Vibration Diagnostic of Steam Turbine Intermittent Rubbing

Abstract

A special purpose steam turbine has started to experience intermittent radial vibration spikes. The turbine is used to drive a Hydrogen Recycle Gas Compressor in Aromatics unit. No abnormality observed on the compressor end. ADRE 408 external data acquisition and diagnostic system was used to collect vibration data during steady state operation. Detailed vibration diagnosis was performed to evaluate overall rotor dynamic behavior of the machine train, assess vibration levels against the OEM set-points and ISO 7919-3 standards, troubleshoot potential mechanical malfunctions that leads to intermittent vibration spike and provide recommendations for safe operation of the unit. From the analysis, the cause of high vibration was determined to be intermittent light rubbing of the steam turbine rotor with the bearing housing oil baffle (seal). Temporary mitigation actions were implemented: (i) increase vibration alarm limit from 30um to 42um and; (ii) commission sealing air to both DE and NDE bearing housing oil baffle, which was later on found to be clogged with oil carbonization. The intent of the mitigation actions is to reduce the probability of tripping the machine. However, it was not adequate and deemed to be too late to prevent the force outage. After the trip, both DE and NDE bearing housing were opened for inspection. Severe oil carbonization was observed on the NDE bearing housing oil baffle which has close clearance and the sealing air port was plugged. The oil carbonization was cleaned without removing the steam turbine rotor (extensive work) in order to minimize the turbine downtime. Top halve oil baffle was completely cleaned while bottom halve oil baffle was cleaned to the best effort due to access constraint. The turbine was restarted successfully. No significant vibration spikes observed after 2 months of running. Proper visual cue (signage) was implemented to ensure sealing air supply valve is opened at all time when turbine is in operation.