Compressor Lubrication Tips PDF Print E-mail
Written by Maintenance Technology International inc.   
Saturday, 12 June 2010 10:55
Article Index
Compressor Lubrication Tips
Cylinder Oils
Guide to Rotary Compressor Lubrication
Sliding vane compressors & Liquid piston rotary compressors
Helical Lobe Screw Compressors
Straight lobe screw compressors & Axial flow compressors
Conversion to Synthetics
All Pages

 

Guide to Reciprocating Compressor Lubrication (Crankcase and Cylinders)

Crankcase Oils

Crankcase oils recommended are ISO viscosity grade 68, 100, 150, or 220 depending upon ambient temperatures. Generally, these lubricants will be paraffin base recirculating oils with rust and oxidation inhibitors and some may have anti-wear characteristics. If mineral base hydrocarbon oils are used where discharge temperatures are below 149°C (300°F), napthenic base oils are frequently recommended because these lubricants have low floc points and will not form wax crystals at low temperatures.

When discharge temperatures are between 150°C–200°C (302°F–392°F), it is recommended that synthetic diester, polyglycol, polyolester or phosphate ester fluids of equivalent viscosity grades be used.

When compressing chemically active gases, such as oxygen or hydrogen chloride, mineral base oils, including synthetic hydrocarbons such as polyalphaolefins and alkylated aromatics, must never be used. (Mineral base oils coming into contact with oxygen will cause explosions). Lubricants recommended for these applications include synthetic chlorofluorocarbons and polybutenes.

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.


Cylinder Oils

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. When these compressors are used in processing hydrocarbon gases such as methane or butane, or where the compressors are processing "wet" gas containing condensed hydrocarbons or moisture, it is recommended that viscosity grades 320 or 460 be used where discharge pressures are 14,000 kpa (2,000 psi), 21,000 kpa (3,000 psi) and 28,000 kpa (4,000 psi) respectively.

Many sour gas or wet hydrocarbon applications may recommend the use of viscosity grade 460 or 680 oils compounded with 3 to 6% fatty oil to ensure cylinder lubrication. Ensure that cylinders receive the correct oil drop feed rate.

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 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.


Guide to Rotary Compressor Lubrication

Centrifugal compressors

require lubrication only at the support bearings, usually an anti-wear oil of a viscosity range of 32 or 46, depending upon the ambient temperature. In units with rolling element bearings, NLGI grades 1 or 2 lithium greases may be used.


Sliding vane compressors

require "flooded lubrication" and because of the high potential for vane to housing contact, oils fortified with anti-wear or mild EP additives are required in a viscosity range of 46, 68, or 100. Some manufacturers recommend polyalphaolefin, diester or polyglycol synthetics.

Liquid (usually water) piston rotary compressors

require lubrication only at the support bearings which are of the rolling element type. Lubricants range from R & O type oils in the viscosity ranges of 32, 46, or 68 to lithium grease of an NLGI grade of 1 or 2, depending upon bearing type and speed.


Helical lobe screw compressors

are primarily of the "flooded lubrication" type where there is major contact between the gas being compressed and the lubricant, thereby causing great potential for oxidation and deposits.

Where discharge temperatures are in the range of 85°C–135°C (185°F– 275°F), lubricant requirements range from high quality R & O mineral oils to synthetic fluids in the viscosity range of 32, 46 or 68. Depending upon the manufacturer’s recommendations, PAO’s (polyalphaolefins), POE’s (polyolesters), PAG’s (polyglycols) and diesters are the primary synthetic lubricants frequently used in these compressor types, depending upon their application.

("Dry or oil free type" helical screw compressors require only that the timing gears and bearings are lubricated. Viscosity ranges recommended are 32, 46, 68 or 100, depending upon temperature, speed and application).


Straight lobe screw compressors

generally require viscosity grade 150 or 220 for higher ambient temperatures. When low ambient temperatures are experienced, viscosity grade 68 is acceptable. All of these oils should be of the R & O type with anti-foaming additives. Depending upon the manufacturer or the application, synthetic lubricants may be recommended.

Axial flow compressors

require lubrication for shaft support journal bearings, axial thrust bearings, usually of the tilting pad type and any seals which may require lubrication. The lubricant generally recommended is a premium rust and oxidation inhibited oil of ISO 32 viscosity grade. In cases where a gear driven speed increaser is used, an ISO 46 or ISO 68 viscosity grade may be required. The synthetics most commonly used are diesters, polyglycols, polyalphaolefins and fluorosilicones.


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 a 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.

  • Viton
  • Thiokol 3060
  • Kalrez
  • Polysulphide
  • Butyl k53
  • Mylar
  • Buna N
  • Polypropylene
  • Neoprene
  • Nylon elastomers
  • EPDM
  • Teflon
  • Polyethylene
  • Acrylonitrile-Butadiene

One exception is diester fluid, which is not compatible with neoprene or "low nitrile content" Buna N. Another exception is polyalphaolefins, which are not compatible with EPDM seal materials.

Where any question or concern exists when selecting synthetic fluids, always confirm your decision by consulting the equipment manufacturer and lubricant supplier.

written by Maintenance Technology International inc.

Last Updated on Saturday, 12 June 2010 11:07