Page 226 - Pipeline Rules of Thumb Handbook

Corrosion/Coatings 213

How can the life of magnesium anodes be predicted?

Multiply the anode weight in lb by 57, and divide by the The projected life is 11.7 years.
current output in milliamperes. The rule is based on 50% current efficiency, and on sta-
bilized conditions. If more anodes are to be added, or if
Example. A 17-lb anode has a current output of 83 mil- time has not been allowed for polarization (one to four
liamperes. What is its projected life? weeks), then the current output will probably fall off, and
the actual life will be 10 to 40% longer than indicated by
17 ¥ 57/83 = 11.7 the rule.

How to find the voltage rating of a rectifier if it is to deliver a given amount of current through a given
ground bed (graphite or carbon)

Multiply the desired current, in amperes, by the total resis- ( 20 ¥ 0 9)+= 20 volts
.
2
tance of the ground bed, in ohm (either calculated or mea-
sured); add two volts, to overcome the galvanic difference Specify the next larger size, which will probably be (depend-
between graphite and protected steel. ing upon the manufacturer) 24 volts.

Note. Where the resistance is measured rather than cal-
culated, and is the total loop resistance between pipe and
Example. A current of 20 amperes is desired from a ground bed, this method is quite precise; for calculated
ground bed whose resistance is 0.9 ohm. values, caution should be observed.

Determining current requirements for coated lines

Survey along the line after a polarization run of three or
four hours, and locate the two points whose pipe-to-soil
potentials are 1.0 volt and 0.8 volt (see Figure 1). 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 line if the section chosen is truly representative; judgment
per mile. Naturally this figure can only be used for an entire must be used at this point.

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 line current can then be adjusted to find the desired quantity;
have the exact values of 1.0 and 0.8, then readings can be this is illustrated in Figure 1; readings were taken at points
taken which approximate these values, and both potential and “A” and “B,” and, by assuming a static potential of 0.6 volt and