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11 111 ........ . . . . . . . . . . . . . . . . . . . . . . Special Pumps

The successive sets of seals form fully enclosed cavities which move
continuously from inlet to outlet. These cavities trap liquid at the inlet
and carry it along to the outlet in a smooth flow.

.4.5 Progressive cavity pump
This pump has been referred to as a single-end, single-rotor type of
screw pump where the pumping elements comprise a single rotor and a
stator. The stator usually has a double helical internal thread with a
pitch twice that of the single helical stator. This results in two leads on
the stator, and one on the rotor.
As the rotor rotates inside the stator, two cavities form at the suction
end of the stator, with one cavity closing as the other opens. The
cavities progress in a spiral from one end of the stator to the other. The
result is a flow with relatively little pulsation, and the shear rates will
also be low in comparison to radial pump styles.
The compressive fit between the rotor and stator creates seal lines
where the two components contact. The seal lines keep the cavities
separated as they progress through the pump with each rotation of the
rotor. The elastomeric stator and stainless steel rotor allow the pump to
handle large solid particles in suspension and a certain percentage of
abrasives.
The manner in which the rotor turns within the stator complicates the
mechanical design of PC pumps. As the rotor turns in the stator, the
centerline of the rotor orbits about the centerline of the stator. This
eccentric motion means the pump must be fitted with universal joints
to transmit power from the concentric rotation of the drive shaft to the
eccentrically rotating rotor. These joints must transmit torsional and
thrust loads. Designs of this drive mechanism range from simple ball-
and-pin mechanisms to heavy-duty sealed gear couplings.

Figure 9.20: Progressive cavity pump (Reproduced with permission of Moyno Inc.)

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