HOW TO DETERMINE EXPLOSIVE ATMOSPHERES. Author: Mihai Magyari, Ph.D.Eng. INSEMEX PETROSANI ROMANIA 32-34 G-ral. Vasile Milea Street, Petrosani, Hunedoara county +40 254 541 621; +40 254 541 622; Fax: +40 254 546 277 e-mail: firstname.lastname@example.org; http://www.insemex.ro.
Author: Mihai Magyari, Ph.D.Eng.
INSEMEX PETROSANI ROMANIA
32-34 G-ral. Vasile Milea Street, Petrosani, Hunedoara county
+40 254 541 621; +40 254 541 622; Fax: +40 254 546 277
e-mail: email@example.com; http://www.insemex.ro
1. Terms and definitions
Continuous grade release - A release that is continuous or nearly so, or that occurs frequently and for short periods. If it is likely to be present for more than 1000 hours per year.
An explosion occurs if a fuel is present in mixture with air (i.e. sufficient oxygen) within the explosion limits, together with a source of ignition.
Figure 1 – Explosion triangle
2. Classification of gases and vapors
Gases and vapors can be classified according to the group or sub-group of equipment required for use in the particular gas or vapour atmosphere.
The general principles used to establish the lists of gases and vapors are given below.
2.1 Classification according to the maximum experimental safe gaps (MESG)
Gases and vapors may be classified according to their maximum experimental safe gaps (MESG) into the groups I, IIA, IIB and IIC.
The groups for equipment for explosive gas atmospheres are:
2.2 Classification according to the minimum igniting currents (MIC)
Gases and vapors may be classified according to the ratio of their minimum igniting currents (MIC) with the ignition current of laboratory methane. The standard method of determining MIC ratios shall be with the apparatus described in IEC 60079-11, but where determinations have been undertaken in other apparatus these can be accepted provisionally.
There is a functional connection between the maximum experimental safe gap (MESG) and the minimum ignition current (MIC).
3 Data for flammable gases and vapors, relating to the use of equipment
3.1 Equipment group
The groups are the result of MESG or MIC ratio determination except where there is no value listed for MESG or MIC ratio. For these, the group is based on chemical similarity.
3.2 Flammable limits
Determinations have been made by a number of different methods, but the preferred method is with a low energy ignition at the bottom of a vertical tube.
If the flash point is high, the compound does not form a flammable vapor/air mixture at normal ambient temperature. Where flammability data are presented for such compounds the determinations have been made at a temperature sufficiently elevated to allow the vapor to form a flammable mixture with air.
3.3 Temperature class - The temperature class of a gas or vapor is given according IEC 60079-14 in the following table
Table 1 – Classification of temperature class and range of auto-ignition temperatures
4. Hazardous area classification
4.1 Sources of release
4.2 Type of zone
The likelihood of the presence of an explosive gas atmosphere depends mainly on the grade of release and the ventilation. This is identified as a zone. Zones are recognized as: zone 0, zone 1, zone 2 and the non-hazardous area.
A continuous grade of release normally leads to a zone 0, a primary grade to zone 1 and a secondary grade to zone 2.
The extent of the zone is mainly affected by the chemical and physical parameters of the flammable material and the process itself, such as:
The greater the release rate, the larger the extent of the zone.
The release rate depends itself on other parameters, namely:
Figure 2 – Example of
spread of liquefied gases
5.1 Natural ventilation
5.2 Artificial ventilation
Figure 3 – Example of the correct arrangement of ventilation openings for heavier than air gases and vapors
5.3 Degree of ventilation
The following three degrees of ventilation are recognized:
Can reduce the concentration at the source of release virtually instantaneously, resulting in a concentration below the lower explosive limit. A zone of negligible extent results.
Can control the concentration, resulting in a stable zone boundary, whilst the release is in progress, and where the explosive gas atmosphere does not persist unduly after the release has stopped.
Cannot control the concentration whilst release is in progress and/or cannot prevent undue persistence of a flammable atmosphere after release has stopped.
Table 2 - Influence of independent ventilation on type of zone
Table 3 - Hazardous area classification data sheet Part I: Flammable material list and characteristics
Table 4 - Hazardous area classification data sheet Part II: Flammable material list and characteristics
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