Problem solved

Compressed air optimization saves energy, improves production at forging plant

The Interstate Forging unit of Citation Corporation implemented a compressed air system improvement project at its Milwaukee forging plant.

The project enabled the plant to maintain an adequate and stable pressure level using fewer compressors, which led to improved product quality and reduced production downtime. The project also yielded annual compressed air energy savings of 820,000 kilowatt-hours (kWh) and $45,000, plus better maintenance scheduling.

With a total project cost of $67,000, the plant achieved a simple payback of 1.5 years. In addition, the project’s success established that no need existed to buy a new compressor. This resulted in avoided capital costs of roughly $60,000 for a new 200-horsepower (hp) unit.

Why did they do it?
Compressed air is vital to Interstate Forging’s production process because it supports grinding and pressing applications as well as the drop-forge hammers necessary
to manufacture various parts. The forging hammers are the most important compressed air application, and require a consistent pressure level of 95 pounds per square inch gauge (psig) to achieve reliable production. Prior to the project, plant operators tried to maintain a system pressure of 100 psig by running five compressors totaling 900 hp that generated up to 3,500 standard cubic feet per minute (scfm) at a discharge pressure of 105 psig.

Despite operating all five compressors and using a 2,500-gallon storage receiver, the system pressure fluctuated between 85 and 100 psig. The pressure fluctuations caused the drop-forge hammers to operate erratically, reducing product quality and increasing cycle time. Convinced that additional compressors were necessary, plant management brought in a U.S. Department of Energy resource partner to review the compressed air system. This resource’s role was to determine how much additional capacity was needed to eliminate the pressure fluctuations and to improve the system’s performance.

However, the resource found that the plant could establish and maintain the required system pressure by operating fewer compressors. The hammers’ intermittent air demand and insufficient compressed air storage were the main causes of the pressure fluctuations at points of use.

Another problem was an air leakage rate of about 20 percent of system output. Most of the air leaked from counterbalance cylinders in the hammers, from point-of-use applications and from some of the system’s distribution piping. The air leakage created artificial air demand, which made the compressors work harder to generate the needed air volume.

How did they do it?
Following the system review, plant personnel implemented a system-level project designed to allow the compressed air system to function effectively without the need to buy additional compressors. The first measure was to stabilize system pressure at the lowest level that met production requirements. To do this, plant personnel installed a pressure/flow controller (P/FL) to separate the demand side of the system from the supply side. In addition, they installed 5,000 gallons of compressed air storage capacity just upstream of the P/FL. Compressed air was set to flow into the storage receivers at 100 psig and to be released into the main header at 95 psig, plus or minus 1 psig.

Next, plant personnel initiated an innovative leak detection and repair campaign. In addition to finding and repairing the largest leaks in the distribution piping, plant personnel redesigned the shaft seals on the counterbalance cylinders so that repairing leaks on those cylinders could be accomplished without having to disassemble the cylinders. This redesign greatly simplified the task of repairing leaks on those pieces of equipment. They also decided to repair leaks on a daily basis instead of waiting until semi-annual maintenance shutdowns.

Conclusion
This compressed air system project yielded important energy savings, improved system performance and enhanced productivity. Currently, the plant operates effectively with three 200-hp compressors, whereas before the project it was unable to meet its air demand while operating five compressors totaling 900 hp at full capacity. The system pressure has been stabilized and lowered to 95 psig, and the remaining compressors (one 200-hp and one 100-hp unit) now serve as backup compressors. The stable air supply has reduced production downtime and improved product quality.

The leak repair effort has reduced artificial demand by almost 600 scfm, lowering the average system flow rate. The system’s average air demand has declined from between 3,000 and 3,500 scfm to between 2,400 and 2,600 scfm.

That’s a significant compressed air improvement!

This article was reprinted with permission from Energy Matters, a quarterly newsletter published by the United States Department of Energy’s Industrial Technologies Program.

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