THE IRRIGATION AGE. 



239 



is no moving part that touches another part with the 

 sole exception of where the shafts run in the bearings. 



VARIABLE HEADS. 



If a motor-pump with a fixed speed is designed for 

 a certain head, say 30 feet, with a capacity of say 1,800 

 gallons per minute, it will operate at 

 its best efficiency at that head. If a 

 valve in the pipe at say 15 feet head is 

 now opened, the quantity of water dis- 

 charged will be greatly increased, prob- 

 ably reaching 2,400 gallons per minute. 

 The power consumed by the motor will 

 increase since the efficiency of the pump 

 is decreased when the speed of the run- 

 ner is too high for the head it is pump- 

 ing against. If a direct current motor 

 is used under such conditions, the speed 

 should be reduced to that required for 

 the lower head, but this is impossible 

 with an induction motor, as indicated 

 above. If, when operating under such 

 conditions, it becomes necessary to re- 

 duce the pump capacity at the lower 

 head to the original capacity for which 

 it was designed, a gate valve may be 

 used upon the discharge side of the 

 pump to throttle the discharge. A cer- 

 tain loss of power is then occasioned. 

 When it is necessary to operate under 

 such conditions, it is advisable to use a 

 motor of ample power to handle the 

 greater quantity of water that will be 

 discharged at the lower head, permit- 

 ting the pump to throw the full quan- 

 tity that it desires without throttling. 

 It is an interesting fact that a motor 

 driving a centrifugal pump at a fixed 

 speed may be overloaded when pumping 

 against a lower head than that for 

 which it was designed, but neither 

 motor nor pump can be injured by at- 

 tempting to pump against a higher 

 head, if the speed is not changed. The 

 pressure created by the pump cannot, 

 exceed that which the runner speed and 

 diameter will give, and if the head is 

 greater than this, the water will rise in 

 the pipe to the point which the pressure 

 will give, and stand there without any 

 flow. Such is always true when a cen- 

 trifugal pump is operated at too low a speed. It is, of 

 course, possible to have a head at which a small quantity 

 of water will be discharged, but at a foot or two greater 

 head no water would flow. 



PRIMING. 



Priming may be accomplished in any of the ordi- 

 nary ways. The use of a pitcher spout pump to suck 

 water from the top of the pump, while the gate valve 

 on the discharge side is closed, is recommended highly. 

 A foot valve may be used if the height of lift and length 

 of discharge pipe are not great enough to be dangerous. 

 Many a pump has been cracked by water hammer in a 

 long discharge pipe with a high head. A check valve 

 may be used instead of a gate valve, but either a check 

 or a foot valve cause a certain amount of drag upon the 



"Vertical Cen- 

 trifugal Pump Di- 

 rect-Connected to 

 Motor." 



waUr while a gate valve gives a clear passage. If a 

 priming pump is used the motor should be started when 

 the [jump is fully primed, the valve being slowly opened 

 when the pump attains full speed. There is no danger 

 of injury by keeping the gate valve closed when running, 

 and if the valve is opened slowly the discharge pipe may- 

 be filled without overloading the motor. It is evident 

 that the pump will discharge a tremendous quantity of 

 water when operating against the small head that it has 

 before the discharge pipe is full. 



ATTENDANCE REQUIRED. 



A motor-pump may be operated continuously 

 through the irrigation season with no more frequent 

 visits than are necessary to see that the oil cups on the 

 stuffing boxes (if any) are filled. It can be run through 

 the night without any attention whatever. 



MAINTENANCE. 



The repairs in a well designed motor-pump are usu- 

 ally considered as being negligible. A very small quan- 

 tity of oil is required for a season. 



POWER REQUIRED. 



In order to determine approximately the power re- 

 quired for any pump, use the following short rule : 



Multiply gallons per minute by feet head including 

 friction, and divide by 4,000. The result obtained is the 

 theoretical horsepower required. Then divide by the 

 efficiency of the pump in per cent. A good centrifugal 

 pump with a capacity of 900 gallons per minute or more 

 will give an efficiency between 60 and 75 percent. Sup- 

 pose we want to pump 1,600 gallons per minute 25 feet 

 high, discharging it through 2,000 feet of 15-inch pipe 

 in which the friction head is about 5 feet. The total 

 head is thus 30 feet, which multiplied by 1,600 gives 

 48,000. Divide this by 4,000, and the result of 12 is 

 the theoretical horsepower required. If the efficiency 

 of the pump is 60 percent, divide by 60 per cent, and 

 get 20 h. p. as the motor power required. A motor of 

 ample capacity should be used, as there is no apprecia- 

 ble loss of power by using a motor that is larger than 

 the actual requirement would seem to indicate. It does 

 not pay to run a large motor at only one-quarter load, 

 but at any point between half load and full load, the 

 efficiency is very high, and the amount of current con- 

 sumed will be just in proportion to the amount of work 

 done. 



TRANSFORMERS. 



Alternating current is usually distributed at high 

 voltages, sometimes exceeding 100,000 volts, but motors 

 are never contructed for voltages higher than 2,200, and 

 even then in no sizes less than 25 h. p. The power 

 company will usually serve current at 6,600 or 2,200 

 volts, and unless a large 2,200-volt motor is to be used 

 on a 2,200 volt line it is necessary to use transformers to 

 reduce the voltage to that of the motors, which is 

 usually 220 volts or 440 volts. If 220 volts is used, 

 there is little danger to the operators, and lamps may 

 be used on the same circuit to light the pumping house. 

 Such is the voltage usually recommended for irrigating 

 plants. It is usually the custom for the power company 

 to serve current at a reasonable voltage, the owner of 

 the motor to furnish transformers, if necessary. The 

 capacity of transformers required for a pumping load 

 requires careful consideration of both efficiency and 



