SAFETY IN THE PLANT
ABOVE: Measures to mitigate the effects of explosions are the basis of explosion protection LEFT: Round venting system on a silo BELOW: A 5 litre bottle chemical supressor
Moreover, all parts of the
enclosure – including valves, access ports, ductwork, etc. – exposed to the explosion pressure, must be taken into account when estimating the design pressure of the vessel. Tis is to ensure that the relief of the explosion pressure is accomplished in a controlled manner. Venting does not prevent an explosion: it limits the explosion pressure. Hence, flame and pressure effects outside the enclosure and flying debris must be anticipated and accounted for. To preclude this, flameless venting devices may be used – this form of venting is particularly useful for plant sited in
(or close to) the middle of the work area; otherwise, long vent ducts would be required to safely vent the explosion outside the confines of the building.
As mentioned above, explosion
suppression does provide containment of the ‘reduced explosion pressure’ (Pred). Tis is achieved by detection of the incipient explosion (i.e. in its early stages). Whilst the combustion is taking place (most of the time quite rapidly over a few milliseconds), once detected, suppressant is injected in to the growing fireball to quench the flame. Te predominant effect is absorption of heat, temperature reduction and
stoppage of flame transmission. Once again, all components of the vessel must be taken into account when estimating the design pressure. In practice, the quantity of
suppressant (number of suppressors) and their location will depend on the violence of the explosion (Pred and Kst from dust testing), the geometry of the vessel and its design pressure. With the exception of some milling operations, as a safety concept, explosion pressure containment is less common. Tis is due to the high design strength needed – typically of the order 8-10 barg for dust explosion containment. Common gases and hydrocarbons have lower peak explosion pressures of about 6-8 barg. It is important to recognise,
however, that any explosion protection system must cover upstream and downstream interconnections.
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