How To Decide Between Synthetic And Petroleum-Based Lubricants
All too often, purchase cost is the deciding factor when buying lubricants.
By Harold Tucker
All too often, purchase cost is the deciding factor when buying lubricants. While the initial cost of synthetic lubricants, often five-times that of petroleum-based products, is certainly a factor, it is just as certainly not the only factor meriting consideration. Type of equipment, severity of service, final product, and the geographical location of a plant should influence the maintenance supervisor's decision on whether to use synthetic or petroleum-based lubricants.
The key difference between petroleum-based and synthetic lubricants is the base fluid. Petroleum-based lubricants are extracted from natural crude oil, and must be refined, desalted, dewaxed, and distilled from crude feedstock. Their base fluids are made up of a great variety of naturally occurring hydrocarbons arranged in many molecular configurations. Families of hydrocarbons are segregated throughout the refining of crude oil.
Synthetic lubricants, which have been in existence for more than half of the roughly 100 yr during which humans have used petroleum-based lubricants, are developed in research laboratories. When something is synthetic, it has been created through the combination of separate parts into a whole (synthesized). As a chemical term, synthetic refers to a compound formed through a chemical reaction that did not occur naturally. The entire molecular structure of a synthetic lubricant is determined by tailored, tightly controlled chemical reactions; hence, the higher cost of synthetic lubricants.
Different synthetic lubricants are made from different base fluids. (The table opposite compares the properties of four base stocks.) Here are some facts about three common synthetic base fluids.
Organic esters are made up of dibasic acid esters or polyol esters. Both can be used at temperatures exceeding 400 F. Dibasic acids are particularly effective as base fluids for compressor lubricants, while polyol esters are used in lubricants for industrial chains and gas turbines.
Phosphate esters serve as the base fluid for many lubricants used where there is a risk of fire.
Synthetic hydrocarbon fluids (SHF) are manufactured by combining hydrocarbons of butylene or ethylene to create a polyalphaolefin (PAO) base fluid. SHF bases are used in numerous applications because of their superior temperature ranges and lubricating properties.
Advantages of synthetics
There is no question that synthetic lubricants have demonstrated advantages over petroleum-based products in several areas.
- Extended oil drains. Due to the thermal and oxidative stability of synthetic lubricants, oil drain intervals can be greatly extended.
- Energy savings. Synthetic lubricants' viscosity index, friction properties, and heat transfer characteristics result in measurably less horsepower required to drive equipment in both hot and cold environments.
- Fire resistance. Because the high flash point of synthetic lubricants reduces the risk of fire, some insurance companies charge lower premiums for plants using synthetic lubricants.
- Fewer deposits. Because of their oxidative stability at high temperatures, synthetics leave very few deposits. The resulting decrease in equipment wear often results in fewer repairs.
- Higher operating speeds. The excellent heat transfer characteristics of synthetic lubricants may allow equipment to be run at higher speeds, resulting in more output.
Best synthetic lube applications
No matter what size the plant, there are several applications for which synthetic lubricants are highly recommended.
Air compressor duty. Nearly every air compressor manufacturer recommends draining a synthetic lubricant after 8000 hr, as opposed to 1000 hr with a petroleum-based lubricant. Aside from drain intervals, air compressors require a lubricant which provides excellent oxidation resistance, because they compress air at very high temperatures, often 250 F and higher.
Extreme temperature duty. Synthetic lubricants have superior viscosity-to-temperature characteristics that make them especially suitable for use in refrigerators, ovens, or plants in hot or cold climates.
Continuous duty. When equipment shutdowns for oil changes absolutely must be minimized, synthetics are the obvious choice because they need to be changed so infrequently. Plants that run an assembly line-type process -- meaning if one piece of equipment is down, all production stops -- may find the use of synthetic lubricants most economical.
Best petroleum lube applications
Here are several applications where petroleum-based lubricants shine.
High consumption applications. The classic example is equipment that has accumulated very many operating hours and become worn, resulting in increased lubricant consumption. Because of their low initial cost, petroleum-based lubricants are the much more economical option in high-consumption applications.
Once-through applications. Petroleum-based lubricants are always the economical option in equipment that has no reservoir -- applications in which oil sprays into the cylinder, lubricates the piston, and then blows down line as in gas compressors.
Product contamination applications. In a natural gas liquid processing plant, neither synthetic nor petroleum-based lubricants can completely escape contamination by the final product when it enters the crankcase. The resulting need for frequent oil changes points to petroleum-based lubricants because of their lesser unit cost.
Very dirty or dusty environments. Unlike automotive motor oils that are designed to suspend dirt and deposits, industrial oils are designed to clean, cool, and seal the equipment they are lubricating. Neither synthetic nor petroleum-based lubricants are designed to suspend dirt and dust blown into the equipment from the exterior. Therefore, petroleum-based lubricants are the cost-effective choice for equipment operated in extremely dirty or dusty environments.
Oil analysis improves maintenance
Oil analysis should be a key tool in any preventive maintenance program, whether the plan incorporates synthetic or petroleum-based lubricants. There are two main components of oil analysis: spectrometric analysis and physical tests.
Spectrometric analysis is used to measure the levels of metals in the oil, and aids in determining equipment condition. With this information, plant maintenance supervisors can plan downtime for inspection and/or repairs.
Equipment interior wear conditions and resulting oil contamination should be monitored, regardless of the lubricant used. It's also important to use spectrometric analysis to check the condition of all equipment scheduled to be switched to synthetic lubricants before making the switch.
Results of physical tests help determine how long oil can remain in equipment.
Viscosity. The internal resistance to flow of a lubricant is perhaps the key determining factor in whether an oil needs to be changed.
Particle count. A high particle count when measuring solids in oil, such as fuel soot and sludge, often indicates that a new oil filter is needed.
Nitration. An indication of contaminants from accumulation of combustion byproducts, such as abnormal blow-by, is also a signal for a change.
The important point to remember is that more factors than just purchase price determine the net cost of using a particular lubricant. Type of application and plant size are the primary factors in determining whether a synthetic or petroleum-based lubricant is most cost-effective.
-- Edited by Gary Weidner, Senior Editor, 847-390-2689, firstname.lastname@example.org
The Phillips 66 Technical Service Department is available to answer questions regarding this article. They can be reached at 800-766-0050. Related articles previously published include: "Lubricant Selection Vital to Maintenance Solutions" (PE, August 1995, p 64, File 8020), "Interchangeable Industrial Lubricants Chart" (PE, October 1995, p 60, File 8010), and "Exclusive Guide to Synthetic Lubricants Chart" (PE, May 1997, p 66, File 8010).
Purchase price shouldn't be the sole deciding factor in choosing a lubricant.
Synthetics are often cheaper to use than petroleum-based lubricants, even though they are priced much higher.
Petroleum-based lubes remain the best choice for certain applications.
Oil drain savings: an example
Consider the lubrication of two identical pieces of equipment in a plastic extrusion plant over an 8000-hr period. Each machine has a 20-gal. sump. (See table below) Machine A is lubricated with 20 gal. of synthetic lubricant, which is drained and replaced at a recommended interval of 8000 hr. Lubricant cost is $25/gal. for a total 8000-hr expenditure of $500 (20 gal. x $25 x 1 oil change). Machine B is lubricated with 20 gal. of petroleum-based lubricant, which is drained and replaced at factory-recommended 1000-hr intervals. The cost of the petroleum-based lubricant is $5/gal. The total 8000-hr expenditure is $800 (20 gal. x $5 x 8 oil changes). Now consider the costs for labor and oil disposal associated with both machines. The oil in Machine A is changed once in an 8000-hr time span. If the oil change procedure requires one man-hour of labor, the plant pays, say, $50 for the hour of labor and $10 for disposal of the 20 gal. of oil (20 gal. x disposal cost of $0.50/gal.). The oil in Machine B is changed every 1000 hr, or 8 times within the 8000 hr that Machine A is running on synthetic lubricant. If each of the 8 oil changes requires an hour's labor, the cost is $400 (8-hr labor x $50/hr). Disposal of the 160 gal. of oil resulting from the drains costs $80 (160 gal. x $0.50/gal. disposal cost). In addition, the plant may have lost revenue due to down time during oil changes -- 8-times as much for Machine B as for Machine A. Plant size plays an important role. For a single machine such as discussed in the example, it may be very difficult to justify the up-front investment and carrying costs of up to $1375 (55 gal. x $25/gal.) for a 55-gal. drum of synthetic lubricant alone. Even at 8000 hr/oil change and three-shift operation, the first 20 gal. would take nearly a year to be consumed. Compare that with a total cost of $1280 (including labor and disposal) to lubricate the piece of equipment for 8000 hr with petroleum-based lubricant. For a larger number of machines, the situation reverses. The savings from using synthetic lubricant for 100 machines far outweigh the extra lubricant cost.
This breakdown of costs for maintaining two identical items of equipment in a plastics extrusion plant illustrates savings from using a synthetic lubricant. However, the considerable up-front cost of synthetics (as much as $1375 for a 55-gal. drum) means that plant size (number of machines) also plays a role in choosing.
Machine A Machine B
Parameter Synthetic Petroleum-based
A .Sump capacity 20 gal. 20 gal.
B. Oil cost/gal. $25 $5
C. Oil changes/8000 hr 1 8
D. Lubricant cost/8000 hr (A x B x C) $500 $800
E. Labor hr/8000 hr (at 1 hr/oil change) 1 8
F. Labor cost at $50/hr ($50 x E) $50 $400
G. Oil disposed/8000 hr (A x C) 20 gal. 160 gal.
H. Lubricant disposal cost (at $0.50/gal.) $10 $80
Total expenditure (D + F + H) $560 $1280
Note: It should be acknowledged that the lubricant disposed of from Machine B could perhaps be burned for heat or removed at low cost by a re-refiner, but the difference in total expenditure would be minor.
Base fluid properties
Property Petroleum oil Organic esters Phosphate esters PAO
Lubricity Good Excellent Fair Good
Elastomer compatibility Good Fair Poor Excellent
Additive response Good Excellent Good Fair
Volatility Fair Excellent Fair Excellent
Oxidation stability Fair Excellent Fair Excellent
Compatibility with petroleum Excellent Excellent Good Excellent
Temperature range Fair Excellent Fair Excellent
Fire resistance Poor Good Excellent Fair