Vibration Analysis
Executive Summary
The motor and gearbox for the compressor were replaced in January. The cold alignment was performed during ambient temperatures < 40°F. In April of the same year, the compressor train shaft vibrations began to increase. With increasing ambient temperatures, the vibration levels continued to increase with operation but had not reached alarm or shutdown levels. A thermal growth study was requested to determine the potential impact that improper hot alignment may have on the unit.
Field Engineering was requested to collect vibration, hot alignment, and cold alignment data. The hot alignment data was collected first while the machine is was in operation. Mounting pucks and fixtures were installed to collect the hot operating alignment data between the compressor-gearbox and gearbox-motor couplings. The fixtures remained in place as installed and undisturbed until a planned shutdown. During the shutdown, cold alignment data was collected, and corrections were provided to align the compressor-gearbox-motor train in the cold condition to achieve optimal alignment in the hot running condition.
Introduction
Visual ODS Equipment History
The motor and gearbox were replaced during a planned outage during the winter months when ambient temperatures are typically very low. The unit was restarted at the end of March, and by early April, vibration levels had increased. As the compressor train continued to operate, vibration levels continued to worsen. There was speculation that the coupling was failing and is the major factor contributing to the increasing vibration levels. Videos and images were collected, which show that some of the coupling shims have developed a “fisheye” distortion at select locations. It has also been observed that the severity of this distortion changes with ambient temperature. This has led the customer to speculate that thermal growth was affecting the changes in machine alignment and subsequently worsening the coupling misalignment, which affected the shaft vibrations.
Rotor Dynamics Objective
The purpose of the Field Engineering study is to perform a rotor dynamics analysis using Dyrobes® and ANSYS® FEA and CFD to determine the exact stimulus of the rotor unbalance and counteract these unbalance forces as much as possible to permit long-term, stable operation.
Conclusions
Severe misalignment in the hot running condition, as well as synchronous and sub-synchronous vibration patterns indicate that, at minimum, accelerated bearing wear was likely occurring, as well as possible internal damage to labyrinth and shaft seals. In addition, the compressor-gearbox gear coupling has a limited range of angular displacement that can be accommodated without excessive wear or damage. The gear-coupling had reached the upper limits of its safe operating range with the As-Found misalignment condition. The Compressor and Gearbox axial vibrations exhibit inflection patterns which appear to coincide with ambient temperature changes. The ambient temperatures compound the shaft misalignment, causing distortion of the shim-pack coupling, resulting in a “fisheye” pattern in between the coupling bolts.
