Lubricants

Analysis of Filters Critical for Turbines

Just as mystics can read the future in the particles in the bottom of a tea cup, the turbine engine specialists at Aviation Laboratories can predict the future health of a turbine engine by examining the debris collected in the oil filter. "When a turbine engine enters an abnormal wear mode, it generally begins making predominantly larger particles and the vast majority get trapped by the filter," explained Rory Hammond, president of Aviation Laboratories (www.avlab.com). "Turbine engine filters are designed to trap particles 10 microns or larger, so the oil is likely to be very clean even if there is abnormal engine wear."

Unfortunately, all too many turbine engine technicians believe that oil analysis programs are enough to gauge the health of their engines, but they're not. The filter needs to be inspected first by the technician, then sent to a company, like Aviation Laboratories, for detailed analysis.

"On an average, turbine engine manufacturers require that the oil filter be cleaned and inspected every 100 to 200 hours," he said. "Unfortunately, when a technician cleans the filter, they rinse all the valuable diagnostic evidence of the engine's condition down the drain. The technician should send the filter to our laboratory so it can be rinsed, and the debris analyzed and documented in a repeatable way."

What takes place during an oil filter analysis? "All of the debris removed is isolated onto an eight-micron filter pad and the total weight of the debris is determined," Hammond said. "Then it's carefully inspected and analyzed by energy-dispersive X-ray fluorescence, under the high magnification of a scanning electron microscope. The amount, alloy type, condition, and form of the debris particles is determined and documented."

Hammond explained that the alloy type determination of the debris particles is reported using AMS (Aerospace Material Specification) numbers. "If the material condition of some particles shows signs of severe oxidation or heating it can point to past engine problems that were not properly flushed from the engine after repair," he added. "The form of the particles often assists with identifying the source of the material as well. For instance, gears can generally produce chunks and bearings generally produce flat platelets.

"Bearing wear is often the most critical wear identified because when a bearing starts to spall and produce large particles the bearing can deteriorate quickly. When we identify abnormal engine wear in some cases engine inspection is necessary, in others, the sample interval is shortened to 10 to 15 hours to monitor the progression of the abnormal wear," Hammond said. "Sometimes an engine component such as a bearing will produce a few particles and then stop and sometimes it's the beginning of a really big problem."

Hammond also cautioned technicians against relying too heavily on chip detectors to signal a problem. "Chip detectors aren't always good at picking up non-ferrous materials and there are a couple of sources inside a turbine engine," he explained. "Also it is important to remember that if there is a chip indication, the oil filter should be immediately inspected or sent to the laboratory for analysis, because if there are a few particles on the chip detector there are likely to be many more in the oil filter." — By Dale Smith