Figure 7. An enlarged auger, attached to the eccentrically rotating coupling rod, and an enlarged extension tube allow a feed rate on the
auger that is 3X greater than the capability of the pumping elements.
so they become a constant point of maintenance concern. entry into the pumping element. During the past ten years,
Progressive cavity pumps have many unique character- this common design has proven quite acceptable and reliable
istics. The cavity created between the rotor and stator does for handling the discharge from belt-filter presses at solids per-
not change its shape as it moves through the pump. It has no centages approaching 30 percent.
valves. It does not inherently pulsate. It has relatively low inter- An enlarged auger attached to the eccentrically rotating
nal velocities because the rotating parts have relatively small coupling rod and an enlarged extension tube allow a feed rate
diameters and it is a pump designed with a bearing on each on the auger that is three times greater than the capability of
end. On the end with the motor, the pump has traditional the pumping elements (see Figure 7). Circulation of the cake
tapered roller bearings, lubricated with oil or grease, like any in the extension tube dramatically reduces the apparent vis-
other rotary pump. cosity of cakes up to 30 percent solids so they can be easily
On the other end, the polished and hardened helical rotor pumped. Still, this pump only requires one stuffing box and
is rotating inside a rubber-lined tube. In other applications, one set of bearings, both being subjected to only atmospheric
this configuration is easily recognized as a marine bearing that pressure.
is common in marine vessels ranging from 21-ft inboard ski However, industry constantly demands more efficient
boats to Dallas-class attack submarines. Rubber, with water as dewatering devices to reduce the amount of water that must
a boundary layer, is amazingly slippery. All of the progressive be transported in sludge cake, thus requiring the feed mecha-
cavity pump manufacturers, at least in their early years, pur- nism into the pump be made increasingly more efficient and
chased stators from manufacturers of marine bearings. effective.
As such, this bearing provides an excellent support for Because the rotor in the pump has to rotate eccentrically,
a feed and/or shearing auger that enables the PC pump to no method really exists to attach an auger to the coupling rod
handle sludge cake and other highly viscous products, with- and create a truly positive pressure in the extension tube. Here,
out adding additional wearing parts or maintenance points. the auger feed system used in a piston pump has a clear advan-
Though it contains as much as 40 percent solids, sludge cake is tage. The latest design for PC pumps overcomes this limitation
still mainly water and maintains the lubricating qualities that (see Figure 8).
allow the stator to operate as a marine bearing. By adding a ribbon auger that is attached to a plate fixed
The cake handling capabilities of the PC pump are easily to the pump drive shaft, behind the concentrically rotating
improved by using the eccentrically-rotating coupling rod and universal joint, a true feed auger mechanism can now be avail-any attachments (like an auger) to more efficiently reduce the able in a PC pump without having to add an expensive, main-apparent viscosity of the cake so that higher viscosities and tenance prone separate feed auger system. This device allows
solids percentages can be pumped. the PC pump to easily handle biosolids up to 40 percent.
By increasing the diameter of the auger and the extension Again, the pump can be fabricated so the hopper matches
tube so that it overfeeds at a rate of 200 percent instead of 30 the discharge dimensions of the dewatering device. The auger
percent, the cake is circulated and sheared a great deal prior to feeds at almost five times the capacity of the rotor and stator. A
Figure 8. A true feed auger mechanism can be available in a PC pump by adding a ribbon auger attached to a plate fixed to the pump
drive shaft behind the concentrically rotating universal joint.
