To show the relations between central station plant, h.p.-hours actually delivered, amount of machinery operated and work accomplished, some actual experiences are here given. The performance at the Roosevelt dam was quite accurately kept during a month when masonry construction was at a maximum. For the other examples cited the conditions are not as accurately known. Some of the data either were not kept or are not available, nevertheless they are believed to be fairly stated approximations that may be used as a basis for rough estimates. At the Roosevelt dam the conditions were as follows:
Electric power at 2300 volts alternating current was metered and delivered to the contractor, a motor-generator set transformed it to 500 volts direct current at which it was used. A small air compressor to supply the drills was belted from the motor-generator set. The quarries were immediately at each end of the dam and at a level higher than the masonry. Quarry derricks passed the stone to points under the cableways, also to the crusher. Two cableways transported all materials to the dam, and a third cableway transported quarry waste some 700 ft. to a dump. A tramway 1700 ft. long brought the cement from the mill to the mixer. At an intermediate station the sand was picked up and transported about 1100 ft. to the mixer. The tramway was about level except for the sag between towers. The tramway worked twenty-nine and one-eighth day shifts and twenty-eight night shifts, taking to the dam 13,538 buckets of sand at 8 cu. ft. and 9822 buckets of cement at 5.2 cu. ft. Two cableways in twenty-eight day shifts took from the mixer to dam 1056 skips of mortar of 2.2 cu. yd. each, 2571 skips of concrete at 2.62 cu. yd., each; they also took from quarries to dam 1876 skips of spalls. The skips were of iron 7 ft. X 7 ft. X 2 ft. and weighed about 2200 lb. Two cableways in twenty-nine night shifts took into the dam 7900 cu. yd. of stone. One cableway in two shifts per day disposed of 3150 cu. yd. of quarry waste. Nine derricks in quarries produced 7900 cu. yd. of stone for dam and 2200 cu. yd. of spalls and passed it to the cableways; passed to the crusher about 2570 cu. yd. of rock, and to No. 3 cableway 3150 cu. yd. of waste. Five derricks on the dam in twenty-eight and one-eighth day shifts laid 18,328 cu. yd. of masonry. One Smith mixer in twenty-eight and one-eighth shifts mixed 2112 batches of 1.1 cu. yd. of mortar and 5142 batches of concrete of 1.31 cu. yd. The rated capacity of each machine, hours actually employed, and h.p.-hours of 100 per cent, load factor assumed, is as follows on page 140.
The motors actually installed and used would total about 980 h.p. The consumption rarely if ever went over 400 h.p. even on momentary peaks. The amount of power actually delivered to the contractor during the month as metered at 2300 a.c. before transforming, was 165,013 h.p.-hr. Assuming an efficiency of 90 per cent, for the motor-generator set, then 165,013 X 0.90 = 148,512 h.p.-hr., were distributed to the various machines, indicating a load factor for the plant as a whole of about 34 per cent.
Two main cableways.............
No. 3 cableways...........
Nine derricks in quarry, day......
8 at 40 each
2,081 total day
Nine derricks in quarry, night....
1 at 35 each
1,988 total night
Five derricks on masonry............
Compressor for drills............
Equivalent to 600
Lights at dam and camp.......
Shop and camp purposes..............
Water supply, ice machine......
The principle is beautifully illustrated at the Arrowrock dam The following information and load curves on that work are taken from an article in Engineering Record for Aug. 24, 1912. At the generating station, 17 miles from the dam, are three hydraulic turbine-actuated 625-kva. generators which deliver three-phase 60 cycle alternating current at 2300 volts. This potential is stepped up to 22,000 volts for transmission. At the dam the current is stepped down to 2300 volts for distribution to the various motors. The feeder panels are equipped with curve tracing wattmeters from which the very interesting curves shown in Fig. 25 are taken.
The apparatus contemplated for use at the dam comprises an installation of 3000 h.p. in rated capacity of induction motors, as follows: Two 300-h.p. cableways, 500 h.p. in various motors in the sand cement plant, 125-h.p. air compressor, 1180 h.p. in units of 50 h.p. to 125 h.p. for pumping, four 75-h.p. derrick motors and a large number of smaller motors for crushers, mixers, shops, etc.
At the Wachusett dam central power station were two 500-h.p. Rand-Corliss compressors. At their normal speed of 75 r.p.m. each compressor delivered 3310 cu. ft. of free air per minute raised to 90 lb. pressure. A 6-in. main 1/2 mile long led to the quarry and an 8-in. main 1 1/4 miles long to the dam. Air was supplied to the following: thirty-one 16-h.p. hoisting engines, two 50-h.p. cableways, fifteen to forty No. 3 Rand drills, three stone dressing machines, three 10-h.p. engines at mixer, screen, etc., three 3-in. to 4-in pumps, ten forges, one riveter in shop and one trip hammer.
With the above plant in operation the compressors were worked between 75 per cent, and about 90 per cent, of their full capacity depending on the number of No. 3 drills in operation. At one time the leakage in the air mains was such that with every machine shut off 11 r.p.m. of one compressor was required to maintain the pressure in the mains; corresponding roughly to 7 per cent, or 8 per cent, of the station output. Undoubtedly there was some additional leakage between the various valves where the machines were shut off and the machines themselves, so that when all were in operation the leakage must have been in excess of that observed when they were shut off. In other words some of the leakage must have been shut off when the machines were shut off.
The hauling of the stone and sand to the dam is not included in the above service. The stone was hauled by locomotive and cars on 2 1/4 miles of standard gage track. The sand was hauled 1/2 mile by teams from the pit to bins near the mixer. At this time ten to eleven derricks on the masonry were setting 500 cu. yd. to 600 cu. yd. of rubble masonry per ten-hour day. At the quarry probably as much stone was wasted as was used.