(Force Mains)

Use of force mains can significantly reduce the size and depth of sewer lines and decrease the overall costs of sewer system construction. Typically, when gravity sewers are installed in trenches deeper than 20 feet, the cost of sewer line installation increases significantly because more complex and costly excavation equipment and trench shoring techniques are required. Usually, the diameter of pressurized force mains is one to two sizes smaller than the diameter of gravity sewer lines conveying the same flow, allowing significant pipeline cost reduction. Force main installation is simple because of shallower pipeline trenches and reduced quantity of earthwork. Installation of force mains is not dependent on site specific topographic conditions and is not impacted by available terrain slope, which typically limits gravity wastewater conveyance.

While construction of force mains is less expensive than gravity sewer lines for the same flow, force main wastewater conveyance requires the construction and operation of one or more lift stations. Wastewater pumping and use of force mains could be eliminated or reduced by selecting alternative sewer routes, consolidating a proposed lift station with an existing lift station, or extending a gravity sewer using directional drilling or other state of the art deep excavation methods.

The dissolved oxygen content of the wastewater is often depleted in the wet-well of the lift station, and its subsequent passage through the force main results in the discharge of septic wastewater, which not only lacks oxygen but often contains sulfides. Frequent cleaning and maintenance of force mains is required to remove solids and grease buildup and minimize corrosion due to the high concentration of sulfides.

Pressure surges are abrupt increases in operating pressure in force mains which typically occur during pump start-up and shut-off. Pressure surges may have negative effects on force main integrity but can be reduced by proper pump station and pipeline design.

Force main design is typically integrated with lift station design. The major factors to consider in analyzing force main materials and hydraulics include the design formula for sizing the pipe, friction losses, pressure surges, and maintenance.

The Hazen-Williams formula 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. Hazen-Williams equation:
f = 0.2083 (100/c)1.852 q1.852 / dh4.8655
where
f = friction head loss in feet of water per 100 feet of pipe (ft h20/100 ft pipe)
c = Hazen-Williams roughness constant
q = volume flow (gal/min)
dh = inside hydraulic diameter (inches)

Force Main Pipe Materials Selection criteria for force main pipe materials include:
* Wastewater quantity, quality, and pressure:
* Pipe properties, such as strength, ease of handling, and corrosion resistance
* Availability of appropriate sizes, wall thickness, and fittings
* Hydraulic friction characteristics
* 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 smaller diameters. Table 1 lists the types of pipe recommended for use in a force main system and suggested applications:
Material:
1. Cast or Ductile Iron
Application – High Pressure/Available in 4 to 54 in pressure surges
Advantages – Good resistance to pressure surges
Disadvantages – More expensive than concrete and fiberglass
2. Steel Cement Lined
Application – High Pressure/All pipe sizes
Advantages – Excellent resistance to pressure surges
Disadvantages – More expensive than concrete and fiberglass
3. Asbestos Cement
Application – Moderate Pressure for 36-inch + pipe
Advantages – No corrosion/Slow grease build up
Disadvantages – Relatively brittle
4. Fiberglass Reinforced Epoxy Pipe
Application – Moderate Pressure for 36-inch pipe
Advantages – No corrosion/Slow grease build up
Disadvantages – 350 psi max pressure
5. Plastic
Application – Low Pressure for 36-inch pipe
Advantages – No corrosion/Slow grease build up
Disadvantages – Suitable for small pipe sizes and low pressure