36 BULLETIN 304, U. S. DEPARTMENT OP AGRICULTURE. 



The above principles have long been regarded as fundamental in the 

 science of mechanical engineering design. Applying them to drainage 

 pumping, where the work is usually confined to a small portion of the 

 year, the labor available is cheap and unskilled, and the other condi- 

 tions are not favorable for effective care of the plant, the fundamental 

 requirements in a drainage pumping plant are reliability and sim- 

 plicity. 



The centrifugal pump is the kind most often used for drainage. It 

 is simple, has no valves, and can be obtained in all sizes. Its dis- 

 advantages are, that it must be primed in starting, that it can operate 

 satisfactorily under a given set of conditions only within a narrow 

 range of speed, and that its efficiency when operated under practical 

 conditions that obtain in drainage pumping is not high. Recently 

 pump manufacturers have given more attention to the design of 

 centrifugal pumps for drainage pumping, and the newer pumps are 

 better fitted for such service. In order to work economically and to 

 maintain a uniform capacity, the speed must be varied with the 

 changes in the total head under which it operates. The size of the 

 centrifugal pump is usually given as the diameter of the discharge 

 opening — for example, a 24-inch pump is one having a discharge 

 pipe 24 inches in diameter as it leaves the pump. Most large pumps 

 are made with a horizontal shaft and are called horizontal pumps. 

 When larger than 24 inches they frequently have a double suction, 

 thus bringing water to both sides of the impeller; this has the advant- 

 age of balancing the side thrust on the impeller and shaft. Without 

 this arrangement some automatic balancing device must be used in 

 connection with the impeller and a collar thrust bearing on the shaft. 



The capacity of a pump is usually reckoned by assuming the water 

 to have a velocity of 10 feet per second through the discharge opening 

 of the pump. However, some manufacturers base the rated capacity 

 of their pumps upon an assumed velocity of 12 feet per second. 

 While it is possible to obtain a velocity of 10 feet per second through 

 the discharge opening of the pump by the use of proper speed at the 

 various heads, as the pumps are usually operated in practice the dis- 

 charge is often 20 per cent less than the theoretical rating. This is 

 due to underestimation of the power required for the various heads 

 pumped against; as the head increases the speed must also be in- 

 creased. During the high-water stages of 1913 on the Illinois River 

 several plants had their capacity reduced about 50 per cent because 

 the engines were not large enough to turn the pumps at the requisite 

 speed. If enough power is furnished, the capacity of the centrifugal 

 pump can be pushed as much as 25 per cent above its rated capacity 

 with a relatively small loss in efficiency. Tests of such plants have 

 been made which showed the amount pumped to exceed the rated 



