Amperes = 45.7 x 0.3597 x 5/100 x 11/1,000
= 0.009 ampere
Amperes = length of pipe (ft) x pipe
circumference (ft)
circumference (ft)
x
percent bare metal
100
x
ma / square meter
1000
,
For the example of 150 ff of pipe in
1,000-ohm-centimeter soil:
1,000-ohm-centimeter soil:
Amperes = 150 x 1.18 x 5/100 × 1/1000
= 0.009 ampere
= 0.009 ampere
B. Number of anodes equal to total current
required times the installation life divided by
the ampere-hour rate of the magnesium
anodes.
required times the installation life divided by
the ampere-hour rate of the magnesium
anodes.
Number of anodes =
current required (amperes x installation life (hr
ampere-hour rating per anode
The ampere-hour rating varies with different
conditions but 0.45 kg (1 lb) of magnesium
can be rated at about 500 ampere-hours.
Thus, a 3.6-kg (8-lb) magnesium anode
would be expected to deliver about 4,000
ampere-hours.
conditions but 0.45 kg (1 lb) of magnesium
can be rated at about 500 ampere-hours.
Thus, a 3.6-kg (8-lb) magnesium anode
would be expected to deliver about 4,000
ampere-hours.
Number of 3.6-kg (8 Ib) anodes required =
0.009 amperes x
x
x 10 yrs
365 days
year
24 hrs
day
400 ampere-hr per 3.63-kg (8-lb) anode
In this case, a single 3.6-kg (8-lb)
magnesium anode would be used with an
indicated useful life of 50 years.
magnesium anode would be used with an
indicated useful life of 50 years.
Problem 2: Determine the sacrificial anode
requirement to protect four bare steel transmission
tower footings in 3000 ohm-centimeter soil. Given
exposed area of each footing as 9.3 m
requirement to protect four bare steel transmission
tower footings in 3000 ohm-centimeter soil. Given
exposed area of each footing as 9.3 m
2
(100 ft
2
).
Required Data
A. Soil resistivity
B. Current demand.
C. Number of anodes required.
Solution
A. Total area to be protected 37.2 m
2
(400 ft
2
).
B. Soil survey shows 3000-ohm-
centimeter soil.
centimeter soil.
C. Assume 11-mA/m
2
(1-mA/ft
2
)
current density requirement.
D. Protective current required = area
to be protected x current density
required
to be protected x current density
required
= 37.2 m
2
x 11 mA/m
2
= 0.4 ampere =
400 ft
2
x 1 mA/ft
2
= 0.4 ampere
E. From the previous problem, 0.45
kg (1 lb) of magnesium will give about
500 ampere-hours or a 7.7-kg (17 lb)
anode will yield about 8500 ampere-
hours. Desired life of the anode
installation is 10 years.
kg (1 lb) of magnesium will give about
500 ampere-hours or a 7.7-kg (17 lb)
anode will yield about 8500 ampere-
hours. Desired life of the anode
installation is 10 years.
Number of 77. kg (17 Ib) anodes required =
0.4 amperes x
365 days
year
x
24 hrs
day
x 10 yrs
8500 ampere hrperanode
4.1
-
=
However, in 3000-ohm-centimeter soil,
a 7.7.-kg (17- lb) anode will only deliver
about 0.028 A (fig. 2); thus,
a 7.7.-kg (17- lb) anode will only deliver
about 0.028 A (fig. 2); thus,
0.4 amperes
0.028 amperes / anode
14.3
=
Therefore, 15 anodes would be required to
completely protect the structure. Installation
life, however, would be raised to over 30
years, determined as follows:
completely protect the structure. Installation
life, however, would be raised to over 30
years, determined as follows:
(FIST 4-. 5)
10