EXPLOSION ANALYSIS AnD MITIGATION

The first step in determining how safe a system is which is processing combustible dust, or flammable vapors or gas, is to have a Process Hazards Analysis done (see the "Risk and Hazards Analysis" section of this website). For dust, this is called a Dust Hazard Analysis (DHA). 

A DHA has been mandatory in most jurisdictions in the US by law for several years  per NFPA guidelines. Have you had yours done?

This basic level analysis includes identification and evaluation of:

• The processes and facility areas where fire, flash fire, and explosion hazards exist
• Specific fire and deflagration scenarios
• Safe operating ranges
• The safeguards that are in place to manage fire, deflagration, and explosion events
• Additional recommended safeguards where warranted.

Detailed design of mitigation systems are not done at this level.

Most large dust explosion industry losses have involved excess levels of dust that was resting on floors, mezzanines, on top of equipment, cable trays, HVAC ducts, and structural members. This is called "fugitive" dust as it has escaped from the processing equipment. ALL dust processes generate fugitive dust in the building compartments in which the dust process equipment is located. 

When an explosion occurs in a a piece of equipment, the equipment either fails or is purposefully provided with explosion vents, releasing the explosion energy. This causes a pressure front to propagate into the room which causes the fugitive dust to be suspended in air, which can then readily explode when exposed to the flame ball which exits the equipment vent. This secondary explosion in the building compartment then causes a chain reaction -  an explosion in the next adjacent building compartment.  

NFPA 654 establishes accepted methods for determining if a particular facility contains too much fugitive dust, allowing thresholds to be implemented for periodic housekeeping. 

Not all combustible dusts present the same explosion hazard. Dusts are characterized by the rate at which energy can be released by a tested parameter called the Deflagration Index, or "Kst". That rate is maximized - a more severe explosion - by smaller dust particulate size, less moisture content, and the chemical nature of the material itself. 

The particular dust hazard that exists at a particular point in the process is determined by these dust material parameters. The actual energy release rate is further affected by the ignition energy and turbulence inside the vessel (see the Equipment Venting section, re, "It's difficult.").

For a DHA, we probably do not need to do dust laboratory testing. But for detailed vent sizing calculations, testing is likely needed. 

NFPA 68 is the default North American standard for deflagration venting design. Unfortunately, it is also the most complicated and mathematically challenging NFPA standard. It's difficult. As a consequence, inadequately rigorous "rules of thumb" are sometimes used, such as the often cited, "One ft2 of vent area for every 15 ft3 of vessel volume". These heuristics are no substitute for a proper analysis per NFPA 68.

The basic concept is easy to understand: an opening (vent) in the compartment in which an explosion occurs allows the developing pressure to escape before the vessel fails. However, the vent cannot open fast enough to prevent the pressure to continue to rise for a while after the vent begins to open, complicating how to design the overall interactive system (i.e., We're back to, "It's difficult."). Also, a a fireball exits the vent. Calculating the "zone of exclusion' for personnel next to explosion vents is an important safety issue.

Explosion suppression systems have a fundamentally different explosion mitigation goal than the more common explosion venting systems. The goal of an explosion suppression system is to extinguish, or "snuff out", the explosion it its incipient stage before it can generate damaging pressures. The developing explosion flame front is detected when it is very small, which causes an inert powder to be injected into the vessel, which "snuffs out" the fire. 

Sound to good to be true? There are, of course, trade-offs. Explosion suppression systems are relatively expensive, and the inspection, testing and maintenance is more costly.  The science behind their application is also not as in-depth as is for venting.    These systems are typically designed and tested for small vessels sizes. The practical result is that the common practice of installing several suppression units on one large vessel is not on a firmly tested foundation.

A building compartment that contains an explosion hazard should be constructed and maintained to withstand the effects of an explosion without incurring significant damage. Designing light-weight deflagration vents in the exterior walls, coupled with the provision of pressure-resistant walls, floor, and roof system, termed Damage Limiting Construction (DLC), will increase the likelihood of resuming operations in a short period. 

Doing this right is tricky, requiring cross-discipline knowledge of architectural and  structural systems, and explosion science.  

These DLC designs do not include fugitive dust that exceeds the threshold established through an analysis per NFPA 654. Any significant amount of fugitive dust will add sufficient energy to an explosion to cause even DLC designed buildings to fail. 

The abovementioned analyses should be part of a wholistic explosion hazards management program which coordinates this information and manages how it will be used. After all, knowledge about hazards is not useful unless it used in some way to make the processes and facilities safer.

A formal Process Hazards Management (PSM) program may be overkill for many systems and facilities, but implementing the concepts that are important within the context of the local operations is important. 

Any change in the process systems or dust can invalidate the hazard analysis, which is a snapshot in time. A program should be in place to implement and maintain the validity of safety systems, including Change Management; Inspection, Testing and Maintenance (ITM); Operator training; Asset Integrity; Contractor Management; and periodic self-audits.

We can help you create and implement these programs. 

Isolating sections of processes from the effects of an explosion in an adjacent interconnected process section is vital.  Explosion isolating devices such as blocking valves and rotary valves should be provided to impede flames from propagating outside of the vessel or duct in which the explosion originates.  

The trick is: how often should isolation devices be located? It would be maximally conservative to install isolating devices on every individual component in which an explosion could occur. For example, if a process includes five small adjacent combustible dust storage bins, it would not be wrong to install an isolation device on the inlet and outlet of every bin to isolate an explosion occurring in one bin from propagating through a bin branch duct to an adjacent bin. But would it be necessary? If management finds the risk of having to replace the entire bank of bins to be tolerable, it would be more cost effective to locate isolating devices on the duct mains leading to and from the group of bins. As always, experience and judgement is important.  

It is important to reduce concentrations of combustible dust or flammable vapors or gas to a level significantly below the point at which it could combust (explode). This is usually attained by adding a sufficient quantity of air to the process vessel with ventilation systems, thereby diluting the combustible material below the Lower Explosive Limit (LEL) for vapors and gases, or the Minimum Explosive Concentration (MEC) for dusts.

However, outside air is often used as the source for clean air, which does not align with energy efficiency goals. This becomes an incentive to reduce the safety ventilation to a minimum.

The resulting design and calculations require cross-discipline knowledge of the process physical parameters, process chemistry, HVAC, and combustion science. Process temperature and altitude are also factors which must be considered.