Determination of explosion indicates (Pmax, KSt)

1. Relevant standards:

• EN 14034-1, EN 14034-2,

• ASTM E1226-10.

2. Differences between standards:

• Specifications concerning the particle size:

  • • ISO: The size of the particles has to be under 63 μm,

    • ASTM: At least 95% of the dust has to be under 75 μm.

    • EN: The median of the size of the dust has to be under 63 μm.

• Delay between the start of the ignition source and the dispersion of the dust:

  • • EN 14034: 0.6 + - 0.01 s,

    • ASTM E1226-10: 0.06 + - 0.005s.

3. Test Apparatus:

A hollow sphere testing chamber with a volume of 20 litres manufactured by Kühner.

4. Amount of sample required:

750 g

5. Aim of the test:

The aim of the test is the determination of explosion indicates of dust and hybrid mixtures. These parameters are the maximal explosion overpressure (p_max) in bar_g and the maximal rate of the pressure rise (dp/dt)_max in bar_g/s. The determined parameters can be used in the design of the preventive explosion protection.

6. Procedure of the measurement:

• Firstly, the pretesting of the dust has to be carried out (moisture content and particle size of the dust). The dust sample should have a median particle size not exceeding 63 μm, if it is over the given value grinding has to be carried out. The moisture content should be under 10 m/m%, if it does not meet this requirement then drying is necessary.

• Before the start of the measurement, the room and the chamber temperature have to be recorded.

• The dust is supplied into the 20 l chamber by a solenoid valve with an overpressure of 20 bar.

• The ignition source required for the explosion is provided by two 5 kJ chemical igniters. The ignition source is automatically activated in a certain time after the dust is dispersed into the chamber. During this time, the appropriate dust cloud forms.

• In the first test series, the maximum explosion overpressure and the maximum rate of pressure rise are determined over a wide range of concentrations, which usually has 7 or more (9-10) measurements. Starting with a low dust concentration of 60 g/m³, the concentration is increased in steps, until the maximum values for the explosion pressure and the rate of pressure rise have clearly been determined. The 20-l-sphere and the dust storage chamber must be cleaned thoroughly before each test. The EN 14034 Standard recommends the usage of the following concentration values: …, 60; 125; 250;500; 750; 1000; 1250; 1500…g/m³.

7. Results, evaluation:

In the first test series, the maximum explosion overpressure and the maximum rate of pressure rise and associated concentration are determined. The second and the third test have to be carried out at the close range of this concentration.

8. Limitations of the test:

• Can not be used directly for the determination of the properties of gases and vapours (separate tests are used for these purposes), only for solids.

• This test only takes into account the ambient temperature and pressure.

• With modifications, a hybrid mixture can also be tested in the KSEP chamber.

• In the case of powders with high concentrations, the loading into the chamber can be problematic if they have a low specific gravity. It may not be possible to load the required amount into the hopper.