ArticleAn End User's Guide To Pump Vibration
by William D. Marscher, P.E.
Causes of pump failures are discussed which include evaluation of vibration, pump operating point influence on vibration, pump suction design influence on vibration, rotor imbalance, shaft alignment, natural frequency and resonance. Lateral and torsional rotordynamic analysis are introduced. A test method using experimental modal impact testing to measure a pump shaft’s 1st critical speed and support natural frequencies while the pump is in operation is discussed. A case history is provided about a hollow drive shaft and gearbox problem.
When failures occur in pumps and their associated systems, they generally fall into one of four categories: fracture, fatigue, rubbing wear, or leakage. Fracture occurs due to excessive loading, for example from higher than expected pressure, or nozzle loading beyond recommended levels. Fatigue requires that the imposed loads be oscillating so that stresses cyclically surpass the endurance limit of the cracking material. Fatigue in pump components is most commonly caused by excess vibration, which in turn is caused by the rotor being out of balance, by the presence of too great a misalignment between the pump and driver shaft centerlines, by excessive vane pass pressure pulsations, or by large motion amplified by a natural frequency resonance.
Rubbing wear and seal leakage imply that the rotor and stator are not positioned relative to each other within design tolerances. This can happen dynamically, and in such a case excess vibration is generally the cause. When the wear or leakage is at a single clock position in the casing, unacceptable amounts of nozzle loading and either resulting or independent pump/ driver misalignment are likely causes. In high energy pumps (especially hydrocracking and boiler feed pumps), another possibility for rubbing at one location on the stator (or for and axial rub or a thrust bearing wipe) is too rapid a change in temperature, which can cause a mis-match in the length and fit of each component, since these change with temperature.
Fortunately, there are certain approaches and procedures that can be followed which minimize the chance for encountering such problems, or which help to determine how to solve such problems if they occur, as will be discussed in this tutorial.
Prediction or troubleshooting of vibration and other unsteady mechanical considerations should include evaluation of:
• rotordynamic behavior, including critical speeds, forced response, and stability
• torsional critical speeds and oscillating stress, including start-up/ shut-down transients
• piping and nozzle load-induced unsteady stress and misalignment-causing distortion
• fatigue of high stress components due to oscillating torque, thrust, and radial load
• bearing and seal steady and dynamic behavior
• lubrication system operation during normal operation and trip coast-downs
• effects of operating range on vibration
• acoustic (e.g. trumpet-like) resonances in combined pumps and systems
The problem vibrations most commonly discussed in the literature are lateral shaft vibrations, i.e. rotor dynamic motion perpendicular to the pump axis. However, problem vibrations also can occur in the pump stationary structure, especially in vertical pumps. In addition to lateral vibration, vibration can occur in the axial direction, or can involve torsion (twisting).”
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