Abstract
Why can even the most reliable turbomachinery get tripped up? In some cases, it's all about bad vibrations--literally. This paper explores the challenges one site had with repeated centrifugal compressor trips caused by high vibration over a period of 25 days. It outlines the troubleshooting attempts made to remedy this issue, it's root cause, and the resulting solution.
This issue occurred at a site with a train configuration of gas turbine driven centrifugal compressors. After 48 hours of its first startup, the machine tripped due to high vibration at the compressor drive end. Initial attempts to fix the problem included checking the vibration probe gap voltage setting, as well as swapping vibrations probes. After these tasks were completed, the site again attempted to run the machine. 48 hours later, it tripped again. During this time, the vibration gradually increased, staying above the alarm of 50 microns and tripping at 70 microns. The trip setting was increased to 75 microns, but after restarting, the unit continued to trip due to high vibrations.
All components were removed and thoroughly examined. Clearances were measured per the standard checklist. The condition of the couplings was found to be normal. The journal bearing pads had minor scratches. The thrust bearing pads were not affected by the failure. Alignment readings were found to be normal. However, residual unbalance was found on the rotor.
This residual unbalance was found to be the root cause of the tripping issue. The journal bearing clearances were thus adjusted from 0.17 mm to 0.13 mm. It was also advised to perform high speed balancing checks in the manufacturer's facility. The lube oil inlet pressure was adjusted to meet the requirements for the new bearing clearance.
Adjusting the bearing clearance, along with the adjustments in lube oil inlet pressure, resulted in improved vibration readings. This case study provides the detailed analysis of the root cause of the frequent trips as well as the process that was followed to fix the problem.
