Oxidation
Oxygen will chemically combine with lubrication oil and its additives to create a variety of oxygenated byproducts. Oxidation raises oil acidity, viscosity, and surface deposits, all of which can shorten bearing life. An open container of oil exposed to atmosphere will absorb about 10% Oxygen.
Oil additives are usually the first to be impacted by oxidation followed by the base stock. The rate at which oxidation occurs in lubricating oils is primarily dependent on the oil, temperature, moisture content, and lubrication environment.
Operating temperature has a tremendous impact on oil oxidation. Figure 1 illustrates the effect of temperature on a variety of oils. For every 10oC (18oF) rise the rate of oxidation doubles. Oil operating at 50oC will last 30 times longer that oil operating at 100oC. The presence of oxidation inhibitors will slow the process. However, these will eventually be consumed and the oxidation will continue unabated.
Moisture
Moisture becomes entrained in the oil either directly through contact with free water or through absorbsion. Water molecules are polar, that is to say they have positive and negatively charged particles that are readily available for bonding with other molecules. Many of the additives and particulates in lubrication oil are polar as well. Water molecules are attracted, and become attached to these available polar molecules. The amount of moisture that oil can absorb is dependent on the amount of additives, as well as temperature and pressure. Once moisture has attached itself to all of the available polar molecules, the oil can be said to be saturated. Beyond this point water will start to collect as condensation of free water in the bottom of the sump.
Free water may enter the sump from leaking heat exchangers, steam from adjacent quench connections, wash downs, and condensation within the bearing housing during temperature cycles
Moisture in any form accelerates oxidation. It reacts with and promotes the breakdown of oil additives, and also forms acids. It promotes the localized fatigue of bearing surfaces, and the oxidation of unprotected metal surfaces within the bearing frame, both of which add to the particulate load within the oil. Figure two shows the life extension that can be achieved by reducing moisture content. For example, oil with 500 ppm moisture content would experience a 3x life increase if the moisture content were reduced to 65ppm.
Particulates
Particulates not only damage the bearings directly, but indirectly as well. Many particulate molecules are also polar and therefore increase the moisture load carrying capability of the oil. In addition Iron and Copper act as catalysts and react with the oil to form acids.
Particulates not only damage the bearings directly, but indirectly as well. Many particulate molecules are also polar and therefore increase the moisture load carrying capability of the oil. In addition Iron and Copper act as catalysts and react with the oil to form acids.
The amount of direct damage that particulates have on the actual bearing surfaces is dependent on the particulate concentration, hardness, size and shape. On the Mohs’s hardness scale1, tool steel is 7.0; unfortunately, dirt, dust, and grinding abrasives, to name a few common contaminants, all have Alumina and Silica with a hardness of 7-9 Mohs. These particles have sharp irregular shapes making them especially abrasive and are often small enough to enter the clearance areas of the bearings. If all is equal in terms of the types of solid contaminants present, then bearing life is certainly dependent on the quantity of solids. This makes particulate counts an acceptable indicator when conducting oil analysis.
Improving Lubrication Quality
Establish Objectives
Set some targets for your lubrication monitoring program. What these targets should be is very dependent on the application, oil selected, and the environment. Targets can later be adjusted based on measurements and operational results. A reasonable start would be 200 ppm of moisture as a maximum and an oil cleanliness standard based on ISO 4406:99 cleanliness codes. (Fig 3)
Measure
The old saying “If you want to improve it, measure it” applies. Institute an ongoing lubrication oil monitoring program. Monitoring and testing equipment and services are readily available, and are a cost effective alternative to increased repair cycles.
Protect
Oil can't become contaminated if the contaminants can't get in.
- Store oil in sealed containers
- Keep oil inventories low so as to keep turnover frequent and oil fresh
- Store oil drums in drum racks where they won't collect water
- Make sure the inside of bearing frames are coated and free of rust
- Work on pumps in a clean environment
- Use labyrinth, or magnetic oil seals, and non vented oilers
- Purchase quality oils
- Lower bearing operating temperatures where possible
Note that many of the contaminant and deterioration factors are interdependent. Increased particulates will increase moisture retention which, in turn, increases oxidation. Increased oxidation, or increased moisture, can increase particulate loading…etc. Unchecked, these factors will compound to work against the life of the equipment. Conversely, reducing any of these factors can compound to improve equipment life. Lubrication related problems can be a silent machinery killer. A high moisture or oxygen content is not always visible. A good proactive lubrication oil program will extend the life of equipment throughout the plant.
1 A scale of 1-10 used in the measure of mineral hardness with Babbitt being 1.0 and Diamond being 10.
Acknowledgements
Jim Fitch, “A much Closer Look at Particle Contamination”. Practicing Oil Analysis Magazine. September 2005
Jim Fitch, Simeon Jaggernauth, “Moisture – The Second Most Destructive Lubricant Contaminate, and its Effect on Bearing Life” 2000 Noria Corporation
Brad Rake, Ray Terrell, “Best Practices in the Application and Maintenance of Oil Sump Lubrication of Horizontal Process Pumps”. Proceedings 19th International Pump Users Symposium, Feb 2002
J.M. Weiksner, “Synchronizing Oil Cleanliness Standards to Machine Working Clearances” 2006 Noria Corporation
No Author, “Understanding Lube Oil and Its Job Functions”, 2002-2005 Gulf Coast Filters, Inc
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