Force mains from the lift station are typically designed for velocities between 2 to 8 feet per second). Such velocities are normally based on the most economical pipe diameters and typical available heads.
For shorter force mains (less than 2,000 feet) and low lift requirements (less than 30 feet), the recommended design force main velocity range is 6 to 9 feet per second. This higher design velocity allows the use of smaller pipe, reducing construction costs. Higher velocity also increases pipeline friction loss by more than 50 percent, resulting in increased energy costs. To reduce the velocity, a reducer pipe or a pipe valve can be used. Reducer pipes are often used because of the costly nature of pipe valves. These reducer pipes, which are larger in diameter, help to disperse the flow, therefore reducing the velocity.
The maximum force main velocity at peak conditions is recommended not to exceed 10 feet per second). List below provides examples of force main capacities at various pipeline sizes, materials, and velocities. The flow volumes may vary depending on the pipe material used.
6 Inches Diameter: 2 fps/GPM 176, 4 fps/GPM 362, 6 fps/GPM 528
8 Inches Diameter: 2 fps/GPM 313, 4 fps/GPM 626, 6 fps/GPM 1252
10 Inches Diameter: 2 fps/GPM 490, 4 fps/GPM 980, 6 fps/GPM 1470
18 Inches Diameter: 2 fps/GPM 1585, 4 fps/GPM 3170, 6 fps/GPM 4755
24 Inches Diameter: 2 fps/GPM 2819, 4 fps/GPM 5638, 6 fps/GPM 8457
36 Inches Diameter: 2 fps/GPM 6342, 4 fps/GPM 12684, 6 fps/GPM 19026
Force mains should be designed so that they are always full and pressure in the pipe is greater than 10 pounds per square inch to prevent the release of gases. Low and high points in the vertical alignment should be avoided; considerable effort and expense are justified to maintain an uphill slope from the lift station to the discharge point. High points in force mains trap air, which reduces available pipe area, causes non-uniform flow, and creates the potential for sulfide corrosion. Gas relief and vacuum valves are often installed if high points in the alignment of force mains cannot be avoided, while blow-offs are installed at low points.
The possibility of sudden changes in pressure (pressure surges) in the force main due to starting and/or stopping pumps (or operation of valves appurtenant to a pump) must be considered during design. The duration of such pressure surges ranges between 2 to 15 seconds. Each surge is site specific and depends on pipeline profile, flow, change in velocity, and inertia of the pumping equipment, valve characteristics, pipeline materials, and pipeline accessories. Critical surges may be caused by power failure. If pressure surge is a concern, the force main should be designed to withstand calculated maximum surge pressures.
Valves are installed to regulate wastewater flow and pressure in the force mains. Valves can be used to stop and start flow, control the flow rate, divert the flow, prevent backflow, and control and relieve the pressure. The number, type, and location of force main valves depends on the operating pressures and potential surge conditions in the pipeline. Although valves have a lot of benefits, the costliness of them prevents them from being used extensively.
Force main performance is closely tied to the performance of the lift station to which it is connected. Pump-force main performance curves are used to define and compare the operating characteristics of a given pump or set of pumps along with the associated force main. They are also used to identify the best combination of performance characteristics under which the lift station-force main system will operate under typical conditions (flows and pressures). Properly designed pump-force main systems usually allow the lift station pumps to operate at 35 to 55 percent efficiency most of the time. Overall pump efficiency depends on the type of pumps, their control system, and the fluctuation of the influent wastewater flow.