Run gearbox vents/breathers high and away from air flow produced by electric motor fans. These fans can blow dirt into the breathers and plug them prematurely. In general, the higher you run breathers in the air (pipe extension), the longer they will last.

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Book Bits: Quick Test Detects Water In Oil

From the book "Practical Plant Failure Analysis":

A crackle test will show only gross contamination but is relatively easy to perform. To perform the test, take a small sample of lubricant and drop it on to a hot plate that is at 160 degrees C (320 degrees F). If the sample crackles immediately after it hits the hot surface, it shows there is water in the sample. The crackling is caused by the boiling of water. This test is only accurate down to about 0.1% water, and we know the quantities below that can cause shortened bearing life, but it does give a good field guide.

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Question: What are some things that can happen when an oil's additives are depleted?

Get the answer.

"We have a 600-liter sump that supplies 150 liters per minute to eight turbine bearings from one reservoir. The oil flows from the reservoir by a single pump and filter, then separates to the eight bearings. We take the oil samples from the return lines before they join for the return to the reservoir. None of the samples shows much wear. On the last sampling when there was a sign of bearing failure, the sample for that bearing showed only 4 ppm iron, 2 ppm tin, 1 ppm aluminum, 2 ppm silicon, 2 ppm sodium, 2 ppm magnesium, and everything else 0. Viscosity was 66.7 on Regal 68 at 1,000 hours (6,200 on the unit). Why would samples not show that there is a problem?"

The answer to your question can likely be summed up in one word - dilution! Assuming, as you state, that you are sampling on the return lines from each bearing, the reason for the low wear metal levels is likely due to the comparatively high oil volume in the return lines. In oil analysis, wear debris is measured in parts-per-million. So when you state "2 ppm tin" what we are actually saying is "2 mg of tin for every kilogram of oil".

So the same amount of wear debris, distributed in a large volume of oil such as a circulating turbine, will generate a much lower ppm than the same amount of wear in a small wet sump system where the volume of oil is typically much smaller.

To minimize the effects, try to ensure the sample is taken as close to the bearings as possible. Also ensure that the sampling method is precisely controlled with the same method used every time, including flushing volumes. You might also need to tighten your alarms considerably to the extent where "normal" really means 0 ppm of tin (and other key elements), and any slight increase, even just 1 to 2 ppm, is consider "cautionary".

Other possible causes of the low wear metals limits could be a failure mechanism which really doesn't generate significant amounts of wear debris, or one that creates larger particles (in excess of 5 microns) which do not show up in conventional elemental spectroscopy techniques. However, given the nature of this application, dilution is the most likely scenario.

Mark Barnes, Noria Corporation

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(918) 749-1400

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