Page 575 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch14_500-554.qxd 8/7/10 8:56 PM Page 531

14.14 Force Main Sewers 531

Column 6 indicates the cumulative area tributary to a line. For example, in line 2, column 6 is
the sum of column 6, line 1, and column 5, line 2, or (2.19 1.97) 4.16.
Columns 7 and 8 record the times of flow to the upper end of the drain and in the drain. For
example, the inlet time to manhole 1 is estimated to be 20 min, and the time of flow in line 1 is cal-
culated to be 340>(60 3.94) 1.5 min from (Column 17)>(60 column 16). Hence, the time to
flow to the upper end of line 2 is (20 1.5) 21.5 min.
Column 9 is the mean intensity of rainfall during the inlet time (time of flow to the upper end)
read from Fig. 14.18.
Column 10 is the weighted mean runoff coefficient for the tributary area.
Column 11 is (column 9) (column 10). For example, for line 1, 2.56 0.508 1.30.
Column 12 (column 11) (column 6). For example, the runoff entering line 1 is 1.30
3
2.19 2.85 ft /s (81 L/s).
Columns 14, 15, and 16 record the chosen size and resulting capacity and flow velocity of the
drains for the tributary runoff and available or required grade. For example, in line 1, a grade of
3
6.42% and a flow of 2.85 ft /s (81 L/s) call for a 12-in. drain. This drain will have a capacity of
3
3.09 ft /s (87.5 L/s) and flow at a velocity of 3.94 ft/s (1.2 m/s).
Columns 17 through 21 identify the profile of the drain. Column 17 is taken from the plan
or profile of the street; column 18 (column 17) (column 14); column 19 is obtained from
Eq. 5.18 [h i (d h v ) kh v ], the required drop in manhole 2 being (d h) 0.2h [(1.5
0.17) (1.0 0.24)] 0.2(0.17 0.24) 0.42 ft; and column 21 (column 20) (column 19),
column 20 furthermore being (column 21) (column 19) for the entrant line. For example, for line
2, (84.28 0.42) 83.86 and subsequently (83.86 0.92) 82.94.

14.13 HYDRAULIC DESIGN OF COMBINED SEWERS
The capacity design of combined sewers allows for a maximum rate of wastewater flow in
addition to stormwater runoff. If entering rainwater is confined to roof water, the waste-
water flows are considerable items in required sewer capacities. The resulting system is
sometimes called a roof-water system rather than a combined system. If the full runoff
from storms of unusual intensity is carried away by the system, wastewater flows become
relatively insignificant items in required combined sewer capacity.

14.14 FORCE MAIN SEWERS
Force mains are pipelines that convey wastewater under pressure from the discharge side
of a pump or pneumatic ejector to a discharge point. Pumps or compressors located in a lift
station provide the energy for wastewater conveyance in force mains. The key elements of
force mains are
1. Pipes
2. Valves
3. Pressure surge control devices
4. Force main cleaning system.
Force mains are constructed from various materials and come in a wide range of
diameters. Wastewater quality governs the selection of the most suitable pipe mate-
rial. Operating pressure and corrosion resistance also impact the choice. Pipeline size
and wall thickness are determined by wastewater flow, operating pressure, and trench
conditions.