What roles do the impeller and casing play in a centrifugal pump?

In a centrifugal pump, the impeller and casing work together to convert kinetic energy into pressure to move fluids.12

The Impeller

The impeller is a rotating wheel equipped with vanes. As it spins at high speed, it imparts velocity to the fluid, throwing it radially outward from the center of the pump. This action creates a partial vacuum at the center, which continuously draws more fluid into the system.12

The Casing

The casing surrounds the impeller and serves as the collection point for the accelerated fluid. In the common volute design, the casing features a spiral-shaped outlet that widens progressively. This geometry is critical because it slows the fluid's velocity, effectively converting that kinetic energy into increased pressure. In other designs, such as diffuser pumps, a ring of fixed vanes within the casing is used to provide a more controlled flow and efficient conversion of velocity head into pressure head.132