Nace CP 1, Course Manual

Page: 465 / 535

RP0200-2000
8 NACE International
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Appendix B: Monitoring Techniques
1. Potential Survey
Purpose
Potential surveys of pipelines and casings are made to
monitor cathodic protection and are the initial tests
conducted to identify shorted casings. The presence of a
metallic contact may also be evaluated by measuring the
pipe-to-electrolyte and casing-to-electrolyte potentials
under the conditions shown below and comparing these
values to a known shorted condition established at the
opposite end of the casing (using test leads and vent
pipes).
Procedure
Potential measurements (surveys) of pipelines and
casings are made using a voltmeter and a reference
electrode (usually a copper-copper sulfate electrode
[CSE]).
More definitive results are obtained if the cathodic
protection current source is cycled on/off while the pipe
and casing potentials are recorded.
1. Measuring pipeline potential: One lead of the
voltmeter shall be connected to the pipeline by way of the
test lead or probe bar. The other lead of the voltmeter
shall be connected to the reference electrode, which is
placed on the ground directly over the pipeline and near
the end of the casing (see Figure B1).
2. Measuring casing potential: One lead of the
voltmeter shall be connected to the casing by way of the
vent, test lead, or probe bar. The other lead of the
voltmeter shall be connected to the reference electrode,
which shall be placed at the same location as where the
pipeline potential was taken (see Figure B1).
V OLT
METER
CARRIER PIPE
CASING
GROUND LEVEL
REFERENCE
CELL
VENT PIPE VENT PIPE
- -
+
Downstream or Downstation
(D/S) End Upstream or Upstation
(U/S) End
TEST
LEA D
TEST
LEA D
FIGURE B1: Potential Survey Measurement
Note: The electrode shall be placed near the end of the casing directly over the carrier pipe. The reference electrode shall
not be placed directly over the casing. The location of the end of the casing can usually be verified with a conductive pipe
locator.
Analysis
A “clear” or not shorted casing is indicated by a potential
difference between the casing and the carrier pipe. For
example, a pipe-to-soil potential of 1.6 V CSE and a
casing potential of 0.65 V CSE has a potential difference
of 0.95 volts and would indicate the casing is clear.
A shorted casing may exist if a small potential difference
exists between the pipeline potential and the casing
potential. This is typically less than 100 mV. Additional
testing should be conducted if the difference in potential
is 100 mV or less.