"We have a gas turbine with a sump capacity of 4,920 liters and 9,825 running hours. In the last oil analysis report, aluminum and potassium increased by 1.29 and 3.31 parts per million, respectively, from the previous value of zero. How can we identify the origin of this wear metal?"
When measuring and analyzing wear debris, the first step is to understand what the elements are and the likelihood of their source. Turbine oils often return a blank slate in terms of elements, so when you begin to find wear debris in the oil, it can be easily deduced that it is from either contamination or machine wear. While these concentrations of potassium and aluminum are quite low, if they have risen significantly over the past few samples, it would suggest that something is occurring with increasing severity.
To make the process easier, create a metallurgical map of the equipment in question. This would include determining the composition of each lubricated component. For a turbine, you would expect to see metals from the bearings in the main circulating system. Depending on the type of bearing, these metals could identify which piece is wearing. Some bearings have overlays that help pinpoint where the metal is coming from and how severe the wear is.
Most turbines have oil circulating throughout the entire system, so not only do you need to know what metals are in the system but also where the oil sample was extracted. Ideally, the sample would be obtained from an area of turbulent flow that is most representative of the oil in the system. For most turbines, taking a sample from the main return line would be ideal for indicating the type of wear as well as possible contaminants.
Potassium can have many sources. You may find it in fly ash and road dust. These contaminants are incredibly small and would be airborne for a long period of time. Headspace ingression is possible if the reservoir isn't sealed properly. Potassium is also a common additive in coolants. Most turbine systems have coolers attached, so this could be a possible source of contamination.
Aluminum can be frustrating to identify because of its prevalence in equipment and dust. You may initially presume aluminum bearing surfaces, but it could also be from alumina found in airborne contaminants. Again, fly ash is a likely source. However, if the lube oil cooler is made of aluminum, there could be another clue as to what is happening.
Without more information, it would be difficult to determine the exact problem, but start with a lube oil cooler investigation. Also, look for water in the oil. An increase in water would be a good indicator in the lube failure. As with most failure investigations, always try to confirm your suspicions with additional testing.