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ETL 1110-9-10(FR)
5 Jan 91
2-6.
Elevated Water Tank (Ice Is Expected).
Impressed current cathodic protection is designed for an elevated
water tank (figure 2-14). The tank is already built and current
requirement tests have been done. Anodes must not be suspended
from the tank roof because heavy ice (up to 2 ft thick) covers the
water surface during winter. The anode cables could not tolerate
this weight,
so another type of support must be used.
An
internally supported hoop shaped wire anode system is selected.
a.
Design data.
1)
Tank is a pedestal supported spheroid with a 10-in.
riser pipe. Only the bowl will be protected because
the riser pipe is less than 30 inches in diameter. For
riser pipes see section 2-7.
2)
Tank dimensions are:
Capacity = 400,000 gal
Diameter of bowl = 51 ft 6 in.
High water depth = 35 ft
Height of bowl above ground = 100 ft
3)
Water resistivity is 2000 ohm-cm.
4)
Design cathodic protection anodes for a 15-year life.
5)
Wire type ceramic anode will be used.
6)
Wetted surfaces are uncoated.
7)
Area above high water level is kept well coated.
8)
Tank is subject to freezing.
9)
The cathodic protection circuit resistance must not
exceed 2 ohms.
10) The available electrical power is 120/240 V AC, single
phase.
11) Based on structure current requirement testing
recently performed on this tank, the current required
for adequate cathodic protection is 25 amp. This high
current requirement indicates that the tank internal
coating is severely deteriorated.
b.
70
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