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Cathodic Protection Design

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- 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.
- 3. Common Alloy Specification
- 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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