Nace CP 1, Course Manual

Page: 416 / 535

RP0193-2001
NACE International 9
5.7 On-grade tanks that are set on solid concrete or
asphalt pad foundations generally require specialized
measures for corrosion protection, because cathodic
protection may be ineffective.
In this circumstance, the
external surface of the tank bottom should be coated. In all
cases, steps should be taken to ensure that water does not
migrate between the tank bottom and the pad.
5.7.1 Providing electrical isolation between the
reinforcing steel and the tank bottom should be
considered if a concrete ringwall or pad is used.
5.8 Design Drawings and Specifications
5.8.1 Specifications should be prepared for all
materials and installation procedures that are used
during construction of the cathodic protection system.
5.8.2 Suitable drawings should be prepared to show
the overall layout of the tank bottoms to be protected
and the cathodic protection system and associated
appurtenances.
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Section 6: Design Considerations for Impressed Current Cathodic Protection
6.1 This section recommends procedures and
considerations that specifically apply to the design of
impressed current cathodic protection systems for on-grade
carbon steel storage tank bottoms.
6.2 Impressed Current Anode Systems
6.2.1 Impressed current anodes shall be connected
with an insulated cable, either singularly or in groups,
to the positive terminal of a direct current source such
as a rectifier or DC generator. The tank bottom shall
be electrically connected to the negative terminal.
Cable insulation should be selected based on the
anticipated environmental conditions and should be
resistant to oil and water.
6.2.2 Anode groundbed configurations may be
vertical, angled, deep, or horizontal, as illustrated in
Figures 4 through 7.
Anodes may be installed in a
distributed fashion under tank bottoms. The selection
of anode configuration is dependent on environmental
factors, current requirements, the size and type of tank
bottom to be protected, whether the tank is of new or
existing construction, and whether it is a single- or
double-bottom tank.
Figure 4: Vertically Drilled Anode CP System
Sand