Troubleshooting Your Compressors (February 01, 2006)
Clean, consistent compressed air and gases are among of the most important requirements of many industrial operations. Compressors are used throughout many processes, such as machine operation, gas pr...
February 1, 2006 | By Lloyd (Tex) Leugner
Clean, consistent compressed air and gases are among of the most important requirements of many industrial operations. Compressors are used throughout many processes, such as machine operation, gas processing, air conditioning and refrigeration, for pneumatic tools and controls, cylinder actuation, and more. In fact, without the reliable operation of compressors, a manufacturing process can stop functioning.
That’s why knowledge about compressor operation and proper troubleshooting techniques is critical for the maintenance team. This three-part article will cover all the key points needed for effective compressor operation and maintenance. Part 1, here, covers types and applications, and includes a guide to compressor troubleshooting.
Compressors are classified as either positive displacement, where intermittent flow is provided, or dynamic, which provides continuous flow of the air or gas being compressed.
Positive displacement compressors include reciprocating piston types of various designs, and rotary types, which include helical screw, straight lobe, liquid piston and rotary vane compressors.
Positive displacement compressors confine volumes of air or gas in an enclosed space that is reduced in size to accomplish compression.
In contrast, dynamic compressors convert energy from the prime mover into kinetic energy in the gas being compressed, which is then converted to pressure.
Dynamic compressors include centrifugal and axial flow types. Some or all of these compressor types may be found in any industrial facility, functioning in one or more primary applications.
Air compressors provide pressurized air to operate tool or instrument air systems. Compressors commonly used for this application include reciprocating piston types and rotary types, such as centrifugal, straight lobe and screw compressors.
Inert gas compressors are used to process gases that do not react with lubricating oils and that do not condense on cylinder walls or compression chambers at high compression pressures. Examples of these inert gases are neon, helium, hydrogen, nitrogen, carbon dioxide and carbon monoxide, as well as ammonia and blast furnace gas.
Compressors used for these applications may include all types of positive displacement compressors — both reciprocating and rotary. Generally, compression of these gases introduces no unique or unusual problems and the lubricants used for air compressors are also suitable for these applications.
Hydrocarbon gas compressors are used primarily in natural gas processing applications, but are also used to process such gases as methane, ethane, propane, butane, acetylene and nitrogen. Where the hydrocarbons being compressed must be kept free of lubricating oil contamination, dynamic compressors are most frequently found, but where high pressures are required, reciprocating types are also used.
Chemically active gas compressors are used to process gases which will react negatively to petroleum lubricants. In these compressor applications, which may use either reciprocating or rotary types, petroleum-based oils should not be used if there is any possibility that the lubricant may come into contact with these chemically active gases. These may include oxygen, chlorine, sulphur dioxide, hydrogen sulphide, or hydrogen chloride.
For example, petroleum-based (hydrocarbon) oils that come into contact with chlorine or hydrogen chloride will result in the formation of gummy sludge, while sulphur dioxide dissolved in petroleum oil may form sludge and could dramatically reduce the oil’s viscosity. Sulphur dioxide gas has also been found to generate carbon deposits when in contact with hydrocarbon oils — either mineral or synthetic types.
In compressors processing hydrogen sulphide, corrosion in the presence of any moisture will occur — including any small amounts suspended in the oil. Mineral-based oils, including synthetic hydrocarbons, that come into contact with oxygen may combine to cause explosions.
Refrigeration and air conditioning compressors: Compressors used in these applications may include reciprocating piston types, as well as centrifugal, sliding vane and screw compressors.
Some compressors driven by electric motors are hermetically sealed with all operating parts, including the motor, inside the sealed unit. The lubricant used in these cases must have good dielectric properties, must not affect motor insulation and should not affect the fluorocarbon refrigerants used in these systems. This is because the motor is completely surrounded by a refrigerant oil mixture.
Where other types of refrigerants are used, such as carbon dioxide, methyl and methylene chloride, polyglycol synthetics are available, however where any question exists, the compressor manufacturer should be consulted.
Compressor operation
To understand the challenges in troubleshooting compressor problems and to more effectively maintain this critical machinery, it is necessary that operators have a full understanding of their particular compressor operation.
In a typical reciprocating piston compressor, air (or gas) is drawn into the cylinder or cylinders through a filter or strainer, where the air or gas is contained, compressed and then released by valve arrangements that operate by differential pressure. The compressor cylinders may have one or more inlet and discharge valves.
Piston rings or packing contain the air (or gas) under pressure within the cylinder and also keep lubricating oil from the pressure chambers above the piston heads (see Figure 1).
The cycle of operation consists of intake, compression, discharge and expansion, (expansion occurs as the small volume of air or gas remaining in the clearance pockets expands as the piston retreats on the intake stroke).
Due to the high compression pressures, the temperature of the discharged air (or gas) increases substantially. In addition, compression causes water to condense in air compressor systems.
These conditions, increased temperature of discharged gas and condensate in discharged air, must be controlled and are important considerations in the maintenance of compressor systems. When very high discharge pressures are required, compression is often carried out in two or more stages to cool the gas between stages in order to limit temperatures to reasonable levels (see Table 1).
With regard to condensate in discharged air, aftercoolers or heat exchangers are frequently used to lower the temperature and precipitate out much of the water in the saturated compressed air. It is very important to remove water from heated compressed air, because water becomes acidic at 180F (82.2C) and can result in corrosive deposits in piping, valves and compressed air reservoirs.
In a typical rotary compressor, such as a flooded helical screw type, the rotors draw air (or gas) through the intake filter; it is then trapped between the rotors and mixed with lubricating oil. In these machines, lubricating oil is required in the rotor set in order to ensure proper lubrication of the screws, but results in an air- or gas-oil mixture during the compression cycle.
Compression raises the temperature of the air- or gas-oil mixture in addition to causing moisture in the air to condense. This mixture exits the outlet or air end of the compressor, where it flows into an oil separator. This tank acts as a reservoir for the separation and recovery of the oil from the air or gas being processed. This oil is filtered and returned to the point of injection.
The compressed air, meanwhile, passes through a heat exchanger or aftercooler to reduce its temperature. The cooled air enters a water separator, which removes condensed water, the air enters the reservoir and the process is repeated.
As can be seen, these compression cycles, depending on the type of compressor or the gas being processed, can create unique problems, resulting in oxidized l
ubricants, the formation of sludge and varnish, contamination, corrosion, rust development and explosion potential, due to hot spots at discharge valves, caused by carbonaceous deposits (see Figures 7 and 8).
In addition, incompatibility issues may result if seals and process gases react chemically with incorrect lubricants. This is precisely why compressor operators must be familiar with the potential problems associated with inadequate compressor system design, excessive operating temperatures and careless selection of lubricants.
Refer to the Troubleshooting Chart (opposite page) for a list of 17 typical compressor problems and 100 possible causes. Additional compressor troubleshooting tips will be included in our next issue.
Lloyd (Tex) Leugner is the principal of Maintenance Technology International Inc. of Cochrane, Alta., a company that specializes in the resolution of maintenance problems and provides training for industry. He can be reached at 403-932-7620 or texleug@shaw.ca.
Look for Part 2 of this article, Compressor Preventive/Predictive Maintenance and Condition Monitoring Recommendations, and Part 3, Compressor Lubricant and Lubrication Considerations, in the April 2006 issue of Machinery & Equipment MRO.
References: Lubrication Fundamentals, J. George Wills; The Practical Handbook of Machinery Lubrication, 3rd Edition, L. Leugner; Handbook of Lubrication, Theory and Practice of Tribology, Volume I, Application and Maintenance, E. Richard Booser, Ph.D., Editor.
Click here to view the tables from the story.