Materials Performance

DEC 2014

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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32 DECEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 12 CATHODIC PROTECTION T The effect of grounding on cathodic protection (CP) potential distribution has been studied using the boundary element method. Infuences from four factors were analyzed: grounding material, relative position between the ground and the pipeline, depth of the ground, and length of the ground. CP designs with and without ground- ing were compared for an oil station. The history of grounding technology for safety began early in the 18th century when Benjamin Franklin introduced the light- nin g ro d . 1 Sin c e th en , g roun din g h a s b e c om e an imp or t ant c omp on ent for protecting the safety of personnel, power systems, and equipment. 2 Increased usage of automatic pipeline controls has made g r o u n d i n g n e tw o r k s a c o n v e n t i o n a l practice for domestic and international long-distance pipeline systems. 3 While enhancing safety, grounding sys- tems can complicate the application of cathodic protection (CP). The low resistance to earth of the grounding system, compared with the piping, tends to accumulate a large portion of the protective current. Numerical methods are powerful tools in the analysis of corrosion problems. These m ethod s include th e f init e dif ference method (FDM), the finite element method (FEM), and the boundary element method (BEM). 4 BEM has been used to model CP sys- tems since the early 1980s. 5 Compared to the other methods, BEM requires fewer equa- tions and a smaller matrix size, and can solve both finite- and infinite-domain prob- lems. 6 The following analysis is based on the boundary element analysis system to simu- late the effect of grounding on CP design. The Mathematical Model of the Problem Assuming that the soil conductivity is uniform with no concentration gradients, Ohm's law applies, and the potential distri- bution is described by the Laplace equa- tion, Equation (1): ∂ ∂ σ ∂φ ∂ + ∂ ∂ σ ∂φ ∂ + ∂ ∂ σ ∂φ ∂ = V: x x y y z z 0 (1) where V = computational soil domain; f = potential in the soil domain; x, y, z = spatial coordinates; and s = soil conductivity. Boundar y conditions can be divided into the anode boundar y condition, the cathode boundary condition, and the insu- lation boundary condition. This is shown in Equations (2) and (3): 7 Γ A : φ = φ a s = φ a – ∆ φ a s or σ ∂φ ∂a + j a = 0 (2) Γ = ∂φ ∂ = n 0 I (3) where Γ A = soil boundary surrounding aux- iliar y anode; f a = potential of auxiliar y anode; f a/s = polarization potential of auxil- iary anode; j a = surface polarization current density of auxiliary anode; and Γ I = insula- tion boundary of soil domain. The only difference between the pro- tected pipelines and the grounding con- ductor is the cathode boundary condition because the pipeline is coated while the ground is bare. The differences of material The Infuence of a Grounding System on Cathodic Protection Gan Cui, Zili li, Xue Bai, and JianGuo liu, China University of Petroleum, Qingdao City, Shandong Province, China

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