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Industrial Fire and Preventive Methods

DEFINITION FOR FIRE:
Fire, combustion or burning of any substance needs basically 4 things to occur.
  1. A fuel [this can be: any organic matter; such as a solvent, oil, wood, paper, etc.].
  2. Oxygen [usually air].
  3. A certain amount of energy in the form of heat.
  4. Free radical reaction. The chemistry of combustion indicates that the union of Oxygen and the fuel is not direct, but through a series of steps wherein the actual reaction taken place between Oxygen and the free radicals liberated by the heated fuel at the point of ignition.

CLASSIFICATION OF FIRES:
Basically all the type of fires can be classified into the following major categories.

a)  Class A Fires:
  • These are fires on ordinary combustible materials, such as paper wood, rags etc. which can be put off by the quenching and cooling effect of water. For extinguishments of such type of fires, use of water, form or dry chemical fire extinguishers would be effective.

b)  Class B Fires:
  • These are fires occurring in flammable liquids such as oils, organic solvents, petrochemicals etc., where a blanketing or smothering effect is essential to put the fire out. Fire extinguishers like Foam, CO2, and Dry chemical are recommended for this type of fires, which provide a blanketing effect around the fire, thus, preventing air from coming into contact with the burning substance.
  • Water is effective for oil fires, only when it is used in the form of a spray or mist, but as such is not advisable to use in fires involving solvents and oils, which are immiscible with water.

c)  Class C Fires:
  • These fires are associated with Electrical fittings, fixtures, such as electrical panel boxes, electric cables, electric ad electronic equipments etc. Non conductive extinguishing agent such as CO2 gas is the most ideal and recommended fire extinguisher for Electric Fires.

d)  Class D Fires:
  • These are fires in metals, such as Li, Na, K, Zn, Mg etc. and mishandling of this type of fires can cause explosion and spread to other areas. Special attention and training is needed to handle metal fires.
  • Normal fire fighting agents used for extinguishing are dry sand, dry powder, graphite etc.
  • It is important to understand the meaning of some of the fire control terms and definitions of certain critical physical characteristics of solvents which will help to understand the degree of flammability of a particular Product.

Flash point (flash p):
  • This is the lowest temperature at which the liquid will give off enough flammable vapors at or near its surface, such that in an intimate mixture with air and a spark or flame, it ignites.
  • The flash point of a flammable liquid is usually determined by the standard method of test for flash point with the Tag Closed Cup Tester (TCC) or with the Tag Open Cup Tester (TOC). The results recorded with Tag Open Cup Tester (TOC) is always found to be 5-10oF higher (less flammable) than that being recorded with Closed Cup Tester.
  • The closed cup flash point value is usually several degree lower (more flammable) than the open cup, as the test in the former case is made on a saturated vapour-air mixture, whereas in the letter case, the vapour has free access to air and thus is slightly this reason open cup values more nearly simulate actual condition.

Fire point (fire p):
  • This is the lowest temperature at which a mixture of air and vapour continues to burn in an open container when ignited. It is usually above the flash point. Wherever the flash point data is not available, fire point figures can be considered as significant as flash point to understand the degree of flammability of the material.

Auto ignition temperature:
  • This is the temperature at which a material (solid, liquid or gas) will self-ignite and sustain combustion in the absence of a spark or flame. This value is influenced by factors such as the size, shape, the material of the hot surface, rate of heating etc. etc.

Vapour Density:
  • This value expresses the ratio of the density of the vapors to the density of air. The vapors of most of the flammable solvents are heavier tan air, with the result that they tend to travel at ground level making them more dangerous, as the risk will be more in case of a fire, since the vapors do not get diluted with air. Hence, in operational areas, where such higher V.D. solvents are used, the ventilating outlets should be at the lower level. Similarly, wherever, the liquids of low V.D. is handled, the ventilator should be at the higher level.

Melting Point (mp):
  • This is the temperature at which the solid and liquid forms of a substance exist in equilibrium. This value also gives the indication at what minimum temperature a flammable solid substance can become flammable solvent.

Boiling Point (bp):
  • This is the temperature at which the vapour pressure on the liquid surface becomes equivalent to the atmospheric pressure and at this temperature a continuous flow of vapour bubbles occur from throughout the liquid body and it is an indication as to how much volatile the liquid could be.
  • As the boiling point is proportionate to the pressure to which it is subjected to, it is apparent that lower the pressure (higher vacuum), lower would be the boiling point and lesser will be the vapour pressure and vis-à-vis.
  • This physical characteristic of liquids can be made use of, in preventing many hazards like Fire, Thermal decomposition of the product etc by subjecting the distillation process to take place under reduced pressure.

Chemical Formula:
  • In the event of lack of data regarding the flammable nature of a product, the chemical formulation of the product can give a clue regarding its flammable nature to a certain extent. For instance, if a product consists only of Carbon and Hydrogen (Hydro carbon), it can be assumed to be flammable and if it is in liquid form and having a low boiling point, it can be considered to be more flammable.

Flammable liquids:
Based on the degree of flammability, liquids of flammable nature are classified broadly into 3 categories as per the Petroleum Act of India.
  1. Class-A solvents having the flash point 0 to 23
  2. Class-B solvents having the flash point >23 to 65℃
  3. Class-A solvents having the flash point >65 to 93℃

FIRE PROTECTION
The two major aspects of Fire Protection are 
1. Prevention 
2. Loss minimization.

1.   Prevention:
  • The most undisputed aspect of Fire protection is it’s prevention. Since, for a fire to occur, all the 3 basic requirements, Fuel, Oxygen and source of Heat are needed to combine, by avoiding any of the 3 basic requirements, Fire can be prevented. For instance let us take an example of an industry which uses large volumes of Acetone in an open atmosphere. Now, for a fire to happen, among the 3 requirements, two of them i.e. Fuel and Oxygen (from air) are already available in the vicinity. The third requirement for Fire is heat.
  • From the flash point data, Acetone has a Flash point of 0oF, which means that at all temperatures above 0oF Acetone would give out enough vapours which on combining with Oxygen of air, can form a flammable mixture in case if it comes into contact with a spark, flame or a hot surface or any other source of ignition. Thus, in an installation using Acetone, following avenues are available from Fire Protection point of view:
  1. The working temperature (ambient) should be kept less than the Flash point, 0oF of Acetone, so that the vapour pressure of acetone at that temperature is lowered to considerable extent and consequently it does to give out enough vapours for the Fire to take place.
  2. The supply of atmospheric oxygen must be cut off. This is possible in two ways.
i)  By applying vacuum in the processes which taken away the air from the liquid surface.
ii)  By providing a continuous stream of Nitrogen gas flow over the surface of the liquid (also termed Nitrogen blanketing).

3. Eliminating all source of ignition in the operational as well as in the surroundings and enforcing following steps:
i)  smoking and carrying of materials of ignition, such as match box, unprotected electric lamps etc should be strictly prohibited.
ii)  Hot operations, such as welding, gas cutting, bracing should not be carried out in and around the operational area.
iii)  All the electrical fittings and fixtures and electric motors used in the process area should be Flame proof in nature.

4.  The area must be ventilated so that even though acetone gives off enough vapour to form a flammable mixture with air, the vapour will be drawn out of the area by means of the fume exhaust as rapidly as it is formed, thus preventing the accumulation of vapour concentration.
Below is a brief account of the 3 essential supporters of fire and the means to reduce their influence for Fire Protection.

A)  OXYGEN:
  • Although under certain specific conditions, chemicals can initiate fire even in the absence of oxygen for short duration (e.g. Fires associated with sulphur / phosphorous chlorine, hydrogen etc.), for sustained propagation of Fire, oxygen is very essential. Also the higher the concentration of oxygen in the atmosphere, more would be the intensity of fire. In industrial atmosphere it is difficult to manipulate the oxygen concentration in the working area, particularly since a concentration of oxygen, far below normal to keep fires from starting, would also be too low to support human life.
  • When it becomes necessary to work with such products, which by mere contact with air of atmosphere, can initiate a Fire, following steps are recommended for their safe handling.
  1. Isolate such products, such as sodium azide, sodium hydride etc. which are highly reactive with atmospheric oxygen from the main stream of materials and store them in containers under vacuum, or follow the guidelines for storage as mentioned in their respective MSDS.
  2. Keep such products under a blanket of inert gas such as nitrogen, helium or argon.

B)  HEAT:
  • In industrial fires the most easily overlooked fact is that all the major Fires that had occurred, had a modest beginning in the initial stages, which might have gone unnoticed and hence uncontrolled. Then, since fires are by definition exothermic, the very small fire started by a tiny heat source, supplies to its surroundings more heat than it absorbs, thus enabling it to ignite more fuel and oxygen mixture, and so on until very quickly there is more heat available than is needed to propagate a large Fire.
  • The heat for the initial Fire to start might have been provided by various sources of ignition such as high environmental (ambient) temperatures, hot surfaces, mechanical friction, spark from a switch, static electric discharge, an open flame or from a jot job, like welding, gas cutting etc.
  • It is worthwhile here to look into the various aspects of the “ignition source” and find ways to effectively curtail them.

a)  Open flames:
  • It is of utmost importance that near the operating areas, wherein flammable solvents are being handled, strict vigilance be exercised to check heat sources like burners, lamps, matches, welding torches, lighting etc. If there are both type of operations, viz. operations involving flammable solvents and hot type of jobs, in the same premises, they should be isolated properly by thick wall of cement and bricks. Never should any flammable material such as wood / plywood / thermacoal be used for the construction of the partition.

b)  Electrical sources:
  • Some of the common source of electric heating are from non certified lightings, cables, switches, starters, electric motors, digital electronic indicators, overloading of supply point, poor earthing systems etc. As a matter of safety, it is advisable to have all electrical switches and electric panel board properly housed in a separate block, away from the main plant / operating premises.
c)  Over heating:
  • Processes which need high pressure steam and involve high temperature reactions, runaway reactions, special processes, etc. should be identified and these are to be segregated from the relatively safer processes. HAZOP study should be undertaken before the implementation of such processes and special attention has to be paid in training the persons operating such plants.
i. Hot surfaces:
  • The most common hot surfaces in a chemical industry are equipment like, drying oven, boiler, steam line, hot oil system etc. These equipments must be housed in a quarantined location preferably away from the operating area (boiler / thermo pack and electrically) heated drying oven in any case should be way from the process block). 
  •  All the supply lines conveying steam, hot oil, hot water, hot air etc. should be properly insulated and maintained. It must be remembered that Flammable solvents, having the auto ignition temperature, lower than any of these hot surfaces, can get ignited themselves in case of their contact.
ii. Spontaneous ignition:
  • Many fires are caused by the heat of reaction produced when chemicals, incompatible in their chemical characteristics come into contact with each other, which are further accelerated by external source of heat and air. Few of such commonly used chemicals, having incompatibility with each other are:
  1. Chromic acid with flammable solvents, such as acetone, alcohols, hydrocarbons etc.
  2. Potassium permanganate with con. Sulphuric acid.
  3. Con. Nitric acid with organic solvents and any organic mass.
  4. Con. Hydrogen peroxide solution with flammable solvents.
  5. Sodium hydride, elements like sodium, potassium lithium etc. with water.
Many of fire investigations have proved beyond doubt that the cause of fire in most of the cases was due to neglect, poor house keeping practices, accumulation of flammable wastes such as cotton rags, residual oil and grease etc.

iii) Sparks:
  • Sparks may be produced from various sources, such as from electric motors, switches, loose electric connections or by static electric discharges. Sparks are also generated from friction between mechanical parts, by hammering and chiseling etc. to avoid electric sparking, some of the recommended precautions are:
  1. All the electrical fittings, such as switches, lighting, induction motors etc. should be flameproof in nature.
  2. Electric panels, main control switches etc must be away from the process block.
  3. Electric cables/wires connected to equipments should be of the right quality and should be in one piece (there should not be any joints on the cable).
  4. Proper earthing should be provided to all equipments, storage vessels etc.
  5. No flammable solvents must be stored or collected in non-conducting containers (such as plastic buckets, PVC/HDPE containers etc.).
Similarly, sparks produced by mechanical operations can be contained to a great extent by adopting following precautions:
  1. All moving parts of the mechanical equipment, such as the shaft of the gear box, shaft of the fan in a drying oven, clutches of a centrifuge etc must be greased and maintained in good condition.
  2. Hammering and chiseling should be avoided as far as possible inside the process plant.

If at all they have to be done, use hammers / chisles made of brass or gun metal, which does not produce sparks on hammering.

a)  Static Electric Discharge:
  • This is a phenomena in which certain products, mostly of organic liquids and non-polar solvents get electrically charged on their surface, by virtue of factors, such as high speed discharge, fall from heights, separation at a higher speed, prolonged storage etc. Causes for fire in many industries especially chemical and paper, are attributed to this unique phenomena. 
  • Most of these occur during the months when the atmospheric humidity is low and artificial heating is don(droplets of water in a humid atmosphere acts as a suitable conduit to carry away the acquired electric charges from the liquid surfaces, thus preventing a spark to generate by static discharge). 
  • Maintaining a humidity of 40 to 50% in rooms where flammable solvents are being handled, will greatly help to reduce the risk of a spark due to static discharge. Electrical grounding of storage tanks, process equipments, discharge pipings etc. are mandatory as per Indian Petroleum Act and it must be strictly enforced.
  • In all the equipments as far as possible the use of belts should be avoided and the use of metallic chains or direct shaft driven systems should be encouraged.
  • Storage of flammable solvents in non-conducting containers allowing a solvent to fall from a height in the form of a spray, pumping of a flammable solvents through a non-conducting pipe etc. are potential causes for static sparks and must be totally avoided.

i. Friction:
  • One of the major causes for industrial Fire can be attributed to the heat / spark generated by friction from the moving parts of mechanical equipments, e.g. fan blades rubbing the sides of outer casting, poorly lubricated bearings of a rotating body like hydro extractor, uncooled mechanical seal of a reactor etc.

ii. Fuel
  • Combustion takes place most easily between oxygen of air and a fuel in its vapour or finely divided particle state. As solid chemicals need preheating for their transformation from solid stage to liquid stage they are relatively safe, particularly if their melting points are high. But in case of liquids, most of them give out sufficient amount of vapours even at lower temperatures which can form a flammable mixture with air and ignite themselves.
  • This particular temperature which is an indicator to measure the hazard potential of a solvent is termed as the Flash point, as already discussed. As it is evident from above explanations, lower the flash point, higher would be fire risk. Indian Petroleum Act of 1934 and 1976, has classified all the petroleum and flammable liquids into three categories on the basis of their flash points; and also has stipulated conditions for their receipt, storage and movement.
  • As a matter of safety and fire prevention, all solvent storage tanks above ground level should have dikes constructed out of bricks and cement so that in the event of any leakage or fire, it can be contained within a limited area, thus preventing it from spreading.
  • An important factor to be considered for safety with respect to flammable liquids, is to prevent accumulations of high vapour concentration of flammable liquids in a closed environment. Adequate ventilation should be provided in the solvent handling area. While designing the ventilation / exhaust systems it should be ensured that for vapours of high vapour density, the exhaust system should be on the lower side of the building and for light density solvents it should be on the upper side and also it must be ensured that the motor and the switch of the system should be of flame proof.
  • It is of utmost importance that a totally comprehensive standard operating procedure (SOP) should be framed and implemented, with respect to receipt, storage and handling of Flammable solvents and all the operating persons should be adequately trained to handle these solvents safely. There is no substitute to Educating and Training the people.
  • It is also worth probing when a new process is developed that, whether or not it is possible to substitute Flammable and Hazardous chemicals with non-flammable/less flammable and non-hazardous chemicals. If cost is the prime factor in favour of a flammable solvent other indirect benefits, such as savings in installation of high cost safety devices, training cost, high insurance premium etc. should be assessed by substituting with a non-flammable solvent. However in any event, usage of flammable solvents in a chemical process cannot be avoided irrespective of the quantum.
  • Besides flammable solvents, other possible disasters in industry, which can be prevented well and only by preventive means are:
  • Dust Explosion: Like static electric discharge, another lurking industrial hazard is dust explosion. Practically any flammable material in the form of the particles or dust, mixed with air at the right proportion, when comes into contact with a spark (mostly generated by static discharges), flame or heat will burn so rapidly as to cause a severe explosion which is termed as Dust Explosion. This kind of hazard commonly occurs in industries handling, plastic, grain, flour, coal dust, metal powders, fertilizers, wood dust, powdered milk, detergent powder, paper dust and industries handling sulphur and phosphorous powders. Some of recommended precautions to minimize dust explosion are:
  • Housekeeping: Good house keeping practices like proper storage, stacking, handling, maintaining of proper temperature conditions in the warehousing and manufacturing blocks, usage of proper equipments for material handling etc. are very important.
  • Do not allow the dust to accumulate to alarming proportions. This can be achieved by providing effective exhaust system.
  • All the dust particles generated in the process, should be taken out through metal ducts, and the vent of such ducts must be outside the block, preferably, taken to a distant location.
  • Proper earthing should be provided to the pulverizing / dust generating equipments / ducting etc.
  • Use of belt driven equipments, conveyors etc. should be minimized as far as possible. Instead metallic chain driven / shaft driven equipments must be encouraged.
  • Cleaning of floors, equipments etc should be done by vacuum methods and cleaning by sweeping, dusting with brooms, dust rags etc should be totally avoided.
  • The use of compressed air to blow the dust off the equipments/floors etc, thus helping the formation of dangerous dust clouds is totally FORBIDDEN.
  • Exposed piping / beams etc. in a powder processing area should be cleaned frequently to prevent the dust from accumulating on them.
  • It is a wrong notion that by providing a false ceiling in a powder processing unit, accumulation of dust on the piping and other projections can be eliminated. But incidents have proved the other way. Unless the ceiling is extremely well designed and installed, there is a great possibility for the dust particles to escape through the crevices and joints of the ceiling and get settled on the upper side of the ceiling. As this will not be visible to human eyes operating on the ground level, there is a great risk of dust accumulation over a period of time, depending on the nature and volume of operations, and it could lead to a serious dust explosion, when a spark is generated in that area.
  • As in the case of Flammable solvents, any kind of source of ignition, such as open flame, smoking, welding/cutting, grinding, electric sparks from loose contacts or from static discharges, should be avoided.
  • Use of inert gas like nitrogen, in closed vessel operations has been found very effective and rewarding in preventing Fire and Explosions.

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