PRACTICAL USE OF ROTORDYNAMIC ANALYSIS AND TROUBLESHOOTING SKILLS TO CORRECT A LONG-TERM SYNCHRONOUS VIBRATION PROBLEM IN AN OVERHUNG TURBOBLOWER

摘要

The two subject blowers operate in parallel to circulate wet chlorine gas. Both units had large synchronous vibrations that led to multiple bearing failures. After simple rotordynamics studies failed to identify the problem, a comprehensive model that accounted for both the motor and blower was successful at identifying the problem as high sensitivity to unbalance loads due to an extremely lightly-loaded (less than one pound) condition at the blower's inboard bearing (refer to Gutzwiller and Corbo, 2001). Based on the results of the rotordynamic analysis, two changes were made to both units. The couplings were changed from disk to gear couplings, and the blower's bearings were changed from plain cylindrical to tilting-pad designs. After implementing these changes, unit "A" ran smoothly for a period of six weeks, in accordance with the predictions of the rotordynamic analysis, and it appeared to all that the problem was completely solved. However, when the "B" blower was then started up the two machines then commenced a three month period of operation in which each suffered from intermittent periods of high synchronous vibrations. During this time, the following behavior traits, which can only be described as bizarre, were observed: 1. After a smooth startup, vibration increased to a high level after about one week. 2. The transition from low vibrations to high vibrations was almost instantaneous. Additionally, this transition was always accompanied by an axial motion of the blower rotor away from the active thrust bearing. 3. The units appeared to be more likely to suffer high vibrations when both were running simultaneously. 4. Starting one machine might result in increased vibrations on the other, and shutting down a machine might lower vibrations on the other. 5. Someone observed that during a rainstorm the vibration levels decreased. (Note: the blowers are located outside.) Spraying water on the bearings sometimes (but not always) had a similar effect. A task force of experts was then commissioned and an extensive troubleshooting effort commenced. Some of the potential root causes that were hypothesized included blower surging, starvation of the tilting-pad bearings, thermally-induced misalignment, insufficient blower thrust, acoustic resonance, axial vibration, seal rubbing, and Morton effects. After an extensive troubleshooting effort that included more rotordynamic analysis, bearing flow analysis, blower thrust analysis, and extensive studying of orbit plots, spectrum plots, and vibration and temperature histories, the task force concluded that the most likely cause was rubbing at the blower's carbon ring seal. Accordingly, the seal was disassembled and its locating pins were found to have come loose and generated a rub. Design changes were then implemented to provide better retention for the pins. The units were then restarted and it was verified that the modifications had finally eliminated the vibration problems. This paper shows how the combination of rotordynamic analysis and troubleshooting skills was employed to identify and generate corrective actions for two independent causes of high synchronous vibrations.