TRADITIONAL, AUTOMATIC fixed FOAM flooding EQUIPMENT
OF FLAMMABLE LIQUID TANKS |
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The eventual explosion of large over-ground vertical cylindrical tanks in the dome space, while containing liquids with enhanced risk of fire and explosion, the fire ensuing as well as the several millions of litres of chemicals discharged into the environment represents a great environmental disaster potentially not for the storage tanks only, but for the region around as well. The intensive technical development of the past 5 years, characterised by the increase of dimensions of tanks and the new storage and material handling closed technology, increased this potential danger by orders of magnitude. Also in our country we had to adjust to the international trend with the introduction of the automatic stationary extinguishing equipment. |
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Traditional, domestic application – semi-stationary EXTINGUISHING SYSTEM |
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Most frequently the fire safety of vertical cylindrical tanks containing flammable liquids was provided with traditional semi-stationary foam extinguishing systems.
The basis of the extinguishing system is a pipeline network integrated with foam jet pipe and dry pipeline mounted into a buttress. With this the traditional method could be applied by connecting a fire engine to the pipe controlling the fire and introducing foam of appropriate in-mixing ratio into the protection pit.
The task of the semi-stationary foam-extinguishing system is to flood the burning tank and its protection pit with foam after connecting the foam extinguishing engine of the fire brigade to the system following the start and detection of the fire. |
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The fire engine, with its integrated pump assures the feeding of the mixture of foam generating substance and water necessary for the operation of the system (foam solution) with the appropriate pressure.
The rapid detection of fire and the exact definition of the fire focus point are of primary significance in controlling the fire, together with the rapid deployment of the fire brigade unit. |
design of the storage tanks |
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In Hungary the protection against explosion of equipment defined as over-ground vertical cylindrical steel tank with rigid roof for the storage of “combustible liquids and melt” is regulated by Chapter IV of the Decree No. 2/2002. (I. 23.) BM (1). Protection against explosion is specified in more detail and from time to time more strictly by the leading foreign regulations [ API 650 (2), DIN 4119 (3) ] .
The definition in the title covers vertical cylindrical tanks of 100 m 3 - 80000 m 3 capacity, typically for the storage of crude oil and petroleum products, as well as of liquid chemical substances, through which the tension of the stored medium permits the production of explosive gas mixture from the combustible gas (vapour) and air at operating temperature.
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The size of the tanks generally increases according to the international trends, in order to make the storage technology more cost-effective, on the other hand, the construction rate of the smaller tanks ( 5000 m 3 or below) is also faster, as the small private storage tanks crop up: the dimensional range is becoming ever wider.
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| According to the traditional solution, floating roof tanks were applied with open top for the storage of explosive media, in which the simple design prevented the explosion in the dome space; having no dome space at all. However, the demand for material transfer of closed technology brought back the stiff roof. At this stage already in our country in most of the cases the tanks are of gas pendulum type or they have internal floating roof. |
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In the case of the explosion of several thousand of cubic metres of explosive gas in the dome space with traditional tanks with stiff roof or with the modern internal floating roof and with gas pendulum, huge destructive energy is released, imposing danger on the tanks and their environment.
Therefore, the tanks have to be designed so that
- no explosive gas mixture can develop in the dome space, however, should this still occur due to any fault, then
- on the occasion of the vapour space explosion an easily ruptured opening surface is produced on the tank roof, in which the explosion in the vapour space can loose its energy without destroying the tank
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| From the point of view of storage technology the absolute solution is the gas pendulum design. The storage technology is implemented in a completely enclosed system. |
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| The internal floating roof is an individual solution, it operates regardless of any other condition, if that is made according to the specifications of PI 650. Due to the ventilation ports of the stiff roof the dome space is a through-ventilated free space, there is no reason to expect a fire starting with an explosion. (A flying roof is still needed in case of volatile flammable liquids!) |
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| From the points above it is seen that no explosive gas-air mixture is produced in the dome space of the tank either with gas pendulum or internal floating roof. However, in case of an unforeseeable failure this may occur, and with the emergence of the initiating energy disaster may occur. Therefore, the flying roof is necessary! |
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| If tank fires do start (and according to the statistics they occur, even if seldom), disaster may be the result without a fire safety equipment. And what construction, project, operating company would be willing to undertake the responsibility for causing risk situations of unimaginable consequences? On top of that – almost like everywhere in the world – even in Hungary it is compulsory to install foam-extinguishing equipment on the tanks containing flammable liquid, and by all certainty, these systems are needed. And if they are compulsory anyway, they can as well be good! |
