Determining current requirements for coated lines including when pipe-to-soil potential values are estimated
To determining current requirements for coated lines:
Start survey along the line after a polarization run of three or four hours, and locate the two points whose pipe-to-soil (P/S) potentials are 1.0 volt and 0.8 volt (see Figure 1 below). The section of line lying between these two points will then be at an average potential of about 0.9 volt, which is approximately the average of a line protected with distributed magnesium anodes. If, then, the line current is measured at each of these two points, the difference between these two line currents will be the total amount of current picked up on the line in the section. This quantity, divided by the length of the section, will then give the current requirements of the line, in amperes per mile. Naturally this figure can only be used for an entire line if the section chosen is truly representative; judgment must be used at this point.
Figure 1 - Current is drained from the line at A, by a welding machine or other source of direct current. Points B and C, having P/S potentials of 1.0 volt and 0.8 volt respectively, are located; the average P/S between them is 0.9 volt. The current required to bring L miles of line to 0.9 volt is then IB -IC, and the average current required for the line is (IB -IC)/L, in amperes per mile.
To determining current requirements for coated lines when pipe-to-soil potential values are estimated:
If it is not possible or convenient to find the points which have the exact values of 1.0 and 0.8, then readings can be taken which approximate these values, and both potential and line current can then be adjusted to find the desired quantity as illustrated in Figure 2 below; readings were taken at points "A" and "B," and, by assuming a static potential of 0.6 volt and linear attenuation, it is determined that points "c" and "D" have the values of 1.0 and 0.8; since line current varies according to the same laws as potential, it is possible to determine the current values at the two latter points, and so to arrive at the desired quantity.
It is not to be expected that the estimate thus made will be highly accurate; it is intended to be merely preliminary.
Figure 2 - Measurements of P/S potential and of Line Current are taken at A and B; it is then assumed (in the absence of other information) that the static potential of the line was originally at 0.6 volt. On this basis, the P/S and current curves are drawn as straight lines on semi-logarithmic paper, and the points whose P/S are 1.0 and 0.8 are located, at C and D. The difference between the current values for these two points then gives the current demand of the line for the distance L.