cen:proj:71479

Generation of flammable dust clouds for testing of explosion protective systems and devices cen:proj:71479

Scope

Standards for assessment of explosion protective systems according to Directive 2014/34/EU for dust explosions often refer to the KSt-value or K-value as a reference value to describe the application range of the system or device. This application range has been verified during experiments performed.

The KSt-value is strictly seen a measure for the reactivity of a dust determined according to experimental procedures described in detail (EN 14034-2) most commonly in a vessel of 1 m3 or 20 l. The explosion protective systems and devices have to be tested for their application range, volume wise, which often are much larger than the aforementioned test vessels. Hence, dust clouds are generated using larger injection systems (1 or more pressured bottles containing dust aiming at a certain normalized dust concentration in the test vessel) creating different turbulence conditions than seen in the standardised tests.

To obtain similar normalized rates of pressure rise at large-scale as seen in the standardised tests the delay time between start of the dust injection and the moment of ignition is varied. It is assumed that when the volume-normalized rate of pressure rise is similar to the KSt-value at the concentration where in the standard test this KSt-value was found conditions are as desired (according to the so-called cubic law).

The delay time shall be such that all dust is released into the vessel at the moment of ignition.

The delay time between start of injection and moment of ignition also determines the degree of turbulence in the vessel: the shorter the delay time the more intense the turbulence is and the stronger the explosion, i.e. the higher the rate of pressure rise. The longer the delay the less intense the turbulence is and the milder the explosions, i.e. the lower the rate of pressure rise. Variation of the delay time is often used to obtain desired normalised KSt-values. Whereas, this is acceptable for tests to determine the behaviour/efficiency of vent panels and to investigate the mechanical strength of protective systems this way of “simulating” KSt-values cannot be used for systems where the ability of flame transmission prevention or flame quenching needs to be investigated (explosion isolation systems, explosion suppression systems, flameless venting etc.). In these cases the delay time variation cannot be used as a means of obtaining desired normalized rates of pressure rise. The reactivity of the dust is determinant and the desired rate of pressure rise shall be determined by this reactivity according to the cubic law. This may imply a search for representative dusts.

The standard aims at describing this demand in detail to assure that explosion protective systems are tested correctly.

Related legislation

Legislation related to this standard