1) Lot of data out there on separation velocities for oil separators on Ammonia refrigerant but for a horizontal that deliberately holds some oil, the limit is about 0.9 m/sec....Which should be calc'd for the Lowest anticipated discharge pressure.

2) the Coalescing Elements have a max effective velocity and a max pressure difference...Lots of little pieces in the Pressure Difference that are associated with the oil itself but the "basic" for an ammonia compressor is going to calc to about 0.5 m/sec. based on the Inside diameter of the cartridge at 80-deg C. gas temp.

3) the "other" methods