A skid mounted Formation water storage tank farm was set up in a remote marginal oil field in North East India. A Cathodic Protection system was installed in the tanks to protect them from Galvanic corrosion over 10 year service life. The tanks had the following design and process data.
Inside diam=6.0m. Height to eaves=4.5m Fixed cone type roof. Liquid capacity=100Kls (Cu.m). Nominal capacity=120Kls. Storage temp=Ambient. Storage pressure = Atmospheric. Max /Min temp (Amb)=38 /3 ᵒC. Design temp = 65 ᵒC. Material of construction = Carbon Steel. Corrosion allowance=3mm Salinity of water=8500ppm. pH=9.5.
The terrain is hilly with heavy slush and mud during monsoon season which extends over April-Dec period every year. This makes the approach to the equipment difficult, requiring stand alone type with least maintenance and low capital investment. So a Sacrificial Anode system was installed.
Design life=10 years. Tank painted with 200 micron DFT Coal Tar Epoxy paint on both sides including structural skid which rests on ground. The system is shown in the above figure the tank steel structure is exposed to stored formation water zone inside, atmospheric zone outside and the base in soil zone.
Calculation of steel areas in various zones Table 1
Atmospheric zone=3.142 x 6.01 x 4.5 + 3.142 /4 x 7^2=123 m^2
Water zone= 3.142 x 6 x 4.5=85“
Soil zone=3.142 x 6.8^2/4=36“
Table 2 Effective areas. Water zone (inside of tank ) no painting damage=85 m^2
Atmospheric zone (outside of tank) 15% painting damage during installation=105 m^2
Soil zone (tank skid and bottom) 50% painting damage
during installation=18 m^2
Assuming uniform 3% painting deterioration per year, the
total deterioration during 10 year service life is 30 %
Painted area in water zone after 10 years = 60 m^2, Bare
Painted area in Atmospheric zone=74m^2, Bare area=31m^2
Painted area in soil zone=13m^2, Bare area=5m^2.
Total current required for polarisation and maintenance
after 10 years
Water zone = 60x38 /1000 + 60 x 25 /1000=3.8 Amps
Atmospheric zone=74 x 15/1000+31 x 75/1000=3.5Amps. Soil
zone=13 x 5/1000+57 x 5/1000=0.35Amps.
Therefore, total current required for the entire structure
after 10 years=3.8+3.5+0.35=7.65Amps.
The anode must provide maintenance current over 10 year
period. So, the current required is average of initial and final
maintenance currents and=(4.92 + 7.65)/2=6.3Amps. The anode
will be of high purity Zinc to stand 10 year service life. The
weight of Zinc is given by
W=CR x L x Amp/n x u
Where W=Weight of Zinc in Kgs, CR is the consumption rate
of zinc in Kg/Amp year. L=Service life, n=Efficiency factor and
u=Utilization factor. In this case W= 10.7x10x6.3/0.9x0.8=936
Using 200mmx200mmx300mm long anodes, weight of each
anode=0.2x0.2x0.3x7073kg/m^3=84.8kgs, 12nos anodes of this
size will provide 1018 kilograms.
Energy capacity of the anodes@810A-hr per kilogram=810 x
Energy required for initial polarisation and maintenance
over 10 year period = (4.92 +7.65) x 365 x 24 x 10 +13.17
= 1,101,145.17 amps. Therefore, the anode system will be
adequate Figure 1