Page 111 - Centrifugal Pumps Design and Application

S2 Centrifugal Pumps: Design and Application

Figure 8-7. Stable operating window vs. suction specific speed.

damage-free operation on high speed pumps. The term "high speed"
does not refer to RPM, but to blade peripheral velocities above 160 ft/
sec. Thus a pump operating at high RPM with a small eye diameter may
be less critical than a slower pump with a larger eye.
A weakness of the 3 % head loss suppression test is that even when con-
ducted at full speed it fails to provide any evidence of the extent of cavita-
tion damage when the pump operates in this 3 % zone for extended peri-
ods. This is particularly true of high-speed pumps operating on water
where cavitation damage can occur at NPSHA values above 3 % head loss
and in some instances, even with 0% head loss. Even the inception of
cavitation detected by acoustic noise testing does not establish the value
of NPSHA for damage-free operation.
To prevent damage and permit safe operation of high-speed pumps, it is
suggested that pumps be supplied with NPSH predicted on a theoretical
basis and that this characteristic performance be termed the "damage
free" or "cavitation free" NPSHR curve. This method of prediction
could be useful in complying with specifications that require a guarantee
of 40,000 hours damage-free operation due to cavitation.