New Ignition Source “Exploding Wire” for the Determination of Explosion Characteristics of Combustible Dusts in the 20-L-Sphere

Tests on Suitability of the Ignition Source “Exploding Wire” for the Determination of Explosion Characteristics of Combustible Dusts in the
20-L-Sphere

Marc SCHEID†, Christian KUSCHE and Volkmar SCHRÖDER

Federal Institute for Materials Research and Testing (BAM), D-12200 Berlin, GERMANY

†Corresponding author: marc.scheid@bam.de, Phone: +49-30-8104-4441

ABSTRACT

Safety characteristics are essential for the determination of explosion hazards during handling of combustible dusts and for the design of safety measures. The characteristics Maximum Explosion Pressure p_max, Maximum Explosion Pressure Rise (dp/dt)_max and Lower Explosion Limit LEL are determined in closed vessels such as the 20-L-sphere (also known as SIWEK-Chamber) according to international standards, for example EN 14034 series or ASTM E1226. The ignition of dust samples is carried out using two pyrotechnical igniters with energy contents of 1 kJ or 5 kJ, which are defined in the standards.

Due to various disadvantages of the pyrotechnical igniter such as high costs, legal requirements concerning its storage and use and high energy input in comparison to most ignition sources in practice, the need for alternative ignition sources arises again and again. Such an ignition source should be less expensive, readily available and the operator should be able to use it without a certificate of competence. In addition, the ignition energy should be adjustable over a wide range. An alternative ignition source which fulfills these requirements is the so called “exploding wire” or “fuse wire”. This type of ignition source is used for the determination of explosion limits of gases and is described in the standard EN 1839.

The principle of the exploding wire is the evaporation and ionization of metal particles from a wire due to a sufficient current flow within milliseconds. During that process a metal vapor is created in which an electric arc is generated between two electrodes because of the electrical conductivity of the plasma.

The full paper presents test results of a comparative study between the ignition sources exploding wire and pyrotechnical igniter for the determination of explosion characteristics p_max and (dp/dt)_max of five selected dusts in the 20–L–sphere. The dusts were selected to allow different reaction mechanism and dust explosion classes to occur. In addition to that high speed recordings allowed comparison of flame front and electrical arc generated from the igniters. Calorimetric and electric measurements obtained information on the ignition energy of the igniters.

So far tests were conducted with a single ignition source with an energy input in the range of 100 J to 1000 J. To archive such energy inputs with the exploding wire a suitable ignition device was built. For all tests a nickeline wire with a diameter of 0.12 mm was used. The wire was tensed between the ends of two electrodes and connected with two springs, see figure 1.

Measurements of the ignition energy of the igniters using two different methods showed that the ignition energy of the exploding wire was reproducible.

High speed recordings showed that the flame generated from the pyrotechnical igniter propagated faster as the flame generated from the exploding wire and reached a larger volume. As a result the igniter should have a significant influence on the determined p_max and (dp/dt)_max values. However, such an effect was not determined during the measurements of p_max and (dp/dt)_max of five different dusts. All tests resulted in comparable values of p_max and (dp/dt)_max for both ignition sources, see table 1.

Table 1 also shows that p_max values determined with ignition energies of 100 J, 500 J and 1000 J were less than 10 % lower than values determined according to the test standard with two igniters of 5 kJ. As a result the influence of the ignition energy of the igniter on the maximum explosion pressure seems to be almost negligible.

In contrast to that (dp/dt)_max values determined according to the test standard led to 30 % higher values. The reason for that could not be solved totally. It is assumed that a combination of different effects has to be considered such as turbulence generated from the ignition source and the flame volume of the ignition source.

At the moment tests with two ignition sources are performed. In addition to that the influence of turbulence on the flame propagation of both ignition sources is determined in a windowed autoclave. The test results will be presented in the full paper. Further tests with ignition energies up to 10 kJ are planned for spring 2013. If available, first results will be presented at the conference.

Figure 1: Exploding wire with electrodes (left side) and pyrotechnical igniter (right side)

Table 1: p_max, (dp/dt)_max and K_St values for all tested dusts determined with exploding wire and pyrotechnical igniter with ignition energies of 100 J, 500 J, 1000 J and 10 kJ.

Dust		100 J		500 J		1000 J		10 kJ
		Ex. Wire	Pyro. Igniter	Ex. Wire	Pyro. Igniter	Ex. Wire	Pyro. Igniter	Pyro. Igniter
Lignite	pmax	8.4	8.1	8.1	7.6	7.9	7.7	8.4
	(dp/dt)max	601	545	681	630	671	608	785
	KSt	163	148	185	171	182	165	213
Maize starch	pmax	8.4	8.5	8.4	8.6	8.2	8.4	8.7
	(dp/dt)max	408	470	528	480	487	468	616
	KSt	111	127	143	130	132	127	167
Niacin	pmax	-	-	8.3	8.2	8.2	8.0	7.9
	(dp/dt)max	-	-	953	942	920	874	1051
	KSt	-	-	259	256	250	237	285
Anthraquinone	pmax	-	-	-	-	8.1	8.0	8.4
	(dp/dt)max	-	-	-	-	1113	999	1379
	KSt	-	-	-	-	307	271	374
Steel dust	pmax	-	-	-	-	4.1	4.1	3.8
	(dp/dt)max	-	-	-	-	398	394	432
	KSt	-	-	-	-	108	107	117