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ELECTRIC RAILWAY JOURNAL · Vol. 53, No. 19 · May 10, 1919 · pp. 906-908.
Interborough Commissions 60,000-Kw. Turbo-Generator Unit
By W. S. FINLAY, JR., Superintendent of Motive Power, Interborough Rapid Transit Company, New York City.
Attention Is Directed in This Article Particularly to the Automatic Control Features of the Installation.
The Interborough Rapid Transit Company, in its Seventy-fourth Street power plant in New York City, has the largest prime mover now in service in the world. The machine is the first of the three-cylinder type of turbine to be put into operation, one of the elements being the high-pressure section and the other two the low-pressure sections. The combined unit has a maximum continuous capacity of 60,000 kw., or 70,000 kw. for two hours. It occupies a floor space of 52 ft. x 50 ft. and at maximum load requires 826,000 lb. of steam per hour.
The unit is designed on what is known as the cross-compound principle. Each element is coupled direct to its electrical generator, and all three elements when in normal operation are tied together electrically. The steam path is such that all of the steam passes through the high-pressure element, then divides equally and flows through the two low-pressure elements. This principle of design, by dividing the work done into separate cylinders, allows the use of smaller individual elements which are inherently stronger than large cylinders; it makes possible an outfit considerably more flexible than a single large unit and more reliable because the turbines are smaller and there is less temperature difference in any one cylinder; and it permits the use of commercially common materials with moderate blade speeds and stresses.
The high and low-pressure turbines are proportioned so that with a total load of 60,000 kw. the load will be equally divided among the three elements. In case of failure of one of the low-pressure elements, it would be called upon to carry an abnormal load since all of the steam from the high-pressure element must pass through one low-pressure turbine. To provide against injury to the generator from this cause there is provided a back pressure valve on the exhaust of the high-pressure element which, when the pressure has reached a given amount, will permit steam to exhaust direct to atmosphere. The pressure selected is that which corresponds to a load of 30,000 kw. on the low-pressure turbine, which it is well able to sustain for a half hour. One half-hour is regarded as sufficient time in which to get other units onto the system, when the load on the high-pressure and one low-pressure element of the triple unit may be reduced to the limits of the continuous capacity of the low-pressure generator.
The high-pressure element contains fifty rows of blades, the height of the first row being 4 in. and that of the last row 9 in. The journals are 10 in. in diameter, and the rotor is equipped with a Kingsbury thrust bearing the function of which is to prevent any axial movement of the rotor. Each low-pressure element contains forty-four rows of blades, the height of the first row being 6 in. and that of the last row 15 in. In this element the turbine rotor journal is 12 in. in diameter. The rotor is, like the high-pressure element, equipped with a Kingsbury bearing.
In connection with the turbine there are four surface condensers installed, two being connected to each one of the low-pressure elements. The total area of cooling surface is 100,000 sq.ft.
The turbine is designed to operate with steam at 220 lb. per square inch, absolute pressure superheated 150 deg. Fahr. and to exhaust into a 29-in. vacuum. At a load of 40,000 kw., which is the point of best economy of the unit, the high-pressure element will exhaust into the low-pressure element at 29.7 lb. per square inch, absolute pressure, at a temperature of 250 deg Fahr. This turbine unit is estimated to operate at load between 30,000 kw. and 60,000 kw. at a water rat which is not more than 5 per cent greater than the minimum.
Each turbine runs at 1500 r.p.m. and its generator delivers three-phase power at 11,000 volts, 25 cycles. Each element has its individual busbars with separate feeders. Installed in the connections between the bus-bars are reactance coils to limit the flow of current between generators.
Although the unit consists of three separate elements the method of starting the elements from rest preparatory to synchronizing is essentially the same as for single-shaft units. First the field current of all three generators is applied; then the throttle valve on the high-pressure element is partially opened. As soon as the high-pressure rotor starts to revolve it will, through the applied field current, set the rotors of the two low-pressure elements revolving. This causes all three elements to come up to speed together and in correct phase with each other, so that when synchronized with the system they can be connected to the main busbars by closing a single circuit breaker.
The new Interborough turbine unit is of interest not only on account of its size but because unusual attention has been given to the development of automatic features by means of which in the event of trouble with any of the elements it will be automatically cut out of service, the remaining two elements continuing to carry the load.
By the use of an ingenious governing arrangement means have been provided that will permit uninterrupted operation of each individual element, should one or the other two be taken out of service by tripping the automatic stop from any cause not affecting all three elements.
For example, if the high-pressure element is shut down, each low-pressure element will automatically receive high-pressure steam direct from the boilers through its own individual high-pressure steam system, whereas in normal operation the low-pressure elements do not receive any high-pressure steam direct. Vice versa, if the two low-pressure elements be shut down, from any cause not affecting the high-pressure element, the high-pressure element will continue operating and automatically exhaust its steam into the atmosphere. Should only one low-pressure element be removed from service, the high-pressure element will exhaust into the remaining low-pressure element. All this governing arrangement is entirely automatic.
Since the governing mechanism must control three units, not only when operating together but also when operating separately, several features novel in steam turbine practice are involved. Each unit is provided with an over-speed stop governor which will immediately shut off the steam from that unit if the speed rises above a predetermined amount. Each unit is also equipped with a speed regulating governor of which that on the high-pressure unit is of the customary form. The speed-regulating governors on the low-pressure units are somewhat more complicated.
A butterfly valve, capable of automatic operation, is provided at each connection between the high and low-pressure units which will be automatically closed should the low-pressure turbine speed exceed a predetermined limit. This is tripped shut first by the speed regulating governor should it go to the outer position, and in the event of its failing then by the automatic stop governor. The high-pressure turbine is provided with another exhaust, having a back-pressure valve, so that when necessary the exhaust from ths high-pressure turbine may pass to atmosphere and the high-pressure turbine continue to carry its load.
Similarly, if the high-pressure turbine loses its load its governor will cut off steam to the whole system. If the governor does not control the turbine and the speed reaches the predetermined limit, then the stop governor on the high-pressure will close the automatic throttle, similarly cutting off steam from the whole system. The whole system will then slow down until it reaches a predetermined speed lower than normal, when the governors on the low-pressure turbine will cause live steam to be admitted directly to them.
There is a butterfly valve in the low-pressure line which is controlled from the governor of the corresponding low-pressure turbine. Each valve is operated by a differential piston to both sides of which steam pressure is admitted. One end of this cylinder is connected to a valve trip mechanism located at the low-pressure governor and so arranged that when the governor reaches a prescribed position a valve will be tripped open, thus releasing steam from that side of the differential position. Steam pressure on the other side will then quickly force the butterfly valve closed. If the turbine is to be shut down the gate valve is then closed by hand. The butterfly valve may be opened or closed also by a hand-controlled valve.
The valve controlling live steam direct to the low-pressure turbine will begin to open when the low-pressure governor reaches a prescribed position. This valve mechanism does not differ in principle from the main high-pressure valve controlling steam in the system, and is in conformity with ordinary practice for such purposes.
The overspeed stop governor on the high-pressure turbine will close the main throttle valve and the main regulating valve, while that on the low-pressure turbine will bring about the closing of the butterfly valve and the governor and throttle valve, admitting high-pressure steam to the low-pressure turbines.
As part of the throttle valve there is a switch which, when closed, will open the main circuit breaker. Should some accident happen to one of the turbine elements, it may be instantly cut out by operating the emergency stop, which will cause the immediate closing of the automatic throttle. This in turn causes the closing of the switch, which opens the circuit breaker.
The new unit was built by the Westinghouse Electric & Manufacturing Company, but unlike most Westinghouse turbines of moderate capacity it is built entirely on the reaction principle, whereas the custom of this company has been to build its turbines with an impulse high-pressure section. This change is due to the enormous volumes of steam which are to be handled and it permits the use of relatively long blades in the first rows of the high-pressure element.
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| TURBINES HAVING A NORMAL CAPACITY OF 150,000 KW. IN INTERBOROUGH POWER PLANT. New 60,000-kw. unit in foreground, three 30,000-kw. units in background. | CIRCULATING WATER PUMP IN INTERBOROUGH POWER PLANT |
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| PLAN AND ELEVATION OF 60,000-KW. TURBO-GENERATOR UNIT |
Sources: Electric Railway Journal, McGraw Hill Company, Digitized by Microsoft, Americana Collection, archive.org.
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http://www.nycsubway.org/articles/erj-1919-irt_60000_kw_generator.html
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