1.
Galvanic Anode CP System Design
• Galvanic anodes are an important and useful means of Cathodic protection to
underground storage tanks, pipelines and other buried or submerged metallic
structures.
• The principle of protection is that one metal (active) more negative in emf series
get consumed or sacrificed while providing protection to less negative noble metal
provided they share same electrolyte and are electrically in contact with each other.
• The electrochemical reaction provides the protective current eliminating the need
of power source.
• Installation and maintenance cost is low.
• Efficient and non interfering because of relatively low current and well distributed
output is maintained and hence no interferences result.
• The small potential difference or driving potential results in a very low current
which severely limits its use in large structures and poorly coated structures.
2.
Characteristic H1 Alloy AZ- 63
Mg Alloy
Hi Pot Mg Alloy Hi Purity Zn Alloy
Soln pot to cu-cuso4
cell
-1.55 -1.80 -1.10
Faradic
consumption rate
8.8 8.8 23.5
Current efficiency 25-50 50 90
Actual amp hrs/lb 250-500 500 360
Actual lb /amp/ year 35-17.5 17.5 26.0
Available Anode Materials: The most commonly used materials for galvanic
anodes on buried structures are alloys of Magnesium and zinc.
4.
Shapes, Sizes and backfill: Galvanic anodes are offered in variety of standard
shapes and sizes and may also be ordered in custom Sizes.
• The use of anode backfill accomplishes the following effects:
Stabilizes anode potential.
Prevents anode Polarization and enhancing current maintenance.
lowers anode-earth resistance increasing current output.
Reduces self corrosion of anode by promoting a uniform corrosion attack and there by
improving efficiency.
• The most commonly used anode backfill mixture is 75% gypsum, 20% bentonite clay,
5% sodium sulfate. This mixture is selected because of wide range of soils encountered, it
has shown best success to achieve desired characteristics.
5.
Anode selection: The criteria of selection is one would expect, an analysis of
performance vs cost.
• The CP design parameters related to anode material performance are:
— design electrochemical capacity, ε (Ah /kg)
— design closed circuit anode potential, Eoa (V)
•The design electrochemical capacity, ε (Ah /kg), and design closed circuit anode
potential, Eoa (V) are used to calculate:
the design anode current output and
the required net anode mass using Ohm’s and Faraday’s laws, respectively.
Cost: The costs involved can be categorized as:
•Material costs: based on alloy, backfill and anode size. Generally heavier the anode lower
the cost per unit of mass. Also Efficient anode materials result in lower cost per ampere hr
of current delivered.
•Installation cost: depends upon number of anodes however it won’t vary greatly on per
anode basis regardless of alloy or size.
• Maintenance Cost: It is not considered in Galvanic Protection.
6.
Design Calculations
Total anode resistance: The resistance R used in the Ohm’s law contains the
anode to electrolyte (Ra/e), structure to electrolyte (Rs/e), internal structure (Rs)
and cabling (Rc) resistances. The last three are normally negligible in a Offshore
sacrificial CP design and the remaining resistance, i.e. anode to electrolyte, is the
most if not the only significant one to define.
Long slender stand off (L > 4r)
Short slender stand off (L<4r)
7.
Long flush mounted (L width and thickness)
Short flush mounted, bracelet and other flush mounted shapes
With:
ρ : Soil or Seawater resistivity (ohm cm)
L : Length of anode (cm)
r : Radius (cm)
A : Exposed anode surface area (cm2)
S : Arithmetic mean of anode length and width (cm)
8.
Protection potential and anode current output:
To predict the current output of protective current from a sacrificial anode the voltage
between anode and cathode (driving voltage) is divided by the resistance of the anode to
the electrolyte.
The accepted criterion for protection of steel in aerated seawater is a polarized
potential more negative than –800 mV measured with respect to silver/silver
chloride/seawater reference electrode. And it is -850 mV with respect to Cu/Cuso4
electrode/soil reference electrode.
Therefore, for design purpose, the protection potential or cathode potential stated above
is used in the equation.
BS EN 13174:2001 states that the driving potential can be chosen from values ranging from
0.3 to 0.15 volts.
9.
Current requirement and protection potential
The current demand required to protect the steel structure is determined by the
following formula:
I= ic * A* f
where
I : Current required in Amps
ic : Current density required in A/m2
A : Surface area to be protected m2
f : Coating breakdown factor
Anode mass requirements
The total net mass of sacrificial anode material is determined from the following
formula:
Where
Im : The maintenance current demand in Amps
t: Design life in years
u : Utilisation factor
e : Electrochemical capacity of anode material in Ah/Kg
And 8760 corresponds to the number of hours per year.
10.
The anode current output is calculated for the initial and final projected life of the
cathodic protection system. In the latter case, the anodes have been assumed to be
consumed to their utilization factor. The final length and final mass are calculated
thanks to the following formulae
m(final) = m(initial) x (1-u)
where
m(final) : Final mass of anode (Kg)
m(initial) : Initial mass of anode (Kg)
u : Utilization factor generally 0.85 (85%)
Final checks
For final verification the anode current capacity is calculated and is defined as:
C = m x e x u
where
C : Anode current capacity (Ah)
m : Net per anode (Kg)
e : Electrochemical capacity of anode (Ah/Kg)
u : Utilisation factor
Anode dimensions and net weight are selected to match all requirements for current
output (initial and final) and current capacity for a specific number of anodes.
11.
In addition, final calculations are carried out to demonstrate that the following
requirements are met:
where
n : Number of anodes
C : Current capacity(Ah)
t : Design life of CP system (years)
and
where
n : Number of anodes
I(ini/fin) : Initial or final anode current output (A)
Ic(ini/fin) : Initial or final current demand (A)
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