Cylindrical steel tanks



Types of steel tanks

Stiffening of membrane of single deck floating roof

Methods of erection

Typical damages of aboveground steel tanks

Inspection of steel tanks

Realized projects of tanks

Typical drawings of tanks


About the author

first page


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.

- 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.

Cylindrical steel tank with double shell
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,  


Minimum thickness tmin ,
D 15 5
 15 < D ≤ 30 6
30 < D ≤ 45 7
45 < D ≤ 60 8
60 < D ≤ 75 9
75 < D ≤ 90 10
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:
Formula of shell thickness

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 γcould 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

Sheme of wind loading of double shell

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.

main EN page