Flame transmission

Flame transmission test procedures for deflagration, detonation, and bnrning tests are discussed in all of the above sections. 

Flame Arrester Element (Matrix) That portion of a flame arrester whose principal function is to prevent flame transmission, usually by quenching the flame front. 

Flow Controlled Aperture An aperture designed to produce flow velocities which exceed the local flame speed of the flammable mixture, thus preventing flame transmission in the reverse direction. 

Hydraulic Flame Arrester A flame arrester consisting of a vessel filled with a seal flnid (often water) and a distribntor which breaks np the incoming gas into discrete bnbbles, thns facilitating qnenching of the flame and preventing flame transmission. 

Static Flame Arrester A flame arrester designed to prevent flame transmission by qnenching gaps (apertnres). These are nsnally dry type flame arresters with elements snch as crimped metal ribbon, parallel plates, wire ganze (mesh), and sintered metal. 

This parabola defines the area of flame transmission through the joint. Its vertex is usually nearby or somewhat below the stoichiometric point. All gases show a behaviour similar to that shown in Fig. 1.3. The knowledge of the vertex MESG values of gas-air or vapour-air mixtures is essential for the construction and use of enclosures, type of protection flameproof... 

The area above the parabolas represents the region of flame transmission. 

Figure 6.9 Prevention of flame transmission through a granule layer above a low energy ignition source (=s10J) [2], Safe granule layer height hs versus granule diameter d.

Even with a zero width of a flameproof joint, there will be no flame transmission if the gap is suitably reduced. 

So, cutting edge-shaped joints do prevent flame transmission to the external atmosphere. For the purpose of manufacturing electrical equipment, this fact may be unimportant, but it is very helpful towards an explanation of the working principle of a flameproof joint. 

Sintered metals show a strong correlation between open porosity and effective gap. (The effective gap weS of a flameproof component is the highest MESG (Maximum Experimental Safe Gap) value of combustible gas mixtures (see Section 1.2.2), for which that component ensures flameproofness. For gas mixtures with MESG < wen, flame transmission occurs.) (see [19]). 

Figure 6.155 Typical record of short-circuit arcing and the flame transmission initiated hereby. Arc voltage UB, arc current I and arc power I x UB versus time, as well as explosion pressure p (results attained in the apparatus according to Fig. 6.157).

The moment of flame transmission (determined by evaluation of film records of camera no. 1) corresponds to the maximum arc power I x UB. 

Figure 6.157 Block diagram of the apparatus for investigations on arc-induced flame transmission according to [56],...

A quite different effect is an arc-induced flame transmission through the joints of a flameproof enclosure. To avoid any misunderstanding these joints do comply with the d-standards and they have been shown to be safe by type testing for the corresponding explosion subgroup and test gas-air mixture. Investigations covering this field have been published in [35] and [56]. 

A very different result has been obtained using aluminium electrodes (Fig. 6.160). There is no correlation safe gap versus arc peak power, but a chaotic distribution of flame transmissions down to very small gap values. This indicates a real particle-induced flame transmission and has been proven by evaluating the films of camera no. 1 the aluminium particles entering the indication chamber act as a very effective ignition source. 

Figure 6.158 Arc-induced flame transmissions. Gap w versus arc peak power Nz at the moment of flame transmission.

Compared with the values for reacting gas-air mixtures (see Fig. 6.77, Table 6.26) this may explain the flame transmission through joints proven as safe for gas-air reactions. 

Remembering the basic principles of flameproof enclosures, two main characteristics of such enclosures shall be proved by type testing they shall withstand the internal overpressure caused by the ignition of a combustible gas-air mixture inside, and they shall prevent flame transmission to the environmental hazardous atmosphere. So, such a test cycle is divided into three parts ... 

Figure 8.3 Oscillogram of a flame transmission (flame transmission test has been failed), according to [17], Pressure (within the test specimen) versus time. EEx dll C motor, without stator iron core and windings.

Flame Transmission Caused by Arcing in Flameproof Electrical Equipment... 

Finally, zinc Is an example of an element whose absorption wavelength Is lower than 230 nm, the range where flame transmission noise dominates. Let us look a bit more closely at the case of zinc. The Instrument performance can be characterized using a precision plot, shown In Figure 2, where the relative standard deviation of concentration Is plotted on the vertical axis and concentration on the horizontal axis. The detection limit Is defined by the Intersection of the precision curve with the RSD which represents the criterion used to define detection, about 30% RSD for k - 3. Note that flame transmission flicker noise limits detection. 

If the burner head Is rotated to reduce sensitivity, we find that the limiting noise Is no longer flame transmission flicker, but source shot noise, since the absorption path has been reduced by a factor of 20. Although the sensitivity Is decreased by a factor of 20, the detection limit Is decreased by only a factor of 10, since the flame transmission noise Is no longer limiting. Thus, referring back to a statement made earlier, sensitivity, or more correctly characteristic concentration [18] cannot be used as an accurate measure of detection limit in AAS. Unlike the case of SBR in emission, because of the complexity of noises In atomic absorption, a general and simple relationship cannot be derived to relate characteristic concentration and detection limit. 

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