Dr. Steve Dye /Business development and marketing manager /Condition Monitoring Business Unit /Parker Hannifin, Kittiwake Developments /www.kittiwake.com
When people think of condition monitoring, they often think of vibration trending. But it’s more than that. Things can happen to a turbine that don’t necessarily generate vibration but do cause detectable wear.
Wear-debris sensors, including simpler particle counters, provide additional insight to gearbox performance by counting ferrous and nonferrous particles in an oil flow and quantifying their size. But they are not just particle counters.
Beyond particle counting
Optical particle counters shine a laser light through a film of oil and look for shadows which they consider particulate matter. They do not differentiate what sort of matter they find, so a particle could be a fleck of paint, sand, metal, or an air bubble. Particle counters, however, can detect small sizes, often 4 to 6 microns.
The recent metallic, wear-debris sensor from Kittiwake is more adept at identifying wear in large oil-lubricated systems, such as wind-turbine gearboxes. The sensor will count the size and number of particles passing through it and stores the measurements into “bins” for retrieval. For example, it might report there are 10 particles between 40 and 60 microns, 22 between 60 and 80 µm, and so on up to about one millimeter. It cannot identify a type of metal, but by looking at deviations in an electric field, it can tell if a particle is ferromagnetic or nonferrous. The sensor has 10 counting bins for ferrous and 10 for nonferrous particles.
When installed in a system for a sufficient period, the sensor can differentiate a rate of change. If a flow typically shows two to four counts per minute and suddenly jumps to 20 or 30 particles/min, then something has changed possibly indicating the onset of a failure event or an impending failure.
Constant measurements, 24/7, provide the advantage of getting an instant indication of a significant event. The device allows setting warning or alarm levels, so if it is typical to see five particles/min, a warning could be set to send an email should the rate increase to 25 particles/min, or it could trigger a severe warning level at 50.
A case history
The chart Wear rates versus wind speed plots real data from a wind turbine. The red trace tracks the number of particles generated per minute while the blue trace identifies wind speed. The red spike on the left tells that 40 to 45 particles/min were generated for a short period on a particular date. The blue trace shows that the wind speed exceeded 22 m/s so turbine controls shut the unit down with a sudden application of its brake. The turbine stayed off line until the high wind subsided.
On restart, the red traces tell of little change in particle generation, but on the 24th the wind hits over 22 m/s again and the trace shows a large increase in the generation of particles. High particle counts continue with increases in wind speed and increases in particle generation, but without a brake incident.
The illustration Total ferrous particles generated gives another perspective of the sudden stop with a total particle count. The first event, the red spike on the previous illustration, appears as a small step. After three weeks, the wear counts begins picking up and climbs to just below 30,000.
Later, a gearbox inspection confirmed what the counter was saying. A borescope showed scuffing on shaft gears and many particles clinging to the dip stick magnet.
A lab examined the filter before and after the event. Back washing the filter in the post-event exam showed about a three-fold increase in material. That let us look to see what the particles were. About 40% were flakes and slivers of iron that measured 0.4 x 1.6 mil, a significant size. The lab analysis provided further information in pictures of the ferrous and nonferrous debris. The crescent shape of the debris generally indicates slicing wear, likely from rough gear teeth sliding against each other. The big difference is in cutting ware that gives a particular shape to debris.
We spoke with operators and found they had indeed signaled the system to halt because of the high wind. And when the system applies the brakes, it does so suddenly. Because the generator is still turning, rotational inertia sends a shock load through the gearbox and to an end-thrust bearing. We know this without taking the gearbox apart because most wear is iron and cast iron and that tells it’s the gearbox housing or casing. The diagnosis is that the shock from the sudden brake application caused the main thrust bearing to twist in its casing. Hence we see the cutting wear of cast iron.
This information gives a repair team time to plan corrective action, either replace the bearing or leave it in service. Wear-debris detection gives an early signal of damage by picking up small increases sooner than other methods. This sort of analysis can change the way a turbine is operated, perhaps with a more gentile stop, or another approach. WPE
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