Is It Time To Change Your Oil?
EP Editorial Staff | March 1, 2022
Extend oil-drain intervals to reduce operating costs.
By Mark Barnes, PhD CMRP, Des-Case Corp.
With the ever-increasing cost of lubricants, industrial and mobile heavy-equipment operators are increasingly looking at extending oil-drain intervals as a way of reducing operating costs. Oil changes not only come with the cost of new oil, but also labor and disposal costs, and recycling fees, all of which can add up to several times the base cost of the new oil. Add to that the strong focus many companies have placed on environmental sustainability and lowering the overall carbon footprint and it’s no wonder that the pressure is on to extend oil-drain intervals.
Extending oil drains requires a systematic approach based on data, science, and a solid understanding of how to optimize the life of a lubricant. Before intervals can be extended, we need to establish a common fact: Lubricants do not last forever. However, with careful consideration, oil-drain intervals can often be increased by as much as two to ten times typical OEM recommendations, provided certain considerations are put in place.
Oil-Drain Extension Factors
First to consider is the type and quality of lubricant in use. While it may be tempting to look at a lubricant as a commodity that should be purchased based on price alone, simply selecting a “better” lubricant—either a premium-quality mineral or, where it makes sense, a synthetic lubricant—is a key first step. Using a premium lubricant, particularly when extreme operating conditions are present, can often extend intervals by two to five times. While it’s true that premium oils typically cost more upfront, the lifecycle cost can often be justified based purely on oil-life extension, without consideration for any of the enhanced lubricating properties premium lubricants may provide. However, a “better,” more-expensive lubricant is not always a silver bullet.
We need to understand how and why lubricants degrade and take steps to eliminate factors that shorten oil life. Of particular importance is the impact of temperature. While there are several factors that determine how temperatures affect oil life, a good rule of thumb is to consider that oil life is cut in half for every 18 F (10 C) increase in temperature above 140 F (60 C) (Fig. 1). As an example, running a standard mineral AW hydraulic fluid at 122 F (50 C) versus 158 F (70 C) can increase oil life by as much as three times.
Likewise, don’t overlook the impact that contamination has on oil life. Contaminants, such as particles, moisture, and aeration, all serve to prematurely degrade fluids, particularly catalytic metals and free and emulsified water. Simply excluding contaminants through proper breather and seal management, as well as pre-filtering new oil prior to use, can help extend oil-drain intervals. Real-time offline filtration to supplement inline (full flow and return line) filters can easily double or triple oil life in many circumstances.
Having optimized lubricant selection, controlled oil temperature to a reasonable range, and implemented contamination-control measures, the final step to effective condition-based oil changes is to use oil analysis to trend lubricant health. In doing so, we need to make sure that we are measuring the correct oil properties. Simply waiting for an oil to fall out of grade due to an increase or decrease in viscosity is a surefire way to machine failure. Oil viscosity is one of the last “oil health” properties to change and typically only occurs during the last 5% to 10% of an oil’s life when sludge, varnish, acids, and other deposits have already started to form.
Fig. 2 shows the typical oxidative degradation pathway of a hydrocarbon oil. The first thing to notice is that the oil oxidation is not linear. This is because oil oxidation is an autocatalytic reaction, i.e., once oxidation starts to occur, the by-products that are formed catalyze the reaction, increasing the effective oxidation rate.
To effectively change oil, based on condition, a series of “oil health” tests should be conducted every two to three months and the results compared to a baseline sample of new oil. The importance of having a baseline cannot be overstated. Oil formulations vary from time-to-time and from brand to brand. As such, a sample of new oil in use should be submitted to the lab as a “new oil reference” that is compared with the physical and chemical properties of the in-service oil.
Testing oil health
For industrial fluids, the most common oil-health tests to perform include:
Viscosity: While viscosity, by itself, should never be used to time an oil change, any shift in viscosity can indicate the addition of the wrong oil or contamination.
Elemental analysis: Most oil-analysis labs report elements such as zinc (Zn), phosphorus (P), and calcium (Ca), which are used in certain oil additives, depending on oil type and formulation. Trending additives elements to measure oil life should be done judiciously since these elements tend to remain in the oil, even when the additive is chemically depleted.
Acid number: As a hydrocarbon oil degrades, the byproducts, such as aldehydes, ketones, and peroxides, can chemically combine to form organic acids. Left unchecked, these acids can start to cause corrosion and serve as a catalyst to further oxidation. Acid number can be used to determine the degree of base oil and/or additive degradation.
FTIR (oxidation and nitration): FTIR can provide a “molecular fingerprint” of an oil. Measuring FTIR oxidation and nitration rates can help determine the presence (or absence) of degradation byproducts due to thermal or oxidative failure.
RULER test: Commonly used for turbine and hydraulic fluids, this test uses cyclic voltammetry to directly measure the remaining useful life of the oil, based on antioxidant levels. The test is very sensitive to having the correct new oil reference.
Rotating Pressure Vessel Oxidation Test (RPVOT): Used almost exclusively for turbine oils, this uses “oxygen uptake” to test an oil’s susceptibility to oxidation. The RPVOT is becoming less favored.
To schedule a condition-based oil change, all the above tests, perhaps with the exclusion of RPVOT, should be trended quarterly. The objective is to try to ascertain where the in-service falls on the oxidation curve depicted in Fig. 2. Condemning limits should be established based on industry standards and oil manufacturer recommendations. Of particular value are the RULER results because the antioxidant additives serve to sacrificially protect the hydrocarbon base oil from chemical oxidation. Once the antioxidant package depletes to approximately 20% to 25% of new oil, they are not able to offer sufficient protection. This is typically the “inflection point” where incipient oxidation starts to become exponential. Condition-based oil changes don’t always make sense. For small sump systems or where the ability to access and shut down equipment for an oil change is limited, it may still make sense to perform oil changes on a calendar cycle. However, even in this case, a small subset of equipment could be tested to scientifically determine the preferred interval before applying that interval to a large cross section of similar assets.
The push to preserve resources and lower environmental footprints is not going away. If your company wants to increase environmental stewardship while trying to reduce maintenance costs, consider switching to condition-based oil changes. EP
Mark Barnes, CMRP, is Senior Vice President at Des-Case Corp., Goodlettsville, TN (descase.com). He has 21 years of experience in lubrication management, oil analysis, and contamination control.