Nace CP 1, Course Manual

Page: 308 / 535

RP0285-2002
4 NACE International
3.3.6.3 All metallic components of the connection
in contact with the electrolyte shall be cathodically
protected.
(Protection of flexible connectors is
often overlooked in nonmetallic piping systems.)
3.3.6.4 Factory-fabricated CP systems are
available for flexible connectors.
3.3.7 All corrosion protection systems should be
monitored in accordance with Section 8 on Operation
and Maintenance.
3.4 Coatings
3.4.1 A high-quality dielectric coating should be
applied to a properly prepared surface of the exterior
areas of the UST including anode connections,
attachments, and lifting lugs. Crevice or corner areas
that restrict coating coverage should be seal welded
prior to coating.
3.4.2 Any type of coating used on a steel tank must
have high dielectric properties.
The purpose of a
dielectric coating is to isolate the tanks electrically from
the environment while reducing demands on the CP
system.
Other properties necessary in a dielectric
coating are resistance to environmental fluids and the
product being stored, impact/abrasion resistance,
adhesion, and resistance to cathodic disbondment.
3.4.2.1 Three types of dielectric coatings
commonly used on steel USTs are polyurethanes,
epoxies, and reinforced plastics. Performance of
these coatings should conform to a recognized
industry standard such as:
(a) STI
(1)
T871,
5
or
(b) UL
(2)
1746,
6
Part 1, Section 10.
3.4.2.2 When flaws, damage, and degradation
occur on coatings, these damaged areas must be
repaired
in accordance with manufacturer’s
specifications.
3.5 Electrical Isolation
3.5.1 A factory-fabricated cathodically protected steel
tank is usually electrically isolated from all other
structures (product and vent piping, hold-down straps,
liquid-level
monitoring systems, interstitial space
monitors, conduit lines, etc.) for the CP system to
perform as designed. Electrical isolation of a factory-
fabricated cathodically protected UST is required
because the CP system design capacity is sufficient
only for the tank.
Isolation devices are normally
installed at the manufacturing plant to eliminate
installation errors and to be compatible with the
product being stored.
The device used to ensure
isolation is dependent on the type of connection
being made to the tank. Electrical isolation devices
should only be used within their temperature and
pressure limitations.
3.5.1.1 Dielectric bushings are used for threaded
connections
in nonpressure tanks. These
bushings shall be as specified in UL Standard
1746, Part I, Section 11, and STI-P3,
7
or meet
equivalent requirements.
3.5.1.2 Flanged connections are used when
conditions preclude the use of dielectric bushings.
Flange isolation kits should be used if flanged
connections are to be made in the field.
3.5.1.3 If hold-down straps are to be used,
effective isolation material shall be used between
the strap and tank surface.
3.5.2 The isolation should be verified after installation
is completed but before backfilling and final paving or
grading.
3.6 Backfill Requirements
3.6.1 The backfill shall be a homogeneous material
that is compatible with the coating and CP system.
The backfill material shall be free of large rocks, trash,
debris, ice, and other nonhomogeneous materials.
3.6.2 The CP design shall consider situations in which
protective current flow is obstructed by geologic
conditions or the presence of other structures.
3.7 Miscellaneous
3.7.1 Tanks are often anchored to prevent buoyant
forces from floating the tanks. Anchor materials shall
consist of flat straps that are isolated from the tank
surface with a dielectric insulating material. Wire cable
or steel round bars should not be used because point-
of-contact damage to the coating can occur. Corrosion
control of the strap components shall be considered.
3.7.2 Each protected UST system should have the
following:
(a) a dedicated test lead wire connected to the
structure;
(b)
access to the electrolyte for portable reference
electrode tests; and
(c)
accessible connection points for all permanently
installed monitoring devices.
___________________________
(1)
Steel Tank Institute (STI), 570 Oakwood Rd., Lake Zurich, IL 60047.
(2)
Underwriters Laboratories Inc. (UL), 333 Pfingsten Rd., Northbrook, IL 60062.