 |
| |
ELECTRIC RAILWAY JOURNAL · Vol. 47, No. 20 · May 13, 1916 · p. 903.
Tests on 30,000-Kw. Turbine
The Most Recent Units for the Interborough Rapid Transit Company, Reaching a Thermal Efficiency of 25 Per Cent, Have Made the Gas Engine Obsolete in Large Stations.
At a meeting of the American Society of Mechanical Engineers in New York on May 9 a paper was presented by H. G. Stott and W. S. Finlay, Jr., in which were given the results of a series of elaborate efficiency tests on one of the 30,000-kw. cross-compound steam turbines recently installed in the Seventy-fourth Street power station of the Interborough Rapid Transit Company of New York. Before the paper was read Mr. Stott made some preliminary remarks on the development of prime movers since the year 1900, when the Seventy-fourth Street station was installed.
At that time the plant was equipped with reciprocating engines of 5000-kw. rating and a maximum capacity of 50 per cent overload. The water rate was 17-1/2 lb. per kilowatt-hour, and the cost of the engine, generator and condenser approximated $40 per kilowatt of rated capacity. The plant was of the unit-type arangement, and 4000 hp. of boilers were furnished for each 5000-kw. turbine. When the new turbines were installed, each one occupied the same floor space as one of the original reciprocating engines, although the turbines was of six times greater capacity. The new machine had a water rate of ll-1/2 lb. and cost about $9 per kilowatt, including generator and condenser, being operated by the same eight boilers that had originally supplied steam for the 5000-kw. reciprocating unit.
 |
| INTERBOROUGH TURBINE TESTS — WATER-RATE CURVE |
This extraordinary development of the turbine, Mr. Stott said, had caused it absolutely to have displaced the gas engine for power station work. The thermal efficiency of the turbine now approximated 25 per cent, as good a figure as could be obtained from the gas engine, while the latter involved very much higher overhead charges and maintenance costs. For the same reason, hydroelectric power, which looked like a gold mine fifteen years ago, even when the cost of development ranged between $200 and $300 per kilowatt, was to-day not a good investment. Even at Niagara Falls, where the development charge is at a minimum, and where the supply of water is practically unlimited, hydroelectric power cannot compete with that obtained from a modern steam-turbine station when the load factor is less than 50 per cent.
The paper of the evening was then presented by Mr. Finlay. This gave a brief description of the installation and presented in detail the results of the tests. From the figures, it appeared that the maximum efficiency was attained with a load approximating 90-per cent capacity, the water rate at this point being 11.25 lb. per kilowatt-hour. Throughout all of the tests, the operating conditions were approximately the same, the figures for the test giving the lowest water rate being as follows: Absolute steam pressure at throttle, 224 lb.; steam temperature at throttle, 500 deg. Fahr. ; superheat, 108.5 deg. Fahr.; absolute steam pressure at primary inlet, 215 lb.; absolute steam pressure at low-temperature inlet, 15 lb. Vacuum referred to 30-in. barometer, 28.86; average load, 26,740 kw.; water per hour, 301,035 lb. The water rate for this test was 11.258 lb. per kilowatt-hour, this figure being corrected to meet standard conditions involving 215 lb. absolute primary-inlet pressure, 120 deg. superheat and 29. in. of vacuum. The Rankine-cycle efficiency under these same conditions was 75.84 per cent and the thermal efficiency was 24.81 per cent.
The load under which the turbine was tested took the swings as normally produced by the railway substations which were being supplied with power, but a number of tests were also made under throttle control to show the influence of the swings upon the economy. The latter results, however, did not differ from the former, showing that swings even amounting to more than 30 per cent of the average load made no appreciable difference in the performance.
In the discussion which followed F. Hodgkinson, of the Westinghouse Electric & Manufacturing Company, who had designed the turbine, discussed the irregularity that appears in the water rate curve above loads of 22,000 kw., this having been found definitely to be due to some other cause than errors in the readings. He ascribed it in part to the action of the separator that was installed between the high-pressure and low-pressure elements for the purpose of removing the water that otherwise would be carried over into the low-pressure blading. This separator was of the centrifugal type, and it was found at times to be inefficient, the removal of the collector plates actually reducing the amount of water carried over at certain loads. R.J.S. Piggott also commented upon this phenomenon, stating that it is impossible to remove the last few per cent of moisture in steam with baffles. It is best to slow down the velocity of the steam below 3000 ft. per minute, at which point the "fog" coalesces into drops which will separate themselves from the flow of steam.
In answer to a number of questions that were raised during the course of the discussion, Mr. Stott stated that the maximum capacity of the turbine was between 33,000 kw. and 34,000 kw., it being rated at 30,000 kw. and guaranteed for a load of 32,000 kw. Beyond this point, the turbine lost speed, so that an addition of 25 per cent overload would produce a slowing down of about 15 per cent from normal speed. The monthly average coal consumption of the plant approximated 1.5 lb. per kw.-hr. With the original reciprocating engine displaced by the turbines, the coal consumption had been 2.5 lb. The thermal efficiency of the station as a whole averaged 17 per cent throughout the month at the present time.
ELECTRIC RAILWAY JOURNAL · Vol. 57, No. 21 · May 21, 1921 · p. 945-946.
Data of New Interborough Turbine
Water Rate of 11 Lb. per Kilowatt-Hour for 30,000-Kw. Turbine Shown by H. B. Reynolds, Research Engineer Interborough Rapid Transit Company, to Have Been Attained in Test — Facts Contained in A. S. M. E. Paper.
In a paper scheduled for presentation at the spring meeting of the American Society of Mechanical Engineers, to be held in Chicago, May 23 to 26, 1921, Herbert B. Reynolds, research engineer, Interborough Rapid Transit Company, New York City, gives information regarding the several types of turbine which have been installed by that company during the past dozen years. He also includes the results of tests on the latest unit installed, namely, the three of 30,000-kw. capacity, the installation of which was completed during the past year.
Mr. Reynolds said that in order to provide additional power capacity for the new subways constructed in New York City during the period from 1913 to 1921, and operated by the Interborough Rapid Transit Company, additional turbine units were installed in both the Fifty-ninth Street and Seventy-fourth Street power stations. He reminded his readers that the original engine-room equipment of the Fifty-ninth Street power plant consisted of nine 7,500-kw. maximum capacity Manhattan-type Allis-Chalmers double-angle compound engine units and three Westinghouse 1,250-kw. turbines, the latter driving 60-cycle generators which supplied current for subway lighting. Later 25-cycle current was adopted for this lighting, the current being taken from the main units. During 1909 and 1910 five low-pressure 7,500-kw. maximum capacity General Electric turbine units were added, taking exhaust steam from five of the engines at atmospheric pressure.
Two of the new 30,000-kw units in the Fifty-ninth Street plant were installed in the space formerly occupied by the three lighting units, while the third turbine was installed at the western end of the station. The concentration of power possible with modern turbines is strikingly shown by the space they require as compared with that for reciprocating engines. The maximum capacity of the engines visible in the background in the photographs reproduced is but 26,250 kw., while that of the turbine in the foreground is 35,000 kw.
The three 30,000-kw. Westinghouse cross-compound turbines which were completed in 1915 were among the new units installed at the Seventy-fourth Street power station.
In the paper Mr. Reynolds gave structural and design details of the new turbines. Among other things, he said that they are of the straight Curtis impulse type, having twenty pressure stages, each consisting of one velocity stage. The normal steam pressure at the throttle is 225 lb. per square inch, abs., with a super-heat of 150 deg. F., exhausting into a vacuum of 29 in. referred to a 30-in. barometer at 58.1 deg. F. The speed is 1,500 r.p.m.
In addition to the primary steam inlet, a secondary valve is provided which opens after the load reaches 24,000 kw. and which enables the turbine to carry a load of 35,000 kw. As all auxiliaries in the station are steam driven, a connection has been provided in the turbine through which any excess auxiliary exhaust steam may be injected. This is at the sixteenth stage of the turbine.
The generators are three-phase, star-connected, generating 25-cycle current at 11,000 volts. The excitation is at 250 volts. The generators are cooled by circulation of air maintained by a fan which forms an integral part of the generator. The air is drawn from the turbine-room basement and discharged from the top of the generator into the turbine room through a short stack.
Each unit comprises one single-shell two-pass Worthington condenser, two Worthington centrifugal circulating pumps, each driven through reduction gears by Kerr turbines; two Worthington centrifugal condensate pumps, each driven by a General Electric turbine, and one Laidlaw-Dunn-Gordon dry vacuum pump. Each condenser contains 50,000 sq.ft. of tube surface in 10,760 tubes 18 ft. long, 1 in. in outside diameter and of No. 18 B.W.G. thickness. The condenser is of the two-pass type, the water entering at the bottom and passing out at the top. As the condensers are mounted on springs, rubber expansion joints are inserted in the circulating water lines.
As no expansion joint was provided between the turbine and the condenser, it was necessary to mount the latter on springs, so as to provide for expansion and contraction. The spring supports are shown in one of the illustrations. To facilitate the setting of the springs and provide a means for detecting and adjusting for fatigue in them, hydraulic jacks were incorporated in the condenser supports.
Mr. Reynolds gave some detail of the procedure followed in setting these springs. He said that after the erection of the condenser and circulating water pipe had been completed, with the exception of making the joint between the condenser and the turbine, the condenser was raised while empty by means of the jacks, leaving 1/2 in. clearance between the face of the turbine outlet and the face of the condenser inlet. The load on each of the four supports was then determined by noting the oil pressure in the jacks. It was decided that with the condenser empty and cold the downward pull on the turbine should not be less than approximately 17 tons. The distance that the joint between turbine and condenser would have to be pulled in order to give this load was estimated from the modulus of elasticity of the turbine and condenser metal. The condenser was then raised to within the predetermined distance of the turbine outlet, which was found to be 0.231 in., after which the lock nuts on the jacks were screwed home and the condenser bolted to the turbine. The load on the springs was then determined with the condenser still empty by noting the pressure required just to raise the lock nuts. Every few months the load carried by the springs will be determined in this manner and compared with the load which existed when the condenser was first bolted to the turbine. Any fatigue which may develop in the spring will be compensated by screwing the lock nuts down.
It was found that the minimum condenser load carried by the turbine with the condenser shell empty was approximately 17 tons. As the water required to fill the condenser weighs about 60 tons the load on the turbine increases to 77 tons when the circulating water pumps are started. This is reduced to about 70 tons due to the compression of the springs under the expansion of the condenser during warming up. Immediately after shutting down, and while the condenser is still warm but drained, the load on the turbine is reduced to 10 tons. Thus, the condenser load on the turbine varies from 10 to 77 tons, out of a total condenser weight varying from 180 to 240 tons.
The equipment used for conducting the turbine tests consisted of two large water-weighing scales for measuring steam consumption, three single-phase rotating standard watt-hour meters for measuring the output, and the necessary thermometers, gages and mercury columns for determining temperatures, pressures and vacua. Most of the tests were of three-hour duration and, with the exception of a few special tests, the turbine was operated under conditions normal as to the type of load.
The results of the tests, in so far as the water rate is concerned, are given in the accompanying curve. The lowest rate obtained while operating under normal conditions was 11.03 lb. per kilowatt-hour. The thermal efficiency, or ratio of the output to the energy in the steam, was 25 per cent. The Rankine efficiency; that is, the ratio of the energy developed to that available within the working range of temperature and pressure, was 75.5 per cent.
In the paper the results of various auxiliary tests were also given, but it is impossible to summarize these within the space limitations of the present abstract.
 |  |
| ONE OF THE THREE 30,000-KW. TURBINES INSTALLED AT THE FIFTY-NINTH STREET POWER STATION OF THE INTERBOROUGH RAPID TRANSIT COMPANY IN 1920. | A COMPLETE TURBINE AND CONDENSER UNIT |
 |  |
| DETAILS OF THE SPRING SUPPORTS FOR THE CONDENSER | WATER-RATE CURVE FOR NEW INTBRBOROUGH RAPID TRANSIT TURBINE. Dotted line shows results that would probably have been obtained if steady-load tests had been conducted within this range of load where the secondary valve is continually opening and closing. |
Sources: Electric Railway Journal, McGraw Hill Company, Digitized by Microsoft, Americana Collection, archive.org.
|
http://www.nycsubway.org/articles/erj-1916-irt_30000_kw_turbine.html
This site is not affiliated with any transit agency or provider. |