Nace CP 1, Course Manual

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RP0169-2002
NACE International 19
7.3.3.7 Interference currents;
7.3.3.8 Deviation from construction specifications;
and
7.3.3.9 Other maintenance and operating data.
7.3.4 Field survey work prior to actual application of
cathodic protection is not always required if prior
experience or test data are available to estimate
current requirements, electrical resistivities of the
electrolyte, and other design factors.
7.4 Types of Cathodic Protection Systems
7.4.1 Galvanic Anode Systems
7.4.1.1 Galvanic anodes can be made of
materials such as alloys of magnesium, zinc, or
aluminum. The anodes are connected to the pipe,
either individually or in groups. Galvanic anodes
are limited in current output by the anode-to-pipe
driving voltage and the electrolyte resistivity.
7.4.2 Impressed Current Anode Systems
7.4.2.1 Impressed current anodes can be of
materials such as graphite, high-silicon cast iron,
lead-silver alloy, precious metals, or steel. They
are connected with an insulated cable, either
individually or in groups, to the positive terminal of
a direct-current (DC) source, such as a rectifier or
generator.
The pipeline is connected to the
negative terminal of the DC source.
7.5 Considerations influencing selection of the type of
cathodic protection system include the following:
7.5.1 Magnitude of protective current required;
7.5.2 Stray currents causing significant potential
fluctuations between the pipeline and earth that may
preclude the use of galvanic anodes;
7.5.3 Effects of cathodic protection interference
currents on adjacent structures that may limit the use of
impressed current cathodic protection systems;
7.5.4 Availability of electrical power;
7.5.5 Physical space available, proximity of foreign
structures, easement procurement, surface conditions,
presence of streets and buildings, river crossings, and
other construction and maintenance concerns.
7.5.6 Future development of the right-of-way area and
future extensions to the pipeline system;
7.5.7 Costs of installation, operation, and
maintenance; and
7.5.8 Electrical resistivity of the environment.
7.6 Factors Influencing Design of Cathodic Protection
Systems
7.6.1 Various anode materials have different rates of
deterioration when discharging a given current density
from the anode surface in a specific environment.
Therefore, for a given current output, the anode life
depends on the environment and anode material, as
well as the anode weight and the number of anodes in
the cathodic protection system.
Established anode
performance data may be used to calculate the
probable deterioration rate.
7.6.2 Data on the dimensions, depth, and
configuration of the anodes and the electrolyte
resistivity may be used to calculate the resultant
resistance
to electrolyte of the anode system.
Formulas and graphs relating to these factors are
available in the literature and from manufacturers.
7.6.3 Design of galvanic anode systems should
consider anode-to-pipe potential, electrolyte resisivity,
current output, and in special cases, anode lead-wire
resistance.
A separate design for each anode or
anode system may not be necessary.
7.6.4 Galvanic anode performance in most soils can
be
improved by using special backfill material.
Mixtures of gypsum, bentonite, and anhydrous sodium
sulfate are most commonly used.
7.6.5 The number of impressed current anodes
required
can be reduced and their useful life
lengthened by the use of special backfill around the
anodes. The most common materials are coal coke,
calcined petroleum coke, and natural or manufactured
graphite.
7.6.6 In the design of an extensive distributed-anode
impressed current system, the voltage and current
attenuation along the anode-connecting (header) cable
should be considered.
In such cases, the design
objective is to optimize anode system length, anode
spacing and size, and cable size in order to achieve
efficient external corrosion control at the extremities of
the protected structure.
7.6.7 When it is anticipated that entrapment of gas
generated by anodic reactions could impair the ability
of the impressed current groundbed to deliver the
required current, suitable provisions should be made
for venting the anodes. For the same current output of
the system, an increase in the surface area of the
special backfill material or an increase in the number of
anodes may reduce gas blockage.
7.6.8 When it is anticipated that electroosmotic effects
could impair the ability of the impressed current
groundbed to deliver the required current output,
suitable provisions should be made to ensure adequate
soil moisture around the anodes.
Increasing the
number of impressed current anodes or increasing the