ignition energy Ignition energy is the spark energy (in joules) which ignites the most easily ignited mixture of the test gas with air at atmospheric pressure. All flammables (including dusts) have minimum ignition energy (MIE) input required to initiate combustion. The MIE depends on the specific chemical or mixture, the Concentration, pressure, and temperature. The MlE’s of a few typical gases are: Gas Ignition energy (pJ)
For most materials, the lowest ignition energy value occurs at a concentration about midway Between those for the LEL and the UEL Ignition energy is related to the minimum igniting current (MIC), which is an important value in intrinsically safe equipment design, and is given in the gas data tables. Density
It is important to know whether a flammable material will rise or fall in the atmosphere. Knowledge of where a flammable material will collect ensures that gas detectors can be located at the correct level, and that ventilation is directed accordingly. The materials are compared with air and allocated a number to denote their relative density, given by the Greek letter ‘p’, (rho). Air has p = 1. A material with p more than 1 will fall in the atmosphere. Materials with p less than 1 will rise in the atmosphere. Typical values of density (p) of some gases are given in the table: . Relative vapors Maternal Density .p. Materials which rise in the atmosphere can collect in roof spaces. Those which fall, such as butane or propane, can 1 avoid ignition due to a stray spark etc. Flash point Flash points are normally associated with flammable materials in the liquid phase. By definition, flash point is ‘the lowest temperature at which a material gives off sufficient vapors to form an explosive atmosphere, when tested in a specified test apparatus. This explosive atmosphere can be ignited by an arc, spark or naked flame. Typical examples are given below. The flash point of a material gives an indication of how readily that material will ignite in normal ambient temperatures. Different materials have different flash points - most are well below 0°C, but some are well above. Materials with high flash points should not be overlooked as a potential hazard since Exposure to hot surfaces can allow a flammable mixture to form locally. If a flammable material is discharged under pressure from a jet, its flash point may be Classification of gases and vapors Gases are classified according to the group or subdivision of the equipment required for use in that particular gas, based on the gas ignition properties. Group I equipment is for use in underground mines susceptible to firedamp (the mining term for methane), and the surface installations of such mines. Group ll equipment is for places with explosive gas atmospheres, other than mines, i.e. the non-mining surface industries. Group ll equipment is subdivided, according to the ignition risk properties of the atmosphere, into subdivisions IIA, IIB, and IIC. The Group ll subdivisions (still referred to by many people as gas groups) IIA, IIB and IIC are based on experimental work conducted with flameproof and intrinsically safe apparatus, and the gases may be categorized into subdivisions by one (or both) of two test methods: - Subdivision based on Minimum Ignition Current (MIC). MIC is of greatest value in Determining standards for intrinsically safe apparatus (in which the energy available in the circuit is insufficient to ignite a gas). v Subdivision based on Maximum Experimental Safe Gap (MESG). MESG is based on work undertaken to develop flameproof equipment. Attest rig is specified in the standards, and this is used to measure the Minimum Igniting Current (MIC) of the gas. Gases and vapors may be classified according to the ratio of their Minimum Igniting Current (MIC) with the ignition current of the laboratory methane. Group ll equipment is subdivided and, for the purpose of classification of gases and vapors, the MIC ratios are: Gas Subdivision Based on MIC Gas Subdivision MIC ratio (Compared to Example (Representative gas) that of laboratory methane) IIA | 2 0.8 | Butane, MIC ratio of 0.94 IIB | From 0.45 5 MIC s 0.80 I Ethylene, MIC ratio of 0.53 IIC | s 0.45 | Hydrogen, MIC ratio of 0.25 The other method determines the ‘maximum experimental safe gap’ (MESG) using an 8 flameproof liter sphere situated inside a gas-tight enclosure. The halves of the sphere have 25mm flanges, and the gap dimension between the flanges can be varied. The area inside and outside the sphere is filled with a gas in its most explosive concentration in air, and the gas inside the sphere is ignited by a spark-plug. The maximum gap dimension which prevents ignition of the gas/air mixture outside the sphere is the maximum experimental safe gap. The more dangerous a gas, the tighter the