Page 225 - Pipeline Rules of Thumb Handbook

212 Pipeline Rules of Thumb Handbook

Install test leads at 162ft spacing. to be had, but if the above rule is used, the spacing will not
The rule works for welded lines of grade B seamless; vari- be too small for satisfactory measurement, or too great for
ations in steel may introduce errors as great as 15%. It is convenience in installation.
always better to calibrate a line section if accurate results are

How many magnesium anodes are needed for supplementary protection to a short-circuited bare
casing?

For 17lb anodes, use A r/50,000 to give the number How many 17-lb anodes would be required for supplemen-
needed for the casing alone; A is the area of the casing, in sq tary protection?
ft, and r is the soil resistivity in ohm-centimeters. The life of
Ar
the anodes may be estimated from r/180. If the life in years, Number of Anodes =
,
as given by this expression, is too short, refigure for 32-lb 50 000
anodes, using A r/55,000 for the number and r/100 for the
2
.
life. If this is still too short, figure for 50-lb anodes; number A =¥ 3 14 ¥ 45 = 283
is A r/60,000 and life is r/65.
Such an installation, grouped on both sides of the line at (284 )(1 100, )
Number of Anodes =
,
each end of the casing, will enable the main line protection 50 000
unit to carry beyond the casing; the values obtained above are
.
approximate only, and should be checked by potential mea- = 621
surements after installation.
Place three anodes at each end of casing. Their life
expectancy will be
,
Example. Suppose the pipeline has shorted out to 45ft of 1 100
= 6 years
24-in. bare casing. Soil resistivity is 1,100 ohm-centimeters. 180

Group installation of sacrificial anodes

When anodes are installed in groups, a spacing correction
factor must be taken into account to determine the current
output. The chart below shows the correction factor for
multiple anode installations.
Example. Single anode 8-in. by 60-in. in 1,500 ohm soil.

Resistance = 4.849 ohm
Potential = 0.85 volts
.
E 17 5 - 0 85
.
.
Current output = = = 0 185 amps
.
R 4 849
Current output of 5 anodes on 15ft spacing under above
conditions.
4 849
.
Combined resistance of group = ¥ 122
.
5
Correction factor = 1.17 ohm
-
1.75 0.85
.
Total current output = = 0 769 amps
1.17