Maximum explosion (over)pressure pmax
Definition
The maximum pressure value occurring during the explosion of a dust/air mixture of optimum concentration in a closed vessel under specified test conditions.
Determination
Reliable values for these explosion characteristics of dusts are obtained if a sphere-shaped vessel with a volume exceeding 20 l is used. The values in the tables are based on tests in the 1 m3 vessel and/or in the 20 l sphere.
In the 1 m3 vessel the dust to be tested is poured into a 5 l dust container located outside the explosion vessel. The dust is usually blown into the explosion vessel via a semicircular perforated 3/4" pipe running along the inside wall of the vessel. The diameter of the 20 or 13 holes in the pipe wall is 5 mm or 6 mm respectively. Compressed air under an overpressure of 20 bar in the dust container ensures sufficiently quick discharge of the dust and a right turbulence inside the explosion vessel after a blasting cap or electro-pneumatically activated valve is opened, so that an adequately homogeneous dust/air mixture with a defined concentration is present in the explosion vessel at the ignition time.
The ignition delay time, i.e. the time between the start of dust injection and triggering of the ignition source, is an arbitrary quantity for the degree of turbulence at the moment of ignition. Turbulence is necessary, on the one hand, to keep the dust in suspension, on the other, it affects the course of the explosion. With diminishing ignition delay time the turbulence increases and the values for the maximum rate of explosion pressure rise and the maximum explosion pressure increase. The latter approaches a maximum value, which is achieved when the dust to be tested has just been fully discharged from the dust container. The associated ignition delay time is 0.6 s. Shortening of the ignition delay time to values below 0.6 s results in an increase in the rate of explosion pressure rise. All tests are based on an ignition delay time of 0.6 s.
The dust/air mixture is ignited in the centre of the explosion vessel. Two chemical igniters with a total energy of 10 000 J are always used as ignition source. The course of the explosion is recorded time resolved via pressure sensors inserted in the vessel wall.
In the course of a series of tests, the dust concentrations are varied, and the explosion pressure of the reaction is determined. Starting with a dust concentration of 500 g m-3 in each case, the concentration is increased gradually by 250 g m-3 and, if necessary, reduced by halving the concentration until the maximum value for the pressure is clearly ascertained.
In the case of the 20 l sphere, the test method and the construction of the equipment are basically the same as for the 1 m3 vessel. They are specified in such a way that values comparable to those in the 1 m3 vessel are obtained within the measuring accuracy limits.
Apart from vessel size, the most important differences are that partial evacuation must be undertaken before the actual test (to achieve atmospheric pressure in the explosion vessel after the dust is blown in), and that the ignition delay time is only 0.06 s. A further difference is that the investigations over the wide concentration range are conducted in three series and the maximum explosion pressure and maximum rate of explosion pressure rise are calculated by forming the mean value of the maximum values of each series.
As in the case in the 1 m3 vessel, two chemical igniters with a total energy of 10 000 J are used as ignition source.
Remarks
For the design of the property of vessels or equipment, overpressure is usually used in practice. This is why the maximum explosion overpressure pmax is indicated in tables.
The information "No ignition (n.i.)" indicates that the respective dust in the tested state is not explosible. It is not permissible to generalise this statement especially to finer or drier dust.
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