Underground Construction - July 2017 - 26

By Mike Bjorkman

Selecting The Best

Dewatering Pump For The Job

When dealing with a construction
site that is underwater or where
the water table is just above sea
level, contractors have to ensure
that the excavation stays dry and
safe for workers. This can be extra
challenging where you have conditions affected by water depth, and
conditions such as silt and sand.
Additionally, these conditions may
vary based on seasonal weather
or tides. It is extremely important to have contractors who have
investigated historical data and
who monitor fluctuations in water
levels and speeds of flow, thereby
decreasing risks to workers and
the success of the project.
One of the primary decisions of
the contractor, based on the hydraulic conditions, is what pumping equipment to utilize. This decision will be based on numerous
job-site conditions, such as flow
required to mitigate water seepage,
depth of a coffer dam and solids
content. The contractor's selection
of pump type usually comes down
to self-priming centrifugal or submersible pumps.

Self-priming centrifugal pumps
One drawback of using end suction
centrifugal pumps is that they do
not fare well when the liquid is
below the pump centerline. Liquid
must be delivered to the pump so
the process can begin. If air is on
the suction side (piping and pump
casing) of the pump, it needs to
be totally evacuated. If air exists
in the centrifugal pump, it will become "air bound" and incapable of
pumping.
The self-priming centrifugal,
a specialized end suction pump
design, creates a vacuum at the
impeller eye to continuously remove air from the suction line. The
pump is not capable of compressing the air during the priming
phase and it must be allowed to

ucononline.com

escape through the discharge. As
air is removed, atmospheric pressure forces water up the suction
piping to the pump, allowing the
pump to operate. The self-priming
process occurs automatically once
the pump is started with the initial
quantity of liquid, and can be complicated when the pumped liquid
contains sand or solids. The practical suction lift limit for self-priming pumps is around 26 feet of
liquid under ideal conditions.
There are some disadvantages
to self-priming centrifugal pumps.
Any small vacuum leak, such as
sealing areas around connectors
in the suction line or pump seals,
can prevent the unit from priming.
The pump will continuously pull
air from the leak instead of the
air in the suction line, thus, never
allowing the priming cycle to be
completed. This is a common cause
of priming failure. The leak can be
very small or invisible to the naked
eye, but can prevent priming.
The diameter and length of the
suction hose or pipe can also affect
prime time due to the volume of
air to be evacuated during pump
priming. This extended priming
time can add heat to the liquid,
which can further extend prime
time.
Self-priming pumps need to be
located as close as possible to the
water source. It is best to be located directly above the coffer dam
with as few restrictions (such as
elbows) as possible to reduce friction. Again, this can present problems dependent on the excavation
location.
Once the pump is primed,
pumping it will require adequate
net positive suction head (NPSH)
to remain pumping without suction limitations. Without sufficient
NPSH, a self-priming centrifugal
pump can cavitate and lose prime.
Lifts through long suction lines,

Engine-driven, self-priming pumps are one of the choices available to contractors.

especially containing obstructions, can be plagued by cavitation
problems resulting in unacceptable
noise levels and possible equipment damage.
If a self-primer is required to
lift water 15 feet or more from the
source to the pump's suction, the
pump capacity could be decreased
significantly. Submersible pumps,
being located below the water level, typically do not have this same
suction limitation.
Most self-priming centrifugal
pumps on remote applications
utilize engine drives, which in itself can present operational issues
such as fuel levels for long run
times, engine oil condition and
other prime mover maintenance
issues.

Submersible pumps
Using submersible pumps can
offer operational and application
advantages. Pumps are submerged
directly into the water for immediate use and unit cooling, thereby eliminating priming issues or
extended prime times. No worker
intervention is required. Pumping
action occurs from direct submer-

sion in the liquid feeding the pump
without the need for a suction line.
Submerged pumps are quiet.
Cavitation issues are extremely rare, but may occur when the
sump is too small for the size of
the submersible pump that is in
it. Submersible water pumps are
lighter in weight and highly portable.
The versatility and low maintenance of submersible pumps make
them a good option for dewatering
service. No regular maintenance
is necessary. Submersible pumps
usually need to be fully submerged.
The water around a submersible
pump actually helps to cool the
motor.

Case history: Galveston
The sea level along the Gulf Coast
is rising faster than most places
on the globe. Galveston, TX, has
experienced a 3-foot rise since the
disastrous flood of 1900 that killed
thousands of people. One of the
toughest jobs in rebuilding the infrastructure of a city that is just a
few feet above sea level is keeping
the water out of your construction
site - particularly water that is
Underground Construction July 2017

http://www.ucononline.com

Underground Construction - July 2017

Table of Contents for the Digital Edition of Underground Construction - July 2017