The process of conveying combustible dusts requires caution and expertise from start to finish.
Good engineering practice, dust-tight design, regular house cleaning and protection methods such as venting panels, flameless venting systems, chemical suppression systems help keep the conveying process safe and efficient.
There are many steps to consider, but in this blog, we are going to cover the following:
- What are combustible dusts?
- OSHA Regulations for Compatible Dusts
- How can I safely convey highly combustible bulk solids?
What are combustible dusts?
According to OSHA, NEP combustible dust is defined as a particulate solid that presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations, regardless of particle size or shape. However, NFPA standards and OSHA also indicated that combustible dust less than 40 mesh (425 microns) poses more hazardous.
A dust explosion can be catastrophic and cause employee deaths, injuries, and destruction of entire buildings.
That’s why the dry bulk material conveying solutions from Pneu-Con are designed to prevent any combustible dust explosions.
Across many industries, the manufacturing, processing, transportation, or storage of combustible materials can result in the formation of gases, vapors, or combustible dust. Unfortunately, this can form an explosive atmosphere. These include but are not limited to:
- Food (e.g., candy, sugar, spice, starch, flour, feed)
- Metal Processing (e.g., aluminum, chromium, iron, magnesium, and zinc)
What are the 5 elements of a combustible dust explosion?
The five elements of a combustible dust explosion include oxygen, heat, fuel, dispersion, and confinement.
Knowing these five elements is crucial in preventing them from being present in a conveyor system.
OSHA Regulations for Combustible Dusts
After the Georgia Sugar explosion incident, OSHA released National Emphasis Program Directive NEP and declared that most of the powders dispersed in the air could cause dust explosion risk. The end-users should take prevention + protection measures. Below, you can watch more about the Georgia Sugar explosion incident:
Any product, regardless of particle size, can cause a dust explosion. However, finer particles pose more explosion risk.
A fire or explosion with victims is not acceptable for the operator, nor within the frame of the OSHA NEP DIRECTIVE NUMBER: CPL 03-00-008 dated 03-11-08 and NFPA Guidelines.
On top of that, an incident may jeopardize the production contingency, which of course, also is not acceptable.
This directive has the following demands:
A risk analysis (or Dust Hazard Analysis) must confirm that the residual explosion risk is acceptable: the protection measures to be taken should conform with the available knowledge and technology.
The product dust explosion characteristics are generally denoted by the following.
- Kst: Rate of Pressure rise in the event of an explosion, Bar/m-sec
- Pmax: Maximum explosion pressure, Bar
- MIE: Minimum Ignition Energy required to cause ignition (MJ)
Products are also classified in Classes ST0: Kst (0-200) ST1(Kst(200-400) St2 (Kst>400)
Generally, food powders are St0, and metal powder are ST1 or St2
Note: SC Vacuum Systems generally have filter receiver volumes of less than 8 Cuft. Hence, they can be excluded from the requirement of explosion protection as per NFPA 652 and other industry-specific combustible dust standards.
How can I safely convey highly combustible bulk solids?
Assuming that the bulk solid is powder or pellets or granules, one must review the MSDS datasheet and the powder’s combustible properties and identify the category it falls by referring to the OSHA fact sheet combustible dust explosions and on how to keep workers safe.
● Agricultural products /dusts
● Carbonaceous dust
● Metal dusts
● Plastic dusts
Comprehensive dust hazard analysis (DHA) should be carried to review the presence of an ignition source that would generate enough energy to over the MIE value. Combustible dusts pose a deflagration or other fire hazard when suspended in air or some other oxidizing medium over a range of concentrations, regardless of particle size or shape; deflagrations can lead to explosions.
OSHA/NFPA recommends the following broad guidelines covered in detail by the relevant industry-specific NFPA standards (Food and Agriculture NFPA 61, Metals NFPA 484, Chemical NFPA 654, sulfur NFPA 655, Wood NFPA 664).
1) Dust Control Methods
Dust Control methods involve regular housekeeping and minimize the accumulation of fugitive dust over the work surfaces. Any open mechanical conveyor is prone to dust spillage, and leakages and proper dust control methods are deployed by providing dust hoods and dust extraction systems and avoiding spillages wherever possible.
2) Ignition Control Methods
Most of the mechanical conveyors (excluding vibrating conveyors) have invariably possible metal-to-metal contact and power transmission drives using bearings. Metal to metal contact generates frictional sparks as an ignition source leading to combustion. Generally, pneumatic conveyors do not require specific ignition control methods as there is no metal-to-metal contact or bearings inside the conveying tubes. Check for any break in the metal-to-metal contact (e.g., Gaskets, flexible connectors, etc.) and ensure continuity by providing bonding and grounding to dissipate electrostatic charges that could be generated.
3) Prevention Methods
Include separation devices such as screens/magnets/metal detectors to remove tramp materials. Open mechanical conveyor systems and screw conveyors are protected against the ingress of tramp metals that otherwise create frictional heat and sparks by metal-to-metal contact. On the other hand, Pneumatic Conveyors have no internal components and no rotating parts and hence less susceptible to foreign material entry.
4) Protection Methods
Protection measures, such as explosion panels or deflagration suppression systems, are required for pneumatic conveyors based on the hazard analysis specified by industry-specific NFPA standards and also needed to provide suitable grounding and bonding on the flexible isolators at the infeed and discharge points.
Of the many benefits when dealing directly with Pneu-Con is our systems predominantly employ vacuum conveying with low risk. The applied vacuum will result in reduced oxygen content and hence less likely to initiate an explosion. At Pneu-Con, we have spent over four decades aggregating a vast amount of dry bulk material knowledge and pneumatic conveying system engineering expertise.
As the preferred one-stop resource chosen by some of the best-known companies in the world to design and engineer pneumatic conveying systems for food, grain, coffee, chemical, pharmaceutical, nutritional, dairy and plastics applications, we have developed the unique ability to solve almost any dry bulk material conveying issue and design.
Our pneumatic conveying systems are built to prevent combustible dust explosions. Learn more about our vacuum systems here.