Mitigate combustion risks & protect from dust explosions

Created Wednesday, October 5, 2022

Almost all industrial manufacturing, processing or packaging applications generate dust as a side effect. Many of these dusts are both combustible as well as explosive, and removing them from the work place can significantly reduce the risk of serious accidents. It is estimated that there are approximately 2,000 dust explosions in Europe annually. Most of these are minor but some are very destructive. So by extracting the dust, risk in the production is reduced, but unless the extraction system is safe the risk is just moved.

What is a dust explosion?

To understand combustible dust risks, take a look at the “dust explosion pentagon” on the right. All five elements need to be present in an industrial facility at the same time to cause an incident:

Combustible dust
Ignition source
Oxygen in the air
Dispersion of the dust in sufficient concentration to be explosive
Containment of the dust cloud within a confined or semi-confined vessel or area

A closed vessel like a dust collector can create the perfect scenario for an explosion if an ignition source enters the collector. When a pulse cleaning event occurs, a suspended cloud of combustible dust is present in high concentration within the collector. An ignition source completes the five elements of a dust explosion and initiates the explosion.

Though some incidents involve a single explosion, it is more common for a series of deflagrations to occur. The initial explosion can dislodge ignitable dust that has accumulated on surrounding surfaces over a large area and trigger secondary explosions that can be ignited from the initial explosion or from other ignition sources. It is these secondary explosions that have historically caused the majority of injuries and damage to property.

Potential ignition sources are sparks, electrostatics, abrasion or dust deposits in the pipes due to wrong transport velocity.

Controlling dust explosions

Combustible dust explosions are a serious risk at many manufacturing and processing plants, and an unprotected dust collection system can be a main cause of an accident. An explosion in an unprotected dust collector produces a high pressure wave that can fragment the housing and send heat flames and dangerous projectiles into the workplace. This can critically injure personnel and damage equipment and buildings. So make sure your dust collector has an effective explosion protection system. The goal of any type of explosion protection equipment is to control an explosion, keep employees safe and minimise damage.

Test your dust & conduct a dust hazard analysis

How do you know if your dust is combustible and/or explosive? Common practice is to assume that it is, unless you have the test results to prove that your dust has a 0 Kst value. If your dust is common - like flour, sugar, etc. - and your particle size and moisture content are the same, you can use documented historical data from other tests. Other dusts must be tested through a notified body and you must keep the test data on file.

If the test is positive, then the explosive index (Kst) and the maximum pressure rise (Pmax) of the dust should be determined. The fact is, any dust above 0 Kst is now considered to be explosive, and the majority of dusts fall into this category. The finer the dust particles are, the more dangerous they are. A dust hazard analysis allows you to determine potential combustion & explosion risks and to determine the required level of fire and explosion protection.

The analysis can be conducted internally or by an independent consultant, but either way the authority having jurisdiction will ultimately review and approve the findings.
Your dust collection equipment supplier will need the Kst and Pmax values in order to correctly size explosion venting or suppression systems.

Atex-directives & zoning

The ATEX Directives apply to new equipment as well as existing equipment, including those items installed prior to the introduction of the directives.

Directive 99/92/EC (also known as ATEX 137 or the ATEX Workplace Directive) on minimum requirements for improving the health and safety protection of workers potentially at risk from explosive atmospheres.
Directive 2014/34/EU (also known as ATEX Equipment Directive) on the approximation of the laws of Members States concerning equipment and protective systems intended for use in potentially explosive atmospheres. The dust collector must comply with this directive if it is located in an ATEX Zone and/or it is collecting potentially explosive dust. Compliance is achieved by incorporating the necessary safety features determined by the explosive potential of the dust.

The higher the probability of an explosive concentration, the higher the level of safety required:

Areas with a low probability of an explosive cloud, classified as zone 22.
When a cloud can occur during normal operation the area, classified as zone 21.
Areas where an explosive cloud is always present, or occurs frequently during normal operation, classified as zone 20.

Technologies for explosion protection

Many different types of devices and systems are used to comply with ATEX Directives for the explosion protection of dust collection systems. They fall into two general categories:

Passive systems: The goal of a passive system is to control an explosion to keep employees safe and minimize plant and equipment damage.
Active systems: Active systems involve much more costly technology and typically requires recertification every three months.

Whichever system you install, make certain to use certified explosion protection devices.

Passive devices

Active devices

Chemical isolation

Reacting within milliseconds of detecting an explosion, this system can be installed in the inlet or outlet ducting. Typical components include explosion pressure detector(s), flame detector, chemical agent, and a control panel. It creates a chemical barrier that suppresses the explosion within the ducting and reduces the propagation of flame through the ducting.

Chemical suppression

Chemical isolation is used to detect and suppress explosions within the ducting and protect the dust collector. It is often used when it is not possible to safely vent an explosion or where the dust is harmful or toxic. The system detects an explosion hazard within milliseconds and releases a chemical agent to extinguish the flame before an explosion can occur.

Fast acting valve

Designed to close within milliseconds of detecting an explosion, the valve installs in either inlet and/or outlet ducting. It creates a mechanical barrier within the ducting that effectively isolates pressure and flame fronts from either direction, preventing them from propagating further through the process.

Depending on the process and the used equipment, there´s a third possibility to protect from dust explosions. The pharma dust collectors of the Quad Pulse Package Series operate with a shock-resistant housing so that no additional venting or suppression is needed to protect the vessel itself. In this case valves are used to protect the ductwork.

Basis of safety

After analyzing the situation and doing the system design, it is crucial to define a safety concept, which describes how the extraction system is maintained and what are the procedures to follow in case of changes in the process.

It is important to regularly inspect the dust collector, safety systems, connected duct work and the surrounding safety zones. You should inspect all of these according to each manufacturer’s installation and operation manual. Also, you should remove any dust build-up within the system, duct work, or safety zone as often as possible. When it comes to the dust collector, a simple but important housekeeping requirement is to change filters properly.

Having a paper that says your system is safe won't help you the day one of the operators makes a mistake and unforeseen events are common reasons behind accidents. Training and work routines help to mitigate the risks caused by the human factor.

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