CENTKIFUGAL PUMPS 667 



1*8, increasing from about 1*2 in the case of a well designed pump with 

 whirlpool chamber or guides and with a value of 7 = 90, to about 1'8 

 in the case of a pump having a volute chamber only, and a value 

 of y = 15. 



This makes / 3 = J *J 2 g H' (approx.). In the case of a pump dealing 

 with a large head in a single chamber this, however, gives an excessive 

 value of / 3 , and in such a case the value may be reduced to as little 

 as I *J % g H'. 



ART. 180. SPEED AT WHICH PUMPING COMMENCES. 



Even if a centrifugal pump be primed, there will be no flow through 

 the delivery pipe until the pressure difference through the wheel, which 

 depends entirely on the speed of rotation, is sufficiently great to overcome 

 the total head of lift, the pump until then merely sustaining a stationary 

 column of water in this pipe. 



If the suction lift is H s feet, the pump will not maintain its charge 

 without a foot valve on the suction pipe, unless the speed is sufficiently 

 great to cause a pressure difference of more than H s feet between the 

 pump inlet and outlet. 



Assuming uniform rotation with no flow through the pump, the 

 pressure difference in the forced vortex becomes 



2 



feet, / 2 and / 3 being zero. 

 For the pump to remain charged when once filled we must have 



while for delivery to take place we must have 



* 9 



Once free flow is established, the state of affairs changes. A further 

 increase of pressure is now necessary to overcome frictional resistances, 

 and in the case of a pump not fitted with vortex chamber or guides this 

 necessitates the speed for free delivery being slightly greater than is given 

 above. A reduction of the speed below this limit will cause pumping to 

 cease. 



With a whirlpool chamber, or guide ring, however, once flow is 

 instituted part of the kinetic energy of flow through the wheel is con- 

 verted into pressure energy, so that the speed may be reduced below that 

 necessary to initiate flow, without pumping being stopped. 



