United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-95/041
March 1995
EPA Project Summary
Improvements in Pump Intake
Basin Design
Robert L Sanks, Garr M. Jones, and Charles E. Sweeney
Pump intake basins (or wet wells or
pump sumps) designed in accordance
with accepted criteria often pose many
operation and maintenance problems.
This report summarizes field surveys
of 3 trench-type pump intake basins
representative of 29 such basins that
have been in satisfactory service for
nearly 3 decades, large-scale (1:4)
model studies made at the ENSR Con-
sulting and Engineering hydraulic labo-
ratory in Redmond, WA and at Montana
State University in Bozeman, MT, and a
full-scale basin study made at
Fairbanks Morse Pump Corporation
plant in Kansas City, KS. Field studies
of three small, round pump inlet basins
are also included. A considerable part
of the report is devoted to recom-
mended procedures and rules for in-
take basin design. The effectiveness of
cones and vanes in reducing swirling
(pre-rotation) is also reported, together
with means for reducing or eliminating
vortexing.
This Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Background
Head-capacity curves for pumps are
obtained by the manufacturer from a single
pump operating in a semi-infinite pool with
no nearby walls or floors and no stray
currents. Hence flow into the pump suc-
tion is symmetrical with no vortices or
swirling. Pumping station designers rely
on these curves to define the operating
conditions for pumps. But various con-
straints (size, cost, and storage time, for
example) often require both the walls and
the floor to be at a distance to the pump
intake no greater than the diameter, D, of
the intake. Consequently, flow toward the
intake cannot be fully symmetrical and
may not develop into symmetrical veloci-
ties in the throat of the intake.
Water usually enters a pump sump from
a pipe or an open channel. Velocity is
reduced as the flow expands in the basin,
and, because rapidly expanding flow is
unstable, localized rotation occurs and can
develop into severe swirling. In many tra-
ditional or common designs (see Hydrau-
lic Institute Standards [1]) the row of pumps
is positioned normal to the inlet pipe so
that the flow to all the pumps except the
center one is asymmetrical. In some de-
signs the incoming flow must make a 90°
turn to reach the pumps. Unless the ap-
proach distances in both designs are long
enough (a distance difficult to quantify),
the water will be swirling before it reaches
the pump intake. Swirling can, in fact,
occur if the flow distribution toward the
pump intake is even slightly off-center.
Using traditional designs is no guarantee
that the pumps will perform to the
manufacturer's curves. Model studies to
improve pump performance have been re^
quired even when traditional designs have
been faithfully followed.
Printed on Recycled Paper
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Asymmetrical flow to an intake is likely
to produce an asymmetrical velocity distri-
bution in the throat. Such dissymmetry
creates an excessive load on one side of
the impeller, and stresses on shafts, bear-
ings, and couplings. It can cause rough
operation, vibration, and loss of head and
capacity. Swirling changes the angle of
attack on the impeller blade and also re-
duces head and capacity. Swirling in the
approach can degenerate into vortices.
Vortices also form as liquid separates from
walls or floors. The pressure in the core of
a vortex is reduced and can cause noisy
operation and vibration. When a vortex is
severe, it results in cavitation that quickly
erodes metals.
Results and Discussion for
Trench-Type Pump Intake
Basins
a) General
A design that largely avoids asymmetri-
cal flow to the intakes is shown in Figure
1. Note that both plan and cross-section
views are symmetrical. The water jet from
the inlet spreads and the velocity is dimin-
ished somewhat before the jet strikes the
rear wall and returns, thereby setting up a
recirculation pattern above the trench. Ve-
locities along the floor of the trench are
very small and water tends to enter the
pump inlet more or less uniformly from all
sides. Swirling is almost always within ac-
ceptable bounds and can be virtually elimi-
nated by cones and/or vanes. Of course,
vortices can form due to the proximity of
walls and floors. But floor vortices can be
eliminated with cones and diminished by
vanes at the pump intake. Wall vortices
can be diminished by vanes. In any event,
vortices in trench designs are not more
severe than they are in traditional de-
signs.
b) Variable Speed Pumping
Trench-type intake basins are suitable
for both variable speed and for constant
speed pumps. In variable speed pumping,
the objective is to match the pump output
to the inflow so that no storage is needed.
Therefore, the volume of the basin is of
no consequence, and the only concerns
are that the cross-section above the trench
be large enough to accommodate the re-
circulation pattern, and that the length be
great enough to accommodate the inlets
and allow sufficient space around machin-
ery for maintenance access. The water
level is used to regulate the speed of the
pumps (2), and the normal operating wa-
ter level is confined between the invert
and the soffit of the inlet pipe (for variable
speed pumping only).
c) Constant Speed Pumping
In constant speed pumping, the pumps
are turned on and off, so some storage
capacity is required while pumps are off.
To avoid overheating motors by frequent
starts and stops, the storage capacity is
often rather large, and if the basin were to
contain all the storage, it might have to be
large, deep, and costly. However, by slop-
ing the approach pipe from some upstream
point at a downward gradient of 2% to the
pump inlet basin over a distance of, for
example, 70 m (200 ft), a normal operat-
ing water level fluctuation of 1.2 m (4 ft)
can be obtained between low water level
at the invert of the inlet and high water
level at the upstream point. The storage
in the pipe augments that in the basin. By
making the pipe larger, the storage can
be increased while the velocity down the
approach pipe is limited to produce no
more than a weak hydraulic jump when
the flowing water contacts the level water
surface somewhere between the upstream
point and the invert of the inlet pipe. The
report contains a table that provides ac-
ceptable flow rates versus approach pipe
size. The sloping approach pipe has an-
other unique advantage: it eliminates the
cascade that occurs in traditional designs
when the water level is below the inlet
pipe. These cascades drive bubbles deep
into the pool below, and pump intake cur-
rents often capture them and draw them
into the pump with devastating effects on
the capacity, head, and efficiency of the
pump. In the trench-type inlet basin, air
bubbles are not introduced into the sump.
d) Solids-Bearing Waters
Many waters (raw water, storm water,
and sewage) contain solids that settle rap-
idly in traditional designs as the scouring
velocity in the intake pipe falls to very low
levels in the approach to the pumps. These
solids can change the hydraulic charac-
teristics of the basin appreciably, and if
any organic material is present, can emit
noxious and corrosive gases. Such solids
can be removed from traditional sumps
only with great expense and difficulty, and
designs for solids-bearing waters are ad-
dressed in the Hydraulic Institute Stan-
dards (1) with the statement "Figures apply
to sumps for clear liquids. For fluid-solids
mixtures refer to the pump manufacturer."
In contrast, an enormous advantage of
trench-type pump inlet basins is the ease
and speed of cleaning them. Solids can
be swept from trench-type basins in a few
minutes at almost no expense and with-
out manual labor. If the inflow is small,
water for cleaning can be stored in the
upstream pipe by shutting off all pumps.
The sluice gate is adjusted to pass about
80% to 85% of the last pump's capacity.
When enough water has been stored, all
pumps are turned on at full capacity. As
the water level in the basin drops, water
running down the ogee spillway acceler-
ates to supercritical velocity and forms a
hydraulic jump that progresses rapidly
downstream from the toe of the spillway,
under the upstream pumps, and then to
the last pump. All solids are swept up by
the jump and carried to the last pump.
During cleaning, the last pump is always
operated at full speed, but because of the
air entrained in the jump, it can only dis-
charge about 85% of its normal capacity.
By controlling the sluice gate opening, the
jump can be made to go downstream at
any desired velocity. Cleaning is normally
accomplished in a minute or less after the
jump forms at the bottom of the spillway.
Conclusions
The trench-type pump intake basin is
eminently successful as has been demon-
strated both with models and in the field
over nearly three decades with pump sizes
ranging from 63 Us (230 m3/h or 1,000
gpm) to 4.7 nrvVsec (17,000 m3/h or 75,000
gpm). No such sump has ever required
retrofitting, and no pump installed in one
has failed to perform satisfactorily. The
authors prefer this type over all others
wherever applicable. But unless the de-
signer has had experience with installa-
tions of larger sizes in trench-type sumps,
this design should not be universally ap-
plied to pump sizes larger than 1,900 L/s
(6,800 rrrVh or 30,000 gpm) without de-
sign-specific testing.
The full report was submitted in fulfill-
ment of Contract No. CR-817937 by the
Department of Civil Engineering, Montana
State University, Bozeman, MT, under
sponsorship of the U.S. Environmental Pro-
tection Agency
References
1. Hydraulic Institute. Hydraulic Institute
Standards for Centrifugal, Rotary and
Reciprocating Pumps, 14th Ed.,
Parsippany, NJ 1983.
2. Sanks, R. L et. al., Pumping Station
Design, Butterworth Heinemann,
Newton, MA, 1989.
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D/4
r— Motorized sluice gate
B
Vane
Cone recommended ^ Vane recommended
Section B-B
Section A-A
Figure 1. Rectangular sump for constant speed pumping and solids-bearing water. For variable speed pumping, high water level is at the top of the
pipe. For clear water, omit the ogee spillway. Pumps can be column or dry-pit types or, with modifications, submersible.
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Robert L. Sanks is with the Department of Civil Engineering, Montana State
University, Bozeman, MT 59717.
Garr M. Jones is with Brown and Caldwell Consultants, Pleasant Hill, CA
94523-4324.
Charles E. Sweeney is with ENSR Consulting and Engineering, Redmond, WA
98052.
James A. Heidman is the EPA Project Officer (see below).
The complete report, entitled "Improvements in Pump Intake Basin Design," .
(Order No. PB95-188090; Cost: $19.50, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
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EPA/600/SR-95/041
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