Sump design for vertical pumps



Vertical pumps in sump (open fit) service rely on atmospheric pressure and
submergence to provide the net positive suction head available (NPSHA) at a
pump site. Thus, a sump requires special design considerations to ensure that it
provides adequate NPSHA and channels the water into the pump with minimum
flow disturbance to obtain optimum pump performance.

The sump should have sufficient depth to provide adequate submergence of the
pump bowl, or casing, as recommended by the pump manufacturer.
Submergence is the vertical distance between the water level and the centerline of
the lowest impeller in the pump. The required minimum submergence is
generally given at sea level and should be adjusted (increased) in higher
elevation location to compensate for the reduction in atmospheric pressure. It
should also take into account the drawdown, or the lowest drop in water level
when the pump has been pumping out the water continuously for some time.

The sump should have enough clearance along the sides and the bottom of the
pump suction bell, or suction bay. A general rule of thumb is that this clearance
should not be less than [ * ] the biggest diameter of the suction bell. The bigger
the clearance, the better it is. In multiple pump installations, each of the pump
suction should be separated by baffles to isolate any flow disturbance, or
turbulence, at the inlet of one pump from affecting the inlet flow of the adjacent
pump.

The sump floor should be such that there are no abrupt changes in the elevation,
such as sharp steps or “drop offs”, within a distance of [ * ] times its diameter,
from the suction bell, to avoid vortices from entering the pump. There should be
no abrupt channels, sudden changes, or corners adjacent to a pump intake.

A suction strainer should be avoided to minimize the suction entrance loss. In lieu
of a suction strainer, a trash rack, or screen, should be provided at the sump
intake to filter out solid particles that are larger than the particle size the impeller
will safely pass to prevent the pump from clogging. If the use of a suction strainer
cannot be avoided, use a large basket-type strainer.

Water should enter the suction bell, or suction bay, at low velocity and with
minimum of turbulence. The recommended inlet velocity is [ * ] feet per second.

[ * ] Some information are excluded in this article. Read more.


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Keywords: sump design, open pit, adequate submergence, site NPSHA, bowl,
casing, sea level, higher elevation, atmospheric pressure, drawdown, suction
bell, suction bay, baffles, flow disturbance, turbulence, inlet flow, drop offs,
vortices, channels, intake, suction strainer, trash rack, screen, sump intake, solid
particle, solid size, basket-type strainer, inlet velocity



Description: Sump design considerations to obtain adequate submergence and
NPSHA, avoid flow vortices and turbulence, and minimize suction entrance loss,
to ensure the optimum performance of vertical pumps in open pit service.


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