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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10 OCTOBER 2016 MATERIALS PERFORMANCE THE BLOG NACE INTERNATIONAL: VOL. 55, NO. 10 materialsperformance.com EDITORIAL MA N AG I N G E D I TO R - I N - C H I E F Gretchen A. Jacobson E D I TO R Kathy Riggs Larsen T E C H N I C A L E D I TO R S Norman J. Moriber, Mears Group, Inc. Jack Tinnea, Tinnea Associates, LLC T E C H N I C A L E D I TO R E M E R I T U S John H. Fitzgerald III, FNACE S TA F F W R I T E R Ben DuBose CO N T R I B U TO R Husna Miskinyar GRAPHICS E L E C T R O N I C P U B L I S H I N G Teri J. Gilley CO O R D I N ATO R G R A P H I C S D E S I G N E R Michele S. Jennings ADMINISTRATION C H I E F E X E C U T I V E O F F I C E R Rober t (Bob) H. Chalker G R O U P P U B L I S H E R William (Bill) Wageneck A S S O C I AT E P U B L I S H E R Eliina Lizarraga ADVERTISING S A L E S MA N AG E R Diane Gross diane.gross@nace.org, +1 281-228-6446 A S S I S TA N T S A L E S MA N AG E R Claudia Archer claudia.archer@nace.org, +1 281-228-6497 ACCO U N T E X E C U T I V E S Pam Golias pam.golias@nace.org, +1 281-228-6456 Jody Lovsness jody.lovsness@nace.org, +1 281-228-6257 Leslie Whiteman leslie.whiteman@nace.org, +1 281-228-6248 A DV E RT I S I N G CO O R D I N ATO R Brenda Nitz brenda.nitz@nace.org, +1 281-228-6219 R E G I O N A L A DV E RT I S I N G S A L E S The Kingwill Co. R E P R E S E N TAT I V E S Chicago/Cleveland/ New York/U.S. West Coast Area– jim@kingwillco.com, +1 847-537-9196 NACE International Contact Information Tel: +1 281-228-6200 Fax: +1 281-228-6300 E-mail: Firstser vice@nace.org Web site: nace.org EDITORIAL ADVISORY BOARD Zahid Amjad Walsh University Susan Borenstein General Dynamics Electric Boat Raul A. Castillo Consultant Ir vin Cotton Ar thur Freedman Associates, Inc. David D. He PG&E; Jerr y Holton Specialty Polymer Coatings USA, Inc. W. Brian Holtsbaum Corsult Associates (1980), Ltd. Russ Kane iCorrosion, LLC Ernest Klechka CITGO Petroleum Corp. Kur t Lawson Mears Group, Inc. Lee Machemer Jonas, Inc. John S. Smar t III John Smar t Consulting Engineer The following are excerpts f rom the NACE International Corrosion Net work (NCN) and NACE Coatings Net work . These are e-mail-based discussion g roups for corrosion professionals , with more than 3 ,000 participants . The excerpts are selected for their potential interest to a large number of NACE members . They are edited for clarit y and leng th . Authors are kept anony mous for publication . Please be advised that the items are not peer-reviewed , and opinions and sug gestions are entirely those of the inquirers and respondents . NACE does not g uarantee the accuracy of the techni- cal solutions discussed . M P welcomes additional responses to these items . They may be edited for clarit y. For information on how to subscribe to these f ree list ser vers , click on the "Corrosion Central " link and then "Online Corrosion Communit y List Ser vers" on the NACE Web site: nace .org. Protecting parallel pipelines Q: Four parallel underground pipelines with only 0.15 ft (0.05 m) between each one will be cathodically protected by impressed current. Each line is 8-in (203-mm) in diameter and 1.9 miles (3 km) long, but their coatings and construction dates are different. Because the previous cathodic protection (CP) system is insuff icient, the owner wants to revamp it. One of the engineers wants us to use one rectif ier with a current divider. The current divider unit will have four out- puts, each sensing pipeline potentials by four separate permanent reference elec- trodes. The aim is to set each pipeline to a constant potential, which is –1,100 mV. The primary goal is to minimize interfer- ence by keeping all pipelines at the same potential. I am considering using only one con- stant potential rectif ier and connecting all the pipelines in parallel. I see no differ- ence between the two systems in order to minimize interference. Their potentials will be the same on the drain point but may be different in other places. Could you please comment on using a current divider? What is your practice to protect multiple pipelines laid in parallel? A: I would bond all the lines to- gether and then bond them to the rectifier negative. ey will then act as one big structure, with currents going to the various defects in the coating. ere will be no interference if they are bonded together properly. I suggest sticking with the NACE International-recommended criteria and not the –1,100 mV. A: I have no firsthand experience with the current divider ap- proach but it sounds like a good idea con- sidering the spacing and the differences in coating. I have worked with pipelines hav- ing more than 0.3-ft (0.09-m) of spacing and found that shielding is a big problem. Connecting all the pipes to a common negative, even on a potential-controlled rectifier, will not prevent inadequately protected areas on lines that do not have a good coating, particularly if the worst coating is on an interior pipeline. A: As short as the lengths are, IR drop from worst case to best (coating condition) would amount to only a few mV. Connect the pipeline drain points together through individual shunts so the current demand can be in- dividually monitored. is would also allow isolation for locating underground shorts. You may f ind that using a permanent reference cell for potential control gives reliability problems over time. If the owner feels the need for potential control, order the rectif ier with constant current as well as manual tap-set options for use as necessary. A: If these lines are electrically con- tinuous at some point upstream or downstream (this could be through electrical apparatus), then the complex system will not work any way. It has been my experience that treating multiple lines as a common unit has presented no signif- icant problems, especially when all are coated lines. A simple system is usually best in the long term. A complex control arrangement invites all kinds of mainte- nance and operational problems. Perhaps there should be more effort placed on getting the groundbed electri- cally remote to all structures so that the potentials would be more uniform. I seriously doubt you can electrically isolate each individual line, and that would be essential for the selective protec- tion. It is common to have electrical devices that sense pressure and monitor

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