The progressing cavity (PC) pump in its most common design has a single threaded screw or rotor, turning inside a double-threaded stator (Figure 1.15). The stator is made of an elastomeric material, and the rotor has an interference fit inside the stator. As the rotor rotates inside the stator, cavities form at the suction end of the stator, with one cavity closing as the other opens. The cavities progress axially from one end of the stator to the other as the rotor turns, moving the liquid through the pump.

Advantages of PC pumps include their ability to pump highly viscous liquids (as well as very low viscosity liquids), shear sensitive liquids, fragile solids, and abrasives. Also, the pump produces very little pulsation and is self priming even when dry. The maximum pressure is limited to about 75 psi, but the PC can be set up with many stages in series using the same driver, so it can achieve upper pressure limits well over 1000 psi. Another advantage is that the packing or seal sees suction pressure rather than discharge pressure.

Disadvantages of the PC pump include the relatively higher cost of replacement parts, its large floor space requirement, the fact that the pump cannot run dry for an extended period, and upper temperature limits of about 300°F. (The material of the elastomeric stator usually sets the upper temperature limit.) Starting torques are quite high, so the pump motor may need to be much larger than would be the case with other types of PD pumps.

Note that as the rotor turns, the centerline of the rotor orbits about the centerline of the stator. There are several methods to connect the PC pump’s drive shaft to its rotor and to account for the elliptical motion of the rotor shaft. Two common configurations are the pin drive connecting rod and the crowned gear drive connecting rod .