Page 230 - Centrifugal Pumps Design and Application

204 Centrifugal Pumps: Design and Application

load, so the ranges associated with gear loading and impeller hydraulic
loads must be considered in detailed analyses.
All this appears a bit intimidating at first glance, but in practice shaft
dynamics has generally shown itself to present neither incessant nor in-
surmountable problems. It should be pointed out that critical speed oper-
ation is not always destructive. Cases exist where a shaft can be made to
run continuously at its critical for long periods, but needless to say, com-
fortable margin should always be provided between critical and design
speed. With a new product family, thorough critical speed analysis is
used in the design phase, and the analysis is confirmed by test experience
in the hardware phase. Need for a fall-blown analysis for each minor
pump variation is alleviated. Normally, each production pump is tested at
foil rated capacity so any dynamic distress can be detected and corrected
prior to shipment. Single-speed machines offer advantage in this regard
since change of shaft stiffness, bearing stiffness, rotating mass, or a com-
bination of all three can often cure a problem with modest hardware al-
teration.
Field problems with shaft dynamics are by far the exception rather than
the rule. Such exception has an increased chance of occurring when full
field operating conditions cannot be duplicated in the manufacturer's test
facility. For example, water is the universally used test fluid, so pumps
designed for low gravity fluids must often be operated at off-design con-
ditions to simulate their full-power or Mi-speed operating characteris-
tics. Or, a pump can interact differently with a user's system or founda-
tion than it does in the laboratory. The computer has proven to be an
invaluable aid in such occasional situations when a problem occurs.
Instrumentation is readily available to continuously monitor machine
health if so desired. Noncontacting probes can directly observe high-
speed shaft motion and can be arranged to provide display, alarm, or
shut-down in the event of trouble. But this option is generally reserved
for large and costly equipment. It is probably safe to say that this instru-
mentation is seldom opted for in machines under a few hundred horse-
power. Experience has shown that the reliability and endurance of high-
speed machines can be assumed to match that of their lower-speed
counterparts, so similar ground rules on protective instrumentation
should apply.

References

Balje, Dr. O. E., Turbomachines; A Guide to Design Selection and The-
ory, John Wiley and Sons, 1981.