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FIRST APPLICATION TO FKED PUMPS of an adjustablespeed fluid drive connect* a constent-epeed qutrrel-cogo motor, rated 700 hp at 1200 rpm. to a booster pump CONSTANT-SPIED MOTOR: One of three 1800-hp rpm unite connected through adjuetable-epeed fluid drive* and geari to 3600-rpm high-pressure boiler-feed pump 3600-Rpm Fluid Drives Adjust Feed-Pump Speed to Boiler Load High-pretsure boiler-feed-system losses are reduced to a min imum with adjustable-speed fluid drives for 3600-rpm pumps. They hold correct boiler level automatically without throttling, By J D ROSEBROUOH Stoll foginter. American 0/ever Corp Adjustable-speed drives for draft fans and boiler-feed pumps form a major consideration in steam-plant de sign. With rising fuet costs and the consequent trend to higher steam pres sures for maximum economy, adjust able-speed drives for boiler feed pumps are even more important than for draft fans. In a high-pressure plant about twice as much power is required to , drive boiler-feed pumps as draft fans. For example, a boiler designed for 900,000 lb of steam per hr at 1500 pai requires 1600-hp on induced-draft fans, 1000 hp on forced-draft fans, with two 1750-hp feed pumps in operation and a third for standby. These pumps de velop about 1800 pai when discharging at maximum rate. When they operate at constant speed and are throttled to reduce flow, pressure may rise to 2000 psi or higher. U the pumps have a Bat head-ca- parity characteristic to keep down pres sure rise when throttled, they are less efficient and require more power at re duced flow, and may be unstable. Also, throttling centrifugal pumps for long periods to reduce flow through them may seriously increase their mainte nance costs. Fluid-Drive Inilollotions. The hy draulic coupling, more popularly known as a fluid drive,- has been used exten sively for draft-fan and feed-pump variable-speed drives with constantspeed motors. It has two identical vsned rotors, shaped like the halves of a grapefruit, and uses oil as the powertransmission medium. One rotor, the impeller, connects to the driving mo tor, and the other, the runner, is at tached to the output shaft and drives the load (fan or boiler feed pump). The impeller pumps oil into the runner driving it like a turbine through a fluid circuit so arranged that oil circu lates continuously from runner to im peller. Efficiency of power transmission at full load is 96 to 97%. Fluid drives were first applied to boiler feed pumps more than ten years ago, photo at left, in the Philadelphia Electric Co's plant. The two 30-in.dia drives, used with 700-lip 1200-rpm motors on booster pumps, are still op erating. The Impeller of each drive is mounted directly on the motor shaft and d heavy costing supports the runner shaft bearings and acts as i manifold for the oil to enter and leave the ro tor. The output shaft connects to the pump ihrough a flexible coupling. A small motor-driven control pump trans fers oil from the sump to the rotor and also reverses to withdrawal from the rotor to slow down the speed. In 1940, three 3600-rpm adjustablespeed boiler feed pumps were installed in the Oswego plant of the Central New York Power Co. These ore driven through step-up gears by 1800-hp 1800rpm motors and fluid drives, photo ot right. The control is so arranged that pump speed can be adjusted (1) diicctly to hold water level constant (2) to control feedwater level by the feed water regulator valve, and then, with control of pump speed, hold a constantpressure drop across the regulating valve. T Oil POWER Jarua'y (<MB 2300 P, 'sj > 1800 1700 1600 1500 !1 -ft'r out >t ptmuW " Ftidiot . 1400 CROSS SECTION through 3600-rpm adjustable-speed fluid drive for boiler-feed pump*. R la runner; !. Impeller; P, ell pump; C, oil eoelera; 8. epeed controller 1300 This-installation marked an advance in feed-pump drives becauso complete control of feedwater flow to the boilers could be obtained by regulatiog pump speed with the automatic feedwater-con trol system. Previously there was doubt as to how last this type of ad justable-speed drive could change pump speed under such severe service. This installation removed the doubts so sev HIGH-PRESSURE bolter feed pump performance dur ing constant- and adjustable-speed operation. With adjuatabfa-apaad control, only re quired feed pres sure need be held A dr 4> 4 ? %$'*0 A L., 200 400 600 600 FUmp dischorge,DOO lb per hr 500 1400 1300 i 1200 j IIOO 1 1000 700 600 eral similar installations were made. These drives led to the development ol the 3600-rpm fluid drive, cross section above, which is directly responsive to feedwater control end-which thus offers full benefit of adjustable speed. Speed Control. The curves, on this page, show why speed control is de sirable for boiler feed pumps. Constantpeed pressure curve is the rising hesdcapacity characteristic of the boiler feed pump as it is throttled lo reduce flow to the boiler. From a flow of 1,100,000 lb per hr down to zero, pump-discharge pressure increases from 1850 to 2300 psi. This sharp rise in pressure is partly caused by decreased friction losses on the suction side of the pump at reduced flow, and by the rising heed characteristic of the condensate pump. Feed pumps must be selected to de velop required boiler-outlet pressure, plus friction losses in superheater, boiler, economiser, feedwater valve, pip ing and high-pressure heaters; with a margin for blowdown. The pump se lected for thesA requirements is good for -much greater capacity than Is needed for normal steaming conditions. The adjustable'speed-pressure eurve, above, shows1 the required discharge pressure.' With adjustable-speed con trol, only `the required pressure need be held and a reduction in pressure of 500 to 700 lb results at low flows, with a corresponding saving in poorer as indicated by the two bottom curves. For example, at 400,000 lb par hr with adjustable-speed drive, pump-discharge pressure is 700 lb less, points A and B, and power input is 300 hp less, points C and D, than with constantspeed drive. Other savings are also important to the operating department These are: (1) flexibility of control for starting up and paralleling pumps (2) reduced pump maintenance as a result of lower pressures between stages and less water churning inthe pump (3) lower aver age pressures in the high-pressure feedwater heaters (4) lest feedwater valve maintenance. - Demands of 3600-Rpm Rumps. To meet the demands for 8600-rpm boiler feed pumps for 500 to 2350 hp, a fluid drive was developed lo give: (1) speed response of less than 30 sec over the lull range of pump capacity--20 to 25% speed change (2) close, ac curate speed control, because of the narrow range of speed over which a pump operates (3)' packaged power unit requiring no accessories other than the linkage to tho automatic control and the water connections to the oil cooler (4) simple design to keep all mainienanee factors out of the adjust able-speed drive. This drive is enclosed, as In cross section above, in a heavy welded-steel housing. This acts as an oil reservoir, a support for the Kingsbury bearings, and a container for the oil cooler and' oil-supply pump. Speed change La by a speed controller that sktmi oil out of the rotating parts and sets the level according to its position. A continuous supply of cooled oil is fed into the im peller by the oil-supply pump. Condensate Ramps. While the most important advantages of stepless speed control ore realized on the main boilerfeed pumps, there are fluid drives on booster condensate pumps or primary (low-pressure) boiler-feed pumps where good1 results are achieved ot relatively low investment. In these applications the main boiler feed pump operates at constant speed. Since booster con densate pumps feed into the suction of the main boiler-feed pump, the com bined total outlet pressure can be low ered by reducing the speed of tho lowpressure pump. Most applications o! fluid-drives, however, have been oo main boiler feed pumps, where full ad vantage can be taken of adjustable speed. POWER January 1440 1431 75