Lesser-Known Option for Reliability Gains and Cost Savings
The quickest way to reduce maintenance costs while improving reliability is to focus on lubricant and airend replacement costs.
1. Switch to Condition Based Maintenance Practices for Lubricant and Airends.
2. Make Informed Choices for Lubricant and Lubricant Purification
Air systems have three primary direct operating cost: energy, repair and maintenance, and rental compressors. This article focuses on cutting repair and maintenance costs for rotary screw compressors. As an independent consultant, it is common to see plants overspend on maintenance while impacting reliability marginally. This paper examines the two primary areas for maintenance savings: air end life and lubricant. These are the two categories with the greatest excess expenditures for most compressor systems.
Step 1 - Switch to Condition-Based Maintenance vs. Time-Based Maintenance
The first step in reducing operating costs while improving reliability is Implementing a condition-based maintenance program. Time-based maintenance depends on predetermined intervals for servicing or replacing equipment, regardless of the components' actual condition, leading to unnecessary expenses and potential equipment failure. In contrast, condition-based maintenance involves monitoring of equipment performance and health, enabling proactive maintenance and replacement when needed. This approach saves costs and improves reliability.
Step 2 – Review Options for Lubrication
Significant compressor OEM markups have opened the door to aftermarket oil suppliers selling comparable, and in some cases superior, products at much lower prices. The OEM's recommended oil change interval (typically 8,000 hours) is meaningless and rarely aligns with actual field experience. When properly monitored, maintained, and only replaced based on actual in-service condition, many compressor oils will greatly exceed the OEM's recommendation, and greatly reduce oil consumption and the labor and downtime needed to replace it.
Compressor lubricant serves four main functions: sealing between rotors, absorbing heat from compression, and lubricating the gears and bearings. The fourth is often overlooked which is protecting metal surfaces against what is a highly corrosive internal environment. Most lubricants do well in sealing and lubricating bearings, but their ability to absorb heat and their behavior during failure varies. Changeouts are usually recommended at one to two years or up to 8,000 hours but that isn’t necessarily because the oil goes bad exactly at 8000 hours. Factors like heat, humidity, local environment, dirt levels, and operational hours per year determine lubricant lifetime. There are now lubricants with 12,000 to 16,000-hour lifetimes and even one with a 24,000-hour recommended change-out. As the time between replacements increases, testing becomes more critical.
Extending Lubrication Life – Four Methods with their Associated Lifetime
Testing - Rotary screw air compressors should have their oil sampled and analyzed every 2,000 hours in service (3 months) in "normal" operating environments. Compressors in "abnormal" environments with a history of short oil life, high temperatures, or acid gas ingestion, should have their oil tested more frequently. Important parameters to include in the oil analysis are Total Acid Number (TAN), Viscosity, pH, Spectrochemical metals analysis, and water content. Other tests, like ISO Particle Counts and DR ferrography should be added on a case-by-case basis if bearing wear is a concern.
Choose a longer-lasting lubricant: historically, lubricants have gone through evolution to improve their ability to perform the primary functions of a lubricant. When Rotary screw compressors first came out, mineral based oils and automatic transmission fluid were quite common. Any plant that uses either of these lubricants should immediately do an evaluation of the value of improving the type of oil used in the machine. This eventually evolved into synthetic lubricants. The three primary synthetics that are still used today can be divided into three categories polyalphaolefins (PAOs) and other synthetic hydrocarbons (SHCs) , Polyglycols, and Poly olesters (POEs). Each type has advantages and disadvantages depending on the application and environment, but what they all have in common is high profit levels for the OEM if they are the source of the lubricant.
Compared to the old mineral oils, PAO and SHC lubricants offer better oxidation stability and longer life. These oils are not very water tolerant, and don't do well in hot and humid environments as the water tends to separate inside the compressor during periods of shutdown. PAOs and SHC are also known to form varnish when run to failure.
Polyglycol (PAG) based oils are often blended with Polyol esters (POEs) to provide superior oxidation and thermal stability, and longer service life than PAOs or SHCs. These base stocks also have a very high water tolerance, making them ideally suited for hot and humid environments. They have a higher viscosity index (VI) than PAOs or SHCs and will not form varnish which can destroy a compressor.
Lubricant Purification 2-4x Improvement:
Compressor oil purifiers are available to purify the oil and control the harmful contaminants that are known to accelerate wear, corrosion and fouling, and shorten the life of the oil, bearings, separators and all oil-wetted components. Using a variety of technologies, from cellulose depth filtration, synthetic ultra-fine filtration, and ion exchange acid adsorption, compressor oil purifiers significantly increase oil service life while removing the contaminants that decrease air end life.
It should also be noted that the air oil separator would have less dirt to contend with and we would expect an increase in lifetime for this too.
Additive Replenishment: 8x + Unknown Improvement
One of the most exciting developments in recent decades for rotary screw air compressors is combining compressor oil purification with additive replenishment. Protective oxidation and corrosion inhibiting additives are used in all compressor oils to improve performance and extend oil service life. But unfortunately, these additives are sacrificial and naturally deplete while in service leaving the oil and compressor internals vulnerable. Purifying the oil will help slow down the rate of additive depletion, but eventually the oil will fail unless key additives, and the protections they provide, are restored.
To accomplish this, preblended additive concentrates are available that can be periodically added to the compressor as make-up oil to return the additives and protection to their new oil levels. Companies that combine these best practices of oil purification and additive replenishment with routine oil analysis can expect to see a 5-10 fold increase in oil service life, accompanied by a 5-10 fold decrease in the oil consumption, disposal, and environmental impact.
Step 3 – Increasing Airend Life
Air end lifetime varies based on local conditions such as heat and humidity as well as the quality of maintenance. The decision to replace the air end should not be made based upon hours but based upon condition.
The Bathtub Curve
The bathtub curve is a useful concept in understanding the value of condition-based maintenance. It represents the typical failure rate of components over time and consists of three phases: infant mortality, normal life, and wear-out. By adopting condition-based maintenance, organizations can better monitor equipment throughout these phases, particularly during the normal life phase, allowing them to make informed decisions on servicing or replacing components. This prevents unexpected failures and reduces costs while improving equipment reliability.
Historically speaking, the OEMs typically design machines to have L 10 lives of 40,000 – 60,000 hours with a few near 100,000 hours. That life though, however, is very dependent upon maintenance and local conditions as previously mentioned.
Air end condition is best understood by monitoring vibration data combined with oil analysis. One of the more common methods is the shock pulse method.
Compressor companies know what the vibration characteristics should be for a typical air end given its size, speed, and pressure. The compressor companies can take this known data and profile your air end against the readings. This allows them to determine if the vibration levels are with intolerance and the speed at which they are changing. Companies with a significant number of compressors also have the option of developing their own internal database benchmarking all machines and developing a database for expected vibration at various frequencies and various hours of operation. And, when vibration monitoring is combined with regular oil analysis, the additional data often provides a better understanding of the condition of the air end.
For instance, oil analysis can detect increases in harmful contaminants like wear metals, acids, and corrosion byproducts, which are normally accompanied by a decrease in the oil's critical protective additives. Used in combination, regular vibration and oil analysis provide users with early warning signs of degradation of both the compressor's air end and the oil it depends on for survival. There are sources external to the compressor companies which can provide some guidance on what to look for and what tests should be conducted. Compressed Air Consultants is one of these sources.
Rotary screw air compressors are extremely vulnerable to their environment. Local ambient conditions and the quality, or purity, of the air being ingested have a direct impact on compressor performance, reliability, and maintenance costs. High temperatures can lead to increased oil oxidation and reduced lubrication effectiveness, ultimately causing premature wear and tear on the bearings. Conversely, low temperatures can result in oil viscosity changes, compromising the lubricant's ability to protect the bearings adequately. Humidity can also have detrimental effects on bearing life, as moisture ingress can cause corrosion, reduce lubricant efficiency, and promote the formation of sludge and varnish within the compressor system. Additionally, dusty or dirty environments increase the likelihood of airborne contaminants entering the compressor, leading to increased wear on the bearings and other critical components.
The importance of proper maintenance practices for extending the bearing life of rotary screw compressors cannot be overstated. Adhering to a routine maintenance schedule that is tailored to your compressors’ unique operating environment is essential in order to optimize the performance and longevity of the compressor's components. One key element in this process is the maintenance of the inlet filter. A filter with an appropriate micron rating ensures that harmful particulates are effectively captured, thus preventing premature wear and tear on the bearings.
Cooler cleaning is also vital especially in air cooled environments with airborne dust as running the lubricant at an excess temperature makes it break down quicker.
Similarly, regular inspection and replacement of the oil filter is crucial to maintaining the compressor's efficiency and performance. An oil filter with a low micron rating and high capture efficiency effectively removes contaminants and debris from the lubricating oil, which in turn reduces the risk of damage to the bearings. Lastly, the oil itself plays a vital role in the overall health of the rotary screw compressor.
Controls and Capacity
Controls and storage capacity also play a critical role in the operation and longevity of rotary screw air compressors. One primary factor that they influence is the frequency of airend cycles, which directly impacts the film thickness of the lubricating oil.
In a rotary screw compressor, the bearings are continuously lubricated by a thin film of oil. The compressor's controls influence the frequency of start-stop cycles. Each time the compressor starts, it momentarily operates with reduced oil film thickness, increasing the risk of metal-to-metal contact and potentially causing damage to the bearings. The more frequent the cycles, the more often this reduced lubrication condition occurs, increasing the likelihood of premature bearing failure. The driver for this force is Pressure x Area. The pounds force increases as a square function as the diameter of the rotor increases. This is why as air ends increase in size, the squeeze on the film thickness increases.
On the other hand, a greater storage capacity in the compressor's system allows for longer operation periods between cycles. This, in turn, reduces the frequency of start-stop events, and thereby decreases the instances of compromised lubrication. As such, with increased storage capacity, the bearings endure fewer cycles and the overall wear and tear on them is reduced. Therefore, the design and management of controls and storage capacity are pivotal in ensuring optimal compressor operation and enhancing the lifespan of the bearings, by minimizing the frequency of potentially damaging air end cycles.
The Net Result
The OEMs are reluctant to officially state an opinion because of the variability of conditions the machines are forced to operate in. (And the fact that they don’t want to go on record with information that could be used against them). For that reason, anecdotal opinions from informed individuals was the best method of setting expectations. In general, the majority of air ends should last 40,000 hours but many will go past that into the 50,000-60,000 hour range with good maintenance practices. Some older designs often lasted 100,000 hours due to the presence of an oil pump but that option is not offered by most manufacturers anymore.
Where there was some disagreement was the length of life on two stage air ends. Two stage means that there are two steps in the compression process where air ends run in series rather than running in parallel. This running in series allows for intercooling which then reduces the amount of energy required to compress to the final pressure. While efficiency is the primary reason to go to a two stage compressor, historically these units had significantly longer air end lives.
Ultimately, the plant should talk to the vendors any time they purchase a new compressor and confirm what is a reasonable expectation for lifetime. The plant would also be wise to discuss with their service providers whether vibration and oil analysis could prevent premature airend changeouts. If they say no , then you need another supplier. If they said yes, it would, then it is valuable to set the parameters up front of what can be expected in terms of data that would indicate it is time to change out an airend and its oil.
Purification and Air End Life
The oil filter for compressor companies typically is rated 10 Microns “nominal” or about 22 microns absolute. The kidney loop purifier previously mentioned has a 3 Micron absolute rating. What this effectively does is takes out some of the finest particulate which can impact bearing lifetime.
According to Timken and other manufacturers, this extra level of cleanliness can improve bearing life and as a consequence could impact air end life. While there is some disagreement regarding the level of impact on line contact bearings such as tapered roller bearings, our most trusted experts suggest that there isn’t when it comes to point contact bearings which most OEMs are moving to today for energy reasons.
So the purification strategy will save on lubricant costs with a high likelihood of extending air end lifetime. Plants may be reluctant to do this for machines under warranty. However, given the implications of the bathtub curve, the risk of a failure becomes even less for machines after a few years of operation.
Compressor companies may recommend practices that bring them exceptional profit levels at the expense of the plant. The two largest maintenance cost abuses are lubricant and air end replacement recommendations. The most important decision is to shift to condition-based maintenance with thorough testing.
For air ends, some type of vibration testing is necessary to monitor the condition of the air end and the bearings within it. Regarding lubricant, plants can explore four levels: testing, lubricant type, purification, and adding additives. These costs are controllable and pose very low risk when addressed intelligently. Being an educated consumer helps keep suppliers honest and potentially more flexible in their pricing. By following these insider tips, plants can significantly reduce rotary screw compressor maintenance costs without sacrificing reliability.