Cylindrical steel tanks


ABOVEGROUND STEEL TANKS

 

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Stiffening of membrane of single deck floating roof

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STEEL TANKS WITH FIXED ROOFS


Fixed roofs are used for tanks with low and middle storage volumes. The most of them are for tanks with diameter D from 6,0 m to 60,0 m but there are not any obstacles to be used for bigger diameters.
Supported cone roof of aboveground steel tank
Supported cone roofs under construction

Fixed roofs could be used in these cases:
a) of all kinds of products (crude oil, gasoline, benzene, fuel oil, water, beer, molasses, etc.);
b) when the temperatures are raised (asphalt, liquid silphur, etc.) by normal and lower temperatures (when liquefied gases are stored in the tank);
c) when there are serious loads from snow and ice there and when ambient conditions are very severe;
d) when requirements about internal hygiene in the tank are exacting. Then roof structure will be placed over the roof plates. Steel plates will be painted with special covers.

1. According to used materials fixed roofs are:
a) aluminum roofs - they begin to be more popular around the world. Their advantages are:
- decreased self weight. They are very light and beautiful structures;
- it is not necessary to be painted;
- aluminum roofs have better resistance to any products that are used in chemical industry;
- they could cover cylindrical tanks with diameter D up to 120,0 m.

b) steel roofs - the most predominate kind of roofs on the tanks around the world. Their advantages are:
- they have lower basic price;
- they are more convenient to prefabrication and mounting;
- the steel has bigger strength than aluminum and clear pronounced yield strength. When a element of structure is overloaded it yields, receives large displacement and re-distribute loads to another members of steel roof structure.

2. According to type (shape) of fixed roof:
a) cone roof;
b) dome roof - it is necessary radial girders to be bended ;
c) umbrela roof - dome roofs without steel rings;
d) hanging roofs;
e) fixed roofs (dome or cone) with internal floating roof.

Cone roof with internal pontoon
Cone roof with internal floating roof

The advantages of using of internal floating roofs are as follow:
- level of evaporation of storaged product is decreased;
- risk of fire is lower - there almost are not cases of fire in the world practice;
- internal floating roof could be aluminum. It has low height and storage capacity is increased;
- the internal floating roof is protected from ambient climatic conditions and they could be used in various earth regions;
- it is not necessary mounting of roof drain.

Dome roof with pontoon
Dome roof with internal floating roof

Tanks with cone or dome roofs and internal floating roofs have reduction loss of evaporation of the product. When the internal pressure is increased the loss is less.
Internal floating roofs could be:
- made of steel - they are similar to external floating roof, but roof drain is not necessary;
- made of aluminum - they are the most popular in the world;
- synthetic - they could not be used when fire is extinguished under layer product.


3. According to static scheme of the roof:
    a) self-supporting roofs - cone and dome roofs;
Self-supporting cone roof
Self-supporting cone roof

Self-supporting roofs, especially dome roof are very suitable when internal pressure is increased.
Latterly internal floating roofs can be mounted when fixed roofs is supported and there are columns inside. When the roof is self-supporting internal space of the tank is free and it is more convenient to install internal floating roof . Conditions of exploitation of internal floating roof is improved.

Advantage of cone roof in comparison with dome roof is that their radial girders are straight. They are not bended.




Sheme of dome roof of aboveground steel tank
Self-supporting dome roof

It is the most light and economical type of fixed roof.
It can be used when:
- the spans are very big;
- the internal pressure is increased;
- aluminum is used as material for construction;

Dome roof can be used by every kind of climatic conditions.

Roof plates usually are welded to supporting structure. According Bulgarian experience cone and dome roofs can be ensured with frangible joints. It can be achieved when part of the welds between cover plates and structures has decreased height.


b) supported ( fixed ) roofs - cone and hanging roofs


Supported cone roof
Supported cone roof
Supported cone roof is the most widespread type of fixed roof in the USA and Arabic world. It can be used for every kind of tank volumes and dimensions.
According to API650 roof plates could not be jointed to roof supporting structures. Plates have to be welded only to top angle. In this reason is certainly that joint between roof and shell is frangible. This type of joint is easy to tear in case of explosion in the tank.

Supported cone roofs are not widespread in Bulgaria, especially for tanks with big capacity.





Hanging roof
Hanging roof
Around the world tanks with hanging roofs are not widespread.
They have advantages as follow:
- roof cover plates work only on tension condition and they could be used sterling . Plates work in two directions;
- this type of roof is easy to prepare;
- hanging roof could cover big spans;
- they are very light.

The disadvantages are:
- hanging roofs are very flexible;
- they have complicated construction details;
- erection of the roof is complex and need experience;
- design of the hanging roofs is complicated and it is not covered by standards;
- hanging roofs could not be used when internal pressure is increased.


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