Dust Explosion Protection

A dust explosion will occur only if a dust is dispersed in air or oxygen within the explosive range and if, at the same time, a suitable source of ignition is present. Preventative measures are aimed at avoiding the presence of either, or preferably both, these factors in the system. In many cases, however, because of the heterogeneous nature of dust clouds and the difficulty in foreseeing or excluding every possible source of ignition, explosion cannot be positively prevented by these means. Although every effort should be made to avoid flammable concentrations of dust and to eliminate all sources of ignition it is usually necessary to supplement such measures by taking precautions designed to reduce the spread and effects of an explosion. There are a number of techniques, listed below, that provide effective protection against the destructive force of a dust explosion. Each method has particular advantages and disadvantages and plant designers must select the appropriate method to provide the necessary basis of safety. Explosion venting is most commonly used due to its low cost and simple maintenance regime.

Explosion Venting

Explosion venting offers a simple and practical method of protecting plant which is not strong enough to withstand the pressure developed by an explosion. If the relief panel is of a suitable size and design, it will safely vent the explosion without significant damage to the vessel or injury to nearby plant personnel. Care must be taken in vent sizing and placement and should be in accordance with the appropriate national safety standard (e.g. EN14491 or NFPA68). Explosion reliefs for plant containing flammable dusts must also be dust tight and have sufficient mechanical strength to resist the wear to which they are subjected and internal pressure fluctuations. On dust collectors in particular, the stress imposed by reverse jet cleaning must be considered in the vent design. Reliefs usually take the form of bursting panels, although explosion doors are common on older plant. Bursting panels provide good, dust-tight seals but may allow the entry of air into plant after an explosion which may give rise to a fire subsequently. Explosion doors can usually be engineered to shut after operation, but are more difficult to make dust tight and have considerably lower venting efficiency due to their weight. Proprietary panels and doors, explosion tested to the appropriate standard (e.g. EN14797) are recommended. Extensive research over the last 20 years has provided soundly based calculation methods to determine the vent area required. Design to is based upon: (a) The volume of the vessel to be protected (b) The shape of the vessel to be protected (c) An estimate of the vessel strength (d) The explosion indices (Kst & Pmax) of the dust hazard (e) The opening pressure of the vent relief (f) The size of the vent relief (g) The vent relief efficiency (g) The length and shape of any associated vent ducting When an explosion vent opens as a result of a dust explosion, a fireball of considerable volume, typically ten times the volume of the vessel, will be ejected over a considerable distance. If the vent opens inside a building, anyone in close proximity is likely to be seriously injured. In addition, any secondary dust (i.e. dust lying as a layer on the tops of bins, ledges etc) may become involved in a much larger secondary explosion. It is therefore normal practice to duct the vent to outside through to atmosphere via a short length of purpose-built ducting. Consideration must be given to flame travel outside and the back pressure effect of the ducting. Alternatively, use flameless explosion vents where a flame arrestor element stops the passage of burning material but still allows the explosion pressure to be abated.

Explosion Suppression

The alternative to explosion vent relief is explosion suppression. In their simplest form, such systems comprise a rapid-acting pressure sensor, control system and a number of pressurised containers of suppressant designed for extremely rapid injection into the protected vessel. The suppressant is commonly Sodium Bicarbonate. The design and installation of suppression systems should be left to specialist suppliers, but compliance with the appropriate standard (e.g. EN14373) should be specified in the purchase contract.

Explosion Containment

Since dust explosions generally develop maximum pressures in the range 8-12 bar, it is not usually practical to produce plant capable of withstanding such pressures unless it is of small volume and round shape. However, certain pieces of equipment, such as hammer mills, are sufficiently robust to contain an explosion. In that instance, it is important to consider the protection of adjoining equipment as containment will increase the explosion severity in what is know as the ‘pressure piling’ effect. Pressure piling is outside the scope of this introduction to the subject and specialist advice should be sought. The design pressure for explosion containment design should be validated by dust explosion testing following EN 14034-1:2004 (determination of the maximum explosion pressure Pmax of dust clouds).