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CP BLOG Continued from page 31
nuity testing should be performed to minimize chances of interference. Careful consideration must given to buried infra- structure, such as ductile iron piping with unbonded joints, isolated metallic storm culverts, and foreign pipelines adjacent to WZ QVKWUQVO \W \PM [Q\M 1 _W]TL LMÅVQ\MTa rule out the 100 mV shift criterion, as dis- similar metals are likely to be involved.
tial of the metal more anodic of the coupling.
A
Potential reading and test hole location
Q
In our plant complex, we have a huge under-
ground piping network. For the past 15 years, this net-
work has been protected by an impressed
In the case where several metals are in electrical contact in an electrolyte, the protection poten- tial must be the protection poten-
current cathodic protection (ICCP) system. Our normal monitoring procedure is po- tential measurement through a number of test holes randomly distributed over the plant. There are no special cables con- nected to each pipeline for each test hole to measure the potential by a copper/ copper sulfate (Cu/CuSO4
) reference elec-
trode (CSE). We connect the voltmeter to the exposed grounding network cables that are connected to the steel structures and all underground piping. I have the following
questions about this system: z Is using the exposed grounding cables effective for obtaining the CP potential measurements on the piping, and does this method give the actual data?
z If the test hole is located between two underground pipelines, what potential reading will we get—the actual for each, the average, or something else?
z What is the extent of the IR drop problem in this case?
z If I connect one terminal of the
voltmeter to one grounding cable, can I measure the potential in sev- eral test holes around that ground- ing cable because I know that all the underground piping is connected to the grounding network?
A Following are my answers to
these questions: z Yes, this method is effective because the grounding grid is
connected to the underground piping network and belongs to the same protected structures.
z and z You will obtain the average of the
readings, but you must be careful because of the distance between where you put the half-cell and where the pipes are located. If the distance is too large, you will measure the pipe potential plus an IR component because the soil is between the half-cell and the pipes. Remember that placing the half-cell closer to the pipe gives the best pipe- to-soil (P/S) potential.
32 MATERIALS PERFORMANCE April 2012
NACE International, Vol. 51, No. 4