Citation Corp.’s Interstate Forging implemented a compressed-air system improvement project at its forging plant in Milwaukee. The project helped the plant to maintain an adequate and stable pressure level using fewer compressors, and led to better product quality and reduced downtime. It also yielded annual compressed-air energy savings of 820,000 kilowatt-hours (kWh) and $45,000, plus better maintenance scheduling.
At a project cost of $67,000, the plant achieved a simple payback of 1.5 years. In addition, the project proved there was no need to buy a new compressor — avoiding capital costs of roughly $60,000 for a new 200-hp unit.
Compressed air is vital to Interstate Forging’s production process. It supports grinding and pressing applications as well as the drop-forge hammers, Interstate’s most important compressed-air application. These hammers require a consistent pressure level of 95 pounds per square inch gauge (psig) to achieve reliable production. Prior to the project, operators tried to maintain a system pressure of 100 psig by running five compressors totaling 900 hp, generating 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-gal storage receiver, system pressure fluctuated between 85 and 100 psig. The fluctuations caused the drop-forge hammers to operate erratically, reducing product quality and increasing cycle time. Convinced that more compressors were necessary, plant managers brought in DOE Allied Partner Pneumatech/
CoflservAir to review the compressed-air system and determine how much additional capacity was needed to eliminate pressure fluctuations and improve system performance.
However, Pneumatech/ConservAir 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% 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 leakage created artificial air demand, making compressors work harder to generate needed air volume.
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 purchase additional compressors. The first measure was to stabilize system pressure at the lowest level that met production requirements. To do this, operators installed a pressure/flow controller (P/FL) to separate the system’s demand and supply sides. Also, they installed 5,000 gallons of compressed-air storage capacity 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 +/- 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 leaks on those cylinders could be repaired without having to disassemble the cylinders. This redesign simplified the task of repairing leaks on those pieces of equipment. It was also decided to repair leaks daily instead of waiting until semi-annual maintenance shutdowns.
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 back-up compressors. The stable air supply has reduced production down time and improved product quality.
The leak repair effort reduced artificial demand by almost 600 scfm, lowering the average system flow rate. Average air demand has declined from 3,000-3,500 scfm to 2,400-2,600 scfm.
Editor’s Note: Adapted from Energy Matters, Fall 2003, a quarterly publication of the U.S. Dept. of Energy’s Industrial Technologies Program. The program offers resources specific to compressed-air systems, including publications, software tools, and training information. Visit www.oit.doe.gov/bestpractices/compressed_air/, or order from the OlT Clearinghouse, Tel. 800-862-2086.