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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Page 14 of 100

12 DECEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 12 Continued f rom page 10 THE BLOG B31.3 stipulates that a vent hole is required for leak detection. To quote directly, "A vent hole shall be provided at the side (not at the crotch) of any pad or saddle to reveal leakage in the weld between branch and run and to allow venting during welding and heat treat- ment. A pad or saddle may be made in more than one piece if joints between pieces have strength equivalent to pad or saddle parent metal, and if each piece has a vent hole." In the latter case, mechani- cally sealing with a pipe plug or seal- welding the vent hole ignores the word "shall." A: We often put a dab of caulk ing or stiff grease in the vent hole (don't f ill up the entire space between the pad and vessel or pipe). This w ill keep the water out to prevent corrosion, and a process leak should push the soft material out of the hole for future leak detection. A: In past inspections, we have seen A PI-certifed inspectors require that the plant drill vent (telltale) holes in required reinforcements that don't have them. I have never seen an application where the amount of liquid entering a telltale hole would cause measurable corrosion compared to what is on the rest of the line. If it was really a concern, you could install a small plastic ftting with a weep hole in the center, which would be a lot less expensive than welding. Performance of PTFE in hydrofluoric acid Q: Does anyone know of any good sources of information on the behavior of poly tetraf luoroethylene (PTFE) in hydrof luoric acid (HF) solutions, the issues affecting the perfor- mance of this material in shipping containers, and the methods of applying it to shipping containers? I am concerned about the transportation of 15% HF by weight. Is PTFE acceptable and how should it be applied? A: I am assuming your question is about aqueous HF solutions. HF is a solution of HF gas dissolved in water. In this way it is similar to hydrochloric acid (HCl), which is hydrogen dissolved in water. I have seen quite a few failures w ith HCl gas permeating PTFE linings in pipe, valves, and f ittings. This results in corro- sion of the metal, behind the liner, and also causes liner failure as the HCl gas in solution increases w ith increasing temperature. When plastic-lined pipe is used for hot, aqueous HCl ser v ice, the pipe must be traced and insulated to keep HCl that permeates the liner from condensing behind the liner. Then, the pipe must be vented to allow the HCl to escape, and different manufacturers use different methods to vent the pipe or

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