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An oil is a bundle of performance properties and its requirements vary by application (Figure 1). Degraded properties compromise the lubricant’s ability to minimize friction, wear and corrosion, thus placing machinery reliability at risk. A lubricant requires properly designed and executed maintenance responses.
Reasons to change
Oil doesn’t last forever. It must be changed or otherwise maintained when it loses the properties that protect the machine, the application and the environment. Oxidative, thermal and hydrolytic degradation permanently change the base oil’s chemical and physical properties, thus altering the lubricant’s performance. In other cases, the additive package becomes depleted or the oil becomes contaminated with material that can’t be removed.
Base oil degradation
Oxidation occurs when oxygen reacts with the base oil, typically a hydrocarbon. Oxidized hydrocarbon molecules are transformed into acid and sludge. The degree to which oil is aerated and the presence of water and reactive metals, such as iron and copper, influences the oxidation rate. Oxidation-inhibiting additives sacrifice themselves to protect the base oil from oxidation.
Thermal degradation occurs when oil comes into contact with very hot machine surfaces or compressed bubbles, such as in hydraulic systems. Thermal failure leaves carbon-rich sludge and deposits. Thermal failure doesn’t produce acid, but it generates deposits that affect the oil’s performance. In some cases, the hydrocarbon chain is cracked into smaller units, which reduces the average molecular weight and its viscosity.
Hydrolysis is the reaction of the oil with water, which permanently modifies its molecular structure. Ester-based lubricating oils, including dibasic acid ester, polyol ester and phosphate ester, are most susceptible to hydrolysis. When exposed to water, esters readily hydrolyze back to the alcohols and acids from which they were synthesized. Many lubricants and hydraulic fluids use esters as the primary base oil component or as a co-base oil to improve the solubility and seal performance of highly refined mineral or synthetic oils.
Oil additives can enhance desirable performance properties, suppress undesirable properties or impart new properties. With use, additives become depleted and must be restored by oil change, sweetening with a partial drain-and-fill or by lubricant reclamation, which ostensibly restores the oil to like-new condition. The rate at which additives deplete depends on its type and environmental conditions, particularly temperature and presence of water. Some additives separate from the base oil at low temperatures. Many additives are susceptible to hydrolysis in the presence of water.
One way to extend life is to select lubes formulated using premium base oils, premium additives or both. The American Petroleum Institute’s (API) standard classification for base oils, called groupings, is intended to reflect oil quality. Groups I, II and III are refined mineral base oils. Group IV covers synthesized hydrocarbon base oils, such as polyalphaolefin (PAO), the most common example.
Group V includes everything that’s not in Groups I, II, III or IV, and includes dibasic acid ester, polyol ester, poly glycol, phosphate ester and a host of other base oils. With a range of base oils included in Group V, API set no specific requirements for this group except that the oil can’t be included in Groups I-IV.
Viscosity index (VI) relates oil viscosity to temperature. A lube with a high viscosity index is favored because it functions across a greater range of temperatures. Generally, high-VI base oil exhibits lower viscosity at cold start and higher viscosity at full operating temperature than low-VI base oils. API Group I has the lowest viscosity index, Group IV the greatest, with Groups II and III in between. Group IV (PAO) base oils generally have a higher viscosity index than Groups I, II or III. The viscosity index in Group V varies.
A mineral base oil that’s highly refined to reduce or eliminate unsaturated molecules resists oxidation and thermal degradation more effectively than a less-refined base oil. This is why the oxidative and thermal life of Group III base oil is superior to that of Group II, which is superior to that of Group I. Refining, however, has a cost. Highly unsaturated base oils don’t dissolve additives effectively and they tend to shrink elastomeric seals. Many Group II, III and IV base oils are formulated with co-base oil, such as diester or polyol ester to improve additive solubility and offset seal shrinkage problems.
Most mineral base oils contain sulfur. Group I has more than Group II or III. Group IV PAO is sulfur-free. Sulfur improves oxidation resistance and natural lubricity. Formulators prefer to start with low-sulfur base oil and add sulfur for the concentration and chemical form they believe is appropriate for the application.
Users often assume premium lubricant means a synthetic base oil. Not every synthetic base oil possesses each advantageous property and, moreover, it might not be required. For instance, Group IV base oil isn’t required for a machine that operates 24 hours a day, seven days a week at a constant temperature. Likewise, there are detrimental aspects associated with synthetic base oil that must be considered in the decision. If you can’t make the decision yourself, seek expert advice.
Base oil is only one relevant variable and additives aren’t all created equal. Some are better than others and these cost more. Many suppliers use additives to formulate specialty lubricants. Naturally, expensive additives, small batches, special sales and application engineering services add cost.
Specially formulated lubes needn’t always use synthetic base oil or highly refined mineral oils. The finished lubricant’s performance depends collectively on the base oil, additives and formulation. It’s important to understand the required performance properties for the application and to match the performance characteristics of the finished lubricant accordingly.
Contamination control is the easiest and most widely used method for extending lubricant life. Moreover, it’s good for the machine. Contamination includes foreign and unwanted matter and energy.
Heat is a lubricant’s worst enemy. The bulk oil temperature in the tank or sump influences a lubricant’s oxidative life following Arhenius’ Law -- oxidative life is halved for every 10°C increase in temperature. The inability to maintain a cool temperature might favor using a premium lubricant. Transient contact with hot surfaces can result in thermal degradation.
Dissolved or entrained air influences the rates of oxidation and thermal degradation. The relationship is approximately linear -- doubling the air concentration roughly doubles the oxidation rate. Hot, compressed bubbles cause thermal failure, especially in high-pressure hydraulic machines. Interfacial tension between oil and air bubbles determines the ease of air entrainment. If interfacial tension is high, air bubbles dissipate and separate readily. Tank design, the lubricant delivery mechanism and other factors influence the air contamination level.
Moisture degrades ester base oils, reduces many additives to acid or sludge, and promotes base oil oxidation, especially in the presence of catalytic metals. Water enters the machine through contaminated oil, breathers, vents and shaft seals. Risks are highest in humid environments in which machines operate intermittently and are subjected to wash-down. The best way to control water contamination is to keep it out using premium seals, desiccant or other water-excluding breathers. What can’t be excluded can be removed by a dehydrating method.
Suspended solids can increase air entrainment, which indirectly increases the oxidation rate. However, iron, copper and some other particles catalyze oxidation. Water then reacts with the metal, forming peroxides and free radicals that cause oxidation. The selection of a particle-removal device is application specific.
There are three reconditioning options: remove the contaminants, refresh the additive system, or reconstruct the additive system. Contaminant removal uses filters to capture particles and moisture. Acid, glycol, fuel and other chemical contaminants are more difficult to remove. Contaminant removal often is limited to large, circulating oil systems.
The purpose for changing oil in a splash-lubricated gearbox is to eliminate contaminants only when the lubricant is physically and chemically fit for service. But, this isn’t very effective. Shutting down a machine allows debris to settle. A drain valve set just above the sump bottom can’t remove residual oil and concentrated contaminants. On restart, the settled contaminants become suspended, despite the expense to complete the oil change. A more effective strategy is to install quick-connect fittings so that a portable decontamination rig or filter cart can clean the oil while the machine is operating (Figure 3). This converts a shutdown task into a runtime activity.
Refreshing the additive system typically involves replacing a percentage of the old oil with fresh oil. Assuming the base oil hasn’t been degraded or contaminated, this method extends the drain cycle. However, if the base oil has been damaged, this method is analogous to sending a healthy person into a roomful of sick people with the expectation that good health is contagious.
Reconstructing the additive system, sometimes called reclamation, involves using a combination of heat and mechanical shear to reblend the lubricant with an additive system that complements the old oil. Assessing the additive condition is tricky. The reconstruction process should be simulated in the laboratory and tested before and after applying it to the in-service oil. These steps ensure efficacy, but add cost.
If you change oil on an interval of time, distance or cycles, be aware that overextending the interval places the machine at risk for wear and failure. If you change oil on the basis of oil analysis, your test slate should reflect the factors that represent a decline in performance properties: appropriate limits, sampling intervals and methods; hardware modifications and staff training.
First, wash the equipment down and inspect it visually. Fluorescent dyes and UV lights make short work of leak detection. Log each leak, assign it a number, tag it so it can be found later, rate its severity, assess its cause and formulate a corrective strategy. Then, decide which to correct and which to ignore, based on severity and the ease with which it can be corrected. Leak elimination isn’t always easy. Containment and guttering can be expensive to install and maintain, and might be only marginally effective. Where possible, leakage elimination is preferred.
Any lubricant will eventually degrade to a point of no return. The Environmental Protection Agency (EPA) regulates used oil disposal under 40 CFR 279. Other codes that also might be relevant are listed in the sidebar. By law, a used oil generator is a person, by site, that causes oil to become subject to regulation. Facilities that need to dispose of used oil should be aware of containment and disposal requirements. The following are general guidelines, but don’t include every regulation associated with used oil.
Used oil generators should comply with Spill Prevention, Control and Countermeasures (40 CFR 279.112) and Underground Storage Tanks (40 CFR 280) in addition to the following regulations.
Storage tanks and containers must comply with 40 CFR 264 and 40 CFR 265. Containers and aboveground tanks that store used oil must have no severe rusting, be structurally sound and should not be leaking. Containers and aboveground tanks, as well as hoses and pipes in used oil service at generator facilities, must be marked clearly with the words “Used Oil.” If the generator detects a leak or spill that isn’t subject to underground storage regulations, the following cleanup steps are mandatory:
- Stop the release.
- Contain the released used oil.
- Clean up and manage the released used oil and other materials properly.
- If necessary, repair or replace leaking storage containers or tanks.
As long as the oil isn’t considered waste oil, generators have two options: dispose of used oils in onsite facilities that comply with government regulations or contract with a disposal company to pick it up for recycling or delivery to a landfill.
Used oil's energy makes it a good fuel either alone or combined with other fuels. One gal. of used oil releases about 140,000 BTU. Facilities that burn used oil on-site should follow regulations defined in 40 CFR 279.
For facilities that don’t generate a lot of used oil, the easiest option is to contract with licensed haulers. These companies sell used oil to reprocessors that reclaim the bulk oil. A third-party contractor should have a valid EPA ID number or a state or local permit for transporting used oil offsite and must comply with RCRA's transport vehicle requirements.
In certain situations, self-transportation without an EPA identification number is allowed. Generators might transport oil from a site to a collection center as long as the generator or its employee owns the vehicle that transports no more than 55 gal. at any time and the collection center is registered, licensed, permitted or recognized by a state/county/municipal government to manage used oil. Also, used oil can be transported without an EPA identification number if it’s reclaimed under a contractual agreement and returned to the generator for use as a lubricant, cutting oil or coolant.
Effective lubricant life-cycle management begins with selecting a suitable lubricant, operating context and environment. Managing temperature, moisture and contaminants can extend lubricant life and oil analysis can provide the information to make the oil change decision. Regardless of diligence, all lubricants will eventually require changing. Be certain to drain used oil completely and dispose of it in accordance with applicable regulations.
Drew D. Troyer is vice president and senior technical consultant and Sabrin Gebarin is project engineer for Noria Corp. in Tulsa, Okla. Contact Troyer at [email protected] and (918) 749-1400 x 106. Contact Gebarin at [email protected] and (918) 749-1400 x 131.