The maximum number of stages per casing should normally be limited to eight. It is usually limited by rotor critical speeds, although in a few cases temperature can be a limiting factor.
Most multistage centrifugal compressors operate between the first and second criticals (flexible shaft rotor). Figure200-29 shows the location of critical speeds in relation to the operating speed range. API specifies the required separation between critical speeds and the compressor operating range. As the bearing span is increased to accommodate additional impellers, the critical speed decreases, with the second critical approaching the operating range. While some manufacturer’s bulletins indicate as many as 10 or more stages per casing, designs exceeding eight impellers per case should be carefully evaluated against operating experience from similar units.
For compound, or sidesteam loads, additional stage spacing may be required to allow for intermediate exit and/or entry of the gas. In these applications, the number of impellers would be reduced accordingly.