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14.14 Force Main Sewers 533
14.14.4 Design Criteria

Force main design is typically integrated with lift station design. The major factors to con-
sider when analyzing force main materials and hydraulics include the design formula for
sizing the pipe, friction losses, pressure surges, and maintenance. The Hazen-Williams for-
mula is recommended for the design of force mains. This formula includes a roughness
coefficient C, which accounts for pipeline hydraulic friction characteristics. The roughness
coefficient varies with pipe material, size, and age.

14.14.4.1 Force Main Pipe Materials Selection criteria for force main pipe materials include

1. Wastewater quantity, quality, and pressure
2. Pipe properties, such as strength, ease of handling, and corrosion resistance
3. Availability of appropriate sizes, wall thickness, and fittings
4. Hydraulic friction characteristics
5. Cost.
Ductile iron pipe offers strength, stiffness, ductility, and a range of sizes and thicknesses
and is the typical choice for high-pressure and exposed piping. Plastic pipe is most widely
used in short force mains and when smaller diameters are appropriate. Table 14.8 lists the
types of pipe recommended for use in a force main system and suggested applications.

14.14.4.2 Velocity Force mains from the lift station are typically designed for velocities
between 2 and 8 ft/s (0.60 and 2.5 m/s). Such velocities are normally based on the most
economical pipe diameters and typical available heads. For shorter force mains (less than
2,000 ft or 610 m) and low lift requirements (less than 30 ft or 9.14 m), the recommended
design force main velocity range is 6 to 9 ft/s (1.83 to 2.74 m/s). Under certain circum-
stances, velocities as low as 2 ft/s (0.60 m/s) can be used, provided precautions are used to
increase the velocity from time to time. This increased velocity for scouring is commonly

Table 14.8 Characteristics of Common Force Main Pipe Materials
Material Application Advantages Disadvantages

Cast or ductile iron: High pressure Good resistance to More expensive
Cement lined Available sizes pressure surges than concrete
of 4–54 in. and fiberglass
Steel, cement High pressure Excellent resistance More expensive
lined All pipe sizes to pressure surges than concrete
and fiberglass
Asbestos cement Moderate pressure No corrosion Relatively brittle
For 36-in. pipe Slow grease
sizes buildup
Fiberglass Moderate pressure No corrosion 350-psi max
Reinforced For up to 36-in. pipe Slow grease pressure
Epoxy pipe sizes buildup
Plastic Low pressure No corrosion Suitable for small
For up to 36-in. Slow grease pipe sizes and
pipe sizes buildup low pressure only
Conversion factors: 1 in. 25.4 mm; 1 psi 6.94 kPa.