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Cathodic Protection Coupon Use for Buried Piping in Plant (i.e. Complex) Facilities

Measurement and interpretation of cathodic protection data in plant facilities present challenges where mixed metals are electrically continuous with the protected structure. This paper address some of the confusion and important aspects when using coupons for buried piping in mixed-metal circuits.

Product Number: 51317--8824-SG
ISBN: 8824 2017 CP
Author: Douglas Gilroy
Publication Date: 2017
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The measurement and interpretation of cathodic protection (CP) data in plant (i.e. complex) facilities present inherent challenges where mixed metals are electrically continuous with the protected structure. Often there is no attempt to electrically isolate the on-plot buried steel piping networks from the facility for safety and practical considerations. Coupons can be used to assist in the evaluation of CP levels on buried steel piping in mixed-metal circuits. However the present industry practice of disconnecting the coupon from the mixed-metal circuit to measure the potential and polarization as described in NACE SP0104-2014 and BS EN 14505:2005 raises concern if the instant-disconnect condition is a true representation of the CP conditions. Another common practice of not interrupting the CP current while measuring coupon-to-soil potentials is also a concern where closely coupled impressed current anode systems are installed adjacent to the buried piping systems. Alternate criteria are often necessary in situations where the polarized -850 mV CSE potential criterion is impractical because of the impact of the buried copper grounding system and other bare metallic structures. Coupons can be used to take advantage of the alternate criteria listed in NACE SP0169-2013 and BS EN 14505:2005. Although coupons provide valuable information in mixed-metal circuits the data obtained can increase confusion and raise additional questions on the protection status. This paper attempts to address some of the confusion and highlights important aspects when using coupons for buried piping in mixed-metal circuits. Keywords: coupons mixed-metal circuits cathodic protection piping criteria

The measurement and interpretation of cathodic protection (CP) data in plant (i.e. complex) facilities present inherent challenges where mixed metals are electrically continuous with the protected structure. Often there is no attempt to electrically isolate the on-plot buried steel piping networks from the facility for safety and practical considerations. Coupons can be used to assist in the evaluation of CP levels on buried steel piping in mixed-metal circuits. However the present industry practice of disconnecting the coupon from the mixed-metal circuit to measure the potential and polarization as described in NACE SP0104-2014 and BS EN 14505:2005 raises concern if the instant-disconnect condition is a true representation of the CP conditions. Another common practice of not interrupting the CP current while measuring coupon-to-soil potentials is also a concern where closely coupled impressed current anode systems are installed adjacent to the buried piping systems. Alternate criteria are often necessary in situations where the polarized -850 mV CSE potential criterion is impractical because of the impact of the buried copper grounding system and other bare metallic structures. Coupons can be used to take advantage of the alternate criteria listed in NACE SP0169-2013 and BS EN 14505:2005. Although coupons provide valuable information in mixed-metal circuits the data obtained can increase confusion and raise additional questions on the protection status. This paper attempts to address some of the confusion and highlights important aspects when using coupons for buried piping in mixed-metal circuits. Keywords: coupons mixed-metal circuits cathodic protection piping criteria

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