It is impossible to talk about air compressor reliability and maintenance without discussing acids. Acids are the silent killer of oil-injected rotary screw air compressors and they’re lurking inside your compressors oil waiting to do harm. The root cause of most routine compressor maintenance, and many major repairs, can be directly traced back to acids. Acids are known to reduce the service life of the compressor’s oil, bearings, coolers, separators, and much more. As a result, acids increase your compressors’ maintenance frequency, downtime, parts consumption, waste disposal / environmental impact, and overall maintenance and repair cost. Given all these negatives, you can afford to ignore acids.
What are acids?
In non-chemistry terms, acids are a highly reactive, normally destructive chemical species that are known to burn flesh and corrode metals. Depending on their concentration and strength (pH), acids will chemically attack most materials they come in contact with, often resulting in harmful physical and chemical changes.
With regard to air compressors, acids are a destructive contaminant that should be avoided and controlled whenever possible. Acids dramatically accelerate oil degradation and additive depletion, which hinders the oil’s performance, reduces its useful life, and impedes its ability to protect compressor internals. And with the oil’s performance compromised, compressor maintenance and reliability suffers.
Where do acids come from?
Acids can come from the air in the form of acid-gases from pollution and chemical vapors that are commonly found in industrial environments. Combustion exhausts, cooling tower spray, welding fumes and industrial cleaners are just a few of the common acid-gases that are routinely ingested by air compressors. These acid-gases tend to be stronger “mineral” acids (sulfuric, hydrochloric, nitric) which are water soluble, and once ingested by the compressor get scrubbed out of the airstream and remain in the oil. Even trace amounts (parts per billion) of strong acids in the atmosphere, once ingested by the compressor will accumulate and concentrate in the oil and can quickly destroy compressor internals.
Acids are also naturally produced inside the compressor as a byproduct of the chemical reaction oxidation. Generally speaking, oxidation is a destructive chemical reaction that naturally occurs between oxygen (from the air) and the oil’s base stock and additive molecules. Common examples of oxidation include wine turning acidic when exposed to air, combustion of a hydrocarbon fuel with air producing CO, SO2, NOx resulting in acid rain, and everyday corrosion that turns metals into a pile of rust (iron oxide).
How do acids impact the oil?
In terms of air compressor oils, acids from oxidation are a leading indicator of oil degradation and greatly limit its useful life. Acids negatively impact the oil’s physical and chemical properties and deplete its protective additives (oxidation and corrosion inhibitors) that are vital to maintaining oil performance and delivering suitable service life. Once in the oil, acids, along with water, elevated temperatures, and fine metal particles, then serve as catalysts to accelerate the oxidation reaction. This “autocatalytic” chain reaction continuously gathers speed accelerating depletion the oil’s protective additives and produces more and more acids at an ever-increasing rate.
The moment a new compressor oil is put into service and exposed to air, humidity, heat, and other contaminants; it begins its race to failure. The question then becomes, how long will this race take before the oil fails or needs to be replaced? Anyone who gives you an answer in terms of time, like 8,000 hours or once a year, doesn’t understand the oil degradation mechanisms and is giving you poor advice. Fortunately, the answer to predicting oil life is easily found through routine oil analysis. By monitoring the oil’s acid concentration (TAN), its acidic strength (pH), and key additive levels, users can quickly determine the oil’s oxidative state and reliably predict its remaining useful life.
How do acids impact air compressor reliability and maintenance?
As described above, acids, oxidation, additive depletion, oil performance and service life are inextricably linked. Likewise, the oil’s in-service condition and performance are inextricably linked compressor reliability and maintenance cost. Rotary Screw air compressor oils are aptly called the lifeblood of the compressor. And the oil’s “health”, as determined by the physical and chemical properties provided by oil analysis, is also an excellent predictor of the compressor’s health and reliability. Today’s air compressor oils are advanced, specially formulated fluids (typically synthetic) that circulate throughout the machine performing critical functions to various compressor components. Some of these functions and the effected components include:
- Lubricating air-end bearings, gears, and seals to prevent wear
- Cooling the compressor by absorbing the heat of compression and rejecting it via the cooler
- Sealing critical air-end clearances to maintain compression efficiency
- Protecting internals against corrosion from acids and the extreme oxidative environment
- Cleaning compressor internals (chemically and mechanically) to minimize fouling and wear
As acid levels rise and oil condition and performance deteriorates, the condition and useful life of every component the oil touches are negatively impacted as well. Acidic oil will corrode bearings, coolers, and other vital components. Corrosion generates rust particles, which then circulate in the oil throughout the compressor to foul air-oil separators, plug oil filters, separator scavenge lines, and air-end injection orifices. This increases separator drops and energy consumption, increases oil carryover and make-up oil, reduces cooling which accelerates oil degradation, and increases bearing wear which reduces air-end life. As a result of acids, expensive compressor oils and air-oil separators must be preventatively replaced more often. Coolers develop leaks which need to be repaired or replaced resulting in more downtime. And the most expensive component of them all, the compressor’s air-end, will experience high vibration levels which leads to costly premature replacement and even more downtime.
3 Steps to Control Acids, Improve Reliability, and Reduce Maintenance Costs
By now it should be clear how acids are directly responsible suboptimal compressor reliability and high maintenance costs. Fortunately, there are effective technologies and steps that can be taken to control acids and dramatically improve compressor reliability and reduce maintenance costs.
- The first step in controlling acids in compressor oils is prevention. Avoiding environments contaminated with acid-gases, and thoroughly draining the compressor’s oil during an oil change will go a long way to reduce the acid load on the oil. If it is not practical to relocate the compressor or its inlet to provide a clean source of air, acid-scavenging inlet air filters are available, but usually as a last resort for rotary screw compressors. Unfortunately, these filters are expensive and typically only make sense on larger centrifugal air compressors where higher maintenance costs justify their use.
- The next step to controlling acids and reducing maintenance costs is to proactively remove the acids from the oil. The best technology for removal of acids is a small filter-like device that utilizes ion exchange acid adsorbing media. “Compressor oil purifiers” were developed specifically for rotary screw air compressors, are extremely cost-effective, and can be installed easily on most compressors. They also include ultra-fine filtration to remove the smallest clearance-size solids responsible for bearing wear and separator fouling.
- The third step in controlling acids is to periodically replenish the oil’s acid-fighting oxidation and corrosion inhibiting additives. When used in tandem with a compressor oil purifier, preblended additive concentrates can be easily added to the oil without needing to shut down the compressor. Specially formulated additives neutralize acids, slow oxidation rates and new acid production, and restore protection to the oil and vital compressor internals.
Combined with a proactive reliability-centered maintenance strategy, compressor oil purification and additive replenishment are proven to produce a 5-10-fold increase in oil service life, or up to a 90% decrease in oil consumption and oil change related downtime.
Fluid Metrics is the industry leader in compressor oil formulation, contaminant control and fluid conditioning technologies. CONTACT US today and let us show you how easy it is to control acids, improve compressor reliability and save time and money.