Explosion protection for zone

Until 1999, there exists no international standard for explosion protected electrical apparatus intended for the application in zone 0. Intrinsically safe circuits according to IEG 60079-11 and EN 50020, category ia, seemed to be an appropriate type of protection for zone 0 for some time. In the same way, for coal mines the first international standard describing the requirements for electrical apparatus that are intended for operation in the presence of methane/coal dust-air mixtures was prepared in 2000. 

In Germany, two attempts have been undertaken in order to establish national standards for this purpose. The first attempt resulted in a draft 

Flat tanks with 12 m diameter and 7 m height, volume 800 m3 each. Horizontal cylindric tanks with 4 m diameter and 16 m length, volume 200 m3 each. Filling rate 120m3/h, flash point 0°C. The value R is tabulated in Table 2.4 with R = 3m. 

The protection area around the exhaust duct extends to a diameter of 6R =18 m. 

The interior of the tanks is classified as zone 0. Dimensions given in metres. 

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Pressurization, with its great diversity of apparatus to be explosion protected for zone 1 (and zone 2) applications, is the favoured protection technique for... 

Apart from inserting the complete gas analyser into a p-enclosure, the analyser head may be explosion protected for zone 1 application (e.g. according to flameproof enclosure - d - ) on its own, whereas the central control unit may be installed in a non-hazardous area, or alternatively, in a pressurized enclosure (Fig. 6.25) when zone 1 installation is required. 

Due to the quite different structures of industrial plants equipped with electrical installations, there are different probabilities for the existence of potentially explosive atmospheres formed by gas-air, vapour-air or dust-air mixtures. With respect to economical reasons, the types of explosion protection for electrical apparatus and systems will depend strongly on the explosion risk at the installation point. It is very unusual to construct and build all explosion protected electrical apparatus in such a manner that they can operate permanently in a surrounding hazardous atmosphere with combustible gases, vapours and dusts in air. The common way is to classify different areas in industrial plants according to the probability for the existence of a hazardous atmosphere and to establish adequate types of explosion protection. In other words, industrial plants with potentially explosive atmospheres are divided and classified into zones. ... 

Table 6.6 Overview about the assignment between equipment categories and zones for the implementation of the explosion protection for the equipment group II.

Following the historical development of electrical engineering and explosion protection, zone classification was the objective of national standards and installation rules. Most of the leading industrial countries established an installation practice for chemical plants and the oil and gas industry with two or three zones for areas hazardous due to gas- or vapour-air mixtures and two zones for areas with hazardous dust-air mixtures. Apart from this philosophy, the coal mining industry in most countries tends to avoid an area classification and defines only one category of explosion protection ( firedamp-proof ). More recent standards or directives present a three-zone concept for areas endangered by combustible gas- (vapour-, mist-) air mixtures and dust-air mixtures in industrial plants (other than coal mines). 

In the same way as for zone 0, a national forerunner in Germany started work for mining apparatus intended for continuous operation in an explosive atmosphere. The work was done by members of the national committees focused on explosion protection and resulted in a decree stated by the national authority. On the basis of clause 30 of ElBergV [7], the following protection concepts are considered applicable for a continuous operation under a permanently existing explosive atmosphere ... 

Explosion protected electrical equipment for zone 1 (Group II, category 2 G) and the classical firedamp-proof apparatus for coal mines (Group I, category M2) are covered by the B module (EC-type examination) and by the C module (conformity to type) or the E module (product quality assurance). Alternatively, with respect to the B module, the notified body issues an EC-type examination certificate (in German EG-Baumusterpriifbescheinigung), whose marking mainly includes ... 

Figure 6.10 shows a typical example for a q-apparatus. It is a power supply for intrinsically safe electric circuits to be installed in zone 1 (and zone 2). The power input (at 24 V AC/DC level) is fed via flameproof plug-and-socket connectors to the apparatus. So, the power supply can be replaced under load without conflicting with the requirements of explosion protection. 

The marking of such s -apparatus has been (Sch)s for mining equipment and (Ex)s G... for explosion protected apparatus (suitable for zone 1 applications). In many cases, protection techniques, which are covered by their own standards today, e.g. encapsulation - m - or static pressurization (as a part of pressurization - p - ), have entered the field of explosion protection as s -apparatus [17]. 

Thus, based on the knowledge of the gases occurred and specified zones, each apparatus can be defined completely (as far as explosion protection is concerned), e.g. Ex II2G EEx ib IIC T4, obviously designed for zone 1 application. 

In the history of the development of explosion protection techniques for electrical apparatus located in zone 2, the non-sparking concept (see Section 2.4)... 

Pressurized enclosures - application in zone 2 and for dust explosion protection... 

In the final part of the book, focal points are financial considerations (Chapter 9), inspection, maintenance and repair of explosion protected apparatus (Chapter 10), apparatus for installation in zone 0 and in zone 2 (Chapter 11) and cable protection systems, especially for underground installations in coal mines (Chapter 12). 

Analogous zones 20-22 exist for the occurrence of explosive dusts.) A place where the occurrence of a dangerous gas mixture is not to be expected normally at all belongs to neither of the zones. No special explosion protection measures are necessary at such a place. 

The selection of electrical apparatus is related to the assignment of risk, e.g. Zone 0, 1 or 2 as specified in International Electrotechnical Recommendation lEC 79-10. The Zone determines the method of explosion protection required for the electrical equipment, and this affects the cost of installation. 

Vacuum pumps of category 2 which pump from zone 1 have to be subjected by the manufacturer to an internal control of production according to annex Vlll of the directive. In addition, the manufacturer has to deposit a technical documentation for the vacuum pump at a Notified Body, including a risk assessment, ignition hazard assessment and notes for the implemented measures to maintain the explosion protection. 

Before vacuum pumps are put into operation, the operating company of a vacuum pump has to examine and assess the explosion risks which can be caused in the connection with the installation site and the processes applied. He must prevent or limit the formation of explosive areas. Remaining explosive areas have to be assigned to zones. Possible ignition sources in the zones must be avoided. The work equipment for the required equipment categories, for example explosion-protected vacuum pumps of the respective category, has to be selected. The results of the risk assessment, the applied protective measures, the zone classification and the remaining risks have to be recorded in the explosion protection document. 

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 ... 

Facilities that are deemed critical or highly manned should be relocated out of the overpressure circles or provided with explosion protection measures. Other systems within these overpressure zones should be evaluated for the specific benefits of providing explosive protective design arrangements. An example is provided in Figure 16.1. 

Siting - The preferred arrangement of bulk transport pipeline systems is for burial underground. This provides for enhanced protection from overhead events. This is even the case for offshore pipelines where there have been numerous incidents of dragged anchors from fishing vessels to pipelines exposed on the seabed. A radius of exposure from a pipeline can also be easily calculated for fires and vapor explosions based on the commodity, pressure, release opening, etc. From these calculations a restricted zone or similar can be designated. 

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