Lubricant Requirements, Properties and Maintenance for Natural Gas Compressors

Compressors are engineered in a variety of types and configurations. The final design selection depends on numerous factors. Gas type and required pressure are significant factors on both compressor and compressor lubricant selection. As temperatures and pressures increase, the stress on the lubricant increases as well. The first part of this two-part series appeared in the May-June issue of ML, and may be accessed at www.machinerylubrication.com. It addressed compressor types, common operating conditions and lubricant volume guidelines. In this second part, the authors address issues that pertain more specifically to the lubricant, with some consideration of natural gas transmission compressor and lubricant issues.

Compressor Lubricants

The choice of a compressor lubricant depends on the type and construction of the compressor, the gas being compressed, the degree of compression and the final outlet temperature. Piston compressors provide the highest gas pressures and are among the most difficult from the standpoint of cylinder lubrication. Rotary compressors with final pressures below 1 mega Pascal (MPa), approximately 145 psi, are less difficult to lubricate. Rotary vane compressors require the use of an antiwear oil1 because an R&O oil is often insufficient for the crankcase splash lubrication of a reciprocating compressor. The selection of the proper compressor and application-dependent lubricant with the appropriate physical-chemical properties is vital to a successful process.2 ISO 6743 - Part 3A provides a classification procedure for compressor lubricants based on the type of equipment and operating conditions.

Some of the most commonly reported oil-related service problems with compressors include:3

• Increase in oil viscosity and total acidity
• Copper corrosion (oil turns green)
• Sludge deposits
• Substantial oil entrainment in discharge gas (air) due to decreased efficiency of the demister element
• Oil strainer plugging
• Bearing failure. For successful operation, compressor oil must exhibit the following properties:4,5,6
• Oxidation resistance
• A wide operating temperature range (high flash point, low pour point, high viscosity index)
• Low volatility
• Superior antiwear performance
• Good demulsibility
• Adequate corrosion resistance
• Thermal/oxidative stability
• Rust and corrosion inhibition
• Hydrolytically stable
• Material compatibility
• Nonsludging performance
• Minimal oil loss to the system
• Nonfoaming behavior
• Nontoxic.

Condensed from the following reference:

George E. Totten, G.E. Totten & Associates, Inc. and Roland J. Bishop, Jr., Dow Chemical Company, "Lubricant Requirements, Properties and Maintenance for Natural Gas Compressors". Machinery Lubrication Magazine. September 2002

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