CEMP-E
EI 11C201
1 March 1997
CHAPTER 7
PUMPING SYSTEM DESIGN
7-1. FORCE MAIN HYDRAULICS.
a. General. The pipeline which receives waste-water from a pumping station, and conveys it
to the point of discharge, is called a force main. Force mains will be designed as pressure pipe,
and must be adequate in strength to withstand an internal operating pressure equal to the pump
discharge head, plus an allowance for transient pressures caused by water hammer. The internal
operating pressure is maximum at the pumping station, and is reduced by friction to atmospheric,
or near atmospheric, at the point of force main discharge. The primary consideration in the
hydraulic design of force mains is to select a pipe size which will provide the required minimum
velocities without creating excessive energy losses due to pipe friction. The most economical
size of force main should be determined on the basis of power costs required for pumping, and
capital investment costs of piping and equipment. In practice however, the size is usually
governed by the need to maintain minimum velocities at low flows to prevent deposition of solids,
and to develop sufficient velocity at least once a day to resuspend any solids which may have
settled in the line. However, regardless of pipe sizes required for minimum velocities, the
minimum diameters to be used are 32-millimeter (1-1/4-inch) for pressure sewers at grinder
pump installations, 100-millimeter (4-inch) for force mains serving small pump stations and
pneumatic ejectors, and 150-millimeter (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:
= K C R0.63 S0.54
V
where:
K
=
0.85 for SI units (1.32 for IP units)
V
=
velocity in meters per second (feet per second)
C
=
coefficient of pipe roughness
R
=
hydraulic radius in meters (feet)
S
=
slope of energy grade line in meters per meter (feet per foot)
(1) Roughness coefficient. Values of C to be used in the formula range from 100 for older
force mains which have been in service a number of years (usually over 10), to 140 for force
mains which are newly constructed. Some manufacturers of plastic and asbestos-cement pipe
report C values as high as 150. However, due to uncertainties in design and construction, plus a
desire to provide a margin of safety, C values greater than 140 will not normally be permitted. At
some installations, force mains may be very old (40 to 50 years) and in extremely bad condition,
with offset joints, broken pipe, or materials encrusted on pipe walls. For these cases, lower C
values may be justified. However, values lower than 80 will not be allowed unless verified by flow
and pressure tests.
(2) Velocity. Velocity criteria for force mains are based on the fact that suspended organic
solids do not settle out at a velocity of 0.6 meter per second (2.0 feet per second) or greater.
Solids will settle at velocities less than 0.3 m/s (1.0 fps) and when wastewater pumps are idle.
However, a velocity of 0.75 to 1.00 m/s (2.5 to 3.5 fps) is generally adequate to resuspend and
7-1
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