and Thoma’s cavitation number, , given by (equation (4.14))
NPSH/Hm, (13.10)is found to be related to specific speed. The NPSHvalues are supplied by
the pump manufacturers.
Defining the suction specific speed, Nss, as
NssNQ1/2/(NPSH)3/4 (13.11)the cavitation number, , is given by
(Ns/Nss)4/3. (13.12)The suction specific speeds for most centrifugal and axial (propeller) type
units range between 4700 and 6700. The critical cavitation number, c, to
avoid cavitation is suggested (from model tests) to be
c0.103(Ns/1000)4/3. (13.13)PUMPS AND THEIR CLASSIFICATION 553
Fig. 13.2 Definition sketch of pump installation