Page 226 - Centrifugal Pumps Design and Application

200 Centrifugal Pumps: Design and Application

with high-speed pumps can be virtually eliminated without efficiency
sacrifice via inducer and staging options. It is interesting to note that a
two-stage machine with equal head split optimizes at 60% of single-stage
speed, which can reduce NPSHR up to about 70%,

Other High Speed Considerations

Aside from hydraulics, a number of other considerations exist in high-
speed design. Several of these design facets are touched upon in the fol-
lowing sections. Each is broad in scope so can only be briefly highlighted
here,

Stress and Deflection. Commercial availability of stage heads up to
about 3,000 feet has been indicated by Karassik for full-emission, high-
speed pump types. As has been indicated, partial-emission design allows
heads to 6,000 feet per stage even with relatively low-strength 316 stain-
less steel material, this potential accruing from rugged blade impeller de-
sign. Simple impeller geometry allows easy extension of this head limit,
if such need arises, through use of high strength-to-weight materials such
as 17-4 PH stainless steel or titanium alloys.
Size reduction associated with high-speed design is dramatically illus-
trated in Figure 11-13 showing high- and low-speed multi-stage rotors
with equivalent pumping capabilities. Fewer high-speed rotors and the
exponential relationships of span and shaft diameter combine to allow
geometries with lower shaft deflection in the high-speed design,

Gears. Speed-increasing gearboxes are generally required for pump
drive speeds above electric motor limits. Integral gear systems are fre-
quently used with single overhung impeller design, where the gearbox
high-speed shaft doubles to support the pump impeller. Multi-stage ma-
chines are usually designed with straddle-mounted rotors and free-stand-
ing gearboxes, so requiring strict attention to alignment and high-speed
coupling design.
The single most important attribute of high-speed, high-pitch line-ve-
locity gearing is precision. Hardened and ground gearing is attractive be-
cause modern gear grinding equipment provides very high precision ca-
pability, along with substantial size reduction over soft gearing.
Hardened gearing does not undergo geometric change during break-in,
so the required gear and mounting precision must exist at assembly.
Industrial gearing is generally designed in accordance with American
Gear Manufacturers Association (AGMA) specifications in which com-
plete design guidelines are presented. Gears corresponding to AGMA