CENTRIFUGAL PUMPS 673 



than that on the exit side, the difference of area being just sufficient to 

 balance the pressure difference (Fig. 316). 



(c) By a rotary balance piston or disc keyed to the pump shaft, one 

 side being exposed to delivery pressure, and the other to the pressure on 

 the suction side of the pump (Figs. 314, 315, 316). 



(d) By relieving the pressure over a certain area behind the shrouding 

 by means of a rotating balance ring (Figs. 306A, 307, and 317). 



Whichever of these methods is adopted, with the exception of (c), the 

 wheel will only be in perfect balance at one speed and with one rate of 

 delivery, and in such cases it is necessary to arrange a thrust block to 

 take the end thrust caused at starting or stopping and in the case of a 

 variable load. 



These methods of balancing will now be considered in further 

 detail : 



(a) Radial Balancing Vanes. Since it is impossible to keep the joint 

 between the outer periphery of the shrouding and casing tight, the space 

 behind the shrouding will be full of water normally at delivery pressure, 

 and since the mean pressure in the wheel is considerably less than this, 

 the resultant effect in a single shrouded wheel (Fig. 305) will be an axial 

 thrust of considerable magnitude, in the opposite direction to that of 

 axial flow. 



If a series of holes be made through the disc, as indicated in Fig. 305, 

 the pressure at this radius on both sides of the shrouding will be equalised, 

 and there will be a constant circulation of water past the rim of the 

 impeller, behind the shrouding to the zone of low pressure, and out to 

 the working side of the shrouding. 



While the axial thrust is then largely balanced, this leakage is produc- 

 tive of inefficient working, and the method, though often adopted in 

 turbine practice, is inadvisable. 



If, however, shallow radial vanes are fitted to the rear face of the 

 shrouding, the water in the clearance space is forced to rotate with vortex 

 motion, and in consequence its pressure diminishes from the outside, 

 where it has a definite value, to the inside, following approximately the 

 forced vortex law. The degree to which it deviates from this law 

 depends on the amount of side clearance between the balance vanes and 

 the pump casing, any increase in this clearance, by reducing the mean 

 angular velocity, tending to increase the mean pressure behind the 

 shrouding. By suitably regulating the clearance and the radial length 

 of the balance vanes, the whole, or any portion of the axial thrust, may 

 .thus be balanced. 



H.A,. x x 



