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"Interborough Power Plant Enlargement" (1915)

ELECTRIC RAILWAY JOURNAL · Vol. 45, No. 16 · April 17, 1915 · pp. 1063-1064.

Interborough Power Plant Enlargement

[Left] FIG. 1-VIEW OF SEVENTY-FOURTH STREET STATION [Right] FIG. 2-INTERIOR BEFORE ALTERATIONS

One-Half of the Seventy-Fourth Street Power Station, Which Supplies Power to the Elevated Railways, Is Being Remodeled, Increasing Its Capacity by 300 Per Cent on the Same Floor Space.

On account of the prospective increase in the demand upon the power-generating plants of the Interborough Rapid Transit Company in New York, due to the third-tracking of the elevated lines, the Seventy-fourth Street station is being remodeled. The changes are now approaching completion, two out of three new 30,000-kw turbo-generator units being in operation, and carrying all of the load except during the peaks when the remaining 7500-kw turbine and four reciprocating engines are sometimes called upon for assistance. One unit, shown in Fig. 3, is finished complete, and the second is undergoing an exhaustive series of tests. The foundations for the third unit are under construction. The remodeling, which is being done with a view to the ultimate installation of a total of eight 30,000-kw units, is being carried on within the original building. For the present it involves principally the substitution of three turbine generators for four engine generators (see Figs. 4 and 5); the making over of one-half of the boiler plant opposite the turbines by replacing Roney stokers with Taylor underfeed stokers, adding superheaters and removing the economizers; the partial replacement of motor-driven triplex, with turbine-driven centrifugal boiler-feed pumps, and the complete rearrangement of the electrical switching system with the addition of current-limiting reactors.

The Power Plant of 1901

The old power plant, built during 1901 [fully described in the issue of the Street Railway Journal for Jan. 5, 1901], was the model steam power plant of that time. Its appearance is shown in Fig. 2. It contained eight Reynolds, Allis-Chalmers double, horizontal-vertical, cross-compound engines of 12,000-hp actual or 8000-hp normal rating each, driving 11,000-volt, revolving-field Westinghouse alternators, the largest built to 1901. Each engine unit was essentially two separate compound engines at the ends of the shaft, each having a 44-in. high-pressure cylinder and an 88-in. low-pressure cylinder and of 60-in. stroke, the speed being 75 r.p.m. By the 135-deg. setting of the cranks eight impulses per revolution were obtained.

The revolving weight on the bearings of each of the engines was 439,000 lb., and an allowance of 70,000 lb. more was made to provide for magnetic pull between field magnet and armature. The shaft itself weighed 63,000 lb. On account of the great mass of the revolving field magnet it was possible to dispense with the flywheel, a notable advance in slow-speed generating-unit design. To these engines was later added a 7500-kw Westinghouse turbine unit.

The engines exhausted into Worthington jet condensers with triplex motor-driven circulating pumps. These were changed, about 1903, to the barometric type.

The boiler house consisted of a basement, two boiler floors and a row of coal bunkers, the height from the basement to the top of the monitor being 128 ft. The basement was divided into three longitudinal compartments for the purpose of protecting the pumps from the dust produced by the ash-handling machinery. On each boiler floor were thirty-two B. & W. boilers, with Roney stokers, each rated at 520 nominal horse-power and containing 5200 sq. ft. of heating surface. These were arranged in batteries of two, eight boilers to an engine, the whole forming a generating unit. The boiler house was provided with four Custodis brick stacks of 17-ft. flue diameter at the top and 18 ft. at the bottom, 278 ft. high above the basement floor, the tallest stacks of the kind constructed in this country to that time. A Green economizer was provided for each four boilers, as it was deemed necessary to have these to heat the feed water, all of the auxiliaries being electrically driven. Each unit was served by an electrically-driven Goulds triplex boiler feed pump. Above the boiler floors was a row of three coal bunkers, separated by 35-ft. spaces for fire protection, having a total capacity of 7500 tons, a ten-day supply.

The building housing this plant, seen in Fig. 1, extends from Seventy-fourth to Seventy-fifth Street along the East River, with a width of 204 ft. 4 in. and an average length of 404 ft. It is divided by a longitudinal wall into engine and boiler houses, respectively 93 ft. 6 in. and 104 ft. 2 in. wide. The basements of these are on the same level, 4 ft. 6 in. above high water and 2 ft. 6 in. below Exterior Street, which lies between the plant and the river. Extending across the west end of the building is a vault 18 ft. wide divided into two parts, one for oil storage and the other for switch control storage batteries. The roof of this vault serves as a roadway from street to street on the level of the lower boiler-room floor.

The coal and ash-handling plant consists of two towers on the river bank for unloading coal and storing ashes, connected by bucket and belt conveyors with the power plant, distributing and collecting in the manner now standard in such plants. A feature of the tower design was the provision for hoisting coal from barges with a 1-1/2-ton shovel just high enough to give the fall necessary for passing it through the crushers and weighing hoppers.

In the engine house were the basement, 21 ft. 6 in. high, the operating floor, 107 ft. to the roof, and on one side three switchboard galleries, under the lower-most of which the engine-driven exciter sets were placed. A 50-ton electric crane traversed the length of this house.

The important features of the switching apparatus were the layout of group switches and the use of General Electric motor-operated switches. There were two complete sets of busbars connected by bus junction switches to permit of the operation of the alternators in either two or four batteries. The feeders from one substation formed one group controlled by a group switch in addition to the individual feeder switches. On the benchboard dummy busbars were placed to give the operator a graphic representation of the connections.

The Power Plant of 1915

The changes listed in the second paragraph, together with incidental changes, will be discussed in the order therein followed. The turbines, fully lagged, will appear as shown in Figs. 3 and 5. They have been rather fully described in the technical press so that only a few salient features need be mentioned. Each cross-compound unit consists of two turbines, a 1500-r.p.m. single-flow reaction turbine, and a 750-r.p.m. double-flow reaction turbine connected as a compound machine with a large receiver between elements. This novel arrangement was chosen to simplify design problems, particularly those relating to temperature range, blade speeds and steam congestion. At the same time the reliability of comparatively small units was secured. The efficiency guaranteed is higher than any heretofore obtained. Taking the amount of heat available in the steam between the conditions of admission and exhaust as a basis, the engines will deliver in electrical form 75.75 per cent of this energy. The total weight of the complete unit is 1,500,000 lb.

The turbine rests upon a foundation consisting of a steel frame encased in concrete, leaving most of the space below available for condensers, receiver and pumps.

The Worthington surface condenser, of 50,000 sq. ft. cooling surface for each unit, is of the twin-shell type, of simple construction and practically self-contained. The tube arrangement is as shown in Fig. 7, passages being provided by "gashing" to give the freest possible access of the steam to the tubes. The condensers are hung directly from the turbine bedplates, a novel arrangement but one conducive to the elimination of stresses due to temperature changes. The weight of the condenser is, however, not carried by the turbine foundation, but upon a number of spring jacks, adjusted to properly share the load. The receiver is a vertical cylinder, of 7 ft. 9 in. inside diameter and 21 ft. long inside, placed symmetrically with respect to the condenser shells as shown in an accompanying plan. This is as large a receiver as could be accommodated in the available space.

Below each condenser shell and forming an integral part of it is a sump 4 ft. in diameter and about 4 ft. high, into which the condensate drains. This is designated as a "hot well" on the drawing, but there is no hot well in the new plant, using the term in its usually accepted sense, that function being performed by the feed-water heater.

The piping to and from the condenser is of unusual construction, designed to minimize the number of bends. As shown in Figs. 7 and 9, the water enters the condenser at the bottom through a 60-in. pipe, which dips under the nearer shell to reach the farther one. A baffle inside deflects a share of the circulating water into its proper channel. A similar outlet pipe above takes care of the discharge flow. This is the simplest possible piping layout for a twin shell condenser. Short rubber sleeve expansion joints are inserted in the intake and discharge pipes near each shell. These joints con- sist of tubes of 1/2-in., 5-ply-insertion rubber, 12 in. long and of 42-in. and 60-in. inside diameter, each clamped between an outer flange and an inner ring.

Circulating water for each pair of condensers is supplied, as shown in several illustrations, through a pair of tri-rotor, centrifugal pumps having a combined capacity of 75,000 gal. per minute. These discharge through separate motor-operated gate valves, the discharge pipes uniting beyond the valves. The full capacity of the pumps will be required in summer when the circulating water temperature is high, but one pump will be sufficient in winter.

The pumps are driven by steam turbines rated at 240 hp each. The pumps draw from a new tunnel 12 ft. 6 in. X 12 ft. 6 in. in section. They discharge into two tunnels, one 8 ft. 6 in. x 12 ft. 3 in., and one, 5 ft. x 12 ft. 3 in., which is the combination of the original intake and discharge tunnels. These will supply condensing water for eight units. The mouth of the intake tunnel is about 150 ft. upstream from the nearer discharge tunnel. New motor-driven revolving screens of the type shown in one of the illustrations have been installed near the river end of the intake tunnel, the driving motor being housed in a small building at the right, not shown. The screens are rotated for cleaning purposes daily. The location of the condensers with respect to the river level makes it possible to circulate the condensing water readily, the power required being only that necessary to overcome a small amount of friction, principally the friction head of the condenser and piping.

The condensate pumps for each unit are of the centrifugal type, turbine driven and of 800-gal. per minute capacity each. One pump is sufficient, the second being a reserve. One reciprocating dry vacuum pump is provided for each unit, with a capacity for two. The vacuum pumps are cross-connected between units.

The general turbine-room piping scheme is shown in Fig. 10. It comprises duplicate 15-in. mains for the turbines and 8-in. auxiliary mains, all suspended overhead in the basement, and provided with long radius curves and goosenecks where necessary. Twelve-inch condensate lines unite in a 16-in. main to the heater, as do 28-in. exhaust lines into a 36-in. main. The illustration shows the essentials of one unit with the exception of the atmospheric exhaust, an important feature on account of the large sizes of pipe involved.

This atmospheric exhaust system is combined with the auxiliary exhaust in the following fashion. From each receiver is a 30-in. line into which the auxiliary exhaust lines are connected. An atmospheric relief valve is in each line near the receiver. This is a standard relief valve weighted to open on about 28 lb. per square inch absolute pressure by means of a hydraulic piston and standpipe accumulator. The three lines lead into a tapered header, from which two 30-in. risers, sealed with back pressure valves, extend above the roof. From the header a 42-in. pipe leads to the feed-water heater. This is also protected with a riser and valve.

Between the receiver and the feed-water heater is a "thermal" or "heat-balance" valve for equalizing the distribution of heat in the system. This valve is shown in cross-section in Fig. 11. Its function is to bleed steam from the receiver into the heater when there is a deficiency of supply to the latter from the auxiliaries, or vice versa. At about 27,000 kw the receiver pressure reaches atmospheric pressure, continuing to rise until at 32,000 kw it is about 8-lb. gage.

In the figure the left-hand opening in the chamber leads to the heater, the right-hand one to the receiver. There are two piston valves, A and B. When heater pressure is above atmospheric, the latter being applied to the upper side through a pipe connection, A rises and bleeds steam through port C to the receiver. Whenever the receiver pressure is higher than that of the heater, B rises, admitting steam through port D to the heater. A dashpot at the top of the upper valve chamber prevents sudden acceleration of A, and through A provides a cushion for B. In action the valves do not lift very high off their seats.

Above the valve is an auxiliary cylinder E with live steam above its piston and with pressure controlled by the turbine automatic stop governor below. The piston is loosely packed and leaky and ordinarily floats. When the automatic stop governor operates, pressure is reduced below the piston which is forced down, restoring A and B to the positions shown and cutting off connection between receiver and heater. Another valve, which has been developed by the company's engineers, is one for shutting off the supply of steam in case the vacuum falls below a predetermined value. It consists of a float, the level of which is controlled by the height of a mercury column connected to the condenser. The valve is adjusted to trip the pilot valve of the actuating piston on the main throttle when the vacuum falls to, say, 15 in.

In the boiler house the original boilers are being retained, but one-half are being rebuilt to the extent of adding standard B. & W. superheaters to give 200 deg. of superheat when the boilers are delivering three times their rated output. The Taylor underfeed stokers which are being installed under one-half of the boilers are standard, but on four of the boilers air tuyeres have been installed along the side walls, directing jets over the fires and protecting the side walls.

One Hoppes open-type feed-water heater 9 ft. in diameter and 21 ft. long, with a capacity of 1,600,000 lb. per hour has been added to do the work of the displaced economizers, there being now a good supply of steam from the turbine-driven auxiliaries. It contains 240 pans, each 4 ft. long, having a total area of 3400 sq. ft. In this heater the surplus heat is being absorbed to such an extent that an occasional gentle puff of steam from the relief valve on the heater indicates how little heat is wasted in this part of the system.

As the remodeled boiler plant provides only superheated steam, provision had to be made for the auxiliary supply of saturated steam for the reciprocating engines. In the line connecting the old and new boiler plants is connected a receiver from one of the old engines. This is placed in a vertical position and the superheated steam is led in at the top, passing downward nearly to the bottom through a 15-in. tube. Near the top water is sprayed into the steam through a spraying nozzle, the surplus collecting in the bottom from whence it is returned to the water system. The wet steam rises in the space between the tube and the casing, drying as it rises, and saturated steam is taken off near the top. This device is known as an "attemperator."

In the boiler-room basement much space has been saved by the removal of four triplex pumps, accommodating three turbine-driven centrifugal pumps sufficient in capacity for the entire plant, and the stoker fans and turbines. For each pair of boilers there is one turbine driving two stoker fans direct and the stokers also through helical reduction gears from the blower shafts. In the ashpit section of the basement, ashpits of expanded metal plastered with cement have been recently put in.

The electrical distribution of the plant has been entirely remodeled with a view to providing adequately large switches, feeders, etc., and to protecting the underground cables from damage due to the large amounts of power which will now be concentrated on short-circuit. There are two sets of buses, main and auxiliary, to which each old and new generating unit is connected. Between the generator and the main bus is a reactor with 5 per cent reactance, and between it and the auxiliary bus is one with 2 per cent reactance. The main bus is sectionalized through oil switches, groups of feeders being taken off from each section.

The reactance coils are placed on the exciter gallery conveniently located with respect to the generators. The 5 per cent coil is about 4 ft. 6 in. in diameter and 8 ft. 6 in. long, and the 2 per cent coil is about 4 ft. 6 in. in diameter by 3 ft. 6 in. long.

The oil switches are of standard General Electric manufacture, type H6, with 10-in. pots.

The generators have been designed for their rated capacity, 30,000 kw, with 55 deg. Cent, temperature rise. They are ventilated by means of fans which form part of the revolving field. The air is taken in through the openings below the bedplates, shown in Fig. 5, and is discharged through ducts to the boiler house, eventually reaching the stoker fans. The generators have about 8 per cent reactance, and the armature windings are braced with extraordinary firmness to withstand the mechanical effects of short-circuits.

The changes described in this article were designed and carried out by the engineers of the Interborough Rapid Transit Company, under the direction of Henry G. Stott, superintendent of motive power. Those in responsible detail charge were Reginald J. S. Pigott, mechanical construction engineer, and Gaylord C. Hall, electrical engineer.

FIG. 3-NEW TURBINE UNIT

FIG. 4-CROSS-SECTION OF ENGINE ROOM BEFORE REMODELING	FIG. 5-CROSS-SECTION OF ENGINE ROOM AFTER REMODELING

FIG. 6-GENERAL LAYOUT OF TURBINE UNIT	FIG. 7-CONDENSER AND SECTIONS OF PIPING

FIG. 8-TRI-ROTOR CIRCULATING PUMPS

FIG. 9a-PLAN AND ELEVATIONS OF CONDENSER

FIG. 9b-PLAN AND ELEVATIONS OF CONDENSER	FIG. 10-PIPING FOR ONE UNIT

FIG. 11-HEAT-BALANCE VALVE	FIG. 12-REVOLVING INTAKE SCREEN

Sources: Electric Railway Journal, McGraw Hill Company, Digitized by Microsoft, Americana Collection, archive.org.