Providing adequate Cathodic Protection (C.P) to existing tank bottoms in a refinery with many
underground structures always pose a challenge. As the original C.P system in the refinery
had been quite old and ineffective, a revamping of the C.P system for the refinery tanks and
underground piping was carried out which excluded the double bottom tanks. However
subsequently due to corrosion of the second bottom of the double bottomed tank, there was an
opportunity and a challenge to include this tank in the revamped C.P. system. The paper
describes these challenges in providing MMO anode system consisting of a series of
concentric loops in a narrow 300mm space between the tank bottom and a polyethylene liner
placed under the new bottom constructed. The concentric loops were closely spaced to
provide desired current distribution and to optimize the “throwing power” of the anodes.
After commissioning the C.P System, the protection level as measured by the permanent
Cu/CuSO4 half cell and the polarization coupon placed under the tank bottom met the
protection criteria as set in NACE standard RP0193-2001. This was done at a fraction of the
energy required for protection of a similar tank using conventional system.
Keywords: Cathodic Protection (C.P), Double Bottomed Storage Tanks, Polyethylene liner,
Concentric circles, MMO, Cu / CuSO4 half cell, polarization coupon, NACE.