ArticleDiagnosing Rubs in Turbomachinery
Rubs are generally defined as contact between rotating and stationary components of a machine. This can result in a wide range of damage severity from minor cosmetic damage to full destruction of the equipment. Rubs, however, are a symptom of some other problem, not the root cause of a machine failure. Rubs may be caused by an unbalanced rotor, clearances that are too tight, differential thermal growth of various components, mis-alignment, instability, or other malfunctions. Often rubs are a result of several malfunctions occurring at the same time. The most common occurrence of rubs in turbomachinery is during startup after an overhaul, since that is when things are most likely to go wrong.
Where do rubs occur?:
Rubs can occur due to contact in either the axial or radial direction. Axial rubs are usually due either to differential axial thermal growth between the rotor and the case or high amplitude axial dynamic motion, as in a compressor surge. On large steam turbines it is critical that the machine be warmed up properly to avoid a rotor to case differential expansion conditions that causes an axial rub. Radial rubs may be due to high radial vibration, shaft centerline position changes that cause contact, case distortion, or insufficient clearance. In the continued effort to improve machine efficiency, a rub due to insufficient clearance has become the most common type. In turbines, this kind of rub is most often due to contact with inter-stage packing seals, and in centrifugal compressors it is due to contact with inter-stage, impeller eye, or balance piston seals. Given the high speed and very tight clearances in most modern turbomachinery, it is surprising that rubs don’t happen more often than they do.
Vibration signal characteristics:
There are two primary types of rub as shown in Figure 1. The first is an impact rebound type of rub. In this case an impact occurs, and then the rotor bounces back, and the process is repeated. The second is a friction rub that causes a fairly constant contact between a rotating and a stationary part. The main effect of this type of rub is heat buildup due to the friction.
The main myths regarding rubs are that a rub will always cause integer fraction sub harmonics of running speed 1/2x, 1/3x, 1/4x, or that rubs will always cause integer harmonics of running speed, 1x, 2x, 3x, 4x, or odd integer harmonics of running speed, 1x, 3x, 5x…”
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