ArticleSources Of Error In The Measurement & Evaluation Of Piping Steady State Vibrations
by Ed Salfelder and Brian J. Voll
This paper describes the general practices recommended when measuring piping vibration. Differences are drawn between vibration measurements on rotating equipment versus those on piping. The need to measure piping vibration in units of displacement is emphasized due to its close correlation with stress – high piping stress being the primary cause of piping failure. Due to the transient nature of flow-induced vibration inherent in piping, the need to measure vibration amplitudes from time waveform data ≥ 10 seconds long is mentioned.
Many similarities exist between rotating equipment and piping vibrations that allow similar types of instrumentation and measurement techniques to be used, however, significant differences also exist. The application of measurement and analysis techniques appropriate for rotating equipment vibrations to piping vibrations without consideration of these differences will lead to measurement and analysis errors. Prior to discussing the specific types of errors that can occur, it is instructive to look at the differences between rotating equipment and piping vibration monitoring in key areas. The purpose for obtaining the vibration measurements, vibration characteristics, acceptance criteria and instrumentation requirements are all areas where important differences exist.
Purpose of Vibration Measurements: Rotating equipment vibration monitoring is performed for three primary purposes. First, vibration monitoring is useful for evaluating the current mechanical condition of the equipment. Vibration levels are compared to pre-established limits to quantify the condition of the equipment. Post-maintenance vibration monitoring quantifies the effects, positive or negative, of maintenance work performed on the equipment. Second, trending of the vibration levels over time is used to monitor the condition of the equipment for predictive maintenance purposes. Third, analysis of the vibration data, typically the vibration signatures (amplitude vs. frequency plots), is performed to identify equipment faults.
For piping the main concern is typically vibration-induced high cycle fatigue of the pipe material. Therefore, the piping vibration measurements need to be correlated to fatigue in some manner. This will be discussed more specifically in later sections. There may also be concern for the effects of the piping vibrations on in-line components (e.g., valve internals or restricting orifices) or piping appurtenances (e.g., external valve components, instrumentation, pipe supports). These could be fatigue-related or wear-related issues, depending on the components involved and the nature of the vibrations.
Similar to rotating equipment, analysis of the vibration data is useful for identifying the excitation mechanisms causing the vibrations. The predominant vibration frequencies can often be correlated to the vibration source(s). Another important purpose for obtaining piping vibration data is for input to detailed analyses that may be required to quantify the effects of the vibrations on the piping or attached components and evaluate the effectiveness of potential corrective actions.
Vibration Characteristics: Rotating equipment vibrations are typically periodic and repetitive, and occur at distinct frequencies related to the rotating speed of the equipment (1X and harmonics). Gear and anti-friction bearing defects and non-linear effects add to the complexity of the vibration signals, but the periodic, repetitive nature of the signals is generally maintained for steady-state operating conditions.”
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