Materials Performance

JUN 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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35 NACE INTERNATIONAL: VOL. 55, NO. 6 MATERIALS PERFORMANCE JUNE 2016 CATHODIC PROTECTION ESSENTIALS Ranking the Risk of AC Interference to Pipelines Many pipeline operators have existing pipeline infrastructure running parallel to high-voltage powerlines that has not been fully assessed to determine its risk due to alternating current (AC) interference, which can present integrity issues for the pipeline as well as a shock hazard to pipe- line personnel. A case study involving AC interference risk ranking of over 6,400 miles (10,300 km) of existing gas transmission piping oper- ated by one of the largest combination gas and electric utilities in the United States was presented by NACE International members Wolfgang Fieltsch and Daniel Silva at CORROSION 2016 in Vancouver, British Columbia, Canada. Te age of these pipelines ranges from 1 to 72 years, and they have a diverse variety of coatings, including hot-applied asphalt, bitumen, coal tar, mastic, fusion-bonded epoxy, and polyethylene tapes. Te scope of this proj- ect was to identify the transmission pipe- lines that were at greatest risk due to AC interference under normal (steady-state) operation, prioritize them based on the severity of risk, and determine what further action was required. According to Fieltsch and Silva, AC corrosion can result when current dis- charges from coating defects (holidays) on a pipeline. To determine if AC corrosion is likely to occur, the industry uses AC current density (CD) as a primary indicator. Re- search has shown that corrosion rates increase exponentially with AC CD. In the presentation, Fieltsch and Silva referred to an AC corrosion test on steel coupons at varying cathodic protection (CP) levels and AC CD, which showed that AC corrosion rates increase with increased AC CD; and on pipelines with adequate CP, the corro- sion rates become elevated as AC CD become >100 A/m 2 . Tis study assessed data that were collected over four years at select test sta- tion locations. Te risk ranking was primar- ily based on historical AC voltage measure- ments taken at pipeline test stations in proximity to high-voltage AC powerlines, and measured soil resistivity data. Fieltsch and Silva comment that measured AC voltage and calculated AC CD (based on local soil resistivity) are the most efective indicators in identifying safety and AC corrosion risks. One of the main drawbacks to this approach, however, is that AC volt- ages can vary signifcantly throughout the day, from day to day, and seasonally. Be- cause of this, they considered powerline load data at the time of the measurement as well as the average and maximum load during the overall time frame of the assess- ment. All test stations within 100 ft (30 m) of high-voltage transmission lines, based on GIS data, were targeted, resulting in a list of over 1,500 locations to be tested. Time- stamped AC voltages were measured at each of these locations. At locations where AC voltages exceeded 2 V, the soil resistivity was measured. Te results of the calcula- tions and assessments for each of the tests station locations are summarized in their paper. 1 Reference 1 W. Fieltsch, D. Silva, "AC Interference Risk Ranking: Case Study," CORROSION 2016, paper no. 7393 (Houston, TX: NACE Interna- tional, 2016).

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