Designation of dust explosion hazard zones — introduction

This article explains the most important terms and phenomena related to the explosion risk caused by the presence of combustible and explosive dusts in production processes. The author also presents legal basis as a starting point for improving explosion safety in industrial plants and, a Dedicated Explosion Safety System prepared on based thereupon.

Explosive atmospheres

A precondition for the occurrence of an explosion hazard is the creation of an explosive atmosphere, i.e. a dust-air mixture (or more broadly, a mixture of dust and an oxidizer) with a concentration within the lower and upper explosive limit. The explosion risk assessment process determines the extent of individual explosive atmospheres, i.e. the volume that can be filled by a dust-air mixture when it occurs. An explosive atmosphere for which a range has been defined is called an explosion hazard area.

With regard to the location of explosive atmospheres one can distinguish:

• internal explosion hazard zones – these are zones located inside equipment and system housings (e.g. in silos, air filters, built-in transfer bays, etc.),
• external explosion hazard zones – these are zones located outside of equipment and installation housings (e.g. open bulk material stockpiles, open dumps, dusty working spaces resulting from system leaks and/or the presence of settled dust).

Explosion hazard zones

To clarify the level of explosion hazard and to enable the selection of equipment with the correct level of protection, each explosive zone is assigned a parameter that informs the user of a system about the frequency of occurrence of a given explosive atmosphere. In the case of dust, these parameters were designated as follows:

• 20 – an explosive atmosphere in the form of a dust-air mixture that occurs continuously, frequently or for long periods of time,
• 21 – explosive atmosphere in the form of dust/air mixture which may occur during normal operation,
• 22 – an explosive atmosphere in the form of a dust/air mixture which does not occur during normal operation and which, if it does occur, persists for a short period of time.

As noted earlier, explosive atmospheres can occur both inside and outside equipment housings. In such cases, their extent and frequency of occurrence should be determined separately for each of these spaces.
Similarly, equipment operating in a given hazardous area must have adequate protection against the ignition of explosive atmospheres. Selection of theses safeguards is the responsibility of the equipment manufacturer.

In the case of electrical equipment intended for use in 20 and 21 explosion hazard zones, and non-electrical equipment intended for use in a 20 dust explosion zone, this selection and the conformity of the equipment with the relevant standards shall additionally be verified by a notified body.

The user/owner of a facility is responsible for determining the explosion hazard zones at the facility and for choosing equipment fitted with appropriate safety devices and certificates. In carrying out both activities, users/owners of facilities often cooperate with external companies specializing in the field of explosion safety.

Equipment operating in two explosion hazard zones

Sometimes a device operates in two explosion hazard zones at the same time. An example of that is a fill-level sensor, the probe of which operates inside a dust container (usually in zone 20) and its electronic system on the outside of the container (usually in zone 22). In such cases, individual components of a device may have different protection measures are appropriate for the identified hazard.

Another example is a rotary valve mounted on a silo outlet. In this case, a 20 explosion hazard zone is designated inside the valve housing (in most cases) and a 22 explosion hazard zone outside the housing. Therefore, individual elements of the valve (internal and external) may have protections adequate to the designated explosion hazard zones.

Generation of dust explosive atmospheres

Fig.1. An extreme case of the so-called settled dust..

Generation of explosive atmospheres inside housings is directly related to the nature of technological processes (crushing, mixing, pouring, etc.). Therefore, their elimination from the production process is difficult and costly or practically impossible.

The equipment and apparatuses inside of which explosions of dust-air atmospheres occurs most frequently include:

• silos (20%)
• dedusting systems (17%)
• grinding systems (13%)
• conveying systems (10%)
• drying systems (8%)
• afterburning systems (5%)
• mixing systems (5%)
• polishing and grinding systems (5%)
• sieving systems (3%)
• other (14%)

External explosive atmospheres are most often created by leaks in systems and by the excitation of the so-called settled dusts, i.e. dusts deposited on floors, equipment housings and steel constructions. Reduction of their formation can be achieved by air-tight sealing of a system, periodic cleaning with vacuum devices (e.g. central vacuum system), use of dedusting systems.

Raising of settled dust may occur as a result of: vibration of steel structures, use of overpressure cleaning systems, uncontrolled release of pressure from a system, explosion (e.g. in an unprotected or incorrectly protected explosion-proof unit), drafts, improper ventilation.

For example, raising of a 1 mm layer of settled dust with a bulk density of 500 kg/m3 may create an explosive atmosphere with a height of 5 metres and a concentration of 100 g/m3.

A less common mechanism for the formation of explosive dust atmospheres is the condensation and solidification of vapours of various chemicals, e.g. soot or welding smoke (condensation dust).

Protection against explosion effects

According to the regulation on minimum requirements for workplaces where explosive atmospheres may occur, the employer should, in the first place, limit the formation of explosive atmospheres and strive to eliminate ignition sources of explosive atmospheres. The above regulation also indicates that “in the event of an explosion, the extent of its impact should be limited only to the workplace and the equipment located there, as a result of the employer’s use of means to prevent its transition to detonation and the propagation of detonation waves”.

In practice, this means that it is necessary to build installations resistant to maximum explosion pressure or use safeguards against the effects of an explosion in the form of:

• explosion relief systems – decompression panels, flameless explosion relief systems, self-closing flaps,
• explosion suppression system – a system using cylinders with suppressing material, sensors and control panel,
• explosion decoupling systems – HRD powder barriers, high-speed gate valves, target valves resistant to wave and pressure of explosion, shut-off valves type Ex-Kop and Ventex.

A properly protected system (equipment) must always be equipped with an explosion decoupling system that blocks the propagation of an explosion to the rest of the system and one of the other protection systems (explosion relief or suppression).

The choice of specific solutions depends on the process parameters (including temperature, pressure), construction of protected devices (including structural strength, shape) and dust explosibility parameters (including p_max, K_st).

Fig. 2. A sugar factory after a sugar dust explosion. The ignition source was an overheated element of the belt feeder.

Secondary explosions

Secondary explosions occur as a result of propagation, or spread, of the primary explosion. Depending on the explosion protection used (or not used), there are three typical cases:

• no propagation after a primary explosion that was initiated inside a plant – this is a model situation that occurs if a process plant is properly protected by a system of explosion suppression or relief and a system of explosion decoupling (isolation). In this situation, the effects of an explosion initiated in the equipment in question shall be limited to its volume and, in the case of pressure relief panels, to its volume and the safety zone which must be delimited in the area of the decompression openings,
• propagation after a primary explosion that was initiated inside a plant and then spread to adjacent equipment and outside the plant – this situation occurs in the case of unprotected or improperly protected process plants:
• unprotected systems: an explosion that originates in a piece of equipment spreads through a system of overflows and ducts to neighbouring equipment, where secondary explosions occur (propagation within the installation). As a consequence of the pressure build-up in a system, it will burst. The released pressure stirs up layers of settled dust, leading to secondary explosions (propagation outside the plant),
• incorrectly protected system: Typical errors made when protecting process plants against explosions include the use of explosion relief panels in halls and buildings and the lack of use of explosion decoupling. In the first case, the explosion released inside a hall raises the settled dust, leading to secondary explosions. In the second case, the lack of decoupling (despite the correct application of explosion suppression or relief) results in the propagation of the pressure wave and flame to adjacent equipment. As a consequence, this phenomenon may lead to their bursting, resulting in transmission of the explosion outside the process installation,
• propagation after the primary explosion initiated outside a system – this phenomenon is most often caused by the presence of settled dust, which is raised as a result of a blast or vibration, creating an explosive atmosphere. If there is an ignition source (e.g. hot surface, cigarette butt, sparks from welding work) in the area of the resulting cloud, an explosion may occur. A vibration and blast caused by a primary explosion causes further portions of dust to be stirred up, which provides fuel for secondary explosions.

Dedicated System for Explosive Safety – DSBW

DSBW is a comprehensive programme of protection of single devices and entire industrial plants against effects of uncontrolled explosion of dusts, gases, liquid vapours and hybrid mixtures. This program was developed by the experts at WOLFF GROUP, a Polish company which has been cooperating with the industry in the field of explosion protection for nearly 20 years.

The purpose of DSBW is to ensure an appropriate level of explosion safety in industrial plants by implementing three levels of protection:

Level 1 – protection through hazard identification and assessment includes:

• development of preliminary and as-built explosion risk assessment,
• designation of explosion hazard zones,
• preparation of an explosion protection document.

Level 2 – protection by reducing the risk of explosion includes:

• selection of devices adapted to work in designated explosion hazard zones,
• design and implementation of a dust extraction/central vacuum cleaning system compliant with Atex (limitation of formation of explosive atmospheres),
• reduction/elimination of ignition sources of explosive atmospheres.

Level 3 – protection by limiting the effects of an explosion to a safe level includes:

• implementation of an explosion suppression system,
• implementation of an explosion relief system,
• implementation of an explosion decoupling system.

The scope of implementation of a Dedicated Explosion Safety System is determined by the actual needs of an industrial plant. In order to determine them, a WOLFF GROUP expert conducts an explosion safety audit of process plants, buildings and halls subject to the ATEX directive. As a result, a report is prepared which clearly indicates the critical points of the tested objects. This report is a basis for determining the extent to which DSBW will be implemented at a specific industrial site.

Our EU membership was conditional upon adaptation of the national legislation to the legal acts in force in the Community. Regulations in individual member states, including Poland, were not uniform and hindered the free flow of goods, so they required necessary harmonization. In implementing these assumptions, Poland undertook to implement in its national system of law the provisions of the directives of the European Parliament and the Council relating to explosion safety.

The most important documents in this respect are Directives 1999/92/EC and 94/9/EC. The former was introduced by virtue of a Regulation of the Minister of Economy on 8 July 2010 on minimum requirements related to occupational health and