Synthetic Lubricants for Air Compressors?... YES!!!
... and the benefits to making the switch are enduring.
S.L. Pearson, Mobil Oil Corporation, Fairfax, VA

Synthetic lubricants offer advantages over their mineral oil-based counterparts in lubricating reciprocating, rotary vane, and rotary screw compressors. The advantage of synthetic lubricants lies in their chemical structure. Conventional lubricating oils start with a barrel of crude oil. Distilling, fractionating, filtering, heating, chilling, and other processes split crude oil into broad product groups– gaseous products, gasoline, middle distillates, lubricants, and residual products. Further processing removes components that do not contribute to satisfactory lubricant base stocks. However, it is impossible to remove all of the unwanted components.

Synthetic lubricants, on the other hand, start with a relatively pure material. Ethylene is one of the most widely used building blocks. Subsequent polymerization, hydrogenation, alkylation, washing, and filtering results in a wide variety of synthesized base stocks with properties that satisfy a number of lubricant product needs.

• Compared with mineral oils, synthetic lubricants offer the following benefits:
• higher resistance to oxidation and deposit formation
• greater film strength and resistance to shear-down
• longer drain intervals
• less wear
• improved oil separation in rotary designs, and reduced internal friction and traction characteristics that lead to cooler operation and lower power consumption.

Taking an infrared temperature gauge reading shows discharge valve temperature on large reciprocating air compressor lubricated with synthetic diester

Oxidation resistance is vital in reciprocating compressors because high-pressure discharge temperatures run from 310º to 355º F. Although discharge temperatures in rotary vane and rotary screw compressors are generally lower, oxidation resistance is still an important factor. Some of the oil that lubricates the pistons, cylinders, vanes, and screws in compressors is carried out with the compressed air and settles on the discharge valves. At temperatures above 275º F, conventional mineral oil-based compressor lubricants oxidize rapidly. The thin film of lubricating oil on valve sur faces makes the oil vulnerable to oxidation by hot discharge air.

Oxidation forms carbon deposits on the valves. The deposits start out sticky or gummy, but then harden into coke-amorphous carbon -- under continued exposure to hot air. These deposits restrict air flow and interfere with valve seating which causes valve leakage and re-compression. This further increases discharge temperature and oxidation. The first stage of deposit formation reduces compressor efficiency. In the second stage, air pressure drops. Then, in the third stage, the unit shuts down as carbon deposits plug the valves.

The experience of one user with more than 30 reciprocating, rotary vane, and rotary screw compressors illustrates this problem and how synthetics solved it.

Every week, mill maintenance people pulled one to three valves for cleaning, rebuilding, or replacement. Naturally, this represents considerable maintenance expense in parts and labor. The maintenance regimen that takes compressors out of production requires substitute compressor capacity, sometimes from rental units. The maintenance department preferred to confine valve maintenance to the planned shutdowns and inspections every 12 to 16 months.

On the advice of its oil supplier, the mill switched from conventional mineral oil-based compressor lubricants to a synthetic diester-based compressor lubricant with and ISO VG (International Standards Organization Viscosity Grade) of 100 formulated to withstand high temperatures. The replacement lubricant has good air separation qualities and a high flash point to prevent fires.

Since making the switch, maintenance on large compressors dropped by 25 to 30 percent and valve maintenance between inspections on the small units dropped 75 percent. The mill found that the synthetic lubricant works in not only the reciprocating but in the rotary vane and rotary screw units. Under normal circumstances, rotary compressors require a lighter viscosity product than do reciprocating units.

The mill also realized the benefits of longer drain intervals and reduced wear. Oil change intervals were extended from the six months to a year or more to match the inspection schedules. Also, there is now 75 percent less compressor maintenance during the annual shutdowns and inspections.

A manufacturing plant using large reciprocating compressors for plant air also realized the benefits of reduced valve maintenance by switching from mineral oil to a synthetic compressor lubricant resulted in lower feed rates in the forcefeed lubrication systems for the compressor cylinders.

Another user with exclusively rotary screw compressors reported similar performance with a second type of synthetic lubricant, a polyal phaolefin-based synthesized hydrocarbon. The plant extended its oil drain intervals by a factor of six. Compressors in this plant exceed industry average unit life by as much as 70 percent. Maintenance people report better air separation and cooler operation.

This last benefit, along with greater film strength, is important to rotary screw compressor operation. In addition to lubricating the gears, rotors, and bearings -- including the heavily loaded discharge rotor thrust bearings -- the oil maintains a film between the rotors and cools the compressed air at the discharge end.

The experiences of these users illustrate the advantage of synthetic lubricants for air compressors and how these advantages easily offset the higher initial cost of the synthetics.