Dust Explosibility

The Occupational Safety and Health Administration, OSHA, has reissued their Combustible Dust National Emphasis Program (NEP) (CPL 03-00-006). Due to serious incidents in industry related to combustible dusts, OSHA is inspecting sites for dust explosion hazards per the NEP using specific guideline documents from the National Fire Protection Association (NFPA). OSHA is targeting (but not limiting) their NEP inspections to companies that handle the following dusts:

  • Metal dust such as aluminum and magnesium
  • Wood dust
  • Coal and other carbon dusts
  • Plastic dust and additives
  • Biosolids
  • Other organic dust such as sugar, flour, paper, soap, and dried blood
  • Certain textile materials.

With the SMS heritage in propellant, explosives, and pyrotechnics, SMS is uniquely qualified to assist clients with all aspects of the NEP. Similar to OSHA’s Process Safety Management Program (PSM) and similar to the SMS’s Fundamental Principles of Explosive Safety, OSHA’s NEP requires the same risk management protocol for combustible dusts, that SMS has been helping clients with for years. SMS core competencies that relate to proper risk management protocol for combustible dusts includes:

  • Understanding the nature of the material – SMS can perform all dust classification testing described in the NEP
  • Risk Assessment – The SMS team of highly qualified engineers and scientists are specifically trained to perform a thorough and accurate process hazards analysis (PHA) incorporating all the sensitivity and reactivity information from the material testing
  • Proper Facility Design and Siting – The SMS team uses explosion and deflagration modeling techniques to design proper mitigating measures and deflagration venting requirements

The purpose of the dust explosibility tests is to determine the minimum amount of concentration and the minimum amount of energy necessary to have an explosion when working with dry, powdery materials. Minimum Explosive Concentration (MEC) is determined by testing various sample weights (dry powders) with a constant energy source. This test will determine how much of the material must be present in a dust cloud to be an explosive hazard. Minimum Ignition Energy (MIE) is determined by using a constant sample weight and varying the energy. From this, data can be obtained to determine how much energy is needed for that material before an explosive hazard exists.

Minimum Auto-Ignition Temperature – Dust Layer

Hot-Surface Ignition Temperature of Dust Layer

Hot-surface ignition temperature of a dust layer is performed in accordance with ASTM E2021, “Standard Test Method for Hot-Surface Ignition Temperature of Dust Layers”. A circular layer of the sample is heated on a hot plate at a constant temperature for a specified duration of time. Both the sample and hot plate are monitored for temperature and the minimum temperature at which the material ignites is considered the minimum auto-ignition temperature.

Simulated-Bulk Auto-Ignition Temperature (SBAT) Test

The SBAT Test consists of:

  • Ramped: Increasing a sample’s temperature 12°C (22°F) per hour to observe the sample’s response to temperature
  • Isothermal Temperature Trials: Exposing the sample to an isothermal temperature until either ignition occurs or it is determined that the sample’s activity is insignificant.

For each of these, the SBAT apparatus is allowed to come to the equilibrium starting temperature. Next, a 3-5 gram sample is placed inside the heavily insulated sample holder of the apparatus. While the sample resides in the SBAT apparatus, the temperature of the sample is monitored relative to the temperature of the heater using sensors. The sample is withdrawn from the apparatus after either a) an ignition has occurred, or b) the sample has shown insignificant activity over a specified time period.

Dust Explosion Severity and Minimum Explosible Concentration

20-Liter Sphere

Dust explosion severity and minimum explosive concentrations are determined using the spherical 20-liter chamber as outlined in the appendix of the current American Society for Testing and Materials (ASTM) Method E1226-10, “Standard Test Method for Explosibility of Dust Clouds”. The chamber is equipped with an injection port, a pressure transducer, a pressure relief port, and an exhaust port. Samples are normally dispersed through a perforated dispersion ring fitted to the bottom port of the bomb. The sample is placed in the injection line and dispersed by a blast of dispersion air. After an injection delay, the ignition source is activated to ignite the dust. The maximum pressure and rate of pressure rise of the dust sample is measured.

Explosion Severity Test (Pmax, dP/dtmax, KSt)

The 20-liter sphere is used to determine the Pmax (maximum explosion pressure), dP/dtmax (maximum rate of pressure rise), and KSt value (explosion index) of a dust cloud, as outlined in ASTM E1226-10, “Standard Test Method for Explosibility of Dust Clouds”. A 10-kJ igniter is used as the standard ignition source. Tests are performed at 250 g/m3 increments, starting at 250 gm/m3, to establish the concentration(s) at which the maximum pressure and the maximum rate of pressure rise occur. Three replicates are completed at the concentration(s) at which the maximums occur, and at the concentration increment below and above it. For incremental concentrations tested below 250 g/m3, the concentration can be cut in half (i.e., 125, 60, 30 g/m3).

Minimum Explosive Concentration (MEC)

The minimum explosive concentration or the lower explosive limit of a dust is determined in the 20-liter sphere and performed according to ASTM E1515, “Standard Test Method for Minimum Explosible Concentration of Combustible Dusts”. A 5-kJ igniter is used as the ignition source. A weighed amount of dust is placed in the injection line. If propagation occurs for a given weight of dust, the weight is reduced incrementally and another trial made until a quantity is obtained which fails to propagate flame in a minimum of two successive trails. The lowest weight at which flame propagates is used in calculating the minimum concentration. The weight is divided by the volume of the sphere for the value of the minimum concentration.

Minimum Ignition Energy (MIE)

The minimum ignition energy (MIE) to ignite a dust cloud is determined using a chamber as outlined in ASTM E2019, “Standard Test Method for Minimum Ignition Energy of a Dust Cloud in Air”. The vessel consists of a modified Hartmann tube with a volume of 1.2 liters. The sample is placed in the base of the dust dispersion system and dispersed into the glass cylinder by a blast of air. A spark at a certain energy level is discharged between two electrodes to determine if the material will ignite. The energy level is varied between 1 to 1000 mJ until the minimum ignition energy required to ignite the dust cloud is determined. (Note: unless otherwise requested, samples will be tested with inductance. Tests conducted without inductance are postulated to be similar to theoretical electrostatic discharges and are thus less conservative.)

Minimum Auto-Ignition Temperature Test of Dust Cloud

The minimum auto-ignition temperature (MAIT) to ignite a dust cloud is determined using the BAM furnace as specified in ASTM E1491, “Standard Test Method for Minimum Autoignition Temperature of Dust Clouds”. The temperature of the furnace or oven is set to a predetermined value. A certain amount of material is placed into the disperser system and dispersed into the furnace. A “Go” is determined by flames exiting the oven. The oven temperature and concentration is varied until the minimum auto-ignition temperature is determined.