When discussing explosion protection for process plants, the ‘protection’ concept is based on the assumption that ignition and an explosion (gas, vapour, mist or dust) may arise and that steps must be taken to mitigate the effects, thereby safeguarding personnel and (as far as practicable) maintaining the integrity of the plant. The options available to achieve this are dependent on the plant vessels/layout and the characteristics of the materials. There is a choice of two designs.
An explosion pressure resistant design (EPRD) does not allow for any deformation of the vessel whereas an explosion pressure shock resistant design (EPSRD) does – i.e. deformation is acceptable, but not total failure.
Explosion pressure relief is perhaps the most common option but essentially there is a ‘loss of containment’, meaning that process material and/or fumes will be emitted from the protected vessel during the explosion. Conversely, explosion suppression and total explosion pressure containment systems do not give rise to a release of process material.
Explosion venting is a protective measure preventing unacceptable high explosion pressure build-up inside vessels/enclosures. Normally explosion venting is applied such that the maximum reduced explosion pressure (Pred, max) does not exceed the known design pressure of the vessel; the lower the vent opening pressure (Pstat) and the larger the vent area, the lower the reduced explosion pressure.
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. This 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). This 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.
The 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.
The Violence Of The Explosion
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. This 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. 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. The 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. (The correct course of action would have been to examine why the explosion doors keep opening!)
Considering Explosion Repercussions
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. The 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).
The 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.
Declan Barry is MD of ATEX Explosion Hazards