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Smart Monitoring Stations installed at Nuclear Power Plants Provide Data on the Corrosion Voltage Potential in Proximity to In-Service Buried Pipe and Tanks to Verify Compliance with NACE SP0169 Criteria
Posted on July 22nd, 2015

For several decades the American National Standards Institute (ANSI) and NACE International (NACE formerly National Association of Corrosion Engineers) direct assessment (DA) standards for monitoring cathodic protection and coating quality have been relied upon by pipeline operators and regulators to accurately monitor the impact of corrosion on transmission pipelines. These same standards have been proven far less useful when deployed in nuclear power plant subsurface environments.

In order to meet regulatory and industry commitments the challenge has been to develop a cost effective method to accurately monitor nuclear plants’ buried pipe. The in-scope piping and tanks are comprised of a dense complex of multiple systems in close proximity at depths sometimes exceeding 30-ft. Most nuclear plants have been in service for over 30 years and rely on original buried pipe systems to perform as designed. These systems were manufactured using various metals, which are bonded together to reduce electrical step and touch potential hazards. Each of these factors reduce the confidence factor of DA, but combined they make the procedure even less effective.

Smart monitoring test stations can be installed during excavation operations or without excavation at depths of up to forty feet directly into the subject pipeline local environment. This provides access to critical condition and assessment information that was previously unavailable. Smart test stations have been designed with the following components: two Electrical Resistance Probes (ER), two CP coupons and a stationary reference electrode. 

In-scope buried piping and tanks are considered protected if the corrosion rate at circuit equilibrium can be demonstrated to be is less than 1mpy (mil per year) or if NACE cathodic protection criteria as described in NACE SP0169 can be demonstrated. Two of the NACE criteria described in SP0169 are a corrosion potential voltage of -850mV or less and a corrosion voltage potential shift of 100mV from the native or static potential. The -850mV criteria is generally considered to be a conservative criteria but several factors make this criteria much more difficult to achieve. The 100mV potential shift is easier to achieve but requires specific data that is not available utilizing indirect assessment technologies from the soil surface and as a result is not an option for most nuclear power plants. The critical data that are required to support the 100mV shift are quantifying the native potential and making sure that the subject pipeline or structure is the most anodic metal in the monitoring circuit. Native potential is the corrosion potential that exists between the subject pipeline and its environment minimizing the effects of other structures, grounding, etc. In plant environments where structures are bonded, most (in the area of 98%) of the CP current supplied to protect the buried pipe goes through the ground circuit. Therefore in a NPP, Utilizing the 100mV shift criteria can reduce the possibility of CP damaging coatings and metallurgy. 

Matt Darois of RSCS recently presented along with Greg Lupia of Exelon Corporation and Hank Kleinfelder of CorrTech Inc. at the ASME 2015 Pressure Vessels & Piping Conference on the installation of monitoring stations at Exelon Nuclear Power Plants. Their presentation described the process from planning to successful completion, including the application of data collected to meet the 100mV shift criteria and support Asset Management Plans and NEI commitments. Their presentation can be viewed here. The associated paper will be published in the ASME 2015 PVP Conference proceedings this year.

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