40 BULLETIN 304, U. S. DEPARTMENT OF AGRICULTURE. 



200 revolutions per minute, and hence may be directly connected 

 to two separate engines. 



Assume that the difference between the river-surface level outside 

 the district and the desired ground-water level within the district 

 will vary from a possible maximum of 20 feet during spring floods 

 to nothing in the late summer. The machinery must have sufficient 

 power to pump against the highest head, although much of the 

 time it will run against a lower head. This is a disadvantage, so far 

 as the efficiency of the machinery is concerned, because every engine 

 runs most efficiently at about its rated capacity, and if it is either 

 overloaded or underloaded its efficiency is reduced. The actual head 

 against which the pumps must force the water, sometimes called 

 the dynamic head or the hydraulic head, is always greater than the 

 difference in water level as described above, which is called the 

 static head. This increase is due to the friction of the water in the 

 suction and discharge pipes and the so-called velocity head — that is, 

 the head necessary to start the water into motion in the pipes. The 

 friction in pipes carrying water at a high velocity is greatly affected 

 by the kind of pipe used and the smoothness of the interior surface, 

 and hence can not be predicted with certainty. For a velocity of 8 

 feet a second the friction in 200 feet of 48-inch pipe would probably 

 amount to 2 feet of head and in a 24-inch pipe to at least 6 feet of 

 head. A bend of 90° adds probably a foot to the friction head. The 

 velocity head amounts to about 1 foot, and if the water passes around 

 a sharp edge in entering the suction pipe there is probably a loss of 

 another foot of head. To be safe, then, we should assume the addi- 

 tion of at least 6 feet to the static head to give the head on the pump. 

 Another small amount might be added to the lift on account of the 

 lowering of the water level in the supply ditch when the pumps are 

 being operated. But it is probably better to ignore this last addi- 

 tion and assume that during the short time when the river is at 

 maximum flood stage the water level in the district will not be 

 pumped down quite to its normal level rather than to provide extra 

 capacity for the very short and infrequent periods of maximum 

 river floods. 



With a total head of 2.6 feet the hydraulic horsepower is calculated 

 by multiplying together the lift (26 feet), the number of cubic feet 

 of water per second (105), and the weight of 1 cubic foot of water 

 (62.5 pounds), and dividing by 550. In this case the result is 310. 

 The power required of the engine will be greater than this amount, 

 depending upon the efficiency of the pumps, which for these condi- 

 tions should be at least 60 or 70 per cent. For this case probably 

 manufacturers would be willing to guarantee an efficiency under 

 test of 70 per cent. Accordingly the maximum brake horsepower 

 required of the engines would be 440. Engines suitable for this 



