Page 575 - Corrosion Engineering Principles and Practice

538 C h a p t e r 1 3 C a t h o d i c P r o t e c t i o n 539

13.3.3 Anode Beds
The type of anode bed used in an application is a key element of the
ICCP design. Anode beds are located either close to the structure
being protected or remote in order to distribute the current over the
area containing the structures to be protected. Anode beds are
classified as follows [9]:
• Local distributed anode-bed system
• Vertical distributed anode-bed system
• Horizontal distributed anode-bed system
• Deep anode-bed system
• Remote anode-bed system
If the anodes are installed vertically in 3 to 6 m deep holes about
5 to 6 m away from the structural elements to be protected, the system
is called a vertical distributed anode-bed system. When the anodes
are installed in a cluster, away from the structures that are being
protected, the system is called a remote anode-bed system. A deep
anode-bed system is a variation of a remote anode-bed system in
which anodes are located remotely from the structures being protected
in a vertical plane.
The location of the anode bed relative to the structure being
protected and other structures in the area is the single most important
factor in minimizing shielding issues. In power plants, for example,
the current discharge from remote anode beds is shielded from
spreading over a large area because of the shielding effects of
reinforcements in the building foundations, duct runs, and other
buried structures. When installing a new CP system, the influence on
foreign structures should be a primary consideration in choosing
the location of anode beds.
Local distributed anode-bed systems are the most common
design. This type of bed can be installed either vertically or horizontally
depending on their position in the soil. Horizontal installations may
be necessary when obstructions or other soil conditions make digging
holes difficult. Horizontal installations are also used where soil
resistivities are very low and the increased resistance of the horizontal
installation is not significant. A minimum of 0.6 m of burial for all
cables and 0.9 m of burial for the anode is recommended. Figure 13.13
shows a typical local horizontal distributed anode bed.
Vertical installations are used when there is a need to obtain lower
resistivities, when there are difficulties with trenching for a horizontal
bed, or when the physical surface area is limited. Generally, due to
their length, vertical bed anodes are brittle and must be carefully
handled to prevent breakage. Figure 13.14 shows a typical local
distributed vertical anode system. The anode cable is prone to failure
if the insulation is damaged in any way. As a result, particular care
should be exercised in handling the anode leads.