Dynamic Stray Current Interference Testing And Mitigation Design For A 90-Inch Water Main

This paper discusses two sections of pipeline installed in a circular rib and lag tunnel.
These tunnels are geographically the closest points between the electric railroad and the
90-inch water transmission pipeline.
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Dynamic Stray Current Interference Testing And
Mitigation Design For A 90-Inch Water Main
By Rogelio de las Casas, Ron Turner, Kristi Roe;
ENTRUST Solutions Group
ABSTRACT
A large number of pipelines are routed around or
through the Chicago metropolitan region of Illinois.
Pipeline operators are faced with operational and
maintenance challenges that include the mitigation
of static and dynamic stray current interference. This
interference is generated by DC current sources
which can include foreign pipelines and DC electric
rail systems. In the Chicago area, the Chicago Transit
Authority (CTA) railway system is one of the sources
of dynamic DC stray current that can affect pipeline
operators.
Preliminary testing conducted on a local 90-inch water
transmission pipeline was observed to indicate the
presence of dynamic stray current interference. Based
on this preliminary testing, more advanced testing was
initiated. Ultimately this activity lead to design services
to address and mitigate the DC stray current found on
this water transmission pipeline.
Field testing and analysis, calculations and the final
mitigation design are presented in this paper.
INTRODUCTION
When testing on a local 90-inch water transmission
pipeline indicates the presence of dynamic stray
current interference, additional confirmatory testing
and design services were initiated. The focus of this
work is to assess the level of stray current interference
and after field testing recommend a design to help
mitigate the effects of the stray dynamic DC current.
Dynamic Stray Current Interference Testing And
Mitigation Design For A 90-Inch Water Main
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PIPELINE CHARACTERISTICS
The water transmission pipeline consists of
approximately 9 miles of 90-inch diameter PreStressed Concrete Cylinder Pipe (PCCP). The pipeline
runs in the vicinity and adjacent to the CTA electric rail
system. The pipeline is directly buried for the majority
of the distance; however, there are two (2) sections
where the pipeline is located in a tunnel. These
sections are located at a point where the 90-inch
pipeline is routed closest to the CTA electrical rails.
Running approximately parallel to the 90-inch pipeline,
but with a separation of approximately ½ a mile, is a
72-inch steel water transmission pipeline. The 72-inch
line has an existing DC stray current system in place.
DC INTERFERENCE CONSIDERATIONS AND
TESTING
Existing Electrical Shielding
As described above, there are two (2) sections of
90-inch water transmission pipeline which have been
installed in a circular rib and lag tunnel. These tunnels
are geographically the closest physical point between
the CTA electric railroad and the 90-inch water
transmission pipeline. The 90-inch water main as
installed is not provided with any designed protection
from the action of DC stray current interference.
If the tunnels act to electrically isolate the 90-inch
water transmission pipeline and there is no electrolyte,
such as water or soil between the external pipe
surface and internal tunnel surface (tunnel annulus),
then stray current cannot be discharged from the pipe
surface to ground within these two (2) tunnel locations.
If isolated, these tunnels act to increase the electrical
path resistance for the DC stray current and may act
to eliminate the stray current from being discharged
from the water transmission pipeline at a point which
is closest to the DC electric rail system. These
tunnels may also act to move the point of electrical
interference to a point upstream or downstream of this
location.
As installed there is no possible means to directly
measure pipe-to-soil potentials or perform any other
tests related to interference at the tunneled locations.
As such, it is unknown whether there is water or soil
in the tunnel annulus. If the tunnel is not acting to
completely shield the pipeline; it is possible that the
90-inch water main may be experiencing interference
due to its close proximity to the DC rail system.
Testing Outside of the Tunnel
Since the section of the 90-inch water transmission
pipeline contained within the two (2) tunnels and
closest to the DC rail system could not be directly
tested, a section of main was selected for more
advanced testing based on a review of the pipeline
route and engineering experience. The test section
selected is the first pipeline section east of the second
tunnel which begins at stationing (156+38) and heads
east. The testing performed on this section of the 90-
inch water transmission is as follows:
Graphing of the Beta curves. This test included
measurement of the pipe-to-rail open circuit potential
Eo, and the pipe-to-soil potential Vg. These two
parameters were measured during the same period
of time and the values were plotted together in an x-y
graph, where the Vg is the y axis and Eo is the x axis.
Determination of Beta values. The slope of the best
fit line formed by the data gathered during the step
above is known as the Beta value and was used to
interpret the level of DC stray current at that point. This
value helped calculate the bond characteristics, such
as the maximum bond resistance and the maximum
allowable current flowing through the bond cable.
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Dynamic Stray Current Interference Testing And
Mitigation Design For A 90-Inch Water Main
Determination of Vg in the case of no interference.
This value is the value of Vg when the pipe-to-rail
open circuit potential Eo is equal to zero. This was
determined as the intercept of each beta curve with
the y axis.
Maximum Eo value: After the point of maximum
exposure was determined, the value of Eo, Vg and
the Beta value were determined over a 24 hour period
of time. By doing this, the variables of the worst case
scenario were determined. These values were used
to calculate the bond characteristics; which were
the maximum current allowed through the bond, the
maximum bond cable resistance and the bond cable
gauge.
TEST RESULTS
Graphing Beta Curves
To obtain the values for the worst case scenario,
the 72” main was disconnected from the existing
reverse current drainage switch. The existing negative
connection to the rail was used as the point of
connection to measure the open circuit potential Eo,
between the electrical rail and the 90” pipeline. The rail
was connected to the negative lead of the data logger
and the 90” pipe was connected to the positive lead of
the data logger.
The data logger was used in a two channel mode,
where one channel was used to measure the Eo
value, and the other channel was used to measure the
pipe-to-soil potential Vg, with respect to a saturated
Cu/CuSO4 reference cell. Both values were recorded
simultaneously and they were plotted on an x-y graph
to obtain the best fit line equation that corresponds to
each set of data.
A set of data was taken at Test Station A and at 100
foot intervals along the pipeline to the east of the test
station, which is located at the eastern end of the
tunnel. The Eo and Vg data was recorded once every
second for a total of ten minutes at each location. A
total of 14 sets of data were obtained and the graphs
were plotted with the pipe-to-rail open circuit potential
Eo on the x axis and the pipe-to-soil potential Vg on
the y axis. Figure 1 below shows an example of one of
the Beta Curves.
Potential when stray current source turned off
The structure-to-soil potential, Vgo, is the pipe-to-soil
potential when stray current affecting the pipe is turned
off. Vgo cannot be determined because the rail system
current source cannot be turned off. Even at night,
when the rail system operates at a lower electrical
load, there is still current flowing on the pipe.
Vgo can only be determined when Eo is zero in the
equation of the best fit line for the data points related
to Eo and Vg. The equation of the best fit line is as
follows:
Vgo was calculated for each Beta plot. The structureto-soil potential measurements tended to become
more electro-negative as the readings were taken
progressively further east of Test Station C. This
tendency might be a consequence of the pipeline
receiving increased current as potential readings are
recorded at greater distances from the location of
maximum current discharge near the rail substation.
See Figure 32
for a graphical explanation of how
current moves in a DC stray current circuit.
The points where the current is being picked up from
the soil to the main will have more electro-negative
potentials and the points where the current discharges
from the main to the soil will have less electro-negative
pipe-to-soil potentials.
Figure 4 shows the Vgo, the tendency of values more
negative as the readings are taken progressively
further east of Test Station C is observed.
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Dynamic Stray Current Interference Testing And
Mitigation Design For A 90-Inch Water Main
Where:
Eo = the open circuit potential between the pipe and
the rail, measured in volts. For the measurement, the
railroad is connected to the negative terminal of a high
resistance multimeter and the pipe is connected to the
positive terminal.
Vg = the pipe-to-soil potential, measured in volts with
respect to a Cu/CuSO4 reference electrode at the
same time that Eo is measured.
ß = the slope of the line, the best fit line, for the scatter
plot representing the relationship between Eo and Vg.
Vgo = the pipe-to-soil potential in volts, when the
source of interference is turned off. This can only be
measured when Eo= zero.
Figure 21
below shows how the connections were
made for the Beta tests. Points A, B and C represent
different points where the structure-to-soil potentials
were obtained while the open circuit potential was
measured at a fixed location. In this case, the fixed
location was at Test Station C. This test station is the
closest test point to the railroad.
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Dynamic Stray Current Interference Testing And
Mitigation Design For A 90-Inch Water Main
Beta Value Determination
The value of Beta (ß) is defined as the slope of the
line represented by Equation 1. This value expresses
how the magnitude of the structure-to-soil potential,
Vg, changes with respect to changes in the pipe-torail open circuit potential Eo. In general, the larger the
value of Beta (the steeper the slope), the greater the
pick-up or discharge of current will be. The polarity of
the Beta value depends on the point of connection for
the voltmeter measuring the pipe-to-rail potential. In
this case, because this connection was made closest
to the railroad substation (close to discharge point),
the value of Beta will always be positive.
Open Circuit Potential Between Main and Rail
The pipe-to-rail open circuit potential is the difference
in potential between the pipe and the rail. It is the
driving force that initiates current movement through
the soil between the pipe and the rail. The test
connections made to determine this value are shown
in Figure 2, where the pipe is connected to the positive
lead of a high resistance voltmeter and the rail is
connected to the negative lead of the voltmeter. When
current is discharging from the pipe to the soil, the
voltage readings will be positive and when current is
being picked up on the pipe from the soil the voltage
readings will be negative.
The open circuit potential (Eo) was measured at the
nearest accessible pipeline test point to the railroad.
This test was performed between Test Station A and
the negative of the existing reverse current drainage
switch installed on the 72” transmission main. Pipeto-soil potentials (Vg) were recorded at each of the
14 locations performed during the Beta value test.
Following the Beta test, the open circuit potential was
recorded over a 24 hour period in order to determine
the maximum value of Eo. The maximum value of Eo
indicates the maximum amount of current that can flow
in the stray current circuit. This value is then used in
the bond characteristic calculations.
DC Circuit Characteristic Determination at
Maximum Exposure Point
Beta Value During 24 Hour Period. Since Eo and Vg
were determined during 24 hours at the closest point
to the railroad, the value of Beta at this location can
be determined. This value was used together with the
Eo value to calculate the bond parameters for design
purposes.
Circuit Characteristics With no External Current
Source. After all necessary values were determined
with no connection between the pipe and the rail,
another series of measurements were recorded
with a direct connection between the rail and the
main installed. Doing this, the parameters such as
the average circuit internal resistance and average
structure-to-soil potential to current ratio could be
determined. These parameters were also used in the
equations for bond resistance and current calculation.
These tests simulate a bond connection between
the water main and the railroad. A current interrupter
and a shunt were connected in series between the
wire that is connected to the pipe and the railroad.
Measurements of the pipe-to-rail short circuit potential,
the current across the shunt and the pipe-to-soil
potential were measured and recorded at the same
time. This testing provided the following information:
a. Average internal resistance: This is the resistance
between the rail and the pipe calculated as the
average closed circuit potential divided by the current
measured during the test.
b. Average structure-to-soil potential to current ratio:
This is the average main-to-soil potential divided by
the current through the circuit measured during the
test.
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Dynamic Stray Current Interference Testing And
Mitigation Design For A 90-Inch Water Main
Circuit Characteristics With External Current
Source. The internal resistance and the average
structure-to-soil potential to current ratio were also
determined while applying an external current source.
The external current source helped to stabilize the
circuit reads and helped to obtain more accurate
values of these parameters.
TESTING PARAMETERS AND RESULTS
Table 1 shows the parameters determined during the
testing and calculated using the procedures mentioned
above. All values listed in Table 1, other than the open
circuit potentials (Eo) which was measured in the field,
are calculated values.
DESIGN PARAMETERS AND CALCULATIONS
Design Methodology
The design methodology utilized for this analysis was
based on the electrical relationship between the driving
force for stray current (the difference in potential
between the DC electric rail and the water main) and
the change in potential of the pipe due to this driving
force. This takes into account that the measured
structure-to-soil potential is a combination of its natural
potential-to-earth plus the sum of all potential changes
caused by the DC sources influencing it.
Design Rationale
Since the pipe-to-rail potential changes with time and
can be positive or negative, the calculations were
divided into two sets. One set of calculations was
performed with the negative pipe-to-rail potentials
and the second set of calculations was performed
using the positive pipe-to-rail potentials. The set of
values that shows the maximum magnitude of current
discharge from or pick-up to the pipe was chosen to
design the bond.
Data Summary
As seen in Table 1, the bond resistance, the maximum
current through the bond and the minimum cable
size for six different cases were calculated. The
calculations take into account all of the available data
and use the parameters that yield the worst case
scenario (maximum current) for bond design purposes.
The average, minimum and maximum parameter
values were derived for each test. A distinction was
made between the values obtained when the current
was draining from the pipe to the soil (positive current
reads) and when the main was picking up current
from soil (negative current reads). Doing it this way,
all of the current flow scenarios were considered.
The primary values used in the bond design are the
positive current values. This is the condition when the
reverse current drainage switch is closed and allows
current to flow through the bond.
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Dynamic Stray Current Interference Testing And
Mitigation Design For A 90-Inch Water Main
Maximum Bond Current Calculation
The maximum current that can flow through the bond
in the case of maximum load in the electric rail circuit
was calculated using the following equation:
Where:
Ib = the current through the bond in amperes.
ß = the Beta value for the worst case scenario at the
point of maximum exposure, where the accessible
pipeline is closest to the railroad (volts/volt)
Eo = the maximum open circuit potential measured
over a 24 hour cycle (volts)
Rpipe = the ratio of structure-to-soil potential to current
applied (volts/amp)
The maximum calculated current flowing through the
bond, from the pipe to the rail was 77 amps.
Bond Resistance Calculation
The bond resistance was determined using the
following equation:
Where:
Rb = the bond resistance (ohms)
Rpipe = the ratio of structure-to-soil potential to
current applied (volts/amp)
Rint = is the internal resistance of the circuit
The bond resistance is the maximum resistance
that can exist in the bond circuit in order to pass the
calculated maximum amount DC interference current.
The maximum calculated allowable resistance of the
bond was 0.038 ohms.
Bond Cable Size Determination
The equation used to determine the cable size was:
Where:
Rcb = cable resistance (ohms)
Rc = cable resistance per 1000 ft. ((ohm/ft)*10-3)
Lb = the distance between the test station and the
connection to rail (feet)
The cable size was selected based on the current
capacity of the cable, the distance between the rail
connection and water main connection and the bond
resistance. In this case, the minimum cable gauge is
3/0 AWG of stranded copper cable. The length of the
directional bore could not be precisely measured due
to the obstruction of the electric railroad and therefore
the exact required length of cable could not be
confirmed. The installation of larger cable (250 MCM)
was recommended to ensure the system drains an
adequate level of current back to the electric railroad.
DESIGN SPECIFICATIONS AND
CHARACTERISTICS
The final design included a reverse current switch
as well as corrosion coupon monitoring test stations.
Even though the 72-inch main has an existing reverse
current switch unit, a separate reverse current switch
for the 90-inch was recommended since the timing of
pick-up and discharge on the 72-inch main and the
90-inch main may not coincide with one another. The
switch will have a remote monitoring unit installed to
facilitate switch monitoring and control.
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RESOURCES
Authors: Rogelio de las Casas, Ron Turner, Kristi Roe; ENTRUST Solutions Group
1
NACE International CP Interference
2
NACE International CP Interference
3
NACE International RP0100-2004
Dynamic Stray Current Interference Testing And
Mitigation Design For A 90-Inch Water Main
Reverse Current Switch Theory
Reverse current switches are used to prevent DC
current from flowing in the wrong direction at DC
transit or other DC generating equipment stray current
mitigation bonds. The drainage switch equipment has
a solid-state circuit that operates an electrical DC
contactor. That circuit monitors the voltage differential
of the stray current mitigation bond between the
structure (water main) and the negative buss (electric
rail).
When the voltage differential of the structure with
respect to the negative buss reaches a positive 150
mV DC (current can move from the main to the soil),
the switch will close. This permits the current to flow
from the main to the rail through the bond cable
instead of draining from the main to soil and back to
the railroad. By limiting the amount of current allowed
to discharge from the surface of the pipeline, DC stray
current corrosion can be mitigated. When the voltage
differential between the main and the railroad reaches
zero mV DC, the switch will open.
FOLLOW-UP TESTING AND ANALYSIS
Once the stray current mitigation system is installed,
a new series of Beta curves should be obtained in
the same locations where the tests were done before
the installation. With this new series of Beta curves,
a graph with all the Vgo in function of the distance
from the connection point to the railroad can be done
again (see figure 4 above). If the Vgo values are more
negative than -1000 mV3, a variable resistor should
be introduced in series in the electric circuit between
the pipe and the railroad to increase the resistance of
this circuit and diminish the amount of current being
drainage, to keep the potential of the pipeline less
negative than -1000 mV.