Nace CP 1, Course Manual

Page: 370 / 535

RP0388-2001
4 NACE International
NOTE: For bolted and riveted tanks, electrical
continuity of all joining components must be ensured if
corrosion control of these components by impressed
current cathodic protection is to be achieved.
4.2.2 For existing tanks, an on-site corrosion
evaluation, current requirement test, performance of
systems in similar service, and/or laboratory testing to
determine polarization characteristics may be useful.
4.3 Direct Current (DC) Power Source
4.3.1 Impressed current cathodic protection requires
DC power. It is commonly obtained from transformer-
rectifier units that step down AC power and convert it to
DC power. However, DC power may be available from
other sources such as solar cells.
Units should be
designed and manufactured to provide continuous,
dependable operation for 10 to 20 years.
Proper
protective and monitoring devices, including disconnect
switches;
circuit breakers; output voltmeters;
ammeters; and surge, lightning, and overload
protection should be provided. Units should be either
manually or automatically controlled over the full
voltage output range.
4.3.2 Output Current Capacity
4.3.2.1 Current capacity of the unit should be
based on the current requirement for cathodic
protection. 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
). Installations involving
high corrosion rates, nonpotable water, and
galvanic metal couples may require considerably
higher current densities.
4.3.2.2 Coating the steel tank surface reduces the
total
current required in proportion to the
effectiveness of the coating coverage. On newly
coated tanks, the initial current requirement may
be less than 1% of the requirement of an uncoated
surface under the same conditions.
4.3.2.3 Current capacity for a coated tank should
be selected so that there is sufficient capacity
available
even after considerable coating
deterioration, typically 10 to 20% for a 20-year
system design life.
4.3.3 Output Voltage Capacity
4.3.3.1 The output voltage capacity is governed
by
the current requirement and the circuit
resistance. The voltage capacity shall be selected
to overcome the wire resistance and highest
anticipated anode-to-water resistance at the total
current demand needed for the life of the system.
4.3.3.2 Power requirements are directly related to
voltage as well as current. Therefore, operating
voltage should be as low as possible (consistent
with other economic considerations) and yet high
enough to maintain the current required for
protection.
4.3.4 Multiple-Circuit Systems
4.3.4.1 A separately controlled circuit should be
provided for energizing other circuits, such as the
riser anode assembly, if necessary.
4.3.4.2 Other applications for multiple system
circuits might include unusual geometries, tanks
with baffles, etc.
4.4 Impressed Current Anodes
4.4.1 Impressed Current Anode Configuration
4.4.1.1 Impressed current anodes shall be
arranged in the tank so that protection can be
provided
to all surfaces without exceeding
potentials (in the vicinity of the anodes) that will be
detrimental to the coating system.
4.4.1.2 The number, diameter, and length of
impressed current anodes shall be sufficient to
achieve an acceptable circuit resistance and
proper current distribution.
4.4.1.3 The design shall prevent shorting of
impressed current anodes to the tank surfaces.
This is particularly important in elevated tank riser
pipes.
4.4.1.4 Where freezing occurs, provisions should
be made for periodic replacement or for an
impressed current anode installation unaffected by
either freezing or falling ice. Alternatively, the tank
may be operated in a manner to prevent ice
accumulation on the anodes or in a solid mass
across the tank, which could damage the anodes
when collapsing.