Dust clouds

Dust clouds

111 less than about 5 mm. They can ignite only the most sensitive atmospheres {e.g. possibly hydrogen or carbon disulphide ). The available means of ...

75KB Sizes 3 Downloads 80 Views

111

less than about 5 mm. They can ignite only the most sensitive atmospheres {e.g. possibly hydrogen or carbon disulphide ). The available means of preventing vent stack ignitions are: • Fit sharp-edged tops to promote non-Incendive glow coronas instead of Incendive brush discharges. The sharp edge may simply be rough cut metal. Sharp edges may not prevent the ignition of sensitive gases such as hydrogen or carbon disulphide that may be ignited by glow coronas. • Fit large radius tops. Large radii reduce the electric fields at the top of the object and can prevent all types of discharge. The radius must, however, be large enough to prevent discharges under all possible atmospheric conditions because any discharges that do occur will be Incendive. As atmospheric potentials are very variable, it is generally more practical to use sharp edges rather than large radii. Large radii may, however, be useful for sensitive atmospheres such as hydrogen that may be ignited by glow coronas. Expert advice should be obtained if the use of large radii is being considered. • Control the position and extent of the flammable zone around a chimney or vent stack. Limiting the extent of the flammable zone can minimise the consequences of ignition and controlling its location may ensure that the atmosphere is non-flammable in the region where discharges occur. The location of the flammable zone can be controlled with fluidic seals (venturis) or by diluting the Flammable Atmosphere with air introduced below the tip of the stack. 6 . 1 1 . Dust clouds

Hazards aI!sing from the accumulation of charge in piles of powder are dealt with in Chapter 5. The present section considers the effect of the charge carried on fine powder (i.e. dust) that remains suspended in a gas. The potentials produced by suspensions of charged ~tust in industrial scale equipment are unlikely to be high enough to generate either lightning-like discharges or Incendive brush discharges. Potentials may be high enough to produce Incendive sparks from Insulated Conductors, but in contrast to the case of water washing {which inherently involves the production of isolated Water Slugs) there is no unavoidable reason why Insulated Conductors should be present in a dust cloud. Electrostatic ignitions from charged dust clouds can thus be avoided simply by ensuring that all conductive objects in contact with the cloud are properly earthed (see Chapter 7 and Appendix I).