Troubleshooting Your Compressors (June 01, 2006)

Part 1: Compressor types and applications, air and gas compressor troubleshooting guide (Feb. 2006 issue). Part 2: Preventive/predictive maintenance and condition monitoring recommendations (April 2006 issue). Part 3: Guide to compressor lubrication and lubricants.

Clean, consistent compressed air and gases are among of the most important requirements of many industrial operations. Without the reliable operation of compressors, a manufacturing process can stop functioning. Knowledge about compressor operation and proper troubleshooting techniques is critical for the maintenance team.

This three-part article covers all the key points needed for effective compressor operation and maintenance. Part 3, here, is a guide to compressor lubrication and lubricants. Part 2 (in the April 2006 issue) discussed compressor preventive/predictive maintenance and condition monitoring recommendations. Part 1, published in our February 2006 issue, covered types and applications, and included a guide to compressor troubleshooting. Previous articles are archived online at www.mromagazine.com.

Lubricants selected for compressor applications generally depend upon eight conditions: 1) the type of compressor, 2) the type of gas being processed, 3) discharge pressures and temperatures, 4) lubricant oxidation, 5) rust and foaming resistance, 6) hydrolytic stability, 7) carbon deposit forming tendencies (particularly at discharge valves) and 8) 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, polyolesters, phosphate esters (for compressors requiring fire-resistant lubricants) and polyalphaolefin 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%. A typical rotary air compressor will discharge air with an average temperature of 93C (200F). Without a proper lubricant, this air temperature could be as high as 370C (700F).

Even well-formulated, oxidation-resistant mineral base oils tend to begin oxidizing at about 70C (160F), with the potential of forming carbon deposits and varnish. At air discharge temperatures of 93C, lubricant life can exceed 8,000 hours of operation. If the discharged air temperature is 110C or higher, lubricant life can be reduced by 60-70%.

Moisture is also 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 it is not controlled. The lubricant must provide excellent hydrolytic stability.

When using mineral-based or synthetic hydrocarbon oils, water content should not exceed 0.5% (5,000 ppm). If polyglycol fluid is used, this lubricant can tolerate about 0.8% (8,000 ppm) of free water. (These are guidelines only. Operators should consult the compressor manufacturer for specific details.)

Reciprocating Compressor Lubrication (Crankcase and Cylinders)

Crankcase oils recommended are ISO viscosity grade 68, 122, 150 or 220, depending upon ambient temperatures. Generally, these lubricants will be 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 149C (300F), 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 150C and 200C (302F-392F), 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 a 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. 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 264, 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 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.

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 (rust and oxidation) 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 85C to 135C (185F-275F), 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 temper
ature, 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 seal materials in Table 2.

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.

Lloyd (Tex) Leugner is the principal of Maintenance Technology International Inc. of Cochrane, Alta., a company that specializes in the resolution of maintenance problems and provides training for industry. He can be reached at 403-932-7620 or texleug@shaw.ca.

Table 2