|Stray shaft currents, or electrostatic discharge, that are caused by an induced magnetic field can be very destructive, yet they can go undetected on their onset. Turbine generator sets can be the most prone. When the detected voltage going to ground exceeds 1.0 volt then it is time to investigate.|
|The sleeve bearing shown to the upper left came out of a steam turbine that drove an 11 megawatt generator set. The frequency of bearing failures had increased for no apparent reason. Closer inspection of the top half of the bearing is shown in the upper right picture. There was an inconspicuous amount of damage that went unnoticed until a different set of eyes inspected the bearing. The damage is circled in the picture. The rest of the bearing looked to be OK so the damage appeared to be meaningless. It wasn't until the damage was looked at under a magnifying high powered camera that the significance became very apparent.
Looking at the picture below there was a large amount of craters that looked like pits. They weren't pits though, they were craters that were formed through arcing from an electrical current. The craters are well defined. Their rims are the result of molten babbitt that solidified from the puddle that was created by the electrical arc.
|The picture below is another view a little further down from the picture above. The white line is approximately 0.001 inches in thickness. The fine radial lines on the bearing are the tooling marks left by the single point tool that finished the babbitt during manufacture. The molten babbitt around some of the craters is quite visible. The long erratic gouge in the upper right is an arc strike that had a long duration of time as compared to the small pits. Notice how the pits in both pictures are in line with each other in the circumferential direction. The arc was traveling, moving from the shaft to the babbitt bearing, and then to ground.|
If you're wondering how stray magnetic currents could be so bad, then look at the next set of pictures below. The top half of the bearing is shown to the lower left. At first glance the damage that is circled appears to be caused by something trivial. Closer inspection shows that the babbitt was spalled off.
A closer inspection of the top half bearing in this area is shown in the picture below. It shows how many of the arc pits connected to each other and caused the babbitt to spall off. The accumulated damage destroyed the smooth surface that the oil film requires. This particular failure had some extenuating circumstances that muddied the waters as to which factor was the true culprit. However, it was clearly obvious that if this bearing was allowed to continue to operate without eliminating the ground currents it would have eventually failed.
EDITORS NOTE - As it turned out, the building grounding system had deteriorated. Apparently the grounding system was in such bad shape that things were grounding through the piping and everything else that could go to ground. In the case of this generator when the grounding line for the generator was disconnected the problem went away. One of the grounding paths that everything was taking was from the building grounding system up through the generator ground wire, through the generator rotor, and out through the turbine pedestal grounding wire that went down into the huge steel reinforced concrete pedestal.The building grounding system is currently being repaired.
|You can determine and measure the amount of an induced magnetic field by using a Gauss meter. A desired level is 2 Gauss. A typical level is between 5 to 10 Gauss. An alert level is around 20 Gauss. If the level exceeds 27 Gauss, as was the case in this particular machine, then the machine should be shut down and demagnetized. You can see how that was done in the article "Machine Design - Case Study No. 133: Electrostatic Discharge Through a Turbine Generator Bearing."|
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