CENTKIFUGAL PUMPS 675 



should work with the minimum possible clearance over the face of the 

 casing, which should be machined and painted. It is probable that vanes 

 about | inch deep and with a side clearance not exceeding *f% inch would 

 give good results. Balancing vanes, however, in any case greatly increase 

 friction losses (p. 190), and in view of this and of the impossibility of 

 obtaining exact balancing by their use, are seldom fitted on modern 

 pumps. 



(b) Here (Fig. 316) the diameter of the shrouding on the side remote 

 from the entrance is reduced so as to make the total area of the 

 shroudings approximately equal. 



The clearance space pressures are thus balanced, and the excess of 

 pressure on the inlet shrouding due to the greater pressure at the outer 

 radius of the runner tends to balance the pressure in the opposite 

 direction due to momentum changes. This method also is impossible of 

 exact calculation, and is therefore only suitable when used in conjunction 

 with a thrust block or balance piston to take up any minor unbalanced 

 thrust. 



(c) Where a balance piston is used (Fig. 316), leakage past this piston 

 is slight, and by adjustment of a valve on the pressure pipe P, which 

 adjustment may be automatic, the pressure may be very accurately 

 adjusted while running, to suit any condition of working. 



In the pumps shown in Figs. 314 and 315, the shrouding area 

 which is exposed to the discharge pressure from each impeller is con- 

 siderably greater on the side removed from the inlet, and consequently 

 the nett pressure is from right to left. To counterbalance this a balance 

 piston or disc P is fitted, exposed to pressure water from the last impeller 

 on its left hand face. This works normally with small clearance over the 

 fixed casing at C. When the pressure to the right becomes equal to the 

 full discharge pressure it is more than sufficient to counterbalance the 

 end thrust on the impellers, the spindle moves to the right, the clearance 

 between piston or disc and casing is increased, and the pressure water 

 escapes into the low pressure chamber E, relieving the end thrust on the 

 piston. The regulation is perfeatly automatic, the spindle taking up a 

 position in which the clearance is just sufficient to maintain the necessary 

 pressure in the space to the left of the balance disc. In view of the 

 simplicity and automatic nature of this device it promises to become 

 general on all modern high speed pumps. 



(d) In this method of balancing (Figs. 306A, 807, and 317). the outer 

 diameter of the balance ring is made slightly less than that of the 

 impeller at the packing ring P, while a series of holes through the 



x x 2 



