MRO Magazine

Troubleshooting Your Compressors (April 01, 2006)

Part 1: Compressor types and applications, air and gas compressor troubleshooting guide. (see Feb. 2006 issue). Part 2: Preventive/ predictive maintenance and condition monitoring recommendations.(...


Part 1: Compressor types and applications, air and gas compressor troubleshooting guide. (see Feb. 2006 issue). Part 2: Preventive/ predictive maintenance and condition monitoring recommendations.(below) Part 3: Guide to compressor lubrication and lubricants (see June 2006 issue).

BY LLOYD (TEX) LEUGNER

Clean, consistent compressed air and gases are among the most important requirements of many industrial operations. Without the reliable operation of compressors, a manufacturing process can stop functioning. Knowledge about compressor operation and proper troubleshooting techniques is critical for the maintenance team.

This three-part article covers all the key points needed for effective compressor operation and maintenance. Part 2, here, discusses compressor preventive/predictive maintenance and condition monitoring recommendations. Part 1, published in our February 2006 issue, covered types and applications, and included a guide to compressor troubleshooting. Finally, our June issue will include our guide to compressor lubrication and lubricants.

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When considering methods for preventive and predictive maintenance and the condition monitoring of compressor systems, there are several steps you should take. One of the first things to do is determine and record the normal full load electric motor current at a specific voltage. This can then be referred to later as a baseline when problems are experienced.

If the compressor has its own receiver, allow the compressor to fill this reservoir from zero to the cutout pressure. Record the cutout pressure and the time it takes to fill the reservoir. This can be used to monitor compressor efficiency at any later date.

Determine the acceptable discharge temperature. Normally, the high-air-temperature switch on a water-cooled, dual-stage reciprocating compressor is set at about 150C-165C (302-328F). This temperature should be recorded and monitored as part of the condition monitoring program. Keep in mind that the higher the discharge temperature, the greater the possibility there is of corrosion, varnish and carbon deposits, lubricant oxidation and possible discharge-line explosions if hot spots develop.

In rotary compressors, the high-air-temperature switch is normally set at about 110C (230F) and is intended to shut down the compressor if the temperature rises. As a rule of thumb, the discharge temperature should be about 38C (100F) higher than the temperature of the inlet air.

Determine the oil operating temperature. It is important to maintain oil temperatures at about 65C (150F). This will ensure that the oil temperature is about 15 to 20 degrees higher than the pressure dew point, which will help reduce the formation of condensate as well as carbon and varnish deposits. Pressure dew point is the lowest temperature to which compressed air can be exposed without causing condensation of entrained water vapour.

For example, in a two-stage air compressor taking in air at atmospheric pressure at a relative humidity of 75%, with a discharge pressure of 120 psi (827 kPa), about 14 litres (3.75 gal.) of water per hour may be condensed for each 1,000 cfm (cu ft/min) of free air compressed. Any unusual increase in the recorded oil temperature should be immediately investigated.

Determine the filter quality necessary to ensure that inlet air enters the compressor uncontaminated and oil and bearing filters are capable of removing contaminants in the 10 micrometer absolute range or smaller.

In flooded rotary compressors, the oil separator is also a critical component. It is a large sub-micronic filter and its quality of operation is far more important than its initial cost. It should be replaced or cleaned when differential pressure reaches about 10 psi (69 kPa).

Determine the normal, expected discharge pressure and record it for future reference and comparison.

Due to the potential for corrosion, rust and varnish deposits, the intercoolers, cylinder water jackets, aftercoolers or heat exchangers should be inspected and/or cleaned at least annually, as part of the PM (preventive maintenance) program.

Air receivers (reservoirs), drains, condensate traps and air line filters should be visually inspected and drained at least once each week to ensure clean, moisture-free instrument or tool system control air. This is a frequently neglected PM activity and the best practices approach recommended is to inspect and drain these components once a day. (Automatic drain valves are of absolutely no use if they are not working properly.)

Inspect and clean lubricators regularly. In most pneumatic systems the lubricant is carried in the air stream and the amount of oil metered, whether as a fog or mist, is usually determined by adjusting the oil feed rate. This oil drip feed rate must be monitored regularly and the most effective feed rate recorded for a maintenance reference.

Some types of chemicals, such as synthetic solvents or lubricants and ketones, may cause deterioration of the plastic or polycarbonate lubricator bowls, causing cracks or breakage, so determine compatibility of these products before their use.

Determine the cylinder lubrication feed rates for reciprocating compressors and record this information in the maintenance files. It is recommended that new or rebuilt compressor cylinders should be run in for five to 10 hours of operation at no-load conditions using at least double the normal oil feed rate. This process will establish normal wear patterns and eliminate the possibility of scoring a new cylinder or its associated components.

In general, the larger the bore and the higher the pressure, the longer the run-in time required. Once run-in is completed, the proper lubricator oil feed rate can be determined using the formula in Table 3.

Once the proper rate has been established, the oil drops may be counted and this information recorded in the maintenance files. If the oil type or specifications are changed, this process must be repeated.

Here’s a typical example. A 12 in. (30.5 cm) compressor cylinder compressing air at a discharge pressure of 10 bar (145 psi) would require a lubricating oil feed rate of 12 drops per minute after run in. If the cylinder has two lubrication points, each point should receive six drops per minute.

Inspect piping to ensure that fittings and drain valves are not leaking, and that the supports are in good condition. A combination of leaks totalling the equivalent of a 1/2 in. diameter hole, escaping at 60 psi of leaking pressure, will cost approximately $30,000 a year. In addition, piping systems tend to corrode and form deposits and today’s best practices suggest that when repairing or replacing piping, smooth bore pipe such as aluminum or plastic should be used.

Interior pipe corrosion, poor piping system configuration and contaminated air or gas can cause inefficient energy use. For example, a 15-psi (100 kPa) pressure drop uses about 10% additional energy and over a 10- to 12-year period, the cost of energy may exceed all other maintenance costs.

Predictive technologies

Use predictive maintenance technologies to monitor compressor system condition on a regularly scheduled basis. The following Pdm (predictive maintenance) technologies are recommended for compressor condition monitoring.

Oil analysis: Using spectroscopic analysis, the levels of wear metal elements will provide information on the rates of compressor component wear. Oxidation levels of synthetic hydrocarbon and mineral base oils can be monitored effectively using infrared spectroscopy.

In addition, pH, acid number and viscosity should be monitored for these lubricant types. A rapid or excessive decrease in pH indicates the ingestion of acidic gases or other contaminants. An increase in acid number suggests that the oil is reaching the end of its useful life. For systems with large lubricant reservoirs, the rotating pressure vessel oxidation test (RPVOT) will reveal the remaining us
eful life of these lubricants. The cost of this test will be far less than the replacement cost of the oil.

For polyglycol and polyolester synthetic lubricants, pH and acid number testing is also recommended.

In all compressor systems, water content should be measured regularly using such accurate methods as the Karl Fischer test. In addition, particulate content should be monitored regularly, using particle-counting technology.

Finally, whenever wear rates or particulate levels increase, analytical ferrography should be carried out. This analysis will provide information as to the type of contaminant or trace element and its possible source.

Compressor condensate analysis is also recommended to detect corrosive or acidic gases in the air that may be harmful to the system. A low pH or high acid number resulting from condensate analysis can reveal potentially serious corrosion conditions that could lead to shortened aftercooler and dryer life.

Vibration analysis: Vibration analysis programs are now available for both reciprocating and rotary machinery. The most common vibration problems are unbalance (of pistons or rotors), misalignment (of drive belts, cylinder rods, or couplings), mechanical looseness (of mounting bolts, couplings, base plates, bearing caps, or drive motor components), resonance (of any component in the system) or bearing failures.

If a vibration is suspected, a stroboscope can be used to confirm if a vibration is present, after which analyzers can be used to determine the source. Often noise is mistaken for a vibration. Ultrasonic analyzers are now available that are so sensitive that they can determine if noise levels are associated with a faulty component, such as early stages of bearing failure, or noise caused by an air leak in a control valve. These testers are invaluable for locating air or gas leaks that are difficult to locate. In fact, resonant conditions may be the result of excessive air or gas leaks, so it is recommended that leaks be corrected before more advanced troubleshooting or repairs are wasted.

Thermographic analysis: This predictive maintenance technology has been used primarily for locating electrical system hot spots, but it has become an extremely useful tool for locating hot spots caused by excessive discharge temperatures, partially plugged components such as intercoolers or heat exchangers, seal rubs, misaligned couplings or drive belts, overheated bearings and faulty lubricating oil pumps. All of these conditions may cause increases in operating temperatures and any higher-than-normal temperature should be investigated immediately.

For example; a slightly misaligned coupling can cause an increase in temperature without any apparent vibration. The temperature increase at the coupling will be high enough to cause premature failure of the bearings nearest the coupling, because the higher-than-normal temperature could cause premature oxidation of the grease in the bearings. This is a common cause of premature bearing failure in both drive and driven machinery.

Lloyd (Tex) Leugner is the principal of Maintenance Technology International Inc. of Cochrane, Alta. He can be reached at 403-932-7620 or texleug@shaw.ca.

More to come

Look for Part 3 of this series, Compressor Lubricant and Lubrication Considerations, in the June 2006 issue. Part 1 appeared in the February 2006 issue, pg. 19.