Industrial explosions are a constant threat to any facility that handles combustible vapors or finely divided combustible dust. Most organic material will burn in a solid form; if this same material is in a dust or vapor form, under certain conditions it will explode. Combustible dust and vapor explosions happen frequently in the processing industry. Sometimes these explosions remain confined to the process vessel in which they originate. However, more often than not, the initial explosion will result in a secondary explosion with devastating results outside the vessel or through interconnecting ducts or pipes.
Having a comprehensive plan to prevent an explosion from happening under normal circumstances and mitigating the effects of the deflagration under upset conditions is critical to the safe operation of any facility that is faced with this threat. The plan must include considering identifying where potential for flame propagation exists and a decoupling strategy to prevent this propagation from occurring. This article will discuss various explosion-isolation options.
For an explosion to occur, five elements must be present:
1. fuel, i.e., a combustible dust or vapor;
2. an ignition source;
3. an oxidizing agent, which usually is the oxygen in the air;
4. confinement, which results in pressure buildup during the incipient explosion; and
5. in the case of dust, dispersion into the airstream.
In the chemical industry, vessels typically subjected to this threat include dryers, mills, reactors, air/material separators such as dust collectors, and storage vessels. In these vessels, it is common for the combustible dust to reach its minimum-explosive-concentration level at least locally within process vessels. All that’s needed to initiate the deflagration is an ignition source. The pressure from the incipient explosion travels at the speed of sound while the growing fireball initially propagates at a much slower speed. A typical sequence for a dust explosion includes:
• The dust cloud becomes ignited.
• The deflagration pressure results in rupture of the vessel.
• The shock wave from the ruptured vessel liberates dust that has accumulated on horizontal surfaces in the process area, such as atop beams, ducts, conveyors and even light fixtures, causing the dust to become suspended in the process area.
• The fireball escaping from the vessel ignites the newly suspended dust in the process area, triggering a secondary explosion that can destroy the building.
• Flame propagation occurs through interconnected ducts, chutes or conveyors to connected equipment upstream or downstream, prompting highly energetic explosions in these connected vessels (Figure 1).
Ignition control, proper housekeeping to remove residual dust, continuous training of plant personnel on dealing with the dust explosion risk, and management of change to address the effects of a process or product change are all critical to helping prevent an explosion from occurring under normal operating conditions. Unfortunately, abnormal conditions that result in an explosion can occur in any process line. This is why the National Fire Protection Association (NFPA), Quincy, Mass., requires the use of explosion mitigation techniques for vessels subjected to an explosion threat. NFPA 69  lists a number of mitigation methods for dealing with this risk. These methods include inerting with a noncombustible gas or dust, building the vessel strong enough to withstand the pressure from a deflagration (containment), explosion venting and explosion suppression. Except for inerting, these protection techniques don’t eliminate the risk of an explosion initiating in one vessel and propagating to interconnected vessels.
The transmission of flame from one vessel to another through an elongated duct results in enhanced burning rates because the turbulence created during propagation increases the mixing of fuel and air. The result can be a flame jet ignition and pressure piling traveling to the interconnected vessel. This often causes a much more energetic explosion in the second vessel than in the source vessel. If explosion protection measures such as venting or suppression are installed on the second vessel, they likely were sized based on a specific deflagration index (KST). KSTvalues are determined using two 5,000-joule igniters, or 10,000 joules of energy. The flame jet ignition can produce a significantly higher energy level, resulting in a much more severe explosion than that for which the protection measures are designed.
When left unchecked, flame propagates through interconnected duct from initial flame speeds of around 10 m/sec to a speed where they transition to a detonation, i.e., the speed of sound (343 m/sec). Documented testing has shown risk of this transition occurs in relatively short distances — 40 duct diameters for vapors, 80 duct diameters for dusts. Few explosion protection measures are designed to withstand the effects of a detonation.