Line Current Monitor
Mr. Joe Maxwell has written on how to
find electrical contacts on gas pipelines
[Materials Performance, April 1999, page 33]. My
emphasis here is related, but somewhat different.
This paper relates to knowing of the contact,
when contact occurred, and a lot more.
Inferred CP potential can now be monitored over a
considerable length of line by measurement of actual
line current. This is best done with two or more
well spaced Sea Clamps™ permanently installed and
buried with the line. An illustration is in Fig. 1.
(Hommema, one of our customers, has
permanently installed
Sea Clamps six feet below ground to measure
interference current flowing in a gas pipeline.)
Fig. 1
Subject Pipeline buried with Sea Clamps set to
measure normal CP current
A suitable Line current monitor is sensitive to a
definite contact with a foreign line. Moreover, it
shows interference current from a foreign line
through soil or water, as well as ground bed current
from a newly activated foreign rectifier. Even a
flange fault on subject line, or a broken cable to a
sacrificial anode will show up in a line current
monitor. Moreover, with 1 milliampere resolution,
there is no need to wait for a complete failure.
Early warning of incipient failure can be available.
Fig. 1 illustrates subject buried pipeline with 3
well spaced clamps permanently installed. The CP
potentials were within acceptable limits when the
above ground indicators I1, I2,
and I3 showed 1 Amp, .2 A,
and .04 A respectively. This is called the "normal"
state.
Currents are indicated as positive because the
direction of current flow is with the polarity arrow
marked on the clamp.
Fig. 2
Subject pipeline with CP current faults F1 & F3
In the illustration Fig. 2, an adjacent foreign
pipeline (not in definite contact) experiences a
line to bare casing short (F1) at about the time the
foreign rectifier is switched on. New foreign anode
bed (F3) is only 100 ft. from subject line which has
adjacent coating defects, so ˝ Amp or so can enter
subject line at F3, and leave near the casing F1.
This fault condition changes the reading of
indicator I1, from + 1 A to +
1.5 A. It represents a big change in CP current in
the 1 mile line sector covered by monitor I1.
Inserting an AC indicator on clamp I1
will likely show a positive going rectified 60 Hz AC
waveform, or perhaps positive going pulses from a
wide throw rectifier. This is a tip off pointing to
an anode bed.
Since a substantial upset of CP potential is
evident, the user will likely soon look the area
over with a view to getting the faults cleared, or
compensating for their presence.
Fig. 3
Subject Pipeline with CP Current fault F2
Fig. 3 illustrates a definite contact fault (F2)
of the sort located by Mr. Joe Maxwell. The CP
current monitor notifies the user that indicator I1
measures double normal current because the bare
water pipe is soaking up most of the available
current.
If an AC indicator and ‘scope are connected to
clamp 1, it will likely show a strong power line
current at 60 Hz (in USA) because water pipe is
often AC ground. This is a hint for the search
party.
Fig. 4
Subject Pipeline with CP Current fault F4
Fig. 4 illustrates and insulating flange faulted
short. The subject rectifier off to the left now
pulls an extra 0.1 Amp from the zinc anode protected
line sector. This increases I1
from 1.0 to 1.1 Amp; I2 from
0.2 to 0.3 Amp; and I3
from
.04 to .14 Amp.
Search is aided by seeing that the .1 A increase
occurs on both sides of the flange. Moreover, I3
now shows positive going AC -60 Hz rectifier
waveform when an AC indicator and oscilloscope are
used. Previously, in the normal state of Fig. 1, I3
was nearly pure DC plus ground noise, which is
characteristic of zinc anodes in a right of way.
Fig. 5
Subject Pipeline with CP Current fault F5
Fig. 5 illustrates a cut anode cable and its
effect on line current. Indicator I3
displays reversed polarity at double magnitude. This
is understandable when we note that in the normal
state, I3 showed near zero
current because the zinc anodes on the isolated line
sector pretty well shared the load. On the other
hand, with the anode cable cut at F5, all current
must go to the right hand anode.
Fault location is aided when we see a change in
only I3. The problem should
be to the right of the insulated flange.
Which anode lead is cut is signaled by the
polarity of I3. More current
is being drawn to the right, so the right hand zinc
appears to be working. Check the left zinc.
AC scope readings will show slightly noisy DC.
The problem is likely associated with sacrificial
anodes.
Conclusion:
Buried Sea Clamps are reliable. They have been
tested for at least 2 years either buried on a 30
inch pipeline or in a salt water container. We are
pleased to find less than 2% gain loss. The change
in zero offset is less than half the He
specification.
A line current monitor can be built to show that
CP potentials are likely within tolerance, or not.
If a pipeline fault occurs, this will also show when
it happened, about where it is, and its nature. |
