Page 70 - Centrifugal Pumps Design and Application

Volute Design 53

Figure 5-2. Radial thrust factor.

though the volute proper is symmetrical about its centerline, the two pas-
sages carrying the liquid to the discharge flange often are not. For this
reason, the pressure forces around the impeller periphery do not pre-
cisely cancel, and a radial force does exist even in double-volute pumps.
Values of the constant K have been established experimentally by actu-
ally measuring the pressure distributions in a variety of double-volute
pumps. The data presented in Figure 5-2 apply to conventional single-
stage double-volute pumps and indicate substantial reductions in the mag-
nitude of K. Tests on multistage pumps with completely symmetrical
double-volute casings indicate that the radial thrust is nearly zero over
the full operating range of the pump.
The hydraulic performance of double-volute pumps is nearly as good
as that of single-volute pumps. Tests indicate that a double-volute pump
will be approximately one to one and one-half points less efficient at
BEP, but will be approximately two points more efficient on either side of
BEP than a comparable single-volute pump. Thus the double-volute cas-
ing produces a higher efficiency over the full range of the head-capacity
curve than a single volute.
Double-volute pump casings should not be used in low-flow (below
400 GPM) single-stage pumps. The small liquid passages behind the long