Select the Right Centrifugal Compressor

Hermetically sealed direct-driven units offer economic and technical advantages

By Amin Almasi, rotating equipment consultant

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Chemical processors increasingly demand centrifugal compressors that not only provide high reliability and availability, and exceed health, safety and environmental requirements, but also that come in simple, lightweight and compact packages. Moreover, the units often are expected to cope with challenging conditions (e.g., high pressure, sometimes extreme temperatures, corrosion, wide operating range, operational flexibility and dynamic loadings). The large capacity of many plants, the remote location of some sites, the long uninterrupted periods of operation expected (often five years or more between plant turnarounds) and other factors can make any downtime very costly.

A classic gear-unit-driven centrifugal compressor using seals and a lubrication oil system requires relatively high maintenance, regardless of whether it uses a gas turbine, steam turbine or conventional electric motor as the driver. A superior option is a hermetically sealed direct-coupled variable-speed electric-motor-driven centrifugal compressor using magnetic bearings. The active magnetic bearings generally support both the compressor and the electric motor shafts. These bearings entirely eliminate the need for a lubrication oil system. The hermetically sealed package also obviates shaft-end seals. Altogether, this modern design simplifies installation and reduces the footprint to less than 50% of that of a conventional centrifugal compressor.

A number of well-known vendors now offer such compressors, but because these units still aren’t that common, let’s go over some guidelines and practical recommendations.

Most compact integrated compressors feature a barrel-type centrifugal compressor casing bolted to a variable high-speed electric motor in such a way as to form a sealed package. An alternative design couples two barrel compressors on each side of a centrally located electric motor.

Hermetically sealed direct-driven centrifugal compressor designs usually are available in standard ratings. For example, one manufacturer offers two ratings: discharge pressures around 20–60 bar for standard applications, and about 70–150 bar for high-pressure services.

The impellers typically are arranged in a back-to-back configuration inside the compressor casing. This allows the compressor to operate with the two impeller groups in parallel or, more usually, in series.

The most compact option is a vertical hermetically sealed compressor with the driving electric motor located above the multistage compressor. This configuration could be provided with a flange that allows lifting of the upper section with the electric motor and the compressor bundle without disconnecting the gas piping from the compressor inlet and discharge flanges placed in the lower section. In another design, the whole compressor module must be lifted for maintenance. Figure 1 shows a vertical hermetically sealed compressor. Figure 2 shows the lower section of the compressor casing in such a compressor.

The impeller number can vary from a minimum of one to a maximum of eight, depending upon the compression ratio required by the service. Usually closed-type impellers are used. The nominal impeller diameter typically ranges from 100 to 1,100 mm. Semi-open 3D impellers also could be used.

The design of such a compressor requires close cooperation between the compressor manufacturer and the electric motor maker. In some cases, the electric motor and compressor are connected via a conventional metallic flexible coupling to ease design, operation and maintenance. This could be a reliable high-torsional-stiffness coupling with a service factor usually above “4.” The flexible connection of the two shafts provides some essential freedom in the design of the compressor and the driver. An alternative is a common shaft for both the electric motor and compressor; vendors that can produce both a high-tech compressor and an advanced electric motor offer this configuration.

Variable-speed (high-speed) electric motors in standard ratings usually are employed. The motor nominal speed frequently varies according to the compressor size. As a rough indication, smaller packages (e.g., those below 8 MW) often rely on motors with speeds up to around 20,000 rpm. Speeds from 6,000 to 11,000 rpm typically are used for above-8-MW units. These electric motors usually can be operated between 60% and 105% (sometimes, 70% to 105%) of their nominal speed.

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