Determination of Lower Explosibility Limit (LEL)
- Relevant standards:
2. Differences between standards:
EN 14034-3: LEL – Lower Explosibility Limit,
ASTM E1515: MEC – Minimum Explosible Concentration.
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.
Particle size specification is only included in the ASTM E1515 standard. Based on this the size of the examined dust should be under 75 μm, however, the received sample can also be used.
3. Test Apparatus:
A hollow sphere testing chamber with a volume of 20 litres manufactured by Kühner.
Hollow sphere testing chamber with a volume of 20 litres manufactured by Kühner
4. Amount of sample required:
5. Aim of the test:
The lower explosibility limit (LEL) is given in g/m3. It determinates the maximum dust concentration at which no explosion occurs after three consecutive tests.
6. Procedure of the measurement:
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 1 kJ chemical igniters.
The pressure as a function of time is recorded on a computer and the maximum values for the pressure and the rate of pressure increase are entered. The maximum pressure pmax during the explosion can be read from the diagram. Its value is at least the initial pressure pi+0,3 bar.
The 20-l-sphere and the dust storage chamber must be cleaned thoroughly before each test.
The test can be started from a concentration of 500 g/m3 or any other value where the explosion occurs, thereafter the concentration is reduced based on the recommendation of the EN 14034-3 Standard: …, 60; 125; 250;500; 750; 1000; …g/m3.
The testing must be continued with the lowered concentrations until for the given concentration value 3 negative tests are observed (negative test = no explosion).
If an explosion occurs during the measurement, the concentration must be further reduced until no explosion occurs for three times at that concentration.
7. Results, evaluation:
Ignition occurs if the explosion pressure is equal to or larger than 0.5 barg.
The determined values of LEL (or MEC) can be used for the design and operation of transport systems carrying dust-air mixture, so the dust concentration would not cause an explosion. However, this needs to be addressed with consideration, as dust clouds that occur in practice are rarely uniform. Locally explosive concentrations, which may be below the LEL, may settle or form under certain conditions in some parts of the system.
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.
In case of powders with high concentrations loading of the hopper can be problematic if they have a low specific gravity. It may not be possible to load the required amount into the hopper.