A European manufacturer of large hydraulic components flushed its newly manufactured components with clean hydraulic fluid at a high turbulence to remove any particles left behind during production. This flushing was carried out using a specified brand of low viscosity hydraulic oil. The oil type, flushing procedures and targeted cleanliness levels were provided to the manufacturer by its customer.

My laboratory analyzed the used flushing oil for particulate contamination using two particle counting methods, automatic particle count (APC) and microscopy (ARP-598b).

From time to time, the results from the two different particle counting methods were very different. We discovered that one of the reasons for the difference was related to water content and the oil’s water-holding capability. The microscopy was not affected by the water content, but the APC occasionally reported several hundred percent differences in the counts. We investigated further by using a Karl Fischer coulometer and a percent water saturation test. Results showed that when the oil had more than 150 ppm to 200 ppm water (greater than 40 percent to 50 percent saturation) the microdroplets interfered with the APC. Once this was discovered, the solution was easy; we purchased an instrument to remove the water from the oil.

Once we began using the water removal instrument, the water dropped to acceptable values. However, we were surprised to discover that even after the water had been removed, there was still a significant difference in the results of the two particle counting methods. The APC showed much higher counts than the microscopy.

We also discovered discrepancies between the new oil and the used oil. We found this unusual because the oil used for flushing was new when it entered the system. We hadn’t expected to see significant changes in the oil or its additives.

We ran an elemental analysis using rotating disc electrode (RDE) on the new unused oil from the barrel and on the oil used to flush the hydraulic power unit (HPU). The sample from the HPU showed more than 200 ppm silicon and approximately 2,500 ppm phosphorus, which was higher than the new oil, which contained 0 ppm silicon and approximately 500 ppm phosphorus.

Searching for Clues
We began to investigate the reasons for the discrepancy. Had the system been topped off with another oil type? Was dirt being sucked in with air? Neither of these possibilities was the problem. The HPU and the environment were clean, and no oil was being topped off.

It turned out that the problem stemmed from poor cooling. The oil was overheating and the heat caused the silicon-based adhesive on the reservoir top plate to melt and, overtime, mix with the oil. The APC method detected the silicon product as particles in the 5 µm to 15 µm range; however, microscopy did not detect these particles.

The lesson we learned from this is that when the APC results are inconsistent, confirm the results with alternative methods.

Submitted by Kevin Hughes
Invicta as, Oslo Norway