Nace CP 1, Руководство по эксплуатации

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TM0497-2002
NACE International 27
quantified by interrupting cathodic protection
sources while the coupon is disconnected and
noting whether there is a shift in the coupon-to-
electrolyte potential.
C3.2.3 A coupon has an insulated test lead
brought
above ground and, during normal
operations, connected to a pipeline test lead. The
coupon receives cathodic protection current and
represents the pipeline at the test site. For testing
purposes, this connection is opened, and the
polarized “off” potential of the coupon is measured.
The time the connection is open to measure the
coupon’s “off” potential should be minimized to
avoid significant depolarization of the coupon. The
“off” period is typically less than 3 seconds. The
coupon is then allowed to depolarize. When
possible, coupon current direction and magnitude
should be verified, using a current clip gauge or
resistor permanently placed in series with the
coupon lead. Measurements showing discharge
of current from the coupon should be reason to
question the validity of using a coupon at the test
site.
C3.3 Comparison with Other Methods
C3.3.1 Advantages
(a) Can measure coupon-to-electrolyte polari-
zation decay with a minimum of specialized
equipment, personnel, and vehicles;
(b)
Can provide an indication of the amount of
polarization present at the test site without
interrupting
the cathodic protection current
supplied to the pipeline;
(c)
Can provide a better indication of cathodic
protection levels due to eliminating the effects of
“long-line” current flow when the pipeline “off”
potentials are measured.
C3.3.2 Disadvantage—Can have high initial costs
to
install a coupon, especially for existing
pipelines.
C3.4 Basic Test Equipment
C3.4.1 Voltmeter with adequate input impedance.
Commonly used digital instruments have a
nominal impedance of 10 megaohms. An analog
instrument with an internal resistance of 100,000
ohms per volt may be adequate in certain
circumstances where the circuit resistance is low.
A potentiometer circuit may be necessary in other
instances.
C3.4.2 Two color-coded meter leads with clips for
connection to the coupon and reference electrode.
C3.4.3 Reference electrode
C3.4.3.1 CSE.
C3.4.3.2 Other standard reference
electrodes may be substituted for the CSE.
These reference electrodes are described in
Appendix A, Paragraph A2.
C3.5 Procedure
C3.5.1 Before the test, verify that:
(a) Cathodic protection equipment is installed
(b) and operating properly; and
(c) Coupon is in place and connected to a
pipeline test lead.
Time shall be allowed for the pipeline and coupon
potentials to reach polarized values.
C3.5.2 Determine the location of the site to be
tested. Selection of a site may be based on:
(a) Location accessible for future monitoring;
(b) Other protection systems, structures, and
anodes that may influence the pipe-to-electrolyte
and coupon-to-electrolyte potentials;
(c)
Electrical midpoints between protection
devices;
(d)
Known location of an ineffective coating
when the line is coated; and
(e)
Location of a known or suspected corrosive
environment.
C3.5.3 Make electrical contact between the
reference electrode and the electrolyte at the test
site as close to the coupon as is practicable.
C3.5.3.1 Identify the location of the
electrode to allow it to be returned to the
same location for subsequent tests.
C3.5.4 Connect the voltmeter to the coupon test
lead and reference electrode as described in
Paragraph 5.6.
C3.5.5 Measure and record the pipeline and
coupon “on” potentials.
C3.5.6 Disconnect the coupon test lead from the
pipeline test lead and immediately measure the
coupon-to-electrolyte potential.
C3.5.6.1 The coupon-to-electrolyte
potential becomes the “base line value” from
which polarization decay is measured.
C3.5.7 Record the coupon-to-electrolyte “off”
potential and its polarity with respect to the
reference electrode.
C3.5.8 Leave the coupon test lead disconnected
to allow the coupon to depolarize.