be done simultaneously using finite element analysis. The
goal is to determine at least all natural frequencies and mode
shapes up to 1. 25 times the number of impeller vanes times
the running speed. The foundation mass and stiffness within
a radial distance at least equal to the height of the top of the
motor, relative to the level of attachment of the baseplate to
the floor, should be evaluated. The vendor having unit responsibility may perform a lateral dynamic analysis.
Typically engineered pumps of 100 hp or greater, especially
variable speed units and/or tall discharge assembly units with
L/D greater than 4.0, should be analyzed.
As discussed, larger vertical turbine pumps, especially
short-set industrial, variable speed units, and units with tall
aboveground structures (and flexible foundations), should
strongly be considered for a vibration analysis prior to installation. In addition, further consideration should be given to
shorter (below critical speed) bearing spans and/or larger line
shaft diameters, as well as rigid, clamp type column shaft couplings. WW
Allan R. Budris, PE, is an independent consulting engineer who specializes in training, failure analysis, troubleshooting, reliability, efficiency audits and litigation support on pumps and pumping systems. With offices in Washington, N.J., he can be
contacted via e-mail at email@example.com.
Marscher, Willian D. “An End-User’s Guide to Centrifugal Pump Rotordynamics,”
Proceedings of the Twenty-Third International Pump Users Symposium, 2007.
Circle No. 240 on Reader Service Card
tolerances necessary for the small shaft ends (which butt together in the typical threaded shaft couplings) to maintain the
desirable shaft straightness would be unreasonable. Such misalignment further adds to shaft vibration. However, if a high
axial thrust is not present, such as with a short-set industrial
pump, this shaft misalignment problem can still be managed
by the use of clamp-type shaft couplings in place of threaded
couplings to insure proper shaft alignment.
VTP ABOVEGROUND STRUCTURE
As mentioned above, the typical VTP aboveground structure tends to be quite flexible, with a heavy motor on top of
a long motor pedestal and discharge head. This can be further
weakened by a light base foundation with a mass less than
five times the weight of the supported equipment. This makes
it fairly likely that the pump will operate at a structure critical speed, especially with variable speed drives, and/or poor
pump piping that can add external forces to the discharge
head. This often necessitates a vibration analysis to avoid having a problem.
When performing a critical speed analysis of an abo-
veground structure, the following should be considered:
• Pump structure
• Pump piping
• Lack of pipe supports close to the pump when piping is
hard-coupled to the pump
• Motor or drive reed frequency (VTPs)
• Operation close to a resonant frequency
• Miscellaneous damping effects
• Variable speed units: With all of the VTP reed and line shaft
vibration modes, the odds of operating at a pump critical
speed is quite high. Often, certain speed ranges may have
to be locked out of the VFD to avoid operating close to a
OTHER CAUSES OF HIGH VIBRATION
In addition to vibration caused by the stationary aboveground pump structure, line shafting and impeller rotational
components, hydraulic disturbances can also add to the vibration.
Although cavitation is not as much of a problem with VTPs
(since the bottom portion of the bowl assembly is submerged
and there is less likelihood of having high suction energy), it
can still add to VTP pump vibration.
Suction pressure pulsations can also increase VTP vibration
when high suction energy pumps are operated in their low
flow suction recirculation region (see Fig. 3).
Normally, a VTP vibration analysis of the stationary structure, the line-shafting, and the pump and motor rotors should
Figure 3: Effect of Flow Rate Region on Vibration Performance
Flow (% BEP)
0 50 100