What is meant by hazardous area?
A “hazardous area” is defined as an area where flammable or explosive gases, dust, or vapors are present or may be present in sufficient quantities in the atmosphere. When three basic conditions are met in such an environment, a fire or explosion can occur. The “hazardous area” or “combustion” triangle is a term used to describe this region.
A method of analyzing and classifying a potentially hazardous area is required to protect installations from a potential explosion. The goal is to ensure proper equipment selection and installation, which will ultimately prevent an explosion and ensure human safety. The methods for classifying an installation vary depending on where it is located in the process area, but there are two main types of classification. This is referred to as Zoning in countries that have adopted the IEC (International Electrotechnical Commission) philosophy, whereas in North America, installations are classified by Classes, Divisions, and Groups to determine the level of safety required.
HAZARDOUS AREA CLASSIFICATION IEC AND ATEX STANDARDS
A Hazardous area is defined based on the following three criteria. They are
• The type of hazard
• The hazardous material’sauto-ignition temperature (temperature or “T” rating)
• The likelihood of the hazard being present in flammable concentrations
The Type of Hazard
The type of hazard either in the form of a gas or vapor or dust or fibre.
The type of hazard is mainly classified into two types based on whether it is mining or above surface industry.
Group I:- Firedamp-prone electrical equipment for use in mines and underground installations Electrical equipment for use in surface installations.
Groups II & III:- Depending on the hazard, they are further subdivided. The amount of energy required to ignite the most explosive mixture of the gas with air is used to group II gases together. The nature of the explosive atmosphere for which Group III dust is intended is used to divide it into subgroups.
AUTO-IGNITION TEMPERATURE OR “T” RATING
The hazard level of gases rises from IIA to IIC, with IIC being the most vulnerable. This group of substances is extremely flammable, with hydrogen being the most flammable. The temperature class, or T-Class Rating, is determined by the gas’s auto-ignition temperature, which is detailed below.
The Likelihood of the Hazard
The probability of the hazard being present in a concentration high enough to cause an ignition varies by location. For most installations, the risk is proportional to the duration of the hazard (gas, vapor, dust, or fiber). To classify this danger, an installation is divided into risk areas based on the length of time the hazard is present. Zones are the names given to these areas. There are three zones for gases, vapors, dust, and fibers.
• ZONE 0
• ZONE 1
• ZONE 2
|Gas||Dust||Hazardous Area Zone Characteristics|
|Zone 0||Zone 20||A hazardous atmosphere is very likely to exist, and it could exist for a long time (>1000 hours per year) or even continuously.|
|Zone 1||Zone 21||It is possible, but unlikely, that a hazardous atmosphere will exist for long periods of time (>10 1000 hours per year).|
|Zone 2||Zone 22||In normal operation, a hazardous atmosphere is unlikely to exist, or only infrequently and for short periods of time (less than 10 hours per year).|
Hazardous areas are defined by DSEAR as “any place where an explosive atmosphere may exist in sufficient quantities to necessitate special precautions to protect workers’ safety.” ‘Special precautions’ should be interpreted in this context as relating to the construction, installation, and use of apparatus, as defined in BS EN 60079-101.
The method of analyzing and classifying the environment where explosive gas atmospheres may occur is known as area classification. The main goal is to make it easier to choose and install apparatus that can be used safely in that environment while taking into account the flammable materials that will be present. The original scope of this analysis was specifically extended by DSEAR to include non-electrical sources of ignition and mobile equipment that poses an ignition risk.
Hazardous areas are divided into zones based on the frequency and duration of the presence of an explosive gas atmosphere, as follows:
• Zone 0: In Zone 0, explosive gases is present in the atmosphere continuously or for longer periods.
• Zone 1: In this zone, explosive atmosphere gas is present in normal operation conditions
• Zone 2: An area where an explosive gas atmosphere is unlikely to occur in normal operation and if it does, will only last a few minutes.
Various sources have proposed time limits for these zones, but none has been formally adopted. The most commonly used values are:
• Zone 0: Explosive atmosphere for more than 1000h/yr
• Zone 1: Explosive atmosphere for more than 10, but less than 1000 h/yr
• Zone 2: Explosive atmosphere for less than 10h/yr, but still sufficiently likely as to require controls over ignition sources.
These values are the most appropriate when people want to quantify zone definitions, but for the vast majority of situations, a purely qualitative approach is sufficient. When a plant’s hazardous areas have been classified, the remaining areas are designated as non-hazardous, also known as ‘safe areas.’
The consequences of a release are not taken into account in the zone definitions. If this is a critical issue, it could be addressed by improving equipment specifications or tightening controls on activities permitted within the zone. The option of specifying zone boundaries more conservatively is not generally recommended because it complicates equipment selection and is illogical in terms of controlling health effects from vapors assumed to be present. When occupiers designate large areas as Zone 1, the practical implications should be discussed during the site inspection.
Apparatus Groups and Temperature Classes for Common Flammable Dusts and Fibres – Group III
When it comes to installations that are at risk of exploding due to dust ignition, the equipment used is classified similarly to how gases are classified. If the surface temperature of the equipment is higher than the ignition temperature of the hazard, it should not be installed. The minimum ignition temperature of some common dust is listed below.
EXPLOSION PROTECTION CONCEPTS
Within these hazardous area zones, various types of equipment can be used to ensure that the risk of an explosion is eliminated or greatly reduced. Protection concepts determine that this equipment be designed and manufactured in accordance with specific construction parameters. These ideas can be divided into four categories.
Ex I Intrinsic Safety
Ex i is an explosion-proofing concept in which the electrical energy within the equipment is limited to a level below that which could cause ignition or limit the heating of the hazardous area equipment’s surface. The “ia” and “ib” subtypes of Ex I protection is the most common. During operation, type “ia” protection allows for the occurrence of two faults. During operation, type “ib” explosion protection allows for the occurrence of one fault.
Ex d Flameproof
The equipment that could cause an explosion is contained within an enclosure that can withstand the force of an explosion while also preventing transmission to the hazardous environment outside. The Ex-d flameproof method of explosion protection also keeps the dangerous atmosphere out of the enclosure and away from the equipment.
Ex m Encapsulation
Ex m is an explosion-proofing concept in which potentially ignitable equipment is encased in a compound or resin to keep it from coming into contact with the explosive atmosphere. Under normal operating conditions, the concept also limits the equipment’s surface temperature.
Ex e Increased Safety
Ex e is an explosion protection concept that is applied to an installation to ensure increased security against the risk of excessive temperatures and sparks from hazardous area electrical equipment. This method of protection forbids the use of equipment that normally causes sparks.
Ex p Pressurised
One process ensures that the pressure inside an Ex p enclosure is sufficient to prevent the entry of flammable gas, vapor, dust, or fibre, which could result in ignition. Another process dilutes any potentially explosive atmosphere by maintaining a constant flow of air (or an inert gas).
Ex o Oil Immersion
Every piece of equipment that has the potential to arc and ignite is submerged in a protective liquid or oil. The oil acts as an insulator, preventing ignition.
Ex q Powder Filling
All equipment that has the potential to the arc is contained within a quartz or glass powder-filled enclosure. The powder filling keeps the contents from igniting.
Ex n Non-Sparking
Electrical equipment in hazardous areas with the potential to the arc is not capable of igniting an explosive atmosphere around it, according to this type of explosion protection. This can be further broken down into the following groups:
Ex nA - Non-sparking materials are used in construction.
Ex nC - Non-incendive materials are used in construction.
Ex nR – Where components are tightly encased to prevent ignition and restrict breathing.
Ex nL– When the energy contained in construction components is insufficient to cause an ignition
Ex s Special As the name implies, there are no specific parameters or construction rules for this type of explosion protection. In essence, it is any method of explosion protection that can provide a pre-determined level of safety to ensure that no ignition can occur. As a result, it does not fall under any one type of protection and maybe a combination of several.
Ingress Protection Safeguarding against the ingress of solid foreign objects and water is another consideration in the protection of equipment in hazardous areas. This is referred to as the IP Code and refers to the degree of ingress protection. IEC 60529 is the relevant standard for the degree of ingress protection. Below is a summary of the IP code as defined by the standard.
BASIC UL / CSA / NEMA ENCLOSURE TYPES
Type 3 An enclosure designed for outdoor use to provide some protection from windblown dust, rain, sleet, and damage caused by external ice formation.
Type 3R An enclosure designed for use outside, primarily to provide some protection from falling rain and damage caused by external ice formation.
Type 3S An enclosure designed for outdoor use, primarily to provide some protection from rain, sleet, and windblown dust, as well as to allow the operation of external mechanisms when ice is present.
Type 4 An enclosure designed for indoor or outdoor use to provide some protection from wind-blown rain and dust, splashing water, hose-directed water, and damage caused by external ice formation.
Type 4X An enclosure designed for indoor or outdoor use to provide some protection from corrosion, windblown rain and dust, splashing water, hose-directed water, and damage caused by external ice formation.
Type 6 An enclosure designed for indoor or outdoor use to provide some protection from falling dirt, hose-directed water, water entry during occasional temporary submersion at a specified depth, and damage from external ice formation.
Type 6P An enclosure designed for indoor or outdoor use to protect against falling dirt, hose-directed water, and water entry during prolonged submersion at a specified depth, as well as damage caused by external ice formation.