Uptime Magazine - October/November 2008 - (Page 67)

Log Magnitude (gn)/(LBF), RMS Log Magnitude (gn)/(LBF), RMS • 1180 RPM • Direct Drive Coupled • 3-Piece Shaft Assembly • 2 Spider Bearings • 6-Blade Impeller • Capacity 6000 GPM, Total Head: 70 ft From experience with vertical pumps of this type, an amplifica0.01 tion of a natural frequency is typically the cause of unexplained vi0.001 bration response(s). The symmetric cantilever design dictates that 100E-6 two natural frequencies should be present (inducing North-South, 10E-6 Even though the operating vibraand East-West motions), i.e., the .0 2E3 4E3 6E3 8E3 10E3 12E3 @1.2E3 tion problems were found only on so-called “reed” modes of the CPM the P4119 pump, natural frequency pump. The biggest dilemma in testing was performed on each unit correcting resonance problems is Figure 2 – P4119 Natural Frequency Measurements for comparison purposes. defining the locations (frequenTable 1 - P4119 Natural Frequency Summary cies) of the modes. The two dominant corrective methods used to alter the natural freExperimental Modal Analysis quency in resonant systems are the addition Freq Mode Mode Shape Margin (CPM) of a stiffness (bracing) and/or addition of mass An Experimental Modal Analysis is a diagto the physical structures. Unless the natural nostic performed on structures to determine 1 737.5 Lateral Cantilever -61.4% frequencies are first defined, the wrong cortheir natural frequencies, damping, and mode 2 1,027 Lateral Cantilever -15.8% rection method may be applied resulting in shapes. This analysis is performed with the increased vibration levels instead of the demachinery in a non-operating condition, which limits background vibration from oth- Vibration levels in the P4119 River Water sired reduction. With symmetric systems (like er machinery sources. An impulse (usually a Pump were found to be considerably above this one) the worst case is when the operathammer blow) is applied to the structure to the vibration alarm levels typically recom- ing speed is between the two modes, which impart broad-band energy to the system. This mended for a pump of this configuration. increases the potential of inducing additional energy functions to excite all of the system‘s The historic overall vibration trend obtained problems no matter what corrective method natural frequencies, similar to the response of from Mr. Schuyler indicated that the levels is chosen. a bell or tuning fork to a sharp impact. This had recently increased. Operational vibration hammer has a load cell built into the head that velocity levels were measured at 1.0 inch per In addition to the Experimental Modal Analymeasures the force parted to the structure and second (ips) on the motor frame, and up to 1.5 sis, this analysis included Operating Deflecallows this signal to be compared to the ac- ips on the outboard end (bell housing) of the tion Shape (ODS) analysis. This test method celerometer response, via a transfer function, motor. Typical alarm levels for vertical pumps is described below. resulting in a measurement of the frequency would be set at roughly 0.290-0.340 ips, deresponse of the structure. pending on vertical elevation. The harmonic Operating Deflection Shape Analysis content (multiples of 1x RPM) were considAn Operating Deflection Shape is measured Given enough response locations, each sys- ered low and acceptable. with the machine at its’ normal operating contem’s natural frequency can be curve fitted dition. This analysis measures the and its mode shape animated machines’ response at a specific with specialized modal analysis 0.01 time or frequency. Both amplitude software. The Mode Shape is an and phase information are coleffective visual representation of lected at various locations on the the deformed structure when am0.001 plified by a driving force. structure and, via special software, the vibrating “shape” or response 100E-6 The natural frequency analysis of the machine can be animated. of each unit revealed some cu10E-6 These animations show the analyst rious results. The problematic .0 2E3 4E3 6E3 8E3 10E3 12E3 how the machine is moving durunit (P4119) appeared to have @1.2E3 CPM ing normal operation. Note that significant operating margin on this is not a resonant response of its fundamental natural frequency Figure 3 – P41195 Natural Frequency Measurements the machine, but its operational modes. The operating speed was response. The forces within the well above each mode. In addition, Table 2 - P41195 Natural Frequency Summary machine are responsible for the motion, or the secondary unit (P4119S) had no amplified shape of motion measured with this analysis response; however it was operating smoothly Freq Mode Mode Shape Margin tool. For example, the unbalance response of at a point between its EW and NS cantilever (CPM) any rotating system will produce a response modes, and with significantly less speed mar1 1,091 Lateral Cantilever -9.07% or driving force at 1x RPM. Misalignment and gin? A summary is presented in Tables 1 and 2 1,257 Lateral Cantilever +5.33% looseness generally produce synchronous 2. Figures 2 & 3 show the measured natural multiples of running speed (2x RPM, 3x RPM, frequencies in relation to the operating speed etc.). (cursor). www.uptimemagazine.com 67 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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