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Rotating equipment is the heart of an industrial installation. The ability to start and stop this equipment on demand is more critical today with the competing supply of energy. The selection of lubricants for use in this equipment must emphasize the impact on reliability.
Industry has been challenged by regulations forcing change in petroleum base stocks. In an attempt to minimize human and environmental damage while using petroleum, the Occupational Safety and Health Administration (OSHA) and the Environmental Protection Agency (EPA) forced major oil companies to develop processes (desulfurization and severe hydro-treating) to remove harmful polynuclear aromatic and other undesirable compounds.
Only 2 percent of crude oil is used to produce lubricant base stocks. With a limited market share of the fuel industry, research and development budgets for petroleum lubricant products that require continuous reformulations are challenged.
In chemistry, likes dissolve likes, i.e., products of a similar chemical structure remain soluble in one another. As the base stocks change, additive selection must change accordingly. For example, the same antioxidant package critical in a Group I petroleum lubricant formulation will not remain soluble in the new Group III base stocks produced today. Regulations will not turn back, forcing oil companies to reformulate to stay ahead of the curve.
Figure 1. Solvency comparison between Group I through VI base stocks
The above chart depicts the evolution of the hydro-treating process, displaying the advantage of higher viscosity index as industry moved from Group I and Group II to today’s Group III petroleum base stocks. This outcome is certainly advantageous, particularly for the large, initial-fill crankcase market for cars and trucks that the industry enjoys, but at a sacrifice in solvency.
As a result, Group III base stocks have more of a propensity to decompose in solids/varnish with less ability to solubilize the solids. In a vehicle in which the crankcase oil only stays in service for a short period of time, the challenges with varnish and solvency don’t rear their ugly head.
Today’s industrial equipment is designed to run efficiently and reliably. Machines usually are operated at higher pressures and have tighter component tolerances. With constraints of footprint size on vessels to requirementsforrobotic-typemovementrequiringtight-tolerancevalvestobeimplemented,selectingthe proper lubricant is critical. Many systems utilize proportional and servo valves that are intolerant of solids and tenacious decomposition products.
Polyalkylene glycol (PAG) base stocks possess unique characteristics in comparison to conventional petroleum base stocks with respect to solid formation. PAGs are designed to have every third atom as oxygen. There is no possibility for oxidation to produce sludge and varnish solids. Products of decomposition within PAGs remain soluble in the base stock for the life of the fluid.
Figure 2. Pencil filters in servo valves for petroleum-based turbine oil (left) and PAG (right)
Figure 2 depicts the comparison of the condition of internal pencil filters. The users of turbine oil were forced to routinely inspect the servos at least twice a year, while PAG users extended these inspections to their major turbine outages that occur every three to five years.
Water is the enemy to lubrication. Free water is a catalyst for oxidation and metal corrosion. The oxygenated PAG molecule bonds water to the backbone, eliminating free water. Therefore, PAGs perform with no sacrifice in integrity or corrosion inhibition with up to 7,500 parts per million of water.
Figure 3. This PAG molecule shows oxygen (red) as the backbone of the molecule. Water seeks out places where it can share a hydrogen atom. The connection must be oxygen-hydrogen-oxygen (red/white/red).
PAGs have natural detergency. The polar chemistry of a PAG readily wets metal surfaces. As a result, the lubricant base stock associates with the metal surfaces and dislodges anything solid that has been deposited on its surface. Lubricants that are not detergent in nature tend to form tenacious solids which require chemical flushing with turbulent flow to cleanse the metal surfaces.
In the power generation industry, PAG turbine fluid just surpassed 10 years of performance in the first gas turbine lube oil installation. The photos below (Figure 4) depict membrane patch colorimetry (MPC) patches (varnish potential) for new PAG fluid compared to the 10-year fluid.
Figure 4. MPC test results for new and 10-year-old PAG turbine fluid
A lubricant must impart satisfactory load-carrying characteristics critical for ongoing performance and reliability of power-producing assets. Figure 5 depicts the frictional force results in Newtons (N) using temperature as a stressor of new versus 10-year PAG turbine fluid. The result indicated no loss in critical load-carrying capacity.
Figure 5. Mini-traction machine load-carrying results of new versus 10-year-old PAG turbine fluid
To improve reliability in its gas turbine fleet, a large power utility developed a rigorous six-stage test slate that was conducted on 17 commercially available turbine oils/fluids to determine oxidation and varnish tendency.
The aim of this power utility’s project was to develop a specific cyclic oxidation test in an effort to compare the different oils/fluids commercially available on the market. To pass this rigorous testing, the defined criteria must be met. The results of the testing ranged from severe oxidation and deposit formation in less robust turbine oil formulations to varnish-free for the synthetic PAG formulation.
In the end, a site’s ability to start up and provide reliable performance is paramount. To do this, the lubricant must remain liquid. Hydrocarbon-based lubricants are challenged with the potential for solid formation. Formulators attempt to overcome the base oil deficiencies with additives.
As petroleum-based lubricants age, the decomposition products are more polar than the base stock and do not go back into solution. The critical additives (antioxidants, corrosion inhibitions, etc.) are more polar and are attracted to the varnish agglomeration.
Many end users have polished varnished turbine oils to remove these agglomerations only to find that additives critical to the ongoing performance of a petroleum-based turbine oil have been reduced.
PAGs are chemically incapable of producing insoluble varnish/solids. The chemistry has proven to provide uptime and system performance in the most severe compressor and turbine installations.
Figure 6. ASTM D7843 MPC testing of mineral oil (MO), thermally stable (TS) turbine oil and high-performance (HP) turbine oil
Figure 7. Results on all testing conducted for PAG turbine fluid only
Figure 8. Photos taken of the finished results for the various oxidation tests conducted
Figures 7 and 8 identify the test results and related pictures for the PAG turbine fluid only. The challenge industry has as it relates to lab testing is that all ASTM tests were written for petroleum-based lubricants. As PAG continues to grow in the market, modifications and new test methods will need to be written to provide end users of the chemistry with accurate results.
A good example of this is the acidity test represented above with yellow caution results after the third cycle. PAGs decompose into mild acidic byproducts that are not harmful to the fluid. The condemning limit for PAG turbine fluid is 2 milligrams of potassium hydroxide per gram of oil sample (mg KOH/g). Using this threshold, the results are well within specification for the fluid’s continued use.
Figure 9. Solubility of the azo dye used in PAG formulations
Visual clarity can be used as an indicator of lubricant quality. The darkening of a petroleum lubricant is the leading indicator of movement toward high varnish potential. In contrast, the azo dye in a PAG darkens the fluid naturally and is not an indication of varnish tendency.
Provided the PAG fluid remains clear/translucent – as evidenced in Figure 9 where a light is shown through, depicting no cloudy or opaque suspensions – the fluid usually remains acceptable for continued use. The acid number (AN) is the primary condemning limit when evaluating a PAG fluid in use.
Figure 10. An independent lab determined the above response by high-performance liquid chromatography (HPLC) for the amount of antioxidant in a PAG fluid after 10 years of service in a gas turbine.
Industry measures antioxidants (amines and phenolics) to measure the anticipated service life of a petroleum-based turbine oil. The remaining useful life evaluation routine (RULER) method is typically conducted in a lab to provide these values for in-use turbine oils.
Figure 10 displays the results of a much more accurate measurement of antioxidant levels. Industry doesn’t embrace this method due to the high cost per test ($5,000).
The PAG turbine fluid in use for more than 10 years as the first installation still shows significant life. HPLC testing has been conducted on this fluid every year starting with year six in service. With the limits set by the original equipment manufacturer as 25 percent of the new value for antioxidant (AO) content as the condemning limit, coupled with the average loss in AO of approximately 2.15 percent per year, this charge of fluid is expected to perform for approximately 20 more years and essentially last the life of the turbine.
In conclusion, as sites operate with reduced workforces, placing more responsibility on personnel, the choice of the lubricant for critical equipment is paramount. Large power utilities are going to the extent of categorizing assets based on tiers, with tier I for the base load, tier II for those that cycle significantly, and tier III considered for peakers (only run in high-energy demand situations).
“Must-run” tier I and, in many cases, tier II assets are being chosen based on reliability, by eliminating the risk of unscheduled failure, shutdown and lost production. Synthetics offer a more forgiving choice, with PAGs having some unique characteristics that set them apart.
This article was previously published in the 2018 Machinery Lubrication Conference Proceedings.