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Cathodic Protection
for Storage Tanks
Cathodic
protection is a standard, cost-effective, accepted method to prevent
corrosion of the submerged surface inside steel storage tanks. Cathodic
protection systems for water storage tanks are almost always impressed
current systems. Technical and economic reasons dictate that galvanic anodes
almost never be used to protect a water storage tank.
Design
Considerations
Design of a
cathodic protection system for a steel storage tank is normally
controlled by the following four considerations.
-
Current Requirements for
Protection
-
Water Resistivity
-
Geometry of Structure
-
Anode Service Life
If anode design
provides a uniform distribution of current, then the current requirement for
protection of a storage tank is calculated by multiplying total surface area by
a predetermined current density. Although many factors can influence current
density, the required current density can usually be determined by examination
of a water analysis. The operating current is never pre-calculated, but is
determined from tank-to-water potential measurements.
Circuit
resistance of the cathodic protection system consists almost entirely of
the anode-to-tank resistance and this resistance is dependent on the length
and size of anodes and water resistivity. In higher resistivity waters,
anode configuration is governed by current distribution, and higher voltage
rectifiers are used.
Geometry of the
structure controls the anode configuration for proper current
distribution. For smaller tanks one ring of anodes will usually provide
proper current distribution to side and bottom of the tank. For larger
tanks, additional inner rings of anodes may be required to provide uniform
current distribution to the bottom. A separate anode with separate controls
is normally used to protect the riser pipe in an elevated tank.
Sound engineering
practice dictates that each cathodic protection system be inspected
annually. For this reason anodes are frequently designed for only one year
of operation.
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