Compressed air is widely used throughout a process plant for automation and control. It can also be used for other process applications like injectors or Venturi tubes. Pneumatic devices such as air tools and cylinders is a third application for compressed air. With so many applications it often times does not get the priority it should. Compressed air is an area within utilities where significant improvement opportunities exist. Included in this are control methodology, storage, leak mitigation and advanced controls.
Most process plants will have two or more air compressors for their compressed air needs. This allows for increased capacity. It also allows for reliability through redundancy. It is recommended to operate these in a lead-lag configuration, rather than in dual load. This means one air compressor will be the primary and supply most of the load for the plant and a secondary compressor to provide additional demand. It is important to remember that compressors are most efficient about an 80% load. Rather than having one 100HP compressor, two (2) 50 HP compressors should be considered. Finally, alternating the primary/secondary compressors will ensure that both are realizing the same operating time, thereby extending the lives of both.
One of the areas of compressed air systems that is commonly misunderstood is the concept of storage. Compressed air best practices utilize a storage vessel or receiver for on-demand capacity. However, the inlet and outlet pressures of the storage vessel are often the same. This only creates a wide spot in the pipe rather than energy storage. Feeding the receiver at a higher pressure and then releasing it at a lower pressure actually creates true compressed air storage. Consider a dam along a river. The outlet rate is lower than the inlet rate until a head is built. If more water is needed downstream, the dam can be opened. If the levels on both sides of the dam were the same, the maximum flow rate is the same as the inlet rate. Compressed air should be treated the same. By implementing this strategy, it lowers the demand on the compressors and allows them to operate more efficiently.
Compressed air leaks are extremely costly to a process plant. Even the smallest leak can cost hundreds of dollars per year in energy consumption. To address this problem there are many companies that offer compressed air audits. Generally, this consists of ultrasonic leak detection equipment. Identifying leaks and eliminating them reduces demand on equipment and provides immediate cost savings. However, this method does not get to the heart of the problem, which is determine where and how the compressed air is being used.
It is critical to measure what equipment or area is using the most compressed air. Naturally, and increase in air demand can alert you to leaks in the system. However, measuring your compressed air consumption will determine how to best operate the compassed air system. Trending system usage will determine how compressors are operated, especially in plants with multiple compressors. Compressed air systems can be quite cyclic in nature. Ramping up compressors during peak usage, and throttling them back during low demand periods reduces utility costs and extends the life of equipment. While general air consumption can be tracked, knowing the specific equipment that uses it allows for a much improved control strategy.
As with other utility systems in a process plant, using instruments with advance diagnostics can provide significant reliability improvements. Compressed air pressure and flow should be measured using instruments with advanced diagnostics. Header temperature from the compressors should also use temperature transmitters with advanced diagnostics to ensure a reliable measurement. For general controls, the air compressor itself will benefit with high density temperature measurement on the compressor, motor, and cooling system. While these measurements will commonly exist as part of the controls package, having more robust measurement will lend itself well to a reliable compressed air system.
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