Part II: Selection
All synthetics are not alike. Selection should be based on the optimum base stock type for the application. The additive system, which also is very important, imparts unique properties to the finished synthetic lubricant. As a result, there can be major differences in performance for the same synthetic type from different suppliers. Case histories and actual field tests are the best way to select a particular synthetic fluid. There are many applications where mineral oils, because of their cost and performance, are perfectly acceptable. Synthetics are problem solvers to be used in applications where their unique properties are cost-justified under the following conditions:
Properties & applications of synthetics
Table I illustrates the most common synthetics and their major applications.
If only one synthetic could be selected in a plant, it would be a PAO. These are the most versatile and most widely used synthetics. They operate over a very wide temperature range, can be produced in a wide viscosity range without changing their basic properties and are compatible with most other lubricant types. Because of their nonpolarity, they have poor additive solubility and cause slight seal shrinkage. Consequently, they must be blended with a polar synthetic such as an ester, which swells seals and gives good additive solubility.
Some of the more common uses for PAOs include:
Diesters are one of the oldest synthetic types—and they are limited in the viscosity ranges produced. The most common ISO VGs are 32, 46, 68, 100 and 150. The viscosity indexes are only high for the ISO 32 while the others are in the 70-100 range, depending on the alcohol and acid used in their manufacture.
The major performance strength for diesters is their excellent solvency minimizing deposit formation. They also have good low-temperature properties and high thermal stability and flash point.
Diesters have a low aniline number and a tendency to swell elastomeric seals. Therefore, resistant seals, such as DuPont Viton, need to be used. Diesters also can hydrolyze in a hot, high-moisture environment—something that
occurs in rotary screw air compressors.
Uses for diesters include:
Polyol Esters (POE)…
POEs have very high thermal stability allowing them to be used in a very high temperature environment. They also are fire resistant with high flash- and fire-point temperatures. Because they are readily biodegradable, they can be used as hydraulic fluids in environmentally sensitive areas.
The major disadvantage of POEs is their cost. They are 50% more expensive than PAOs, PAGs and diesters. Although they have a tendency to hydrolyze at hightemperature and high-moisture conditions, POEs are more stable than diesters.
Primary uses for POEs include:
Polyalkylene glycols (PAG)…
As discussed in the first article in this series, PAGs are quite versatile. They can be designed to produce a wide variation in water solubility by adjusting the ratio of ethylene and propylene oxide during manufacturing. They have very high viscosity indexes exceeding 250, as well as excellent polarity for metal surfaces that gives them good lubricity. PAGS don't produce deposits and can be designed to minimize hydrocarbon gas solubility. Their major weakness is compatibility with hydrocarbon lubricants like mineral oils and PAOs. They also shrink many elastomeric seals and attack certain paint types.
Some primary uses for PAGs include:
This following list highlights several applications where synthetics provide major cost justifications. Many more applications could have been presented:
Rotary screw compressors…
Most of today's industrial air compressors are rotary screws like that shown in Fig. 1.
In a rotary screw compressor, air is compressed, high temperatures are generated and, along with the moisture that is present, a severe oxidative environment is present for oil. The lubricant in this equipment performs four major functions: cooling, lubricating (bearings, gears and screws), sealing and corrosion prevention. This requires an oxidatively stable lubricant with high VI and good lubricity. Many OEMs have their own fluids—which are mainly synthetics. As shown in Table II, the different lubricants used can be classified based on fluid life.
The expected hours shown in Table II are OEM recommendations on expected life. Depending on the conditions, synthetics may exceed these numbers if the temperature and moisture are lower than normal.
The most common fluids used for air compressors for extended service are ISO VG 46 PAO and PAG/Ester. The esters most commonly used with PAGs are diesters and POEs that swell seals to counteract shrinkage caused by PAG.
POE gives the longest life extension for the fluid and is being used for extendedwarranty applications. Some POEs on the RPVOT test, which is a measure of the oxidative stability of a fluid, give results in excess of 3000 minutes—that's nearly double the results obtained with PAOs and PAGs. POEs can be used at temperatures up to 240+ F, which is above the shutdown temperature of an air compressor. PAO can handle temperatures up to 220 F and PAGs are lower at 200 F. PAGs have the added advantage of very high viscosity indexes that gives a thicker film at high temperatures which minimizes wear. Furthermore, they don't form deposits at high temperatures when they oxidize.
Two major cost justification areas for the use of synthetics is in fluid life extension and energy savings. Consider the following case study.
An evaluation was performed on a 300 hp compressor with a 60-gal. sump capacity operating at 180 F. Running mineral oil required change-out every 1000 hours, while a PAO greatly exceeded the OEM recommendation of an 8000-hour change by running 15,000 hours. This resulted in a 67% savings—or more than $1700—in lubricant costs in one year. (Data courtesy of Dr. Ken Hope, Chevron Phillips.)
Energy savings can be significant with air compressors. A number of studies have shown savings between 3-5% with rotary screw compressors. Combining energy savings and longer fluid life, along with less wear and better operation, synthetics make sense for air compression applications.
While reciprocating compressors (Fig. 2) are not used much in air compression today, there are still many old compressors working in the industry.
Because of high temperatures, the cylinder region in a reciprocating compressor is the most difficult area to lubricate. One major problem associated with the use of mineral oils for this application is that they form hard deposits when they oxidize and coat the exhaust valves, thus keeping the valves from seating properly. As a result, hot gas is drawn back into the cylinder to be recompressed. This dangerous condition can lead to high heat generation and a possible fire.
The lubricant of choice for reciprocating compressor applications is an ISO 100 or 150 diester with excellent solvency. Fig. 3 shows two actual exhaust valves. The valve on the left had been running for six months on diester. The valve on the right had been running four months on mineral oil. The valve on diester continued to run with no coking, saving over $10,000 in valve replacement costs.
Hydrocarbon compression Rotary screw compressors…
Non-flooded rotary screw compressors running at temperatures below 180 F can use mineral oils without major problems. Users, however, may want to turn to a synthetic like a diester or a PAO for their energy-saving potential. Flooded screw compressors with hydrocarbon gas will quickly lose their viscosity with most mineral oils and synthetics because the gas dissolves in the lubricant, thus lowering the viscosity.
PAGs are the most resistant lubricant to hydrocarbon gas dilution and are recommended for flooded screw compressors. More resistant to dilution than mineral oils, PAGs will, however, be diluted to an extent with hydrocarbon gases, a fact that must be taken into consideration in selecting the initial viscosity to arrive at the correct viscosity at the operating temperature. PAGs have the added advantage of having very high VIs.
Ethylene high-pressure reciprocating compressors…
PAGs are the lubricant of choice for high-pressure ethylene compressors because of their minimal dilution by hydrocarbon gas. The typical viscosity of PAGs used in this application is 270 cSt. The film integrity at a reasonable viscosity is maintained at very high pressures, leading to low lubricant consumption and very low wear rates.
Low-pressure hydrocarbon compressors…
Mineral oils at ISO VG at moderate temperatures have been used successfully. As conditions become more severe, though, synthetics need to be considered. Both PAGs and diesters are good alternatives. PAOs, however, are not recommended because of their tendency to form hard deposits when they oxidize.
Enclosed gearboxes Helical, herringbone and spiral bevel…
Gearboxes experience EHL lubrication through sliding and rolling motion. A key criterion in lubricating gear teeth is to have thick enough film for the high sliding and shock loads. In many cases, EP additives are effective as anti-scuffing agents and are used in many loaded gear reducers. Parallel and right angle shaft gears such as helical, herringbone and spiral bevel are lubricated normally with an ISO VG 220 with EP. Under abnormal conditions, such as high temperatures and high shock loads, an ISO VG PAO with EP is used. Although PAGs can be used, because of their incompatibility problems, PAOs are preferred. Energy savings are more difficult to attain with high-efficiency gears like helicals, herringbones and spiral bevels. Normally, synthetics have shown efficiency improvements of 3% or less. Therefore, the use of synthetics for these gears is not justified by energy savings alone. A better way to justify in these applications is to take into account how dramatically gear performance is improved under difficult load or temperature conditions when synthetics are used.
Worm gears (Fig. 4) are highly inefficient. They also are good candidates for synthetics. A worm gear is a right-angle gear with non-intersecting shafts. These units consist of a steel worm and a sacrificial bronze wheel. There is very little rolling motion. Most motion is sliding—which causes the high wear and high heat. Worm gears typically can run 90 F degrees or higher than ambient temperatures. Since EP additives can attack bronze, very few EP gear oils had been used in the past. The only alternative had been to use a compounded high-viscosity mineral gear oil—such as ISO 460—containing animal fat for lubricity to protect the teeth during boundary lubrication. These types of lubricants oxidized quickly at high temperatures and didn't provide a high level of wear protection.
The two most popular synthetics used in worm applications today are ISO VG 460 PAO and PAG. Each will perform very well. Neither of these synthetics contain EP and they both provide a high film strength and score very high on the FZG test that measures scuffing of gear teeth at different load stages. Mobil SHC 634, which is an ISO 460 PAO with no EP, exceeds 13 stages, the highest level on the test. This results in very low wear rates and energy savings.
Efficiency savings in excess of 7% have been documented. Because of their lower traction coefficient (which is the internal friction in the lubricant) PAGs often provide higher efficiency savings—but PAOs do very well. Temperature drops with a synthetic can be 20-30 F degrees. While PAGs are more common in gearboxes in Europe, more are being used in the United States. A PAG, because of its greater energy efficiency, is a good choice for new gears and can be used on other gears only with the proper flushing procedures. Moreover, PAGs attack some paints. A safer choice to convert a worm gear from mineral to synthetic is to use a PAO.
The following is a case history of the conversion from mineral oil to PAO:
A major can manufacturer used double-enveloping worm gears with an average reduction ratio of 60:1. The company was using a compounded ISO 460 mineral oil. On average, the company was experiencing four gear failures per year, each costing an average of $12,500 to repair. Temperatures typically were 200 F—and in some cases got as high as 215 F. The mineral oil was replaced with an ISO 460 PAO and the failures were eliminated. In fact, to date, 18 months later, there still have been no failures in this equipment. In addition, the average temperature dropped across the worm gears by more than 20 F degrees.
Synthetics are real problem solvers. While they can work well and be cost justified, there are many applications where mineral oils will do just as well. Three applications where synthetics can improve equipment operation and provide major cost savings are air compressors, high-pressure and hot reciprocating compressors and worm gears.
Deciding which synthetic to use is very important. Each candidate will have advantages and disadvantages that need to be considered before a final decision is made.
Keep in mind that the same synthetic type from different manufacturers can give different results. Even though the base stocks may be similar, the additive package may impart different properties from one supplier to another. Make comparisons between the data sheets, but let your final decision rest on field performance. Look at case histories and, if possible, run a carefully controlled plant test where meaningful data can be collected. Even though this will not be possible in some cases, definite equipment improvements can still be observed without rigorous testing and data collection. Be sure to document this data. Since synthetics are more expensive than mineral-based oils, you will want to be very accurate in your cost justifications.
The author wishes to thank Tim Taylor of Summit and Dr. Martin Greaves of Dow Chemical for their assistance in the preparation of this article. LMT