PC Pumps
The feeding mechanisms in progressive cavity pumps have
changed substantially over the last two decades and are headed
in at least two different directions. One direction is towards
more positive feeding. The other is towards higher shear forces
to further reduce apparent viscosities.
The original pumps used 40 to 50 years ago had simple
open inlets with an auger welded to the eccentrically rotating
coupling rod, between the drive shaft and the rotor. The most
basic open hopper PC pump (see Figure 4) has an auger on the
coupling rod, no extension tube, and cast iron fixed dimension
inlet housing. This design is suitable for discharge from gravity
thickeners and solids of no more than 10 percent.
The next generation PC pump (see Figure 5) typically
included an extension tube that enclosed the auger in an area
between the inlet hopper and the cavity created by the rotor
and stator. The auger is designed to feed at a rate about 30
percent greater than the capacity of the pumping cavity to
ensure more positive feeding. The circulation of the cake in the
extension tube imparts additional shear that helps to reduce
the apparent viscosity of the cake and promote better filling
of the cavity.
This design is often provided with a fabricated housing
that allows the sludge cake to fall vertically into the pump,
instead of building up on the sides of a shallow sloped transition hopper in a standard housing. This design could handle
sludge to about 17 percent and works very well on the discharge from gravity thickeners.
Counter-rotating bars with paddles were added to the
pumps to prevent bridging of sludge cake above the feed auger
(see Figure 6). Without these bridge breakers, it was common
for the auger to drill a hole in the cake. But when placed in
close proximity to the auger, these bridge breakers could generate substantial amounts of additional shearing energy that
could also significantly reduce the apparent viscosity of the
sludge cake.
This design was generally effective at handling centrifuge
discharge at over 35 percent solids. It has also been successfully
used to mix calcium oxide (CaO) into the sludge to produce
an exothermic reaction in the sludge, or increase the pH significantly to produce a Class B or Class A biosolids. Total solids
could exceed 52 percent.
The most effective of these designs include a separate variable speed drive for the bridge breakers (since their primary
role is to generate shear), while the pump speed can be regulated to meet the discharge rate from the dewatering device.
Until about ten years ago, all PC pump manufacturers
had similar designs until some of them decided to adapt the
system used by piston pump manufacturers. The advantage of
this approach is that the screw feed mechanism can be conveniently located directly below the discharge of the dewatering
device and the pump, like a piston pump, can then be placed
in a more suitable position.
The unfortunate aspect of this design is now the user
really has two pumps to maintain. The screw feeder must now
generate a positive pressure that not only pushes cake into the
pumping cavity, but also overcomes any friction loss in a transition chamber or pipe between the screw feeder and the pump.
This is a difficult job for a simple screw-feeding device.
As a result, these twin-screw feeders have become quite
complicated and maintenance-prone. Each shaft must be
reinforced with bearings, driven with a motor and gears and
sealed to prevent leakage. Naturally, bearings are not typically
designed to operate in an environment of pressurized sludge,
Figure 6. Counter-rotating bars with paddles, or bridge breakers, were added to the pumps to prevent bridging of sludge cake above
the feed auger.
