Cathodic Protection Study 2
A sensitive voltmeter will measure the balance (voltage) between the potential of a reference
electrode and the potential of metal, but only when arranged in closed circuit.
A pipeline is a length of insulated metal submerged in an electrolyte. The direct electrical current passes through the electrolyte to complete the circuit of a corrosion reaction. Direct electrical current is impressed into the electrolyte by a cathodic protection system.
Voltages are measured between an electrode placed on the ground (in contact with the electrolyte) and a conductor in contact with the pipeline at a test post facility.metal.
An example of a typical 'pipe-to-soil potential' plot is shown on worksheet CPSTUDY1.XLS .
The 'equivalent circuit' of a pipeline with impressed cathodic protection is shown schematically on the snapshot of the spreadsheet below.
The negative terminal of the TR is connected to the pipeline metal by a low resistance insulated conductor.
OHMS LAW APPLIED TO SAMPLE READINGS
Ohms law states E = IR, that is, voltage is current times resistance.
It is, therefore, possible to calculate the current from a given voltage and resistance.
Cathodic protection 'potential' records consist of voltages recorded at intervals along the pipeline
Each voltage is measured with reference to a 'standard electrode' and are
presented as if the electrode is of constant value wherever it is placed. This means that the pipeline metal potential must vary by the value of the voltage ('E' in the formula E=IR )
The potential of the 'half-cell' is used as a datum when plotting the readings. The half-cell is used as the 'base potential' or zero.
An 'attenuation curve' is plotted to assess the current distribution along a section of pipeline.
It is blindingly obvious that the half-cell is not a reference potential when used in this way.
Steel pipelines are welded into continuous lengths which have a very low electrical
resistance, normally a fraction of an Ohm. For example, one mile of 24" dia, half inch
wall steel pipeline has a resistance of less than 0.001 Ohms.
Cathodic protection test posts are normally placed at 1 mile intervals on pipelines of this specification.
Assuming that the electrode has a constant potential of zero (the reference potential against which the
potential of the pipeline metal is measured), we can calculate the amount of
current which must be flowing through the pipeline to maintain the electrical status in the
cathodic protection records.
For example, the first voltage is recorded as 1.95 volts, the next is 1.85 volts, and that the
two are separated by 1 mile of 24" dia 1/2 wall steel pipeline with a resistance of 0.001
It is seen that the voltage difference between the two test positions is 0.1 volts and,
as the pipeline is a continuous conductor, this voltage would require 100 amps to be
passing through this section. The output of the transformer rectifier, serving a single
coated, pipeline is unlikely to reach this value.
No one with any sense would use this method to calculate the amount of current flowing in a pipeline.
CPSTUDY2.XLS calculates the current being applied down the pipeline from one test
post to the next, assuming that the half-cell is the datum. The user can replace the sample readings with those from their own
records. It will probably be found that the current will be calculated to run in both
directions along the pipeline, and this is beyond credibility.
Kirchoff's Law says that the current into a node equals the current out, so where is all this
current coming from or going to?
It is difficult to imagine how anyone has been able to justify such a concept and it is interesting
to hear the explanations offered.
This simple logic destroys the credibility of the 'half-cell' electrode being of constant value against which
we can measure the potential of the pipeline metal.
This is not to say that the half-cell is of no use. It's known internal reaction makes it a good method to contact the electrolyte.
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