Rotary lobe pumps are self-priming, valveless, positive-displacement pumps.

• A. False
• B. True
• C. Not applicable
• D. Sometimes

Answer: (B)

Rotary lobe pumps move liquid by trapping fixed volumes between rotating lobes and the casing. As the lobes turn, these sealed pockets form on the suction side, drawing liquid into the pump, and then migrate to the discharge side where they release the liquid. This creates a predictable, volume-based flow, which is the hallmark of a positive-displacement pump.

They are considered valveless because there are no internal valves that open and close during the pumping cycle to control flow; the movement is governed entirely by the lobes’ geometry and the casing. Any backflow prevention is usually provided by external components or system design, not by internal pump valves.

The self-priming aspect comes from the pump’s ability to evacuate air and draw liquid into the suction line as it starts, without requiring the suction line to be completely filled. In practice, self-priming performance depends on factors like suction lift, fluid viscosity, and proper venting, but rotary lobe pumps are generally viewed as self-priming within typical operating ranges.