Page 664 - Dust Explosions in the Process Industries

Research and Development 63 I

Experiments have indicated that water can be an effective suppressant, if injected at
a temperature >180°C. According to Tyldesley (personal communication, November
1993, optimum suppression requires about 0.5 liters of water per m3 of vessel volume.
Of the superheated water, 16-18% is flashed to steam, the remainder forms very small
droplets. Reduced maximum explosion pressures of 0.3-0.4 bar(g) were obtained in
experiments in a 28 m3 experimental vessel, using a dust of Ks, = 150 bar.m/s. Moore
(1996a) considered the various types of suppressants employed in automatic dust explo-
sion suppression systems. After the use of “halons” became highly restricted due to the
environmental problems caused by these substances, the chemical industry identified a
range of alternative agents that are environmentally acceptable and at the same time have
good fire-extinguishing properties. Moore discussed the desired properties of an explo-
sion suppressant in general and compared the effectiveness of available suppressants.
Dastidar et al. (1998) determined the minimum concentrations of inert powder/powdered
suppressant (MIC) required to effectively prohibit flame propagation at any concentra-
tion of the explosible powder or dust. Knowing this parameter can facilitate the design
of optimal explosion suppression systems. Moore and Dunster (2001) discussed the
influence of various key parameters on the performance of automatic explosion sup-
pression systems, with reference to results from experiments and modeling. Tailoring,
accounting for the intimate relation between hardware design and suppressant groper-
ties, is essential to ensure optimal system performance.
Gieras et al. (1994) conducted a series of experiments to optimize the shape and mass
of the explosive charge used for automatic release of suppressant (powder) for sup-
pressing dust explosions. The overall aim of the research was to minimize the powder
ejection time after the onset of the dust explosion had been detected. Gieras, Klemens,
and Wolanski (1996b) and Klemens et al. (1998a, 2000a) developed and tested this
system further and reported that the exit seals of their pressurized suppressant contain-
ers could be blown open within a fraction of a ms. In a subsequent experimental study,
Gieras and Klemens (2002) compared the effectiveness of water spray, imrt dusts, and
common pulverized suppressants for suppressing explosions of organic dusts in a closed
1.25 m3 chamber.
Siwek and Moore (1995) and Moore and Siwek (1998) summarizedtheir extensivework
on suppression of dust explosions and added some further data that filled significant
gaps in previous knowledge and enabled thorough revision of some commonly used
desigiz nomographs. The papers provide useful updated guidance to engineers having to
specify suppression system requirements for specific industrial applications. Moore
(1996b) paid particular attention to the choice of suppressant and the close interaction
between the chemical and physical properties of the suppressant and the optimal design
of the system for injecting the suppressant into the flame to be suppressed. Chatrathi and
Going (1998) compared the effectiveness of three different suppressants-sodium bicar-
bonate, monoammonium phosphate, and rock dust-for suppressing dust explosions.
The dusts used in the tests were of coal, maize starch, polyethylene. anthraquinone, and
aluminum.It was concludedthat specificheat, thermal conductivity,paxticle size and shape,
and the mechanism of particle decomposition were important parameters deciding the
effectiveness of the suppressants tested.
Lebecki et al. (1998, 2000) investigated the possibility of using solar panels as flame
detectors for activating triggered barriers of water or stone dust for suppressing coal mine
explosions. The yellow light emitted by the burning coal dust particles is an effective