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

See other Maintenance Minders articles here.

Pages