Dynamic sealing -- or hydrodynamic sealing -- is accomplished by a combination of pumping vanes on the rear shroud of the impeller and a secondary expeller located behind the impeller, which rotating with the shaft, impart a rotary movement on the liquid in the pump resulting in the liquid centrifugally moving away from the center shaft penetration in the case. An equilibrium in the back of the pump is achieved between the pressure of the liquid in the pump and the atmosphere, creating an air void at the shaft penetration of the case while the pump is running.
