I have only seen this problem one time and the situation was very unusual. In our system, the seal oil and lube oil system were side by side. They both used the same product. The seal oil system included a large overhead tank that served as the head tank to provide the necessary differential pressure for the iso-sleeve seals. It also served as a run-down tank to provide seal oil for coast down in case both seal oil pumps were lost. Since the run-down tank was quite large, a cross over line existed to allow the operators to transfer oil from the seal oil tank to the lube oil tank to keep it from overflowing on shut down. This cross-over line had a valve that was leaking by. Since the seal oil reservoir was maintained at a level lower than the lube oil reservoir, oil was migrating from lube oil into seal oil. I recognize that this is probably an obscure example and may not be relevant to your situation. I would still check your P&ID and do a field walk-down to check for cross over lines.
Otherwise, a restricted drain line on the lube oil return could cause oil to back-up and cross over through the separation seal into the clean seal-oil return line. A plugged breather vent could cause poor drainage on the lube oil drain. A plugged breather on the seal oil drain cavity could cause this cavity to pull a slight vacuum and draw lube oil across the separation seal.
If work was done recently, I would verify that all lube oil supply orifices were correctly reinstalled. A missing orifice could cause excessive lube oil flow which might overwhelm the drain. I would verify that the lube oil supply pressure is still set correctly. A malfunctioning pressure controller could result in excessive lube oil flow.
As already noted, more information would likely result in more useful responses.
Johnny Pellin
