Hydraulic Unit for Kamyr Diffusion Washer
Hidden Failure Mode causes Filter By-Pass
The following case study demonstrate the ability of oil analysis to reveal hidden failure modes. In this case study a large hydraulic reservoir in a pulp and paper mill suffered a filter by-pass indicator failure. The historic particle count (farthest chart to the left) shows that the typical ISO 4406:199 operating cleanliness code for this unit is 19/16/13. When the oil analysis testing indicated that the cleanliness code had jumped from 19/16/13 to 22/20/17 (middle chart) an operator was sent out to check the filter indicators on the unit. The electronic filter indicator was not lit, indicating that the filter was still performing its function and not operating in by-pass. Another oil sample was taken from the hydraulic unit in two months time (chart on right). This time the cleanliness code had jumped even higher, up to 24/23/22. Again the operator reported that the filter by-pass indicator was not lit.
Figure - Over the period of six months the filter plugged, went into by-pass and the oil cleanliness code increased from 19/16/13 to 24/23/22.
Further investigation by the reliability team revealed that the indicator light had failed, and that the filter had, in fact, been operating in by-pass for many months. The filters were changed, the filter indicator was repaired and a number of months later (and several additional filter changes) the cleanliness codes were approaching their original level. The PMs for this unit were revised to include an electronic check of the filter indicators, as well as scheduled filter changes and a yearly clean-out and inspection of the reservoir.
Figure - After the hidden failure was discovered (failed filter pressure indicator) and corrected, and after several subsequent filter changes, the oil cleanliness was reduced from 23/22/21 to 18/13/10.
Marine Vessel - Controllable Pitch Propeller
Dirty Hydraulic System Reservoir
Marine vessels use condition monitoring to extend overhaul periods on ship machinery. Routine analysis on the Controllable Pitch Propeller (CPP) hydraulic system showed a sudden increase in particle count results. The ISO 4406:1999 cleanliness code increased from 22/17/11 (far left chart) to 23/22/19 (middle chart) within a six month period at sea. The CPP hydraulic system was scheduled for inspection at the next docking to determine the cause. It`s worth noting that the visual screen photograph of the oil showed decreased clarity (top middle photograph).
Figure - Routine oil analysis detected a dramatic increase in particulate in the critical CPP hydraulic system. The ISO cleanliness Code increased from 22/17/11 (left chart) to 23/22/19 (middle chart) and the oil clarity reduced visibily.
As a result of the most recent particle count results, the oil was drained from the Controllable Pitch Propeller (CPP) hydraulic system during the recent emergency docking. The sump tank was opened up, cleaned, inspected and found to be very dirty, and the system filters were changed. New oil was added to the system through 3 micron filters with the use of a portable filter cart. Offline filtration was carried out for eight days using 10µ and 3µ (micron) filters. A re-sample was sent to WearCheck for rush analysis. The results showed that the CPP hydraulic system cleanliness code decreased from 23/22/19 to 21/17/14 due to the reliability maintenance crews efforts and the vessel was approved for return to service. As a result of the critical nature of the CPP sytem, the oil was put on an increased sample schedule frequency.
Figure - During an emergency inspection, the CPP hydraulic system reservoir was found to be very dirty. A sump cleaning followed by filter changes and off-line filtration for eight days returned the oil to the target cleanliness code level.