Rotor Rubbing in Pumps

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:

• appearances of sub-synchronous fractional components (particularly 1/2× sub-synchronous vibration)
• brief appearances of components with natural frequencies (because of the transient character of the rubbing)
• appearances of high harmonics of the fundamental speed component (1×, 2× and 3×)
• changes in shaft centerline positions. Partial or fully backward orbiting of the rotor is one of the important characteristics of rubs, distinguishing this malfunction from others.

The vibration amplitudes could become limited at the rubbing location, and the dynamic amplitudes may increase in other shaft sections. The thermal effect of rubbing can cause ever-changing vibrations of the shaft. These mechanical and thermal effects may lead to vibration amplitude fluctuations and continuous phase-lagging as a function of time resulting in spiral characteristics. The friction forces generated during heavy contacts or rubs could produce a considerable amount of heat. Sometimes, depending on characteristics of the rub phenomena, this heat is transmitted to the rotor through a small portion of the circumferential surface; of course, the effect of pumped liquid should always be considered. This local heating (locally high flux of heat) could cause a shaft thermal bow and sudden changes in synchronous vibrations.

Special Considerations for Rubbing

The rubs can significantly affect the vibrations of a pump. These dynamic effects could be particularly pronounced on a pump shaft portion near the rub location. The heat introduced into the rotor assembly by friction forces and impact effects could induce time-varying excitations to a pump’s shaft. This can generate time-varying (transient) vibrations. The time-varying characters should be considered in analytical models and condition monitoring exercises. The rate of magnitude increase of the time-dependent bending moments induced by rubs can give very helpful information in the monitoring exercises on a pump rotor system. An accurate nonlinear method that combines thermal equations and motion equations of a pump system should be employed for an accurate modeling of the rub. The nonlinear modeling is absolutely necessary to obtain satisfactory results. The sub-synchronous vibrations (1/2×, 1/3×, 1/4×) and super-synchronous vibrations (2×, 3×, 4×) show clearly the importance of nonlinear effects in a rubbing incident. The evaluations of the rate of changes in vibration amplitudes, shaft bow and bending moments (and stresses in a shaft) are important in a rub incident modeling. The heaviest rubs usually occur on limited portions of a pump rotor where some small clearances are located. The shaft can be affected by one (or more) local bow(s), the amplitude of which is time-dependent. A higher vibration is expected at the rotor side near the rub location. The rub location could be identified by comparing the vibrations of two sides of the rotor (where vibration sensors are usually installed).