Integral geared compressors for spared applications

January 14 2013 - TI Staff

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Compressors are the highest producers of revenue among driven equipment, in upstream and downstream industries. They are usually un-spared, and are considered as critical equipment items. Compressor reliability is therefore a high priority, and is directly proportional to company profit. The best practice is to use integral geared centrifugal compressors only for spared compressor applications.

The typical applications of a multistage, integral gear compressor casing are plant and instrument air and inert gas compressors. The casing consists of a cast or fabricated gear case and bolted on individual stage cast casings. Some designs allow rotor removal without having to disconnect the process piping. The advantages of an integral geared compressor are the reduced number of stages required and higher efficiency. This is because all stages are intercooled, and the integral speed increasing gear allows all stages to operate in a higher specific speed.

Multistage, integral geared compressor manufacturers have begun to offer these units for hydrocarbon service. Before purchase, one has to confirm satisfactory field operating experience on similar gases. The requirements for gas or oil seals, varying gas composition and flow rates and the possibility of fouling can significantly reduce the reliability of this type of compressor opposed to air or inert gas applications.

Un-spared integral geared compressors are not used for un-spared process duty because they were originally designed for plant and instrumental air services which would use spared compressors. They are maintenance intensive since they use multiple bearings, seals, gear meshes and operate at high speeds (above 50,000 RPM for the last stage. Because the gas is cooled after each impeller stage, their performance characteristics and reliability are dependent on intercooler condition. Since they are typically supplied with only an overall surge protection system and not individual impeller stage protection, they are prone to surging if intercoolers do not attain design heat removal requirements.

Un-spared integral geared compressors have lower reliability than between bearing centrifugal compressors (95 percent) and greater MTTR due to their many components. Many integral geared compressor failures as well as additional maintenance time required to inspect and install bearings and seals confirm acceptable gear mesh contact. Though some clients do not use un-spared integral geared compressors for process applications including process air applications, this best practice has been used since the 1990s and has successfully convinced project teams not to use integral geared compressors for un-spared process applications. Between bearing compressors were justified on the basis of optimum reliability (99.7+ percent compared to 95 percent) and less mean time to repair (MTTR).