The high cost of low-grade lubricants
A compressor’s ‘life blood’ is an important, but sometimes overlooked, component.
As with anything with moving parts, from our knee and elbow joints to bearings in wheel assemblies, lubrication helps equipment operate smoothly and last longer. The performance of industrial machinery, such as compressors, depends largely on the reliability of its moving parts. A critical factor in achieving a highly reliable and long lasting compressor is proper lubrication.
Today's newer compressors use synthetic lubricants, such as diesters, polyglycols, polyalphaolefins, polyol esters, and silicon-based fluids to run efficiently, stay cool and extend product life. These newer lubricants are capable of handling wide temperature ranges and high flash points. Whatever lubricant you choose for your compressors should be well matched with the application and your airend's moving parts and compressor's seals.
Lubrication keeps compressors moving
Fluid acts as a lubricant for the compressor's meshing rotors, roller and plain bearings, as well as a sealant and coolant. Rotary screw air compressors use a fixed volume of air that when compressed, increases in temperature, along with the increased pressure of the gas. This increase in temperature and exposure to compressed gas creates extreme demands on the lubricant. There are four key attributes that distinguish a quality lubricant from a low-grade lubricant and can make a significant difference in the performance and longevity of compressors.
- Oxidation stability extends compressor lifespan: Lubricant is designed to extend the life of and maintain optimum performance of compressors, and a high quality nonfood grade rotary lubricant should last 8,000 to 16,000 hours at normal operating conditions.
- Protection against wear: A high-grade lubricant will protect a compressor's components and, in particular, a compressor's rotors and bearings.
- Corrosion protection: A quality lubricant will ensure the internal airend components and lubrication system are protected from corrosion.
- Performance with air and water: The way a superior lubricant performs in the presence of air and water will extend a compressor's lifecycle.
Lubricants impact compressor components
A quality lubricant will coat seals and allow parts to move smoothly, while simultaneously preventing the formation of varnish, extending the compressor reliability and lifespan. With a high thermal conductivity value allowing greater levels of absorption of the heat generated by the air compression cycle and friction in the bearings, this type of lubricant helps the compressor run cooler and more efficiently, even when the operating temperature is high.
For rotary screw compressors, fluid is injected into the compression chamber to lubricate the intermeshing rotors and associated bearings. The lubricant also takes away most of the heat caused by compression and acts as a seal within the areas between the meshing rotors and rotor housing.
Most oil-injected rotary screw compressors use the air pressure in the oil sump or separator, after the discharge of the airend, to circulate oil through a cooler and a filter prior to re-injection to the compression chamber. Alternatively, some designs may use an oil pump. Bearings at the end of each rotor carry the radial and axial thrust loads, and they are lubricated directly with the same filtered oil that is injected into the compression chamber.
A similar configuration is used with built-in spiral and turn valve for capacity control. It's critical that the lubricant be maintained on a regular basis to ensure the compressor's moving parts, seals and bearings are well protected against heat, friction and corrosion.
Lubricant is injected to the compression chamber to mix with the incoming suction air and absorbs heat from the compression cycle. The injection temperature and subsequent airend discharge temperature are controlled to avoid condensation of moisture. To avoid condensation of moisture that would mix with the lubricant, the discharge temperature must remain above the pressure dew point. A thermostatic bypass valve allows the lubricant being circulated to flow through or bypass the oil cooler to maintain the desired temperature over a wide range of ambient temperatures.
An appropriate temperature and viscosity of the lubricant are required for optimal lubrication, sealing and to avoid condensation in the oil sump. In addition to oil cooling, an air aftercooler is used to cool the discharged air and remove excess moisture. In the majority of oil-flooded applications, an opportunity exists to recover the heat of compression either for heating or other types of energy recovery.
A proper routine maintenance schedule will ensure reliable compressor operation and maximize the lubricant effectiveness. This can be done by regularly sampling the lubricant. It's important to regularly check the lubricant analysis for potential metal wear components, additive content, acid level and water in the system, and if elevated levels are found, to take corrective actions. Other scheduled maintenance practices should include: checking the pressure drop across the oil filter and replacing when necessary, checking the connections between the hoses and the airend for leaks and making sure to change the lubricant before it reaches its end-of-service life.
When topping off lubricant, it's important not to mix different types of base fluid chemistries and to use a product that matches the quality of the existing lubricant. Fluid analysis is an important part of a proactive maintenance program that can increase a compressor's reliability while revealing problems before they result in system failure.
Protecting your investment
A quality nonfood grade lubricant should last at least 8,000 hours or two years, whichever comes first, depending on the operating environment and how frequently the compressor is used and maintained. While the initial price of a low-cost lubricant will save money upfront, it will cost more in the long run as many low quality lubricants need to be changed up to eight times more often than a high quality lubricant.
It's important to be aware that low-cost often means lower quality. And when it comes to protecting your investment in compressors and maintaining them, it doesn't pay to skimp on the life-blood of your compressor. This can lead to problems such as shorter fluid life, reduced component life, higher operating temperatures and reduced operating efficiencies, which can significantly shorten the compressor lifecycle.
There are other reasons low-cost lubricants cost more over time. By purchasing a high-quality lubricant, operators can actually save money by reducing lubricant carryover and lowering the maintenance and energy costs that are inevitable with inferior lubricants.
There are customers in the field that have chosen low-cost lubricants to save money and ultimately paid a high price in the end. These lubricants created a varnish around the airend, coolers separator tank and oil filter which eventually ruined the airend, requiring it to be replaced. The customer saved a few hundred dollars on the lubricant each month, but ended up spending $25,000 to replace the compressor's airend.
Three kinds of lubricants
Quality is key when it comes to lubricants and the longevity of your compressor. A quality lubricant will help rotary screw compressors maintain their peak performance at a full range of operating temperatures and extend the lifecycle of the compressor. Many factors should be considered when selecting the appropriate lubricant for your application, including: lubricant properties, corrosion protection and service life.
Below are three levels of lubricant quality that can be used to protect your airend and lubrication system.
A typical, standard quality, engineered synthetic lubricant will last two years, or 8,000 hours, and will deliver up to 50% lower carryover than mineral oils and polyalphaolefin (PAO). With a high thermal conductivity value, it will also provide better compressor performance and maintain an appropriate operating temperature.
Extended life lubricant
Extended life lubricants can last up to 16,000 hours, or three years, and are formulated for high-demand, multi-shift operations. This higher quality lubricant will reduce the frequency required for changing the lubricant, which reduces disposal costs. Extended life lubricants will also lower varnish build up that leads to early component failure and additional cleaning for more frequent condensate drain clogging.
Another advantage is better cooling capability so compressors are protected and operate more efficiently, even in high-ambient temperature environments. These lubricants are also highly efficient with anti-corrosion and anti-oxidation additives. Extended life lubricants should have a flash point at least of 262 C or of 505 F, and a pour point of at least -30 C or -53 F.
Concerns over food safety have never been higher, and industry regulations have never been so stringent. Food processing and pharmaceutical industries use several levels of food-grade lubricants, H1 to H3, depending upon whether the lubricant will come in contact with the product. This lubricant should support all rotary screw compressors applications that require a USDA/NSF food-grade lubricant.
Food-grade lubricants can last 6,000 hours, depending on the application and operational conditions. They also resist the formation of foam, sludge, varnish and corrosive acids. A food-grade lubricant should have a flash point at least of 262 C or 505 F and a pour point of -30 C or -53 F.
Quality lubricants lead to quality performance
To keep compressed air systems working at their best, quality lubricants that have been tested and rated for high-quality performance will pay off in the long run. When you depend on reliability and long-term performance, look no further than the quality of your lubricant—it's the best investment to reduce lubricant disposal costs, improve efficiency and extend the life of your compressor.
Trying to save a few dollars each month by using a low-quality lubricant ultimately runs the risk of compressor failure and costly plant downtime, which will far exceed any initial savings garnered by using a low-cost lubricant.
David Gonzalez is channel program manager for Ingersoll Rand compression technologies and services.
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