Learn the risk factors and how to mitigate damage.
by Amin Almasi
November 22, 2017

The rotor-to-stator rub has been an important malfunction in many pumps. The rotor rub could result in serious damages such a shaft failure. The bending movements associated with the shaft bow caused by rubs could result in very high stresses on a pump shaft. Proper vibration-based condition monitoring systems can be employed to identify the rub severity and rub location. The rub phenomena and related vibration signature are discussed.

Introduction

The study of rotor-to-stator rub phenomenon is one of the necessary items for proper operation, reliability and monitoring of a pump. It is important because of relatively small clearances present between the rotor assembly and various static parts. Many types of malfunctions or degradations such as a high value of unbalance, a misalignment, high shaft vibrations or some induced dynamic instabilities can cause rotor-to-stator rubs in a pump.

Rubs can cause impacts, chaotic motions, sub-synchronous vibrations and super-synchronous vibrations. On the other hand, light partial arc rubs and full annular rubs often cause major progressive changes in synchronous vibrations. Sometimes, depending on the mechanical and thermal characteristics of a pump and the shaft rotating speed, stable or unstable synchronous spiral vibrations could occur.

Often, the rub phenomena that occur in operating conditions can be prevented by properly modifying some suitable design and operation parameters of a pump. Sometimes, modifications of the shape and wideness of rotor orbits (and consequent changes in the rotor centerline position inside the bearings and seals) can allow the available radial clearance to become sufficient to avoid rubs. When rotor-to-stator rubs occur during a transient situation such as a start-up or a shutdown, the shaft bow evolution can become more complex, particularly considering some high-speed pumps passing through shaft flexural critical speeds in the start-up and shutdown.

Rotor Rubbing

Advanced vibrational condition monitoring methods have been used for effective operation of pumps. Understanding basic factors of the pump vibrational behavior goes beyond pump dynamic modeling. It should involve deep understanding of the pump’s dynamic behavior during its operation and any malfunction situation. A pump vibration monitoring requires appropriately selected and strategically located vibrational sensors for capturing the pump vibration.

The rubbing between the rotor assembly and stationary part of a pump is a serious malfunction that could lead to a catastrophic failure. The rubbing involves several physical phenomena, such as the friction, stiffening effects, coupling effects and impacting. The rubbing could affect the fluid and thermal balance inside a pump.

The rubbing usually occurs as a secondary effect. The primary malfunction could be an unbalance, misalignment, fluid-induced excitations and self-excited vibrations, which all result in high vibration amplitudes (or changes in the shaft centerline position within available clearances) and eventually result in the rubbing.

The unbalance and misalignment are relatively easy to model. Other effects are relatively difficult to analytically model. The common rub models reflect an intermittent action of the rub. The simplified mathematical formulations become piece-wise continuous with variable stiffness. The modal stiffness is split into several sections. For simplified models, usually three sections are used.

The friction is commonly approximated by using the Coulomb model. The friction force is oriented in the tangential direction opposite to the direction of rotation. The effect of short-lasting, impact-related rotor/stator contact could be considered (estimated) in terms of rotor free-vibration response, following each impact.

The rubbing usually presents fractional sub-synchronous vibrations, often with backward directions (resulting in external loops on the rotor orbits). In other words, because of particular system nonlinearity, the unbalance force excites fractional frequency responses. The rubbing could cause self-excited vibrations (known as “dry whip” or “full annular rub,” occurring mainly in seals) independently from the excited vibrations. Pump seals could offer the minimum clearance in a pump rotor assembly.

The seals are usually the first places to rub. In a typical high-speed pump rotor rubbing incidence, a rotor is lightly rubbed on a surface of a seal. Fractional sub-synchronous vibrations (particularly 1/2, 1/3, 1/4) and the synchronous vibration (1×) are observed in such cases. At a high-speed pump, a full annular rub could occur. In relatively low-speed pumps, a rotor could bounce inside a seal, producing multiple higher harmonics (2×, 3×) in addition of the synchronous vibration (1×).

High radial (normal) and corresponding friction (tangential) forces at the contacting surfaces could lead to extremely severe damages of the seal and rotor surfaces in a very short time. In addition, because of the backward mode of vibrations in a rubbing case, the rotor operates under severe alternating stresses with relatively high frequencies. The rub-related failures of pumps occur quite often.

Any dynamic response of a pump rotor rubbing usually contains a spectrum of higher harmonics (besides the fundamental component). The impact, as a nonlinear mechanism involved in the rubbing, can increase the strength of higher harmonics spectrum even more.

The rotor-to-stationary element rubbing is actually a very harmonic rich phenomenon resulting in rapidly changing system parameters with a tendency to chaotic motions. The diagnosis of rotor rubbing using vibrational data is mainly based on:

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