Which pump type is known for being essentially self-priming?

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Self-priming means the pump can draw liquid into the suction side and expel any air without needing someone to pour in priming water first. Rotary pumps achieve this because their positive-displacement action and internal valves trap a small pocket of liquid as they rotate. That trapped liquid helps evacuate air from the suction line and then fills the pump so flow can start, even if the line isn’t pre-filled with liquid. Centrifugal pumps, by contrast, rely on a full suction line and can’t develop suction if air is present, so they aren’t considered essentially self-priming. Diaphragm pumps can be self-priming in many designs as well, but the rotary type is the one commonly described as essentially self-priming due to its continuous, displacement-based action and valve arrangement.