|Content Type||Conference Paper|
|Title||New Cp Maintenance Concept For Buried Steel Pipelines - Current Density-based CP Criteria, and On-line Surveillance System for CP Rectifiers ? -|
|Authors||Yuji Hosokawa, Fumio Kajiyama, Tokyo Gas Co., Ltd.|
|Source||CORROSION 2004, March 28 - April 1, 2004 , New Orleans, La|
|Copyright||2004. NACE International|
|Keywords||Cathodic protection, Pipeline, Coupon, Current density, Rectifier, On-line surveillance|
New cathodic protection (CP) criteria based on DC and AC current densities have been developed to assess the increasing risks of AC corrosion and overprotection on cathodically protected buried steel pipelines. Furthermore, in order to perform the risk assessment of the pipelines subject to these risks with respect to the new CP criteria, a new hand-held instrument that enables the simultaneous measurement of DC and AC current densities in the field was developed. The risk of rectifier failure due to lightning surge and the aging of ground beds is also increasing. An on-line surveillance system for rectifiers was then developed in order to immediately detect the failure of rectifiers and allow corrective action when required. New CP maintenance program consisting of 1) CP protection level monitoring using the new criteria and instrument, and 2) remote monitoring of the rectifiers using on-line surveillance system, was conducted on a newly installed pipeline in close proximity to overhead high-voltage AC power transmission lines. AC corrosion risk was successfully detected and eliminated using solid-sate DC decoupling devices. Reliable remote monitoring of the rectifiers was also successfully performed, thus ensuring the adequate supply of CP currents. Finally, a new concept of CP maintenance using these new techniques was established.
CP maintenance is one of the most important activities necessary to ensure the long-term integrity of buried steel pipelines. CP maintenance mainly consists of two activities of 1) CP potential monitoring, and 2) CP system inspection. CP potential monitoring is conducted to evaluate the adequacy of the CP level of pipelines. Conducting a periodic pipe-to-soil potential survey is one representative method of CP potential monitoring, and the results are evaluated with respect to the CP criteria based on pipe-to-soil potential (potential-based CP criteria) such as -0.85VcsE polarized potential criterion ~. CP system inspection is conducted to determine the operating condition of the CP equipment. With regards to rectifiers, periodic checks of output voltage and current have been generally performed at the site.
Several risks increase on cathodically protected buried steel pipelines due to various changes in factors. Figure 1 shows the relationship between the changes in the factors and the increasing risks.
The risks of AC corrosion and overprotection are increasing due to the changes in burial conditions, the characteristics of coatings and pipe materials. Authors have revealed that potential-based CP criteria are not always appropriately indicative of these increasing risks 2, 3. New cathodic protection criteria based on DC and AC current densities (current density-based CP criteria) have then been developed through laboratory and field tests by the authors, that can assess these increasing risks as well as the conventional corrosion risks such as stray DC corrosion 2,3. Furthermore, in order to perform CP protection level monitoring on the pipelines subject to these risks with respect to the new CP criteria, authors have developed a new hand-held instrument that enables the simultaneous measurement of DC and AC current densities in the field 4.
The risks of rectifier failure due to lightning surge and the aging of ground beds are also increasing. Rectifiers often provide CP currents for long lengths of pipelines, and therefore early detection is required of rectifier failures. Moreover, as for the rectifiers installed in proximity to DC powered rail transit systems, immediate action should be taken in case of failures to prevent stray DC corrosion. On-line surveillance system for rectifiers have been developed by the author
|File Size||751 KB||15|