This design solution is applied when there is little space there for
the
facility or when the requirements for environmental safety are
increased.
The advantages of the tanks with double shell are as follow:
- smaller space occupied by the tank and catching basin;
- smaller surface which must be extinguished
during fire;
- in case of the incident it is granted that
stored product does not penetrate into the soil.
Disadvantages:
- the price of the facility is higher;
- it is necessary to construct complementary
foundation or to enlarge the existing one, upon which the steel
catching shell will
be erected;
- the anchors of the tank passed through the
bottom between the main shell and catching shell, and it impedes the
hermetical
seal of the tank.
Aboveground steel tank with double shell
Constructive characteristics of the tanks with steel catching basin:
- the distance between two shells should be
approximately 1,8 - 2,5 m for free movement of the personnel. The minimal
distance is 1,5m;
- the height of the second shell should be not
less from 80 % height of the shell of the main tank;
- it is necessary that the space, limited
by the catching basin is
big enough to contain the whole stored product and the maximum level
reached by
the liquid must be not higher then 1,0 m from the top angle of the second
shell;
- compensators should
be put on all pipelines in order to neutralize the influence of the
movements
of the shells under loading;
- the steel catching basin is usually designed
and constructed as open top tank. The space between two shells is open
for
atmosphere influence. It is necessary that the intermediate bottom to
be
constructed with slope toward the new erected drain system for drain of
atmosphere waters;
- it is possible to put the protective roof above
the intermediate space. Then the reliable ventilation must be assured
through the
vents or
the joint between second shell and its roof
must not be tightly;
- the shells of the main tank and steel catching
basin are put on its own annular bottom plate, which is calculated
according to
the corresponding standards. It is recommended construction of double
bottom of
the tank with continuously monitoring vacuum system.
According
to German standard DIN 4119, the minimal thickness of the steel catching shell are as follow:
| № |
Diameter D of tank,
m
|
Minimum thickness tmin
,
mm
|
1
|
D ≤ 15 |
5
|
2
|
15 < D ≤ 30 |
6
|
3
|
30 < D ≤ 45 |
7
|
4
|
45 < D ≤ 60 |
8
|
5
|
60 < D ≤ 75 |
9
|
6
|
75 < D ≤ 90 |
10
|
7
|
90 < D ≤ 105 |
11 |
Steel
protective catching basin will be
designed for loading from:
- loading from product
The open top steel catching basin is loaded
only be stored there liquid, i. e. the overloading with overpressure is
not
included. The formula for determining necessary shell
thickness will be the following one:
where:
D is nominal
diameter of the tank, m;
H – height of
maximum level of filling up to the
lower edge the measured course, m;
G – calculated
specific weight of the stored
product, t/m3 ;
Ry –
design value of yield strength of the material, MPa;
γc
– coefficient of condition of work of the steel catching
basin from the dynamic
loading from
the product spilling inside.
The only one unknown value in the above
mentioned
formula is coefficient γc . It is
necessary to pay attention to the following factors:
a) if the shell is collapsed, the stored product
rush violently in the tank. The pressure in of the liquid in the
catching basin
has hydrostatic and hydrodynamic components which depend on mode of
destroying
of the internal (basic) shell;
b) the loading on the protective catching basin (if there would be any)
would last a short time, and
because of it for coefficient γc could
be accepted higher values;
c) if the spilling product is in fire, the catching
basin will be thermally loaded.
In the examined from author German projects of steel tanks, the values for coefficient of condition of work γc
are increased with 25% comparing with of the main tank.
- wind loading
Wind loading on cylindrical tanks with steel catching basin
The figure above shows that the steel catching
basin complementary
decreases the wind loading upon the main tank. This is favorable for
thinned by
the corrosion shell.
- loading by earthquake
The
space between the outside shell and the main
tank is filled in the case of accident and the loading would take short
time.
It is unlikely catching basin to be loaded by spilled product and
earthquake on same time. On the other hand the protective catching
basin must be enough reliable
in
order to bear the earthquake impact, when it is filled with liquid.
The
design for
earthquake loading should be done in two main conditions:
1. The main tank is full with liquid up to its maximum level and the catching
basin is empty;
2. There is the liquid in the main tank and in the
catching basin. The level of liquid is in
hydrostatic equilibrium.
The main characteristic of calculations of
the catching basin in the condition of earthquake is that there is
internal tank
there. The tank bears all seismic waves exerting the hydrodynamic
influence
upon the steel basin.
The distance between the two shells is little
and the generated waves are small. Their influence could be neglected.
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