Plant Services - February 2008 - (Page 38)

RELIABILITY/PREDICTIVE Prevent catastrophic motor failures with current signature analysis urrent signature analysis (CSA) detects broken rotor bars by analyzing the sidebands around line frequency. Demodulated current spectrum analysis (DCSA) enhances the ability to detect broken bars, especially on 2-pole motors. The polarization index (PI), a metric for insulation testing, is the ratio of measured resistance at 10 minutes divided by the measurement at 1 minute. Plotting the resistance measurement at 5-second intervals produces a polarization index profile (PIP), which can be used for additional analysis of the insulation system that can’t be obtained from the standard PI. Power quality is a measure of the quality of the motor’s voltage and current. Analyzing the harmonics, voltage and current unbalance, over- or under-voltage, and over-current conditions reveals what might be causing nuisance trips, voltage swells or sags, and other power system problems. Routine EMAX testing of a 3,500-hp, 2-pole, 4,160volt, 3,590-rpm AC induction motor revealed a 0.74 dB peak level of the pole pass sideband, which exceeded the alarm setpoint of 0.3 dB. This suggested rotor bar problems, but vibration analysis indicated a healthy motor. The decision was to monitor the motor and trend the test results. The motor was retested periodically and the peak level of the pole pass sideband rose from 0.18 dB at 3,591 rpm to 2.59 dB at 3,592 rpm. This exponential increase is indicative of at least one broken rotor bar. Also, load variation increased from 0.855% to 2.345%, when it should be constant from test to test under normal operating aconditions. The CSA showed an increase in sideband activity, which indicated broken rotor bars. A rotor influence check (RIC) indicated a rotor anomaly. Disassembly revealed 22 of 51 rotor bars were broken or cracked. The root cause was bad brazing between the bars and end rings from a repair performed years earlier. The Increase quality production by identifying underperforming process control assets cost to repair the motor now was $90,000. Had it run to failure, the cost would have been $370,000. A PIP test on another motor showed a low PI value and short rise-time to a relatively low value overall, when insulation resistance should be at least 100 megohms and PI greater than 2.0. This is indicative of damp insulation. After the cables and components were dried, a PIP indicated healthy insulation. Contaminated insulation exhibits significant spiking in the profile. A third motor had voltage harmonics greater than 5% and full load amps (FLA) of 107%. A 480-volt, 2,500KVA transformer was feeding eight motors and five DC drives. This suggested the DC drives were degrading the power quality. With no load, current harmonics (THD) were <2%, voltage harmonics were 5%, voltage harmonics >7%, full load current of 108% of rated current, and a system voltage drop of 21 volts. The report was filed away. Then, the plant installed a microprocessor-based lock system for the loading dock. It worked fine most of the time, but periodically the locks opened and closed spontaneously. The system was under warranty so the controller manufacturer replaced several cards and two complete controllers. The plant then realized the controller feed came from the motor control center feeding the motors and drives. That earlier report suggested using line filters on one of the new controllers. The problem for the dock lock controller disappeared. The lessons are that broken rotor bars can be detected using online and offline testing, a modification to the polarization index test helps analyze the health of insulation, and that you can use power quality to analyze a power system and correct harmonics that motor drives introduce. PDMA (www.pdma.com) Using real-time data directly from existing control systems, software focuses attention on underperforming components of the control system. By exceeding expected production targets and discovering hidden process performance opportunities, a A cracker operation is on track for a full return on investment within six months of start-up. Expertune (www.expertune.com) tion, triggering online or e-mail alerts and driving corrective work orders into the CMMS. Significant improvements in asset performance include an 88% reduction in downtime at the company’s rod mill and a 30% increase in meltshop throughput. Ivara (www.ivara.com) Improve refinery reliability with alarm rationalization Reduce downtime by coordinating maintenance activities with reliability software A steel company was able to bring together a wide range of equipment condition data, including maintenance inspections, operator rounds and online information, using reliability software. The more complete picture of equipment health and performance, online in real time, allows automatic tracking and alarming on asset condi38 Years of configuring alarms at multiple levels without rationalization led to more than 5,000 tags configured with alarms, each having as many as six or more alarms. Managing alarms was becoming increasingly time-consuming and problematic for the process control team. Instead of embedding alarm management in a new DCS, the plant implemented a separate system to manage alarms for both new and existing DCSs. The plant performance level went from stable/reactive to robust in four months, with 261 tags rationalized. Matrikon (www.matrikon.com) February 2008 www.PLANTSERVICES.com http://www.pdma.com http://www.ivara.com http://www.expertune.com http://www.matrikon.com http://www.PLANTSERVICES.com

Table of Contents Feed for the Digital Edition of Plant Services - February 2008

Plant Services - February 2008 Contents From the Editor Letters The PS Files Up and Running Crisis Corner What Works Asset Manager: Why Six Sigma? Technology Toolbox: Ensuring a Calm Response Cover Story: Best Practices Awards Lubricants: A Slick Success Story Motors: Reliable Torque Compressors: Before You Call for Help Web Hunter: Exploiting Intellectual Capital In the Trenches Product Picks Classifieds Fast Facts Energy Expert: Energy, Climate and Food

Plant Services - February 2008

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