Approaches Toward Achieving Optimum Efficiency in Rerates of Large Centrifugal Compressors

Abstract

Centrifugal compressors are commonly used within the process industries as prime movers. For many years, plants have been designed with "single train", unspared, major machinery in order to reduce construction, operating, and maintenance costs. This has resulted in many of the compressors being quite large, operating at power levels in the 10,000 to 50,000 horsepower range. When new, the machines are aerodynamically and mechanically designed and built to the state of the art as it exists at that time. However, these compressor casings are commonly rerated in later years when process modifications or plant expansions dictate duty changes, and the state of the art has advanced. But because of design constraints imposed by the characteristics of the reused hardware, the assumptions of the machinery supplier, and less than thorough definition by the owner, the best available new components are not always offered or utilized. Unfortunately, the constraints that result in using "the good", rather than "the best", state of the art, components are often not actually valid. Significant additional operating costs can result, though, when the small efficiency differences between "the good" and "the best" are multiplied by large power consumptions. These unrecoverable, lost opportunities are preventable.