« PreviousContinue »
isfaction which prevails among turbine users.
We will now consider the specific points of design.
Principle of Operation. Like the hydraulic turbine, the steam turbine is simply a mechanical means of transferring to a rotating bucket wheel the kinetic energy of a rapidly moving fluid jet.
These principles have already been outlined in sufficient detail in previous papers before the Association. One fact, however, has not been made sufficiently clear, viz. : that, with high velocity steam, mechanical erosion of all metal in contact with the steam, is bound to occur, especially with steam containing entrained moisture. Realizing the difficulty of constructing turbines of the single-wheel impulse type, possessing the desired strength, high efficiency and long life, Mr. PARSONS finally developed the multi-stage expansion turbine. In this system, the steam is only partially expanded in each row of blades, i. e., the energy of the steam is removed step by step. Hence, the bucket and jet velocities need be but a fraction of those necessary in the single-stage turbine, and it becomes possible to construct commercial machines of high economy and operating at comparatively low bucket and shaft velocities. The Parsons multi-stage turbine may be classed as embodying the combined impulse and reaction principle, i. e., the steam acts upon the moving blades partially by impulse and partially by reaction. This will be better understood by examining Figure 2, a diagram of the two consecutive states of a Parsons turbine.
The steam enters the first row ( 1 ) of blades at boiler pressure and expands a few pounds to a lower pressure (P 1), acquiring a velocity and impinging upon the first row (2) of moving blades. Here its velocity is reduced and the energy of the jet partially given up to the moving blades. But, while passing through the first row of moving blades (2), the steam expands further in the moving blades and again acquires velocity by the time it reaches the exit of the first row of moving blades. Thus, the force required to accelerate the steam in the moving blades (2), is transmitted back to them and thus delivers more energy to the first row by reaction. This process is repeated
. a sufficient number of times (from fifty to seventy, depending upon the size of the turbine) to abstract the energy in the steam and transfer it to the moving blades. What is the result? By means of this many-stage principle, we have effected complete expansion of steam from boiler to condenser pressure without having exceeded at any point in the expansion a velocity of 600 feet a second, or about 450 average, as compared with 4,000 in a single bucket wheel and about 2,000 in the turbine of not more than four stages. Furthermore, we have transferred a greater quantity of heat energy in the steam into useful work than is possible with the average reciprocating engine; sixtyfive to seventy per cent, and even higher in the low pressure turbine.
Construction. Returning now to the actual construction, we will consider the 1,000 kilowatt turbo unit shown in Figure 3, as representative of the present Westinghouse-Parsons system up to capacities of about 3,000 kilowatts.
Turbines of this type are already operating in the field up to 7,500 kilowatts capacity, and several machines of 10,000 kilowatts are either under erection or building, for the Kent Avenue Station of the Brooklyn Rapid Transit Company, New York, one of the largest stations in the country and exclusively equipped with Westinghouse turbines. These machines represent the largest single turbo units ever constructed for power work, and will have a guaranteed over-load capacity of 22,000
brake horse power, which would be equivalent in the reciprocating engine, to an output of 25,000 indicated horse power. It is unnecessary, however, for the purposes in view, to consider the special construction of these large units. In Figure 4 a turbine is shown with some of its parts removed for inspection. This view should be studied in connection with Figures 5 and 6. These three views speak volumes for the man who thinks it worth while to inspect his machinery at intervals.
Stator. Briefly, the turbine consists of two parts, the stator or cylinder and the rotor or spindle. The stator is built in two parts, as shown, the upper carrying the admission valves and distribution passages, the lower cast into two pedestals. At the exhaust end the cylinder is anchored to a rigid, continuous bed plate, while the outer pedestal is permitted to slide between guides, thus providing for expansion and contraction of the turbine along its axis. Inasmuch as no live weight comes upon the cylinder corresponding to the moving weight of the reciprocating engine piston, the structure is simply that of a cylindrical envelope carrying its own weight from two supports. The cylindrical section gives the most rigid structure and avoids entirely any allowance for sag or vertical deflection.
Rotor. For a similar reason the spindle is built up from a hollow central shell, into which are fitted slide shaft ends forced into place and retained by shrink links. This allows a light but rigid structure in which the sag is practically negligible. In spite of this fact, the horizontal design of turbine has been unjustly criticized as being greatly affected by sagging between bearings; a practically non-existent factor. The rotor is not
a built in one piece, but consists of a number of sections threaded in succession upon the spindle. Each section is not only balanced individually, but also the entire spindle after completion, so that the effects of unsymmetrical distribution of metal are brought to a minimum.
Contrary to the general impression, the three diameters of blade drums are employed simply for mechanical convenience in blading, and not on the triple expansion principle. Note