What to Know About Your Potentially Explosive Process

  • Is my dust explosive?
  • What am I supposed to do with the explosive dust?
  • How can we protect against potential explosions?

When in doubt, a Keller Pro is available to help you out!

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Dust Explosions and What Causes Them.

A dust explosion is an exothermic, unstable reaction assigned to the explosion category deflagration.

Deflagration, more commonly known as instantaneous combustion, occurs suddenly from some form of heat transfer, spreads quickly, and is characterized by considerable pressure increase caused by rapidly expanding gases. Flames from an instantaneous combustion expand at a rate of 100 m (330') per second and pressure typically increases by 10 bar (1 MPA) or higher, depending on the fuel.

The intensity of a dust explosion is determined by dust volume, ratios of reactants, and the housing conditions (pressure vessel) it is initiated within. 

A dust explosion does not occur by chance; reactive substances with proper mixing ratios create a potentially explosive situation.
For dust explosions, organic and combustible metallic dusts are potential reactants. Air supplies the additional reactant, oxygen.

When a mixture of dispersed dust particles in various sizes interact with air molecules, the differences in densities generate a continuous circulation that creates a homogeneous mixture. As soon as a specific dust density is achieved below certain particulate dimensions, an explosive environment is produced.

The existence of an explosive environment, however, does not immediately create an explosion. To release an explosion, an ignition source must be present and a substance (homogenous mixture) must have the active ability to spontaneously combust.

Pressure curve in a 1 meter-cubed, round pressure tank

The maximum explosion pressure pmax is significantly influenced by the housing.

The rapid pressure increase dp/dt reflects the potential severity of an explosion. It is defined as primary point of origin in the deviation point of the pressure/time curve and used as parameter (KSt value) for classification into dust explosion classes St 1, St 2 and St 3.

In addition to the maximum pressure, the dust explosion classifications establish the basis for the practical design of protection measures.


Explosion parameters of previous Keller projects:

Explosion parameters of previous Keller projects.

Ignition Energy Graph
The minimum ignition energy level is usually temporary.
Minimum Ignition Classes
The classes of the minimum ignition potential.

Minimum Temperature

The lowest temperature of a hot surface at which the most sensitive mixture in an explosive environment (dust cloud) ignites is determined under regulated testing conditions.
The minimum ignition temperature is of great importance for filtration systems, since a fan motor can heat up surfaces. Hence, the maximum surface temperature within a filtration system has to lie 2/3 below the minimum ignition temperature of the dust cloud.

Industrial examples: Lowest temperature required for an ignition.

Paint overspray

350 °C

Stitching and milling dust plastics

430 °C

Rubber milling dust

500 °C

Aluminum dust with aerosols from MQL

550 °C

 

Minimum Energy

The minimum ignition energy is an important factor for the design of protective measures. The least amount of energy stored in a condenser, which cannot ignite the most ignition-prone mixture in an explosive atmosphere at 20 times the discharge in succession, is determined under regulated testing conditions.

Please take into consideration that the most critical point for an ignition often occurs during system start and stop times. The ignition temperature is higher for heavy or sparse dust-air-mixtures.

Industrial examples: The least amount of energy required to create an ignition.

Aluminum milling dust

5 mJ

Fabric fibers (laminate residues)

3 - 10 mJ

Zinc dust

10 mJ

Aluminum dust with aerosols from MQL

> 10 mJ

Polyethylene dust

10 - 30 mJ

Stitching and milling plastic dust

10 - 30 mJ

Rubber milling dust

100 mJ

Tin-copper dust

100 mJ

Paint overspray

100 mJ

Pressure and Temperature

The upper explosion limit raises if pressure and temperature increase.
An increase in the start pressure (system’s working pressure) leads to an increase in explosion pressure and explosion severity. This can escalate quickly to a detonation.

Dust Particle Size

The combustive reaction of an explosion occurs on the particle surface. The finer the dust, the more surface available for a reaction – and the greater the explosion pressure and explosion severity. The seemingly obvious solution of coarser dust is not suitable as a protective measure since only a 5–10 % ratio of fine dust is sufficient to ignite an explosion.

Instability

Any prevailing instability in an explosive atmosphere can lead to higher explosion pressures upon ignition and to a 10-fold higher explosion severity.

Spontaneous Combustion

Since large dust accumulations cannot be prevented, their ignition temperature must be controlled.

Organic Dusts
Dusts from the treatment and processing of these products and substances are flammable and are assigned to dust explosion class ST 1.

  • Plastics similar to CFC, GRP, GMPU
  • Carbon
  • Powder
  • Cocoa
  • Starches and Fibers
  • Lumber
  • Feedstuff
  • Paint and Spray

 

Metallic Dusts
Typically, metallic dusts are minerals such as magnesium, aluminum and fine blasting dust such as steel.
With a particularly small particle size (i.e. dust) even substances that are not normally combustible in solid form, can suddenly ignite.
The very large surface area of metallic dust particles generates heat quickly and presents an explosion hazard.

General Information on Explosion Prevention / Explosion Protection in the Workplace.

To avoid hazards through explosions, explosion protection must occur systematically. It often makes sense to combine the possibilities of explosion prevention with advantages of explosion protection. With this combination, a very high safety level can be reached which can withstand malfunctions and workplace oversights. 

Explosion Prevention

Added measures to reduce the creation of an explosive environment and ignition sources.

Avoiding a potential explosion seems to be an obvious safety measure – however, it may not be suitable for all processes. 
 

1. Wet Separation

For this process, possible ignition sources are rinsed with water, preventing the creation of an explosive environment in a traditional dust collector. Because of the variety of processes, this technology is not universally applicable for all dust collection needs.

To learn more about Keller's Wet Scrubber technology, click here

 

2. Inertisation

When using inertisation, the collector is supplied with an inert material to avoid the creation of an explosive situation.

Material especially suited for explosion prevention are inert gases (nitrogen, carbon, or noble gases) or powdery inert material, such as limestone powder. When using powdery inert material, the addition of nine times the amount of the explosive dust amount can be required to achieve the inertisation effect. For example, one cup of potentially explosive dust may require up to nine cups of inert material in order to be effective at preventing an explosion within the dust collector.

The mixture ratio must ensure that the combination of inert material and explosive dust/air cannot set off an explosion.
The economic feasibility depends on the volume of dust created and the need for inert material.

An explosion can only be set off in the presence of an ignition source.
 

1. Procedural measures

These are to avoid common ignition sources such as welding, smoke and other sources of open flames.

The explosion pentagon diagram below shows that despite an explosive environment, an explosion is impossible without the existence of an ignition source. 

 

2. Eliminating ignition sources in the separation system

Inadequate for especially ignition-prone dusts with MIE < 3 mJ and hybrid mixtures.

All components inside EX zones must be approved according to ATEX in order to be ruled out as an ignition source.

If the entry of an ignition source from outside of the separation system can be prevented safely, grounding the system components is sufficient. The system is safeguarded from current leakage and stray currents.

Grounding prevents electrostatic spark discharge.

Additional conductive and grounded filter elements can be installed.

2.1 Ignition source monitoring

For processes with possible spark creation (e. g. aluminum processing), the system can be monitored by a spark sensor.

If entry is detected, the SPS can prevent the next cleaning cycle.

Further information: Click here

3. Cleaning during system downtimes

If the separation process is interrupted to clean the filter elements, no ignition sources exist.

Explosion Protection

Explosion protection according to BGR 104 and VDI 2263

If the above mentioned measures are inadequate to prevent an explosion and its effects, additional explosion protection methods can be installed.

Directives BGR 104 and VDI 2263 differentiate among the following protection measures:

– Explosion-proof construction
– Explosions pressure relief
– Explosions suppression
– Prevention of flame and explosion transfer

Explosion pressure-surge resistant

For smaller explosion hazards, the system is designed according to this category.

Explosion pressure resistant

Calculated according to the maximum explosion pressure or the reduced maximum explosion pressure, the filter housing should resist an explosion without buckling.

Explosion pressure relief for organic dusts

In outdoor installation or adjoining outer walls, it is feasible to use burst panels as an explosion pressure relief to cover a large safety zone. Please consider a possible flame outlet.

Burst panel

By integrating a flameless pressure relief, flames and heat can be adsorbed by a stainless steel mesh screen. Please take into consideration adequate safety distances.

Dry separator VARIO with ProVent

Flameless explosion pressure relief 
for metals

ProPipePlus and TR-1 ProVentPlus ensure a danger free explosion relief in closed rooms.

ProPipePlus is a patented system with flame trap and dust retention made of stainless steel.

This system does not require a protection zone. There exists a special design for aluminium dusts.

ProPipePlus at VARIO 2

TR-1 ProVentPlus is specially suited to separate explosive dust from machine tools with dry processes or MQL.

The combustion products of the flameless pressure relief are guided upwards by means of a deflection channel. A safety zone is not required.

However, an individual assessment according to DIN EN 16009 is required.

TR-1 ProVentPlus

 

Suitable for reduced maximum explosion pressure

A dynamic pressure sensor is activated when it detects the pressure in the filter area and the pressurized tank which stores the extinguishing agents.

The extinguishing agent is dispensed within 50 ms and an explosion is prevented. The emerging explosion pressure normally reaches 0.4 bar excess pressure when terminated.

The system is approved for organic dust.

Perfect for toxic dusts

Since the effects are limited only to the interior of the system, this procedure is perfectly suited for toxic dusts.

Round filter with explosion suppression

Protection against consequences of explosions and personal injury

Air pollution control systems with explosion-proof construction, explosion suppression or explosion pressure relief must be explosion-decoupled.

This is to prevent successive explosions and personal injury from the airborne shock wave and possible flames.

It must be clarified if a complete or partial flame and pressure reduction is adequate or necessary.

Decoupling the clean air duct

Normally, the advance of flames into the clean air duct is prevented by filter elements. However, filters are not considered as a protective system since they can be flammable.

The clean air duct should resist the reduced explosion pressure and may only terminate in a non-hazardous location.

As an alternative to decoupling the clean air duct, pressure relief vents 
with burst panels can be installed.

Quick closing valves can also be used to decouple flame and pressure.

Dirty air duct decoupling

A verified method is our ProFlap back pressure flap.

During operation, the downstream mounted back pressure flap is kept open by the air flow. During an explosion inside a protected system, the flap is closed by the pressure surge inside the ductwork.

ProFlap back pressure flap
Additional information: Click here

Extinguishing barrier (Flame barrier)

This device is activated by a sensor and dispenses depressurized extinguishing agent into the duct in the event of a dust explosion.
The flames are suffocated however the shock wave is not contained, which is the reason why the downstream system components must be designed to withstand a reduced explosion pressure.

Protection against the consequences of an explosion and personal injury

Decoupling of dust discharge

The locking function of a rotary valve can be used to avoid an explosion from spreading inside the dust collection tank. The modified design is flameproof.

The same result can be achieved with a lock.

As an alternative, pressure-surge proof design of the dust collection tank can also be selected.

Rotary valve

Residual Risk

A remaining risk due to unpredicable incidents cannot be avoided despite all measures taken. VDI 2236, page 6, explains the performance of risk evaluations and the results of suitable protection measures.

 

Guidelines for Potentially Explosive Dust

Zone division

The areas of operation must be divided into zones (VDI 2263-6/BGR 104):

Environment: Gas/mist/fumes

Hazardous if content is more than 50 % of the operation or continuous: Zone 0

Occasional hazard if less than 50 % in the process: Zone 1

No hazard during normal operation, or rare and short-term: Zone 2

Environment: Dust

Hazard if content is more than 50 % of the operation or continuous: Zone 20

Occasional hazard if less than 50 % in the process: Zone 21

No hazard during normal operation, or rare and short-term: Zone 22

Device group I

Includes systems for underground and surface mining.

Device group II

Classification of all systems in remaining explosion prone areas. All the following information pertains to device group 2.

Device category 1

Hazard potential: continuous, frequent or over an extended period of time
Requirement: very strict safety procedures

Device category 2

Hazard potential: occasional
Requirement: strict safety measures

Device category 3

Hazard potential: rare or short-lived
Requirement: normal safety measures

In Zone 20 use only 
device category 1.

In Zone 21 system in 
device category 2 (+1) are permissible.

In Zone 22 suitable 
device category 3 (+2 and 1).

List of process-related ignition source types according to ATEX 114

– Sparks
– Flames
– High surface temperatures
– Arcing
– dB levels
– Radiation in the visual range
– Electromagnetic waves
– Electrostatic charges
– Stray or leaking electrical currents
– Heating by friction
– Heating from impact
– Shock waves or compression
– Chemical reaction

Keller's Experience with Explosion Protection

It is essential to consider all physical circumstances (ex. environmental conditions) and legal regulations when designing effective and safe air filtration systems. Keller provides extensive knowledge based on years of research and practical experience with explosive situations.

As a result of  the implementation of Europe's ATEX 137 guideline, system operators of machines at risk of explosion are required to determine and evaluate the risk of explosion by filing an explosion protection document. To fulfill this requirement it is usually necessary to test explosive dust parameters. Keller is capable of providing the necessary support for the evaluation of the explosion hazard, the detection of the explosive parameters and establishing explosion protection measures. 

Are explosion protection measures required?

Scope of the examination:
– Sieving on dust particle size < 63 µm
– Determining explosiveness with the modified Hartmann device
– Not explosive
– Explosive
– Dust explosion class St1
– Dust explosion class St2

– Total disposal volume
– Volume of dust particles: approx. 100 g

Provided that ignition sources are not created during the extraction process, the prevention of ignition sources as an explosion protection is sufficient (according to regulations for explosion protection for dust with a MIE of > 10 mJ). There are no explosion protection measure requirements for pressure relief, for example, or for explosion suppression.

Scope of the examination: 
– Sieving on dust particle size < 63 µm
– Definition of particle size distribution
– Determination of the minimum ignition energy without electrical source
– Total disposal volume
– Volume of dust particles: approx. 300 g

Pressure relief or explosion suppression. Operational parameters are of vital importance.

Scope of examination:
– Sieving on dust particle size < 63 µm
– Definition of particle size distribution
– Determining the pressure increase 

(KSt value) and of the max. explosion pressure pmax in the 20 L ball (screening means that only one test is made)

– Total disposal volume
– Quantity of dust particles: approx. 200 g

Inert limestone powder is added to prevent an explosive environment. Additional explosion protection measures in the separator are not necessary.

For optimal results when supplying limestone, the suitable dust – limestone powder combination ratio must be determined. The ratio can be reduced to 1:1.

Scope of the testing: 
– Sieving on dust particle size < 63 µm
– Definition of the particle size distribution
– Determining the necessary mixing ratio of the limestone powder: explosive dust in the 20 L ball in 12 % increments

– Total disposal volume
– Quantity of dust particles: approx. 500 g

Keller Solutions for Ex-Prevention / Protection

Keller dust extraction systems with constructive explosion protection according to ATEX can be found in all areas of industry throughout the world. Depending on the application, various technological solutions can be applied.

Keller Products Specializing in Explosion Protection

To ensure customer safety and that all products operate to the highest performance level, Keller tests all products making sure they meet explosion protection guidelines. 

For further assistance finding your explosion protection solution, click here