EARLY CRACKED SHAFT DETECTION IN PUMPS USING ROTOR LATERAL VIBRATION ANALYSIS

摘要

This paper presents a chronological account of a successfulcrack detection on a large vertical pump using rotor lateralvibration data analysis. It then looks at one of the primaryforces occurring in a pump, the steady state radial load forcegenerated by moving the fluid, and explores the interaction ofthis force with the changes in the dynamic stiffness of thesystem caused by a shaft crack. The vibration behavior of anysystem depends on the complex interaction between the forcesapplied to the system and the dynamic stiffness of the system.These effects depend heavily upon the original dynamicstiffness parameters, the operating speed of the pump, and thelocation of the crack. The primary effects can be categorizedinto creating additional vibration components at harmonics ofoperating speed, and shifting the values of selected parametersin the stiffness matrix. These interactions can produce profoundchanges in the rotor lateral vibration response which can beused to diagnose crack propagation in the rotor. Unfortunately,other mechanisms that can produce similar results exist,preventing the construction of simple vibration analysis rulesfor shaft crack detection, such as, 'when the twice rotativefrequency (2x) lateral vibration increases, the shaft is cracking.'To solve the above dilemma, methods are presented forevaluating the possibility that a particular vibration componentchange is produced by a cracked shaft, or is the result of othersystem parameter changes. To accomplish this, traditionalsingle axis signal processing is expanded to include complexvariable processing of the data from two orthogonal transducersat one measurement plane, and modeshape analysis whichconcurrently uses the vibration information from several axiallocations. The use of these advanced rotordynamic signalprocessing techniques provides much more insight into theprimary cause of changes in the vibration response of anymachine, including pumps. As shown by Muszynska et al.(1997), complex variable processing decomposes the responseat any location into a forward and backward circular rotatingresponse. The relative magnitudes and phases of thesecomponents can give much more insight into the systemdynamic stiffness than using measurements from a single axisalone. The effectiveness of these signal processing techniquesis demonstrated using data obtained from numericalsimulations, rotor rigs capable of modifying individualparameter values within the stiffness matrix while beingsubjected to the common forces produced in a pump, andvibration data from pumps with cracked rotors in the field. Byusing good rotordynamic principles and some advanced signalprocessing techniques, analysis of the lateral vibration of pumprotors can be a powerful tool in the early detection of shaftcracks.