Once-Through Oil Vacuum Pumps. These pumps, also called OTO pumps, are the sliding-vane type that uses once-through oil to seal clearances and lubricate moving parts. Vanes are in slots in a rotor, mounted eccentrically to the pump chamber. As they rotate, centrifugal force pushes the vanes against the chamber walls, creating pockets whose size varies. This size variation creates suction which draws process gas into the pump from the vessel being evacuated. Compression occurs as the vanes rotate toward the discharge side of the device, decreasing the area and forcing the gas and lubricating oil against the discharge valve. The discharge valve opens slightly above atmospheric pressure. These pumps can handle acidic or otherwise corrosive vapors, because the once-through oil continuously flushes the vapors out. OTO pumps are usually found in pharmaceutical plants and chemical plants, for drying, evaporation, distillation or other vessel evacuation tasks.