Materials Performance

OCT 2016

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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11 NACE INTERNATIONAL: VOL. 55, NO. 10 MATERIALS PERFORMANCE OCTOBER 2016 f lows and control valves. The grounding in these devices bonds each line together. It is not possible to create electrical disconti- nuity in an electrical grounding system. A: It might be worth running a cur- rent requirement test on each in- dividual pipeline. is would give a much better picture of relative coating condi- tion, not to mention isolation. A: e current divider approach, or bonding all four pipelines to- gether and using a constant potential rec- tifier may work just fine. A key consider- ation in designing the CP system is the actual quality of the coatings on each line. If the coatings are in good shape, even though the lines were installed at dif- ferent times and with different coatings, then both approaches should work. If the coating is in bad shape on some of the lines, shielding and current distri- bution may be the major concern. It may be diff icult to adequately protect the lines without some ty pe of distributed ground- bed or multiple rectif iers. A: e last time I managed a set of parallel pipelines with widely varying coating conditions, we first ran a survey to locate unintended cross con- nections and bond cables and then re- moved them all. en we installed indi- vidual rectifier connections ("negatives") to each and routed them through a bond box with shunts for each pipeline so that we could measure the current drain. e reason we did this was not because the old, bonded system didn't work. Instead, ownership of one of the lines changed and the new owner insisted on knowing how much current was used to protect it. ey also wanted the ability to conduct sur- veys on each pipe individually. Pipes needing more current naturally received more current because of their poor coating condition. In our case, the pipes were ~31 miles (50 km) long. We did need to install additional rectif iers to overcome excessive pipe-to-soil poten- tials that were encountered in order to satisf y NACE criteria on the poorly coated line. The coating conditions were wildly different: one line was a 1940s-vintage pipe with poor coating and the newest was a modern fusion-bonded epoxy- coated pipe. We also expected to have to "resistor back " some of the current, so we built junction boxes big enough to house resistors, but they were never necessary. A: One method of lowering the ap- plied voltage to the well-coated pipelines without resistors is to install a silicon diode in series. e 0.7-V forward voltage drop reduces the voltage on the pipeline and the diode prevents reverse current flow if the rectifier fails. e diode does not regulate current; it just reduces the applied voltage because of its resistance. The variable resistor may be required. The purpose of the resistor is to control the amount of current supplied to indi- vidual pipelines—it does this by reducing the applied voltage to an individual pipe- line. The diode does the same thing, but it is not variable. naceinstitute.org/MPOct Take the Next Step... • Prove your skills • Gain industry recognition • Drive your career path Get NACE CIP Certified!

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