I am assuming this unit has a remote condenser. Is ther liquid refrigerant coming back down the discharge line during the off cycle? Is the oil seperator in a cold enviroment where the refrigerant can be condensing inside the unit during the run cycle? Also the have had diferential pressure valve leak by during off cycle filling oil seperators with liquid refrigerant.

 

I am assuming that we are having liquid coming back down the discharge line into the oil sep during the off cycle.
The strange thing is that when i arrived on site the oil return line was freezing for a few start ups, but then eventually warmed up. I think that maybe because the oil strainer was dirty(which caused the system to turn off on oil failure, without pumping down) and then I started it
it had refigertant where it shouldnt have?

 

all you need,is a check valve on the discharge line after the oil separator.....

 

I had this happen to me years ago on a Hussman twin rack. Liquid is free draining from the roof top condenser and loading the oil seperator. Liquid refrigerant is heavier than oil, it goes to the bottom, the float on the oil level control opens and fills the crankcase with liquid. I installed traps in the condenser lines and that solved the problem. I've seen check valves used also, but they were a source of major problems, sticking shut, busting compressors.

 

Oil seperator efficiencies are all about Discharge superheat,Discharge superheat<22degrees you will loose oil