(43d) 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.