The Enemy of Rotary Screw Compressors

Rotary Screw air compressors are excellent scrubbers of their environment.  By design, they continuously ingest high volumes of airborne contaminants and force them into the oil.  This makes the compressor’s lubricant extremely vulnerable to acidic and oxidizing gases ingested from the atmosphere.  As these contaminants accumulate and concentrate in the oil, they chemically react to produce harmful acidic byproducts and degrade the oil’s ability to lubricate, cool and protect compressor internals against corrosion.  Even the slightest concentration (parts per billion) of the wrong pollutants or chemical vapors can destroy compressor fluids and internals, and negatively impact compressor reliability and  maintenance costs.

Many “oil-related” problems are not the oil’s fault at all, but often can be traced back to contamination ingested from the air. Ingested Contaminants are equally destructive to both petroleum and synthetic base stocks alike. Knowing how to detect, avoid, and control these contaminants gives compressor users the ability to eliminate, or at least minimize, the harmful impact these contaminants can have on their compressor fluid.

Detecting harmful contaminants and identifying their source can sometimes be tricky. However, with routine oil analysis and simple observations of the compressor’s operating conditions and environment, compressor users can gain important clues to detect the presence of harmful vapor contaminants. Below are some of the symptoms and effects which may indicate you have a problem with ingested contaminants:


Fluid SymptomsShort Term EffectsLong Term Effects
High TANShort Fluid Life CorrosionHigh Oil & Filter Usage High Maintenance Costs
Low pHCorrosion Bearing Wear Short Fluid LifeAir-end Failure Corrosion of Bearings, Coolers and Condensate Drains
High ViscosityHigh Temperatures High Amp DrawOil Degradation, Leaks, Loss of Performance
Varnish / DepositsWear & Fouling High Temperatures High Amp DrawAir-end Failure Plugged Coolers and Separators, High Oil Carryover


Industrial environments are often rich in harmful pollutants and chemical vapors that can wreak havoc with expensive compressor fluids. Strong oxidizing and acidic gases are extremely destructive to compressor fluids and internals, and have shown to reduce fluid and air-end service life by more than 90%. In documented cases, as little as 100 ppm of Chlorides present in the air has resulted in air-end failures in less than 18 months.

Many of the harmful contaminants, like those listed to the left, are commonly found in many types of industries. These contaminants should be avoided at all costs. Some of the industries specifically known to have problems with airborne contaminants include:

Power Plants, Fertilizer Plants, Plastics and PVC Manufacturing, Pulp & Paper Mills, Oil Refineries, Petrochemical Plants, Ceramics Manufacturing, Carpet & Textile Extrusion Plants, Waste Water & Sewage Treatment, Aluminum Manufacturing, Steel Mills, Foundrys, Salt Mining and Refining, Automotive Paints and Coatings Plants, Sulfuric Acid Plants, Mining & Aggregate Plants, Dairy Processing Plants, and more…

Residual Oil Contamination left behind from a previous oil fill can also be destructive to compressor fluids and severely impact fluid life. During a typical compressor oil change, as much as 20% of the old acidic fluid is unavoidably left behind to contaminate and reduce the life of the new oil. Compressor fluids with a history of high TAN, low pH or those that have varnished, if not thoroughly drained can have a dramatic impact on the service life of the new fluid. Even when a flush is performed after draining the old fluid, varnish and other baked-on acidic deposits will slowly dissolve into the new fluid, increase its acid level, and reduce its useful life.

The best strategy for dealing with compressor fluid contaminants is to eliminate them at the source by supplying the compressor with the cleanest, driest and coolest air available. Specific care should be taken to locate compressors as far away from the equipment and areas listed below, that are known sources of problem contaminants:

Cooling Towers, Water Softeners, Boilers, Thermal Oxidizers, Smelters, Loading Docks, Busy Highways, Chemical Storage Areas, Kilns, Welding Areas, Maintenance Shops, Parts Cleaners, and Paint Booths

Sometimes it is not practical to eliminate contaminants from being ingested by the compressor. In situations like this, controlling the accumulation and concentration of contaminants in the oil is the only strategy available to limit their impact on fluid performance and life. Since fluid contamination manifests itself primarily as an acid problem, controlling fluid acid levels can be an extremely effective course of action to minimize the impact of atmospheric contaminants. Fluid Metrics’ Compressor Oil Purifier (COP) with ion exchange acid removal technology is proven to control fluid acid levels, improve fluid performance, and increase useful service life. There are many documented cases where fluids suffering short fluid life from ingested contaminants have realized more than a 5 fold increase in service life after employing a COP.



About us

Fluid Metrics’ technologies are utilized and endorsed by many of the world’s largest compressor users, and are widely recognized as the best practice in rotary screw air compressor maintenance.

Fluid Metrics, LLC

4514 Chamblee Dunwoody Rd. #252

Dunwoody , GA 30338

Phone: (770) 393-8636

Fax: (770) 393-8631


Viscosity Index ASTM D2270 153
Viscosity, centistokes – cSt
@ 0° F / ‐17.8° C ASTM D445 690 cSt
@ 100° F / 37.8° C ASTM D445 26.9 cSt
@ 104° F / 40° C ASTM D445 24.9 cSt
@ 210° F / 98.9° C ASTM D445 5.2 cSt
@ 212° F / 100° C ASTM D445 5.3 cSt
Pour Point, ° F / ° C ASTM D97 ‐40 / ‐40
Flash Point, (COC) ° F / ° C ASTM D92 470 / 243
Flash Point, (PMCC) ° F / ° C ASTM D93 410 / 210
Copper Strip Corrosion, 3 hrs. @ 212° F / 100° C ASTM D130 1B
Ferrous Metal Corrosion, Rust Test
Distilled Water ASTM D665A Pass
Synthetic Sea Water ASTM D665B Pass
Foam Tendency (Sequence I, II, III) ASTM D892 0 (Nil)
Density, g/cc @ 25° C ASTM D941 0.98
Total Acid Number (TAN), mg KOH/g ASTM D664 0.18
pH Dow Method 7.9

Viscosity – cSt @ 40°C ASTM D445 44.4
Viscosity – cSt @ 100°C ASTM D445 7.3
Viscosity Index ASTM D2270 126
Pour Point – °F / °C ASTM D97 -58 / -50
Flash Point – °F / °C ASTM D92 501 / 261
Demulsibility ASTM D1401 Excellent
Foaming Sequences I, II, III ASTM D892 Nil
Specific Gravity ASTM D1298 0.89
Properties subject to change