ETL 1110-9-10(FR)

5 Jan 91

be seen that stub anodes are required for this

design. For a two-ring anode design (main and

one-stub anode ring), which is usually sufficient

for tanks up to 1 million gal storage capacity,

the 4-ft long stub anodes are located on a radius

one-fourth of the bowl radius, or 7 ft (28 ft x

0.25 = 7 ft). Typically, there will be about one

half as many stub anodes (two ring design) as

there are main anodes so we will plan for five 4-

ft long stub anodes on a 7-ft radius.

The

outside radius of the area to be protected by the

stub anodes is approximately 13 ft and the inside

radius is 2.5 ft (riser radius). The stub anodes

are thus located on an 7-ft radius to place them

in the center of the area to be protected. (Note:

For smaller diameter tanks, stub anodes may not

be required.)

c)

Find the current division between main and stub

anodes.

(1)

The area of tank bottom protected by stub

anodes (As) is found by equation 2-6 (see

figure 2-20):

ASB = B (r22 - r12)

(eq 2-6)

Where:

r1 =

2.5 ft (Riser radius)

r2 =

13 ft (Radius of protected segment.)

This is based on the fact that the

main anode string is 6 ft from the

tank wall and that the anode will

protect another 9 ft (1.5 x 6 ft in

toward the center on the tank bottom

due to arc shape of the tank

bottom). Outside radius of the area

to be protected by the stub anodes

is, therefore:

28 ft (tank radius) - 6 ft - 9 ft = 13 ft

3.1416 [(13 ft)2 - (2.5 ft)2]

ASB =

ASB =

3.1416 x 162.75

ASB =

511.3 sq ft; use 512 sq ft as the area to

be protected by the stub anodes.

92

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