Wastewater Technology Fact Sheet: Sewers, Force Main

1 United StatesEnvironmental ProtectionAgencyOffice of WaterWashington, 832-F-00-071 September 2000 WastewaterTechnology Fact SheetSewers, Force MainDESCRIPTIONF orce mains are pipelines that convey wastewaterunder pressure from the discharge side of a pump orpneumatic ejector to a discharge point. Pumps orcompressors located in a lift station provide theenergy for Wastewater conveyance in Force key elements of Force mains surge control main cleaning mains are constructed from various materialsand come in a wide range of diameters. Wastewaterquality governs the selection of the most suitablepipe material. Operating pressure and corrosionresistance also impact the choice. Pipeline size andwall thickness are determined by Wastewater flow,operating pressure, and trench conditions. Common ModificationsForce mains may be aerated or the wastewaterchlorinated at the pump station to prevent odors andexcessive corrosion.

2 Pressure surge control devicesare installed to reduce pipeline pressure below asafe operating pressure during lift station start-upand shut-off. Typically, automatically operatedvalves (cone or ball type) control pressure surges atthe pump discharge or pressure surge , Force main cleaning includes running amanufactured pigging device through the line andlong Force mains are typically equipped with pig insertion and retrieval stations. In most cases,insertion facilities are located within the lift stationand the pig removal station is at the discharge pointof the Force main . Several launching and retrievalstations are usually provided in long Force mains tofacilitate cleaning of the mains are used to convey Wastewater from alower to higher elevation, particularly where theelevation of the source is not sufficient for gravityflow and/or the use of gravity conveyance willresult in excessive excavation depths and highsewer pipeline construction costs.

3 Ductile iron and polyvinyl chloride (PVC) are themost frequently used materials for Wastewater forcemains. Ductile iron pipe has particular advantagesin Wastewater collection systems due to its highstrength and high flow capacity with greater thannominal inside diameters and tight joints. Forspecial corrosive conditions and extremely highflow characteristics, polyethylene -lined ductile ironpipe and fittings are widely used. Cast iron pipe with glass lining is available instandard pipe sizes, with most joints in lengths upto meters (20 feet). Corrosion-resistant plasticlined piping systems are used for certain wastecarrying applications. polyethylene -lined ductileiron pipe and fittings known as poly-bond-lined pipe is widely used for Force mains conveyinghighly corrosive industrial or municipal types of thermoplastic pipe materials used forforce main service are PVC, acrylonitrile-butadiene-styrene (ABS), and polyethylene (PE).

4 The corrosion resistance, light weight, and lowhydraulic friction characteristics of these materialsoffer certain advantages for different Force mainapplications, including resistance to microbialattack. Typically, PVC pipes are available instandard diameters of 100 to 900 mm (4 to 36inches) and their laying lengths normally rangefrom 3 to 6 meters (10 to 20 feet). The use ofcomposite material pipes, such as fiberglassreinforced mortar pipe ( truss pipe ), is increasingin the construction of Force mains. A truss pipe isconstructed on concentric ABS cylinders withannular space filled with cement. Pipe fabricated offiberglass reinforced epoxy resin is almost as strongas steel, as well as corrosion and abrasion types of asbestos-cement pipe areapplicable in construction of Wastewater forcemains.

5 The advantage of asbestos-cement pipes insewer applications is their low hydraulic pipes are relatively lightweight, allowinglong laying lengths in long lines. Asbestos-cementpipes are also highly corrosion resistant. At onetime it was thought that many asbestos containingproducts (including asbestos-cement pipe) would bebanned by the Environmental Protection , a court ruling overturned this ban and thispipe is available and still used for Wastewater forcemain applications (Sanks, 1998). Force mains are very reliable when they areproperly designed and maintained. In general, forcemain reliability and useful life are comparable tothat of gravity sewer lines, but pipeline reliabilitymay be compromised by excessive pressure surges,corrosion, or lack of routine maintenance. ADVANTAGES AND DISADVANTAGESA dvantagesUse of Force mains can significantly reduce the sizeand depth of sewer lines and decrease the overallcosts of sewer system construction.

6 Typically,when gravity sewers are installed in trenches deeperthan meters (20 feet), the cost of sewer lineinstallation increases significantly because morecomplex and costly excavation equipment andtrench shoring techniques are required. Usually, thediameter of pressurized Force mains is one to twosizes smaller than the diameter of gravity sewerlines conveying the same flow, allowing significantpipeline cost reduction. Force main installation issimple because of shallower pipeline trenches andreduced quantity of earthwork. Installation of forcemains is not dependent on site specific topographicconditions and is not impacted by available terrainslope, which typically limits gravity construction of Force mains is less expensivethan gravity sewer lines for the same flow, forcemain Wastewater conveyance requires theconstruction and operation of one or more liftstations.

7 Wastewater pumping and use of forcemains could be eliminated or reduced by selectingalternative sewer routes, consolidating a proposedlift station with an existing lift station, or extendinga gravity sewer using directional drilling or otherstate-of-the art deep excavation methods. The dissolved oxygen content of the Wastewater isoften depleted in the wet-well of the lift station, andits subsequent passage through the Force mainresults in the discharge of septic Wastewater , whichnot only lacks oxygen but often contains cleaning and maintenance of Force mainsis required to remove solids and grease buildup andminimize corrosion due to the high concentration surges are abrupt increases in operatingpressure in Force mains which typically occurduring pump start-up and shut-off. Pressure surgesmay have negative effects on Force main integritybut can be reduced by proper pump station andpipeline CRITERIAF orce main design is typically integrated with liftstation design.

8 The major factors to consider inanalyzing Force main materials and hydraulicsinclude the design formula for sizing the pipe,friction losses, pressure surges, and Hazen-Williams formula is recommended forthe design of Force mains. This formula includes aroughness coefficient C, which accounts forpipeline hydraulic friction characteristics. Theroughness coefficient varies with pipe material,size, and main Pipe MaterialsSelection criteria for Force main pipe materialsinclude: quantity, quality, and properties, such as strength, ease ofhandling, and corrosion of appropriate sizes, wallthickness, and friction iron pipe offers strength, stiffness, ductility,and a range of sizes and thicknesses and is thetypical choice for high-pressure and exposedpiping.

9 Plastic pipe is most widely used in shortforce mains and smaller diameters. Table 1 lists thetypes of pipe recommended for use in a Force mainsystem and suggested mains from the lift station are typicallydesigned for velocities between to metersper second (2 to 8 feet per second). Such velocitiesare normally based on the most economical pipediameters and typical available heads. For shorterforce mains (less than 610 meters or 2,000 feet) andlow lift requirements (less than meters or 30feet), the recommended design Force main velocityrange is to meters per second (6 to 9 feet persecond). This higher design velocity allows the useof smaller pipe, reducing construction velocity also increases pipeline friction lossby more than 50 percent, resulting in increasedenergy costs.

10 To reduce the velocity, a reducer pipeor a pipe valve can be used. Reducer pipes areoften used because of the costly nature of pipevalves. These reducer pipes, which are larger indiameter, help to disperse the flow, thereforereducing the maximum Force main velocity at peakconditions is recommended not to exceed 3 metersper second (10 feet per second). Table 2 providesexamples of Force main capacities at variouspipeline sizes, materials, and velocities. The flowvolumes may vary depending on the pipe 1 CHARACTERISTICS OF COMMON Force main PIPE MATERIALSM aterialApplicationKey AdvantagesKey DisadvantagesCast or Ductile Iron,Cement LinedHigh pressureAvailable sizes of 4-54 inchesGood resistance to pressuresurgesMore expensive thanconcrete and fiberglassSteel, Cement LinedHigh pressureAll pipe sizesExcellent resistance topressure surgesMore expensive than concrete and fiberglassAsbestos CementModerate pressureFor 36-inch + pipe sizesNo corrosionSlow grease buildupRelatively brittleFiberglass ReinforcedEpoxy PipeModerate pressureFor up to 36-inch pipe sizesNo corrosionSlow grease buildup350 psi max pressurePlasticLow pressureFor up to 36-inch pipe sizesNo corrosionSlow grease buildupSuitable for small pipe sizesand low pressure onlySource.