Page 112 - Centrifugal Pumps Design and Application

NPSH 93

Figure 8-8. Performance curve showing NPSHR cavitation free and NPSHR 3%
head loss.

"Cavitation-free NPSHR" cannot be demonstrated by a suppression
test, as no head loss will be evident. Therefore, to satisfy the normal re-
quirement for testing on the pump manufacturer's test stand, it is sug-
gested that two NPSHR curves be offered with the pump quotation.
These would be "cavitation-free NPSHR" and the conventional 3% head
loss NPSHR (Figure 8-8).

Cavitation-Free NPSHR

As described earlier, cavitation is the formation of vapor-filled cavities
in the pumped liquid resulting from a sufficient reduction of the liquid
pressure to vaporize a proportion of the liquid. To prevent this vaporiza-
tion and the damage associated with it, the pump designer must first con-
sider the head losses in the most critical area, which is between the inlet
nozzle of the pump and the leading edges of the first-stage impeller
blades, This head loss is a result of the following factors:

1. Head loss due to friction.
2. Head drop due to fluid acceleration, which is the energy required to
accelerate the flow from the suction nozzle to the impeller eye.
3. Head shock loss due to blade entry, which is the localized drop at
the blade leading edge and is a function of the angle of attack and
blade entry shape.