Plants use a lot of air. Select the right lubricants and additives to ensure compressors operate efficiently.
The precision machines that produce compressed air for air tools or gas for refrigerants all require delicate care and proper maintenance to ensure optimum performance, and lubrication is key.
It serves three distinct purposes: lubricating, sealing and cooling. Compressor lubricants are exposed to high temperatures, compression of reactive gases, dilution, condensation and abrasive contaminants. Fluids must be of the proper composition and contain the correct type and amount of specially selected additives to ensure long life and smooth operation.
Using synthetic hydrocarbons, silicones or polyglycol oils prolong compressor service life. Their advantages include high oxidation resistance, high natural viscosity index, low pour point, increased film strength, lower volatility and lower carbon-forming tendencies.
Good compressor lubricants depend on the selection of appropriate lubricant base stock and additives, correct application and diligent monitoring, said Inga Kuksis, Petro-Canada Lubricants Inc.’s product and fluids specialist who discussed air compressor fluids at a Society of Tribologists and Lubrication Engineers (STLE) section meeting in Toronto.
Service life is influenced by composition, operating temperatures, film thickness, viscosity, volatility and shear ability. Mineral oils that meet these requirements are base oils such as polyalphaolefins (PAO), polyalkylene glycol (PAG) industrial gas compression esters, polyol esters, and silicone diesters. Phosphate esters are often added for fire resistance. Bear in mind oxidation stability demands on lubricants increases as plant operations expand because of hotter discharge temperatures and higher cooling requirements, and that high ambient air temperatures reduce the cooling ability of oil.
Corrosion control and water separation are also important, and oil compatibility is definitely an issue. Many compressor fluids are compatible, said Roy Hoppe, a technical advisor for fuels and lubrication at Shell Canada Products. For instance, mineral oils are compatible with PAO and diester synthetic fluids, but the addition of mineral oil has an adverse effect on the performance of synthetic fluids. Mineral oils and PAO synthetic fluids are not compatible with PAG and silicone-based fluids. Be sure to do a thorough cleaning and before changing oil or adding new lubricant.
There are various laboratory tests to ensure that compressor oils are compatible and meet high standards. These tests check pour point, shear stability, anti-wear, volatility, flash point and corrosion resistance. Hot room tests and field trials are also available. It’s a good idea to request a list of test results before buying compressor oils.
Additives – chemically active or inert – impart or reinforce a desirable property of the lubricant. Since they affect how well the oil performs, it’s also important to choose them carefully.
In a recent presentation to the Society of Tribologists and Lubrication Engineers (STLE) in Hamilton, Don Johnston, regional manager for Lubrizol Canada, explained the functions of the nine most common oil additives – detergents, dispersants, anti-wear agents, extreme pressure agents, foam inhibitors, friction modifiers, oxidation inhibitors, corrosion inhibitors and viscosity modifiers.
Detergents clean internal engine parts, neutralize combustion acids while inhibiting corrosion and oxidation. They contain metals that produce ash and they’re based on various substrates. Certain types of detergents react with water or acidic contaminants to cause the oil to thicken and ultimately gel.
Keeping parts clean
Dispersants stabilize colloidal dispersions, preventing them from agglomerating and coming out of oil as dirt. They also help to keep parts clean at lower temperatures, and they protect against contaminant particle growth. But dispersants interact with other additives and affect the viscometrics of the oil, making it difficult to meet certain low temperature criteria.