By far the most common cause of premature airend failure is contaminated or insufficient lubricant supplied to the airend. The lubricant supplied to the compressor airend serves several very important functions. Most immediate is the removal of heat of compression.
If this term is new to you, it refers to the heat released when a gas is compressed. As an example, a 100 horsepower rotary screw compressor will typically generate from 350,000 to 400,000 BTUs per hour of running time.
Without an adequate flow of lubricant, the airendís bearings may hold up for hours or even days. But within a few moments of running time, without the constant flow of lubricant/coolant through the airend, the rotors will become extremely hot, lose their critical discharge end clearances and seize as in the following photo. Notice the bluing of both rotors and dry rotor profiles indicating a total lack of oil flow.
In this case, both rotors seized to the discharge bearing housing. Notice the sealing strip damage to the male rotor (on right) indicating that the rotor had also contacted the cylinder. Failures of this type are most often due to a complete blockage in the oil flow such as a bad oil stop valve or oil pump (where applicable), plugged oil filter assembly, cooler, airend injection port or oil pipework.
This failure of a new airend could have been prevented with some common sense applied during commissioning. The compressor ran several minutes before seizure. During that time, the technician should have noticed that:
1. This compressor is making very little compressed air. (Lubricant is also required to seal the rotors to each other and to the airend castings.)
2. There is no hot air blowing from the oil cooler. Normally, within very few minutes a significant amount of heat is released into the cooler by the circulating lubricant.
3. The oil pipework is not getting hot since no lubricant is returning to the airend. (A compressor package with a thermostatic bypass valve will circulate lubricant from the airend to the sump through the thermal valve, oil filter and back to the airend until the lubricant reaches operating temperature. Heat should be present throughout this pipework.)
4. The discharge end of my airend is getting very hot. Donít rely on oil temp shutdown switches to protect against lubricant starvation. If there is no lubricant flow, hot oil temp switches and even some hot air temp switches may not function properly.
5. It would also appear that this airend was never primed with lubricant at startup. It would likely not have prevented this catastrophic failure, but it may have given the technician a bit more time to locate the problem.
Contaminated lubricant claims more airends prematurely than any other cause. By their nature air compressors ingest airborne contaminants every minute they are operating. While inlet filters help slow the process, they usually only catch particles over 20- 30 microns, allowing anything smaller to pass by. If the filters plug, dirty air can suck past the inlet filter gaskets pulling larger debris inside. Below are photos that show two different machines with damage likely caused by debris in the lubricant.
You must not assume that your lubricant will remain clean for those advertised 8000-hour oil change intervals. Most oil filters have some sort of pressure bypass designed into them to prevent oil starvation should the filter element become plugged. Usually these operate at from 15 to 30 psig of pressure drop. We have seen dusty installations where this occurred consistently within a week of filter replacement. Oil analysis and careful monitoring of compressor vital signs must accompany extended oil change intervals.†
The photographs above show the dramatic effects of contaminated lubricant on the thrust bearings of a large rotary screw airend. The spalling shown in the left photo can eventually give way to the splitting of the rolling elements or balls as shown on the right.