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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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