Which type of pumps are essentially self-priming and differ from centrifugal pumps?

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Rotary pumps move fluid by trapping a fixed volume with rotating parts and then displacing that volume from suction to discharge with each revolution. That positive-displacement action lets the pump draw liquid into the suction line and push it out even when the suction side isn’t perfectly filled, which is what we mean by self-priming. Because the pump doesn’t rely on creating a high-energy jet to pull liquid up from the source, it can start moving fluid under less-than-ideal priming conditions and continue as long as liquid is available, up to its lift limit.

This behavior is fundamentally different from centrifugal pumps, which depend on imparting velocity and pressure to the fluid with an impeller. Centrifugal pumps need a liquid column in the suction to develop flow and are much more sensitive to air or vapor in the suction line; they generally require priming to remove air before starting. In short, the rotary (positive-displacement) design provides intrinsic self-priming capability and a different flow mechanism than centrifugal pumps, especially useful for viscous liquids and varying pressures.