Hazardous areas

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. 

In order to determine the location and extent of the hazardous areas it is necessary to establish the sources of release and the likely dispersion of the flammable material from the source in any direction to the point where its concentration is below the LEL. Factors such as the rate of release, concentration, volatility, ventilation, topography and density of the gas/air mixture will need to be taken into account. A contour line joining the points so established, both vertically and horizontally, is the boundary between adjacent zones. 

BS EN 60079-10 1996 provides guidance on a systematic approach that can be used to identify the volumetric characteristics of the hazardous areas, taking due account of the factors listed above. The standard contains a number of example calculations and shows how the zones can be marked around possible sources of release (see Fig. 15.1 for an example). 

Establishing the boundaries between the various zones in a hazardous area is a difficult task because of the assumptions and variables involved and it is best carried out by those concerned who have the appropriate knowledge, such as the process, production and chemical engineers and the safety officer. It will usually be found relatively easy to determine the Zone 0 and Zone 1 boundaries where the release of the material is controlled. These boundaries are usually within a few metres of the release source, but Zone 2 boundaries are more difficult because the emission is not controlled as it is due to an abnormal occurrence, such as a leaking pump gland or perhaps a burst pipe, and may occur in unmanned areas where immediate discovery is unlikely. 

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The explosion-proof enclosure is designed such that an explosion in the interior of the enclosure containing the electronic circuits will be contained. The enclosure will not allow sufficient flame to escape to the exterior to cause an ignition. Also, a surface temperature rating is given to the device. This rating must indicate a lower surface temperature than the ignition temperature of the gas in the hazardous area. 

Because self-regulating heaters are parallel heaters, they may be cut to length at any point without changing their power output per unit of length. This makes them much easier to deal with in the field. They may Be terminated, teed, or spliced in the field with hazardous-area-approved components. 

Except for areas with fire or explosion hazards (hazardous areas), motor enclosures are designed to provide protection to the internal working parts. The development or improved insiilating materials and finishes has affec ted the required degree of protection and consequently the design and classification of enclosures. Examples of several types of enclosures are shown in Fig. 29-4. 

A restricted temperature rise as for a spinning mill, a refinery or i hazardous area... 

In India, the Central Mining Research Institute, Dhanbad carries out this testing and provides the necessary certilication for motors used iti explosive atmospheres. But for approval of the equipment, whether it is worthy of use in a particitlar hazardous area, there are accredited agencies.. Some of these are Directorate General Mines Safety. Dhanbad, Chief Controller of Explosives. Nagpur and Directorate General of Factory Advice Service and Labour Institute. Bonibav. 

Electrical apparatus for explosive gas atmospheres Classification of hazardous areas 5572/1994 BS EN 60079-10/1996 ... 

Electrical installations in hazardous areas (other than mines)... 

A normal enclosure is meant for a reasonably clean atmosphere and a relative humidity not more than 50% for LT and 95% for FIT indoor enclosures. Where the atmosphere is laden with fumes or steam, saline or oil vapours, heat and humidity, excessive dust and water or contaminated with explosive and fire hazardous gases, vapours or volatile liquids (Section 7.11) a special enclosure with a higher degree of protection is required as in lEC 60529 or lEC 60079-14. For non-hazardous areas, the enclosure can be generally one of those discussed in Tables 1. 10 and 1. 11, and when required can be provided with special treatment to the metallic surfaces. For hazardous areas, however, special enclosures will be essential as discussed in Section 7.11. 

For hazardous areas flameproof enclosures alone are recommended, except in areas with moderate intensity of contamination and where such assemblies are located away from the affected area and in a separate well-ventilated room, when pressurized enclosures may also be. safe. The reason for this precaution is that frequent arcing takes place within the enclosure on each switching of a contactor, switch, breaker or an OCR etc. and also during operation of power and auxiliary contactors. 

Since it is not practical to manufacture a llameproof enclosure due to its size and bulk and the number of knockouts and openings on the doors for switches, metering, indicators, and pushbuttons (PBs) etc., it is common practice to locate the.se assemblies some distance from the affected area in a separate well-ventilated room. Depending upon the location and intensity ol contamination, it may be permissible to meet the requirement by using a pressurized enclosure by maintaining a positive pressure inside the enclosure similar to that for motors (Section 7.1.3..3). When there arc many switchgear assemblies, the room itself can be pressurized, which is safer and easier. Small enclosures, however, such as a PB station, switch or a switch fuse unit or an individual starter unit etc., which can be easily made of MS plates or cast iron, as discussed in Section 7.13, can be mounted in the hazardous area while the main MCC can be installed in the control room, away from the contaminated area and from where the process can be monitored. 

The storage and processing of powder paints must be carried out under well-controlled conditions, and preferably away from fire hazardous areas. The powder on mixing with air becomes inflammable and can cause an explosion. Powder paints should be stored at about 2.6 C. 

These transformers may be PVC taped, thermoplastic (polypropylene) moulded, fibreglass taped, polyester resin cast or epoxy resin cast depending upon the system voltage and the surroundings. HT indoor transformers, for instance, are generally polyester or epoxy resin cast, and are economical with good dielectric properties. They are resistant to humid, chemically contaminated and hazardous areas. Outdoor HT transformers, how-ever. may be epoxy... 

Table 21.3 Maximum impedances of ground loop lor protection by ground leakage relays in hazardous areas...

Protection of a domestic or an industrial single phase system Ground fault on an LT system Ground fault protection in hazardous areas. Ground leakage in an HT system Core-balanced current transformers (CBCTs). Ground fault (G/F) protection schemes... 

Today loading piers are mostly cathodically protected with impressed current. At moorings for tankers, cathodic protection rectifiers are installed on extinguisher bridges as far as possible from the hazardous area. Otherwise, they must be of an explosion-proof type. 

Agahi, R., Ershaghi, B., Bosen, W., and Leonhard, M., Turboexpanders with Magnetic Bearings in Air Separation and in Hazardous Areas, MAG 92, Alexandria, VA, 1992. 

Must be air purged, explosion proof, or intrinsically safe to be used in hazardous areas. 

This chapter will describe all common compressor drivers, but as a practical consideration, details on selection or sizing, hazardous area... 

Classification of hazard areas according to the probability of a flammable concentration of vapour occurring (to BS 5345 Part 1)... 

AMD 1 Electrical apparatus for explosive gas atmospheres. Electncal installations in hazardous areas (other than minesj. Comgendum No. 1 (Corr. 10013J dated April 1998, superseded BS 5345 Part 1 1989, Parts 2 and 7 1979, Part 4 1977, Part 5 1983, Part 6 1978 and Part 8 1980... 

Wlien a contractor/subcontractor is hired to perform work in a potentially hazardous area on one of your facilities or that of a client, the project manager shall provide the contractor/subcontractor a copy of the contractor site safety rules checklist for completion. The form and any other rules specific to that site must be signed, dated, and returned prior to any work being performed at a particular site. An executed copy will be made a part of the project file. 

The fire department blamed the accident on welders cutting in hazardous areas without a fire watch, highly combustible structural components (fiber-glass-resin), high-density storage of highly flammable and detonable material, spilled ammonium perchlorate about the plant, and high w ind conditions. 

Figure 17-3. Hazardous area cbssHkalions used in U.S. and Canada, in accordance with Arride 500, NEC Cock—1984. [Courtesy of R. Siahl, Inc.)...

Figure 17-15. Hazardous area location diogram for a typical offshore production platform.

LIMITATIONS ON INSTALLATION OF ELECTRICAL DEVICES IN HAZARDOUS AREAS... 

A motor used in standby operation mode should be equipped with a space heater to keep the motor windings dry. In classified areas these space heaters must meet the surface temperature requirement of the specific hazardous area. 

Instrument Purging for Reduction of Hazardous Area Classification... 

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