Nominal diameter

What is the relation between the real diameter and the nominal diameter of centrifugal compressor?
For example: BCL307 (after the Operating manual of the machine, 30 it is the nominal diameter =300mm, but the real diameter of impeller is 370mm)


They have standardized frame sizes for the casing. In this particular case, 300 size can accomodate a range of impeller diameters [D2min,D2max]. Note that this range may vary by impeller type (2D or 3D) and/or family.
An impeller of 370mm should be inside this range and fits inside the selected casing size.

Actually this is more of a tagging issue. It matters if you ask a reference list for example, you would want to compare BCL 30x.

Above is a snippet.