Publisher’s Note
Lubricants selected for compressor applications generally depend upon eight conditions; the type of compressor, the type of gas being processed, discharge pressures and temperatures, lubricant oxidation, rust and foaming resistance, hydrolytic stability, carbon deposit forming tendencies (particularly at discharge valves) and compatibility (with seal materials and the gas itself).
Even though today’s top-quality mineral base oils are frequently used as compressor lubricants, the trend is toward synthetic fluids, most notably; polyglycols, diesters, polyol esters, phosphate esters (for compressors requiring fire-resistant lubricants) and polyalphaolefins hydrocarbons. The primary reasons for their use are their extremely high viscosity indices and superb oxidation resistance. A synthetic lubricant with a high viscosity index can reduce power consumption by up to 12%.
Moisture is a factor, particularly in air compressors, when they are allowed to run unloaded. This is because condensation occurs during unloaded periods when the cylinders cool below the dew point of the air remaining in them. This condensate can cause severe corrosion and rust deposits if not controlled. The lubricant must provide excellent hydrolytic stability. When using mineral-based or synthetic hydrocarbon oils, water content should not exceed .5% (5,000 ppm). If polyglycol fluid is used, this lubricant can tolerate about .8% (8,000 ppm) of free water. In self-driven integral engine compressors, both engine and compressor pistons are connected to the same crankshaft. The running gear may also share a common crankcase. As a result, diesel engine oils are frequently used and may be mineral base or synthetic of similar viscosity grades as noted previously. Cylinders used in single and two-stage crosshead or trunk-type compressors processing air or inert gases are usually lubricated using the same oil found in the crankcase.
Another factor that determines cylinder oil selection is the operating temperature. Thin films of compressor cylinder oil will inevitably reach the discharge valves. The hot metal surfaces create severe oxidizing conditions and the formation of carbon deposits. These deposits restrict the discharge passages, further increasing discharge temperatures and contributing to more deposits. Eventually, a hot spot will develop which may result in a fire or explosion. Lubricant selection and condition monitoring are critical considerations in reciprocating compressor operation and not enough attention is paid to these requirements for safety and insurance reasons.
Conversion To Synthetics There are two very important considerations when converting any compressor system to synthetic lubricants. The first is that some synthetics will dissolve mineral base oil deposits and a viscous tar-like substance may develop, plugging piping, valves, intercoolers, and heat exchangers. Conversion to synthetics therefore may require complete flushing and cleaning of the entire system before installing the new fluid. Diester fluids in particular have excellent solvency and are frequently used as flushing fluids. Secondly, all synthetic fluids may not be compatible with all seals or sealing materials. It is also necessary to determine if the synthetic fluid being considered is compatible with machine coatings or paints often found on the inside surfaces of reservoirs or other components. In general, polyglycols, diesters, polyalphaolefins and alkylated aromatics are compatible with the following seal materials.
In addition to the cover story on Oil & air compressor oils, we have several other articles on lubricant monitoring, reception & storage and handling application. It also includes articles on lubricant contamination control & reconditioning, analysis & troubleshooting and lube tips and others.
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Wish you a very happy Dussehra & Diwali.
Warm regards,
Udey Dhir
