centrifuge or belt press into the pump.
The air gets compressed and sometimes
collects before it can be expelled, making
it often seem as though the pump is pulsating. The pump does not really pulsate;
it continually pumps.
Reciprocating pumps pulsate. Each
cylinder must be evacuated. The piston
must recover its entire length, then the
cylinder must be filled. Most reciprocating
piston pumps are hydraulically actuated.
They often remain in an open position to
allow filling for an extended period, compared to a mechanically operated reciprocating pump, where pistons simultaneously pump and recover.
This results in a relatively sporadic
discharge from the end of the pump. The
screw takes some time to feed and fill the
piston. The piston must accelerate to a
midpoint, decelerate to a stop and recover
in the opposite direction, but with the
same velocity profile. This not only results in sporadic flow,
but the peak velocities must also be a minimum of twice the
average of a constant flow profile – and they can reach three
times that of a constant velocity pump due to the fill time.
Piston pumps must inherently generate two to three times
the pressure of a PC pump to produce the same flow rates per
minute or per hour as that of a PC pump for the same application. Piston pumps can no doubt generate more pressure than
a PC pump, and an eight-stage or nine-stage PC pump is not
typically recommended for any run of pipe longer than 450
linear feet, unless a boundary layer injection system is used.
Typically a PC pump manufacturer tries to size the discharge pipe for only 1-psi/ft of pipe and use a larger pipe diameter than a piston pump manufacturer. The piston pump manufacturer may recommend 2-psi/ft of pipe friction loss and a
smaller pipe size because they can generate more pressure.
Figure 3. Typical velocity and flow profile with a reciprocating piston-type cake
pump.
Pipe Sizing
I have a firm stance regarding pipe diameter sizing. In a PD
pump (GPM x psi)/1714 = HP, so any decrease in pressure
results in a power savings, along with less wear and tear on the
pump.
A designer should take any opportunity to reduce wear
and tear and power consumption, so selecting a larger pipe
diameter always yields a long-term savings. Going to larger
pipe sizes certainly diminishes the return, but this is an easy
exercise to undertake and must be done on every cake pump
application.
Nomograms estimate friction losses for sludge cake in
pipe, but some quick rules-of-thumb can also be used for initially estimating friction losses. For sizing PC pumps, I recommend an estimated apparent viscosity of 250,000-cPs for 20
percent solids and 450,000-cPs for 30 percent solids with a
simple computer program utilizing the well-known Hazen and
Williams formulas.
This rule has proven relatively successful over time as an
initial estimate. Most pump manufacturers offer demonstration or trial equipment, and it is always desirable (if possible)
to run a trial with a length of straight pipe to measure friction
loss and infer an apparent viscosity.
