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that turbine blading can be made too rigid, and the design here illustrated was adopted with this point in view. It possesses ample strength and rigidity for all normal conditions of operations, yet in an emergency, its failure will not tend to spread over a considerable region, but will be confined to the locality of the defect.
Governing. In the Parsons type, two methods are in general use for governing the admission of steam according to the load. In one the steam supply is simply throttled so that the inlet pressure varies with the load. In the Westinghouse system, steam is admitted in puffs of approximately full pressure and occur at regular intervals. The duration of the puff, however, is controlled by the governor according to the load, by allowing the valve to seat sooner or later, as the case may be. Figure 10 shows this governing system comprising a sensitive centrifugal governor working against adjustable spring pressure, and varying the position of the moving fulcrum (F). In so doing it changes the position of a small pilot valve (A) which controls the exhaust from a cylinder (B) actuating the double poppet valve shown in detail in Figure 5.
In addition to this motion, however, a reciprocating motion of uniform range is communicated to the governor linkage from an eccentric rod (C) acting about a fixed point (D). This eccentric determines the number of puffs of steam admitted (about 165 puffs per minute), while the governor fixes the relative position of the pilot (A). This system entirely avoids the
possibility of sticking of the governor, as all parts are constantly subject to a slight motion. It also avoids to a great extent, wearing of the valve seats which occurs when steam is simply throttled. And, as the governor is not loaded with the friction work of moving heavy valves, a high degree of sensitiveness is possible, in fact, more than is necessary for average conditions of parallel operation in alternating current systems.
The accompanying report (Table 1) of a test on a turbine now in service in a southern cotton mill shows a drop in speed of less than three per cent, at full load, or a total speed variation from one-half load to load and one-half, of only 1.84 per cent. of the speed at full load. Within the more usual range of load three-quarters to load and one-quarter, the speed variation would approximate one per cent., and can be made less, if desired.
SPEED REGULATION TEST.
500 K.W., Westinghouse, Parsons Turbine No. 209. From report of test by Messrs. Ludwig & Co., Engineers, Atlanta.
Test No. 8
4 Nominal load,
24 full 112 Speed Rev. per min., 3663.7 3623-3 3,604.4 3,555.3 3,539.0 Variation from no load run,
I 24.7 Variation from no load %,
1.84 per cent.
Speed variation from } to it load per cent. full speed,
3.0 per cent. 2.6 per cent. 3,780 RPM. 5 per cent.
Safety Speed Limits. As in all engines of high speed, some form of safety stop is desirable to check the speed should the turbine governor become inoperative through neglect or damage. Figure 11, which is self-explanatory, illustrates a well proven design of the centrifugal type. Positive action is secured by mounting the stop directly on the end of the turbine shaft, thus avoiding belts and gears. The tension of the plunger spring is adjusted by a guide bushing. As soon as predetermined overspeed is reached, usually about seven per cent., the projecting plunger trips a small valve which releases a quick closing throttle in the steam line. Once set, this speed limit cannot change adjustment, and it is only necessary to trip the throttle by hand occasionally to keep it free and in good working order.
Bearings and Lubrication. From the standpoint of reliability, this detail of the turbine is of grave concern to the operator. In the Westinghouse design two principles are rigidly adhered to: First. — Bearings of ample proportion to carry weight of rotor without resort to forced lubrication. Second. — Continuity of oil supply dependent only upon the turbine itself and not
upon independent auxiliaries.
In spite of protestation to the contrary, past experience has proven that we must not allow the least outside interference with the integrity of the lubricating system: in other words, turbine and oil pump must run together. These principles can best be embodied in a horizontal design of turbine, and past experience has fully justified the attitude of Parsons turbine designers. Third – The oil pump of the simple plunger type is driven directly from the turbine shaft by a worm gear. Hence, so long as the turbine runs, the bearings will receive oil.
Fourth. — No forced lubrication is employed, simply a static head of one or two feet, from the reservoir shown at the extreme right, Figure 4. This head is just sufficient to thoroughly flush the bearings.
Figure 12 shows a type of bearing used on all machines above 1,800 revolutions per minute, consisting of three concen