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Explosion indices

The maximum explosion pressure, the maximum rate of pressure rise and the lower explosion limit are determined in a standard test apparatus with a content of 20 litres. The maximum rate of pressure rise (dp/dt)max measured in the 20 litre-sphere is used to obtain the Kst-value. This Kst-value can be used to calculate the maximum rate of pressure rise for other volumes by means of the cubic law: (dp/dt)max . V1/3 = Kst

The maximum explosion pressure and the Kst-value describe the explosion behaviour of a combustible dust in a closed system. The explosion limits describe the range of dust concentrations in air within which an explosion is possible. Generally only the lower explosion limit is determined.
By means of the measured combustion time in the 20 litre-sphere, a value for the minimum ignition energy can be estimated.

Importance

Maximum explosion pressure

Most process equipment will not be sufficiently strong to withstand the typical pressures generated by unvented dust explosions. In principle, strengthening of the equipment can prevent it from bursting, but in general the structures required for achieving the sufficient strength will have to be so heavy that this approach is not generally recommendable, neither from the point of view of capital cost nor with respect to running and maintaining the plant. Exceptions are cylindrical dust extraction ducting, which can be made pressure resistant with reasonable wall thicknesses, and certain types of equipment which is heavy anyway, such as some mill types.

It nevertheless happens that the concept of fully pressure resistant process plant is adopted, e.g. when the powders are highly toxic and therefore in no circumstances can be admitted to outside the equipment. In such cases it is important to know the highest pressures to be expected, should a dust explosion occur within the equipment.

Maximum rate of pressure rise

Industrial enclosures such as conventional process equipment, are normally far too weak to withstand the pressures exerted even by only partly developed, confined dust explosions. Consequently a primary objective of fighting an explosion after it has been initiated, is to prevent the build-up of destructive overpressures.

At least three techniques for preventing destructive overpressures are in current use in industry. The first and probably most widely used is venting. Another technique is automatic suppression. In case the explosion starts in an enclosure that is strong enough to withstand the explosion pressure, such as certain types of mills, isolation by high-speed valves to prevent the explosion from propagating to other, weaker enclosures constitutes a third means of protection.

Regardless of which protective technique is adopted, the violence of the dust explosion, i.e. the rate of heat generation inside the enclosure where the explosion is initiated, is a deciding factor as to whether a given protection system will perform adequately.

Lower explosion limit

For a given type of explosible dust, dispersed as a cloud in air, there is a well defined minimum quantity of dust per unit volume of air below which the dust cloud is not able to propagate a flame. In theory, therefore, one could eliminate the possibility of dust explosions by ensuring that the dust concentration does not exceed this lower explosion limit.

Test equipment & procedure

The maximum explosion pressure, the maximum rate of pressure rise and the lower explosion limit are determined in a standard test apparatus with a content of 20 litres.

The dust sample is dispersed into the explosion chamber with compressed air from a storage container via a special distribution system. The tests are performed with two pyrotechnic igniters of respectively 1 kJ for the determination of the lower explosion limit and 5 kJ for the determination of the maximum explosion pressure and the maximum rate of pressure rise as ignition source. The course of the explosion is recorded as a function of time (with two quartz pressure sensors), and from the pressure- time curve the explosion pressure and the rate of pressure rise are recorded. The dust concentration is varied over a wide range until there is no further increase in either the explosion pressure or the rate of pressure rise.

The explosion behaviour in the 20 litre-sphere is evaluated by means of the corrected explosion pressure.

Explosion indices

A first correction of the measured explosion pressure is necessary to account for the influence of the igniters. Even without an explosive dust in the 20 litre-sphere an important overpressure is recorded, which is caused by the heat liberation from the chemical igniters. The influence of the igniters on the measured explosion pressure diminishes as the pressure effect of the explosion itself becomes larger.

A second correction, which is only important at high explosion pressures, correlates the measured explosion pressure in the 20 litre-sphere with the explosion pressure which would be measured in a 1 m3-vessel. The latter explosion chamber is considered to be the international reference. Because of the higher surface/volume ratio which results in greater losses of heat, the explosion pressures measured in the 20 litre-sphere are slightly lower than those measured in the 1 m3-vessel.
If the explosion pressure exceeds 0.4 bar, an explosion is said to have occurred. It is on the basis of this criterion that the lower explosion limit is determined.

The maximum rate of pressure rise is volume dependent. By applying the "cubic law" it can be converted to the Kst-value, a characteristic which is independent of the vessel volume.
The following dust explosion classes are assigned to the Kst-values:

Kst-value (bar.m/s) dust explosion class
> 0 to 200 1
> 200 to 300 2
> 300 3

If the corrected explosion pressure exceeds 0.4 bar, an explosion is said to have occurred. It is on the basis of this criterion that the lower explosion limit is determined.

Starting with an explosible dust concentration, the concentration is stepwise reduced (e.g. by halving the concentration) until there is no more explosion. To establish no ignition, there must be at least three tries with the same dust concentration resulting in no ignition. The dust concentration in g/m3 which did not result in an ignition is defined as the lower explosion limit.
The test is performed on the sample fraction having a particle size less than 63 µm.

Bibliography

  • EN 14034:1-2-3: Determination of explosion characteristics of dust clouds
  • VDI-Richtlinien 2263, Blatt 1: Untersuchungsmethoden zur Ermittlung von sicherheitstechnischen Kenngrössen von Stäuben (1990)
  • Kühner AG, Operating instructions for the 20 litre apparatus
  • W. Bartknecht, Staub Explosionen: Ablauf und Schutzmassnahmen (1987)
  • R.K. Eckhoff, Dust explosions in the process industries (1997)
  • Handboek explosiebeveiliging, Kluwer-Editorial