SAFETY IN THE PLANT
It is imperative to provide isolation to prevent propagation of burning particles, flame and pressure. Isolation can take the form of an
ATEX-certified rotary valve or active or passive slam-shut (rapid action) valve, chemical extinguishing barrier, product choke, etc. as described in prEN 15089 Explosion Isolation Systems. For complete isolation (e.g. by use of a slam-shut valve), the design pressure must be applied up to the isolation device. Hence, any ducting or pipework up to this point would need to withstand the maximum anticipated pressure, i.e. Pred or Pmax. Te closure time of the rapid action valve, together with the response time of the detection/control system and flame speed, defines the required minimum distance (L) from the source of the explosion – typically L > 5,000mm. A vented explosion must discharge to
a safe area and this often requires the use of a vent duct. In most cases, the action of venting will be accompanied by ejection of burned and unburned gases and flames, and measures must be taken to ensure that nearby plant and personnel will not be at risk from the vented fireball. It is important also to note that a vent duct will increase the back-pressure during the relief process, requiring a greater pressure resistance for equipment and vessels. As part of the isolation concept, equipment must be shut down automatically, in the event of an explosion, to prevent transfer of burning material, etc. With venting, this is normally achieved by sensors fitted to the vent panel. Of course, this should not result in frequent spurious shutdowns, since some will find ways of by- passing the problem – by means of wood and scaffolding poles to keep vent doors shut, for example. (Te correct course of action would have been to examine why the explosion doors keep opening!)
Detectors and suppressors on a process containment
THE BIGGER PICTURE When fitting explosion protection, from a process point of view, it is important to think about any repercussions. One example is the use of rotary valves for explosion isolation purposes, as this is often contentious due to the likely ‘wear rates’ and the need to maintain certain tolerances (in particular, the gap between the blades and the casing).
Another example is the use of flap
valves in dust-laden ductwork. Te ongoing ‘user obligations’ regarding
inspection and maintenance of such items is not always appreciated. For example, periodic inspection checks must be undertaken to ensure that the explosion isolation capability does not deteriorate (for instance, due to corrosion, abrasion, or dust built up on the flap or inside the body of the flap valve). Te positioning of explosion vents on dust filters is important too, to ensure that the internal filter membranes do not obstruct the protection. Vent panels sited close to membranes can result in an increase in the ‘reduced explosion pressure’ (Pred value) and over-pressurisation of the vessel – the filter bags can be blown out of the vent on activation. A further consideration is that of providing automatic fire suppression, since filters can be terminally damaged by secondary thermal stresses due to burning bags or product, following the explosion.
a) Ignition, b) Detection, c) Suppressant release, d) Extinguishment, e) Suppressed 56
www.engineerlive.com
Declan Barry is MD of ATEX Explosion Hazards.
www.explosionhazards.co.uk
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