To understand oil carryover, we must examine compressor operation and lubrication systems. In rotary screw compressors, air enters through the intake filter, mixes with oil, and is compressed within the screw element. This oil-air mixture then enters a separator tank. Here, centrifugal force and impact separate most of the oil, which collects at the bottom. Remaining fine oil mist is captured by a specialized oil separator filter (coalescing filter). The separated oil is returned to the compressor element or crankcase via a return line (scavenge line). Clean compressed air then passes through the minimum pressure valve (MPV) before entering the system. Any disruption in this cycle can cause oil to enter the air lines.

Reciprocating compressors have a different lubrication setup. Oil in the crankcase is splashed onto moving parts by the crankshaft and connecting rods, or delivered by an oil pump. Piston rings are designed to scrape oil off the cylinder walls, returning it to the crankcase and preventing it from entering the compression chamber. Worn rings or damaged cylinder liners can allow oil to leak into the compression area and subsequently be carried into the air lines.

Key technical reasons for compressor oil carryover include:

• Clogged or Failed Oil Separator: The separator filter can become blocked with particles over time or its filter media can degrade. This reduces its efficiency in separating oil from air, allowing oil to pass into the compressed air stream. Regular maintenance and replacement of the separator are critical.
• Blocked Oil Return Line: The scavenge line, which returns oil from the separator to the crankcase, can become clogged with carbon deposits or debris. This blockage causes oil to accumulate in the separator, which can then be entrained by the airflow.
• High Oil Level: An oil level above the manufacturer’s recommended maximum can cause oil foaming and overwhelm the separator’s capacity. This hinders effective oil-air separation, leading to oil carryover.
• Faulty Minimum Pressure Valve (MPV): The MPV maintains a minimum system pressure necessary for proper compressor operation and efficient oil return from the separator. If the MPV is faulty or opens too early, system pressure drops, preventing adequate pressure differential for oil return and causing oil carryover.
• Worn or Damaged Oil Seals and Gaskets: Shaft seals in the compressor element or valve seals in reciprocating compressors can wear out or get damaged. This allows oil to leak into the airflow.
• Incorrect Oil Type or Viscosity: Using an oil type or viscosity different from the manufacturer’s specifications can lead to foaming, increased evaporation, or reduced separator performance. High viscosity oils may cause flow issues at low temperatures, while low viscosity oils can evaporate more readily at high temperatures.
• Clogged Air Filter: A blocked intake air filter creates excessive vacuum, potentially drawing oil from the crankcase into the intake and then into the system. It can also contribute to compressor overheating.
• Cooling System Malfunction: Blocked or faulty oil coolers or aftercoolers can cause the compressor to overheat. High temperatures increase oil vaporization, making it easier for oil to mix with the compressed air.
• Worn Piston Rings and Cylinders (Reciprocating Compressors): In piston compressors, worn rings or scored cylinder walls are common causes of oil leaking into the compression chamber and then into the air lines.

Diagnosing and resolving these issues may vary depending on the compressor type, age, and operating conditions. However, regular maintenance and accurate troubleshooting can overcome these problems.