3. DC through ground from foreign CP systems or
grounded electrical equipment; AC through ground from
grounded electrical equipment; Background - Induced AC;
AC induced from overhead power lines; AC induced from
overhead power lines - Coating Effect; AC induced from
overhead power lines - Soil Resistivity Effect; AC
induced from overhead power lines - Power Line Voltage
Effect; AC induced from overhead power lines - Parallel
Path Effect; AC induced from overhead power lines -
Effect of Load Balancing; AC induced from overhead power
lines - Time Variations
DC through ground from
foreign CP systems or grounded electrical equipment
Cathodic Protection
systems flow DC current into the soil. Other grounded
equipment including traction systems and mining systems
can also put DC currents into the ground.
DC Currents flowing
onto and off a pipeline can cause a specific form of
corrosion called interference. Interference is also
called Electrolysis or Stray Current Corrosion.
Interference corrosion, its detection, mitigation and
monitoring is not covered in detail in this document.
AC through ground
from grounded electrical equipment
Grounded AC electrical
equipment can put AC currents into the ground.
AC Currents flowing
onto and off a pipeline can also cause corrosion on a
pipeline. Per amp-yr of current, AC causes less
corrosion than an amp-yr of DC current. However, large
AC currents are possible.
Background -
Induced AC
AC induced from
overhead power lines
Overhead power lines
are often placed in the same right of way as buried
pipelines. These power lines are often very high voltage
(thousands of volts) and carry large currents.
The alternative
current of the power lines can induced very large
currents and voltages on buried pipelines.
The following factors
tend to increase the amount of induced AC
- good coating
- high resistivity
soil
- large voltage on
power lines
- long path of power
lines paralleling pipeline
- Inbalanced Load
AC induced from
overhead power lines - Coating Effect
A poor coating or bare
pipe allows the draining of AC induced current to ground
with very little resistance. Voltage buildups are small.
Any corrosion is spread over large surface areas of the
pipe.
AC induced from
overhead power lines - Soil Resistivity Effect
High resisitivity soil
increases the resistance of the pipe line to ground.
High resistance to ground make is possible to build up
large voltages on the pipeline and its components.
AC induced from
overhead power lines - Power Line Voltage Effect
The larger the voltage
on an overhead line the greater the amount of induced
current on the pipeline.
AC induced from
overhead power lines - Parallel Path Effect
The longer the length
of pipeline that parallels the power line, the greater
will be the induced current on the pipeline.
AC induced from
overhead power lines - Effect of Load Balancing
Transmission AC power
lines are three phase. That is to say they have three
wires conducting AC currents. If the loads are balanced
(the same) very little induced AC effect occurs on the
pipeline. The more imbalanced the load, the greater that
induced AC effect.
AC induced from
overhead power lines - Time Variations
It is important to
note that since the load balancing changes from minute
to minute, the induced AC effects also change from
minute to minute. Studies of the period of several
hours, several days or even several weeks may be needed
to detect problems |
