Uptime Magazine - October/November 2008 - (Page 69)

tion problem. The relatively new bolted joints in the base plate are considered acceptable based on this analysis. The final analysis performed was an “added mass trial” on P4119. Mass is added to resonant systems to alter the natural frequencies. Adding mass (with sand bags) should both reduce the natural frequency, as well as dampen its response via the shifting sand. Since the natural frequency for this unit was below operating speed, adding mass should increase the operating margin. If a stiffener support were applied instead, the likely result would be an increase in vibration since the operating margin would be reduced. Figure 5 – 1X RPM Response Indicating Orbital Motions Figure 7 – 1X RPM Foundation Plate Response The Xcel Energy maintenance support acquired several sandbags totaling close to 120 lbs of weight. The unit was started up and the bags were stacked on the motor. No change in vibration levels were noted regardless of the amount of sand applied. The unit was shutdown for a natural frequency (impact) test. The test indicated that the natural frequency at 1027 CPM was shifted lower by the added 120 lbs of mass to 990 CPM, or approximately 6%. If resonance were the root cause of the elevated vibration levels, a significant drop in the 1x RPM amplitude would have been noted during pump operation. This proved that resonance was not the dominant contributor to the excessive vibration levels. Based on this analysis and discussions with Xcel Energy management and maintenance personnel, it was recFigure 6 – 1X RPM Whirling Motion with ommended that the unit be removed Motor Removed and inspected. It was suspected that some internal unseen fault was rethe plates are moving in unison (in-phase). Out-of-phase motion between plates (grouted sponsible for the elevated vibration levels. interfaces) or bolted joints would imply looseness response. These animations confirm that Disassembly and inspections of the hardware the foundation is not the source of the vibra- showed that a locked coupling was likely the root cause of the excessive vibration. Continued operation in this “locked” state resulted in significant wear in the spider bearings, the shaft, and rotor bushings. This extra clearance allowed the rotor to “whirl” as a unit, inducing the orbital motion noted above deck. The operating animations (ODS analysis) proved to be invaluable in isolating the root cause of the rotor problems. Dan Ambre, P.E. is a Mechanical Engineer and founder of Full Spectrum Diagnostics, PLLC, a Full Service Predictive Maintenance Consulting company. Dan specializes in Resonance detection, Experimental Modal Analysis, and Operating Deflection Shape machinery diagnostics. Full Spectrum Diagnostics provides Vibration Analysis level I, II, and III training and certification, as well as training in advanced diagnostic techniques. Dan is a certified software representative for Vibrant Technology, Inc., the creators of ME’scope VES software tools. He also provides ME’scope VES Software Training targeting the In-Plant Vibration Analyst. Animations referenced in this article are available via email at modalguy@aol.com. Please visit our web site at www.fullspec.net. www.uptimemagazine.com 69 http://www.fullspec.net http://www.uptimemagazine.com

Table of Contents Feed for the Digital Edition of Uptime Magazine - October/November 2008

Uptime Magazine - October/November 2008 Contents Upfront Upclose: An Asset Management Oasis in the Desert Information Technology: Streamlining Through Simplicity Lubrication: Phases for Framing a Solid Foundation Infrared: Do Your Homework Before You Buy Maintenance Management: Get It Together by Getting Together Motor Testing: Key Elements in Motor Decision Making Precision Maintenance: Shedding Some Light on Tolerances Reliability: RCM & the Mortgage Meltdown Ultrasound: A Multidimensional Tool Vibration: The Whirling Pump Mystery, Solved with ODS Upgrade: Are You Looking in the Right Place for Your Wear Debris?

Uptime Magazine - October/November 2008

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