Naturally, the thinner the
liquid, the faster it can flow
through the restrictions that
block the entrance into the pump
cavity. A suitable analogy would
liken the entrance into the cavity
of a positive displacement pump
to the hole in the bottom of a
Zahn cup that measures the viscosity of paint thinned for use in
an aerosol spray gun. The thinner the liquid, the faster it flows
through the hole. The lower viscosity measurement will be registered in SSU (Seconds Saybolt
Universal).
The pump cannot move the
liquid internally any faster than
atmospheric pressure will allow
the liquid to flow through the
restrictions on the inlet side of
the cavity. If the viscosity can be
lowered, then the amount of time to fill the pump cavity can
be reduced and the cycling activity of filling cavities can be
increased.
Functionally, this means that the pump speed can be
increased and a smaller, less expensive pump can be used if
the cake apparent viscosity can be decreased. Again, the feed
devices used on these pumps can have two functions: to fill
the cavity and reduce the viscosity of the cake. This is a major
difference between the function of the screws in a piston pump
and their function in a PC pump.
Piston pumps typically use feed screws to fill the cylinder without creating a great deal of shear, while PC pumps
typically use devices that try to minimize the viscosity of the
cake. Increasing the diameter of the cavity and reducing intake
restrictions is easier and often more advantageous for piston
pump manufacturers than for a PC pump OEM. Conversely,
adding a feed auger to the already rotating coupling rod on the
inlet of the pump is easier for the PC pump manufacturer. For a
piston pump OEM, a separate rotating auger mechanism must
be added to feed the reciprocating pumping mechanism.
Figure 1. Schematic of a screw feed device used on either a piston or PC-type cake pump. A
motor, speed reducer, timing gears, four sets of bearings, and at least two sets of stuffing boxes
are required. The device is really one pump designed to feed another pump with higher-pressure
capabilities.
The tradeoff here is that the piston pump is almost always
more expensive and has more moving parts, but the cake being
discharged from this pump is usually of a higher viscosity. A
typical complaint with a PC pump is that its cake looks runny
and acts more like a liquid than a cake. Note, though, that the
solids percentage has not been reduced. No water has been
added. In a few hours, the sludge will revert to a more cake-like condition.
But without precautions, the cake from a PC pump can
cause more of a housekeeping problem. To some users, this is
an advantage: when filling any container, including a dump
truck, a more liquid-type product has fewer air pockets and
flows easier into corners than a true cake. However, some land-fill operators may want a more cake-like product.
Continuous vs. Pulsating Flow
Another big difference between PC pumps and piston pumps
is evidenced in system pressures. PC pumps produce flow on a
continuous basis. In theory, they should not pulsate. In practice, air pockets are often captured in cake as it falls from a
