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).
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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
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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.
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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
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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
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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.
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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
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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
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