Page 231 - Pipeline Rules of Thumb Handbook
P. 231

218     Pipeline Rules of Thumb Handbook
































         Figure 6. Combination of Figures 4 and 5. The length along the pipeline where a fixed level of cathodic protection can be used
         and those regions where controlled protection is required for any value of HVDC earth current. In this example, variable protection
         is required along the pipeline for approximately 20 to 30 miles on either side of the point closest to the HVDC electrodes.





         The required correction for this detrimental effect is similar  tion, a portion of the HVDC earth current can be collected
         to the effect for the previous HVDC polarity: namely, the  in a protective ground bed and conducted to the pipeline to
         uncorrected electric field must be modified by an added   satisfactorily modify the original field into an acceptable
         field to give an acceptable total. Theoretically, in this situa-  one.







         Troubleshooting cathodic protection systems: Rectifier-ground bed


           When the monitor, or a pipe-to-soil potential survey, indi-  on the line itself. Any area in which work has been done
         cates inadequacy of protection, the first place to look is at the  recently should be investigated; for example, if a new lateral
         protective unit. The current output of the rectifier should be  has been connected, the insulation should certainly be
         checked; if it is normal, the trouble is on the line itself; if it  checked. If investigation of such suspected sources discloses
         is high, and accompanied by low voltage, the trouble is cer-  nothing, then a more detailed search must be made. First, the
         tainly on the line, and is caused either by increased current  pipe-to-soil potentials should be studied, to see if the failure
         demand or by a short-circuit to parasitic metal. If the current  seems to be localized. A more thorough, but slower, approach,
         output is low, with voltage normal or high, the trouble is in  is that of making a detailed line current survey; find out where
         the ground bed or connecting cables. A pipe-to-soil potential  the drained current is coming onto the line. This will be much
         over the ground bed will show a peak over every anode which  more easily interpreted if a similar survey, made when the line
         is working; a disconnected anode will not show at all. This test  was in satifactory condition, is on record; a comparison will
         is particularly useful if comparison can be made to a similar  often very quickly locate the offending parasite. On the other
         test made at the time of the original installation. When inac-  hand, if the current collected in each section is greater than
         tive anodes are found, only digging will uncover the cause.  on the earlier survey, with no pronounced differences, then
           If the rectifier and its anode bed appear to be performing  the trouble is simply that of increased overall current require-
         satisfactorily, the source of the low potentials must be sought  ment, probably due to coating deterioration.
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