As part of their lubrication best practices, world-class reliability and maintenance organizations filter all oil prior to or during its installation into a system. They also dehydrate and deaerate samples to prevent water droplets and air bubbles showing up as particles in optical particle count tests.
However, even after following these practices, many still find that the fluid they just filtered still appears to contain particulates, as shown in particle evaluation tests. They are left questioning what could be causing it. Is the filtration equipment not functioning properly? Is the laboratorys particle counter malfunctioning? Why would new lubricant – even after filtration – be contaminated with particulates? Perhaps there are ways the particle count tests are tricked by certain samples.
Recent research has proven the existence of ghost particles. Additional research has generated new techniques to aid laboratory analysts in busting these ghost particles.
Lubricant Life Cycle
Depending upon type, a lubricant can be composed of one or several base fluids, an additive package and various component additives. Each ingredient must move through several processes as it is being manufactured, transported and transferred. Each process for each ingredient, as well as the lubricant manufacturers own processes, presents an opportunity for introducing contamination into the finished product. Most of this contamination can be mitigated through filtration and other purification techniques, and many users filter their new fluids for verification purposes.
If users are unable to reach predetermined cleanliness goals even after subjecting the lubricant to multiple passes through a filtration system, the first reaction is to think that the filtration system is not working or something is wrong with the particle counting equipment. Flowmeters and pressure gauges on filtration systems provide a way to verify whether the system is functioning properly. Also, particle standards are available that can be used to verify whether the particle counting device is working properly.
After those two possibilities are cleared, the only other possibility is that something in the lubricant is giving a positive – or false positive – response in the particle count tests.
Testing Techniques
One of the best ways to evaluate particle contamination in a lubricant is to use a particle counter. Some in the industry have proposed cleanliness limits for new oil based on a combination of fluid viscosity and equipment application. For many years, the ISO 4406 standard has been used as the main guideline for lubricant cleanliness.
Various types of particle counters are employed to evaluate lubricating fluid cleanliness. Probably the most common is the optical particle counter. This type of device uses a light source and a detector, between which a sample of lubricant is passed (Figure 1). Any particulate material that either reflects or refracts the light will cause the detector to collect electronic information on the number and size of particles present.
Another common type of particle count technique is the pore blockage technique, in which the lubricant is passed through filters of controlled size. The amount of particulates is determined by evaluating the pressure differential as the oil sample is forced through several filters.
The final type, and probably least used, is the visual optical counter. In this case, the fluid is passed through a filter patch and the user physically counts particles using a microscope.
Various studies have shown that certain ingredients used in lubricant formulations have a tendency to trick some particle count test methods. These ingredients have been described as ghost particles, phantom particles or soft particles.
Finding the Ghosts
To better understand this phenomenon, the author conducted an experiment on new fluid ingredients in an effort to determine how they would affect various particle counting tests.
The hypothesis was that certain ingredients in new lubricant samples give positive readings in some particle counting tests, making new oil seem to contain particulate contamination even though the lubricant has not been used. It has been shown that certain base fluids have better solubility for additives. Perhaps, it was suggested, these base fluids would affect the ghost particle response in particle count tests.
Lubricating fluids are formulated using various ingredients: base fluids, corrosion inhibitors, oxidation inhibitors, viscosity index improvers, defoamants and antiwear additives. Samples were collected from inventoried containers in an operating lubricant plant, including base fluids, commercially available additive packages and various components that would be used to formulate typical industrial lubricants.
Three particle counting methods were used to evaluate these lubricant components. Two were optical particle count methods, and the third was the pore-blockage technique. The first optical particle count apparatus was in the authors lab at Lubrication Engineers, manufactured by Hiac Royco. The second optical particle count technique (a LaserNet Fines from Spectro Scientific) and a pore-blockage technique called the Entek Contam-Alert system, were commissioned through an outside contract lab.
Several typical base fluid types and common additives that could be used to formulate hydraulic fluids, listed in Figure 2, were evaluated in the authors lab. Each of the base fluids was first evaluated in un-additized form, then blended with each of the additive components at typical treat rates.
Next, two commercially available hydraulic oil additive packages were blended into each base fluid, and the samples were sent to the contract lab for the second optical particle count evaluation and the pore-blockage evaluation.
False Alarms?
The data gathered from the testing supported the hypothesis that ingredients in lubricants could interfere with particle counting. Some hydraulic fluid formulations contain ingredients that provide false positive or ghost particle readings.
All base fluids seemed to have similar particle count readings, except that they appeared to be slightly cleaner when tested with the pore-blockage technique than when tested with the optical techniques.
After the base fluids were additized, the ghost effect was definitely more pronounced when using the optical particle counters than with the pore-blockage technique. Therefore, it can be surmised that the pore-blockage technique is less affected by additives.
Surface-active additives, such as defoamants, detergents and antiwear agents, seem to produce more positive particle response than other additives, such as viscosity modifiers and antioxidants. This should not come as any real surprise, as these surface-active chemistries function because they have limited solubility in the base fluid or they form micelle structures. Thus, it was also no surprise to find that the commercially available hydraulic package products provided a positive response, as they contain defoamants and antiwear additives.
Two colorant dyes were evaluated to determine whether they would trick the tests, since they obviously change the visible appearance of a lubricant. Although they caused a slight increase in particle count, it was no more dramatic than most additives and much less than the defoamant additives.
Defoamant additives – both silicone and non-silicone types – appeared to provide the most dramatic positive response.
Banishing the Ghosts
The industry has responded to the challenge of ghost particles with a new analysis method that can be used to produce reliable results, whether analyzing new or used lubricating fluids.
In late 2010, ASTM International Subcommittee D02.96 published ASTM D7647, Standard Test Method for Automatic Particle Counting of Lubricating and Hydraulic Fluids Using Dilution Techniques to Eliminate the Contribution of Water and Interfering Soft Particles by Light Extinction. In this method, samples are diluted using certain solvents as diluents to overcome the false positives caused by certain additives and oxidation residues. Once results are obtained, calculations are performed to adjust the results, accounting for the dilution ratio. Therefore, the data from D7647 can be compared to data from traditional optical particle count methods.
Building upon preceding studies, the authors experiment proved that certain lubricant components create a ghost particle response in various particle counting tests.
While it was not originally a goal to compare particle counting techniques, the data also showed that optical particle counters are more susceptible to this effect than pore-blockage particle counters. This is important because many equipment operators use optical particle counting tests to evaluate new and in-service hydraulic fluid cleanliness to monitor the condition of their equipment and fluid.
Does this mean that particle count data is flawed and useless? No, to the contrary, it just means that end users need to be aware of the ghost effect if they have difficulty obtaining the desired cleanliness level with new hydraulic fluids.
Lubricating fluid cleanliness is still extremely important to the continued health of operating systems, and the ASTM D7647 analysis method can be used to overcome the ghost effect.
John Sander, vice president of technology at Lubrication Engineers in Wichita, Kan., has 26 years of lubricant industry experience. An STLE Certified Lubrication Specialist and past winner of the NLGI Authors Award, he has authored or co-authored more than 20 papers and one book chapter. Email him at j.sander@le-inc.com.