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that each drum carries shorter blades at the beginning than the end of the preceding drum. Were only one drum employed, from fifty to seventy different lengths of blades would be required for each size of turbine. This increased diameter reduces the stock to a few sizes, still maintaining the proper steam areas.
Referring to Figure 5, steam enters through the strainer (S) and primary admission valve (V) to the distribution port (A) which delivers to the entire periphery of the rotor. From this point the steam expands down to condenser pressure through successive stages, already represented by diagram in Figure 2. Here the cntire space between rotor and stator walls is filled with a positive flow of working steam, this steam area increasing by gradual steps according to the volume which the steam occupies at the various pressures. There are no idle spaces in which eddies can exist and fluid friction ensue.
The drop in pressure from stage to stage in the Parsons system would naturally result in a very considerable end thrust along the axis, varying with the load and the pressure distribution. Marine turbines utilize the reverse thrust of the propellers
. for equalizing this steam thrust, but in the stationary machine three rotating balance pistons (P), (P), (P), are used, each of just such diameters as to exactly balance the thrust of the steam for any condition of load, steam pressure, or vacuum.
Equalizing passages (E), (E), (E), serve to maintain this exact pressure balance which is so sensitive that the turbine shaft practically floats in its bearings and can be moved either way by the pressure of a pencil point. These balance pistons (P), (P), (P), retate without friction, yet are steam tight owing to the deep corrugations on the periphery, forming what is known as labyrinth packing. Although no actual end thrust exists, it is naturally necessary to maintain the rotor in a definite axial position. This is accomplished by an adjustable collar bearing (T). The two halves of this bearing are adjusted by set screws above and below, each graduated to register one one-thousandth of an inch movement of the shaft. By this means, running clearances may be readily checked at any time, a valuable feature.
Over-load Valve. The normal steam passages are designed to handle steam up to a point where the governing valve is wide open and where would occur the limit of the machine's capacity, but, at this point a secondary valve (Vs) admits steam from inlet (A) around into the second drum, which is of larger diameter. This in effect, serves to increase the mean effective pressure of the expansion through the later stages, in some such manner as high-pressure steam is by-passed into the intermediate cylinder of a triple expansion engine to increase the maximum capacity. Here we have added little to the turbine, yet have secured an enormous overload capacity with little loss in economy, as will be outlined later; and, in addition, full load capacity when forced to operate without a condenser.
Blading. Owing to the relatively minute pressures actually exerted upon each individual blade, the most desirable characteristics of a Parsons blading material, are not maximum physical strength, but rather ability to withstand high temperatures without deterioration, and the corrosive effect of impure water. Moisture always occurs at some point in the expansion of steam, even if super-heated, hence, toughness is to be desired in the blade. Several materials have been successfully used in Westinghouse turbines, viz., bronze, steel and hard copper. None of these materials combine all the desired physical characteristics to be universally applicable. With pure water, steel has unquestionably proven an excellent material. With corrosive water, bronze or copper has been used to better advantage.
In the last year, however, a special material, known as duplex metal, has been developed, and is now exclusively used in Westinghouse turbines. This material is manufactured by a secret process and combines the structural strength of a steel center with the protective qualities of the copper sheath. The two metals are welded so perfectly that the material can be drawn to the required blade section without altering the thickness of the sheathing, an enlarged section of this blading is shown in Figure 7, in which the uniform thickness of the sheathing can be readily discerned. This copper sheath provides great resistive qualities against chemical erosion without altering the strength of the blade.
This blading is drawn into long strips cut to length and mounted in tapered slots in stator and rotor with soft steel spacers. These are of a precise section, so proportioned as to give the proper steam channels between blades, and, after assembling, are caulked firmly in position, the soft metal spreading into the dovetail, and gripping the blade so tightly that it will pull apart before pulling out. Furthermore, with steel in blade, spacer and spindle, there is no opportunity for the blading to become loose by unequal expansion at high temperatures.
One of the most effective details of the present design is the method of lashing together the tips of the longest blades for the purpose of re-enforcing them and maintaining the proper spacing. Figure 8 shows this in detail.
The lashing is drawn in long strips with a section resembling an inverted "comma." The tips of the blades are punched to a corresponding section and threaded on to the lashing in sections two or three feet long, which considerably facilitates assembling of the blading. When the blades are finally caulked in position, the “tail of the comma” is sheared over by a tool, as shown, wedging tightly between the contracting faces of the blades, while leaving intact the round section of the lashing, as well as the short strip or key within the punched blade. This results in several important features: First. A continuous tie wire of circular section re-enforcing the blade tips and forming practically no obstruction to the steam flow. Second.—A strut or spacer wedge between the adjacent blades. Third. -A key or wing left within the blade section, serving to prevent a broken blade twisting around the lashing wire. Fourth.— The lashing may be located at any desired point on the blade to prevent intermediate or nodal vibration of the longest blades, should there be any such tendency. Fifth.-It is easily replaceable, permitting the insertion of one or two new blades without necessitating the removal of an entire ring. Sixth.—The lashing cannot warp so as to distort the blading, nor can it release itself as the result of rubbing and thereby communicate serious damage to other parts. The blades only require punching, no riveting being necessary, and the work of final gauging of the blades is accomplished before the lashing is finally secured, and, hence, without weakening its grip. An average sample of the finished blading is shown in Figure 9, which illustrates the striking uniformity possible with blading assembled by hand. It also shows the lashing wire in position. Past experience has demonstrated