19 - Critical speeds and rotor response

Abstract

This chapter analyzes the critical speeds and rotor response. In the case of turbo-compressor rotors, their natural frequency must be excited by some external force to produce a response that will result in increased amplitude of vibration. The term critical speed defines the operating speed at which a natural frequency of a rotor system will be excited. Every turbo-compressor that is designed must have the rotor system critical speeds determined prior to manufacture. It is found that if a turbo-compressor with oil seals experiences a significant change in critical speeds, it is usually an indication of seal lock-up. The seal does not have the required degrees of freedom and supports the shaft acting like a bearing. In order to accurately calculate the rotor critical speeds, the entire rotor system stiffness, masses and damping must be considered. It is observed that if the vibration at the probe locations is high, the best advice is to stop the compressor fully loaded which will reduce the time in the critical speed range as much as possible. It is suggested that Bode plots should be recorded during every planned start-up and shutdown of every turbo-compressor.