Air Compressor: Lubricate Properly
In recent years, there has been a tremendous increase in the use of high pressure respiratory-type air compressors by fire departments. Because of this increase, helpful information is needed for the operators of these compressors. One of the most important operational items (as well as one of the most abused) is proper lubrication.
High pressure respiratory air compressors, by their very nature, are precision machines. Wear tolerances in the high pressure cylinders, for example, usually cannot exceed .002 of an inch. A slight malfunction or improper preventive maintenance can result in rapid destruction of a critical operating part. A better understanding of the part good lubrication plays in this will help increase the operational reliability of the compressor.
The lubricated reciprocalng-type compressors for respiratory purposes are still the most common. They contain oil in a central reservoir such as a crankcase. From here, oil is distributed to all parts of the compressor which require lubrication. The purpose of this oil or lubricant is to:
- Separate rubbing parts
- Reduce friction
- Dissipate heat
- Absorb moisture and move it to noncritical operating areas of the compressor
- Minimize wear
- Flush away dirt and wear particles
- Improve efficiency of metal-to-metal seal in valves and piston rings
A good lubricant should have the following important characteristics:
High autoignition temperature. This is the temperature at which the oil will ignite spontaneously with air. If this occurs, carbon monoxide and excessive carbon deposits can be produced. The temperature in the cylinder heads of high pressure compressors will be in the range of 360 to 400°F. It is important that the lubricant being used has an autoignition point above these temperatures.
Multiviscosity. Simply speaking, viscosity is the ability of an oil to flow. For proper lubrication of bearings and other critical parts, the oil should be of a mult iviscosity grade to compensate for the wide range of temperatures encountered from initial starting of a compressor through prolonged continuous operation.
Moisture absorption. A lubricant’s ability to remove moisture from critical parts and then separate and leave these moisture droplets safely in the bottom of the crankcase will extend the compressor’s service life.
Types of lubricants
There are several types of available lubricants.
Vegetable and mineral oils are unacceptable for use in high pressure compressors due to their inability to function satisfactorily as a lubricant at high temperatures and pressures.
Petroleum-based lubricants have been commonly used in high pressure compressors for years. They have the drawback of producing carbon deposits as well as having a low affinity for removing moisture.
Synthetic diester-based lubricants have been accepted in recent years by virtually all manufacturers of high pressure compressors due to their superior lubricating qualities under a wide range of conditions. They are more expensive than a petroleum-based lubricant, generally costing four times as much. But they have a good affinity for moisture removal, a high autoignition temperature in the range of 750°F, and are powerful solvents which reduce the forming of carbon deposits. Synthetic oils designed for high pressure reciprocating compressors are highly recommended.
In years past, as well as occasionally now, some operators of respiratory compressors used mineral oil as their lubricant. This was usually done in the mistaken belief that this oil would not produce carbon monoxide. Mineral oil can and will produce carbon monoxide if the oil’s flash point is reached.
Synthetic oils recommended for high pressure compressors, as compared to mineral and petroleum-based oils, will have a longer interval between changes. Greater safety is maintained as a result of higher autoignition temperatures. Extended parts life and lower crankcase temperatures are due to the synthetic oil’s better thermal conductivity and film strength. Reduced corrosion is the result of the synthetic oil’s ability to handle moisture.
Caution should be used when considering the use of synthetic oils on older high pressure compressors. These older compressors may have internal rings and gaskets made of neoprene or rubber, both of which are not compatible with the synthetic oils. The more modern compressors utilize high nitrile buna N and other compatible materials in lieu of neoprene and rubber.
Compressors that have been operating on petroleum lubricants more than likely have extensive carbon buildup in the compressor and intercoolers. Keep in mind that the synthetic diester oils are powerful solvents and will loosen these carbon deposits. This can cause clogging of valves and oil lines, which can result in the malfunctioning of the compressor as well as cause possible damage. Basically, all compressor parts that will be in contact with the synthetic oil will have to be thoroughly cleaned before making the change.
Good lubrication procedures are very important during the break-in of a newly overhauled compressor. The reason for this is that all new parts have certain rough spots that need to be worn or rounded off in order to form good mating surfaces. This is probably the most critical period in the operational life of these parts. For the typical high pressure compressor used by fire departments, a thorough break-in period can take six to eight hours.
Because of the superior properties of the synthetic oils, the normal glazing period will take longer than with petroleum-based oils. For this reason, greater care must be taken during this period when using the synthetic oils. A method that is definitely recommended and will produce excellent results is to use a recommended petroleum-based oil for break-in, then switch to the synthetic oil.
Compressors, like any piece of machinery, will eventually experience operating problems. The accompanying table shows oil-related problems, as well as causes and symptoms, that occasionally arise during the operational life of a high pressure compressor.
Since this information does not pertain to a specific manufacturer’s compressor, it has to be taken in a general fashion. An operation/maintenance manual is sent by the manufacturer with every compressor. It is a detailed guide to the proper operation of your compressor and it should be studied and followed diligently.