Acid gas ingestion in rotary screw air compressors is a frequent problem that poses significant challenges, primarily due to the destructive nature of these gases and their impact on both the compressor’s mechanical components and the lubricant. Acid gases, like those contained in combustion exhausts, cooling tower emissions, welding fumes, process vapors, herbicides, pesticides, and cleaners, are common in industrial environments and can enter the compressor through the intake air, leading to a range of problems. Below, we will describe these problems and the strategies to mitigate them:
Unique Problems Caused by Acid Gas Ingestion
1. Corrosion of Metal Components:
– Acid gases can react with moisture in the air to form acidic compounds, which then corrode metal surfaces within the compressor, particularly in the rotors, bearings, casing, sump tank, coolers, and discharge piping. This corrosion can lead to premature wear, increased clearances, and, mechanical failure.
2. Lubricant Degradation:
– Acid gases can function as both reactants and catalysts leading to accelerated oxidation, the breakdown of the oil’s additive package, lowering of the oil’s pH, and the formation of sludge and varnish in some oil types. This reduces the lubricant’s ability to protect the compressor and dramatically reduces its useful service life, while leading to increased corrosion, friction, wear, and the potential for catastrophic failure.
3. Seal Degradation:
– Acidic environments and the lowering of the oil’s pH can degrade elastomeric seals, causing them to harden, crack, or lose their sealing ability. This can result in leaks, reduced efficiency, and contamination of the compressed air.
4. Decreased Compressor Efficiency:
– As acid gases corrode the internal components and degrade the lubricant’s physical, chemical and performance properties, the compressor’s efficiency decreases. Increased oil viscosities and fouling of separators, coolers and oil injection orifices leads to higher temperatures, energy consumption, and reduced output, increasing operational costs.
Remedies and Technologies to Mitigate Acid Gas Problems
1. Identifying and Eliminating the Source of Acid Gases:
– Conduct a thorough analysis of the compressor’s operating environment to identify the source of acid gases. This may involve air quality testing and investigating nearby industrial processes that could be emitting these gases. Once identified, steps should be taken to eliminate or reduce the source, such as improving ventilation or altering nearby processes.
2. Relocating the Compressor or its Inlet to a Cleaner Environment:
– If the source of acid gases cannot be controlled, consider relocating the compressor or adding a remote intake to pull air from a cleaner environment. This might involve moving the compressor to a different part of the facility or installing it in a dedicated, clean-air room with controlled environmental conditions.
3. Installing an Acid Scrubbing Inlet Air Purifying System:
– An inlet air purifying system with acid gas scrubbing capabilities can be installed to remove harmful gases before they enter the compressor. These systems typically use filters impregnated with alkaline materials that neutralize acid gases or activated carbon filters that adsorb the gases. These systems can be cost prohibitive, particularly when used on smaller rotary screw air compressors.
4. Installing an Acid Removal Compressor Oil Purifier:
– To maintain lubricant integrity and increase service life, an acid removal compressor oil purifier can be installed. These systems, specifically those that utilize ion exchange technology, continuously filter the oil, removing acidic compounds and other contaminants that degrade the lubricant and harm compressor internals.
5. Periodically Replenishing the Oil’s Protective Additives:
– The protective additives in compressor oil, such as antioxidants and corrosion inhibitors, naturally deplete over time, but this depletion and loss of protection accelerates acidic environments. This dictates that oil analysis be conducted more frequently to monitor the buildup of acids (TAN, SAN and pH) and declining additive levels. With routine oil analysis, compressor users can replenish these additives as needed by topping off oil levels with an additive replenishing concentrate designed to restore the oil’s protective properties.
6. Performing a Thorough Flush when the Oil Becomes Abnormally Acidic:
– Often when compressor oils are contaminated by acid gases and their acid levels and/or acidic strength become abnormally high (high TAN, SAN or low pH), the first instinct is to just change the oil. However, due to the compressor’s design and the difficulty in draining all the old acidic oil trapped in dead zones and low-lying areas, a large volume of “bad” oil is often left behind to contaminate the new.
In many rotary screw air compressors, as much as 25% of its oil fill volume can be left behind during an oil change. This residual contamination immediately degrades the new oil and depletes its protective additives to negatively impact the performance and dramatically shorten the life of the “new” oil. Therefore, whenever oil analysis indicates the oil has become abnormally acidic and needs to be replaced, a thorough drain, flush, drain, and refill should be performed. Additionally, an often overlooked but critical step is to take a new baseline oil sample shortly after the oil is changed to verify the effectiveness of the flush and that the oil looks good as new.
7. Consider Changing the Oil Type:
– Most rotary screw air compressor oils in use today fall into one of two categories, Polar and non-Polar, each with its own inherent strengths and weaknesses. In the polar category are the PAG (polyalkylene glycol) based fluids, which are normally preferred due to their non-varnishing chemistry but are vulnerable to attack from water-loving acid gas contaminants. PAGs tend to attract the acid gases and scrub them out of the air stream to become sacrificial, while minimizing the acid gases effects on downstream components.
By contrast, in the non-polar oil category are the PAOs (polyalphaolefin), SHCs (synthetic hydrocarbons), and mineral oils. These oils, which are prone to forming varnish that can destroy a compressor, are less susceptible to attack from acid gases ingested by the compressor. Unlike the PAGs, these oils tend to repel acid gases and are much less impacted, but the corrosive gases continue downstream with the air where they condense out to attack the compressor’s aftercooler, discharge piping, condensate drains, and refrigerated air dryers. With these non-polar oils, the acid gas problem often goes undetected until corrosion of downstream components eventually surface resulting in component replacements (often with material upgrades), major downtime, and costs.
Conclusion
Addressing acid gas ingestion in rotary screw air compressors requires a multi-faceted approach. By identifying and eliminating the source of acid gases, relocating the compressor or air intake if necessary, and implementing technologies like compressor oil purifiers and additive replenishment, the harmful effects of acid gases can be significantly mitigated. Regular monitoring and maintenance, including oil analysis and additive replenishment, are also crucial to ensuring long-term compressor reliability and performance in environments where acid gases are present.