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CP BLOG Continued from page 40 We sometimes even install shunts and diodes and occasionally bury coupons and permanent reference cells between the pipes (the number of cells depends on the distance between pipes). If we have sacrifcial anode groundbeds, we always leave cables connected to the pipes for the pipe-to-soil (P/S) potential measurements. When you connect two cables to a test point, one cable could be used to install sacrifcial anodes. When you end your lineal pipe route drawing with all of those test points, I've typically had to place one test point at intervals of 500 m or less. In addition, conducting P/S potential close-interval surveys requires that the distance be small enough to allow for connections to the pipes. If you need to install test points near places infuenced by induced voltages from high-voltage power lines, remember to ground them in accordance with NACE International and IEEE standards. To my knowledge, there is no regulatory requirement governing the spacing of test stations. U.S. Cod of Federal Regulations (CFR) Title 49, Part 195, Section 195.416 only states that "Each operator shall maintain the test leads required for CP in such a condition that electrical measurements can be obtained to ensure adequate protection." A must have suffcient test stations or other contact points for electrical measurement to determine the adequacy of CP." In my opinion, technologies such as close interval surveys and direct current voltage gradient can provide very reliable data to determine the level of CP that complies with one or more of the criteria and/or requirements given in the regulations. A It has been my experience that some regulators want test stations at one-mile intervals. On a recently installed long-distance gas pipeline, two-mile (3.2-km) intervals were quite acceptable. AC corrosion of buried pipelines Most of the literature that I have read regarding alternating current (AC) corrosion on buried steel pipelines is related to overhead AC power transmission lines. If, however, Q there is an underground high-voltage (14.4-kV) AC duct bank near a steel pipeline, does it have the same effect as overhead AC power lines? In other words, does it matter whether the AC lines are overhead or underground when deterBecause of the expense of adding mining if they might cause AC corrosion test stations to existing pipelines, on the steel pipe? we try to locate all points of conIn general, the induced AC volttact that are available—including age is lower from underground valves, drips, outside meters, vent lines, cables than overhead power casing test stations (sometimes they conlines. The main reason for this is tain pipe wires), compressor stations, etc. When necessary we have used steel that most of the high-voltage power caprobes. Record searches or feld surveys bles have some kind of metal armor that can turn up many points of contact, provides some degree of shielding of the magnetic feld. However, induced AC which in turn save money. voltages depend on many factors: groundAccording to CFR 49, Part ing design, separation, parallelism, cable 192.469, External Corrosion type, load conditions, soil conditions, etc. Control: Test Stations, "Except Every case should be addressed sepawhere impractical on offshore rately. I would defnitely take a closer and wet marsh area pipelines, each pipelook. line under CP required by this subpart A A A NACE International, Vol. 52, No. 5 May 2013 MATERIALS PERFORMANCE 43