Water & Wastewater Asia July/August 2022

HOTSEAT
greatly. Pumping coal slurry at
30-40% solids concentration may
be possible but pumping a fibrous
slurry such as dairy manure at
8-10% solids is often the upper
limit, and municipal sludges often
have an upper limit of 6.7% solids.
Regardless of what operators are
pumping, they generally need a
fair amount of water to suspend
and transport the solids. The
slurry must be a liquid that flows.
Head loss for pumping slurries
can be two to 10 times higher than
pumping clean water, so tapping
into the chopper pump supplier’s
expertise is critical. Choosing the
proper flow rate can ultimately
avoid contamination of local
waterways or plant flooding, save
on electrical costs and incorporate
reliability into the equipment
design.
ENSURE THE CHOPPER PUMP
AND SYSTEM ARE OPTIMISED
FOR SOLID HANDLING
While choosing the correct pump to
address solid handling challenges
is a good start, operators will also
need to optimise their system for
handling solids reliably. To do this,
they need to ensure the head flow
characteristics of their pump are
properly matched to their system
so that both operate reliably.
Pipeline velocities
Reliable solids pumping should
be accomplished at turbulent
velocities. This approach keeps
solids stirred up as they move
through the pipeline to avoid
settling and blockage issues.
Typically, turbulent flow is assured
at 3-5ft/sec. This velocity is a good
guideline for pumping through
horizontal piping and fittings.
When pumping through vertical
sections of piping, however,
we found that 8-10ft/sec is
recommended to avoid settling
heavier materials like sand or grit in
the vertical run of piping. Settling
of heavier materials over time can
and has caused pipeline blockage
in some slurries and may cause the
pump to pump the same sand and
grit over and over. Velocities over
8-10ft/sec may result in high flow
noise and wear.
Suction piping recommendations
As a general rule, the minimum
suction piping diameter should
be equal to or one size larger pipe
size than that used at the suction
entrance to the pump. For example,
if an 8-inch discharge horizontal
pump has a 10-inch suction flange,
then we recommend that at least
10-inch piping be used, and in
many cases, 12-inch piping will be
needed to achieve our recommended
3-5ft/sec of velocity in the suction
piping.
To reduce piping down to match the
pump suction flange size, an eccentric
reducer is needed. The eccentric
reducer should be oriented so that the
flat side of the reducer is on the top
side of the piping so that air or gas is
not trapped in this fitting. Do not make
suction changes greater than 4-inch in
pipe size with only one reducer. A rapid
change in size causes the non-uniform
flow to enter the pump and causes
rapid wear and vibration, excess noise,
vibration and wear.
Avoid the use of tees or short radius
elbows near the pump on the suction
side of piping. Tees induce swirling
flow in the piping downstream of the
tee, causing reweaving of hair and fibre
into rags, which can then plug pumps.
Plugging on rewoven rags is less of
a problem with chopper pumps, but
the swirling flow also impacts pump
performance. Y-type fittings provide
considerably better flow.
For the lowest noise and vibration
in the pump and longest parts life,
provide uniform, straight-line flow to
the pump and ensure all leaks are
sealed. Air leaks in the suction piping
of self-priming pumps make them very
noisy and could prevent them from
WATER & WASTEWATER ASIA | JULY/AUGUST 2022 47