Combustible Dust &amp; Powder Testing

BS EN ISO IEC 80079-20-2

This test is conducted to assess whether a dust/powder will form a flammable atmosphere or support flame propagation when dispersed as a dust cloud in air. It’s a purely qualitative test that categorises a material into one of two groups:

Combustible (Group A) – Flammable or explosible as a dust cloud

Non-Combustible (Group B) – Non-flammable or explosible as a dust cloud

Varying concentrations of dust are dispersed in into a 20L explosion vessel and exposed to a 2-kJ ignition source. Piezo electric transducers then monitor the vessels atmosphere for ignition, in the form of pressure rise.

Materials that are found to be combustible (Group A) will present a flammable atmosphere when dispersed as a dust cloud. Therefore, if a sufficient ignition source is present then a hazardous situation could arise. This can include an explosion or fire that may result in product loss and, potential operator injury or fatality. Further testing is required to establish a suitable basis of safety for each process or operation handling a combustible (Group A) dust and/or powder.

Materials that are found to be non-combustible (Group B) will not present a flammable atmosphere when dispersed as a dust cloud. As a result, materials that are found to be Group B are not subject to the conditions stipulated within DSEAR (Dangerous Substances and Explosive Atmospheres Regulations) or ATEX (Atmosphere Explosive) in regard to dust cloud explosion hazards*. This screening approach can save the need to conduct further dust dispersive testing to establish a suitable basis of safety for processing the material.

* Ignition of a material in layers or bulk deposits through thermal decomposition may still present a risk and further testing should be considered. Discover more about our Air Over Layer, Bulk Powder (diffusion cell), Aerated Cell and LIT tests, below.

Minimum Ignition Energy (MIE)

BS EN ISO IEC 80079-20-2 / ASTM E2019 / BS EN 13821 (withdrawn)

The Minimum Ignition Energy (MIE) test is conducted to determine the lowest spark energy that can ignite a powder when dispersed in the air as a dust cloud. A purely capacitive generated spark is used to assess electrostatic discharges (brush, spark, transitional / propagating brush discharges). An inductive generated spark is selected for mechanical (friction, impact and grinding) or electrical (wire short, electrical contact) spark incendivity analysis.

Working through a range of powder concentrations, materials are dispersed into a 1.2L litre glass vessel using dry compressed air. These momentarily suspended dust clouds are then exposed to sparks of known energy and for a selected duration. Following this, energy is then lowered, while powder concentration and time delay is varied until the ‘MIE’ is established

MIE data is typically used to define the minimum spark energy needed to ignite a dust cloud of a process material in the working environment. MIE is one of three core tests where ‘Avoidance of Ignition Sources’ is the ‘Basis of Safety’ for the operation. This approach tests materials to determine how sensitive they are to ignition sources such as electrostatic discharge, mechanical & electrical sparks and hot surfaces. The Basis of Safety is often used when charging vessels from sacks, IBC’s or FIBC’s, pneumatic conveying, milling and tabletting operations. Many materials that can create a flammable atmosphere are found to be insensitive to ignition and therefore, it is easy, practical and cost effective to ensure that an ignition source capable of initiating a reaction is not present.

A sub-group of ‘Avoidance of Ignition Sources’ are the electrostatic properties of products and materials used in a process environment. This may include the electrostatic properties of materials being processed, electrostatic properties of containers or Personal Protection Equipment (PPE). This is particularly important where process materials are found to be very sensitive to ignition.

Minimum Ignition Temperature (MIT)

BS EN ISO IEC 80079-20-2” to the standards before BS EN 50281-2-1 / ASTM E1491

The Minimum Ignition Temperature (MIT) test is conducted to determine the lowest temperature at which a hot surface will ignite a powder when dispersed in air.

Working through a range of powder concentrations, the material is dispersed through a heated vertical ‘Godbert-Greenwald’ furnace using compressed air. Once an ignition temperature is observed, the furnace temperature is reduced. Dispersion pressure and powder concentration is then varied until a minimum ignition temperature is established.

The MIT test is one of three core tests used to access a materials ignition sensitivity. It is generally used for when the avoidance of ignition sources’ is the ‘Basis of Safety’ for an operation. MIT test data is required for defining the maximum operating temperature for electrical and non-electrical equipment used in areas where test material is present.

Avoidance of ignition sources as the ‘Basis of Safety’, is often used for processes such as charging vessels from sacks, IBC’s or FIBC’s, pneumatic conveying, milling and tabletting operations. Many materials that can create a flammable atmosphere are found to be insensitive to ignition and therefore, it is easy, practical and cost effective to ensure that an ignition source capable of initiating a reaction is not present.

Layer Ignition Temperature (LIT)

BS EN ISO IEC 80079-20-2” to the standards before BS EN 50281-2-1 / ASTM E1491

The Layer Ignition Temperature (LIT) test is conducted to determine the lowest temperature at which a hot surface will ignite a powder when settled as a dust layer.

A material is placed onto an isothermally heated hotplate and formed into a 5mm layer. Once an ignition temperature is observed, the hotplate temperature is reduced and tested. This is then repeated with fresh materials, until the lowest ignition temperature is established.

The LIT test is one of the three core tests for accessing a materials ignition sensitivity It is generally used for when the avoidance of ignition sources’ is the ‘Basis of Safety’ for an operation. This data is required for defining the maximum operating temperature for electrical and non-electrical equipment used in areas where test materials are present.

Explosion Severity (Pmax & Kst)

BS EN 14034 parts 1 & 2 / ASTM E1226

The explosion severity test is conducted to determine the maximum pressure (P_max), maximum rate of pressure rise (dP/dt)_max and the dust constant K_st value of an ignited powder, dispersed in the air, as a dust cloud.

Working through a range of powder concentrations, a powder is dispersed into a 20L explosion vessel (sphere) using dry compressed air. These suspended dust clouds are then exposed to a large 10 kJ chemical ignition source and the following explosion monitored, measured, and recorded, using high accuracy piezo electric transducers. Testing is conducted over a wide range of powder concentrations which enables flammability curves of P_m, dP/dt & _Kstto be generated, along peak values.

Data generated by the explosion severity test is predominantly used for either the design of explosion protection systems (venting, containment or suppression) or to confirm if a material is appropriate for processes within an established explosion protection system.

Minimum Explosion Concentration (MEC)

BS EN 14034 part 3 / ASTM E1515

The minimum explosive concentration or lower explosive limit test is conducted to determine the minimum quantity of powder dispersed in the air, as a dust cloud that will form a flammable atmosphere.

Working through a range of decreasing powder concentrations, a powder is dispersed into a 20L explosion vessel (sphere) using dry compressed air. These suspended dust clouds are then exposed to a large 2 kJ chemical ignition source and the following explosion monitored, measured, and recorded, using high accuracy piezo electric transducers. Once ignition is achieved, the concentration of the powder is then reduced until an ignition is not possible and the MEC established.

Data generated by the minimum explosive concentration test is predominantly used to ensure that levels of airborne dusts are kept below the minimum threshold by which a flammable atmosphere can be created. It may also confirm if a material is appropriate for processes with an established LEV or set dust extraction rate.

Limited Oxygen Concentration (LOC)

BS EN 14034 part 4

The limiting oxygen concentration test is conducted to determine the minimum quantity of oxygen, within an atmosphere, that will enable a powder dispersed in the air to ignite.

Working through a range of powder concentrations, a powder is dispersed into a 20L explosion vessel (sphere) using dry compressed air and nitrogen. Using partial pressures, the oxygen content within the sphere at the time of ignition can be controlled. These suspended dust clouds are then exposed to a large 2 kJ chemical ignition source and the following explosion monitored, measured, and recorded, using high accuracy piezo electric transducers. Once ignition is achieved at a specific oxygen concentration, the oxygen concentration is then reduced, powder concentration varied, until ignition is not possible, and the LOC is established.

Data generated by the limited oxygen concentration test is predominantly used for either the design of nitrogen inerting systems or to confirm if a material is appropriate for processes with an established limited oxygen atmosphere. Many systems are set at a much lower oxygen level than required, resulting in large nitrogen costs. This cost can be avoided or reduced, by knowing a materials specific LOC and setting oxygen levels accordingly.

Burning Behaviour

Standards

VDI 2263 and BS EN 17077

The test is conducted to assess the behaviour (severity) of combustion propagation through a material deposit when ignited by an external ignition source. A small powder strip is formed onto a heat resistant, non-porous plate and ignition trials are performed using a hot (1000°C) platinum wire. The 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.

Type of Reaction		CC	Reference Product
No spreading of fire	No ignition	1	Table Salt
	Brief ignition and rapid extinction	2	Tartaric Acid
	Localised combustion or glowing with practically no spreading	3	Lactose
Fire spreads	Glowing without sparks, (smouldering ) or slow decomposition without flame	4	Tobacco
	Burning with flame or spark generation	5	Sulphur
	Very rapid combustion with flame propagation or rapid decomposition without flame	6	Black Powder