Nace CP 1, Course Manual

Page: 307 / 535

RP0285-2002
NACE International 3
3.2 Factory-Fabricated CP Systems
3.2.1 Single- and Double-Wall Tanks
3.2.1.1 Factory-fabricated galvanic anode CP
systems are available for new USTs. The design
and
specifications for the factory-fabricated
galvanic anode CP systems consider three
important factors:
(a) the galvanic anodes;
(b) a dielectric coating; and
(c) electrical isolation.
The components are designed together as a
system to achieve corrosion protection for various
tank sizes and most site conditions. Bonding of
this system to other structures may violate the
manufacturer’s warranty.
3.2.2 Anodes
3.2.2.1 Packaged zinc or magnesium anodes are
used for factory-fabricated systems.
Aluminum
anodes usually are not effective in underground
applications.
The size and number of anodes
required to protect a tank from corrosion is
predetermined by calculations based on the
desired design life, the tank surface area, the
quality of the coating, anode type, anode size, and
the range of soil resistivity in which it is anticipated
that the tank will be installed.
3.2.2.2 The anodes can be attached directly to
the tank or wired through a test station to the tank.
The type of anode used is determined by site
conditions and operational factors.
Zinc is the
most common type of anode used for factory-
fabricated systems. Magnesium anodes are more
effective in high soil resistivities and shall be used
on tanks where the anode temperature exceeds
49
°
C (120
°
F).
3.3 Piping for New USTs
3.3.1 Corrosion protection of all underground piping
associated with the UST system can be achieved
through a combination of material selection, system
design, and coatings.
3.3.2 Piping within a secondary containment piping
system may require other corrosion control methods in
lieu of CP.
3.3.3 Several design parameters to be considered in
selecting materials include:
(a) compatibility with the environment;
(b) compatibility with the product to be contained;
and
(c)
pressure and temperature.
3.3.4 Metallic Piping
3.3.4.1 Metallic piping in contact with an
electrolyte shall be protected from external
corrosion through the application of coatings and
CP.
3.3.4.2 When available, factory-applied coatings
are preferred. The following NACE International
standards may be helpful:
(a) RP0190;
1
(b) RP0185;
2
(c) RP0375;
3
and
(d) MR0274.
4
3.3.4.3 All metallic components to be cathodically
protected should be made electrically continuous.
When using galvanic anode CP, all metallic
components to be protected should be electrically
isolated from all other metallic components.
3.3.4.4 Metal piping with mechanical joints may
require bonding to ensure electrical continuity.
3.3.4.5 Recommendations for CP monitoring,
including facilities and configurations, are given in
Section 8.
3.3.4.6 Metallic secondary containment systems
exposed to an electrolyte should be cathodically
protected.
3.3.4.7 CP systems shall be designed to mitigate
any adverse effects from stray current on foreign
metallic structures within the influence of the CP
system.
3.3.5 Nonmetallic Piping
3.3.5.1 Nonmetallic piping cannot be protected by
CP; however, any metallic components of the
product piping system that are exposed to soil
shall be protected.
3.3.5.2 Materials selection should assure product
compatibility not only with the basic pipe material,
but also with any adhesives or joint compounds.
3.3.6 Flexible Connectors
3.3.6.1 Flexible connectors are used on rigid
piping systems to accommodate pipe movement.
These connections, depending on materials of
construction, may create dissimilar metal couples
in buried applications.
3.3.6.2 Flexible connectors may not provide
electrical continuity. Verification and bonding may
be required based on the system design.