This one is the most stubborn hydraulic myth in the world. If you ask me - there's no explanation for it. No explanation at all. I can understand where it came from, but I can't understand why it is still here in this day and age.
The myth is - since a hydraulic pump (or a motor) is connected to oil - there is no need to fill the respective case with oil before its commissioning because when the equipment begins operation, the case will fill by itself. (Of course, I am referring to pumps that have oil-filled cases).
Just a week ago I had a conversation with a mechanic responsible for service and maintenance of several pieces of hydraulic equipment, and as we were discussing a recent premature pump failure, I (very diplomatically) mentioned that during the pump replacement procedure it wouldn't be a bad idea to make sure its case is filled with clean hydraulic fluid - to which I got a very quick reply that could be resumed to:
"You don't need to teach me how to replace hydraulic pumps. I've done it many times, and never have I had to waste time filling cases! Hydraulic pumps are already connected to suction lines, dah! Relax - I am a trained professional. I know what I am doing!"
Well, my very trained friend, you are not entirely correct! You can't start a "dry" unit and hope that it will "lubricate itself" immediately. Of course, the case will fill up, eventually - it's just that the "eventually" may be long enough to cause damage!
I have worked with pumps and motors for a long time, and I can come up with dozens of equipment commissioning scenarios where, indeed, there is absolutely no need to fill the case with oil - but this only means just that - there is o need to fill - simply because there is already enough oil inside the case to guarantee a problem-free start.
I can equally come up with dozens of scenarios when it will take minutes to fill the case with oil, subjecting the internal bearings and sliding surfaces to insufficient lubrication and reduced heat removal - which will guarantee a premature or even immediate failure.
Maybe a car analogy can help here. Everybody knows cars, right? Which of the two procedures sounds better?:
1) drain the crank-case
2) put the new oil in
3) start the engine
1) drain the crank-case
2) start the engine
3) put the new oil in
See the point?
When I start a new or an overhauled unit and don't fill its case with oil - I can explain why I am one hundred percent sure that there is already enough oil inside to start the system safely. So, when you commission a new pump or a motor and you can't do the same - do yourself and your pump a favor - and fill the case with hydraulic fluid!