Expander over-design factor

[DieguitoI]

Checking over-design criteria in a company standard, I found over-design factor for centrifugal compressor expressed as % of the volumetric flowrate while over-design factor for expander expressed as % of the mass flowrate.
Why expander over-design is based on mass flowrate and not on volumetric flowrate?

 

[georgeverghese]

Not clear to me either. If P and T and composition at inlet remain the same, then % volume design margin would be the same as % mass design margin.

 

[DieguitoI]

Over-design factor should be applied upon the design case, max volumetric flowrate case can be different from max mass flowrate case.