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A totally enclosed outer shell may be added
resulting in a double wall construction. Both the
inner and the outer tanks are vented separately
and are pressure tested at 5 psi.
When "double wall" enclosing a U/L Listed day
tank, an additional emergency vent is added to
the outer tank the same size as the inner tank.
(The U/L Listing is "Secondary Containment
Aboveground Tank for Flammable Liquids",
File MH12807.)
Day Tanks with a capacity of 25 gallons and more
have all threaded connections located on the tank
top (except the overflow, drains, and the double
wall vent). The inner tank drain is piped through
the outer shell.
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