Vibration Analysis

The motor and gearbox for the compressor was replaced in early 2020. The cold alignment was performed during cold ambient temperatures < 40°F. In March of 2020, the compressor train shaft vibrations began to increase. Vibration levels continued to increase with operation but had not reached alarm or trip 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 perform a thermal growth study to collect vibration, hot alignment, and cold alignment data. The hot alignment data is collected first while the machine is 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. At that time, 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.

The motor and gearbox were replaced during a planned outage in February 2020. The unit was restarted at the end of March 2020. By early April, vibration levels had increased. As the compressor train continued to operate, vibration levels continued to worsen. There is speculation that the coupling is 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 “fish-eye” distortion at select locations. It has also been observed that the severity of this distortion changes with ambient temperature. This has led to the hypothesis that thermal growth is affecting the changes in machine alignment and subsequently worsening the coupling misalignment during running conditions.

The purpose of the Field Engineering site visit is to collect Visual ODS®, shaft vibration, casing vibration, and hot alignment readings to diagnose the severity of the condition. The hot alignment data will indicated the severity of misalignment during the running condition. When cold alignment data are subtracted from the hot alignment readings, corrected values will be obtained to set the train in the proper cold condition so that there is minimal misalignment at the hot running condition.

Corrections for cold to hot alignment will differ for the compressor-gearbox and gearbox-motor trains. That is, the movement, or rate of change for thermal growth is expected to be linear with temperature, however the rate of change for the compressor-gearbox will be different than the gearbox-motor thermal growth. The linear thermal expansion can be plotted for the high-speed (HS) and low-speed (LS) shaft alignment. The slope of these lines is not expected to be the same. Once the final cold alignment readings are obtained, the thermal growth rates will be verified, and corrected as needed.

Severe misalignment in the hot running condition, as well as synchronous and sub-synchronous vibration patterns indicate that, at minimum, accelerated bearing wear is occurring, as well as possible internal damage to labyrinth and shaft seals. Compressor and Gearbox axial vibrations exhibit inflection patterns which appear to coincide with ambient temperature changes. The ambient temperatures appear to compound the shaft misalignment, causing distortion of the shim-pack coupling, resulting in a “fish-eye” pattern in between the coupling bolts. It is not known if any of the coupling shims have yielded permanently, or if the distortion is temporary and can be corrected with proper alignment.