Generally,
the double bottoms of cylindrical steel tanks could be classified by following criteria:
a) according to used material for construction
for the second (complementary) bottom:
- from steel;
- from polymer material.
b) according to the kind of the system which detects
leaks:
- passive system - it means that there are
pipes under internal (upper) bottom, by which the product felt among the two bottoms, comes out to the
spot
determined for this purpose and easily accessible. The liquid movement
is due
to its own weight. On this reason detecting of leakage has delay;
- continuously activated vacuum system - pressure between the bottoms is decreased. The system observes without interruption
the increase of the pressure between the two bottoms. The discovery of
the
breakage on the bottom is immediate.
c) according to position of the
second (complementary) bottom:
- above the basic steel plates;
- under the basic steel plates.
If
it is necessary to change whole area of the bottom of tanks in service,
or new tanks will be erected, it is recommended to mount leak detection systems. They ensure
continuous control of entirety of the bottom and minimize likelihood of
soil pollution.
The systems for leaks detection are obligatory
for the tanks where ecologically dangerous products are stored. These systems must assure canal for outflow of
the liquid toward to the spot where the liquid will be found easily and
the
result can be shown out of the tank.
In American standard API Std. 650 are shown various decisions of double bottoms :
Various types of double bottoms of aboveground steel tanks
Contemporary
tendency
in Europe leads to mounting of additional (second) steel bottom and
active vacuum
system. Level
of pressure between steel bottoms is decreased with vacuum pump and it
is continuously monitored. When pressure going to be height it
is sign
that one of the bottoms is punctured.
This kind of
monitoring is one that guarantees uninterrupted control and immediate
reaction
if there are holes on the bottoms.
Joint between
upper bottom and shell has been made with welding to them of angle section with
low
thickness . The thickness of angle section should be equal to
thickness of
the upper bottom. This guarantees comparatively easy little movements
and bending
of angle section.
It is
recommended minimum thickness of upper bottom to be t = 6 mm.
Double steel bottom with active vacuum system
The
sheets of two bottom are separated with wire net putted between them. Diameter
of these rods is approximately 3 – 4 mm. The steel net prevents full
contact
between steel plates. On this reason is impossible creation of areas
where
movement of liquids and gases is not free.
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