Sampling Cleanliness

The method and cleanliness by which an oil sample is taken has a critical effect on the accuracy of the laboratory results. If the sample is falsely contaminated by taking it from the wrong point or in the wrong way, or if the sample-taking equipment or method introduces contaminants, then false contamination levels will be reported.

Agood sample is one that is cleanly taken from the circulating oil flow. The proper sample-taking method and procedure should be agreed with the laboratory and if necessary the laboratory should be asked to provide training for the sample takers.

Many original equipment manufacturers have accepted the indisputable evidence from numerous field and laboratory trials that oil cleanliness has a major effect on wear within their equipment. Some of them are now specifying how clean the oil used in their equipment must be if warranty claims are to be honored.

For example, Caterpillar Inc. specifies new oil to have a particle count of ISO 16/13. If new oil is above this level of contamination it will not warranty the equipment. When new oil from a leading international oil manufacturer was tested before putting it into new Caterpillar equipment, the solid particle contamination was found to be 17/14. This was new oil from a never previously opened container. In this case the new oil had to be further filtered to bring it to below the required specification

Table 2 is a list of the recommended target oil contamination levels for close tolerance equipment from a survey of hydraulic oil equipment and oil filter manufacturers.

For extremely low wear rates and long equipment life, the evidence indicates that oil needs to be filtered down below 5 micron size and preferably down to 1 micron size. Care needs to be taken that the filter does not remove any solid additives, such as graphite, in the oil. Additives dissolved in the oil will not be removed unless the additive is attached to a solid particle.

Oil filtration can be done under full oil flow or with bypass flow or offline. There are several filter types such as pleated paper and wrapped fibre cord. In all cases the filter must capture a large proportion of greater than 5 micron particles if it is to clean the oil.

The correct way to measure filter performance is by use of the Beta Rating. which compares the number of particles entering a filter to the number leaving. It is an accurate way to measure true in-service performance. Nominal filter micron size ratings from manufacturers are meaningless. And absolute filter micron size ratings are unreliable since the softer particles in the oil can be squeezed through the filter and reappear as contaminants.

Numerous tests on a range of hydraulic (e.g., piston pump) and oillubricated equipment (e.g., truck engine) have been conducted that confirmed filtering oil and removing particles deliver exceptionally long equipment life. The cost of suitable filtration systems is not expensive. For expensive hydraulic and oil-lubricated equipment the cost of filtration is easily and quickly returned by the large gain in equipment working life and reliability.

This article is an excerpt from “Rotating Machinery Essentials” by Mike Sondalini (1999). Detail at www.feedforward. com.au/life-cycle-management.htm

Mike Sondalini is an equipment longevity engineer and author. He edits the subscription-based newsletter Process & Plant Equipment UPTIME (www.feedforward.com.au). He can be reached at Lifetime Reliability, P. O. Box 578, Bentley, WA, 6102 Australia; (+61 8) 9457 0742; www.lifetime-reliability. com. Don Irvine is with Donamar Filters that supplied the data for this article.