There are three main hazards in chemical industries. These are Fire, Explosion and Toxic Release. In many instances, more than one of these hazards occur either simultaneously or in tandem of each other. For example, a fire may lead to explosion which subsequently causes more fire and toxic release.
2.1 Fire Triangle
Removal of any one of the sides of this Fire Triangle will extinguish the fire.
If fuel is removed, the fire will starve and be extinguished. If air is removed, the fire will suffocate – because of a lack of oxygen – and go out. The removal of heat is the most common form of fire suppression.
There are methods of suppression, however, that work on the basis of suffocating the fire. In most cases water is used to essentially soak up the heat generated by the fire. Without energy in the form of heat the fire cannot heat unburnt fuels to ignition temperature and the fire will eventually go out. Chemicals can be added to water to improve the heat removal properties of water, or to improve the ability of the water to stick to unburnt fuel.
2.2 Types of Fire
i. Pool fire
Pool fire can be defined as a turbulent diffusion fire burning above a horizontal pool of vaporising liquid fuel under conditions where the fuel has zero or very low initial momentum. The fuel pool is not necessarily static. It may be spreading or contracting.
In this case, fire will heat and pressurised the car, weaken the body and possibly generate BLEVE.
ii. Jet Fire
Jet fires occur following the ignition and combustion of flammable fluids issuing continuously from a vessel, pipe or orifice, which burn close to the release plane. Jet fires dissipate thermal radiation away from the flame’s visible boundaries. The energy transmitted could be hazardous to both life and property.
Jet fire can occur in the process industries, either by design or by accident. The principal situations in which flames occur by design are in burners and flares. Ejection of flammable fluid from a vessel, pipe or pipe flange can give rise to a jet flame if the material ignites. An intermediate situation, and one which particularly concerns the designer is where the jet flame result from ignition of flammable material vented from a pressure relief valve.
Scenario involving jet flames are not easy to handle, since a large jet flame may have a substantial reach, sometimes up to 50 m or more. Jet flames have been involved in a number of accidents, perhaps the most dramatic were the large jet flames from the gas riser on the Piper Alpha Oil platform. In other cases jet flames from pressure relief valves have caused adjacent vessels to overheat and burst, giving a boiling liquid expanding vapour explosion or BLEVE.
iii. Flash Fire
A vapour cloud fire or flash fire, occurs when a vapour cloud forms from a leak and is ignited, but without creation of significant overpressure. Released a flammable vapour from a process equipments or pipe followed by ignition is a not uncommon occurrence. If the ignition is prompt, the cloud may be modest in size, but if the cloud has time to spread over an appreciable part of the site and is than ignited, a major vapour cloud fire may result. This occurs in only a very small proportion of ignited releases.
The hazard from a vapour cloud fire is usually assessed by considering dispersion of the vapour cloud and ignition of this cloud and making some relatively simple assumption concerning the effects inside and out side the cloud.
2.3 Radiation Heat Effects
In the case of any fire situation, heats are released from the surface to the surounding. A convenient way to compute the heat accumulated by the recipient is to make use of the black body radiation computation. Based on the surface temperature and the duration of the exposure, the energy dose received by the recipient can be computed. Based on the thermal dose, skin damage can be estimated.
The of heat effects on skin damage based on BS5908 (Lees, 1996) is summarised in the following table:
Table: Skin Damage Due to Radiant Heat
|Burns||Criterion||Thermal dose kJ/m2|
|First degree||Persistent redness||125|
Thermal dose can cause more damage including additional fire to the surrounding. For example, if the thermal dose received by a building material is above the limit where spontaneous ignition can be triggered, the building will be burning. In this manner, fire may spread further into uncontrolled situation. This is the rationale why firemen spray water to surrounding buildings as a strategy of fire fighting.
Explosion is a rapid release of energy causing development of pressure or shock wave. Energy may be pressure energy or chemical energy.
Shock wave: An abrupt pressure wave (energy front) generated due to sudden release of energy, which move in the medium.
Blast wave: A shock wave in open air generally followed by strong wind, the combined shock and wind is called blast wave
Overpressure: The pressure on an object as a result of an impacting shock wave
Deflagration: An explosion in which the reaction front (energy front) moves at a speed less than the speed of the sound in the medium.
Detonation: An explosion in which the reaction front (energy front) moves at a speed greater than the speed of the sound in the medium.
3.1 Types of Explosion
i. Boiling Liquid Expanding Vapour Explosion (BLEVE)
A BLEVE can be defined as a major failure of container at a moment in time when the contained liquid is at a temperature well above its atmospheric pressure boiling point. The definition does not qualify the cause of the container failure, or the chemical and physical properties of the container liquid.
The most common type of BLEVE occurs when a pressure vessel that is partially filled with liquid is exposed to a fire. The fire weakens the portion of the tank shell that is contacted by the flame and that is not in contact with liquid. Simultaneously, the flame heat the liquid in the tank, increasing the equilibrium pressure and ultimately the tank pressure. At some point the tank weakens so much that internal pressure is sufficient to cause the vessel to rupture. Fragments of the tank are propelled away from the tank location with great force. The liquid remaining in the tank at the time of rupture is subject to rapid flash vaporization that atomizes much of the liquid. A fireball is created by burning vapour and liquid at it expands outward.
Second type of BLEVE is that which occurs mechanically damage tanks. In some cases, the damage done to a pressurized tank of liquefied gas in a transportation accident has been sufficient to cause an immediate catastrophic failure of the tank. In other cases, the damage to the tanks appeared to be minor, but the stresses imposed on the mechanically damaged areas were sufficient to cause the tank to fail catastrophically at a later time. In this instances, BLEVE’s can occur with out the presence of a fire and might or might not followed by fireball.
A third type of BLEVE can occur if a pressure vessel is allowed to become completely filled with liquid. As the temperature rises, the pressure relief capacity is insufficient to keep the internal pressure from exceeding the strength of the tank. This type of BLEVE can occur with out the presence of a fire and might or might not be followed by a fireball.
When BLEVE occurs, fragments of the ruptured tank are propelled away from the tank location. Much of the released liquid flashes to vapour and the remaining liquid is atomised to vapour by the expanding vapour. The hazard includes flying pieces of the tank shell, overpressure of the surrounding and the heat generated by the fireballs. Among these, fireball is by far most hazardous.
The thermal hazard created by the BLEVE of a pressurized tank combustible liquid is due to the fireball created by the combustion of liquid and vapour expelled from the tank. After most of the combustion has taken place, the fireball become lighter than air and lifts from the ground. The maximum diameter of the fireball and the length of the time that elapses before lift-off are function of the weight of combustible materials. The following equation are used to compute the fire ball diameter and lift-off time.
Example of BLEVE (Video)
ii. Vapour Cloud Explosion (VCE)
When flammable vapour is released to the atmosphere, it will form a cloud suspended in the air, filling the atmosphere to a height limited by its density. If it is ignited at some early stages, flash fire will be formed. However if ignition is delayed and triggered within the explosive limit, it will generate more devastating effect known as the VCE.
Under certain conditions, VCE can result in widespread damage to plant and property and injury to people. It is in fact the most hazardous of all fires and explosion as the area within and under the cloud can be considered fatal.
Probably the best known VCE incident in the UK is the explosion at the Nypro (UK) Ltd plant at Flixborough on 1 June 1974. The explosion followed the release of cyclohexane, resulted in the death of 28 employees and caused extensive damage to the plant and surrounding area. A more recent incident was the explosion at the Texaco Refinery, Pembroke, on 24 July 1994. In this case there was a release of hydrocarbons, following the failure of flare drum.
iii. Dust Explosion
Any combustible material (and some materials normally considered noncombustible) can burn rapidly when in a finely divided form. If such a dust is suspended in air in the right concentration, it can become explosive. The force from such an explosion can cause employee deaths, injuries, and destruction of entire buildings. Such incidents have killed scores of employees and injured hundreds over the past few decades.
Materials that may form combustible dust include metals (such as aluminum and magnesium), wood, coal, plastics, biosolids, sugar, paper, soap, dried blood, and certain textiles. In many accidents, employers and employees were unaware that a hazard even existed.
A combustible dust explosion hazard may exist in a variety of industries, including: food (e.g., candy, sugar, spice, starch, flour, feed), grain, tobacco, plastics, wood, paper, pulp, rubber, furniture, textiles, pesticides, pharmaceuticals, dyes, coal, metals (e.g., aluminum, chromium, iron, magnesium, and zinc), and fossil fuel power generation.
4.0 Toxic Release
There is no such thing as safe chemicals. They are all hazardous if contacted above certain concentrations. Chemicals enter human body through four routes – ingestion (oral) into stomach and digestive system, inhalation into lungs and respiratory system, skin absorption and skin injection directly into blood. Hazardous toxicants include chemical agents, physical agents such as dusts, fibers (e.g. asbestos) and radiation.
From industrial installation, toxic gases can spread to the surrounding community through venting and flares as well any accidental release or releases following fires and explosion. The most well-known example of toxic release is the Bhopal incident that killed thousands and injured more.