Chemical Processing - June 2008 - (Page 26)

Tips and Traps in implementing cm techniques, the first rule of thumb is to be sensitive to in-house levels of experience. Because many maintenance staffs aren’t expert on such methods, here’re some practical pointers. VibraTion moniToring: 1. recognize the vast differences between detecting a machinery problem and analyzing the cause. For example, replacing a bearing that indicates a high level of vibration may or may not be the solution to bearing failure. Usually, a secondary issue is the primary contributing factor. such root causes could include misalignment, looseness, imbalance or others; proper analysis will drill down to the right solution. 2. specify sufficient frequency range. it should be high enough to capture all defect frequencies of interest. For example, sleeve bearing equipment typically is measured to a frequency range of 20 times running speed and rolling element bearing equipment to 50 or 60 times running speed. 3. carefully select the measurement point. When collecting machinery vibration data, avoid painted surfaces, unloaded bearing zones, housing splits and structural gaps. These areas cloud response and compromise data integrity. Take measurements at the same precise location for comparison and, when measuring vibration with a hand-held sensor, pay close attention to sensor position, angle and contact pressure. 4. optimize the conditions. ideally, take measurements while a machine is operating under normal conditions — for example, when the rotor, housing and main bearings are at regular steady operating temperatures and it’s running within the rated voltage, flow, pressure and load. For variable-speed equipment, take measurements at the same point in the process or manufacturing cycle. periodic measurements at all extreme rating conditions can confirm the absence of outlying problems that only appear at extreme conditions. 5. check equipment status. make sure the measuring equipment collecting vibration data is in good condition and the transducer has sufficient time to settle after initial power-up. FFT specTrum analysis: 1. minimize spectral leakage. applying the FFT method to finite-duration sequences can fall short due to “spectral leakage.” This occurs when the signal doesn’t result in a sequence containing the whole number of periods. applying weighted “windows” to the data sample, you can plug such leaks. caution: while weighted windowing (configured as rectangles, triangles or others) can improve the quality of a spectrum in particular cases, some distortion may still occur. 2. Factor in appropriate resolution settings. The FFT frequency spectrum has some basic settings that determine the usability of the data. The FFT is collected with a given resolution that is defined by the frequency span being measured divided by the number of bins or lines collected. closely spaced defect frequencies may require higher resolution to distinguish. as resolution is raised, the amount of time necessary to collect the data increases. Time domain analysis: 1. Establish appropriate measurement intervals. conventional thinking holds that vibration signals from rotating machinery are stationary and continuous. This implies that the vibration signal is the same from rotation interval to rotation interval, which may not be true. many impulsive responses will change in amplitude or slide rather than roll over a defect. so, determine a suitable measurement time (typically encompassing 10 to 15 rotations) for the machinery. bump TesTing: 1. perform a “background” test. This can show that the vibration present during the bump test is due to the user’s impacting of the structure. in some cases vibration from nearby equipment can transmit through foundations into the equipment being tested. simply collect data while the machine is off and no impacting is being done on the structure; then compare these to the data collected during operation and impacting. 2. check the accelerometer. placed onto a machine’s measurement position it can skew results unless operators take care. Follow these guidelines: firmly screw the magnet onto the accelerometer (any looseness between the magnet and accelerometer will corrupt the reading); gently slide the accelerometer onto the measurement position (slamming the transducer onto the machine may cause data overload); the magnet should be in firm contact with the machine’s surface (any movement will be falsely recorded as vibration data); and operators should avoid knocking or disturbing the accelerometer while taking the measurement. lubricanT analysis: 1. confirm the lubricant is correct. critical machinery can benefit from tags placed at grease points identifying the proper lubricant to be analyzed and, if necessary, replaced. 2. sample properly. The possible introduction of a contaminant during the sampling process is the most likely cause of invalid analysis results. When taking lubricant samples from storage tanks or equipment in service, workers should adhere to proper sampling techniques. 3. Document the results. Detailed documentation makes sharing of test results more widely easier and, in the process, may telegraph useful information for other parts of an operation. over time, it enables meaningful comparisons based on prior history to help solve current problems. Using any of these techniques, situations may arise where the analysis data can’t clearly identify the cause of a problem. in these cases, outside expertise or a working partnership with a knowledgeable supplier can prove invaluable. JUNE 2008 chEmicalprocEssiNg.com 26 http://chemicalprocessing.com

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Chemical Processing - June 2008 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Achieve Model Operations Bolster Your Condition Monitoring Toolbox Particle Analysis Makes Solid Progress Improve Batch Reactor Temperature Control Improve Your Job Security Plant InSites Process Puzzler Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - June 2008

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