DUST TESTING.

Hazard classification test for combustible dust

The classification test offers a qualitative assessment of whether or not a suspended dust cloud is capable of initiating and sustaining an explosion in the presence of four different ignition sources; 10kV electric spark, hot coil, 1000°C hot surface, and low energy chemical igniters. Explosion testing for hazard classification is made on a visual observation of flame propagation in the first two tests and on explosion pressure increase for the chemical igniters in an enclosed volume. The test is carried out to EN ISO/IEC 80079-20-2:2016 Explosive atmospheres – Part 20-2: Material characteristics — Combustible dusts test methods and results in one of two possible outcomes:

• Group A (hazardous) – the dust cloud propagates flame from the ignition source and is classed as a combustible and potentially explosible material.
• Group B (non hazardous) – the dust cloud does not propagate flame from the ignition source and is classed as a non-combustible material.

Group A dusts (combustible) may form Zone 20, Zone 21 and Zone 22 hazardous areas, depending upon the frequency and duration of the explosive atmosphere.

Explosion indices test (Kst value and P max)

Dust explosion classes ST1, ST2 & ST3

Kst value and Pmax are explosive properties measured in the laboratory to quantify the severity of a dust explosion. The explosion indices test follows EN 14034-1:2004 (determination of the maximum explosion pressure Pmax of dust clouds) and EN 14034-2:2006 (determination of the maximum rate of explosion pressure rise of dust clouds Kst). The tests are carried out in a 20 litre sphere apparatus which reproduces a high state of turbulence to simulate worst case process plant conditions. A weighed quantity of combustible dust is placed into the dust container. The main explosion chamber is then evacuated to 0.4 bar absolute. An automatic test sequence is initiated to pressurise the dust container to 20 bar gauge, and then the fast acting valve on the dust container outlet is opened to allow material into the explosion chamber. The rebound nozzle ensures an even distribution of dust within the explosion chamber and the control system activates two 5 KJ chemical igniters at the centre of the sphere 60 ms after the dust has been dispersed. Explosion pressures are measured for a range of dust concentrations using piezo-electric pressure transducers. The tests are carried out over three series to ensure a thorough investigation of the explosion properties. From the tests, the arithmetic mean of the maximum values (both maximum pressure and maximum rate of pressure rise) is obtained. The Kst value is calculated as the equivalent pressure in a 1 m3 sphere from the cube law (Kst value = cube root of volume x explosion pressure rise). The ST class is based on the Kst value as follows:

• ST class 0 – Kst value = 0
• ST class 1 – Kst value less than 200 bar m/sec
• ST class 2 – Kst value between 200 and 300 bar m/sec
• ST class 3 – Kst value greater than 300 bar m/sec

Explosion testing for Kst value & Pmax is essential to validate protection design (explosion venting, explosion suppression and explosion containment). 

Testing for minimum ignition energy (MIE)

Dusts sensitive to ignition by electrical spark

The minimum ignition energy (MIE) test measures the ease of ignition of a dust cloud by electrical and electrostatic discharges. The test is carried out according to EN ISO/IEC 80079-20-2:2016 (Explosive atmospheres Part 20-2: Material characteristics – Combustible dusts test methods). The MIE apparatus consists of a borosilicate tube placed over a dispersion cup and fitted with two electrodes. The electrodes are connected to a circuit that produces an electrical spark of known energy. A weighed sample of combustible dust is placed in the dispersion cup, which is blown up through the tube with compressed air past the ignition source. If flame propagation is observed, the energy of the spark is reduced until no flame propagation is seen for 10 consecutive tests. At that spark energy, a range of dust sample weights is tried to thoroughly investigate potential dust explosion concentrations. The test result is stated as an energy range, indicating that ignition took place at the higher value and no ignition took place at the lower value. The MIE is a measure of how sensitive an explosive dust cloud is to electrical spark ignition. Testing for minimum ignition energy gives guidance on whether ignition by electrostatic discharge from plant personnel or process conditions is likely to occur in practice. 

Minimum ignition temperature (MIT)

Minimum ignition temperature of a dust cloud

The minimum ignition temperature (MIT) is the lowest temperature of a hot surface that will cause a dust cloud, rather than a dust layer, to ignite and propagate flame. The test follows EN ISO/IEC 80079-20-2:2016 (Explosive atmospheres Part 20-2: Material characteristics – Combustible dusts test methods). Approximately 0.1 g of combustible dust is placed in a dust holder at the top of a temperature controlled furnace with an open bottom. The dust is dispersed by compressed air downwards past the hot surface of the furnace to see if ignition occurs and flames are produced. If the dust does not ignite, the furnace temperature is increased and the test repeated until ignition of the dust occurs. Once ignition has been established, the mass of the dust sample and injection pressure are varied to find the most vigorous explosive flame discharge. The temperature of the furnace is then reduced incrementally until flame propagation is no longer observed. At this temperature, the dust mass and injection pressure are varied to confirm that no ignition is found over ten consecutive tests. The minimum ignition temperature (MIT) is the lowest temperature of the furnace at which flame is observed minus 20°C for furnace temperatures over 300°C or minus 10°C for furnace temperatures under 300°C. For items of plant such as driers, testing the minimum ignition temperature is important to prevent a dust explosion occurring through contact with a hot surface. The internal temperature is generally limited to two thirds of the MIT when measured in degrees Centigrade. For example, a dust with a MIT of 450°C would require a maximum operating temperature in the drying process of 300°C, giving a safety margin of 150°C.