Managing Fire related Processes in a Steel
Managing Fire related Processes in a Steel Plant
Steel plant is a fire hazard industry. In a steel plant heat meets flammable materials almost everywhere. People, plant and equipments, and production all are exposed to high risk of fire almost in every area of the plant. Fires result into service disruptions and production downtimes. It causes damage to the plant and equipments and burn injuries, many times fatal injuries to the people. Even a small fire can paralyze the entire plant with disastrous economic damage.
The fire risk in the steel plant is immense. There are processes with very high temperatures and at many places with open flames. There are control rooms, motor control rooms, hydraulic rooms, transformer rooms, oil cellars, cable tunnels, and rubber conveyor belts etc. which are fire prone. Handling of liquid metals and slags, products with high temperatures, hot waste materials, and sediments containing hot materials are involved. There are also handling, storages and transport of flammable materials such as fuel oils, fuel gases, coal and coke, cryogenic liquids, etc., as well as requirement of cutting and welding everywhere for meeting the maintenance needs. There is presence of electrical ignition sources. There are areas where there is significant unit related fire loads are there. Due to these reasons, fire related processes such as fire protection, fire prevention, fire safety, and firefighting all have important significance in a steel plant.
Classification of fires
Fires are classified in the following four classes (Fig 1) as per the internationally accepted classification.
- Class A – These are fires involving solid materials normally of an organic nature (compounds of carbon), in which combustion generally occurs with the formation of glowing embers. Class A fires are the most common. Effective extinguishing agent is generally water in the form of a jet or spray.
- Class B – These are fires involving liquids or liquefiable solids. For the purpose of choosing effective extinguishing agents, flammable liquids may be divided into two groups namely (i) those that are miscible with water, and (ii) those that are immiscible with water. Depending on (i) and (ii), the extinguishing agents include water spray, foam, vapourizing liquids, carbon dioxide, and dry chemical powders etc.
- Class C – These are fires involving gases or liquefied gases in the form of a liquid spillage, or a liquid or gas leak, and these include methane, propane, butane, etc. Foam or dry chemical powder can be used to control fires involving shallow liquid spills. Water in the form of spray is generally used to cool the containers.
- Class D – These are fires involving metals. Extinguishing agents containing water are ineffective, and even dangerous. Carbon dioxide and the bicarbonate classes of dry chemical powders may also be hazardous if applied to most metal fires. Powdered graphite, powdered talc, soda ash, limestone and dry sand are normally suitable for this class of fire. Special fusing powders have been developed for fires involving some metals, especially the radioactive ones. Presently special dry chemical powders have been developed for extinguishing metal fires.
Fig 1 Classification of fires
Electrical fires are not considered, according to present day ideas, as a separate class, since any fire involving, or started by, electrical equipment, is in fact, be a fire of class A, B or D. The normal procedure in such circumstances is to cut off the electricity and use any extinguishing method appropriate to what is burning. Only when this cannot be done with certainty, then special extinguishing agents are required which are non-damaging to the equipment. These include vapourizing liquids, dry powders carbon dioxide, and other gaseous extinguishing agents.
With the wide diversities of technologies being employed is the steel plant, there is not only the factor of susceptibility, but the complexity of fires, explosions and the hazards which the steel plant employees, plant and equipments are exposed to. These hazards have been instrumental in causing heavy losses in lives and property throwing up fresh challenges to the plant designers, fire protection and fire-fighting equipment providers, as well as plant operators for evolving better and improved methods of design, fire prevention and fire-fighting methods in order to mitigate losses due to the fires.
Steel plants use the concept of defense-in-depth to achieve the required high degree of safety. With respect to the fire protection system also, the defense-in-depth principle is to be applied to achieve an adequate balance in (i) preventing fire from starting, (ii) detecting fires quickly, suppressing those fires which occur, putting them out quickly and limiting the damage, and (iii) designing plant safety systems, so that a fire that starts in spite of prevention programme shall not prevent essential plant safety functions from being performed.
The first objective requires that the design and operation of the plant be such that the probability of initiation of a fire is minimized. The second objective concerns the early detection and extinguishment of fires by a combination of automatic and/or manual fire-fighting techniques and therefore relies upon active systems. For the implementation of third objective, particular emphasis is to be given to the use of passive systems, which is to be the last line of defense, if the first two objectives were not effective.
Fire protection in the steel plant is to be accorded a very high priority since the risks involved are enormous and since the plant and equipment of all types (mechanical, chemical, metallurgical, electrical, instruments, and automation equipment, hydraulic and lubricating equipments, testing equipment, pipelines, and storages of fuels and materials etc.) are to be located within the restrictions provided by the plant and shop boundaries. There are several areas in the steel plant which are vulnerable to fires. All types of fires are possible in a steel plant. There may be many fire prone areas in the plant which are not even manned. The consequences of fire in a steel plant can be very serious, severe, and even disastrous since it can be accompanied with explosions. Further there is the added risk of release of poisonous gases and hence involving gas safety.
Because of these reasons, the fire safety assumes great importance even when the steel plant is in design stage. The plant layout, shop layouts, buildings’ designs, equipment layouts, location of fire prone facilities, provision of fire detection, fire alarm, fire suppression, and fire protection systems, and provision of fire-fighting facilities all have to be taken care during the plant design and construction stage for reducing the fire risks during the operation of the plant. Provisions are to be made in the plant designs to avoid fire spreads in case of fires. Escape routes and escape lighting need to be provided for people in case they get caught in the fires. Also there is a necessity to provide fire-fighting access. Various standards, codes and best practices need to be followed during the plant design and construction to make the plant fire safe during the plant operation. Substantial investments in fire related facilities are required to be made to make the plant fire safe during operation.
Fires, their extinction and suppression
There are three factors which are essential for a fire to occur. It has been shown from the triangle of fire that the three factors which are essential for combustion are namely (i) the presence of a fuel, or combustible substances, (ii) the presence of oxygen(usually as air) or other supporter of combustion, and (iii) the attainment and maintenance of a certain minimum temperature. Fire continues as long as these three factors are present. Removal of one of them leads to the collapse of the triangle and the fire is extinguished.
Fire extinction, in principle, consists in the limitation or elimination of one or more of the three factors, and the methods of extinguishing fire may be classified conveniently under the headings namely (i) starvation (or the limitation of fuel), (ii) smothering / blanketing (or the limitation of oxygen), and (iii) cooling (or the limitation of temperature). In practice, specific methods of fire extinction often embody more than one of these principles.
The triangle of fire representing three basic constituents of fire is the conventional concept. Fire scientists have now found that there is a fourth constituent in all flaming fires which play a vital part in the fire growth and sustenance. This is the unbroken or uninhibited chain reaction. Thus, as per modern concept, the previous figure of triangle of fire has been transformed into a tetrahedron of fire, each of its four sides representing one of the four basic requirements namely (i) fuel, (ii) temperature, (iii) oxygen and (iv) unbroken or uninhibited chain reaction. This last factor comes into play only in flaming mode of combustion which is normally applicable in the case of flammable liquids and gases.
Despite the many techniques which are available for the extinguishing of the fires, water is still the most efficient, cheapest and most readily available medium for extinguishing fires of a general nature. The method of applying water to a fire varies according to the size of the fire. For major fires, greater quantities of water are necessary, and the built-in pump driven by the fire vehicle’s engine is used for giving the necessary energy to the water to provide adequate striking power. A variation in the application of water can be made by means of nozzles that produce jets or sprays ranging from large sized droplets down to atomized fog effects. Judicious use of this type of application can not only cut down the amount of water used, minimizing water damage, but will ensure that it is used to greater effect. Water extinguishes a fire by a combination of mechanisms cooling the combustible substance, cooling the flame itself, generating steam that prevents oxygen access, and as fog blocking the radiative transfer of heat.
Foam used for extinguishing fires is usually generated by the mechanical agitation of a diluted foam compound solution in the presence of air. The desirable characteristics of foam are resistance to radiant heat, to fuel vapours and to loss of water content by drainage. It should flow readily and recover a surface if disturbed, without being too sloppy. The most satisfactory measure of the efficiency of the foam as a firefighting agent is the minimum rate of application at which a fire is controlled by the agent. Foam concentrates can be classified in two ways namely (i) classification by expansion such as low expansion (expansion ratio up to 50:1), medium expansion (expansion ratio between 50:1 and 500:1), and high expansion (expansion ratio more than 500:1), and (ii) classification by constituents such as protein foam concentrate, fluro-protein foam concentrate, fluro-chemical foam concentrate also known as aqueous film forming foam, synthetic foam concentrate, and alcohol resistant foam concentrate.
Halogenated extinguishing agents, though a relatively recent innovation in fire protection, are already being phased out, since they have very high ozone depletion potential. Several Halon alternatives have been developed. In addition to clean total flooding gaseous Halon alternatives, new technologies such as water mist and fine solid particulates are also being introduced.
Carbon dioxide (CO2) possesses a number of properties which make it a good fire extinguishing agent. It is non-combustible, does not react with most substances and provides its own pressure for discharge from the storage container. Being a gas, it can easily penetrate and spread to all parts (including hidden) of the fire area. It does not conduct electricity and can be used on energized electrical equipment. Also it leaves no residue. It is easily liquefied and bottled, where it is contained under a pressure of around 51 kg/sq cm at about 15 deg C. As the fire extinguisher is discharged, the liquid boils off rapidly as a gas, extracting heat from the surrounding atmosphere. The gas, however, extinguishes by smothering, or reducing the oxygen content of the air. The gas is heavier than air and a concentration of 9 % in air is the maximum that people can withstand without losing consciousness within a few minutes. The extinguishing concentration of CO2 required for various types of fuels vary from approx. 30 % to 62 % depending upon the fuel.
Steam is the oldest among the smothering agents. Now extinguishing systems based on steam are rarely used. Steam based systems are not effective for total flooding, but only for local application by hand held branches or lances. Steam is taken from boilers through fixed piping. The control valves are opened slowly. A by-pass is opened first to warn occupants. Manual systems with flexible tubing and lances are more common. These systems may still be seen in some of the benzol plants, and oil quenching tanks etc.
There have been at least four inert gases or gas mixtures developed as clean total flooding fire suppression agents. Inert gases are used in design concentrations of 35 % to 50 % by volume which reduces the ambient oxygen concentration to in the range of 10 % to14 % by volume. It is known that for most typical fuels oxygen concentrations below 12 % to 14% do not support flaming combustion. The inert gas mixtures developed so far contain nitrogen and/or argon; and one blend contains CO2 (approx. 8 %). They are not liquefied gases, but are stored as high pressure gases. Hence they require high pressure storage cylinders. These systems use pressure reducing devices at or near the discharge manifold. Discharge times are of the order of one or two minutes.
The result of applying water on most fires involving burning metals can be explosively disastrous, hence in such fires dry chemical powders are used. The base chemical of most dry chemical powders is sodium bicarbonate. This, with the addition of a metallic stearate as a waterproofing agent, is widely used as for fire extinguishing, not only in portable extinguishers, but also for general application in large quantities. Apart from stearates, other additives like silicones are also used to decrease the bulk density, and to reduce packing in the cylinder. Dry chemical is expelled from containers by gas pressure and, by means of specially designed nozzles, and is directed at the fire in a concentrated cloud. This cloud also screens the operator from the flames, and enables a relatively close attack to be made.
Fire safety and fire-fighting systems and procedures
During the plant operations prevention of fire is an important issue for the management. For fire prevention, it is necessary fire safety procedures are made for each area of the plant. These procedures are to take into account, fire dangers present in the area and availability of fire alarm, fire detection, fire protection, and fire-fighting facilities in the area as per the plant design. These procedures should also include regular inspection, testing and maintenance of fire alarm, fire detection, fire protection, and fire-fighting facilities. Further procedures are also needed for handling, storing and use of flammable materials, control of cutting, and welding and open flame work.
Good housekeeping is very important for the fire safety. It is not only to be part of the fire safety procedures but the employees are to be made responsible to ensure good housekeeping in their area. Further plant is to be self-sufficient in handling any kind of fire inside the plant. For this a fire hydrant network in the plant with sufficient number of fire hydrants to cover the entire plant is to be available. Also sufficient facilities are to be available in fire station with trained fire men and fire tenders to tackle all the types of fires.
Instrumentation associated with fire protection systems are to be inspected and calibrated annually as per the documented procedure. If the plant telephone system is being used as a portion of the fire alarm system then it is to be tested at least twice per annum. Also self-contained breathing apparatus (SCBA) in sufficient numbers are to be made available for use during the fire emergencies and these are to be inspected monthly and after each usage.
Employees are to be initially trained and also to undergo refresher training at regular intervals in these procedures for ensuring fire safety of the area. Also employees are to be trained in the use of the fire-fighting equipments, as well as precautions which they have to take in case of fires. Employees are also to be aware of the techniques of first aid which is necessary to be given for minor burn injuries. In case of major burn injuries victim is to be immediately shifted to the hospital.
It is necessary to carry out mock drills at regular intervals to ensure that the fire-fighting machinery remains always in active condition to fight the fire when it takes place.
Fire safety and fire-fighting procedures are to be regularly reviewed and updated so that they meet the current fire safety and fire-fighting needs. Also regular audit checks are to be performed on the working condition of fire alarm, fire detection, fire protection, and fire-fighting facilities as well as on the fire safety and fire-fighting procedures. If during audits non conformities are found then corrective actions are to be taken without delay.
It is essential that a standing fire order is established and documented for both plant and fire station personnel. This standing fire order is to outline the communication / responsibilities / responses of the various agencies like the plant operating personnel, plant medical team, plant management, and fire station personnel etc. It shall also contain the specific hazards of the plant and the type and locations of the fire suppression equipment/system provided. It shall specify the emergency equipment to be maintained at the plant and the procedures to assign responsibility for inventory, inspection and replacements of the same. The standing fire order shall also have procedures for assessing the effectiveness of the periodic mock drills.