Nace CP 1, Руководство по эксплуатации

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TM0497-2002
NACE International 5
5.6 Meter Polarity
5.6.1 Pipe-to-electrolyte potentials are usually
measured by connecting the instrument negative
terminal to the pipe and the positive terminal to the
reference electrode, which is in contact with the pipe
electrolyte.
With this connection the instrument
indicates that the reference electrode is positive with
respect to the pipe. Because the reference electrode
has a positive value with respect to the pipe, the pipe
voltage is negative with respect to the reference
electrode (see Figure 1a).
This negative pipe-to-
electrolyte potential is the value used for NACE criteria.
5.6.2 Pipe-to-electrolyte potential measurements are
sometimes
made with the reference electrode
connected to the instrument negative terminal and the
pipeline to the positive terminal. Figure 1b illustrates
this connection.
5.6.2.1 If the instrument is a data logging device,
the recorded data may be printed out with a
negative symbol unless a polarity reversal occurs.
5.7 The pipe-to-electrolyte potential measurement of a
buried pipe should be made with the reference electrode
placed close to the metal/electrolyte interface of the pipe.
The common practice, however, is to place the reference
electrode as close to the pipe as practicable, which is
usually at the surface of the earth above the centerline of
the pipe. (See Figure 1a.) This measurement includes a
combination of the voltage drops associated with the:
(a) Voltmeter;
(b) Test leads;
(c) Reference electrode;
(d) Electrolyte;
(e) Coating, if applied;
(f) Pipe; and
(g) Pipe metal/electrolyte interface.
5.8 The pipe-to-electrolyte potential measurement as
described above is a resultant of the:
(a) Voltage drop created by current flowing through the
electrical resistances of the items listed in Paragraph 5.7;
and
(b)
For coated pipe, the influence of coating holidays,
depending on their location, number, and size.
5.9 Pipe-to-electrolyte potential measurements made to
determine the level of cathodic protection at the test site
should consider the following:
(a) Effectiveness of coatings, particularly those known or
suspected to be deteriorated or damaged;
(b) Bare sections of pipe;
(c) Bonds to mitigate interference;
(d) Parallel coated pipelines, electrically connected and
polarized to different potentials;
(e) Shielding;
(f) Effects of other structures on the measurements;
(g) History of corrosion leaks and repairs;
(h) Location of impressed current anodes;
(i)
Unknown, inaccessible, or direct-connected galvanic
anodes;
(j)
Location of isolation devices, including high-resistance
pipe connections and compression couplings;
(k) Presence of electrolytes, such as unusual corrosives,
chemical spills, extreme soil resistivity changes, acidic
waters, and contamination from sewer spills;
(l)
Location of shorted or isolated casings;
(m) DC interference currents, such as HVDC, telluric,
welding equipment, foreign rectifier, mining equipment, and
electric railway or transit systems;
(n) Contacts with other metals or structures;
(o) Locations where the pipe enters and leaves the
electrolyte;
(p) Areas of construction activity during the pipeline
history;
(q) Underground metallic structures close to or crossing
the pipeline;
(r)
Valves and other appurtenances; and
(s) HVAC overhead power lines
.
5.10 Voltage drops other than those across the pipe
metal/electrolyte interface shall be considered for valid
interpretation of pipe-to-electrolyte voltage measurements
made to satisfy a criterion. Measurement errors should be
minimized to ensure reliable pipe-to-electrolyte potential
measurements.
5.11 The effect of voltage drops on a pipe-to-electrolyte
potential measurement can be determined by interrupting
all significant current sources and then making the
measurement.
This measurement is referred to as an
“instant-off” potential. The measurement must be made
without perceptible delay after current interruption to avoid
loss of polarization.
The voltage value measured is
considered to be the “polarized potential” of the pipe at that
location.
Because the current interruption may cause a
voltage spike, recording the spike as the “instant-off
potential” must be avoided. The magnitude and duration of
the voltage spike can vary; however, the duration is usually
within 0.5 second. The following are examples of when it
may not be practical to interrupt all current sources to make
the “instant-off potential” measurement.
5.11.1 Galvanic Anodes
5.11.1.1 Galvanic anodes connected directly to
the pipe without benefit of aboveground test
stations or connections.
Interruption requires
excavation of the connections.
5.11.2 Impressed Current Systems
5.11.2.1 Galvanic anodes directly connected to
piping protected using an impressed current
system;
5.11.2.2 Multiple impressed current sources;