TM 5-814-2/AFM 88-11, Vol. 2
that a minimum velocity of 2.5 to 3.5 fps be provided
b. System head-capacity curve. To determine the
once or twice daily. Large pumping stations which serve
head required of a pump, or group of pumps, that would
the entire installation or major portions thereof, and
discharge at various flowrates into a force main system,
which are designed to pump continuously, will usually
a head-capacity curve must be prepared. This curve is
have a greater number of pumps operating over a wider
a graphic representation of the total dynamic head, and
range of flowrates. Since the pumping range may vary
will be constructed by plotting the TDH over a range of
from 7 or 8 to 1, it will generally be sufficient to design
flowrates from zero to the maximum expected value.
for velocities of 0.5 up to 7.0 or 8.0 fps. Maximum
Friction losses can be expected to increase with time,
velocity is set at 10.0 fps.
thus affecting the capacity of the pumping units and
(3) Slope. The value of S in the formula is
their operation. Therefore, system curves well reflect
equivalent to the kinetic energy loss due to pipe friction
the maximum and minimum friction losses to be
divided by the length of conduit, or S = Hf/L. Minor
expected during the lifetime of the pumping units, as
energy losses from fittings and valves will be converted
well as high and low wet well levels. The typical set of
to equivalent lengths of conduit for use in the formula.
system curves will generally consist of two curves using
Conversion tables for fittings and valves can be found in
a Hazen-Williams coefficient of C = 100 (one for
standard hydraulics textbooks. The total kinetic energy
maximum and one for minimum static head), and two
loss in a force main will be computed by multiplying the
curves using a Hazen-Williams co-efficient of C = 140
slope of the energy grade line by the total length of
(for maximum and minimum static head).
These
conduit including equivalent lengths, or Hf = S x L.
coefficients represent the extremes normally found in
wastewater applications.
c. Pump head-capacity curve. The head that a
5-2.
Pump analysis and selection
a. Total dynamic head. The head in feet against
particular pump can produce at various flowrates is
established in pump tests conducted by the pump
which a pump must work when wastewater is being
manufacturer. The results of these tests are plotted on
discharged is termed the total dynamic head (TDH).
a graph to form the pump characteristic curve. Along
The two primary components of TDH in wastewater
with the discharge head developed, the pumps
applications are the static discharge head and the
operating efficiency, required power input, and net
kinetic losses due to pipe friction. Velocity and pressure
positive suction head are generally included on the
heads are also present, but are usually insignificant.
same diagram.
The TDH will be calculated with the use of the Bernoulli
(1) Efficiency and power input. Pump efficiency is the
energy equation which can be written as follows:
ratio of the useful power output to the input, or brake
2
horsepower, and is given by:
TDH =
(Pd/W + V d/2g + Zd) -(P8/W +
2
V 8/2g + Z8) + Hf
E = wQ TDH
(bhp)(550)
where
where
E = pump
efficiency
(100
E
=
Pd, P8 =gage pressures in pounds per
percent)
square foot
w = specific weight of fluid in pounds
per cubic foot
Vd, V8 =velocities in feet per second
Q = pump capacity in cubic feet per
Zd, Z8 = static elevations in feet
second
TDH = Total dynamic head, and
Hf = kinetic energy loss from pipe
bhp = brake horsepower
friction, fittings, and valves, as
calculated in paragraph 5-1b (3).
Pump efficiencies usually range from 60 to 85 percent.
w = specific weight of fluid in pounds
Most characteristic curves will indicate a best efficiency
per cubic foot, and
point (BEP) at which pump operation is most efficient.
g = acceleration due to gravity 32.2
2
Where possible, pumps will be selected to operate
ft/sec )
within a range of 60 to 120 percent of the BEP.
(2) Net positive suction head. When pumps
All head terms are in feet. Subscripts d and 8 represent
operate at high speeds and at capacities greater than
force main discharge and pump suction, respectively. In
the BEP, the potential exists for pump cavitation.
order to determine hydraulic conditions at the pump
Cavitation can reduce pumping capacity and may in
suction, it will be necessary to write an energy equation
time damage the pump impeller. Cavitation occurs
from the liquid level in the wet well to the pump suction
when
nozzle.
5-3
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