United States Environmental Protection Agency Office of Water Washington, D.C. EPA 832-F-00-071 September 2000 Wastewater Technology Fact Sheet Sewers, Force Main DESCRIPTION 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 offeree mains are: 1. Pipe. 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 material. Operating pressure and corrosion resistance also impact the choice. Pipeline size and wall thickness are determined by wastewater flow, operating pressure, and trench conditions. Common Modifications Force mains may be aerated or the wastewater chlorinated at the pump station to prevent odors and excessive corrosion. Pressure surge control devices are installed to reduce pipeline pressure below a safe operating pressure during lift station start-up and shut-off. Typically, automatically operated valves (cone or ball type) control pressure surges at the pump discharge or pressure surge tanks. Normally, force main cleaning includes running a manufactured "pigging" device through the line and long force mains are typically equipped with "pig" insertion and retrieval stations. In most cases, insertion facilities are located within the lift station and the pig removal station is at the discharge point of the force main. Several launching and retrieval stations are usually provided in long force mains to facilitate cleaning of the pipeline. APPLICABILITY Force mains are used to convey wastewater from a lower to higher elevation, particularly where the elevation of the source is not sufficient for gravity flow and/or the use of gravity conveyance will result in excessive excavation depths and high sewer pipeline construction costs. Ductile iron and polyvinyl chloride (PVC) are the most frequently used materials for wastewater force mains. Ductile iron pipe has particular advantages in wastewater collection systems due to its high strength and high flow capacity with greater than nominal inside diameters and tight joints. For special corrosive conditions and extremely high flow characteristics, polyethylene-lined ductile iron pipe and fittings are widely used. Cast iron pipe with glass lining is available in standard pipe sizes, with most joints in lengths up to 6.1 meters (20 feet). Corrosion-resistant plastic lined piping systems are used for certain waste carrying applications. Polyethylene-lined ductile iron pipe and fittings known as "poly-bond-lined" pipe is widely used for force mains conveying highly corrosive industrial or municipal wastewater. The types of thermoplastic pipe materials used for force main service are PVC, acrylonitrile- butadiene-styrene (ABS), and polyethylene (PE). ------- The corrosion resistance, light weight, and low hydraulic friction characteristics of these materials offer certain advantages for different force main applications, including resistance to microbial attack. Typically, PVC pipes are available in standard diameters of 100 to 900 mm (4 to 36 inches) and their laying lengths normally range from 3 to 6 meters (10 to 20 feet). The use of composite material pipes, such as fiberglass reinforced mortar pipe ("truss pipe"), is increasing in the construction offeree mains. A truss pipe is constructed on concentric ABS cylinders with annular space filled with cement. Pipe fabricated of fiberglass reinforced epoxy resin is almost as strong as steel, as well as corrosion and abrasion resistant. Certain types of asbestos-cement pipe are applicable in construction of wastewater force mains. The advantage of asbestos-cement pipes in sewer applications is their low hydraulic friction. These pipes are relatively lightweight, allowing long laying lengths in long lines. Asbestos-cement pipes are also highly corrosion resistant. At one time it was thought that many asbestos containing products (including asbestos-cement pipe) would be banned by the Environmental Protection Agency. However, a court ruling overturned this ban and this pipe is available and still used for wastewater force main applications (Sanks, 1998). Force mains are very reliable when they are properly designed and maintained. In general, force main reliability and useful life are comparable to that of gravity sewer lines, but pipeline reliability may be compromised by excessive pressure surges, corrosion, or lack of routine maintenance. ADVANTAGES AND DISADVANTAGES Advantages Use offeree 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 6.1 meters (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 offeree mains is not dependent on site specific topographic conditions and is not impacted by available terrain slope, which typically limits gravity wastewater conveyance. Disadvantages While construction offeree 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 offeree 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. DESIGN CRITERIA 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 offeree 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. 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 smaller diameters. Table 1 lists the types of pipe recommended for use in a force main system and suggested applications. Velocity Force mains from the lift station are typically designed for velocities between 0.6 to 2.4 meters per second (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 610 meters or 2,000 feet) and low lift requirements (less than 9.1 meters or 30 feet), the recommended design force main velocity range is 1.8 to 2.7 meters per second (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 3 meters per second (10 feet per second). Table 2 provides examples of force main capacities at various pipeline sizes, materials, and velocities. The flow volumes may vary depending on the pipe material used. TABLE 1 CHARACTERISTICS OF COMMON FORCE MAIN PIPE MATERIALS Material Application Key Advantages Key Disadvantages Cast or Ductile Iron, Cement Lined Steel, Cement Lined Asbestos Cement Fiberglass Reinforced Epoxy Pipe Plastic High pressure Available sizes of 4-54 inches High pressure All pipe sizes Moderate pressure For 36-inch + pipe sizes Moderate pressure For up to 36-inch pipe sizes Low pressure For up to 36-inch pipe sizes Good resistance to pressure surges Excellent resistance to pressure surges No corrosion Slow grease buildup No corrosion Slow grease buildup No corrosion Slow grease buildup More expensive than concrete and fiberglass More expensive than concrete and fiberglass Relatively brittle 350 psi max pressure Suitable for small pipe sizes and low pressure only Source: Sanks, 1998. ------- TABLE 2 FORCE MAIN CAPACITY Diameter (inches) 6 8 10 18 24 36 Velocity gpm 176 313 490 1,585 2,819 6,342 = 2fps Ips 11 20 31 100 178 400 Velocity gpm 362 626 980 3,170 5,638 12,684 = 4fps Ips 22 40 62 200 356 800 Velocity gpm 528 626 1,470 4,755 8,457 19,026 = 6fps Ips 33 60 93 300 534 1,200 Source: Metcalf and Eddy, 1981. Vertical Alignment Force mains should be designed so that they are always full and pressure in the pipe is greater than 69 kiloPascals (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 blowoffs are installed at low points. Pressure Surges 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, 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 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. PERFORMANCE 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. OPERATION AND MAINTENANCE The operation offeree main-lift station systems is usually automated and does not require continuous on-site operator presence. However, annual force ------- main route inspections are recommended to ensure normal functioning and to identify potential problems. Special attention is given to the integrity of the force main surface and pipeline connections, unusual noise, vibration, pipe and pipe joint leakage and displacement, valving arrangement and leakage, lift station operation and performance, discharge pump rates and pump speed, and pump suction and discharge pressures. Depending on the overall performance of the lift station-force main system, the extent of grease build-up and the need for pipeline pigging are also assessed. If there is an excessive increase in pump head and the headloss increase is caused by grease build-up, the pipeline is pigged. Corrosion is rarely a problem since pipes are primarily constructed of ductile iron or plastic, which are highly resistant to corrosion. Buildup can be removed by pigging the pipeline. COSTS Force main costs depend on many factors including: 1. Conveyed wastewater quantity and quality. 2. Force main length. 3. Operating pressure. 4. Soil properties and underground conditions. 5. Pipeline trench depth. 6. Appurtenances such as valves and blowoffs. 7. Community impacts. These site and system specific factors must be examined and incorporated in the preparation of force main cost estimates. Construction Costs Unit force main construction costs are usually expressed in $ per linear foot of installed pipeline and costs typically include labor and the equipment and materials required for pipeline installation. Table 3 unit pipeline construction costs for ductile iron and plastic (PVC) pipes used for force main construction. These costs are base installation costs and do not include the following: 1. General contractor overhead and profit. 2. Engineering and construction management. 3. Land or right-of-way acquisition. 4. Legal, fiscal, and administrative costs. 5. Interest during construction. 6. Community impacts. All unit pipeline costs are adjusted to 1999 dollars. TABLE 3 CONSTRUCTION COSTS FOR DUCTILE IRON AND PLASTIC PIPES Pipe Diameter (inches) 8 10 12 14 16 18 20 24 30 36 Ductile Iron Pipe ($/linear foot) 23 29 36 46 53 66 72 84 142 190 PVC Pressure Pipe ($/linear foot) 15 20 26 33 41 48 56 65 90 135 Source: James M. Montgomery Consulting Engineers, 1998. Operation and Maintenance Costs Force main operation and maintenance costs include labor and maintenance requirements. Typically, labor costs account for 85 to 95 percent of total operation and maintenance costs and are dependent on the force main length. The ------- maintenance costs usually vary from $7 to $20/meter ($2 to $6/linear foot), depending on the size and number of appurtenances installed on the force main. An internal inspection using TV equipment can be completed, if visual inspection is not sufficient. TV inspection can be costly, ranging from $1,000 to $11,450 per mile with an average cost of $4,600 per mile (WERF, 1997; Arbour and Kerri, 1997). Table 4 summarizes force main construction costs Other EPA Fact Sheets can be found at the following web address: http://www.epa.gov/owmitnet/mtbfact.htm 1. Arbour, R. and K. Kerri, 1997. Collection Systems: Methods for Evaluating and Improving Performance. Prepared for the EPA Office of Wastewater Management by the California State University, Sacramento, CA. TABLE 4 FORCE MAIN CONSTRUCTION COSTS Project/ Location Compton, CA Oceanside, CA Eugene, OR CMCWD 1, CA CMCWD II, CA Goleta, CA Gillette, WY Force Main Average Capacity (mgd) 8 18 12 42 30 56 30 Construction Costs ($US/linear foot) 70 85 90 510 260 365 120 Source: James M. Montgomery Consulting Engineers, 1998. from several projects, adjusted to 1999 dollars. REFERENCES Other Related Fact Sheets Sewers, Lift Stations EPA 832-F-00-073 September 2000 Pipe Construction and Materials EPA 832-F-00-068 September 2000 Sewer Cleaning and Inspection EPA832-F-99-031 September 1999 2. Bethany R. B. May 1994. Pressure Reducing Stations - A Key to Networked Interceptor System Operations. Operations Forum, Vol. 11, No. 5,8-12. 3. Casada, Don. May 1998. Pump Optimization for Changing Needs. Operations Forum. Vol. 9, No. 5, 14-18. 4. Huges D. M. and R.G. Cornforth. May 1997. The Importance of Surge Protection in Avoiding Pump Station Failures. Operations Forum, Vol. 14, No. 5, 25-28. 5. Horton A. M. October 1996. Protective Linings for Ductile Iron Pipe in Wastewater Service. In Proceedings of the Water Environment Federation. 69th Annual Conference & Exposition, Dallas, Texas, Vol. 3. 6. Jackson J. K. May 1996. Variable Speed Pumping Brings Efficiency to Pump Systems. Operations Forum, Vol. 13, No. 5, 21-24. 7. James M. Montgomery Consulting Engineers, 1988. "Sewerage System Preliminary Cost Estimating Curves." 8. Metcalf & Eddy Inc., 1981. Wastewater Engineering: Collection and Pumping of Wastewater, McGraw Hill Book Company. ------- 9. Moody T. C. October 1998. Optimizing Force Main Odor Control Chemical Dosage: A Tale of Two Systems. In Proceedings of the Water Environment Federation, 71st Annual Conference, Orlando, Florida, Vol. 2. 10. Paschke N.W. May 1997. Pump Station Basics - Design Considerations for a Reliable Pump Station. Operations Forum, Vol. 14, No. 5 15-20. 11. Prasuhn, A.L. 1987. Fundamentals of Hydraulic Engineering, Holt, Rinehart and Winston, New York. 12. Robinson, P. E., Aguiar G., Grant M. October 1995. Fast Tracking the Critical Dade County Cross Bay Transmission Line. In Proceedings of the Water Environment Federation. 68th Annual Conference & Exposition, Miami Beach, Florida, Vol. 3. 13. SanksR. L., TchobanoglousG.,NewtonD., Bosserman, B.E., Jones, G. M. 1998. Pump Station Design, Butterworths, Boston. 14. Seigal S.E. May 1994. Upgraded to the World's Largest. Dry-Pit/Submerged Pumps Make the Grade. Operations Forum, Vol. 11, No. 5,24-28. 15. Smith E. C. July 1994. Don't Lose the Pump Efficiency Game. Operations Forum,Vol. 11, No. 7, 18-21. 16. U.S. Environmental Protection Agency. October 1985. Design Manual. Odor and Corrosion Control in Sanitary Sewerage Systems and Treatment Plants. EPA/625/1 - 85/018. 17. Water Environment Federation. 1994. Existing Sewer Evaluation and Rehabilitation. Manual of Practice No. FD6. 18. Water Environment Federation. 1985. Operations and Maintenance ofWastewater Collection Systems. Manual of Practice No. 7. 19. Water Environment Federation. 1992. Wastewater Collection Systems Management. Manual of Practice No. 7. 20. Water Environment Research Federation (WERF), 1997. Benchmarking Wastewate Operations - Collection, Treatment, and BiosolidsManagement. Project 96-CTS-5. 21. Workman G., and M.D. Johnson. October 1994. Automation Takes Lift Station to New Heights. Operations Forum, Vol. 11, No. 10, 14-16. ADDITIONAL INFORMATION Luis Aguiar, Assistant-Director Miami-Bade Water and Sewer Department 4200 Salzedo Street Coral Gables, FL 33146 Eileen M. White East Bay Municipal Utility District P.O. Box 24055 Oakland, CA 94523 Richard R. Roll Wastewater Treatment Plant P.O. Box 69 Niagara Falls, NY 14302 Gary N. Oradat City of Houston DPW& Engineering Utility Maintenance Division 306 McGowen Street Houston, TX 7706 David Jurgens City of Fayetteville 113 West Mountain Street Fayetteville, AR 72701 ------- Bruno Conegliano Water & Wastewater Utility City of Austin, P.O. Box 1088 Austin, TX 78767 The mention of trade names or commercial products does not constitute endorsement or recommendations for use by the United States Environmental Protection Agency (EPA). For more information contact: Municipal Technology Branch U.S. EPA Mail Code 4204 1200 Pennsylvania Avenue, NW Washington, D.C. 20460 MTB Excellence fri compliance through optimal technical solutions MUNICIPAL TECHNOLOGY BRANCH ------- |