Case StudyThis case study demonstrates the steps taken to solve a structural resonance issue on a VFD driven vertical pump installed in the field. Topics discussed include problem validation, steps taken to solve the issue, the resulting reduction in vibration amplitude, and key takeaways

Patterson, Donnie

English

Texas A&M University

Structural Natural Frequency 
Tuning on a Vertical Pump
Donnie Patterson, PE
Author: Donnie Patterson
Donnie Patterson has been with Sulzer since 2007. 
He has held various positions within Sulzer including 
Design Engineer and Rotordynamic Engineer. He 
currently serves as a Field Engineer with Sulzer Pump 
Services Rocky Mountain Service Center in Denver, 
CO.  He received his BSME from John Brown 
University and is a licensed PE in the State of Texas. 
Abstract
Several pumps were experiencing high vibration in the 
field. They are VFD driven, VS1 style pumps. Vibration 
evaluation indicated an above grade natural 
frequency within the VFD operating range. Design 
changes to the discharge head and foundation were 
analyzed and predicted to raise the natural frequency 
above operating speed. The proposed changes were 
made to one unit and successfully reduced the 
vibration amplitudes to within acceptable levels.
Background Information
• Existing VS1 style in condensate water 
service
• Rated for 148 feet at 9000 GPM running 
at 900 rpm
• 400 HP VSS motor
• VFD operated at 65%-100% speed (585-
900 rpm)
• Waterfall plot showed high vibration amplitude in 
the horizontal plane (0.22 in/s) at approximately 
12 Hz when running at 80% speed.
• Bump testing indicated a natural frequency at 12 
Hz, which correlated with operational vibration 
data.
Preliminary Review
WATERFALL PLOT FROM INITIAL VIBRATION EVALUATION
Preliminary Review
0.225 in/s 
@ 720 rpm
INITIAL BUMP TEST RESULTS:
PEAK AT 12 Hz
Preliminary Review
Preliminary Review
• Pump foundation showed poor grout conditions, 
with excessive voids and 100% delamination.
• Pump mounting plate was resting on nuts installed 
on the anchor bolts between the grout and 
mounting plate.
Preliminary Review
Grout 
voids100% Grout delamination
Analytical Review
• Motor reed frequency of 28 Hz reported by vendor.
• As the motor RCF was significantly above run 
speed, it was decided to stiffen the discharge head 
to “tune” the natural frequency above run speed.
• A calculation based method was used to calculate 
natural frequencies of combined pump and motor.
• Different stiffening rib combinations were evaluated 
to determine the configuration that contributed the 
most stiffness.
Analytical Review
• Anchor bolt pattern modified from four to six bolts, 
with reduced bolt circle.
• Recommended re-grouting on the pump 
foundation. 
• Removed nuts on anchor bolts under the mounting 
plate.
Modifications
Before After
Added 
ribs
Extended 
ribs
Added 
anchor 
bolts
Results
• Modified discharge head installed on one unit
• 1x vibration levels at 80% run speed (~720 rpm) 
decreased from 0.22 in/s RMS to 0.10 in/s RMS
• Customer happy with results and plans on making 
modifications to remaining units
Results
WATERFALL PLOT AFTER MODIFIED UNIT PUT BACK IN SERVICE
Key Takeaways
• Bump testing can be an effective tool for 
determining structural natural frequencies of 
equipment.
• If considering VFD retro fit on existing fixed speed 
equipment, recommend bump test or similar testing 
prior to retrofit to avoid “surprises” .
• Modifications to tune structural natural frequencies 
can be an effective method to shift natural 
frequencies away from operating speeds. 
Key Takeaways
• Modification results depend on accurate field data! 
If on-site vibration equipment not adequate, 
consider third-party.
• Good practice to do vibration testing on modified 
equipment to verify results and provide new 
vibration baseline.
• Foundation conditions can impact structural 
frequencies (e.g. grout delamination).


Structural Natural Frequency 
 Tuning on a Vertical Pump

https://oaktrust.library.tamu.edu/bitstream/1969.1/174979/1/09_Patterson.pdf

Structural Natural Frequency Tuning on a Vertical Pump

Abstract

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Texas A&M University