Nace CP 1, Course Manual

Page: 314 / 535

RP0285-2002
10 NACE International
6.3.2.2 Nonwelded joints may not be electrically
continuous.
Electrical continuity between all
components of the protected system must be
verified.
6.3.3 Anode Systems
6.3.2.3 CP systems shall be designed to mitigate
any adverse effects from stray current on foreign
metallic structures within the influence of the CP
system.
6.3.3.1 A variety of materials such as (a) high-
silicon cast iron, (b) graphite, (c) mixed-metal-
oxide-coated titanium, and (d) platinum-coated
titanium or niobium are used for impressed current
anodes. These anodes are normally installed with
low-resistivity carbonaceous backfill.
6.3.3.2 Anode lead wires shall be constructed
with insulation that meets the mechanical and
chemical
resistance requirements of the
environment. Impressed current anodes shall be
connected either singularly or in groups to the
positive terminal of a DC source. The protected
system components shall be connected to the
negative terminal with insulated cable.
6.3.3.3 Cables between anodes, rectifiers, and
negative returns from the structures to the rectifier
require special insulation.
It is good practice to
install cables in plastic conduit. If installed in soil,
the
following cable insulation qualities are
required:
(a) abrasion resistance;
(b) low moisture absorption; and
(c) resistance to tank product spills.
6.3.3.4 The life of impressed current anodes can
be
extended by the use of low-resistivity,
carbonaceous, conductive backfill around the
anodes.
The most common of these backfill
materials are metallurgical coke breeze made from
coal, and calcined petroleum coke. Low-resistivity,
carbonaceous, conductive backfill also reduces
the anode-to-earth resistance.
6.3.3.5 Anode groundbed configurations may be
vertical, horizontal, or angle drilled. The selection
of
anode configuration is dependent on
environmental factors, current requirements,
current distribution, and the size and type of
structure to be protected.
Caution should be
exercised to ensure that anode placement results
in uniform distribution of protection current to the
protected system surfaces.
6.3.3.6 The current requirement for achieving a
given protection criterion can be determined by
preliminary testing on existing structures through
the use of temporary or simulated CP systems.
The current requirement can be estimated by
calculating surface areas and applying a minimum
protective current density based on experience
and sound engineering judgment.
6.3.3.7 Although there are many sources of DC
for impressed current CP, rectifiers are most
commonly used. Various types of rectifiers such
as (a) fixed voltage; (b) constant current; (c)
automatic potential control; and (d) combinations
of the above are available.
Separate terminal boxes with the DC power supply
that can accommodate multiple circuit outputs that
can be varied to individual circuits or anodes are
available. These come equipped with shunts so
that individual anode current outputs can be
monitored.
6.3.3.8 All impressed current systems shall be
designed with safety consideration as a priority.
Caution should be exercised to ensure that all
cabling is protected from physical damage and
from the possibility of arcing.
When required,
rectifiers and junction boxes shall be explosion-
proof.
6.4 Test Stations
6.4.1 Test stations for potential and current
measurements should be considered for each system
at sufficient locations to facilitate CP testing.
6.4.2 Test stations have a number of different
configurations, including the following:
6.4.2.1 The test station can be cast iron or
impact-resistant plastic but shall be set at grade in
a manner to ensure its long-term durability. The
test station may contain a terminal block. Wires
should be color coded or otherwise permanently
identified.
Wire shall be installed with slack.
Damage to insulation shall be avoided, and proper
repairs must be made if damage occurs.
6.4.2.2 A test station can consist of a test lead
continuous with the structure surface, secured by
a nonconductive strap to a fixture, and accessible
in the manhole opening.
6.4.2.3 If a portable reference electrode is used
for monitoring, then an area of clean, unshielded
backfill or soil should be made accessible in the
manhole area for electrode placement.
6.4.3 Provisions should be made to monitor potentials
at the bottom of tanks. Such facilities can include:
(a) permanent reference electrodes, and
(b) portable reference electrodes inserted in access
tubes.