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


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Following the errors occurred during the erection and continued exploitation, the membrane of single deck floating roof has been deformed. One part of the water does not reach roof drain, leaves on the roof and accelerates corrosion process. The installation of additional stiffening girders approximates the shape (the slope) of the membrane toward the projected one. These girders stabilize the roof in the case of wind blows and suction.

Radial girders on membrane of single deck floating roof
Radial girders on membrane of single deck floating roof

When the tank has the diameter D > 40 m the classical single deck floating roofs without any stiffening on the membrane is not recommended solution, because during the exploitation big deformations will appear in the membrane. Serious deviations from the projected form of the membrane are possible especially if they are situated in the area with increased wind speed or membrane is made by rolls method.
Purpose of using of the stiffening girders:
- decrease the vibrations and deformations in the central part of the roof;
- the installation of girders which are high enough and which are tightly connected to stiff ring in the center of roof, separates membrane on sections and it prevent the pouring of the product all over the membrane if there is any gap in the membrane. When this solution is preferred it is necessary every new formed section to have its own water drain which through the pipes under the roof is connected to the central roof water drain;
- when the girders have big stiffness and their  joints are rigid the work of the central part of the roof is changed. It is not the round hanging membrane any more and becomes round orthotropic plate. The complementary stiffening probably could be necessary if the girders and pontoons work together.

Radial stiffening girders

The external floating roof which membrane is stabilized by radial girders exist in Bulgaria, Netherlands, Germany, Austria and USA. The kind of the girders and their joints depend on the diameter of the tank. The kinds of the stiffening girders are:
- girders from steel sheets - it can be complementary cold rolled in order to become stiffer. It is used for the tanks which have smaller volumes;
- girders from  hot-rolled sections
- it is used angular, U-sections and I-sections. Their mounting is very convenient and easy;
- the steel sheets or the gilders with gaps in the web - they have increased constructive height and the anti corrosion defense become more difficult;

- girders with box section - they usually are used when the tanks have bigger dimensions. When the box section has space it is possible to decrease volume of the pontoons, because these girders enable roof to float when there is the liquid upon the roof . In other words they are complementary pontoons;
- the radial girders on the roof can have different positions. Some possible solutions
are shown on the figure bellow.

Radial stiffeners on membrane of single deck floating roof
 Location of the radial stiffening girders on the membrane

- girders which are not connected between them - this option is used when the girders are made from steel sheets. They can be connected to the pontoons but can be situated at several distance from them;
- girders connected through the central ring - when this solution is used almost always the girders are connected with the pontoons and work with them. The central ring can be formed as a cylinder toward which the girders are connected frontal
or to be cut as a flat disk which lay above the ribs. When this solution is used, the girders upon the membrane  are high enough and the central ring  has the shape of cylinder, independent sections can be formed;
- girders which are not connected and are dislocated between them - the girders stabilizing the membrane have the length = 0,25.Dr. Due to this construction roof becomes more stiff, but works as a hanging membrane. The forming of the independent sections above the membrane is impossible.

 Stiffening girders as rings
The stiffening girders as rings must be put under the membrane according to the following scheme:

Circular stiffeners on membrane

Location of the circular stiffening girders

Many tanks which volumes are V = 50 000 m3, which are stiffened with girders made as a rings, and the girders dimension are L100x100x110 mm, are constructed in Poland. When the measurements for determining the moving in the center of the membrane have been done this mode for stiffening showed very good effectiveness.
Circular stiffening girders are used in 7 steel tanks with V = 20 000 m3 capacity in PB "Polykraiste", Bulgaria, see picture bellow:

Membrane stiffening
Stiffening of membrane with circular girders

The disadvantage of this kind of stiffening is that the girders must be put under the membrane. It causes the following consequences:
- all joints made on site must be done in upper condition – the welders are below ceiling when the welding joint must be done;
- it is impossible to form smaller sectors on the membrane.

Radial and circular stiffening girders

When the radial and circular stiffening ribs are used the consumption of the metal and labor is bigger comparing with the other methods for membrane stiffening.

Research of the influence of the girders upon the deformation of roof membrane.
The author made 3D computer models of roof membrane and stiffening girders and after that he made science research and analysis.
Conclusions from the obtained results:
- when the membrane is not stiffened with diameter D ≥ 40 m the deviation from loading and deformation are considerable and the complementary stiffening of the construction is necessary. When the stiffening girders are put correctly the deformations can be decreased two times;
- decreasing of angle between the radial put girders (increasing of their density) leads to the stiffening of the membrane and decrease of the deformation in it;
- when the stiffening girders are not connected to pontoons and when joints between pontoons and girders are flexible, the deviation from loading of the membrane is the same. More favorable for the mounting is to put the girders which are not connected to the pontoons, with which the stiffness which is achieved is approximately the same as with the flexible joint ribs;
- rigid joint between the stiffening girders and the pontoons decrease the deviation from loading of the membrane. This is better expressed when the tank diameters are bigger. In other part the rigid joint between the girders and the pontoons changes their static scheme and tensioned status. So if the pontoons are not prepared for complementary bending from these additional girders it is better to avoid the rigid joints to the pontoons;
- the putting of central stiffening ring, joints all radial girders is more favorable for deviation from loading of the membrane then the rigid joint of this sections to the pontoons;
- the circular stiffening girders under the membrane decrease its deviation from loading but more material is used and the weld joints which must be done are on the ceiling.   

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