Which description best matches rotary lobe pumps?

• A. They are non-priming, centrifugal pumps
• B. They are axial-flow pumps used for air handling
• C. They are self-priming, valveless, positive-displacement pumps
• D. They are designed for heating fluids only

Answer: (C)

Rotary lobe pumps move fluid using rotating lobes that trap a fixed volume of liquid in pockets between the lobes and the casing. As the lobes rotate, these pockets form on the suction side, fill with liquid, and then move to the discharge side where the liquid is expelled. This action creates a steady, controllable flow because the amount pumped with each revolution is essentially fixed, which is the hallmark of a positive-displacement pump.

Because the flow is generated by the lobes themselves rather than by opened and closed valves, there are no internal valves controlling the intake and discharge—hence the term valveless. The design also lends itself to self-priming behavior, since the pump can fill the suction pockets with liquid from the source and start pumping without requiring a separate priming step, at least under typical conditions.

This combination—self-priming capability, absence of internal valves, and positive-displacement action—fits rotary lobe pumps well. They differ from centrifugal pumps, which rely on impellers and centrifugal force and are not typically self-priming; they aren’t axial-flow air-handling devices, and they aren’t limited to heating fluids.