Page 156 - Offshore Electrical Engineering Manual

Cathodic Protection Calculations 143

length is extensive in comparison with cross-section as in subsea pipelines, R should
S
be evaluated.
A similar calculation is required for galvanic anodes as follows:
E d E a − E cu
I a = = A
R a R a

where I , Anode current output (A); R , Anode to electrolyte resistance (Ohms);
a
a
E , Open circuit potential between anode and electrolyte, measured by reference
a
electrode (V); E , Structure to electrolyte potential when fully protected. (V); E ,
d
cu
Driving voltage between anode and protected structure.
(V) = E a −E cu = V s + V a
where V , structure to electrolyte potential and V , anode to electrolyte potential.
s
a
Typical values of E are:
a
Galvomag 1.7 V
Magnesium 1.5 V
Zinc 1.05 V
Galvanum III 1.05 V

Calculation of Anode Consumption: the expected life of an anode in years is
given by:
CR × W
Y =
8760 × A

where Y, Life (years); CR, Consumption rate in Ah/kg; W, Anode weight (kg); A,
Anode output (A); A typical value of CR for magnesium is 1230 Ah/kg.