that a minimum velocity of 2.5 to 3.5 fps be provided

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.

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.

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