The process industries manufacture many types of unique compounds and substances but uses a select range of processing equipment for manufacturing the end products. The storage, processing, handling and packaging of material create potentially hazardous condition in the form of dust in the workplace with a risk of fire and explosion.
What is Combustible dust testing?
Combustible dust testing refers to laboratory testing for determining main Basis of Safety approaches related to Dust explosion.
Go/No Go Test or Explosibility Screening Test
The test is conducted to assess whether a dust/powder will form a flammable atmosphere, supporting flame propagation, when dispersed as a dust cloud in air. It is a purely qualitative test and categorises a material into one of two groups:
GO/NO GO testing is conducted in accordance to the following standards
1. British Standards BS EN ISO IEC 80079-20-2
2. American Society for Testing and Materials (ASTM) E1226
A) Minimum Ignition Energy (MIE)
The test is conducted to determine the lowest spark energy that will ignite a powder when dispersed in air, as a dust cloud. The spark energy may be either Electrostatic discharges or Mechanical sparks
Minimum Ignition Energy (MIE) – Capacitance: A purely capacitive generated spark is used for assessment of electrostatic discharges (brush, spark, transitional / propagating brush discharges) incendivity.
Minimum Ignition Energy (MIE) – Inductance: An inductive generated spark is selected for mechanical (friction, impact and grinding) or electrical (wire short, electrical contact) spark incendivity analysis.
Minimum Ignition Energy (MIE) testing is conducted in accordance to the following standards
1. British Standards : BS EN ISO IEC 80079-20-2
2. American Society for Testing and Materials: ASTM E2019 - 03
B) Minimum Ignition Temperature (MIT) : Dust Cloud
The test is conducted to determine the lowest temperature at which a hot surface will ignite a powder when dispersed in air, as a dust cloud. The data is required for defining the maximum operating temperature for electrical and non-electrical equipment used in areas where the test material is present.
Minimum Ignition temperature (MIT) testing is conducted in accordance to the following standards.
1. British Standards : BS EN 50281-2-1
2. American Society for Testing and Materials: ASTM E1491
C) Layer Ignition Temperature (LIT) : Dust Surface
The test is conducted to determine the lowest temperature at which a hot surface will ignite a powder when settled as a dust layer. The data is required for defining the maximum operating temperature for electrical and non-electrical equipment used in areas where the test material is present.
Layer Ignition Temperature (LIT) testing is conducted in accordance to the following standards
1. British Standards : BS EN 50281-2-1
2. American Society for Testing and Materials: ASTM E2021
A. Minimum Explosive Concentration (MEC)
The test is conducted to determine the minimum quantity of powder dispersed in air, as a dust cloud that will ignite. The test is conducted in a 20 L (explosion vessel) sphere with 2 kJ chemical ignition source Data generated by the minimum explosive concentration test is predominantly used to either ensure that levels of airborne dust are kept below that to form a flammable atmosphere or to confirm a material is appropriate for processes with an established LEV or set dust extraction rate.
Minimum Explosive Concentration (MEC) testing is conducted in accordance to the following standards
1. British Standards : BS EN 14034 part 3
2. American Society for Testing and Materials: ASTM E1515
B. Limiting Oxygen Concentration (LOC)
The test is conducted to determine the oxygen concentration below which combustion is not possible, independent of the concentration of fuel. The test is conducted in a 20 L (explosion vessel) sphere with 2 kJ chemical ignition source Data generated by the Limiting Oxygen Concentration test is predominantly used to either ensure that levels of oxygen to fuel (dust) concentration to kept below that to not form a flammable atmosphere or to confirm a material is appropriate for processes with an established LEV.
Limiting Oxygen Concentration (LOC) testing is conducted in accordance to the following standards
1. British Standards : BS EN 14034 (pt.4)
2. American Society for Testing and Materials: ASTM E2931 -13 (2019)
C. Explosion Severity Test (Kst & Pmax)
The test is conducted to determine maximum pressure (Pmax), Maximum rate of pressure rise ((dP/dt)max) and Kst of an ignited powder, dispersed in air, as a dust cloud. The test is conducted in a 20 L (explosion vessel) sphere with 2 kJ chemical ignition source. Data generated by the explosion severity test is predominantly used for either design of explosion protection systems (venting, containment or suppression) or to confirm a material is appropriate for processes with an established explosion protection system.
Explosion Severity Test (Kst & Pmax) testing is conducted in accordance to the following standards
1. British Standards : BS EN 14034 parts 1 & 2
2. American Society for Testing and Materials: ASTM E1226
A) Powder Volume Resistivity (PVR)
The test is performed at two controlled relative humidity conditions, an upper humidity of 50% and a lower humidity of 15%, in order to see the changes in resistivity as humidity falls. In pharmaceutical production many of the processes operate at low humidity.
The resistivity value will determine whether a material is conductive, static dissipative or insulating. With the latter case, care must be taken during movement of the powder to minimise charge accumulation which could increase the risk from electrostatic spark discharges.
B) Charge Relaxation Time
The Charge Relaxation Time compliments the Powder Volume Resistivity measurement as it indicates how long a material can retain its electrostatic charge. This is especially important with materials that are found to be insulating so as to minimise the risk from electrostatic spark discharges. Again the tests are performed at two relative humidity’s.
This test data is useful for operations that involve powder movement or storage such as pneumatic conveying, blending, milling, big bag storage etc.
A) Burning Behaviour (BB)
The test is conducted to assess the behaviour (severity) of combustion propagation through a material deposit on being ignited by an external ignition source. Its burning behaviour is numerically rated from 1 (material does not ignite) to 6 (supports rapid combustion) in accordance with the test standards characterisation table. The rating is known as “CC” or Combustibility Class and testing is performed at ambient and elevated environmental temperature conditions.
When the burning behaviour of a solid material is known, fires and explosions caused as a result of the introduction of hot embers or friction heat (on bearing failure) can be predicted and therefore either avoided or mitigated. It also can be used as a screening tool to highlight materials that should be classified as a ‘flammable solid’ for transportation
Burning Behaviour (BB) testing is conducted in accordance to the following standards
1. British Standards : BS EN 17077
2. American Society for Testing and Materials: ASTM E1515
B) Burning Rate (BR)
The ability of a substance to propagate combustion is tested by igniting it and determining the Burning Rate. This test adequately asses the relative hazard of readily combustible solids so that an appropriate classification for transport can be made (determination of Packaging Group (Groups II and III)). Powdered, granular or pasty substances should be classified in Division 4.1 (Dangerous goods) when the time of burning of one or more of the test runs is less than 45 s or the burning rate is more than 2.2 mm/s. Powder of metal or metal alloys should be classified when they can be ignited and the reaction spreads over the whole length of the sample in 10 minutes or less.
Burning Rate (BR) testing is conducted in accordance to the following standards
1. British Standards : BS EN 17077
2. American Society for Testing and Materials: ASTM E1515
A) Air Over Layer Test
The test is conducted to establish the onset temperature of exothermic activity of thin layers of material when exposed to elevated atmospheric temperatures. It replicates thin layer (up to 15 mm) deposits in any drying situation i.e. drier walls and roof where hot air rushes over its surface.
Not only is the air over layer test a necessity in evaluating whether current or proposed drying and process temperatures are safe, it can also confirm the possibility of using a higher temperatures to increase productivity.
Air Over Layer testing is conducted in accordance to the following procedure given by - Prevention of Fires and Explosions in Dryers – John Abbott
B) Bulk Powder Test
The test is conducted to establish the onset temperature of exothermic activity of a material when exposed to elevated atmospheric temperatures. It replicates small bulk deposits in any drying situation where air is naturally available.
Not only is the bulk powder diffusion cell test a necessity in evaluating whether current or proposed drying and process temperatures are safe, it can also confirm the possibility of using a higher temperature to increase productivity.
Bulk Powder testing is conducted in accordance to the following procedure given by - Prevention of Fires and Explosions in Dryers – John Abbott
C) Aerated Cell Test
The test uses the same bulk powder test cell but a lid is attached with a sintered glass base. Pre-heated air is then pushed through the sample mimicking a condition where there is a bulk of powder with large amount of air availability such as a fluid bed dryer or rotating dryer.
Aerated Cell testing is conducted in accordance to the following procedure given by - Prevention of Fires and Explosions in Dryers – John Abbott
