Uptime Magazine - October/November 2008 - (Page 66)

vibration upload Case of the Whirling Pump Finding Hidden Machinery Faults With Operating Deﬂection Shape Analysis by Daniel T. Ambre T his article is a vibration case study using Operating Deflection Shape (ODS) and Experimental Modal Analysis (EMA) tools and computer animation techniques to solve difficult rotating machinery problems. Mr. Tim Schuyler and Mr. Steve Werness from Xcel Energy Fort St. Vrain Generating Station requested the services of Full Spectrum Diagnostics to perform a vibration investigation on a vertical river water supply pump whose vibration had significantly increased recently for unknown reasons. used to detect natural frequencies, an Operating Deflection Shape (ODS) analysis was also planned to investigate the operating characteristics. This analysis is helpful in resolving unbalance, looseness, misalignment, and bowed rotor faults. Operating analysis also provided an evaluation of historical repairs to the pump foundation on these units. Xcel Energy provided periods of both operating and non-operating test conditions for the ODS and Modal analyses, respectively. Machine Design Information Drive: • Westinghouse AC Induction Motor • 200 HP @ 1180 RPM • Rolling Element Bearings: Driven Machine: • Worthington Single Stage VTP River Pump The Fort St. Vrain facility is located in Platteville, Colorado, and was both that state’s only nuclear power plant, and the United States only commercial High Temperature Gas Cooled (HTGC) reactor design. The plant was decommissioned in 1989 and converted from a nuclear fueled facility to a natural gas fueled steam generation facility in 1996. The site added heat recovery steam generator (HRSG), and a second gas turbine in 1998. A third combustion turbine with HRSG was added in 2001, providing a current combined rated output of 720 megawatts. Two more combustion turbines are planned to supply up to 300 additional megawatts. The generating station has a pair of vertical river water pumps used to control the cooling water in the facility. The P4119 and P4119S units are similar in design, both manufactured by Worthington and driven by Westinghouse 200 hp AC electric motors. Overall vibration levels on the P4119 unit were noted to have “jumped” from a stable trend of 0.150 ips to levels approaching 1.0 ips. The dominant frequency was found to be the 1x response at 1180 RPM. A resonance condition was suspected, however, a history of foundation issues suggested a deeper underlying problem or contributing factor. Cantilever mounted vertical pump designs will typically have natural frequencies near operating speed points. These so-called “reed” modes are a function of the height of the cantilevered structure (above deck), the length of the pump shaft (below deck), the mass of the drive motor, and the “footprint” of the mount interface structures. Each of these parameters effect the mass and/or stiffness of the overall structure, thus the location of the natural frequencies. Also, as a note, symmetric systems (like this unit) will have two fundamental cantilever natural frequencies (N-S and E-W). These modes are fairly close together, but may appear below operating speed, above operating speed, or even straddle the running speed point. This makes identification of the modes a top priority in establishing a proper structural correction, if required. In addition to the planned Experimental Modal Analysis Figure 1 – P4119 (left) and P4119S (right) 66 october/november 2008

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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