ArticleCarbon Seal Rub On A Steam Turbine
by Joe McCollum
This brief paper gives an account of how the author and his other plant personnel dealt with a carbon seal rub on a steam turbine following rebuild of the unit. The paper goes thru how vibration analysis was used to detect and then help to resolve the problem. Corrective actions for this problem were discussed.
“A steam turbine unit driving a 32-inch gas blower at a chemical plant had just undergone yearly maintenance; the bearings and seals were replaced. When the unit was brought back on line, the vibration levels increased to trip conditions (2,920 RPM) and shut down the unit. This seemed odd because the critical speeds of the turbine and blower were not at 2,920 RPM. Several attempts to start the unit failed. A multi-channel monitoring system was set up to measure all of the X-Y proximity probes installed in a Bently Nevada 3300 system; the case vibration at all the bearing cap locations was measured with magnetic base accelerometers.
The data captured during the startup revealed high vibration amplitudes at the drive-end X-bearing turbine probe that eventually increased to trip conditions and shut down the unit (Figure 1). During the excursion the amplitude overloaded the set point of 10 mils on the data collector. The bearing clearance for the drive end bearing is 0.008 in. There was concern for the bearing condition after this 10 + mil excursion. Case vibration at the same bearing position was extreme at 0.8 IPS pk. It was also noted that the drive-end Y-probe was not working correctly.
An internal inspection of the bearings was recommended. The drive-end Y-probe was examined. The proximity probe had an incorrect gap setting that was corrected Figure 2). The drive-end turbine bearing had indeed been damaged during the startup and was replaced.
During the inspection several rub spots were found on the carbon seals. This extra set of carbons had been manufactured by a source other than the turbine manufacturer. The internal diameter of the seals was found to be tighter than that suggested by the turbine manufacturer; in addition, the temperature rating was not to the recommended specifications. The carbons used on the previous run were from the same manufacturer but had snore clearance. In order to get the machine up and running, the seals from the previous run were used.
The next run showed a steady increase in amplitude with speed that eventually reached 5.02 mils on the drive-end X-probe at 2,040 RPM. To avoid another bearing failure the speed was decreased then slowly increased. There was no repeat of the 5.02 mil excursion, but the amplitude did reach 3.37 mils at 2,200 RPM before it leveled off to 2.88 mils at 2,580 RPM.”
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