TM 5-814-2/AFM 88-11, Vol. 2
CHAPTER 5
PUMPING SYSTEM DESIGN
5-1.
Force main hydraulics
0.63 0.54
a. General.
The pipeline which receives
V = 1.32 C R
S
wastewater from a pumping station, and conveys it to
the point of discharge, is called a force main. Force
where
mains will be designed as pressure pipe, and must be
V = velocity in feet per second
adequate in strength to withstand an internal operating
C = coefficient of pipe roughness
pressure equal to the pump discharge head, plus an
R = hydraulic radius in feet, and
allowance for transient pressures caused by water
S = slope of energy grade line in feet per foot
hammer. The internal operating pressure is maximum
at the pumping station, and is reduced by friction to
(1) Roughness coefficient. Values of C to be
atmospheric, or near atmospheric, at the point of force
used in the formula range from 100 for older force
main discharge. The primary consideration in the
mains which have been in service a number of years
hydraulic design of force mains is to select a pipe size
(usually over 10), to 140 for force mains which are
which will provide the required minimum velocities
newly constructed. Some manufacturers of plastic and
without creating excessive energy losses due to pipe
asbestos-cement pipe report C values as high as 150.
friction. The most economical size of force main should
However, due to uncertainties in design and
be determined on the basis of power costs required for
construction, plus a desire to provide a margin of safety,
pumping, and capital investment costs of piping and
C values greater than 140 will not normally be
equipment. In practice however, the size is usually
permitted. At some installations, force mains may be
governed by the need to maintain minimum velocities at
very old (40 to 50 years) and in extremely bad condition,
low flows to prevent deposition of solids, and to develop
with offset joints broken pipe, or materials encrusted on
sufficient velocity at least once a day to resuspend any
pipe walls. For these cases, lower C values may be
solids which may have settled in the line. However,
justified. However, values lower than 80 will not be
regardless of pipe sizes required for minimum velocities,
allowed unless verified by flow and pressure tests. A
the minimum diameters to be used are 1Y4-inch for
solution to the Hazen-Williams formula is given in figure
pressure sewers at grinder pump installations, 4-inch for
force mains serving small pump stations and pneumatic
ejectors, and 6-inch for all other force mains.
b Design formula and chart. Force mains will be
designed hydraulically with the use of the Hazen-
Williams formula as follows:
5-1
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