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

6.0 OPERATION AND MAINTENANCE
This section reviews best practices for O&M that can be effective in either prolonging the life of a buried
sewer force main or allowing a utility to monitor real-time performance so action can be taken as needed
to repair, rehabilitate, or replace the force main before a catastrophic failure occurs. Proper cleaning can
improve the capacity and hydraulic performance of a sewer force main. Several other methods are
available for improved O&M, including cathodic protection to arrest the effects of external corrosion,
linear polarization resistance (LPR) to determine a rate of external corrosion, ultrasonic transducers to
monitor loss of internal wall due to corrosion, pressure monitoring, leak detection, and acoustic
monitoring of PCCP for wire breaks. These are all methodologies that a force main owner can employ to
better manage and prolong the life of the buried asset. As discussed below, the ability of a utility’s repair
crews to skillfully carry out emergency repairs on lined force mains is also cited as one deficiency that
limits the use of renewal technologies over replacement, even when their life-cycle costs are lower.
6.1 Procedures to Prolong the Life of Existing Force Mains
6.1.1 Cleaning. Unlike gravity sewers, force mains are not designed to be self-cleansing. As a
result, solids, fats, and greases can accumulate in the main. Operational records can show when a force
main is in need of cleaning. One useful indicator is the volume of flow per unit of electricity consumed
(m 3 /kW); if this reduces significantly then it indicates greater resistance to pumping, which is likely to be
build-up of debris or encrustation on the pipe surface. Many contractors report that cleaning of force
mains, usually by pigging, produces a greater impact than cleaning of water mains. Cleaning methods fall
into two categories: those that dislodge the solids so that they are carried away with the wastewater flow
and those that remove the solids from the pipeline. Pigging, vacuum jetters, and bucket dredging remove
the solids. High pressure water jetting and mechanical rodding are methods that dislodge the solids.
Pigging
Water jetting may be used for cleaning pressure pipelines, but there is also a wide range of tools ranging
from wire brushes and squeegees to motor-driven flails for use in iron pipes. In some cases, the pressure
in the main may be used to force a pig through the entire line, with no need for winching. The bulletshaped
pigs are commonly made of polyurethane and have surface coatings and/or wire brushes that
scrape off the deposits as they travel through the pipeline.
The most commonly used pig for cleaning is the versatile poly pipeline cleaning pig, which is thrust
through the pipeline by hydraulic or pneumatic pressure to clean the interior walls, remove debris, and
flush liquids from the pipeline. A selection of such pigs is shown in Figure 6-1.
Pigging requires high volumes of water at high pressure to force the pig through the line. Volumes above
100 gal/min may be required even in quite small mains and a significant pressure should be applied. Care
must be taken not to exceed the design pressure of the pipe itself during pigging. Large volumes of debris
and sludge can accumulate at the downstream end of the pigged pipeline and these must be captured to
avoid their flowing into the sewer system and simply moving the problem further downstream.
Pigging also requires access to the force main for pig insertion. Force main systems are seldom, if ever,
designed with pigging in mind, so this may be a major operation unless access can be provided at the
pump station. Since force mains generally terminate at a manhole or discharge into a gravity sewer,
recovery of a pig is relatively straightforward.
71