Kst value

Definition

The Kst value – also known as the dust explosion constant, K-dust, deflagration index or specific explosion parameter – is a safety parameter that characterises the explosion behaviour of combustible dusts in closed systems. It describes the maximum rate of pressure rise over time during a dust explosion, normalised to a test volume of 1 m³. The unit is bar·m/s.

The Kst value is calculated using what is known as the cubic law:

Kst = (dp/dt)max × V^(1/3)

Here, (dp/dt)max is the maximum rate of pressure rise in bar/s and V is the volume of the test vessel in m³. The Kst value is therefore a volume-independent parameter specific to the dust and the test method, which, together with the maximum explosion overpressure pmax, describes the reaction behaviour of a dust.

Determination of the Kst value

The Kst value is determined under standardised conditions, typically in accordance with ISO 6184-1, VDI 3673 and VDI 2263.

Standard test vessels are the 20-litre spherical vessel and the 1 m³ vessel. In simple terms, the procedure involves:

• Dispersing a defined dust-air mixture in the container
• Ignition by a chemical igniter or a spark of a specified energy, typically 10 kJ
• Measurement of the pressure profile and determination of (dp/dt)max

• Calculation of the Kst value using the cubic law; the maximum measured value is taken as the substance-specific Kst value

   

Classification into dust explosion classes

On the basis of the Kst value, combustible dusts are classified into dust explosion classes (St classes):

A higher Kst value indicates a faster build-up of pressure and, consequently, a more violent and dangerous explosion. Whilst the Kst value is the measurable parameter, the St classes group the values into design categories; the technical implications for construction methods and protective measures are described in the entry on dust explosion classes St1 to St3.

Key characteristics and influencing factors

Material-specific parameter

Every combustible dust has its own Kst value. This depends on:

• chemical composition
• Particle size – finer dusts usually have a higher Kst value
• Moisture content
• Dust concentration
• Turbulence in the system
• Ignition energy

Volume independence

As the relationship (dp/dt)max × V^(1/3) = constant holds, the Kst value can be applied to any tank volume and is therefore suitable for the design of real-world plants.

Relationship with pmax

Whilst pmax describes the maximum explosion pressure, the Kst value indicates how quickly this pressure builds up. Together, these two parameters form the basis for calculating and designing structural explosion protection measures such as pressure relief and resistance to pressure surges.

Practical significance in mixed media

In the processing of powders and bulk materials, mixing causes clouds of dust to be stirred up at concentrations that are potentially explosive. The Kst value is therefore a key parameter for the safety-related design and ATEX-compliant operation of such plants.

Design of the mixing tank

The Kst value of the processed products determines whether a mixer must be designed to be pressure-resistant, resistant to pressure surges, or fitted with pressure relief devices such as burst discs. The higher the Kst value, the faster the pressure rises in the event of an explosion, and the greater the demands on the vessel’s strength or pressure relief areas.

Determining the appropriate level of protective measures

• Pressure relief: The size of the relief area and the response pressure are calculated primarily on the basis of Kst and pmax.
• Explosion suppression: The quantity of extinguishing agent, the arrangement of the nozzles and the response time are determined by the expected rate of pressure rise.
• Explosion decoupling: The Kst value is taken into account when selecting and designing suitable decoupling devices for supply and discharge lines.

Process design and inerting

Due to their design, mixers often generate clouds of dust in the headspace of the container. For products with Kst values in St 2 or St 3, additional measures are advisable:

• Inerting, such as operating under a nitrogen atmosphere, to reduce the oxygen content below the flammability limit
• Pressure relief and explosion suppression
• Optimisation of filling and emptying processes to minimise dust generation
• Control of ignition sources from hot surfaces, sparks and electrostatic charge

Product changes and formulation changes

If different products or formulations are processed in a plant, the mixer must always be designed for the most hazardous product with the highest Kst value and pmax. Changes to the product portfolio or to the degree of fineness are grounds for reviewing the existing explosion protection concept. As the Kst value depends heavily on particle size and moisture content, even a change from granules to very fine powder of the same substance can significantly alter the explosion class.

Illustrative Kst values for typical products

The figures below are indicative and may vary depending on product quality, particle size distribution and moisture content. Values for the original material, determined by laboratory analysis, are required for the design of protective measures.

Practical and expert advice

• Do not use generalised values from the literature: references are for guidance only. For the definitive interpretation of explosion protection measures, substance- and production-specific tests should always be carried out in certified laboratories.
• The state of the material is crucial: a fine dust may have a significantly higher Kst value than a coarser granulate of the same substance. Even minor changes in moisture content or formulation can have a noticeable effect on the explosion parameters.
• Test hybrid mixtures separately: if flammable gases, vapours or solvents are present, the characteristic values for the pure dust are no longer valid.
• Professional risk assessment: The planning, design and evaluation of explosion protection measures, including ATEX compliance, should always be carried out by qualified specialists on the basis of a comprehensive risk assessment.