Basics of rotary screw compressor lubricants - …

1 Basics of rotary screw compressor lubricants By: Wayne Perry, Technical Director, Kaeser Compressors Lubricant is a critical component in rotary screw for this reason. Mineral oils have the disadvan- compressors. Understanding the characteristics tage of a complex mix of natural hydrocarbon of lubricant types helps ensure their proper ap- molecules. There are waxes that solidify at low plication and increases customer satisfaction by temperatures, volatile components that vaporize extending the service life of compressors. Be- and natural mineral oils tend to oxidize quickly, low we discuss the types of lubricants and then forming varnish and sludge, when exposed to discuss factors that reduce the effectiveness of high temperatures and elevated pressures.

2 lubricants and cause premature wear or damage in rotary screw compressors. Synthetic Hydrocarbons Synthetic hydrocarbon lubricants are engineered Types for particular applications. For compressor ap- There are seven basic types of lubricants used plications, polyalphaolefin (PAO) base stock is in compressors today. Each type of lubricant has most commonly used. PAOs provide many of the advantages and disadvantages for specific appli- best lubricating features of a mineral oil without cations. the drawbacks. Although PAO components These are: are derived from petroleum base stock, they are 1. Mineral oils chemically re-engineered to have a consistent, 2. Synthetic hydrocarbons controlled molecular structure of fully saturated 3.

3 Organic esters hydrogen and carbon. Because their molecular 4. Phosphate esters structure is homogeneous, their properties and 5. Polyglycols characteristics are predictable. PAOs separate 6. Silicones from water extremely well, are chemically stable 7. Blends (Semi-synthetics) and have low toxicity. PAOs, however, are not good solvents. The additive chemistry must be Mineral Oils adjusted for that fact. Additive packages in PAO. Mineral oils (petroleum oils) have long been used lubricants are usually blended to beyond their in various types of compressors. Their use in saturation point at cool temperatures. Under the rotary screw compressors was common until the normal operating conditions of compressors, this 1980's.

4 Some manufacturers factory filled with is not a problem. The elevated temperature and motor oil and some used automatic transmission constant motion keep the additives dissolved. fluid. Mineral oils began to lose favor when oil When PAO lubricants are stored for prolonged recyclers began to charge for oil disposal. With periods of time, it is possible for some of the addi- oil change intervals as low as every 1000 hours, tives to condense and cause the lubricant to have many manufacturing plants had to change eight pockets of cloudiness in the storage container. times per year. An advantage of frequent oil These additives will return to solution with a bit of changes is that contaminants in the compressor agitation/stirring.

5 In a paper presented to an en- are removed with the waste oil. In highly con- gineering meeting in Sweden several years ago, taminated environments, mineral oil is still used a major bearing manufacturer shared results of Kaeser Compressors, Inc. 877-586-2691 White Paper: Basics of rotary screw compressor lubricants their testing of lubricants for use in rotary screw concentrations. Although the primary market for compressors. Their conclusion was that PAO polyol esters has been for use in turbines, these lubricants were the best lubricants for rolling ele- fluids are being used in some high-pressure ro- ment bearings used in screw compressors. tary screw applications.

6 Pure PAO fluids have a tendency to cause certain Phosphate Esters low nitrile Buna-N elastomers and some other Phosphate esters have very limited application. synthetic materials to become brittle and crack. Phosphate esters have a poor viscosity index, For compressor applications, a small amount of poor tolerance of heat and moisture and become ester fluid is usually blended in the mix to counter corrosive as they degrade. For that reason, this tendency. The ester also helps keep addi- service life is very limited and fluid changes are tives in solution. required often. They can, however, be made fire resistant, and once ignited, they will only continue Organic Esters to burn if the source of ignition remains present.

7 The next class of synthetics is the organic esters. Petro-chemical plants, refineries and other high- The two primary types used in compressors are risk facilities form the primary market for this type dibasic acid and polyol. Dibasic acid lubricants of lubricant. are better known as diesters. Diesters were the most popular synthetic lubricants when synthetic Polyglycols lubricants first became common in compressor Polyglycol fluids, sometimes called PAG fluids applications in the 1970's and 1980's. They have (polyalkylene glycol), were first developed for a number of desirable qualities for compressor natural gas compressors. They can be manufac- applications. They have a stable viscosity over a tured from either of two primary chemical stocks, wide temperature range, high film strength, good or a blend of these stocks.

8 Propylene oxide has metal wetting characteristics and a low vapor the characteristic of being hydrocarbon soluble pressure at high temperatures. and water insoluble. Ethylene oxide has opposite characteristics, being water-soluble and hydrocar- Diesters are strong solvents. As such, they readi- bon insoluble. With a gas analysis, blenders can ly accept additives, making them customizable for adjust the amount of each type to produce a final particular applications. Being a strong solvent, product with the correct characteristics required however, is also one of their primary drawbacks. by an individual application. The solubility differ- Diesters will dissolve paint and varnish, cause ences in the two basic fluids make the chemis- certain common elastomers to swell and deterio- try of the additive package very important.

9 For rate and can damage some common downstream natural gas applications, the fluids are blended so components of compressed air distribution sys- they are not diluted by the particular hydrocarbon tems. Esters can react with water to form corro- gas being compressed. sive acids and volatile alcohols. Polyglycol fluids for use in air compressors are Diesters are not commonly used in rotary screw compromise fluids. They were developed to compressors today, but still enjoy wide popular- reduce complaints about poor lubricant life and ity among large reciprocating compressor users varnish deposits. The formation of varnish is an where the diesters' solvency and resistance to indication of a petroleum or synthetic hydrocar- carbon formation is a plus for extended valve bon lubricant that has failed.

10 Since polyglycols life. Polyol esters have similar characteristics to do not form varnish, there is no visual indication diesters but have the added advantage of be- that the lubricant has failed. Polyglycols attract ing stable at higher temperatures and oxygen water and mix readily. This characteristic makes Kaeser Compressors, Inc. 877-586-2691 White Paper: Basics of rotary screw compressor lubricants separation from condensate difficult and reduces Causes of bearing damage in rotary screw their ability to protect bearings. The saturation compressors point for these lubricants is much higher than for mineral or synthetic hydrocarbon fluids, but water entrained in the fluid beyond this point will sepa- Water rate into free water.