Dust Explosion Mitigation & Prevention

Dust Explosion Mitigation and Prevention

Dust explosion pentagon with ignition, dispersion, confinement, fuel, and oxygen around it.

In order to understand dust explosion mitigation and prevention, you must understand what combustible dust is. A combustible dust is a finely divided combustible particulate solid that presents a flash fire or explosion hazard when suspended in air. Particle Size and Moisture Content play a pivotal factor in explosibility and ignition potential. Materials considered to be a combustible dust can be found in a variety of industries and can arise from handling everyday materials such as sugar, plastics, and sawdust.

Dust Explosions occur when all five of the legs of the Dust Explosion Pentagon are present.

  • Fuel – Combustible Dusts serve as fuel for combustion. Examples include grain dust, plastics, cornstarch, coal, wood dust, and
    metals.
  • Oxygen – Air is the most common oxidizing medium. O2 must be present in sufficient concentration to support the combustion.
  • Dispersion – A suspended cloud of explosive dust with sufficient concentration will allow combustion to occur more rapidly than in a layer.
  • Confinement – A suspended dust cloud must be enclosed in order to build pressure that is characteristic of a dust explosion. Without confinement, the rapid combustion is a Flash Fire.
  • Ignition – Ignition source is often the only leg of the pentagon not readily present. Possible ignition sources include open flame, hot surfaces, static or electrical discharges, and heat from friction or mechanical impact.

Dust Explosion Mitigation Techniques

NFPA 652, Standard on the Fundamentals of Combustible Dust (2019), requires that facilities and processes are designed to mitigate the dangerous consequences of fires and explosions that are a hazard of handling combustible dust. To meet this . requirement, facilities can elect to utilize either a Prescriptive approach or a Performance-Based approach.

 

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Performance Based Approach

Using a Performance-Based approach involves the design of a system or facility that is assessed and evaluated against specific design criteria to demonstrate safety against each identified Fire and Explosion Scenario relevant to the system. The system must be able to adequately address the areas of Life Safety, Structural Integrity, Mission Continuity, Mitigation of Fire Spread and Explosions, and the Effects of Explosions. When utilizing a Performance-Based design, the proposed system or facility must be tested using appropriate methods and calculations acceptable to the Authority Having Jurisdiction (AHJ).

Perspective Approach

Using a Prescriptive approach involves designing Explosion Mitigation systems as laid out in Chapter 9 of NFPA 652. This means that each vessel that is deemed to have an explosion hazard shall be protected using one of the following six acceptable protection methods:

  1. Inerting – Oxidant Concentration Reduction (sometimes called Blanketing or Padding) involves introducing inert gas (such as
    Nitrogen, Argon, or CO2) into the system in order to reduce the oxidant concentration to a point at which combustion
    cannot occur. This method requires that the Limiting Oxygen Concentration (LOC, EN14034-4:2004) be known for the
    combustible dust. Inerting is a method of explosion PREVENTION.
  2. Explosion Venting – Deflagration Venting in accordance with NFPA 68 is a method of preventing the overpressure of a protected vessel or building through the use of a rupture panel or similar device designed to release at a low pressure and expel the flame ball out of the protected volume. Explosion venting must be applied to outdoor vessels directed to a safe location or indoor vessels may be vented with a vent duct directed to the outdoors. The required vent area of the explosion vents are described by calculations found in NFPA 68. The Pmax & Kst values (ASTM 1226) for the combustible dust are required in order to properly design a venting solution. The strength of the vessel or building (Pred) must also be known. Explosion Venting is a PASSIVE mitigation strategy.
  3. Flameless Venting – Flameless Venting is a method by which the a deflagration is vented out of the protected vessel through a flame arresting mesh (quench tube) designed to quench the flame which makes it safe for use indoors. Like explosion venting, the design of the flameless vent should be determined in accordance with the calculations found in NFPA 68 and an efficiency factor should be applied. Pmax, Kst and Pred values will all be needed in order to complete the design of this solution. Flameless Venting is a PASSIVE mitigation strategy
  4. Containment – Deflagration Pressure Containment is a method by which the vessel is built to withstand the maximum pressure (Pmax) of a deflagration within the vessel. In most cases, this means building the vessel to withstand between 8 to 12 barg (115 to 175 psig) and this pressure is determined by Dust Testing (ASTM 1226). Typically, containment is used with milling equipment (e.g. hammermills) and smaller process equipment found in the pharmaceutical industry. It is important to include fast-acting mechanical isolation rated for containment to prevent the pressure and flame from exiting via the inlet/outlet of the protected vessel. Containment can be PASSIVE or ACTIVE, depending on the type of isolation.
  5. Explosion Suppression – Explosion Suppression is a method for detecting and extinguishing an deflagration in its early stages, before pressure has a chance to build. This is typically achieved by using pressure detectors and chemical suppression bottles mounted directly to the protected vessel. These chemical suppressant bottles will usually contain a powder suppressant such as sodium bicarbonate and high pressure gas (e.g. nitrogen) to propel the suppressant into the protected volume. When designing a chemical suppression solution, the Kst and Pmax of the material must be known (ASTM 1226), the Pred of the vessel must be determined, and the process conditions must be considered to avoid false activation. Explosion Suppression is an ACTIVE mitigation strategy.
  6. Dilution – Dilution with a noncombustible dust is the least commonly employed of these approaches. This mitigation strategy involves the addition of a noncombustible solid to a combustible dust mixture to render the mixture noncombustible as a whole. When designing a dilution scheme, it is important to confirm that the mixture created is within the noncombustible region by having your mixture tested using the Salt Lake Go/No-Go Screening Test over a range of mixture concentrations. Also, it is important to consider if and how the combustible component will be separated from the noncombustible component downstream in your process. Dilution is a method of explosion PREVENTION.

Combustible Dust Explosion Isolation

In most cases, the Prescriptive approach for vessels with Combustible Dust Explosion Hazard does not stop with the Mitigation Strategies listed above. To mitigate the risk of Secondary Explosion and the effects of pressure piling as a deflagration moves down a constricted pathway (e.g. duct), NFPA 652 requires that explosion isolation devices be installed in accordance with NFPA 69 to contain the effects of pressure and flame. These isolation devices are commonly used on inlets, clean air outlets, product outlets, aspiration ducts, and any other duct or chute to/from a vessel.

There are several different methods for isolation that are acceptable under NFPA 69. Below are commonly utilized isolation methods:

  • Isolation Flap Valve – The Flap Valve is a mechanically operated device similar to a check-valve design that is actuated based
    on the pressure front from a deflagration that precedes the flame front. These flap valves are becoming increasingly
    common for explosion isolation of inlet ducts to dust collector and cyclones. Flap Valves should be chosen carefully for the
    application considering the process design and any limitations outlined in the valve’s Approval. Ensure to utilize a valve with
    a valid ATEX EN16447 Approval Listing as there have been losses from valves that were not tested to this new Approval
    Standard. The Flap Valve is a PASSIVE form of Explosion Isolation.
  • Chemical Isolation System– A chemical suppression bottle containing a solid extinguishing agent (typically sodium bicarbonate) is connected to the duct and triggered using either Pressure Detection or Optical Detection. The suppressant is deployed into the isolated duct in order to extinguish the flame front. This type of isolation system offers the most flexibility for installation and can be used both on small pipes (3”) and large (90”) diameter ducts. False activation concerns and ongoing maintenance requirements should be considered. Explosion Isolation is an ACTIVE form of Explosion Isolation.
  • Isolation Float Valve – The Float Valve is a mechanically operated device which operates on many of the same principles as the Flap Valve, but which protect from explosion traveling in the same direction as normal air flow. Due to the design of the float valve with a large poppet in the center of the valve, this should only be used with air flows with very low solids concentrations. Again, ensure that the valve is designed and approved for explosion isolation with a valid ATEX Approval Listing. The Float Valve is a PASSIVE form of Explosion Isolation.
  • Isolation Pinch Valves – The Pinch Valve consists of a rubber bladder which is actuated by compressed air stored in a reservoir tank above. The pinch valve may triggered based on the opening of a rupture panel or by detectors sensing the pressure or light from a deflagration. These valves are ideal for use in pneumatic convey operations and are limited to small duct sizes up to about 12”. Care should be taken to design the valve to minimize wear to the bladder.
  • Fast Acting Mechanical Valve – Typically a slide gate, this fast acting valve is triggered based on pressure or optical detection at the vessel. These valves can be built for containment and prevent both the pressure and flame ball from traveling past the valve. Some valves will utilize an ignitor to actuate the valve gate while newer designs utilize compressed air to actuate the valve. Using compressed air allows the valve to be repeatedly opened and closed without need for rebuild. Fast Acting Mechanical Valves are an ACTIVE form of Explosion Isolation.
  • Rotary Airlock – Rotary Airlocks are a common device found at the bottom of hoppers to provide explosion isolation between the vessel above and the drop chute below. However, in order to provide effective explosion isolation, rotary airlocks must conform to the criteria outlined in NFPA 69 regarding material chokes. The criteria include strength of the valve to withstand Pred, the minimum required number of vanes, and the required clearance of the tips. Rotary Airlocks are a PASSIVE form of Explosion Isolation.

Explosion Prevention

While Explosion Mitigation methods aim to minimize the negative effects of a deflagration with the assumption that ignition will occur, Explosion Prevention methods aim to keep the deflagration from occurring in the first place. Explosion Prevention is achieved by removing one or more of the legs of the Dust Explosion Pentagon. Two methods of explosion prevention, Inerting and Dilution, are described above. The following are some of the Prevention methods commonly employed:

  • Spark Detection & Extinguishing
  • Housekeeping
  • Bonding and Grounding
  • Magnetic Separation
  • Preventative Maintenance
  • Electrical Classification
  • Hot Work Permitting
  • Administrative Controls

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