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Suction Velocities and the Clogging Non-Clog Pump

Water: The Next Oil?
Suction Velocities
and the Clogging
Non-Clog Pump
Richard Barile, Cornell Pump Company
Here are some ways to overcome clogging that can
occur from the settling of organic and inorganic
solids in the suction piping of a pump station.
ver the last ten years, the “constant level wet well”
phenomenon has developed into a cost savings measure that has dramatically increased the number of
pump stations utilizing variable frequency drives (VFDs) to
maintain wet wells at a constant level.
Smaller wet wells mean less excavation and smaller structures for pump stations. As new pump stations are built in
developing areas, many are sized for future growth with pumps
designed for current needs. This means the suction pipe
through the wet well/dry well separation wall gets sized for the
future – sometimes as much as 40 years future – anticipated
demand. The VFDs are programmed to vary the pump speed
in an effort to closely match the pump station influent rate.
During normal-to-peak flow periods this configuration
typically works well. Low flow cycles, however, demand the
same pumps to operate well below the minimum velocities
required to keep solids in suspension through these large horizontal suction lines.
The following is a quotation from Hydraulic Institute regarding line velocities for pump intakes:
“For many common solids-bearing liquids, a velocity
of about 1.0-m/s (3.0-ft/s) is required to prevent sedimentation in horizontal piping. A velocity as low as 0.6-m/s
(2.0-ft/s) is generally sufficient for organic solids.”
Hydraulic Institute – American Institute Standard for
Pump Intake Design Section – Recommendations
june 2006
Pumps & Systems
As is the case with nearly every municipal sewage application, there is a combination
of organic and inorganic solids present in the
pumpage. Horizontal line velocities below 3-fps
can cause settling of solids and roping of stringy
materials that might later be pulled up into the
pump when velocities increase. The pump’s solids
handling design is then overwhelmed,
and a stoppage or plug occurs.
The following example reflects this
sort of problem.
Recipe for Failure
The Borough of Pumperville needs a new
pump station for their urban sprawl. As
neighborhoods are developed, a pump
station is designed and built to accommodate their waste requirements.
It is estimated that at peak flow
a pump would be required to handle
2000-gpm at 125-ft TDH. This would
make the pump of choice a large 6-in
or small 8-in non-clog pump running
at 1800-rpm. A small wet well is constructed and 16-in piping is installed
from the wet well to the pump pad in
anticipation of future requirements. A
6-in non-clog pump is purchased with a
VFD to maintain the wet well at a constant level.
The velocity at 2000-gpm is
approximately 22.7-fps at the suction
and discharge nozzles of the pump.
Since space is at a premium, the pump
is mounted vertically on a 10-in x 6-in
suction base elbow. The 2000-gpm produces a velocity of approximately 8.16fps at the 10-in flange of the suction
elbow. An eccentric reducer transitions
from the 16-in suction pipe to the 10in suction elbow flange. The velocity in
the 16-in pipe at this same 2000-gpm
“peak” flow is 3.19-fps.
During the course of the day, flows
vary from 1600-gpm to 2000-gpm.
At 1600-gpm, the 16-in line velocities drop to 2.56-fps. This is marginal,
but the flow ramps up often enough to
2000-gpm to keep solids from concentrating enough in the suction pipe to
plug the pump.
During the early morning hours,
the incoming flow drops off significantly
and averages about 600-gpm (.96-fps).
Horizontal line velocities below 3-fps can cause
settling of solids and roping of stringy materials that
might later be pulled up into the pump when velocities
increase. The pump’s solids handling design is then
overwhelmed, and a stoppage or plug occurs.
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june 2006
Water: The Next Oil?
A wet well should be sufficiently sized to allow the pump to keep
the suction line above minimum acceptable velocity levels.
Solids begin to drop out of the flow stream in the 16-in suction pipe, settling along the bottom. Pre-rotation occurs in the
pipe, allowing stringy and fibrous materials to wrap themselves
into ropes and settle also. Flow still occurs as the liquid portion
of the pumpage makes its way into the pump.
As the neighborhood begins to awaken with kitchen and
bathroom activities, system demands increase significantly. The
VFD signals the pump to match the incoming flow rate. The
pump comes to life and responds with
peak performance. All of those solids
– along with the roped and ragged fibers
that have been laying along the bottom
of the 16-in pipe during the course of
the night – now begin to move out into
the flow. Quickly, the increased velocity
rushes them into the eye of the pump,
causing a plug.
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One answer to this problem is to design
for a wet well that is sufficiently sized
to allow the pump to keep the suction
line above minimum acceptable velocity levels.
Another solution is to install
a “jockey” pump to handle the low
flow conditions with a suction pipe
no larger than 10-in (preferably 8-in).
Still another answer is to sleeve the 16in pipe with 8-in or 10-in for current
demand conditions so the interior pipe
can be removed as the area develops and
minimum demands increase.
When designing solid-handling
systems, be aware of pipeline velocities
– and make your colleagues, customers
and vendors aware of them too. When
providing specifications to pump vendors for quotation, always supply drawings and all relevant design criteria,
including minimum flow requirements.
Education will help ensure proper
pump station design and save untold
time and expense in troubleshooting
and correcting these situations in the
Richard Barile is the municipal sales
manager for Cornell Pump Company,
Sunrise Corridor Business Center,
16261 S.E. 130th Avenue, Portland,
OR 97015, 503-653-0330, Fax: 503653-0338, www.cornellpump.com.
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june 2006
Pumps & Systems