Skip to main navigation Skip to main content Skip to page footer
amixon GmbH amixon GmbH
Amixon
Request Trial
Contact

Hybrid mixtures

 

Definition

A hybrid mixture is an explosive atmosphere that contains both flammable dust and a flammable gas, vapour or mist. The term thus describes the borderline case between the two traditional areas of explosion protection – gas and dust explosions – each of which is subject to its own zones, equipment categories and characteristic values.

From a safety perspective, a hybrid mixture is not simply the sum of its components, but must be assessed independently. The crucial point is that a hybrid mixture can be ignitable, even though neither the dust fraction nor the gas fraction, taken on its own, reaches the respective lower explosive limit.

 

Effect on explosion parameters

Even small proportions of a flammable gas or vapour significantly alter the behaviour of a dust-air mixture:

  • The minimum ignition energy (MZE) decreases significantly. Ignition sources that would be inconsequential for pure dust – such as weak electrostatic discharges – may be sufficient. This is the most important change in practical terms.
  • The rate of pressure rise increases. The Kst value of the mixture is higher than that of the pure dust; the reaction time available to protective systems is reduced.
  • The maximum explosion overpressure pmax increases. The load assumption for vessel design increases.
  • The explosion limits shift. The lower explosion limit of the mixture lies below the individual limits of dust and gas.
  • The minimum ignition temperature may decrease. Consequently, the permissible surface temperatures and temperature classes become more stringent.

These effects are not linear and cannot be calculated from the individual characteristic values. Reliable values can only be obtained by testing the actual mixture under realistic conditions.

 

Where hybrid mixtures are produced in mixing processes

In powder processing, hybrid mixtures are more common than they appear at first glance. Typical scenarios:

  • Addition of liquids to powders: Spraying or dosing of oils, flavourings, oleoresins, alcohol or alcohol-containing solutions into the ongoing mixing process
  • Granulation and instantisation: Formation of agglomerates using solvent-based binders
  • Coating and coating processes: Application of solutions or suspensions to particle surfaces
  • Vacuum mixing and drying, and synthesis: Evaporation of organic solvents in a dust-laden apparatus
  • Residual solvents in the starting material: Powders from upstream processes that still contain volatile components and release these into the mixing chamber
  • Outgassing products: Substances that release flammable decomposition products when subjected to increased temperature or mechanical stress

Particular attention must be paid to the last case: a process that is designed to be purely dust-based may unintentionally become a hybrid process due to residual moisture, residual solvents or decomposition.

 

Consequences for zone classification and equipment selection

If a hybrid mixture is present, classification based solely on dust is no longer applicable. Both atmospheres must be assessed:

  • The affected area must be classified with regard to both gas zones 0, 1 and 2 and dust zones 20, 21 and 22.
  • Equipment and protective systems must meet the stricter of the two sets of requirements – both in terms of equipment category and protection level, as well as in terms of type of protection and temperature class.
  • The marking must indicate suitability for both atmospheres, i.e. for both equipment groups G and D.

The effectiveness of individual protective measures also varies: measures designed for dust – such as boundary layer analyses relating to dust deposits – are not effective for gases, and conversely, ventilation concepts for gases are only of limited effectiveness in the presence of dust.

 

Design implications for equipment

  • Pressure-shock-resistant construction: The design pressure must be determined on the basis of the characteristic values of the hybrid mixture. An apparatus designed for pure dust may be undersized.
  • Pressure relief: The required relief areas become larger because the pressure rises more rapidly. In the case of solvent vapours, venting to the open air is often not permitted; flameless systems must then be considered.
  • Inerting: With hybrid mixtures, reducing the oxygen content is often the most economical and safest solution, as it addresses both atmospheres simultaneously. This requires a controlled gas supply and oxygen monitoring.
  • Explosion decoupling: Due to the higher flame speed, decoupling from upstream and downstream plant sections is particularly critical.
  • Stricter prevention of ignition sources: Owing to the low minimum ignition energy, continuous earthing, equipotential bonding, the prevention of mechanical sparks and the limitation of surface temperatures take on added importance.

  • Leak-tightness and gas management: The apparatus should be designed to be gas-tight so that vapours do not escape into the surrounding environment and create an additional gas zone there.

     

Directives and standards

The assessment of hybrid mixtures is carried out within the framework of the same directives as for other aspects of explosion protection, in particular Directive 2014/34/EU on equipment and protective systems and Directive 1999/92/EC on operators’ obligations. Methodological principles are provided, amongst others, by EN 1127-1 for the principles and methodology of explosion protection, and by VDI 2263 for dust fires and dust explosions. Hybrid mixtures must be explicitly identified as such and assessed in the explosion protection document.

 

Practical guidance

  • Fully characterise the process: All substances fed into the mixer or which may be generated within it must be taken into account – including cleaning agents, residual solvents and decomposition products.
  • Determine the mixture’s characteristic values: Literature values for pure dust are not reliable for hybrid mixtures. Testing of the actual mixture in a certified laboratory is required.
  • Determine at an early stage: Whether a process is hybrid determines the construction method, design pressure and protection concept. A subsequent conversion is significantly more complex than taking this into account during the planning phase.
  • Check formulation changes: Changing a binder or flavour carrier can transform a process that was previously purely dust-based into a hybrid process.