As the Piston rod passes up through the stuffing box, the oil from the crankcase is scraped off by the lower sets of rings and is returned via drillings to the crankcase. Any oil that passes this primary set is scraped off by another set of rings, and is led away through a drain to a tell tale open ended pipe into a tun dish outside the engine from where it drains to a recycling tank.
As the piston passes down through the stuffing box, the top set of scraper rings will scrape off the contaminated oil into the bottom of the scavenge space, where it is drained away via the scavenge drains. However if these rings are faulty, then the oil may drain into the recycling tank.