Hazardous Area Location Classification

Group G atmospheres conutmng combustible 1 dusts having resistivity of iOO.OOOohm-cemimeier or greater 

The European philosophy on area classification varies from that of the United. States and Canada. Specifically, in Europe and most other inter national areas, the Zone concept is utilized. An area in which an expio sive gas-air mixture is continuously present, or present for long perioiK of time, is referred to as Zone 0. The vapor space of a closed, but vented, process vessel or storage tank is an example. An area in which an explosive gas-air mixture is likely to occur in normal operations is designated Zone 1. An area in which an explosive gas-air mixture is less likely to occur, and if it does occur will exist only for a short time, is designated Zone 2. Zone 0 and Zone 1 correspond to Division 1 in the U.S. and Canada System. Zone 2 is equivalent to Division 2. 

In order to properly classify areas surrounding production equipment, not only must the specific items of equipment (separators, pumps, compressors. etc.) be identified, but also the degree of ventilation must be 

T1 = temperature of indoor air, °R = temperature of outdoor air, °R H = height from the center of the lower opening to the Neutral Pressure Level (NPL), ft 

NPL is the point on the vertical surface of a building where the interior and exterior pressures are equal. It is given by  

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NFPA 497 Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas, 1997 edition. National Fire Protection Association, Quincy, MA. 

FPN No. 1) As a guide in determining when flammable gases are present continuously, for long periods, or under normal conditions, refer to. Recommended Practice for Classification of Locations for Electrical Installations of Petroleum Facilities, API RP 500-1991, Electrical Apparatus for Explosive Gas Atmospheres, Classifications of Hazardous Areas, lEC 79-10 and. Institute of Petroleum Area Classification Code for Petroleum Installations, IP 15. 

NFPA-325 Guide to Fire Hazard Properties of Flammable Liquids, Gases and Volatile Solids, (1994 ed.), NFPA-321 Basic Classification of Flammable and Combustible Liquids (1991 ed.), NFPA-497A, Classification of Class 1 Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas (1992 ed.), and NFPA-497B, Classification of Class II Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas (1991 ed.), National Fire Protection Association, Quincy, MA. 

Hazardous (classified) locations have traditionally been designated by Class, Division, and Group. Equipment used in areas so designated is selected and systems are designed, based on requirements established for the classification. This approach is defined in Article 500 of the NEC and API 500. 

NFPA 497A—Recommended Practice for Classification of Class I Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas contains recommendations for wiring requirements in an enclosed area where lighter-than-air flammable gases may be released [5.12]. While these recommendations are for chemical processing areas, the releases are similar in the hazard they present to what would be found for natural gas released in repair garages. These recommendations are also illustrated in Figure 5-1. 

NFPA 497 (2004) Recommended practice for the classification of flammable liquids, gases or vapors and of hazardous (classified) locations for electrical installations in chemical process areas. (National Fire Protection Association). 

Design for Hazardous Areas. This type of area classification tends to simplify the electrical design by directive means, but poses alternate choice problems, dictated by economics. The cost of an explosion-proof installation is as much as twice that using standard equipment. As an example of more costly electrical equipment, rigid metal conduit with threaded explosion-proof boxes and fittings are required for all Class I, Division 1, hazardous locations either rigid metal conduit or electrical metal tubing is approved for both Class I and II, Division 2, locations. 

The discussion on hazardous area standards is concluded with the recommendation that the applicable standard part must be read thoroughly before starting any equipment selection. Since area classification is mainly carried out by mechanical and safety engineers, good coordination is helpful. Gas detection systems for hazardous locations is another area of concern for instmmentation and control engineers so brief discussions on combustible gas detection systems will be dealt with in the next clause. 

The plant operator must be able to identify those areas of the plant presenting identifiable hazards and must maintain wiring systems which conform to the probable hazard present. Great skill is needed to keep to a minimum the electric devices actually located in the highest hazard areas. Careful arrangement keeps control and switching devices in the area with the lowest classification. Explosion-proof wiring is expensive. It is often less expensive to remove or reduce the hazard than it is to provide the appropriate electric system. 

Where there are flammable materials that constitute an explosion risk, the locations in which the explosion risk exists are called hazardous areas. These areas are classified into zones according to the extent of the risk, using guidance published in BS EN 60079-10 1996 Electrical apparatus for explosive gas atmospheres. Part 10 Classification of hazardous areas. The zones are also defined in the European Directive on minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres. The zones are ... 

Employers in control of hazardous areas need to determine the locations and extents of these zones in each of the hazardous areas, an exercise known as hazardous area classification. At present, in 2001, there is no specific legal duty to carry out hazardous area classification, apart from the general duties to carry out risk assessments and, in the context of mines, in Regulation 19 of the Electricity at Work Regulations. However, the Protection of Workers Potentially at Risk from Explosive Atmospheres Regulations expected to be enacted soon, will contain specific provisions relating to area classification and the need to record the area classification information in an explosion protection document. 

When the facility uses or stores hazardous chemicals or gases in sufficient quantity, it must be considered a Hazardous, Group H occupancy. A summary of the quantity, location and type of hazardous materials located within a given control area is required in order to make the correct occupancy classifications. If a review of the hazardous production material (HPM) inventory indicates quantities above the exempt amounts set in UBC Tables 3-D or 3-E (pgs. 1-62 to 1-64) for a control area, an H occupancy classification is mandated. Refer to Tables 1 and 2 which have been taken from the 1994 UBC. 

National Fire Protection Association (NFPA), NFPA 497A. Classification of Class I Hazardous Locations for Electrical Installation in Chemical Process Areas. NFPA, Quincy, MA, 1992. 

Conducting an electrical area classification An electrical area classification involves identifying the extent (if any) of location(s) in a facility where combustible materials could be present during normal and/or abnormal conditions. According to Article 500 of the NEC, major categories of hazardous locations are ... 

Review of surrounding plants/facilities Existing pipe racks Maintenance and operations access Sewers and storm water drainage Existing fire and radiation circles Hazardous chemical releases from upwind locations Existing electrical area classification Close access to substations... 

For Case 2, each DVP requires 72.66 kWh of electricity, and operating cost savings due to this modification are US 257 801/year. Installed cost of two DVPs, based on vendor quotation, is US 400 000. So, payback period for this retrofit modification is 1.6 years. Thus, replacing the existing SJEs by DVPs is attractive for low capacities and non-hazardous applications such as a steam turbine s surface condenser vacuum system. DVPs are efficient, they require less electricity and so modification required at the substation is minimal. They are very compact and can be installed at ground level or on elevated platforms. Depending on the area classification of their location, explosion-proof motors may be required. 

The local site effects play an important role in the evaluation of seismic hazard. The proper evaluation of the local site effects will help in evaluating the amplification factors for different locations. This article deals with the evaluation of peak ground acceleration and response spectra based on the local site effects for the study area. The seismic hazard analysis was done based on a probabilistic logic tree approach and the peak horizontal acceleration (PHA) values at the bed rock level were evaluated. Different methods of site classification have been reviewed in the present work. The surface level peak ground acceleration (PGA) values were evaluatedfor the entire study area for four different site classes based on NEHRP site classification. The uniform hazard response spectrum (UHRS) has been developed for the city of Bangalore and the details are presented in this work. 

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