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ETL 1110-9-10(FR)
5 Jan 91
2-3.
Underground Storage Tanks (USTs).
The service station shown in figure 2-5 has three existing
underground tanks and associated pipe. The quality of the coating
is unknown and it is not feasible to install dielectric insulation
to isolate the UST system.
Because of the anticipated large
current requirement, an impressed current protection system is
chosen.
To distribute the current evenly around the tanks and
piping, and to minimize interference effects on other structures,
a distributed anode surface bed using vertical anodes is selected.
Vertical anodes can be installed with relative ease in holes cored
through the paving around the UST system. Wiring can be installed
several inches below the paving by cutting and hand excavating
narrow slots/trenchs through the paving.
a. Design data.
1)
Soil resistivity is 4500 ohm-cm.
2)
Pipe is 2 in., nominal size.
Total length of all
buried piping is 750 ft.
3)
Tanks are 8000 gal, 96 in. diameter by 21 ft 3 in.
long.
4)
Electrical continuity of tanks and piping has been
assured.
5)
It is not feasible to install dielectric insulation;
system is therefore not isolated electrically from
other structures.
6)
Design cathodic protection anodes for 20-year life.
7)
Coating quality is unknown, assume bare.
8)
The cathodic protection system
circuit
should not exceed 2.5 ohms.
9)
Electric power is available at 120 V single phase in
the station building.
10)
Current requirement test indicates that 8.2 amperes
are needed for cathodic protection.
b. Computations.
1)
Find the external surface area (A) of the storage
tanks and piping.
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