CENTRIFUGAL PUMPS 



645 



Leakage from stage to stage of a high -lift pump is prevented by brass 

 packing rings surrounding the shaft between each pair of chambers, while 

 to prevent corrosion of the pump spindle this is usually protected by a 

 brass sleeve. 



Admission of air on the suction side of such a pump is to be guarded 

 against with the greatest care, as being productive of inefficiency and of 

 considerable shock in the pump casing and delivery pipes. 



ART. 174. SUCTION AND DELIVERY PIPES. 



The area of the suction pipe is commonly made equal to the discharge 

 area of the impeller. A foot valve and strainer should always be fitted 

 to this pipe, the area through the valve being not less than half that 

 of the pipe. No part of the pipe should be above the pump inlet in order 

 that the formation of air pockets may be prevented, while when the pump is 

 circulating water through a pipe line forming a syphon, an ejector or air 

 pump should be fitted to the highest point of the pipe, for priming the 

 pump and for the removal of any accumulation of air. 



The suction lift should be as small and as direct as practicable, and 

 should never, if possible, exceed 20 feet. Where hot liquids are to be 

 handled, the vapour pressure reduces the possible suction lift, the effect of 

 temperature in the case of water being as follows : 



Practically the limit is reached much before this because of the libera- 

 tion of air when the pressure is reduced nearly to that corresponding to 

 the saturation temperature, and because of the increasing importance of 

 slight air leakage at joints, and at the stuffing boxes of the pump spindle 

 at these low pressures. In no case should the suction lift exceed two- 

 thirds of the values given above, while where hot liquids are to be lifted it 

 is advisable to place the pump below the supply level. 



The above remarks may be taken as applying with equal force to the 

 reciprocating pump. 



When parallel, the area of the discharge pipe should, in generrl, be not 

 less than that of the suction pipe, and may with advantage be increased 

 to three times this area where the working head is high. It is, moreover, 

 advantageous to have a main discharge pipe of greater area than the 



