Suppliers have long touted energy savings as a good reason for switching from mineral oils to synthetics. Are those savings real?
By Ray Thibault, CLS, OMA I, OMA II, MLT, MLT II, MLA II, MLA III, Contributing Editor
Synthetics offer users a number of clear advantages over mineral-based lubricants, including: better film strength (to reduce wear); the ability to operate at high temperatures (resulting in lower oxidation and longer drain intervals); good flowability at low temperatures (because no wax is present); and high viscosity indexes (allowing consolidation of lubricants over various viscosity ranges). When it comes to energy savings related to the use of synthetics, however, not all claims can be substantiated. That could be a result of poor measurement techniques.
Making the case for a switch from mineral-based products to synthetics based on energy savings can be a rigorous exercise—especially in equipment operating at high efficiencies. Careful planning between the lubricant supplier and end user is needed before embarking on such a program to document actual savings. Objectives must be established and operational guidelines developed to make the test runs as meaningful as possible. There’s much more to this type justification initiative than using a power meter to measure amperage drop.
A point of reference
In the context of lubricants, a “synthetic” is produced by turning low-molecular-weight materials into those of a higher molecular weight through a chemical reaction. Formulators are able to control these reactions and produce lubricants with uniform consistency and targeted performance properties. Mineral-based lubricants don’t exhibit the consistency and uniformity that synthetics do, nor do they have the performance properties of synthetics. Table I lists synthetic lubricant types and applications.
Polyalphaolefins (PAOs)—also known as synthetic hydrocarbons (SHCs)—are the most common type of synthetic. PAOs are manufactured by reacting ethylene to produce a C10 hydrocarbon called decene. Decene is reacted with itself to produce high molecular weight of hydrocarbons that are linked in groups of 10 carbon atoms (so we can produce any molecular weight in groups of 10). The initial reaction involves a reaction of a linear alpha olefin to produce molecular weights in groups of 10 carbon atoms. The final reaction saturates the double bond to produce a PAO. (It is also possible to react dodecene that has 12 carbon atoms to produce increasing molecular weights in groups of 12 carbon atoms.)
(Most studies conducted on the use of synthetics for energy efficiency have been performed with PAOs—and it’s PAOs on which this article will focus.)
Frictional losses lead to energy consumption: Controlling these losses leads to greater efficiencies. The two major types of friction are solid and liquid. Metal-to-metal contact not only causes wear, it leads to high consumption of energy. This needs to be controlled by maintaining an adequate lubricant film. The major lubrication regimes for separation of metal surfaces are hydrodynamic, elastohydrodynamic and boundary/mixed.
- The hydrodynamic lubrication regime is characterized by development of a full film with no metal-to-metal contact.
- The elastohydrodynamic (EHD) regime is characterized by:
- Non-conforming surfaces that occur during rolling motion
- Thin, solid-like film, usually less than one micron
- Very high contact pressure, resulting in deformation of metal surfaces to better distribute pressure
- The boundary/mixed regime results when a lubricant film can’t be developed to properly separate the metal surfaces. This usually occurs during start-up of machinery and also occurs in shock loading of gear boxes. This is controlled by the use of additives, such as those for anti-wear and extreme pressure.
Fluid friction—which is dependent on the molecular structure of the base stock and its viscosity—can be characterized as a series of molecular plates sliding over each other like the spreading of a deck of cards. The resistance to the sliding results in energy losses and heat generation. Figure 1 illustrates this point.
Compared to that of a mineral-based product, the molecular structure of a synthetic (like a PAO) is more consistent, thus making sliding between the metal and lubricant film and sliding between the layers of molecular structures easier. Mineral oils have a variety of molecular components based on size. As shown in Fig, 1, this makes shearing or sliding of the lubricant film more difficult, resulting in higher energy consumption.
During the EHD lubrication regime—where a solid-like film is produced—the force required to shear the film is called traction coefficient. PAOs have a much lower traction coefficient than mineral oils resulting in energy savings in shearing of the film in rolling-element bearings and the pitch point during meshing of gears.
While base stocks contribute to the lubricity and shearing of the lubricant film, resulting in energy savings, another important factor is additives, such as friction modifiers, that go into the finished product. In some cases, additives can have more of an effect than the base stocks. A well-formulated lubricant will incorporate a synthetic base stock with the proper additives to reduce friction, resulting in the maximization of energy savings. Some small, specialty-lubricant companies promote energy-savings potential from the use of PAO base stocks with proprierty additives such as liquid moly.
Testing methods can be among the most difficult factors to deal with when attempting to justify a switch to synthetics based on energy savings. Some companies turn to sophisticated power meters and try to control as many variables as possible; others do a quick study that only measures amperage drop without any adjustments. Although there seem to be countless energy-saving “success stories” out there, it’s difficult to assess the accuracy of their results without acutally evaluating the data. The fact is, the more efficient a piece of equipment or system component is, the more difficult it is to see significant energy improvements. Take, for example, non-worm gears that have efficiencies from 90-95%, and worm gears that operate with efficiencies in the 65-80% range: It’s usually easier to see energy improvements in worm gears than other industrial gear types.
Here are several accounts of real-world evaluations that attempted to make the case for using synthetics based on the energy savings they could generate.
I. The case of a coal pulverizer plant worm gear. . .
A major lubricant company conducted a study on the efficiency improvements in worm gears through the use of a PAO.
- Initial laboratory tests were conducted on a worm gear with a 20:1 reduction ratio operating at 1750 rpm.
- ISO 460-compounded gear oil was used. Based on the viscosities at the operating temperature, an ISO 320 PAO was used to more closely match the viscosity of the ISO 460 oil. This resulted in a 3.5% efficiency improvement in the laboratory test. Initially when an ISO 460 PAO was used, the efficiency improvement was only 0.9%. This illustrated another advantage of synthetics because with the high viscosity index one lower ISO grade could be used, giving the same protection with less fluid friction.
- Under carefully controlled conditions, the actual field trial tested a utility’s coal pulverizer with reduction ratio of 22:1, running at various load levels. Because the operating temperatures were low, it was decided to compare an ISO 320 EP mineral oil with an ISO 320 non-EP PAO (so the viscosities would be fairly similar at the operating temperature).
- At different loadings, energy savings of 9.8%, 8.7% and 8.5% were realized—far exceeding the laboratory test results.
II. The case of a crushed-rock-mining PUG mill…
Energy savings were evaluated on a PUG mill using an ISO 150 PAO and ISO 150 PAO/mineral oil. The major difference between these two lubricants was their additives.
- Fifteen-minute runs were conducted with each product, keeping the variables on production as close as possible.
- Electric consumption was measured with a Summit Technology Powersite Energy Analyzer.
- Adjustments were made for tonnage processed for the two lubricants.
- Energy cost per ton was $.0218 for the PAO and $.0134 for the PAO/mineral blend.
- Total yearly cost savings were $5650, which was a 35.6% savings for the PAO/mineral oil blend.
It’s interesting to note that the semi-synthetic blend outperformed the 100% synthetic PAO. Additives like friction modifiers can have a major effect on energy savings.
III: The case of a 1250 hp 3600 rpm electric motor. . .
Working with six lubricant suppliers, a major electric-motor manufacturer conducted a comprehensive, two-day testing pro-gram on a 1250 hp unit with sleeve bearings running at 3600 rpm.
- Seven lubricants were evaluated. All of them were ISO 68 viscosity grades:
- Three mineral oils
- Two Group III synthetics
- Two PAO synthetics
- The following variables were monitored:
- A Yokogawa Darwin unit was used for measuring bearing temperatures (which were checked a 15-minute intervals).
- The electrical data was collected with the use of a Yokogawa Power Meter.
- Velocity transducers were used for monitoring bearing housing vibration.
- Testing protocol
- Reference oil was used to verify repeatability between runs and flush between additions of different test oils.
- Once bearing temperature stability was established with the reference oil, the motor was run for another 30 minutes to verify stability. Vibration and temperature readings were taken continuously.
- All oils tested were added from sealed containers in the presence of all lubricant-company participants.
- The reference oil was used between each run to flush the previous oil and to verify it ran similarly to the reference run before the next oil was evaluated.
- No differences stood out from one test run to another.
- Synthetics ran one degree hotter than the mineral oils.
- Vibration values repeated themselves from run to run.
- Power consumption was nearly identical for each run.
The final conclusion from this study was that given a specific ISO grade, no one oil in this study under real-world conditions was more efficient than another oil.
The case of a large centrifugal CO2 compressor. . .
A manufacturing facility in the upper Midwest evaluated a MAN Turbo 8-stage 20,000 hp, 2700 psig discharge CO2 compressor with a sump capacity of 1300 gallons. The goal was to compare an ISO 32 turbine oil with an ISO 32 PAO synthetic for energy-savings potential.
- This comprehensive study was conducted by the plant’s own reliability engineers.
- The test was run for six months on the facility’s previous product, an ISO 32 turbine oil. Another six-month test was run with the ISO 32 PAO.
- To ensure a meaningful comparison, polytropic efficiency was used—something that’s more difficult than adiabatic assumptions, because gas flows in and out of the system, and this added energy changes some of the gas properties.
- Efficiency gains of 2.7% across the range of flow were realized from the PAO, which translated into annual energy savings of $184,151.
This evaluation demonstrated that the use of a particular synthetic PAO with unique additives led to a marked reduction in energy consumption. (The results are illustrated in Fig. 2.)
Fig. 2. Results of the Midwest manufacturing facility’s evaluation of its MAN Turbo 8-stage 20,000 hp CO2 compressor that compared the energy-saving potential of an ISO 32 PAO vs. that of an ISO 32 mineral-based turbine oil.
Synthetics lubricants can be real problem solvers. They operate in high- and low-temperature environments, provide good wear protection and generate energy savings. This article focused only on the energy-saving potential of polyalphaolefins (PAOs)—the most common synthetic type. Not all finished lubricants consisting of a PAO base stock will perform the same. Additives play a strong role in performance, as illustrated by one of the case histories where a PAO/mineral blend outperformed a 100% PAO in saving energy.
Although many suppliers promote energy savings as a key reason for switching from mineral-based products to synthetics, justifying those savings in your operations and/or specific application(s) could be difficult. Studies have to be carefully designed with the use of proper power meters taking into account the variables in comparing the tested lubricants.
Keep in mind that the lower the efficiency of a piece of equipment (or system component), the easier it is to see efficiency gains. For example, spur and helical gear types are highly efficient—making it difficult to measure small efficiency gains. Worm gears, on the other hand, are the least efficient gear type: It’s easier to see energy improvements in them.
A well-designed study based on rigorously evaluated data can measure even small efficiency improvements that generate significant dollar savings. This was illustrated in the 2.7% efficiency gains in the large centrifugal CO2 compressor with yearly savings close to $200,000.
It’s unrealistic, however, to expect synthetics to save energy in every case. This was documented in the account of the 1750 hp electric motor study that evaluated several ISO 68 VG products—synthetic and non-synthetic. No significant differences were observed among the seven different lubricants that were tested.
Remember what you’ve been told about things sounding too good to be true: When it comes to synthetic lubricants, energy-savings claims very often can be justified. . .
but not always. LMT
Long-time Contributing Editor Ray Thibault is based in Cypress (Houston), TX. An STLE-Certified Lubrication Specialist and Oil Monitoring Analyst, he conducts extensive training for operations around the world. Telephone: (281) 250-0279. Email: email@example.com.