Page 610 - Corrosion Engineering Principles and Practice

572 C h a p t e r 1 3 C a t h o d i c P r o t e c t i o n 573

very accurate way to make potential measurements. In reality, how-
ever, making measurements completely free of voltage drop error is
not always possible because some currents cannot be easily inter-
rupted. Uninterruptible current sources may include: sacrificial an-
odes directly bonded to the structure, foreign rectifiers, stray cur-
rents, telluric currents, and long-line cells [16]. Modern interrupters
are based on solid-state switches and are programmable to perform
switching only when the survey is performed during the daytime.
This feature minimizes the depolarization of the pipeline that may
occur gradually due to the cumulative effects of the “off” periods.
When several rectifiers protect a structure, it is necessary that all
rectifiers be interrupted at the exact same instant in order to obtain
meaningful measurements. Pipeline operators usually specify that at
least two rectifiers ahead of the survey team and two rectifiers behind
the survey team have to be interrupted in a fully synchronized
manner. The amount of time between current interruption and
depolarization can vary from a fraction of a second to several seconds,
depending upon details of the structure. In addition, capacitive spikes
that occur shortly after current is interrupted may mask the instant-
off potential. Measurements made with a recording voltmeter are
preferred as they can be subsequently analyzed to determine the real
instant-off potential [16].
An example of graphical CIPS data is presented in Fig. 13.36 [15].
In the simplest format, the “on” and “off” potentials are plotted as
a function of distance. The usual sign convention is for potentials to
be plotted as positive values. The difference between the “on” and
“off” potential values should be noted. As is usually the case, the “off”
potentials are less negative than the “on” values. When the relative
position of these two lines is reversed, it indicates that some unusual
conditions such as stray current interference may be at play.
13.9.2 Pearson Survey
Pearson surveys, named after its inventor, are used to locate coating
defects in buried pipelines. Once these defects have been identified,
the protection levels provided by the CP system can be investigated
at these critical locations in more detail.
During a Pearson survey, an AC signal of approximately 1000 Hz
is imposed on the pipeline by means of a transmitter (Fig. 13.37),
which is connected to the pipeline and an earth spike as illustrated in
Fig. 13.38. Two survey operators make earth contact either through
aluminum poles or metal studded boots (Fig. 13.39). A distance of
several meters typically separates the operators. Essentially, the signal
measured by the receiver provides a measure of the potential gradient
over the distance between the two operators. Defects are located by a
change in the potential gradient, which translates into a change in
signal intensity.