You haven't said what type of pump (assuming it is a pump)what type of bearings and their installation configuration etc. A bit more info might get more meaningful input.
pump is Centerline mounted end suction pump oh2, with antifriction bearing single row, deep groove, radial bearings and duplex, single row, 40° angular contact thrust bearings. i wondering which are criteria to select ext. lub system?
Not really my area of expertise, doesn't sound all that difficult for a standard oil lub system unless there is a heat removal problem, however, my suggestion would be to contact either the pump manufacturer or one of the major bearing manufacturers for their advice.
At my facility we have several 1000+ bhp installations which use splash lubricated bearings with no difficulties. However, these are BB3 pumps which do have different rotordynamic characteristics than the OH2 pumps.
I would contact the pump OEM and determine what the pump needs.
Are you having bearing problems in this particular installation and believe an external system would work best? Sometimes a bearing problem is a symptom of something else. We had a rash of bad thrust bearings in some BB3 pumps after they had been re-staged. Turned out the pump shop induced a hydraulic thrust imbalance by blanking the wrong stage.