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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34 DECEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 12 CATHODIC PROTECTION Discussion According to the mathematical model, the inf luence of the grounding conductor material, the relative position between the grounding conductor and the pipeline, burial depth of the grounding conductor, and the length of grounding conductor on CP can be considered. Grounding Materials There are many types of grounding materials, including non-metallic modules of low resistance, galvanized flat steel, and zinc-clad steel. 9 Three materials were con- sidered: copper, steel, and galvanized steel. These were compared with CP potential di st r i b u t i o n o n t h e pi p e li n e w i t h o u t grounding. In each simulation, the pipe length was 500 m, the diameter was 0.3 m, and the burial depth was 2 m. The auxiliary anode, with a diameter of 0.1 m, length of 4 m, burial depth of 2 m, and output current of 20 mA, was located at one end of the pipe at a hori- zontal distance of 10 m. The ground, with a diameter of 0.1 m, length of 0.5 m, and burial depth of 0.2 m, was located near the center of the pipeline at a distance of 2 m. The simulated result, shown in Figure 4(a), shows that the mixed potential associ- ated with each ground material is strongly influenced by the open circuit potential of that material, with intensified effects near TABLE 1. COMPARISON OF EXPERIMENTAL DATA WITH NUMERICAL RESULTS E (mV CSE ) X (m) Experiment Data Numerical Results Relative Error (%) 0.5 –930.8 –907.6 2.49 1.0 –927.4 –903.3 2.60 1.5 –925.8 –900.4 2.74 2.0 –922.3 –897.4 2.70 2.5 –919.8 –892.5 2.97 3.0 –912.4 –883.8 3.13 3.5 –916.6 –891.7 2.72 4.0 –915.5 –895.7 2.16 4.5 –914.9 –897.4 1.91 5.0 –914.1 –898.4 1.72 5.5 –913.7 –899.0 1.61 the ground connection. Copper and steel grounds cause the potentials to shift in the more noble direction compared to the ungrounded pipe, while the galvanized ground actually behaves as a sacrificial anode. Relative Position Between Grounding Conductor and Pipeline Th e rel ativ e p o sition b etw e en th e grounding conductor and the pipeline refers to changing the horizontal distance of the grounding conductor from the pipe- line while maintaining a constant depth. In this simulation, the parameters of the pipe- line and anode remain the same, but the anodic output current was adjusted to 30 mA. The buried depth of the grounding conductor remained near the middle of the pipeline, and the material used was steel. The results for four horizontal distances— 5, 10, 15, and 20 m—are shown in Figure 4(b). The CP potential distribution curves are almost identical except near the ground connection. Buried Depth of Grounding Conductor In general, equipment grounding is bur- ied in a shallow position located at the top of the buried pipelines. Four grounding conductor depths were analyzed: 0.2, 0.6, 1.0, and 1.4 m (Figure 4[c]). As with the distance of the ground from the pipeline, the CP potential curves are almost identical for the various ground depths. Potentials remain fairly uniform except near the ground connection, where the local influence is ~5 mV. Length of Grounding Conductor Increasing the length of the grounding conductor has th e great est impact on reducing the ground resistance to earth. This increases the costs of material and installation while having a great impact on CP. 10 The length of grounding conductors was analyzed at lengths of 0.5, 1, 1.5, and 2 m (Figure 4[d]). The simulation results show that pipe- line CP potentials get more positive as the grounding conductor length increases in length, and the changes are large. Note that increasing the length of the grounding con- ductor by 1.5 m caused a potential change of 50 mV, indicating that the length of the grounding conductor has a huge impact on CP. The increase in the length of the ground- ing conductors reduces the percentage of CP current that distributes to the pipeline. As a result, additional current must be pro- vided to maintain effective CP. If that is even possible, the cost of the CP system increases. Therefore, it is important to restrict the length of the ground so that it meets safety requirements but does not unnecessarily affect CP effectiveness. Application These principles were applied to the CP design for an actual oil station. The buried pipelines of the station without grounding are as shown in Figure 5(a) and with steel grounding in Figure 5(b). The pipeline model was created in the boundary element analy- sis system software, according to the real shape and dimensions of the pipelines. CP for the pipelines without grounding u sed sa cri f i ci al an o d e s p o sition ed a s shown in Figure 6(a). The result of the sim- ulation, shown in Figure 6(b), indicates that the pipe-to-soil (P/S) potentials of the bur- ied pipelines are between –927 and –947 mV vs. a copper/copper sulfate (Cu/CuSO 4 ) electrode (CSE), indicating that the pipe- lines would receive effective CP from a rela- tively small number of anodes.

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