Abstract
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.
