Rotary piston vacuum pump is suitable for air and other ordinary gases, with a gas ballast valve, witch could be applied to pump condensable gases (such as vapor). The pump should be equipped with appropriate accessories if the gas pumped has the following features, such as rich in oxygen, explosive, corrosive to ferrous metal and chemical reactive with the vacuum pump oil. This series of pumps could be operated in single unit or act as the pre-process pumping unit for the next high vacuum pumps, such as diffusion or roots pumps. It is widely used in the industries of vacuum coating, vacuum heat treatment, vacuum impregnation, vacuum smelting, vacuum metallurgy, vacuum drying, molecular distillation and altitude simulation testing.