Avoid Bad Turns with Rotating Equipment

Identify damaged and worn components before they cause problems.

By Amin Almasi, WorleyParsons Services Pty. Ltd.

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Diagnosing fluid whirl/whip vibrations may be relatively easy, particularly when transient startup or shutdown vibration data are available. However, identifying the actual cause of the instability is more difficult, especially when only the whip vibrations are present.

Because of the bearing/fluid interactions, the fluid whirl vibrations have frequencies slightly lower than ½× (for most bearings, whirl speeds usually range from 0.38 to 0.47 of the rotor speed). The fluid seal or stator/blade tip whirl vibrations may exhibit quite different vibration behavior and frequencies. This is especially true when the seals are equipped with swirl brakes or anti-swirl injections as well as when there's a significant level of fluid recirculation, as often happens in process pumps and compressors. The whirl vibrations even may exhibit frequencies higher than the rotating frequency (1×).

LOOSE PART
Looseness between the rotor supporting pedestal and the foundation (for example, due to a loose bolt) is a common malfunction. The unbalanced force carried by the rotor occasionally may exceed the gravity force (or other forces) applied to the machine. This can cause a periodic lifting of the pedestal, resulting in a reduction in system stiffness, its cyclic variability and impacting. As a result, the rotor may exhibit changes in synchronous responses, and fractional sub-synchronous vibrations (most commonly ½×) may appear. The diagnosis of pedestal looseness (particularly foundation bolt looseness) usually is based on the appearance of ½× vibrations and visual inspection of the bearing, pedestal and foundation fastenings.

Looseness in rotating elements such as disks or thrust collars mounted on rotating shafts (or bearings untightened in bearing pedestals) represents another type of malfunction. A disconnected disk or a thrust collar still may rotate but at a different speed than that of the machine's shaft. A loose bearing may start rotating (dragged into motion by the rotating shaft) or may stop suddenly. The clearances, friction conditions between the shaft and the loose part, and the tangential external force applied to the part (such as the external fluid dynamic drag) play important roles in the rotor-dynamic response. A loose rotating part usually carries an unbalance that changes the balance state of the rotating machine. This results in a modification of the synchronous vibrations (1×) and also appearance of the loose-part unbalance-related vibration components.

The loose part rotating frequency usually is a function of the shaft/loose part clearance, the surface friction, and the tangential drag coefficient. Depending on the particular machine, the latter can make the loose part spin faster or slower than the rotor rotating frequency. In both situations, friction and fluid drag act in opposite directions. At steady-state conditions, the two may balance each other, giving a constant loose-part rotating frequency. If loose part speed doesn't differ very much from the rotor rotating speed, the resulting vibrations exhibit the characteristic pattern of a beat. Most often, however, the looseness of a rotating part leads to transient conditions.

BEARING PROBLEMS

Hydrodynamic and rolling-element bearings used in rotating machines feature a small bearing clearance that's appropriate for normal operation of the rotor. Poorly lubricated or worn bearing surfaces inevitably increase the clearance — and thus decrease system stiffness. Excessive bearing clearance can cause variable bearing-system stiffness, thereby providing nonlinear conditions for unbalance-related excitation that may lead to rotor instability.

The physical phenomena occurring in the worn (or oversize) bearing system resemble a mirror image of those during rotor-to-stator rubbing. In rubbing, during a cycle of vibration the system becomes periodically stiffer, which can lead to an increase in the average stiffness. In the worn (or oversize) bearing system, the average stiffness decreases. The rubbing occurrences are described as "normal-tight" situations, while the worn (oversize) bearing is "normal-loose." In both situations, two other physical phenomena, namely friction and impacting, are involved. However, their role and strength differ in each specific case. Distinguishing a worn (or oversize) bearing malfunction from rubbing requires careful study of the recorded vibration of the rotor.

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