Pipeline Protection
with Clamp-On
DC & AC Ammeters
Contents:
Introduction: Why Measuring Current Is Important
Corrosion generally occurs where current leaves a line into an electrolyte, so we
need to know the direction of current flowing in a line. The amount of metal lost
by a line is proportional to the magnitude of the current leaving the line into
soil or sea water. As Dr. John Leeds has said, "Industry would do well to shift
to methods that read Cathodic Protection (CP) current amperage and direction..."
[Pipeline & Gas Industry - April 1997 - page 55.]
Real ammeters -- both AC and DC -- are now available in non-contact or clamp-on
form. These Sea Clips® and Sea Clamps™ measure the actual CP current in a line at
the spot where you put them. In one form or another, they measure 5 milliamperes
to 500 amperes in conductors from ¼ inch to 5 feet in diameter. Resolution of 1 milliampere DC (0.1 mA AC) can be provided from ¾" to 30" diameter with sensitivity
accuracy typically 1% of reading. Adjacent conductor interference and zero offset
error due to residual local magnetism are reduced by new technology.
Five figures are presented to illustrate the use of these ammeters. Specific data
refers to the ammeters known best: DC and AC Amp Clips and Clamps manufactured by
the William H. Swain Co. under the marks Sea Clip, Sea Clamp, and MER™ -- the new
Magnetic Error Reduction form for improving accuracy.
Real CP Current Measured in situ, Quickly and Accurately
Fig. 1A illustrates a method for finding the resistance of an insulated flange by
accurately measuring real flange current at the flange using the normal CP system
with an interrupter. "...faults consuming large amounts of current are top candidates
for repair". [Pipeline & Gas Industry - April 1997 - page
57.] The Sea Clamp is used to evaluate the seriousness of a fault, and
to set repair priority.
Fig. 2 shows how the actual anode current during normal use is accurately measured
at an offshore tower. Assessing anode life expectancy, and integrity of mount are
but two of the reasons for this work.
In Fig. 1 and Fig. 2 the current measured is that from the operator's rectifier
or anode, through his pipe to a specific access, preserving influences from foreign
lines, shorts, excess resistance, etc., intact.
Fig. 3A and 3B illustrate the use of a portable current generator to locate a pipe
to rebar short in a crowded sector where line tracers don't work.
Fig. 4 shows how to detect, evaluate, and find a fault on an adjacent foreign line
which causes a sharp increase in the corrosive action of current leaving our line.
Fig. 5 is a common corridor illustration of how to detect, evaluate, and find a
fault. Both AC and DC are considered. Both gas line and URD line troubles are evaluated.
Fig. 1A: Resistance of Flange
Fig. 1A illustrates how the resistance RF
of a leaky insulating flange in a pipeline can be measured. The change in flange
voltage VF is divided
by the change in flange current iF.
Accuracy is enhanced by interrupting the CP rectifier if there is residual system
voltage, and/or local line magnetism which causes a zero offset in the indicated
line current.
The interrupter on the CP rectifier produces the necessary change in VF and iF.
If these are too small, a portable interrupted current supply can be connected closer
to the clamp.
Polarity
The indicator in Fig. 1 reads -.7 A because the current iF is shown flowing through the aperture of the Clamp
and out on the bridle side. This is the reverse of our direction for positive current.
The ammeter reads positive when conventional current flows from left to right through
the aperture of the sensor when the cable to the indicator is up. If the sensor
is a clamp, the bridle is on the left when the indicator cable is up.
Fig. 1B: Interrupter Used to Change Flange Voltage & Current
Fig. 1B illustrates the changes in voltage and current in Fig. 1A over time. When
the interrupter is in the OFF state for 30 seconds, the rectifier's current output
is zero, but the voltage VF
and current IF are likely
to have residual magnitudes; for example; -0.4 V & -0.3 A. This can be due to
polarization potentials, interference current, zero offset, etc. We only require
that they be reasonably constant for about 60 sec.
When the interrupter goes to the ON state for 15 seconds, the rectifier's output
may be -10 Amp. In this example, the flange is some distance from the rectifier
so the flange voltage VF
changes by -1.4 V and the flange current iF
changes by -0.7 A. Then in this example the flange resistance RF is:
Fig. 2: Sacrificial Anode Current
In Fig. 2 a single Sea Clip is used to measure the anode current flowing in each
4" standoff secured to an offshore tower. In this example, the total output current
is 2 A + .5 A = 2.5 A.
The current in the lower standoff is much less than the upper because of poor electrical
contact to the tower leg.
Tests such as these are used to verify electrical contact of retrofit electrodes,
and to estimate the operational life of similar anode installations. The indicator
may be on the surface with up to 700 feet of cable to the clip. Or the indicator
may be in the ROV or diver's bell so a separate long cable is not required.
Longevity in Sea Water
Sea Clip and Sea Clamp sensors are intrinsically stable and reliable, whether in
air or under water. To demonstrate stability in water, we
tested a bare 5" sensor
having no epoxy coating and no special waterproofing -- first in air, and then immersed
in about a foot of natural sea water. It
was stable in the air and also under water
for over a year.
Buried Sea Clamp
To sense instant defects and to measure their severity, CP current may be continually
measured for many months. Sea Clamps are now considered suited for long term burial
on a pipe. This will also permit measuring gradual coating degradation.
More Stable
Sea Clamps are generally more stable when subject to mechanical and electrical stress
than Hall devices because the sensing action is distributed over practically the
whole core. Sensors for DC and AC Amp Clips resemble the familiar current transformer.
There is a fine steel core, split into two halves, with a sense winding wound around
the core. This forms a distributed sensor, because the magnetic field set up by
the current flowing through the aperture acts on the whole circumference of the
core.
Fig. 3A: Finding a Short to a Pipeline
A systematic method for finding a short to a pipeline is outlined in Fig. 3A and
3B. We are told that this direct current method works in a congested urban environment
where alternating current fault locators are ineffective.
Fig. 3A
At Pipeline access #1, a single Sea Clamp and a single indicator measure the interrupted
current from a portable current source, first to the right (-2.5 A) of the current
drive point on the line, and then to the left (+0.4 A). The major current entering
the drive point from the right indicates that the short is to the right.
Polarity
Current flowing into the drive point from the left reads positive (+) because it
goes through the aperture of the sensor from left to right when the cable from the
sensor to the indicator is up, as shown. Current flowing into the drive point from
the right is indicated as negative (-) because it is flowing in the reverse direction
from our convention for positive.
Fig. 3B
Since the major current (-2.5 A) in Fig. 3A showed that the ground fault
must be
to the right of access #1, the search is continued using access #2 which is well
to the right of #1.
Fig. 3B
The same Sea Clamp and indicator and portable current source are set up and used
at access #2 as shown in Fig. 3B. Here the major current (+ 2.6 A) is to the left
of the drive point. It
appears that the short is between access #1 and #2.
Search is made at other accesses closer together, between 1 and 2. The process is
repeated until the short is found.
Or experience is
used. Since it is known that the pipe goes through the wall of
a building having grounded rebar, a search is made at the wall as soon as it is
seen that the major currents bracket the wall.
Fig. 4: Interference
Earth current leaving our pipeline is 0.9 - 0.4 = 0.5 Amp.
["Their pipeline" here means any foreign pipeline, or another pipeline
owned by us which is not now the center of our attention.]
Fig. 4 illustrates interference to our pipeline when an adjacent foreign line draws
much more than normal earth current from its anode bed. This can be due to a pipe
to casing short, or some serious flaw in their coating.
The earth current fans out, and some enters our pipe where it has a few coating
defects. The damage is done where this current leaves our pipe to go to the shorted
casing.
The severity of a flaw may be judged by measuring pipeline current at several access
points. The Sea Clip is used to find high concentrations of current loss along our
line.
The current loss over a pipeline span between two access points is the difference
between the current measurements at each one. In this example, a single Sea Clamp
and indicator are used to find the 0.5 Amp (.9 A - .4 A)* leaving our line. The
weight of the steel leaving our line each year is directly proportional to the current
leaving the line into the soil**. Ten pounds loss in a year is serious if it all
occurs in a small length on a small pipe. However, if our line is large and the
0.9 A and 0.4 A sectors are widely separated, the 0.5 A current leaving our line
may be widely distributed and hence less important. If not very much steel is lost
in any one place, the pitting may not be deep enough to be serious.
* Direction of current flow i.e., polarity, is important. An unexpected fault could
cause a reading to be (-) 0.6 A. Then the loss in the +.9 A to -.6 A span is 1.5
A.
** Twenty pounds of steel lost due to 1 Amp leaving the line for one year is a widely
accepted figure.
However, if a relatively short span has most of the current loss (for example, 0.8
A - 0.5 A = 0.3 A loss), then our line may be in danger.
Fig. 5: Short to Gas Line in Common Corridor
Fig. 5 illustrates how a Sea Clip or Sea Clamp can be used to discover that there
is a sudden defect, to locate the short, and also to evaluate the damage that
is being done.
In this example, the gas main in the center of the common corridor is shorted to
a conductor making good earth contact. The short may be a new pipe in a bored hole
that nicked the gas line, a metal tray, or a bridge support. With the short, the
URD and water pipes act as channels for anode current of the gas pipe.
The corrosive effect of current lost by the URD line will likely increase the resistance
of the return conductor at this position. If allowed to continue, this will cause
alternating current to flow in the soil and gas line. This trouble can regenerate,
with AC and DC corrosion becoming ever more serious. An AC indicator will enable
the Sea Clip to measure AC interference in the gas line.
The table below summarizes the URD and water line currents lost to earth. These
are corrosive. The negative loss currents to the gas line are likely not corrosive.
|
Summary of current lost to earth from each of the 3 lines |
|
|
URD |
Water |
Gas |
|
Current loss between rectifier and short |
1.1 A |
1.5 A |
-2.6 A |
|
+ Current loss on far side of short |
0.2 A |
0.2 A |
-0.4 A |
|
= Total current leaving the line to the earth |
1.3 A |
1.7 A |
-3.0 A |
In Fig. 5 a bare URD coaxial power cable and a water pipe are on either side of
a gas line -- all buried in a common corridor between accesses. Normally this is
made to work fine, primarily because although under CP, the gas line is well coated.
Typically, very little rectifier current from the anode bed enters or leaves the
URD or water lines because the gas pipe is well insulated by its coating.
However, everything changes for the worse when a bare pipe in a bored hole nicks
the gas line, cutting through its coating and electrically connecting the gas line
to the bare pipe. An instant defect such as this can suddenly endanger lines. A
current sensor can be installed permanently and connected to sound an alarm if a
defect occurs.
In Fig. 5, the defect is most apparent as a large increase in rectifier output,
from perhaps 0.5 A to 3 A. At least three problems are created:
- The gas line does not have enough voltage drive from the
rectifier for effective CP, especially beyond the short. The gas line may start
to corrode further on down.
- Anode current is leaving the bare outer conductor of the
URD power line in the span adjacent to the short. This will eventually strip out
a lot of return conductor and cause power current to flow in the earth. This can
be a hazard to persons or property nearby.
- In like manner, current is leaving the bare water pipe
in the span near the short. This can even endanger the water main if it persists.
A single Sea Clamp and indicator can be used at several access points as shown in
Fig. 5 to locate the fault and stop corrosion.
Bond Current
One or more bond cables may be added to fig 4 so that most of the earth current
flowing between our pipeline and theirs is rerouted through a copper path. This
bond current should
be measured because as Dr. Leeds reminds us, significant bond
current can introduce an IR drop error in measuring pipe-to-soil potential.
A Sea Clip may be used to measure the current magnitude and direction of flow in
each bond -safely, quickly and accurately. Lifting the bond or adding a series shunt
resistance is not necessary. This is important, because adding resistance changes
the current.
The 5 inch clip used in Fig. 2 will do for measuring current in higher current bonds.
A ¾" or 2 ½" Sea Clip is better because the zero offset error is less important
when measuring a current less than 1 Amp.
Alternating Current
Short Location
One of our lines in Fig. 5 may make intermittent contact (the "short" in Fig. 5)
with a foreign pipe. This could be from a bored hole containing a new water pipe,
etc. When heavy trucks go over the contact, the pipe and our line can be joined
and then separated, causing a
considerable change in the AC in our line. This can
also cause an arc, and eventually burn a pit or even a hole into our line or the
pipe.
To find the short, the AC in the gas line is measured at several accesses. This
can be done accurately using the same Sea Clamp if a suitably calibrated AC indicator
is inserted instead of the DC indicator shown. Resolution of 0.1 mA AC is available
up to 13" diameter, and 1 mA up to 82" diameter.
AC Interference
A single AC indicator and Sea Clamp can be used to find 60 Hz AC flowing in a common
corridor such as that illustrated in Fig. 5. Here a gas line and a water main are
adjacent to a URD line having a locally corroded or damaged outer braid conductor.
Personal safety may prompt a search, or potential corrosion. It is heard that 1%
to 3% of AC may act as DC. Thus 10 A of AC may have the corroding effect of 100 mA of DC.
Conclusion
Clamp-on DC and AC ammeters are used by corrosion engineers and others to accurately
measure actual pipe and cable current -- on the line -- during normal operations.
This saves time and trouble.
