Nace CP 1, Course Manual

Page: 434 / 535

RP0196-2004
4
NACE International
(k) Other pertinent information, including age and
history of the tank, and whether the tank is of welded,
bolted, or riveted construction.
4.2.2 In most cases, an on-site corrosion evaluation
and preliminary design survey is useful.
4.3 Design Considerations
4.3.1 Current Output
4.3.1.1 The galvanic anode CP system must be
capable of providing sufficient current to protect
the tank to the minimum criterion for the full design
life of the system, including any anticipated coating
deterioration.
4.3.1.2 Calculated current output should be based
on the current requirement for CP. This current
requirement is expressed in terms of current per
unit area of total submerged bare surface area,
and depends on the corrosiveness of the water. In
fresh waters, the current requirement is usually
between 5 and 27 mA/m
2
(0.5 and 2.5 mA/ft
2
) of
submerged bare surface area. Installations
involving high corrosion rates, nonpotable water,
and galvanic metal couples may require
considerably higher current densities.
4.3.1.3 The driving voltage used with the circuit
resistance to calculate the current output shall be
the difference between the open-circuit potential of
the anode and the minimum polarized potential of
the tank.
4.3.1.4 For circuit resistance calculations, refer to
Anode Resistance Fundamentals and
Applications—Classic Papers and Reviews.
4
4.3.1.5 Circuit resistance variation as a result of
temperature and water conductivity changes must
be considered. Maximum circuit resistance shall
be calculated based on the physical shape of the
galvanic anode when it is 85% consumed.
4.3.2 Galvanic Anode Selection
4.3.2.1 Galvanic anodes shall be manufactured
from magnesium (Mg) or zinc (Zn) alloys that are
formulated specifically for this use. Table 1 lists
typical galvanic anode materials and
characteristics.
4.3.2.2 Galvanic anode materials shall be chosen
to be compatible with the water composition and
intended use.
4.3.2.3 All galvanic anodes shall have a steel
core that extends throughout the full length of the
anode and is centered in the galvanic anode, and
on which the anode can be supported either
vertically or horizontally.
4.3.3 Galvanic Anode Arrangement
4.3.3.1 Galvanic anodes shall be arranged to
provide as close to uniform current density as is
practical. When uniform current density cannot be
achieved, the total system current output must be
increased to compensate by adding anodes or
adjusting the system current output through the
test box described in Paragraph 4.3.4.1.
4.3.3.2 Galvanic anodes shall be suspended or
mounted in such a way as to minimize damage to
the system when ice conditions are anticipated or
measures must be taken to prevent ice formation.
4.3.3.3 Galvanic anodes shall be installed and
supported to prevent damage during high flow
rates and when the tank is drained for inspection
and cleaning.
4.3.3.4 Note: Some coatings have been damaged
by excessive current density at coating holidays
and excessive potential gradients across the
coating. The coating selected shall be compatible
with CP. Coating failures have been observed at
polarized potentials more negative than -1,100 mV
vs. saturated copper/copper sulfate reference
electrode (CSE).
4.3.4 Installation, Monitoring, and Testing
4.3.4.1 One preferred method of installation is to
use galvanic anodes with lead wires that connect
to a collector (header) cable terminating in a test
box with a tank connection cable, a tank test lead,
a shunt, and a reference electrode lead. The use
of a calibrated shunt between the anode collector
cable and the tank connection cable facilitates
measuring the current output of the system. The
test box allows for interrupting and adjusting the
current and obtaining a polarized potential vs. the
reference electrode, free of IR drop. Although the
other reference electrodes are sometimes used,
CSE is preferred.
4.3.4.2 If the galvanic anodes are directly
connected to the tank, a coupon and reference
electrode shall be configured to evaluate the IR
drop and approximate the polarized potential of
the tank.
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4.3.5 Galvanic Anode Life
4.3.5.1 Galvanic anode life shall be calculated
using the nominal operating current density.
4.3.5.2 Galvanic anode current capacities used in
the system life calculations for any galvanic anode
shall not exceed the consumption rate values
given in Table 1.