0041im.jpg)
ETL 1110-9-10(FR)
5 Jan 91
2-4.
On-Grade Tank Bottoms.
Four on-grade fuel oil storage tanks are to be constructed with the
configuration shown on figure 2-6. This design may be prohibited
if secondary containment uses a nonconductive membrane beneath the
tanks.
The membrane would not allow the cathodic protection
current to flow from the remotely located deep anode through the
nonconductive membrane to the tank bottoms.
If this situation
exists, a distributed anode design with the anodes located between
the membrane and the tank bottoms would have to be used.
All
piping will be above grade. To minimize the extent of underground
cable, it was decided to use a deep anode groundbed, located just
outside the spill containment dikes. (Note: Some county, state,
or federal agencies such as the EPA may have regulations that
affect the use of deep anode beds because they can provide a
conduit for the mixing of water between aquifer levels. In such
cases, regulations have sometimes required cementing of the annulus
between the deep anode bed casing and the augered hole to prevent
this water migration. The system designer should check with the
applicable agencies before committing to a deep anode design.
Figures 2-7, 2-7A, 2-8, and 2-8A illustrate a typical deep anode
groundbed using ceramic rod anodes. The tank bottoms will be bare.
All piping will be above ground. The tanks will be dielectrically
insulated from the structures. Field tests were made at the site
and the subsurface geology was determined to be suitable for a deep
anode groundbed (reference 25).
a. Design data.
1)
Soil resistivity at anode depth is 1500 ohm-cm.
2)
Tanks are 75 ft in diameter.
3)
Design cathodic protection anodes for a 15-year life.
4)
Design current density is 2 A per sq ft of tank bottom.
5)
Since the tanks are electrically isolated from each
other, intertank bonds will be required.
6)
The cathodic protection system
circuit
should not exceed 0.75 ohm.
7)
Electric power is available at a switch rack in an
unclassified (nonexplosion proof) area 125 ft from the
desired groundbed location, 230 V AC, single phase.
39
Previous Page
