By Mark Harrop, BA.Sc., CFPS Randal Brown & Associates
Electrical Equipment in Hazardous LocationsEngineering
Specifiers often make conservative, costly, errors when specifying electrical equipment for hazardous locations. The person may lack information regarding the operations conducted in an area, or they...
Specifiers often make conservative, costly, errors when specifying electrical equipment for hazardous locations. The person may lack information regarding the operations conducted in an area, or they might not understand the behaviour of materials or the applicable code requirements. Whatever the reason, they fail to apply the requirements of the Electrical Code in a practical fashion.
Under-specification of equipment can reduce the level of life safety and property protection in the area. Over-specification leads to unnecessary costs to owners. Electrical equipment designed for use in hazardous locations can be five to 15 times more expensive than unclassified electrical equipment, depending on the type.
A typical hazardous location might be an automotive spray painting operation, a pharmaceutical manufacturing plant, cement plant, coal processing operation or even a carpet factory. Hazardous locations also include rooms in which flammable liquids are used or stored, fuel dispensing stations, and repair garages.
Classified electrical equipment is equipment designed to be used in a specific hazardous location without presenting an ignition hazard. It can be made suitable in a number of ways. “Intrinsically safe” electrical equipment is designed to limit the electrical energy flowing into the device so that in the event of a fault, the energy released is not sufficient to ignite a hazardous atmosphere. In other types of classified equipment the interior of the device is filled with a substance — perhaps sand or oil — that will prevent an ignitable mixture from forming inside. There is also classified electrical equipment that uses a protective gas fed into the enclosure to prevent flammable or combustible materials from entering. The wiring that feeds an electrical device in a classified area has to meet strict guidelines; these may require running wire inside threaded rigid metal conduit and sealing the conduit at electrical enclosures and unions.
Though people often complain and question the necessity of providing classified equipment in certain process areas, it is possible to achieve cost-effective installations that comply with building codes and standards by applying critical evaluations during the planning and design phase.
The legislated requirements concerning electrical installations in hazardous locations are contained in the Canadian Electrical Code (CEC), which is Canada’s model code document.
The hazardous location classification system in use in Canada for areas containing vapours of flammable liquids is the “Zone” system. The zone system is slightly different from the previous “Division” system of classification, which remains in use for areas containing combustible dusts and ignitable flyings.
The Canadian Electrical Code defines three classes of hazard:
* Class I – Flammable Vapours
* Class II – Combustible Dusts
* Class III – Ignitable Fibres and Flyings.
Once the class of hazardous location is known, it is then identified as a zone within the plant or the facility.
Class I hazardous locations are identified by zones as follows:
* Zone 0 — a location in which explosive gas atmospheres are present continuously or for long periods of time.
* Zone 1 — a location in which explosive gas atmospheres are likely to occur during normal operation, or which is adjacent to a Zone 0 location.
* Zone 2 — a location in which explosive gas atmospheres are not likely to occur in normal operation, and if they do occur will last for only a short time.
The Canadian codes unfortunately give limited guidance on how to define the extent of the hazardous class and zone. The Canadian Electrical Code provides four generic figures illustrating the extent of typical Class I hazardous locations, but these figures are not specific and cannot be expected to apply to all potential process conditions.
While the Canadian codes are complex and somewhat difficult to apply, additional and more practical guidance for determining the requirements for a hazardous location are published by the U.S. National Fire Protection Association (NFPA).
NFPA 497 “Recommended Practice for the Classification of Flammable Liquids, Gases or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas” (2004 edition) and NFPA 499 “Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas” (2004 edition) address the classification of areas containing flammable liquids and those containing combustible dusts, respectively.
NFPA 497 provides 37 diagrams that illustrate common plant processes and situations and the required extent of classified areas around these locations. The diagrams address indoor and outdoor vapour sources, vapour sources at grade and above grade, and variations in the extent of areas based on the size, pressure and flow of flammable liquids in a process. Diagrams specific to tank loading and unloading operations, and areas containing lighter than air vapours, are also presented.
NFPA 499 has nine diagrams related to dusty atmospheres. The diagrams provide guidance for applying the concepts detailed in the guideline to determine the required classification in an area. The classification is based on the reduction in visibility in the room caused by dusts in suspension in air, and on the typical thickness of dust layers that accumulate on horizontal surfaces in an area over a 24-hour period.
While the classified areas described in NFPA 497 and 499 are not legislated requirements in Canada, the information represents good engineering practice and is a starting point for analyzing the required extent of hazardous locations.
Flammable Liquids — The vast majority of vapours produced by flammable liquids are heavier than air (i.e. vapour density >1). Some of the very few flammable liquids listed by NFPA that emit vapours lighter than air are 2-chloropropionyl chloride, hydrocyanic acid and jet fuels (e.g. JP-6). Therefore, the majority of Class I hazardous locations should have well defined upper boundaries since heavier-than-air vapours are unlikely to rise to the ceiling.
In a typical scenario, a flammable liquid dispensing room might occupy between 10 and 100 square metres in an industrial facility with drums stored on the floor. Dispensing may be conducted in the room from the drums into smaller containers, which can be transported more easily to various areas in the plant. The liquid is transferred to the smaller drums by hand operated pumps inserted into the drum in an operation that creates two normal vapour sources in the room — one at the open bung of the drum, and the other at the opening of the receiving container.
Figures contained in the Canadian Electrical Code show that the hazardous location surrounding an indoor vapour source extends to a height of 1.5 metres (4.9 ft.) above the source and is a Zone 1 area where adequate ventilation is not provided. Where adequate ventilation is provided, the area extends 1.0 metre (3.3 ft.) above the source and is classified as Zone 2. Adequate ventilation is defined in the code as natural or mechanical ventilation that is sufficient to prevent the accumulation of flammable vapours in concentrations greater than 25% of their lower explosive limit (LEL).
The height of a standard 55 US gal. (208 L) drum is approximately 0.9 metres (3 ft.). Based on the extent of classified areas illustrated in the Canadian Electrical Code for heavier-than-air vapours, even in an inadequately ventilated room, the classified area should extend to a maximum height of 2.4 metres (8 ft.). Despite this fact, lighting equipment suitable for a Class I hazardous location is frequentl
y provided in dispensing rooms for flammable liquids where ceiling heights are 3 metres (10 ft.) or higher, at an increased cost to the owner. See page 22 for an illustration of the classified areas required in this scenario.
This type of conservative specification is the result of a lack of understanding of the applicable requirements and a failure to apply reasonable physical principles in classifying a location.
A similar misconception occurs in rooms that contain a classified area, where electrical equipment through the entire volume of the room often is specified as classified. Careful analysis of the operations conducted in a space and the location of equipment and fixed vapour sources present will result in clearly defined classified areas. Unclassified electrical equipment can then be installed outside the classified areas at a substantial cost savings.
As an example, in a large processing room containing four tanks of flammable liquid with heavier-than-air vapours, each tank is provided with its own dike for spill control. Since the tank is fixed in place and if the walls of the dikes are far enough away from the tank shells (approximately 1.5 m. (5 ft.)), the required classification area could be contained in each dike (since the vapours released will be heavier than air). This design would leave the remainder of the room free for mounting electrical panels and other equipment.
However, this type of approach cannot always be accomplished due to the presence of mobile vapour sources.
Dusts — Applying hazardous location classifications to dusty areas can be equally difficult. The presence of dusts is not enough to warrant classification as a hazardous area. Rather it depends on the type of dust present, the method in which the dust is handled, the frequency of handling and the resulting quantity of dust that might be released into the air.
The guidelines contained in NFPA 499 provide simple quantitative rules for these situations. These rules are based on the combustible range for the dust. They use the visibility in the room during dust producing processes to determine the required classification area, as well as the thickness of the accumulated dust layer on horizontal surfaces after a 24-hour period.
The NFPA 499 guideline specifically looks at the atmosphere that results from a process in operation. This makes applying its rules to the electrical classification of a new process a difficult task. Providing unclassified equipment in the expectation that a process will not produce much dust can, if the expectation turns out to be wrong, result in a costly replacement of electrical equipment or the need to increase the capability of the dust collection system. Conversely, specifying classified electrical equipment for a process that turns out to be relatively dust free is an unnecessary cost.
The best method for determining the appropriate classification for an area containing a proposed dusty process is to draw a comparison to a similar existing location.
To conclude, in addition to knowing the legislated requirements, consultants need to evaluate the physical properties of various dusts and liquids in order to properly determine the extent of any classified area. Careful analysis is a key component of specifying electrical equipment.
Mark Harrop, BA.Sc., CFPS is an engineering intern at Randal Brown & Associates, a building code and fire protection engineering firm in Toronto.