ETL 1110-9-10(FR)

5 Jan 91

b. Computations.

1)

Find the surface area to be protected.

Area of each tank bottom = B r2:

A = B x 37.52 = 4418 sq ft

Area of the four bottoms:

A = 4 x 4418 = 17,672 sq ft

2)

Determine the current requirement (I) from equation

1-1:

I = (A) (I')(l.0-CE)

Where:

A

=

17,672 sq ft (Total surface area from

previous calculation)

I'

=

2 mA/sq ft (Current density from item

4 of paragraph 2-4a)

CE

=

0.0 (bare) (Coating efficiency from

item 8 of paragraph 2-4a)

I

=

17,672 sq ft x 2 mA/sq ft x (1.0 -

0.0)

I

=

35 amp

3)

Select an anode and calculate the number of anodes (N)

required to meet the design life.

The deep anode

groundbed will consist of a series of rods connected to

a continuous header cable. For physical strength, the

1/8-in. or 1/4-in. diameter rods are usually chosen. To

ensure good transmission of current in to the backfill

column, long anodes, typically 6 ft or 8 ft are used.

For this example, 1/4-in. diameter by 72-in. long rods

have been chosen. Using equation 1-2:

N=

I

IA

Where:

I = 35

amp

(Current

requirement

from

previous calculation)

IA = 6.6 amp/anode (Current rating per heavy

duty coated anode from table 3-3)

N=

35

= 5.3; use 6 anodes

6.6

46

Integrated Publishing, Inc. |