Standardization of Compressor Components
Process centrifugal compressors of the single-shaft type are characterized by an almost infinite number of variation options. They compress gases with molar masses of between 2 and 100, covering volume flows within the range of 0.1 to 200 m3/s and dischar
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Process centrifugal compressors of the single-shaft type are characterized by an almost infinite number of variation options. They compress gases with molar masses of between 2 and 100, covering volume flows within the range of 0.1 to 200 m 3/s and discharge pressures from below atmosphere to 800 bar, many with side streams or extractions. From one impeller up to 25 impellers are arranged inline or back-to-back in up to four casings. Compressors are either uncooled or have a maximum of three intercoolers per casing, resulting in a maximum number of eight nozzles per casing. Process compressors are driven by electric motors, steam or gas turbines either directly or via a gear box between the drive and compressor or between the casings. Such a vast array of combinations excludes any package-unit standardization as is the case, for example, for plant-air or water-chilling compressors, where the gas never changes and the pressure ratios are within narrow limits. In spite of all this, there were early attempts in the industry to standardize, i.e. to limit the design options of impellers and of stationary components; nobody wants to reinvent the wheel for every new compressor to be built, because extrapolating means reducing prediction accuracy. There is, however, no doubt that any attempt to bring about a standardization has to concentrate on the stage as the smallest element common to all process compressor variants. Basically, there are two philosophies for standardizing impellers: meridional impeller blade cutoff and scalable fixed-geometry impellers.
10.1 Meridional Impeller Blade Cutoff A wide-spread conventional method is to work with a predetermined scalable impeller blade shape in space; from contract to contract, cut-outs from this singlecurved (2D surface) or twisted (3D surface) confined area are calculated that fit the special application. Once the impeller outer diameter is set, the meridional impeller contour is adapted in this way to arrive at the following necessary parameters: • • • •
hub diameter (to attain the necessary shaft stiffness), eye diameter (to attain the specified volume flow), impeller exit width (to attain the envisioned head), impeller diffusion (to attain anticipated efficiency and operating range).
K. H. Lüdtke, Process Centrifugal Compressors © Springer-Verlag Berlin Heidelberg 2004
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10 Standardization of Compressor Components
The method is straightforward for impellers having 2D-blades, where the meridional hub contour can be regarded as fixed. The blade cutoff is carried out on the shroud side only, giving the impeller the calculated inlet and exit widths: highly convergent with high molar masses and (almost) parallel with hydrogen. Of course, this requires certain base master models at different flow-coefficient levels, e.g. one for qJ= 0.07-0.05 through shroud-side cutoff, the next one for qJ =0.05-0.03, and so on. This takes care of the different eye and hub diameters and lower blade inlet angles at lower flow coefficients. And it requires most likely a second master-mo
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