Explosion Factors

Explosion Factors

 

Oxygen

 

The amount of oxygen in the air can only oxidize/burn a certain amount of flammable material. This ratio can be determined theoretically; it is called the stoichiometric mixture. When the amount of flammable material and available atmospheric oxygen are close to the correct ratio, the effect of the explosion – the increase in temperature and pressure – is the strongest. If flammable material is too small, combustion will be difficult to propagate or stop altogether. A similar situation occurs when flammable material is too high for the amount of oxygen present in the air. All flammable materials have an explosive 5 www.sigma-industries.kz EXPLOSION FACTORS range, depending on the available activation energy. This range is usually determined by igniting the mixture with an electric spark. The lower and upper explosive limits limit the explosive range, and this means that explosions do not occur below and above these limits. This fact can be exploited by sufficiently diluting combustible agents with air or preventing air/ oxygen from entering a part of the equipment. However, the latter option is not possible, or only viable with limitations, in an environment where people work regularly and therefore should be reserved for manufacturing equipment.

 

Inflammable substance

The flammable substance can be gaseous, liquid, or solid. Its reactivity with atmospheric oxygen is considered for a general discussion about workplaces.

 

Flammable gas

The flammable gas may be an element such as hydrogen, which can be made to react with oxygen with very little additional energy. Flammable gases are often compounds of carbon and hydrogen, and these flammable gases and vapors require only small power to react with atmospheric oxygen.Vapor is the part of the liquid (when it comes to explosion protection of flammable liquids) that has evaporated into the ambient air due to vapor pressure above the surface of the liquid, around a jet of this liquid or liquid droplets. Mist is a special kind of liquid that, because of its explosive behavior, in tandem with vapors, can be included to meet safety considerations.

Inflammable liquids (vapors)

Inflammable liquids are often hydrocarbon compounds such as ether, acetone, or naphtha. Even at room temperature, enough of them can go into the vapor phase so that an explosive atmosphere forms at their surface. Other liquids form such an atmosphere at their surface only at elevated temperatures. Under atmospheric conditions, this process is strongly influenced by the temperature of the liquid.
For this reason, the flashpoint is an important consideration when handling inflammable liquids. The flashpoint refers to the lowest temperature at which an inflammable liquid, under specified test conditions, produces sufficient vapor on its surface for an effective ignition source to ignite the vapor-air mixture.
Flashpoint is essential for the classification of potentially explosive atmospheres. Inflammable liquids with a high flashpoint are less dangerous than those with a flashpoint at or below room temperature.
Spraying a flammable liquid can produce a mist consisting of tiny droplets with a massive surface area, like spray cans or automotive spray stations. Such vapor can explode, and in this case, the flashpoint is less critical. For a fine mist from an inflammable liquid, the behavior relevant to safety can be approximated from the known behavior of vapors.

Inflammable solids (dust)

Inflammable solids (dust or volatile particles) can react with atmospheric oxygen and produce catastrophic explosions. Usually, more energy is required to activate a blast in the air than in the case of gases and vapors. However, once combustion begins, the energy released by the reaction creates high temperatures and pressures. In addition to the chemical properties of the solid itself, the fineness of the particles and the total surface area play an important role, which increases with fineness. Properties are processes that occur directly on the surface of a solid. Lighting and extinguishing a paraffin candle makes it possible to demonstrate several processes occurring in a solid material in a short period, which are not easily represented in a simplified form.
The experiment shows that when the wick of a candle is ignited, the paraffin melts, then evaporates, and this vapor feeds the flame. After the candle is extinguished, the paraffin fumes are still felt, the molten paraffin solidifies, and the paraffin fumes dissipate. The paraffin candle is now harmless again. Dust reacts differently depending on whether it is in a deposited dust layer or a suspended dust cloud. Layers of dust can begin to smolder on hot surfaces, while a cloud of dust ignited locally or on contact with a hot surface can explode immediately. Dust explosions are often the result of smoldering layers of dust that rise and can cause ignition. If such a layer is agitated, for example, by mechanical cleaning methods during transportation or incompetence extinguishing attempts, this can lead to a dust explosion.
A gas or vapor/air explosion can also stir up dust, which often results in a gas explosion occurring first, followed by a dust explosion. In deep coal mines, methane/flame explosions often caused coal dust explosions, the consequences of which were more severe than those of the initial flame explosion.

Ignition sources

A large number of ignition sources are possible using technological facilities.

Hot surfaces (5.3.2) 

Hot surfaces result from energy losses in systems, equipment, and components during regular operation. In the case of heaters, they are desirable, and usually, these temperatures can be controlled.
In the event of a fault – for example, in an overload or tight bearings – the energy loss, hence the temperature, inevitably increases. Technological facilities should always be evaluated to see if it stabilizes, i.e., whether it can reach the final temperature or an unacceptable temperature rise is possible, which must be prevented by taking appropriate measures.
Examples: coils, resistors or lamps, hot equipment surfaces, brakes, or overheated bearings.

Flames and hot gases

Flames and hot gases (including hot particles) (5.3.3) can be generated inside internal combustion engines or analyzed devices during regular operation and malfunction. Here, protective measures are required to prevent them from leaving the housing for a long time.
Examples: exhaust gases from internal combustion engines or particulates that form as a result of sparks during switching circuit breakers and corrode the material of the circuit breaker contacts.

Electrical devices

Electrical devices (5.3.5) should typically be suitable ignition sources. Only sparks of shallow energy with an energy of only microwatt seconds can be considered too weak to start an explosion.
For this reason, appropriate measures must be taken to prevent such ignition sources. Examples: sparks during switching, sparks on commutators, or slip rings.
Busbars and other earthed voltage sources, such as electrical corrosion protection of equipment, can lead to stray electric currents cathodic protection (5.3.6), leading to a voltage difference between different grounding points. Therefore, it is necessary to provide a highly conductive connection to all electrically conductive parts of the equipment so that the voltage difference is reduced to a safe level. It does not matter whether the electrically conductive equipment is an electrical or non-electrical part of the installation, as the cause of the current may be outside the equipment.
Equipotential bonding must always be ensured, regardless of whether such currents are expected or not and whether their sources are known.
Regardless of the presence or absence of electric tension, electric sparks can be caused by static electricity (5.3.7). The stored energy can be released in sparks and serve as an ignition source. Since this ignition source can occur completely independently of the electric tension supply, it must also be considered when working with nonelectrical devices and components. This is due to separation processes; therefore, it is necessary to evaluate the cases where this ignition source should be considered.
Friction during regular operation can cause an electrostatic charge. For example, portable devices cannot be grounded or connected to an equipotential bonding ring. A static charge may occur during regular operation when interacting with the wearer’s clothing.
Prevent static electricity from becoming an ignition source by taking appropriate measures. Examples: transmission belts of plastic materials, portable device housings, synthetic clothing material. Separation processes when rolling out paper or plastic film, plastic pipe systems.

Lightning (5.3.8) and a lightning strike can ignite an explosive atmosphere. Lightning invariably ignites an explosive atmosphere. However, there is also the possibility of ignition due to the high temperature reached by lightning.

Large currents flowing from a lightning strike can cause sparks near the strike.

Radiofrequency (RF) electromagnetic waves have a frequency of 104 Hz up to 3x1011 Hz. Among the ignition sources in which radiation energy affects an explosive mixture, the following should be highlighted:

• Electromagnetic radiation – radio waves (5.3.9)
• Electromagnetic radiation – IR radiation, visible spectrum (5.3.10)
• Ionizing radiation – UV radiation (5.3.11)
• Ultrasound (5.3.12)

Systems, devices, and components using radiation can be installed and operated in a hazardous area if their parameters are permanently and reliably limited, and this equipment is checked.
Examples: transmitting and receiving equipment, mobile phones, photoelectric beam barriers, and scanners.

Finally, adiabatic compression and shock waves (5.3.13) inside tubular structures operating under negative pressure can also become an ignition source.
Examples: rupture of a long fluorescent tube in a hydrogen atmosphere.