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

could also be used in force mains. Pressure sensors are attached to the pipe and monitored over a period
of time. This monitoring can be done remotely and the data captured and analyzed to provide a graphical
display of pressure over time. Pressure monitoring can indicate potential risk due to fatigue loading of the
pipe and to transient pressure surges. Force mains, in which pumps are frequently started and stopped,
undergo significant cyclical loading. Transient pressures, also known as surge or water hammer events,
are the result of sudden changes in the velocity of liquid in a pipeline. Even very short-term (less than 1
second) pressure transients can cause damage to pipes if they are repeated frequently. Transient pressures
that exceed the structural strength of the pipe can cause damage in the form of rupture or more frequently
cause latent damage that results in a failure in the future. PVC can be especially vulnerable to fatigue
failure, especially older pipes where formulations are different from those used today. Similarly, negative
pressures can also be a source of damage to pipes.
6.1.5 Leak Monitoring. Leakage in a pressurized main can be a precursor of failure. Typically,
over time, leakage will increase and eventually may disrupt the bedding and support of a buried pipeline.
That can lead to differential settlement and either a failure at a joint or in the barrel of the pipe.
Consequently, identification of the location of leaks in a force main could be an important element in any
condition assessment program, as well as in the normal operation of a force main.
Leaks can fall into two categories: joint leaks and pipe wall leaks. Leaks arising from perforations in the
pipe wall have greater significance. Not only can they be a source of pollution, but they are also a clear
indicator of significant corrosion and an indicator of potential future failure. At the very least, they
provide a focus on where to use structural integrity investigation methods.
Just 21% of the responding utilities to the WERF research project on inspection of force mains (WERF,
2009) stated they have performed leak detection on force mains. The most common method for leak
detection reported was a field hydrostatic test, where either loss of pressure or fluid is used to identify the
presence of a leak in a line. Of course, this requires isolation of the line for the duration of the test and
will not pinpoint the actual location of the leak. The hydrostatic field test is usually limited to new lines
as part of the acceptance requirements.
Leak detection uses a variety of techniques including acoustic, hydrophones, tracer gas and infrared
thermography. Leak detection is of importance in the investigation of force mains and a number of new
developments are providing cost-effective tools for this purpose. External technologies fall broadly into
two types: acoustic sticks (listening devices) and correlators. Ferrous force mains have similar leakage
defects as are found in ferrous water mains and some of the tools from the water industry could be used
with some adaptation. In-line leak detection can be an effective means of undertaking leak surveys on
ferrous force mains. It is a relatively new development for force mains having been initially developed
for potable water main leak investigation. Virtually all diameters and pipe materials can be investigated
with the mains in normal operation. The two principal technologies used in water mains are Sahara ® ,
offered by Pressure Pipe Inspection Company (PPIC) and SmartBall ® by Pure Technologies, Ltd. Both
have been used successfully in force mains and have the added benefit of being able to detect and locate
air pockets, which can be indicators of corrosion in force mains. Sahara ® employs a tethered hydrophone,
while the SmartBall ® is free swimming. The minimum pipe diameter is 12-inch (300-mm) for Sahara ®
and 10-inch (250-mm) for SmartBall ® .
6.1.6 Acoustic Monitoring for Wire Breaks in PCCP. One of the primary failure mechanisms
for PCCP is wire breakage. Real-time monitoring for ongoing wire breaks can enable a picture to be built
up over time of the rate of wire breaks and more importantly, any concentrations of breakage. These can
indicate locations where corrosion is causing rapid deterioration of the pipe and where intervention is
necessary to prevent a failure. The wire breaks can be detected acoustically. This technique can be used
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