Polyalkylene Glycol (PAG) lubricants have a rich history and a prominent place in the world of industrial lubricants, particularly in the realm of rotary screw air compressors. Known for their exceptional performance and versatility, PAG lubricants have become a preferred choice across a wide variety of applications. This blog highlights the origins, history, applications, and properties of PAG based synthetic lubricants, illustrating why in most environments they are the best option for rotary screw air compressors.
Origins and Historical Context
PAG lubricants were first developed during World War II, born out of a necessity mandated by the U.S. Navy. Hydraulic fluid fires on ships, resulting from ordnance strikes, highlighted the need for a safer, more reliable lubricant. In response, PAG-based water glycol hydraulic fluids were created in 1942. These fire-resistant fluids operated effectively over a wide temperature range, leading to their exclusive use by the Navy for the next three decades.
Following their initial military application, PAGs found extensive use in the textile industry and metal heat treating processes. Building on these successes, PAGs were patented in 1981 for use as a lubricant/coolant in rotary screw air compressors. A few years later PAGs were adopted by leading compressor OEMs Ingersoll Rand and Sullair as their factory-fill for new compressors. And nearly 40 years later, PAGs remain their factory-fill lubricant still today.
PAG Development and Composition
PAGs are synthesized through polymerization by combining oxides of ethylene, propylene, or alkylene with an alcohol or water. Each raw material provides its own distinctive chemistry which enables formulators to vary their amount to impart a wide range of polymeric structures, viscosities, water solubilities, and functionalities. Unlike other synthetic oils, the versatility in the PAGs synthesis allows for customization of the lubricant’s properties to accentuate the positive, mitigate the negative, and meet the specific demands of the application.
The evolution of PAGs as lubricants/coolants for rotary screw air compressors has led to an optimized blend of PAG and POE (pentaerythritol or polyol ester) synthetic base stocks. The addition of POE instead of other types of esters provides many benefits including improved oxidation, thermal, and hydrolytic stability, increased solvency, water tolerance, and additive performance, and greater compatibility with elastomers and seals.
PAG Key Properties and Advantages
1. High Oxidation Stability: PAGs have excellent oxidation stability and resist degradation when exposed to air, water, high temperatures, and other catalytic contaminants.
2. High Temperature Stability: PAGs exhibit high natural VIs (viscosity indices) and perform well in both high and low-temperature environments, making them well suited for varied climatic conditions and industrial applications.
3. Low Volatility: PAGs are also characterized by their low volatility in high-temperature applications, which reduces the amount of oil carryover and thickening, and ensures long-term efficiency.
4. Biodegradability: Their biodegradability makes PAGs an environmentally friendly option, essential for applications where ecological impact is a concern.
5. Deposit Resistance: Distinct from hydrocarbon-based lubricants and/or PAOs, PAGs do not form varnish, sludge, or carbon deposits. This is due to their chemical structure, which prevents the formation of insoluble byproducts.
6. Heat Transfer Efficiency: PAGs have about 10% higher thermal conductivity compared to Polyalphaolefins (PAOs) and mineral oils, providing superior cooling and maintaining lower operating temperatures.
PAGs in Rotary Screw Air Compressors
Rotary screw air compressors demand that their lubricants/coolants can withstand high pressures and temperatures and the oxidative stresses caused by the air, water, and other contaminants that are continuously ingested by the compressor and forced into the oil. PAGs excel in these areas, offering many critical benefits for rotary screw air compressors.
1. Zero Varnish and Sludge Formation: The inability of PAGs to form insoluble byproducts ensures clean operation and reduces maintenance costs associated with cleaning and replacing parts affected by deposits.
2. Superior Cooling: PAGs’ thermal conductivity is about 10% higher than PAOs and other synthetics. This improves heat rejection, which typically lowers discharge temperatures by 10-15°F, while reducing the risk of overheating and increasing the life of the oil and other compressor components.
3. Superior Water Tolerance: PAGs have exceptional tolerance to water which, due to the rotary screw’s unique design, is a constant threat to the compressor’s oil and internals. Unlike other compressor oils, PAGs are hydrophilic and can hold large amounts of dissolved water in solution (8,000 – 10,000 ppm) without separating into free water, negatively impacting lubrication, or needing to be drained between compressor starts.
4. Higher Flash Point: With naturally higher flash points than mineral oils, PAOs and other synthetics, PAGs provide increased safety reducing the risk of sump fires at elevated pressures.
5. Higher Viscosity Index: PAGs have a naturally higher viscosity index (VI) than other oils, without the need for harmful viscosity improving additives that can breakdown over time and contribute to deposit formation. PAGs higher VI provides excellent film strength at higher temperatures when viscosities decrease, and good oil flow at lower temperatures when viscosities increase.
6. Lower Oil Carryover: PAGs exhibit minimal lubricant carryover (as low as 1 ppm) which reduces oil make-up and consumption and minimizes contamination of downstream compressed air and condensate systems.
7. Increased Operating Life / Lower Consumption: While “OEM-grade” PAG/POE oils are typically formulated for 8,000-hour service life (in normal environments), today’s advanced “performance grade” PAG/POE oils are formulated for extended drain intervals with a normal operating life up to 16,000 hours. And due to the PAG/POE’s varnish-free performance and high solvency, they are ideally suited for supplemental oil purification and conditioning for even longer life. Utilizing compressor oil purifiers with ion exchange technology to control acid buildup and periodically replenishing the oil’s protective additives, compressor users are able to dramatically increase useful oil life up 80,000 hours, or 10 years, and slash their oil consumption as much as 80%.
8. Environmentally Friendly: Condensate from air compressors using PAG lubricants is biodegradable (EPA test method 796.3100) and is suitable for discharged directly into most sanitary sewer systems. Consult your local water treatment facility for approval or pretreatment requirements.
Don’t Be Fooled by “Replacements”
“True” PAG/POE compressor fluids, like those originally adopted by Ingersoll Rand (Ultra Coolant) and Sullair (Sullube), are formulated with approximately 70% PAG, 25% POE (pentaerythritol ester), and 5% aromatic amines and additives. But don’t be fooled by low cost “substitutes”; aftermarket PAG formulations and performance can vary greatly. Some aftermarket PAG “blends” may contain less than 20% PAG, and instead of blending it with a POE, they will often use an inferior and cheaper diester fluid to cut costs. Meanwhile, there are other aftermarket PAG “Replacements”, or PAG “Compatible” fluids, that contain absolutely no PAG or POE whatsoever, and therefore deliver none of their benefits. Compressor users must be careful and ask questions to ensure they are getting a “True” PAG that will deliver the desired performance.
Conclusion
Polyalkylene Glycol (PAG) lubricants have a long-proven track record of performance, reliability, and versatility. Their unique physical and chemical properties, and ability to be customized for optimum performance, make PAGs ideally suited for the extreme demands of rotary screw air compressors. PAGs provide compressor users with a wide range of benefits over other types of compressor oils. These include varnish-free performance, extended service life, superior cooling, and increased safety and reliability. And when combined with reduced oil consumption and purchases, reduced downtime for oil changes, and reduced waste disposal and environmental impact, properly formulated PAG/POE lubricants are proven to be the best available lubricant/coolant for rotary screw air compressors operating in normal environments.