What are the types and causes of fire hazards in textile industry?

Dust Related Explosion and Fire HazardsInflammable and Explosive DustsThe three ingredients that cause fires are heat, oxygen and combustible material. When all three components are present, combustion takes place. Many dusts and powders that emanate out of the industrial processes are easily flammable. These dusts carry characteristics that differentiate them such as:

Dusts that explode easily. Since they carry an oxidant, they do not need oxygen from the atmosphere to explode. (Eg. Chemicals that pump up automobile air bags, gunpowder)

Dusts that ignite easily. They require little or no external heat to ignite.

An explosion is defined as the process in which combustion occurs and spreads so rapidly as to create a high pressure. In this case, the fire expands from a source of ignition and develops high pressure when restricted in an area.

Explosions have lower and upper limits (of dust concentration). A lower limit of explosion indicates that the concentration of dust particles is below the lower limit of explosion. Below this level, the dust concentration will not explode even on ignition. This happens because the heat produced in such a circumstance is not sufficient to affect other dust particles. Alternatively there could be a dust concentration of explosive levels but there may not be enough oxygen to start the fire.

To counter the hazards of fire and explosions, it is necessary to know the characteristics of the dust. For example, fine aluminum dusts explode at very low limits of explosion whereas coarse aluminium dusts do not catch fire even under the influence of another source of heat.

Understanding Explosions

By mixing gas (air) and compound (dust) in concentrations that create an explosion, combustion proceeds at high speed (sonic speed or supersonic speed) from the point of ignition. The face of this growing ball of fire is called ‘flame front. If there are no obstructions, this high pressure flame burns itself out after traveling a distance. But if it restricted in any way, the pressure increases tremendously. By knowing the rate of increase of this pressure for a particular compound, the hazard that it can cause can be known.

The bag house has an inherent advantage with its air cleaning process that may keep dust concentrations below the lower limit of explosion. On the other hand, shaker dust collectors are susceptible to explosions since static charges could set off a spark while ‘shaking’ or bag filter cleaning is in process.

Explosions are of two types. Primary explosions are those ignited by dust concentrations being at explosive levels. These primary explosions could take part in one part of the system. But then they move rapidly along the system ductwork. When the flame front reaches the dust collector it could cause further explosions (by creating explosive dust concentration by unsettling all dust particles on the filter bags). This would cause a secondary explosion.

Explosion VentsTo reduce the damages that could be caused by explosion, dust collectors are provided with exhausts or vents that allow the explosion to go out of the system before further damage to other parts. Common designs of explosion vents are:

Membrane: A membrane that breaks at a preset pressure on the explosion vent to allow the flame front to exit from the vent in high pressure situations (explosion)

Hinged/ Restrained panels: By installing Hinged or Restrained panels with springs that open at preset pressure, the flame front is allowed to exit the system

NFPA – 68 Guide for Venting DeflagrationsInformation on controlling explosions and reducing their structural effects on dust collectors can be obtained in the NFPA – 68 guide ‘Guide for venting of Deflagrations’ prepared by the National Fire Protection Association. Deflagration is defined as the propagation of a combustion zone at a velocity that is less than the speed of the unreacted medium. Simply put, deflagration is nothing but burning with great heat.

Early vent designs used an assumed vent ratio of 40:1 (vent ratio is). In time however, many dusts and powders have been analyzed for their individual rate of combustion (burning) and combustibility. Based on these analyses, a value of ‘Kst’ has been assigned to each dust, which indicates the deflagration index of the dust (expressed as bar-m/second). Higher ‘Kst’ value indicates faster rate of combustion.The NFPA – 68 guides is a useful document that provides ‘Kst’ values for varieties of dusts and powders. It also provides the method to calculate vent area for various applications.

Shaker Collectors and ExplosionsShaker collectors carry the hazard of explosions due to the ‘shaking’ of the bags that could cause static charge on the bags to set off a spark, leading to an explosion. Since dust concentrations invariably reach explosive limits during cleaning operations in shaker collectors, precautions need to be taken to avoid explosions. A common method is by coating bag fibers with a conductive coating that grounds the static charge thus neutralizing danger from static charge. Shakers are prone to secondary explosions as well because these bags tend to accumulate more dust (combustible material or fuel) when compared to other collectors.

Fabric Jet Collectors and ExplosionsThese collectors posses an inherent advantage in mitigating explosion risk due to static charge since their design uses reverse flow of air. This reverse flow of air (that is not ionized) through the dust, depletes all static charge from the dust particles. The cleaning system is thus, constantly cleaning up any charges from building up as well. Fabric jet collectors also have the design capability to retard secondary explosions.

Assume a cylindrical bag in a high ratio reverse air system (4 ½ inch diameter and 96 inch length). The total air pumped into the bag during a cleaning cycle is one cubic foot. This air forms a hollow cylinder, lined with one-inch thick dust.

Controlling Secondary ExplosionsThe energy produced in the primary deflagration must be reduced to control secondary explosions. This can be achieved by controlling factors that contribute to explosion i.e. fuel and oxygen. Some of the methods are:

1. Reduce oxygen in the air: Use of new generation compact high ratio fabric designs and use of multiple hoppers (reduces hopper volume at product end) reduces oxygen levels in the system considerably.

2. Reduce fuel (dust particles) in the air: Smooth finish filter media (such as eggshell or singed) on bags, reduces dust accumulation on bag surfaces.  Use of PTFE laminated bag with frequent cleaning reduces dust concentrations.It is also advisable to remove dust from hopper as it could form a potential concentration for explosive levels. Another method of reducing dust concentration (fuel) is by maintaining a low pressure drop (indicated by the magnehelic gage) through the filter media by frequently pulsing air cleaning valves. In centrifugal and pleated bag media the pressure drop must be kept low by cleaning the media frequently.

3. Other techniques: By mixing inert air produced in another part of the system with the combustible dust of another machine a less combustible mixture can be formed.

Fire Hazards

Collectors are highly susceptible to fires due to the presence of fuel (dust particles) and oxygen. Collectors have fan flow that further contributes to the fanning of a fire and helping it grow. Since collectors contain all these fire-friendly characteristics it is necessary that proper filter media is chosen and operating techniques are followed with care. There are many dusts that burn (form a fire) but they have a low rate of combustion.

Causes of FireThe main causes of fires are (I) sparks and (ii) spontaneous combustion.Sparks: Sparks find their way into dust collector systems from the industrial process. They enter through the hoods and ducts in the collector system. One way of tackling sparks was by having a long duct. Traveling along the long duct would enable the spark to burn itself out or cool off. However it is not an effective solution since sparks are known to travel over a 100 feet and survive the cyclone before igniting the bags and dust in the dust collector. Fires in dust collectors start when the system is in process.To understand the nature of the sparks we may look at the example of a campfire where sparks fly over the fire. Though sparks consist of a heavy particle, they ‘fly’ because they are surrounded by a layer of ‘hot air’. This layer of hot air around it makes the spark behave like a hot air balloon and it travels long distances easily. Dust collector systems have a smooth flow duct design which helps sparks to travel unaffected by gravity and centrifugal forces of the system.

Spontaneous CombustionsFume dusts are a classic case of fine dusts that have large surface areas. Fume dusts are known to oxidize and the process of oxidation produces heat which is a factor that can start a fire. Oxidation does not cause a problem when the dust collector is in operation since the heat is removed by the flow of dust through elements. However when the dust collector is not in operation, heat generated by the oxidation of these dusts forms hot spots on the filter cake. These hot spots in the filter cake

could ignite when the flow is restarted in the system. Once ignited the fires are fanned by the airflows and cause extensive damage.

Ways to Control SparksTo extinguish sparks, it is essential that the layer of hot air surrounding the spark is removed. This can be achieved by creating a change in air velocity. The change in air velocity creates eddies in the air stream and removes hot air from the spark. Once the layer is disturbed, sparks can be cooled in a fraction of a second. The eddies can be created by

abrupt change in duct sizes in the system single/multiple plates with orifices placed near the hood inlet change in direction of duct such as a square elbow instead of a smooth turning vane spark trap

Sprinkler SystemsWater sprinkler systems could be installed in collectors in the area where clean air is recycled into the work place. However, one must remember to turn off the water sprinkler system when the fire is put off because water could collect in the hopper and bins. If a large amount of water accumulates, the structure of the dust collector could be affected badly involving expensive reconstruction.

Smoking

Smoking is the leading cause of home fire deaths in the United States.

Never smoke in bed. Always look under cushions and in trashcans for burning cigarettes before going to bed. Check carpeting where ashtrays have been used.

More to think about

Get rid of stored newspaper or other unnecessary materials. Newspapers stored in a damp, warm place may ignite spontaneously.

Install smoke detectors on every level of your home and outside of sleeping areas.

Mount a fire extinguisher in the kitchen, garage and workshop. Agree in advance on an escape plan. There should be at least two

exits in every room.

Care and Maintenance of your Fire ExtinguisherInspect fire extinguishers at least once a month (more often in severe environments).Fire extinguisher maintenance is important for everyone’s safety.

You must ensure that:

The extinguisher is not blocked by equipment, coats or other objects that could interfere with access in an emergency.

The pressure is at the recommended level. On extinguishers equipped with a gauge (such as that shown on the right), the needle should be in the green zone - not too high and not too low.

The nozzle or other parts are not hindered in any way. The pin and tamper seal (if it has one) are intact. There are no dents, leaks, rust, chemical deposits and/or other signs

of abuse/wear. Wipe off any corrosive chemicals, oil, gunk etc. that may have deposited on the extinguisher.

Some manufacturers recommend shaking your dry chemical extinguishers once a month to prevent the powder from settling/packing.

Fire extinguishers should be pressure tested (a process called hydrostatic testing) after a number of years to ensure that the cylinder is safe to use. Consult your owner's manual, extinguisher label or the manufacturer to see when yours may need such testing.

If the extinguisher is damaged or needs recharging, replace it immediately!