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ETL 1110-9-10(FR)
5 Jan 91
b. Computations.
1)
Calculate the external surface area of the pipe (A) to
be protected.
Unit Area*
Pipe diameter
Length
(sq ft/lin ft)
Area (sq ft)
1 1/4 in.
28,000 ft
0.434
12,152
4 in.
1,200 ft
1.178
1,414
6 in.
3,600 ft
1.734
6,242
*from table A-2
Total
19,808
2)
Determine current requirement (I) from equation 1-1:
I = (A)(I')(l.0-CE)
Where:
A
=
19,808 sq ft (Surface area of pipe to
be protected from previous calculation)
I' =
2 mA/sq ft (Current density, from item
4, paragraph 2-5a)
CE =
0.0 (Coating efficiency [bare pipe])
I
=
19,808 sq ft x 2 mA/sq ft x (1.0 - 0.0)
I
=
39,616 mA or 39.6 amp; use 40 amp
3)
Select an anode and calculate the number of anodes
required (N) to meet the design life.
For this
design, tubular anodes have been chosen.
The
groundbed will consist of a series of anodes attached
to a continuous header cable. A typical size tubular
ceramic anode used in deep anode beds, 1-in. diameter
by 39.4 in. long, is selected. The number of anodes
are determined using equation 1-2:
N=
I
IA
Where:
I=
40
amp
(Current requirement
from
previous calculation)
IA =
8.0 amp/anode (Current rating per anode
from table 3-3 [in coke breeze])
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