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

2.5 Redundancy
Many force mains are located at critical points in sewerage systems. As a result, if they have to be taken
out of service due to a failure, then by-pass pumping or the use of honey trucks is necessary. One way in
which utilities plan for this is to install dual parallel force mains thus providing redundancy. During an
emergency, such as a failure in one line, the parallel line can be used to maintain a minimal level of flow
such that any upstream wetwell does not overflow or sewers become surcharged, or at least the use of
honey trucks (required to keep the sewage from swamping any lift stations) is minimized. The other
advantage in having a redundant line is the ability to take one line out of service for an intrusive (internal)
inspection and/or repairs.
Over 45% of utilities contacted indicated that they had at least one redundant force main in their system.
For those reporting in the affirmative, an average of 14% of the systems had some redundancy built in.
However, some utilities actually reported in the WERF survey having a larger number of redundant
systems than total force mains so this number is suspect. This limited sampling would suggest that
perhaps about 5% of the total number of force mains do have some redundancy built in.
Parallel mains to enable one main to be out of service are seldom installed. Those utilities that do have
parallel mains generally installed them because extra capacity was needed over and above that available
from an existing main. The parallel main is then installed to provide additional capacity and not the total
capacity. Thus, the opportunity for redundancy and operational security is not taken.
Current practice is not to install parallel mains to provide redundancy. Installing alongside existing mains
considered critical is done only very occasionally.
2.6 Cause of Failure in Force Mains
Based on the WERF survey, almost universally, with 92% of the responders in the affirmative, utilities
attempt to identify the cause of the failure in the force main. The utilities were asked to identify the most
common factors relating to failures in their force mains. Various causes are discussed below. It is
important to understand the typical failure modes and mechanisms of force mains in order to select the
most appropriate repair, rehabilitation, and replacement technologies.
2.6.1 Ferrous Force Mains. Figure 2-6 shows that internal corrosion was rated as being
responsible for ferrous force main failures 26.2% of the time, ahead of all other known causes. External
corrosion at 19.2% and third-party damage at 19.4% are the next most common causes of failures.
Combining internal and external corrosion, corrosion appears responsible for failures in force mains in
nearly 46% of cases. After third-party damage, joint leakage at 15.2%, surge pressure at 10.2%, and
insufficient capacity at 9.8% are the next most significant causes of force main failures.
2.6.2 Non-Ferrous Force Mains. Figure 2-7 shows the most common single cause of failure in
non-ferrous force mains is third-party damage, which accounts for 36% of failures. Corrosion and
structural failure together account for 54% of failures. It is considered likely that some of the structural
failures reported for cementitious pipes (AC, RCP, and PCCP) are due to corrosion. Figure 2-7 is based
on a weighted average of failure causes for each pipe material. The main causes of failure vary
significantly by pipe material. For each pipe material type, the main causes of failure are as follows: (1)
PVC – third-party damage, (2) PCCP – corrosion, (3) RCP – corrosion, (4) AC – third-party damage, and
(5) PE – third-party damage.
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