North African Pipeline Vibration ConsultancyHGL Dynamics was asked to investigate increased amplitudes of vibration within the second of three Power Turbine units of a Gas Compression System, and to assist in identifying the possible cause of this increased vibration for rectification. The turbine sets were located on a gas pipeline in North Africa, where the trend plots of the proximity probes show that the vibration levels have increased from 20 microns to 46 microns. This increase in amplitude had been seen over approximately a 15 hour period with a sharp increase over the last 6 hour period. It was also reported that excessive noise and rumble was coming from the PT coupling end area. Therefore the second unit was shut down pending further investigation. There are three identical units at the site. Usually, only 2 of these are running at any one time with the third unit on standby for 8 months (covering high demand periods); for the remaining months only one unit would run with the other two on standby. Each unit operates approximately 4000 hours a year typically at base load (PT rated speed 4950 RPM, max speed 5200 RPM).
The rated speed of the Compressor and Power Turbine are the same, therefore there is no gear box between the Compressor and the Power Turbine. There is a drive shaft between the Power Turbine shaft and the Gas Compressor; this shaft has two flexible couplings, one at each end. Study of the schematics shows an auxiliary drive train coming off the Power Turbine rotor. This drives two lubrication pumps, (Compressor and PT fluid bearings and auxiliary gearbox) and three hydraulic pumps (cooling fan motors and the Gas Generator lubrication oil pump). From the incorporated trend monitoring system, increased vibration was seen on the PT rotor disc end, high vibration was also seen on the PT coupling end, and a slight increase was seen on the gas compressor drive end (coupling end). This suggested further investigation around the PT coupling end bearing. This is also the area where the auxiliary shaft is located. Measurements were taken at 10 points along the radial area between the coupling casing and the PT compressor end bearing. Also data sets were recorded from one axial position for completeness. The data from the 10 points acquired indicated peak amplitudes between the frequencies 1621 Hz to 1626 Hz. This indicated a fault within the gear meshing of the power turbine auxiliary drive. There were also side bands around 1499 Hz and 1713 Hz which also indicated a number of other possible faults:
It was recommended that the auxiliary drive shaft should be inspected. Inspection of the auxiliary drive shaft found excessive end float on the auxiliary drive and increased backlash on the gear meshing, which confirmed HGL’s prediction.
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