The right compressor lubricant for any application is one that gives maximum performance from the compressor, allows for the longest maintenance-free operation, is compatible with equipment parts such as paint and seals as well as the compressed gas, and does all this at the right price.

Operators must consider lubricant performance in the particular application and the value that any given product delivers. As with all things in life, you either pay now or pay more later. In the case of ocean-going vessels, the consequences of making a choice based solely on price might range from more frequent maintenance intervals to unexpected breakdowns, either in port or at high seas-including both financial risk and risk to the safety of the vessel and crew.

For the lubricant selection process, compressor design is the starting point, as this will dictate the main oil characteristics. Viscometrics, pour point and flash point, coking potential, oxidation resistance, demulsibility, foaming characteristics and elastomer compatibility are a few of the parameters that need to be considered. There are two types of compressors that are typically found on board vessels: screw compressors and recip­rocating (piston) compressors. These have significant differences in design and lubricant requirements.

A typical screw type air compressor will operate at around 80 degrees Celsius. If the temperature is too low, condensate will form and the resulting oil and water emulsion will reduce the sealing and lubricating ability of the oil. Air release properties of the oil are also important, as settling times are quite short in screw compressors. Piston compressors operate at much higher temperatures-up to 210 C. As oil is carried into the gas phase, deposits on the cylinder inlet and outlet valve reduce the performance of the compressor.

Diverse challenges for compressor oils that arise from equipment design and operating conditions imply that no single product is suitable for all applications. To get a picture of the performance characteristics that different base oil chemistries have to offer in compressor oil formulations, a number of commercially available products were evaluated for coking potential, oxidation resistance, elastomer compatibility and gas solubility. Tested oils included two diesters, a polyol ester, an API Group II mineral oil, a polyalphaolefin and a polyalkylene glycol.

The coking potential of the oils was measured using the Ramsbottom method, as it gives better differentiation at low deposits compared to the Conradson method. It was clear even by visual inspection of the coking bulbs that the worst results were generated by the polyol ester and the mineral oil based products. The other tested oils showed a limited tendency to form deposits and excellent performance, each with a Conradson Carbon Residue (CCR) value of 0.01 percent or less. Nevertheless, it has to be observed that there was more than a 25 percent difference between the two diester fluids, and that the PAG based fluid exhibited less than half the deposit formation potential compared to the PAO based fluid. This is also observed in field trials where very limited formation of deposits is visible when using the premium diester based compressor oil.

Greater oxidation resistance leads to longer service life. Oxidation resistance in compressor oils is traditionally measured using the Rotating Pressure Vessel Oxidation Test method, during which the oxidation process is accelerated by elevating the temperature and pressure, and adding water and a copper catalyst to the system. These conditions are often unrealistic for compressor oil applications.

Alternatively, the Rapid Small Scale Oxidation Test does not call for water or catalyst to be added to the sample. This means that the oxidized oil can be further analyzed using traditional methods such as Fourier transform infrared spectroscopy or total acid number measurements to determine the actual extent of the oxidation process, rather than simply observing the induction time. Non-polar oils give similar results in both test methods, while more polar oils tend to give a longer induction period in the RSSOT as there is no water present to accelerate the degradation process.

The parameter that is most commonly measured when carrying out a preventive maintenance program for compressor oils is the total acid number. Using the RSSOT approach allows for TAN measurements of the fresh and conditioned oils. In order to obtain a reference point, the oils tested were aged at a 104 C for 96 hours. The difference in the start and end point of the acidity highlighted the difference in the additive and base oil chemistry that was used for each oil tested. The polyol esters acidity jumped by 65 percent, followed by one of the diesters at nearly 35 percent. The remaining oils changed by 20 percent or less.

Since the introduction of the 2013 Vessel General Permit regulations by the United States Environmental Protection Agency, there has been a shift towards the use of lubricants with minimal environmental impact. One could argue that compressor lubricants are not affected by the VGP-there is no oil-to-sea interface under normal operating conditions-but the VGP regulates all effluent streams on board vessels. This means, for example, that even compressed air condensate that contains some oil cannot be released into the sea or mixed with ballast water as in normal practice. These air compressor condensate discharges make the VGP a matter of concern in relation to compressor oils, as well.

One of the major issues that the industry has dealt with is the way that environmentally acceptable lubricants (in most cases synthetic or biobased ester products) interact with elastomers commonly used as sealing material. With mineral oils and synthetic hydrocarbons such as PAOs, some shrinkage of elastomers is observed in most cases. In contrast, ester based products tend to cause elastomers to swell.

The elastomer compatibility of the tested diester and polyol ester based products shows that different base oil chemistries will react differently to the same elastomer. Typically, these products will cause the elastomer to swell at an initially high rate, but eventually result in a volume increase between 2 and 6 percent, which is acceptable in most cases. A suitable ester base fluid can also cause the finished oil to perform similarly to a hydrocarbon, with which some shrinkage of the elastomer is recorded.

The increasing use of very large gas carrier (VLGC) vessels, as well as ocean-going vessels fueled by liquefied petroleum gas or natural gas, has created an increased interest over the last few years in the use of compressor fluids that are non-miscible in hydrocarbon gases. Gas solubility in the compressor oil will significantly affect the viscometrics, which in turn will affect the lubricity and the wear protection the oil can provide in the compressor. Gases that will typically need to be considered are methane, ethane, propane, NG, LPG, ethylenes and propylenes. More polar base fluids will tend to be less affected by such gases. Esters and polyalkylene glycols are commonly used, while mineral oils and PAOs are not recommended.

Also in order to counteract the solvency effect of gases, higher viscosity oils are used. As pressure increases, so does the solubility of the gas, thus reducing the viscosity of the compressor oil. Tests on the ISO 150 viscosity PAG fluids performance with propylene gas showed the viscosity of the oil-operating at 200 pounds per square inch-reached as low as 30 centiPoise, which provides borderline lubrication protection.

Considering the above operational parameters can help make selection of the best fluid easier for the operator and the compressor designer. It is clear that each fluid technology offers advantages in particular operating conditions, so both the compressor design and the compressed medium need to be considered when choosing a particular oil.

It is often desired on board vessels to limit the number of lubricants available. This simplifies sourcing and reduces the possibility of a lubricant mix-up during day-to-day operations. Based on the described selection process, operators together with their lubricant suppliers need to decide if this is a viable option and the extent to which it can be applied for their business case.

Andreas Dodos is a chemical engineer with Eldons SA. He has over 15 years of experience in the field of lubricating greases and specialty industrial lubricants. He is a member of a number of professional bodies, has been chairman of the European REACH Grease Thickener Consortium since 2010 and currently serves on the ELGI board of directors. He can be reached at andreas.dodos@eldons.gr.