This article is the last in our year-long series on the most important components of lubrication certification exams administered by the Society of Tribologists and Lubrication Engineers (STLE) and the International Council for Machinery Lubrication (ICM). However, even if you’re not pursuing certification, having a thorough understanding of fluid-conditioning principles is crucial in dealing with rotating equipment. Here, we examine the two most destructive contaminants: particulates and water.
It’s been said that 75% of equipment failure in circulated systems is caused by contamination. The major cause of equipment wear is three-body abrasive wear caused by particles—according to some estimates, up to two-thirds of the wear in equipment. And, it’s what you don’t see that causes the major problems. The smallest particle size that can be seen with the naked eye is 40 microns. Unfortunately, the most destructive particles are much smaller in size: you just can’t see them.
In dealing with equipment cleanliness, one must be as proactive as possible. It costs at least 10 times more to remove particles than to keep them from entering a system. The following are some of the ways to be proactive:
General filtration principles
Even if your operations are being proactive in fluid cleanliness, it’s still necessary to have proper filtration to achieve targeted cleanliness levels. Outside particulate ingression is usually not totally controlled, and wear metals from operation need to be removed. The most effective filters have the following characteristics:
The following are key factors in filter selection:
Filter sizing is extremely important. Pressure drop determines the filter’s size. The following variables determine pressure drop:
Measuring filter performance…
Filter performance should be based on the absolute filter rating which is determined in a laboratory with the Multi-Pass Filter Performance Test. Dirt in milligrams per liter is introduced in a test fluid at a constant rate. The fluid is circulated at a constant or variable rate through a test filter. The test concludes when the terminal pressure drop of the filter is reached. This is the point when the filter manufacturer designates the filter as no longer operable and needing to be changed. The filtration efficiency of the test filter is expressed as the filtration ratio.
For example, a filter with a beta ratio of ß6=200 indicates that for every 200 particles greater than six microns in size entering the test filter one will pass though. Therefore, 199 will be captured. The efficiency of the filter is calculated as ß-1/ß x 100. The efficiency of a filter with a beta ratio of 200 is 99.5%.
ISO Cleanliness Code…
Fluid cleanliness is expressed as a three-number code according to ISO 4406. The code is expressed as all particles > 4µ[c], > 6µ[c] and > 14µ[c]. The numbers are obtained from the ISO 4406 Chart in Table I.
As an example, consider a fluid where the following particles per milliliter of fluid were measured:
Find the range number that expresses the number of particles per milliliter. For instance, 7500 particles are found at the range number of 20 where the range is 5000 to 10,000 particles. Notice that for every increase in range number, the number of particles can double. Thus, even a moderate increase in the range number can result in a large introduction of particles.
Target equipment cleanliness code…
To extend equipment life, the correct cleanliness must be maintained. Different equipment components require different cleanliness levels. The clearances of the equipment determine the level of cleanliness. For example, a servo valve in a hydraulic system requires cleaner oil than a gear pump. Many tables are available as a guide to equipment cleanliness requirements. For purposes of this article, we use one from from Eaton (Table II).
Utilizing Table II, let’s determine the cleanliness level required for a variable inline axial piston pump operating at a pressure of 3500 psig with a servo valve. The most sensitive component in the system determines the cleanliness code selected. The piston pump requires a fluid cleanliness of 16/14/12 while the servo valve requires a cleanliness of 15/13/10. Thus, the cleanliness required for this hydraulic system is 15/13/10.
Filter location placement is a key variable in achieving target cleanliness. Usually a combination of locations is employed. Work with your filter manufacturer to determine the optimum combination of filter types and location to achieve your desired cleanliness level.
Figure 1 (from Pall Filters) illustrates the most common filter-placement locations.
The second most destructive contaminant—water—is introduced in the system the following ways:
Water contamination manifests itself as follows:
Small amounts of water can reduce rolling-element bearing life via hydrogen blistering and embrittlement. Aim for 200 ppm or less; close to 100 ppm is better. Journal bearings in turbines should be maintained not to exceed 200 ppm.
The three water forms are dissolved, emulsified and free. Dissolved water usually isn’t a problem, unless the saturation point is reached at a lower temperature, resulting in free water which can become emulsified and can only be removed though vacuum dehydration or passing hot air through a special type of filter or system. The most destructive form is emulsified water, which can be removed through centrifuging, coalescing filters and vacuum dehydration. Free water is the easiest to remove: Reservoirs and sumps should be routinely drained of free water, which, if circulated, can become emulsified.
The following proactive practices can help prevent water from entering a system:
The following are ways to remove and minimize effects of water ingression in a system:
This entire five-installment series of articles was designed to help prepare you for the various certification exams through the STLE and ICML. Be aware, though, that “help prepare” is the operative term. Reading this series of articles will be a step in the right direction, but that alone won’t be enough. Much greater preparation—through significant self-study and formal training—will be required to seriously pursue certification. For complete details on STLE and ICML certification, go to: www.stle.org and www.lubecouncil.org. LMT