State of Technology Report for Force Main Rehabilitation, Final ...

5.0 DESIGN AND QA/QC
This section will review the existing design concepts that pertain to the renewal of a force main. The
design methods currently employed are for either interactive or independent liners and depend upon the
condition of the existing (host) pipe. A complete review of most applicable ASTM standards is included.
These standards are broken down by material type and functionality including product/material, design,
and installation practices. The section also covers the QA/QC aspects of renewal by looking at short-term
factory and field requirements, as well as long-term qualification requirements. The latter are notably
lacking for many products offered for both a pressurized and corrosive environment.
5.1 System Design
5.1.1 Redundancy in System Design. Force mains are often located at critical points in a utility’s
sewerage system. They typically carry a large percentage of a utility’s untreated wastewater and in a few
cases all of the flow to the wastewater treatment plant is conveyed through force mains. Despite the
critical nature of sewer force mains, less than 5% have any redundancy built into the system (WERF,
2009). The use of redundancy in system design appears to be a more frequent practice in Europe than the
US. If a line is taken out of service due to a failure, by-pass pumping or the use of honey trucks is
necessary. Those that do have a redundant line can use it during a failure to maintain a minimal level of
flow, avoiding overflowing wetwells, and surcharging upstream sewers. The other advantage to having a
redundant line is the ability to take a force main out of service for an extended period of time to allow for
either an intrusive (internal) inspection or for extended repairs or rehabilitation.
With redundancy very limited, most utilities have not been in a position to carry out in-depth inspections
and assessments of their force main systems. Consequently, many utilities in the past have merely reacted
to problems, such as failures, when they occur rather than being proactive in attempting to assess the
force main’s in situ condition. Little is known about the condition of sewer force mains in US utilities,
but that is gradually changing, primarily as a result of EPA consent decrees requiring municipalities to
carry out condition assessment of their force mains.
Heretofore, there has been very little rehabilitation of sewer force mains due to lack of redundancy and
the availability of reconstruction products for pressure sewers. The redundancy issue isn’t going to
change overnight, although more utilities are considering investing in redundancy with new projects. Any
attempt to rehabilitate a sewer force main, aside from outright replacement while the main remains in
service, is going to either require by-pass pumping or a renovation technology that can be installed very
quickly during a limited outage. The latter doesn’t exist at the moment so by-pass pumping is the norm.
As a result of vendor success in the gravity field with CIPP and reformable thermoplastic pipe products,
adaptations of these products are now starting to find their way into the pressure sewer field.
5.1.2 Pig Launchers/Retrieval for Cleaning and Inspection. Like redundancy, pig launchers and
retrieval systems are not commonly designed into sewer force mains. Some utilities, like St. Petersburg,
FL, have installed pig launchers and retrievals on most of their sewer force mains and use them on a
regularly scheduled basis to clean each main. Keeping the mains clean maintains capacity and reduces
pumping (energy) requirements.
It is only within the past few years that some leak detection tools have become available to allow for
limited assessment of a force main with little interruption in service. These tools can be introduced into
the flow stream and are either tethered (Sahara ® ) or free swimming (SmartBall ® ) and will locate leaks and
gas pockets in a force main. These tools can be launched through 2 inches (50 mm) (Sahara ® ) or 4 inches
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