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Thursday, March 31, 2011

Preventive Maintenance Strategies for Compressed Air Systems

A brewery neglected to perform routine maintenance on its compressed air system for years. As a result, two of its centrifugal compressors, whose impellers had been rubbing against their shrouds, were unable to deliver the volume of air they were rated for and one of those units had burned up several motors during its lifetime. In addition, plant personnel did not inspect the system’s condensate traps regularly. These traps were of a type that clogged easily, which prevented the removal of moisture and affected product quality. Also, the condensate drains were set to operate under the highest humidity conditions, so they would actuate frequently, which increased the system’s air demand. As a result, energy use was excessively high, equipment repair and replacement costs were incurred unnecessarily, and product quality suffered. All of this could have been avoided through regular maintenance.

Like all electro-mechanical equipment, industrial compressed air systems require periodic maintenance to operate at peak efficiency and minimize unscheduled downtime. Inadequate maintenance can increase energy consumption via lower compression efficiency, air leakage, or pressure variability. It also can lead to high operating temperatures, poor moisture control, excessive contamination, and unsafe working environments. Most issues are minor and can be corrected with simple adjustments, cleaning, part replacement, or elimination of adverse conditions. Compressed air system maintenance is similar to that performed on cars; filters and fluids are replaced, cooling water is inspected, belts are adjusted, and leaks are identified and repaired.

A good example of excess costs from inadequate maintenance can be seen with pipeline filter elements. Dirty filters increase pressure drop, which decreases the efficiency of a compressor. For example, a compressed air system that is served by a 100-horsepower (hp) compressor operating continuously at a cost of $0.08/kilowatt-hour (kWh) has annual energy costs of $63,232. With a dirty coalescing filter (not changed at regular intervals), the pressure drop across the filter could increase to as much as 6 pounds per square inch (psi), vs. 2 psi when clean, resulting in a need for increased system pressure. The pressure drop of 4 psi above the normal drop of 2 psi accounts for 2% of the system’s annual compressed air energy costs, or $1,265 per year. A pressure differential gauge is recommended to monitor the condition of compressor inlet filters. A rule of thumb is that a pressure drop of 2 psi will reduce the capacity by 1%.

All components in a compressed air system should be maintained in accordance with the manufacturers’ specifications. Manufacturers provide inspection, maintenance, and service schedules that should be strictly followed. Because the manufacturer-specified intervals are intended primarily to protect the equipment rather than optimize system efficiency, in many cases, it is advisable to perform maintenance on compressed air equipment more frequently.

One way to tell if a compressed air system is well maintained and operating efficiently is to periodically baseline its power consumption, pressure, airfl ow, and temperature. If power use for a given pressure and flow rate increases, the system’s efficiency is declining. Baselining the system will also indicate whether the compressor is operating at full capacity, and if that capacity is decreasing over time. On new systems, specifications should be recorded when the system is fi rst installed and is operating properly.

Types of Maintenance

Maintaining an air compressor system requires caring for the equipment, paying attention to changes and trends, and responding promptly to maintain operating reliability and effi ciency. To assure the maximum performance and service life of your compressor, a routine maintenance schedule should be developed. Time frames may need to be shortened in harsher environments. Proper maintenance requires daily, weekly, monthly, quarterly, semi-annual, and annual procedures. Please refer to the Compressed Air System Best Practices Manual for the types of procedures that are relevant to the compressors and components in your system. Excellent maintenance is the key to good reliability of a compressed air system; reduced energy costs are an important and measurable by-product. The benefi ts of good maintenance far outweigh the costs and efforts involved. Good maintenance can save time, reduce operating costs, and improve plant manufacturing efficiency and product quality.


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