When surveying a coated pipeline system, data on the electrical strength of the coating and on current requirements for CP can be taken concurrently. If the coating is in rea-sonably good condition, current requirements are much smaller than on bare lines. This makes it possible to test many miles of pipeline with one test set-up and a modest power supply. Batteries are usually sufficient.
Although the same coating specification may be used throughout the length of a given pipeline, the effective electrical strength of that coating (in terms of its ability to resist the flow of current) may vary considerably along the route. Variations may be due to the type of terrain, construction difficulties (rocky pipe-laying conditions may result in many more accidentally damaged areas), changes in average soil resisti-vity, and different degrees of quality of pipeline construction work and inspection. In any event, knowledge of areas where abnormally low coating resistances are preva-lent on a given line will assist the corrosion engineer in planning the corrosion control system.
TEMPORARY DETERMINED FOR THIS SECTION OF PIPELINE TEST POINT
Figure 5.16 Coating resistance and CP current requirement tests.
When making the combined survey on a coated line, a test arrangement may be used as illustrated byFigure 5.16. At the test battery location, a current interrupter (see Chapter 6) is used to automatically switch the current source on and off at a convenient time inter-val (such as 10 s on and 5 s off). This way the data needed for coating resistance calcula-tions are obtained. At the same time this procedure assures the test engineer at remote lo-cations that the battery installation is still operating properly as long as the potential and line current measurements continue to change in accord with the established on–off cycle.
On a reasonably well-coated pipeline, test data taken at intervals of, typically, 3 to 5 miles will give satisfactory information on the average coating resistance within each section tested. Testing section by section can be continued in each direction from the temporary CP location until the changes in the observed currents and potentials (as the current interrupter switches on and off) are no longer large enough to result in accurate data. The limits of the area that can be maintained above the protected ctiterion of−0.85 V or better will be established at this same time.
Coated pipelines polarize very rapidly. The better the coating, the faster the polariza-tion. This means that conditions stabilize within the first few minutes (and sometimes in a matter of seconds) after the test current is applied.
On coated pipeline systems provided with test points for potential and line current measurement, a survey will proceed rapidly. Data may be taken with reasonable accu-racy by a single test engineer. For maximum accuaccu-racy, however, two engineers in radio communication can observe data simultaneously at each end of each section tested. This becomes essential if the pipeline under test is affected by stray current. Strays may make it necessary to take a series of simultaneous readings and average them to obtain usable data.
To obtain data for calculations of coating resistance, readings are taken along the pipeline to a copper sulfate electrode with the interrupter on and off, and pipeline cur-rent is measured with the interrupter on and off at each end of each line section. From these readings one can determine the change in pipe potential (1V) and the change in line current (1I ) at each end of the test section. The difference of the two 1I values will be equal to the test battery current collected by the line section when the current interrupter is switched on. The average of the two1V values will be the average change in pipeline potential within the test section caused by the battery current collected. The average1V in millivolts, divided by the current collected in milliamperes, will give the resistance to earth, in ohms, of the pipeline section tested. From the length and diame-ter of pipe in the section tested, its total surface area in square feet may be calculated.
Multiplying the pipe-section-to-earth resistance by the area in square feet will result in a value of ohms per average square foot, the effective coating resistance for the section tested. Some workers express coating condition in terms of conductivity (in mhos or micromhos). This is simply a matter of conversion. The reciprocal of the resistance per average square foot is the conductivity in mhos. The reciprocal of the resistance per aver-age square foot is the conductivity in mhos. The reciprocal times 106is the conductivity in micromhos.
Here is an example of data and its treatment as described in preceding paragraph.
Referring toFigure 5.16, assume that the section between test points 1 and 2 is under test and that this section consists of 15,000 ft of coated pipeline having a total external pipe surface area of 50,070 ft2. Test data taken at test point 1 are as follows:
Pipe to CuSO4 = −1.75 volts, ON and −0.89 volts, OFF 1V = −0.86 volt
Pipe span potential drop = +0.98 MV, ON and +0.04 MV, OFF
With span calibrated at 2.30 amps per mV (discussed earlier in this chapter.), Pipeline current= +2.25 amps, ON and +0.09 amps, OFF
1I = 2.16 amps
Test data taken at test point 2 are
Pipe to CuSO4 = −1.70 volts, ON and −0.88 volts, OFF 1V = −0.82 volt
Pipe span potential drop = +0.84 MV, ON and −0.02 MV, OFF With span calibrated at 2.41 amps per MV,
Pipeline current= +2.03 amps, ON and −0.05 amps, OFF (negative off currents indicate current flow in opposite direction)
1I = 2.08 amps
Calculation of coating resistance is therefore:
Average1V = (−0.86 + −0.82) ÷ 2 = −0.84 volt Current collected = 2.16 − 2.08 = 0.08 amp
Pipe to earth resistance = 0.84 V ÷ 0.08 A = 10.5 ohms Effective coating resistance = 10.5 ohms × 50, 070
= 526,000 ohms for an average square foot (approx.) (ohms-ft2)
Note that the average soil resistivity has an effect on the effective coating resistance measurement. In part this is because the apparent pipeline resistance to remote earth measure in the procedure described is a combination of the coating resistance and the resistance to remote earth of the pipeline itself. In the example given, if we assume that the test section was in 1000 ohm-cm soil, the resistance to earth of the 15,000 ft of 12-in-diameter line, if bare, would be in the order of 0.0062 ohm. If the average soil resistivity were 100,000 ohm-cm, this resistance would be 0.62 ohm. If the difference,∼0.6 ohm, is added to the 10.5 ohms of pipe-to-earth resistance calculated in the example, the new total of 11.1 ohms, multiplied by the 50,070 ft2 surface area, would give an indicated effective coating resistance of 606,000 ohms per average square foot.
Actually, however, the resistance to earth of exposed steel at coating defects may have a much greater effect on the apparent coating resistance with variation in soil resistivity. Using the example again, if the coating were perfect (1013ohm-cm resistivity), the resistance of the pipeline section to remote earth would be in the order of 50,000 ohms—whereas the measured value was only 10.5 ohms. Now if we assume that the 10.5 ohms (in 1000 ohm-cm soil) is primarily the resistance to earth at pinholes distributed along the 15,000 ft section, this resistance will vary in approximate proportion to the soil resistivity. The resistance in 100,000 ohm-cm soil, then, would be in the order of (100,000/1,000) × 10.5 or 1050 ohms × 50,070 ft2, or 52.5 × 106 ohms for an average square foot (ohms-ft2). This relationship is not rigorous and depends on the relative size and spacing of coating defects as well as the ratio between the section resistance with perfect coating and that as actually measured (when the ratio is high, as in this case, pinhole resistance prevails). This does, however, demonstrate the effect that soil resistivity can have on apparent coating resistance. In particular, it shows that something must be known about the soil resistivity when evaluating a section of pipeline coating.
The method described for obtaining an approximation of effective coating resistance depends, for accuracy, on the precision with which field data are taken. The potential mea-surements pose no particular problem (unless erratic stray current effects are present), but the line current measurements are another matter. Unless current-measuring test points of a type that can be calibrated are permanently installed, errors in span length or variations in pipe span resistance may make calculated currents erroneous. Also, as was demonstrated earlier, it may not be possible to detect small currents or current differ-ences that are below the sensitivity range of the millivoltmeter being used. Nevertheless, as long as these limitation are recognized, the procedure is fully practical in establishing relative coating quality from section to section.
An initial coating resistance profile along a new pipeline will serve as a reference to which similar data taken in later years may be compared. Such comparisons reveal information on the long-term performance of the coating. For example, detrimental effects caused by such things as high pipeline operating temperature, areas of abnormal soil stress, areas subject to a high degree of bacterial activity, or any other condition that may affect the coating.