Stopping the Big Bang

Industry today handles many different materials that are combustible and it comes as no surprise that fires occasionally occur. What is less well known, however, is that fine particulate material (dust) not only has the potential to burn, but also to explode.

The risks from both fire and explosion are very real. In the UK alone, nearly 300 incidents were reported in a nine-year period. More than a quarter of these cases resulted in injuries and/or fatalities.

The conditions for a dust explosion include a certain fineness of dust - particle sizes typically 200 micron or less; dispersion in air with a concentration above the LEL (Lower Explosible Limit) - this value varies widely with different materials, but is not normally less than 30 g/m3; an ignition source of sufficient energy to ignite the dust cloud; and enclosure of the dust cloud within plant equipment or building.

Pressure increase

The flame front expands with a flame speed of around 10 m/sec. The sudden heating of the surrounding air by the fireball generates a pressure wave that travels outwards at the speed of sound (330 m/sec), much faster than the flame front itself.

Laboratory tests show that the explosion generates a steady increase in pressure up to a maximum value of about 9 barg. The rate of this pressure increase - given by the slope of the pressure/time graph, shown over the page - is an important measure of the dust characteristics, and is a key element in designing suitable explosion protection.

The pressure rise measured in tests is normalised to 1m3, using the Cube Root Law:

3=volume1 x (dP/dT)volume1 = 3=volume2 x (dP/dT) volume2 = Kst

With units of bar m/sec, the Kst value then determines the explosion class of the dust, denoted as an `St number'. This ranges from St0, indicating a non-explosible material, for Kst of 0 bar m/sec, up to St3, which is a very strongly explosible material, for Kst above 300. A high percentage of the St 3 class materials are metals and this perhaps reflects in the high number of metal dust explosions being reported, despite other industry sectors being much larger and handling more material.

Legislation requires that employers take suitable preventative and protective steps to deal with dust explosion hazards. These measures are currently embodied in the Factories Act 1961 and Health & Safety Work Act 1974. In essence, this means eliminating one or more of the conditions for an explosion, or using explosion vents, suppression or containment.

Vents provide a pathway through which the pressure and combustion of an explosion can escape. To prevent dust escaping during normal operation, the vent is sealed using a door, a bursting panel or a rupture membrane.

Nowadays, purpose-built burst panels are most frequently used, allowing a high-degree of venting efficiency and repeatability of burst pressure. In some cases, flame arrestor devices are added to the vent. These are designed to stop combustion beyond the vent location. This is particularly useful when venting inside the workspace, in locations where it is not practical to duct the discharge to a safe area.

Work on dust explosions over the last 20 years has led to a bewildering array of vent sizing methods. The current UK code of Practice (IChemE Dust Explosion Prevention and Protection, Part 1 - Venting) contains no fewer than 11 different methods.

One method which has found widespread application in industry is the Scholl equation. This is a complex calculation which derives the area of the vent as a function of the maximum and minimum pressures produced by the explosion and the static opening pressure of the installed bursting panel. It is likely that the Scholl equation method will be adopted under ATEX 100a (Directive 94/9/EC) as part of harmonisation across the EC in July 2003.

Suppression

An explosion suppression system can be thought of as super-fast fire extinguishing. Pressure sensors mounted on the protected vessel detect that an explosion is underway. Following confirmation of the signal by the central control unit, suppressors are activated to flood the vessel with extinguishing powder - usually sodium bicarbonate, Dessicarb or monoammonium phosphate. Halogenated hydrocarbons have now been phased outl. These substances not only extinguish the explosion, but also render inert the unburnt portion of the dust.

Needless to say, the effectiveness of an explosion suppression system is determined by its speed of reaction. Modern electronic pressure sensors and high rate discharge containers have response times measured in fractions of a millisecond.

In practice, sophisticated computer models are used to simulate the explosion and validate the protection system design. By this means, design parameters such as detection, pressure setting and suppressant throw distance can be varied. This optimises performance and reduces overall costs.

Containment

Perhaps the simplest of the protection methods, containment relies upon brute strength to contain the explosion. That is to say, the protected vessel is built to withstand the maximum explosion pressure the dust can generate. Pressure vessel codes, such as BS 5500 and ASME section 8, are often used for containment design and some form of proof testing would normally be required. In addition, it is important to consider the effect of containment on surrounding equipment. Explosion isolation - using multi-blade rotary valves, slam-shut valves or barrier valves - is essential to prevent the spread of combustion flames and the effects of high pressure on other parts of the process.

Michael Ward, MA is an explosion protection specialist with Fike UKin Maidstone, Kent