It describes a test method intended for the measurement of the maximum experimental safe gaps MESG for gas- or vapour-air mixtures under normal conditions of temperature1 and pressure so
General
Gases and vapours 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 vapours in the table of Annex B are given below.
Classification according to the maximum experimental safe gaps (MESG)
Gases and vapours may be classified according to their maximum experimental safe gaps (MESG) into the groups I, IIA, IIB and IIC
The standard method for determining the Minimum Explosible Concentration (MESG) is outlined in section 6.2, although provisional acceptance is allowed for results obtained from an 8-liter spherical vessel with ignition near the flange gap.
The groups for equipment for explosive gas atmospheres are:
Group I: equipment for mines susceptible to firedamp
Group II: equipment for places with an explosive gas atmosphere other than mines susceptible to firedamp
Group II equipment is subdivided and, for the purpose of classification of gases and vapours, the MESG limits are:
Group IIB: 0,5 mm < MESG < 0,9 mm
NOTE 1 For gases and highly volatile liquids the MESG is determined at 20 °C
For MESG determination at temperatures exceeding ambient levels, a temperature of 5 K above the required vapor pressure or 50 K above the flash point is utilized The resulting MESG value is then recorded in the table, which informs the classification of the equipment group based on this outcome.
Classification according to the minimum igniting currents (MIC)
Gases and vapors can be categorized based on the ratio of their minimum igniting currents (MIC) to the ignition current of laboratory methane The standard procedure for measuring MIC ratios is outlined in IEC 60079-11; however, results obtained from alternative apparatus may be provisionally accepted.
Group II equipment is subdivided and, for the purpose of classification of gases and vapours, the MIC ratios are:
Classification according to MESG and MIC
For most gases and vapours, it is sufficient to make only one determination of either MESG or MIC ratio to classify the gas or vapour.
Classification according to a similarity of chemical structure
In the case of gases or vapours belonging to a homologous series, their classification can be tentatively determined based on the properties of other lower molecular weight members of the series However, conducting a direct test is recommended whenever feasible.
Classification of mixtures of gases
Gaseous mixtures should be classified into specific groups only after accurately assessing their Minimum Explosible Concentration (MESG) or Minimum Ignition Concentration (MIC) ratio A reliable approach to estimate the appropriate group involves calculating the MESG of the mixture using a variant of the Le Châtelier principle.
One determination is adequate when:
Group IIA: MESG ≥ 0,9 mm, or MIC > 0,9;
Group IIB: 0,55 mm ≤ MESG < 0,9 mm, or 0,5 ≤ MIC ≤ 0,8;
Group IIC: MESG < 0,5 mm, or MIC < 0,45
To determine the MESG and MIC ratio, specific conditions must be met: for IIA, the MIC should be between 0.8 and 0.9, confirmed by MESG; for IIB, the MIC must range from 0.45 to 0.5, also confirmed by MESG; and for IIC, the MESG should be between 0.5 and 0.55, confirmed by MIC.
This method is unsuitable for mixtures or streams containing acetylene or similar hazards, oxygen or strong oxidizers, or high concentrations of carbon monoxide (over 5%) Caution is advised with two-component mixtures that include an inert component, like nitrogen, as they may yield unrealistically high Minimum Explosible Concentration (MESG) values.
For mixtures containing an inert such as nitrogen in concentrations less than 5 % by volume, use an MESG of infinity For mixtures containing an inert such as nitrogen in concentrations
5 % and greater by volume, use an MESG of 2
An alternate method that includes stoichiometric ratios is presented in the paper by Brandes and Redeker
5 Data for flammable gases and vapours, relating to the use of equipment
Determination of the properties
General
The compounds listed in this standard are in accordance with Clause 4, or have physical properties similar to those of other compounds in that list.
Equipment group
The groups are determined by the MESG or MIC ratio, except in cases where these values are not available In such instances, the grouping is based on chemical similarity, as outlined in Clause 4.
When determining the Minimum Explosible Concentration (MESG) at temperatures exceeding ambient levels, a temperature of 5 K above the required vapor pressure or 50 K above the flash point is utilized The MESG value obtained is then listed in Annex B, and the classification of the equipment group is based on this result.
Flammable limits
Various methods have been employed for determinations, with the most favored approach being low energy ignition at the base of a vertical tube The results, expressed as percentages by volume and mass per volume, can be found in the table in Annex B.
A high flash point indicates that a compound does not produce a flammable vapor-air mixture at normal ambient temperatures Flammability data for these compounds are typically obtained at elevated temperatures, which enable the vapor to create a flammable mixture with air.
Flash point FP
The values presented in Annex B refer to the "closed cup" measurement; if this data is unavailable, the "open cup" value is used instead The symbol "