Chemical Processing - June 2008 - (Page 24)

vibration pen Figure 1. This small hand-held monitoring tool provides a convenient means to collect data on overall vibration. Many plants already rely on some CM methods, particularly overall vibration monitoring and lubricant analysis. However, the CM toolbox also includes under-appreciated techniques such as time domain analysis and bump testing. So, we’ll look at what various methods involve and the insights they provide. The sidebar gives some pointers for making the most of these techniques. They (and others) can help promote successful CM programs. Overall vibratiOn A typical starting point is to trend a machine’s overall vibration level. This is the total vibration energy measured within a specific frequency range. In the case of a rotor, for example, the overall vibration would be measured and then compared with its normal value to assess any inconsistencies. A higher-thannormal overall vibration reading would indicate that “something” is affecting the rotor. Further analysis can identify the actual cause. Hand-held units such as low-cost vibration pens (Figure 1), overall vibration meters or more sophisticated portable data collectors (Figure 2) and related instruments combining compact size with data storage capabilities make data collection for overall vibration analysis easy. Other options include online surveillance systems to perform round-the-clock monitoring of machinery, regardless of equipment location. This type of technology has been highly engineered to collect data continuously (or at a predetermined datacollection frequency) from permanently installed sensors. Findings then are transmitted to a host computer for subsequent analysis. FFt spectrum analysis Vibration can be defined as the behavior of a machine’s mechanical components in response to internal or external forces. Because most rotatingequipment problems cause excessive vibration, this operating parameter generally is considered the best to initially assess a machine’s condition. Vibration monitoring can detect fault conditions such as imbalance, misalignment, rolling bearing degradation, mechanical looseness, structural resonance and soft foundation. When analyzing vibration, frequency and amplitude of the signal should be evaluated. The frequency at which the vibration vibration monitoring can detect fault conditions such as imbalance, misalign- occurs indicates the type of fault (certain ment, rolling bearing degradation, mechanical looseness, structural reso- types of faults typically occur at certain nance and soft foundation. frequencies). By establishing the frequency, a clearer picture can emerge regarding cause. Amplitude typically determines the severity of the fault (the higher the amplitude, the higher the vibration and the bigger the problem). Amplitude depends on the size of the machine and must be considered relative to the vibration level of the fully functioning equipment. JUNE 2008 chEmicalprocEssiNg.com 24 Among methods for viewing vibration and noise signals and pinpointing the causes, a Fast Fourier Transform (FFT) spectrum is perhaps the most useful. Vibration and noise signals are broken down into specific amplitudes at various frequencies. Because each equipment component vibrates at a certain individual rate, maintenance personnel by processing these signals can distinguish between several different rates and then determine which rate coincides with which component. The resulting FFT spectrum can point the way to the location, cause and stage of a problem. User-friendly FFT analyzers have been developed to measure vibration and noise signals and separate them into their component frequencies. These tools can display spectrum information in simplified formats to enable a first-pass diagnosis of machinery condition or identify areas for further scrutiny (Figure 3). The placement of FFT analyzer sensors, setup, and the process of taking measurements can be performed without taking machines out of service. time dOmain analysis Time domain (or time waveform) signals offer one of the few methods to detect certain types of problems. Time domain analysis also can bolster confidence that data in the frequency spectrum have been http://chemicalprocessing.com

Table of Contents Feed for the Digital Edition of Chemical Processing - June 2008

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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