A positive displacement pump's typical parts include which pair?

• A. Valve and Seat
• B. Shaft and Bearing
• C. Diaphragm and Piston
• D. Impeller and Volute

Answer: (C)

In a positive displacement pump, the fluid is moved by changing the volume of a chamber with a moving boundary. The essential pumping element is either a piston moving inside a cylinder or a flexible diaphragm that expands and contracts to trap and release fluid. A piston pump uses a rigid piston to displace fluid, while a diaphragm pump uses a membrane to achieve the same effect. These moving boundaries are what create the fixed, controllable volumes that define PD pumps.

The other parts listed aren’t what gives a PD pump its characteristic action: valves and seats manage flow direction but aren’t the displacement mechanism; shafts and bearings are support and drive components; impeller and volute belong to centrifugal pumps, which rely on fluid velocity rather than fixed-volume displacement.