The vacuum in the foreline siphoned the oil out of the pump.When a vacuum pump is shut off there is a significant differential pressure between the oil case of the pump and the foreline pressure at the inlet of the pump. This differential pressure will cause the air and oil to be siphoned up the foreline until the pressure equalizes. Most modern direct drive pumps incorporate an “anti-suckback “ valve to prevent this. These valves will fail over time if the foreline isn’t vented. If you have a belt drive pump they don’t incorporate an “ASV” and will backflow oil every time you shut them off unless you vent the foreline. We offer auto off safety vent valves for these pumps. The part numbers start with the letters ASVQF.
Sub-Category: Oil-Sealed Mechanical Vacuum Pumps