CATHODIC PROTECTION COUPON USE FOR BURIED PIPING IN PLANT (I.E., COMPLEX) FACILITIES

摘要

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 millivolt (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 issues and offers a method on how coupons can be used to evaluate the 100 mV polarization criterion in mixed-metal circuits.