Double Valves for Safe Isolation Double Valves Promote Safe Powder Handling at Kst Values up to 1000 Bar·m/s
Closed plant and equipment handling flammable materials in powder form carries a risk of explosion whenever the particle size is less than 0.5 mm. Minimizing this risk places stringent demands on equipment manufacturers and plant operators. The use of double valves of appropriate design prevents explosions from propagating by creating a reliable flame-tight barrier.
A dust explosion can occur during the handling of any flammable solid if the airborne concentration is above a threshold value and a source of ignition is present. The risk is addressed by a system of standards which began with EN 500 14 and since 2003 has been covered by Guidelines 94/9/EC (Atex 95) for manufacturers of devices and safety systems for conventional application in potentially explosive areas. 99/92/EG (Atex 137), for instance, applies to owners of electrical facilities with potentially explosive atmospheres.
Potentially explosive areas for flammable dusts are classified as zones 20, 21 and 22, respectively, according to the probability that an explosive concentration of dust will exist at all times, “occasionally”, or “seldom”. The likelihood of an explosion also depends on the ignition temperature and ignition energy of the dust.
Once ignited, the severity of an explosion depends on the maximum explosion pressure and rate of pressure rise associated with the dust in question. The rate of pressure rise (Kst) is expressed in units of bar·m/s, and gives rise to three hazard classes:
- St1 (Kst>0–200 bar·m/s),
- St2 (Kst>200–300 bar·m/s),
- St3 (Kst>300 bar·m/s).