Nace CP 1, Руководство по эксплуатации

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TM0497-2002
12 NACE International
9.4 Comparison with Other Methods
9.4.1 Advantages
(a) Voltage drops associated with the protective
currents being interrupted are eliminated.
9.4.2 Disadvantages
(a) Additional equipment is required;
(b) Additional time, personnel, and vehicles may be
required to set up equipment and to make pipe-to-
electrolyte potential measurements; and
(c)
Test results are difficult or impossible to analyze
when stray currents are present or direct-connected
galvanic anodes or foreign impressed current devices
are present and cannot be interrupted.
9.5 Basic Test Equipment
9.5.1 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 in which the
circuit resistance is low. A potentiometer circuit may be
necessary in other instances.
9.5.2 Two color-coded meter leads with clips for
connection to the pipeline and reference electrode.
9.5.3 Sufficient current interrupters to interrupt
influential cathodic protection current sources
simultaneously.
9.5.4 Reference electrode
9.5.4.1 CSE.
9.5.4.2 Other standard reference electrodes may
be substituted for the CSE.
These reference
electrodes are described in Appendix A,
Paragraph A2.
9.6 Procedure
9.6.1 Before the test, verify that cathodic protection
equipment
has been installed and is operating
properly. Time should be allowed for the pipeline
potentials to reach polarized values.
9.6.2 Install and place in operation necessary
interrupter equipment in all significant DC sources
protecting the pipe at the test site, and place in
operation with a synchronized and/or known “off” and
“on” cycle. The “off” cycle should be kept as short as
possible but still long enough to read a polarized pipe-
to-electrolyte potential after any “spike” as shown in
Figure 3a has collapsed.
9.6.3 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 potential;
(c) Electrical midpoints between protection devices;
(d) Known location of an ineffective coating when
the pipeline is coated; and
(e) Location of a known or suspected corrosive
environment.
9.6.4 Make electrical contact between the reference
electrode and the electrolyte at the test site, directly
over the centerline of the pipeline or as close to it as is
practicable.
9.6.5 Connect voltmeter to the pipeline and reference
electrode as described in Paragraph 5.6.
9.6.5.1 If spiking may be present, use an
appropriate instrument, such as an oscilloscope or
high-speed recording device, to verify that the
measured values are not influenced by a voltage
spike.
9.6.6 Record the pipe-to-electrolyte “on” and “off”
potentials and their polarities with respect to the
reference electrode.
9.7 Evaluation of Data
Cathodic protection shall be judged adequate at the test site
if the polarized pipe-to-electrolyte potential is negative 850
mV, or more negative, with respect to a CSE.
9.8 Monitoring
When the polarized pipe-to-electrolyte potential has been
determined to equal or exceed a negative 850 mV, the
pipeline “on” potential may be used for monitoring unless
significant environmental, structural, coating integrity, or
cathodic protection system parameters have changed.