the solid forging, whereas in the impulse type machine it is attached to the rims of the wheels.
The blades of an impulse turbine may vary in size from the smallest at the steam inlet end to the largest at the final exhaust end, and may range, according to size of machine, from, say, 2 in. to about 3 feet long in the last stage low-pressure wheel of a turbine of 50,000 or 75,000 kilowatts.
For ordinary steam temperatures the blades may be of bronze or mild steel ; but, when subject to high temperatures of superheated steam, or to the erosive action of particles of water and impurities in the steam, they are made of stainless steel or stainless iron.
The blades are cut from bars which have been rolled or extruded to approximately their final shape, thereby saving material and machining. The cross-section form of the blade is of extreme importance, especially on the face on which the steam impinges. The blades are carefully finished by various machining operations.
Diverse methods are adopted for fixing the blades to the rim of the wheel. One of these is a form of dovetailing. The top, or outer end of the blades has a small projection cut out of the solid. This fits into a hole or slot in an outer shrouding band made up in sections and encircling the whole wheel of blades. These little projections are then riveted over the outside. The shroud ring is made slightly wider than the blades, so that, should the running wheel be inclined to touch the diaphragm sides, the shroud will touch first and little, if any, damage will be done.
The blades are fitted into the dovetailed slot turned in the periphery of the wheel and of a form corresponding to the root of the blade. A small space is left cut out of the slot, forming a gate through which each blade is entered in turn, and then pushed round the slot. The blades are correctly spaced in the wheel rim by distance pieces, either separate from or solid with the blade.
After all the blades have been inserted, the last one to be entered through the gate is made perfectly rigid by a caulking piece between the first and last blades assembled, and then it is pinned.
The cross-section of the blades of the reaction turbine is quite different from that of the impulse turbine blades, but there is a general resemblance in the methods of manufacture and assembly. In some reaction turbines, however, the root of the blade is serrated. Such blades may be either of the single integral type or of the segmental type. In this type the individual blades are assembled in groups, with their distance pieces, before being fitted in the rotor, each group consisting of eight to twelve blades. Both the fixed and the moving blades are shrouded, the shrouds projecting on one side of the blade with a sharp knife edge which is almost in contact with the barriers formed by the roots of adjacent blade rows.
The edge of the shrouding seals the blade assemblies against short-circuiting of the steam so that they are quite as effective when thin as when thick. But should contact occur between the moving parts, these thin edges will wear away without setting up serious trouble from expansion or heating. Parsons from the first recognized the advantage of this method of shrouding and it is now nearly always fitted.
Blading of the segmental type is used almost exclusively for the high-pressure section of the reaction turbine. The integral type is used towards the low-pressure end. The whole operation of blading a turbine is a mechanical one, that is, the angles and spacing of the blades are all determined carefully beforehand and the blades are made to close limits, so that the men who have the job of putting them in are not called upon to do other than exercise care in assembly.
In one type of impulse turbine steam passes through a number of nozzles bolted to the interior of the high-pressure casing. These nozzle segments may consist of segmental castings with steel guide plates cast in position, or alternatively they may be built up from forged steel blanks in which the nozzle openings are milled out, the nozzles being then assembled in steel segments and finally welded together. The sides of the nozzles are highly finished to afford the least possible resistance to the flow of steam.
Between each two adjacent wheels of the turbine, except in the pure reaction stages of such a turbine, a diaphragm is provided in which are nozzle openings through which the steam passes from one wheel to the next.
The diaphragm is made in halves with a tongued and grooved steamtight joint at the horizontal division. It is either of mild steel or of high-grade cast iron, the choice depending on the temperature conditions. The diaphragm is located in position in the top and bottom half casings by shoulders machined inside the casing, and is located axially by pins which fit into holes in the diaphragm on the inlet side near the periphery and bear against the shoulders in the casing. The central opening in each diaphragm through which the shaft passes is sealed by the yielding labyrinth type of packing gland. In the reaction turbine, such as the Parsons, there is no gland of any kind between the rows of blades. The fixed blades extend in a ring from the inner surface of the casing nearly to the surface of the rotor, and the seal against leakage from one row of blades to the other is provided by the shrouding rings.
The arrangement of the wheels on the rotor shaft depends on the design in question. These may take the form of one or more of the impulse type, followed by a number of the reaction type when the machine is a combined impulse and reaction turbine. A somewhat similar arrangement may be used in the high- or the low-pressure cylinders, or in both. Again, in some designs there may be two rows of running blades on a single wheel, with a row of fixed guide blades between them.
The length of the blades increases along the direction of the steam flow to allow for the increase of volume as the steam expands, but in some instances the steam is admitted to the centre of the cylinder. The sizes of the blades then increase to right and left, corresponding to the divided flow of the steam. This is more pronounced with the low-pressure cylinder, because when the steam gets near to the point of exhausting into the condenser it has increased enormously in volume. Otherwise it would be difficult to obtain sufficient area in the final steam passages for the free exit of the steam at nearly perfect vacuum. Another advantage of a central inlet is that the axial thrust is balanced. Both these practices were introduced by Parsons and followed by the makers of impulse turbines. The Ljungström turbine is shaped so that expansion takes place from the shaft radially outwards.
Economy of Working
The higher the vacuum the lower the steam consumption of a turbine, and the last inch or so (vacuum being measured in inches of mercury) makes a difference out of all proportion to the pressure drop. At 300 lb. pressure one pound of steam occupies If cubic feet. At atmospheric pressure it takes up nearly 27 cubic feet, but at 29 in. of vacuum, which is about the limit for normal atmospheric conditions, the volume has increased to about 640 cubic feet, or more than 400 times its original volume at 300 lb. pressure.
Hence the large exhaust pipes seen on the low-pressure end of turbines where they connect up to the condenser. The feature of the possibility of carrying expansion to low limits makes it advantageous in some instances to run steam turbines from a low steam pressure supply. For example, certain types of marine engines are a combination of reciprocating engine and turbine. The former uses the steam from its initial working boiler pressure down to about atmospheric pressure, and then exhausts all its steam into a steam turbine which operates between that pressure and the condenser vacuum. This effects greater economy than is possible with the reciprocating engine alone working under the extreme range of pressures.
The casing, or cylinder in which the rotor works, is made of close-grained cast iron when the steam pressure is not particularly high. For temperatures above about 450 degrees Fahrenheit cast steel is used.
The casing is made in halves, top and bottom, divided along the centre line of the shaft and bolted together along the joint. The machining of this joint has to be accurately done, and the surfaces of the joint are scraped by hand and bedded down to each other after they have been planed in the planing machine.
Some of these casings are so large as to require a planing machine capable of taking work up to 13-1/2 feet wide and 12 feet high, and having a cutting stroke of 25 feet. The casing ends are then bored out for the main bearing seatings, with circumferential slots to take the diaphragms that carry the fixed guide blades, or nozzles, where diaphragms are used. A great number of bolts, sometimes with a diameter of as much as 4 in., is required to hold the top and bottom halves together with sufficient rigidity and steamtightness. Several removable long steel guide pillars are provided to fix in the bottom casing to prevent injury to the blades when the top half casing is lifted off and replaced.
With the high steam temperature used there is an appreciable amount of expansion and contraction of the casing due to changes of temperature. Provision has therefore to be made throughout the whole machine so that the parts maintain their correct alinement.
All water in the steam on its passage through the turbine must be got rid of as quickly as possible by suitable drainage arrangements or other means. Otherwise it causes erosion of the blades. To prevent such erosion, which is most likely to be found at the low-pressure end, the blades in the Parsons reaction turbine are protected by a hard steel shield which is brazed to the leading edge of the blade.
Labyrinth and Carbon Glands
Where the rotor shaft passes through the ends of the casing, packing glands must be provided to prevent steam from leaking out at the high-pressure end, and air. from getting in at the low-pressure end, where a small quantity of air would reduce the vacuum. These glands may be either of the labyrinth type or of the carbon type, the former comprising internally sharp-edged metal rings embracing a portion of the shaft, which is provided with corresponding grooves, so that a tortuous passage results. The principle of the labyrinth gland is that the pressure drop between one side and the other is reduced in a series of steps by successive throttling at each step.
The glands are made in halves, and fit in T-shaped slots in the surrounding casing of the gland. In the carbon gland steamtightness is obtained by rings formed of segmental blocks of carbon running in contact with the shaft. The blocks are held together by garter springs and the complete ring is spring-supported so as to relieve the shaft of its weight.
The packing glands are sealed with steam supplied from a series of pipes and valves connected up to a regulator which automatically controls the amount of steam supplied to each gland according to its requirements. The sealing steam escaping from the glands may be led to a small auxiliary condenser so as to avoid any inconvenience arising from its escape into the engine room.
The main bearings in which the shaft of a turbine runs are spherically seated and self-alining ; they consist in each instance of a cast-iron shell with white metal lining. They are lubricated with oil under a pressure of from 5 lb. to 10 lb. per square inch. The oil is delivered from a pump through one main distributing pipe, from which branch pipes connect to each bearing. With the heaviest rotors a supply of oil at a pressure of from 1,200 lb. to 1,500 lb. per sq. in. is provided for a second or two to float the rotor in its bearings at the moment of starting. A regulating valve for adjusting the oil supply is provided in each bearing, and also an inspection box through which the return oil passes. This box has a glass window in the lid to facilitate inspection of the oil flow, and a thermometer is fitted for taking the exit temperature of the oil.
Any escape of oil from the main bearings is prevented by oil guards, consisting of internally grooved labyrinth rings attached to the bearing housings, and closely embracing oil-throwers provided on the shaft. Vent pipes are fitted to the bearing caps to carry away oil vapour.
In both impulse and reaction turbines a locating device is necessary to keep the successive rows of moving blades on the rotor in correct relative axial position with the diaphragm in the one instance and with the fixed blades in the casing in the other. This locating device consists of a thrust block generally attached to the spherically seated shell of one of the main bearings. The thrust block is generally of the type in which the thrust surfaces are fitted with segmental tilting pads lined with white metal, and which engage with a single thrust collar machined on the shaft. The thrust block can be adjusted longitudinally by a screw mechanism to maintain the desired clearances at the blading.
Admission and Control of Steam
The arrangements for the admission and control of the steam to the turbine affect materially the degree of success in obtaining the desired economy of steam consumption, as well as the governing qualities. Various designs are adopted for this particular purpose. In one form of construction steam is admitted to the first stage of an impulse turbine through a number of nozzles, divided into groups, and the admission of steam to the groups is controlled by several valves, one to each group. Generally, the governor, which is of the centrifugal type, does not have to actuate the big steam control valve, but only to move a light plunger or sleeve of a pilot valve. This valve allows oil at a pressure of about 60 lb. per square inch to pass to a form of servomotor which opens one or more of the valves controlling the amount of steam admitted to the turbine as required by the demand of the turbine load. Conversely, as the load on the turbine decreases, the control valves are made to close and reduce the amount of steam supplied. An emergency governor is also provided which, in the event of a failure of the main governor to prevent the speed from rising above the normal, will at once reduce the oil pressure, thereby causing to close all governor valves and also an emergency stop valve. Such emergency control can be arranged to be operated from the switchboard control room, should the set have to be shut down quickly.
The maintenance of an unfailing supply of oil to all the bearings of a turbine is of vital importance. This is almost invariably effected by a valveless rotary oil pump driven from the turbine shaft. The oil is circulated through strainers and an oil cooler.
During construction many of the parts are assembled independently for fitting to the complete machine on erection. The rotor shaft is carefully bedded down in its main bearings and lined up before the casings are closed up. Clearances at all vital points are examined and measured. When completely assembled the turbine is run under steam for several hours, and all necessary adjustments are made. The casings, steam chests and all steam piping connexions are eventually lagged with non-conducting material, to be covered finally with planished blue steel plates which can be. easily removed when necessary.
In the larger sizes, much dismantling is necessary for transport. When the plant has been completely erected on the site exhaustive tests are carried out.