Combustible Dust Testing Overview

According to NFPA 652, it is the responsibility of the owner/operator of a facility to determine if the dust in their facility is combustible/explosible or not. This determination is made by collecting a sample of dust from the facility and sending it to a lab for combustibility tests. The collection of the sample needs to be performed following a written sampling plan. A more detailed discussion of a sampling plan will be presented in a later entry. The following outlines some of the most common dust tests that are available. Other tests may be necessary depending upon the material in your facility.

Explosibility Screening Test: Commonly called the “Go/No” Go test, this test determines if the dust presents an explosion hazard. If the results from the test are negative, i.e. the dust does not present an explosion hazard, no further action is required other than documenting the fact that the dust is not explosive. If the test is positive, i.e. the dust does present an explosion hazard, further testing is required to characterize the explosion severity and risk. The test procedure is described in ASTM 1226-12a.

Deflagration Index (K_St): The deflagration index is a measure of how “explosive” a material is. The value is determined from test data and is found using the following relationship:

V = volume of the test vessel

This value represents how quickly the pressure rises during an explosion within the test vessel.

Maximum Pressure (P_Max): This is the maximum pressure created by a dust explosion. This value is calculated with an optimal dust concentration, i.e. the concentration that gives the highest pressure.

Minimum Explosible Concentration (MEC): This value indicates the minimum concentration of dust which can sustain a deflagration. If the concentration is too low, there is not enough energy released by a single dust particle to bridge that gap to the next particle. It is similar to the Lower Explosive Limit (LEL) or Lower Flammability Limit (LFL) of vapors. There are some important differences between the LEL and the MEC. However in a big picture sense they are equivalent.

Minimum Ignition Energy (MIE): This represents the minimum amount of energy required to ignite a dust cloud. The lower this value is the easier it is to ignite a dust cloud.

Flash Fire Demonstration Video

The previous posts were all about fire and dust. I wanted to post an example of a flash fire caused by a dust cloud.

This fireball was created with less than 1 teaspoon of dust. Imagine the size of the fire if hundreds of pounds of dust were ignited. Many of the deaths resulting from dust incidents are due to the burns from the flash fire not from injuries caused by an explosion. The happens so quickly there is no way to get out of the way of the flames. You will be suddenly engulfed in flame.

The purpose of the Dust Hazard Analysis (DHA) is to examine your facility and processes so that risks that can lead to flash fires are managed and controlled.

Why Dust Makes an Excellent Fuel

There are a lot of factors that affect how quickly a fuel oxidizes, i.e. burns. In the following discussion we will assume everything about the reaction is constant: the reaction rate, the fuel is homogeneous, moisture content is the same, etc. These simplifications will help keep things simple and allow us to concentrate on the major issue dust presents: incredibly large surface areas.

The burning reaction essentially takes place on the surface of the fuel. As more surface is exposed to the oxidizing environment the reaction takes place over a larger area, more material is consumed in a shorter period of time and the burning rate increases. As an example, assume we have a perfectly spherical fuel that has a diameter of 1 (of whatever unit you choose). The surface area of that sphere will be 3.14159 square units of area.

Now assume that we have the same amount of fuel but this time it is made up into four spheres, i.e. each sphere is ¼ the volume of the original sphere. The surface area of the four spheres is 4.99 square units, or an increase of approximately 58%. If the fuel is dispersed with the surrounding air the total surface area exposed to air is 58% more than with the original fuel and there is 58% more reaction locations. This increase in reaction locations has the effect of increasing the reaction rate.

Now assume that we have the same amount of fuel made up into 25 identical spheres. The new surface area is 9.19 square units. This is almost three times (2.92) the original area and will allow the reaction rate to be approximately three times the original reaction rate.

Now assume that the same amount of fuel is divided into 10,000 spherical particles. In this case the original surface area increases approximately 232 times! When these particles are dispersed in the air, all 232 times the surface area is available to the burning process.

This exercise helps to demonstrate why a combustible material which is in the form of dust form is more energetic and presents a danger: the surface area available for the oxidization reaction is much greater than found with a large chunk of fuel.

This greater surface areas isn't of great concern if the dust is in a pile. The particles pack fairly tightly and reduce the amount of surface area exposed to oxygen. The problems arise when the dust particles are suspended in a cloud. These dust clouds can easily be formed from normal operations.It is the responsibility of the owner/operator of a facility to identify the locations where dust clouds are likely to form and to implement the engineering controls and administrative controls to prevent these clouds from forming. This identification of risks and how they will be managed form the basis for the Dust Hazard Analysis (DHA).

The Triangle, the Quadrilateral, and the Pentagon

One of the tools used to teach people about the danger of fire is the fire triangle. The triangle is used because there are three elements needed for a fire to occur: fuel, oxidizing agent, and an ignition source. The fuel is what burns be it a solid (wood, coal) or a gas (natural gas, gasoline or alcohol vapors). (Liquids don’t actually burn; it is the vapor above the liquid which burns.) The oxidizing agent is typically the oxygen present in the atmosphere. The ignition source can be an open flame, an electric spark, or even a hot surface. Combining these three elements typically results in a fire. If one or two of these elements are taken away, a fire will be extinguished.

In order to have a dust fire, a fourth element is typically required: dispersion. If a dust is in a pile or a layer, it will typically not burn due to the limited amount of oxygen and surface area exposed to the oxygen and ignition source. This statement is generally true and depends upon the flammability of the particular powder that is being exposed to the ignition source. But when the dust or powder is dispersed in the atmosphere, the amount of surface area exposed to the oxidant and the ignition source increase by orders of magnitude. The large amount of surface area of the dust can experience a rapid oxidation reaction and take the form of a flash fire. Technically speaking, this flash fire is a deflagration. If deflagration occurs in an open area the heat and products of combustion are free to expand. This expansion creates a large amount of radiant heat and a pressure wave. But since it is uncontained, the effects are primarily localized to the vicinity of the flash fire.

In order to have a dust explosion, a fifth element is required: containment. When a deflagration occurs within a contained area, the rapid expansion of the heat and products of combustion cannot freely expand. This causes the pressure within the containment structure to rise rapidly. If the pressure rise is sufficient the containment structure will fail violently and rapidly. This failure is the explosion.

It is important to realize that if these five components are present and dust explosion is likely. In order to avoid a dust explosion one or more of these components need to be removed from the area.

In manufacturing plants that create or handle dusts or powders, these five elements are present in various amounts throughout the facility. It is the responsibility of the owner/operator of every facility to carefully consider the processes in their plant and determine how these five elements are to be controlled, both within the process stream and within the building itself. 

Press Release

Dust Explosion: Is Your Facility at Risk?

June 2, 2017

Colorado Springs, CO – Dust explosions occur every year, killing and injuring workers and damaging property. The National Fire Protection Association (NFPA) has issued new standards that apply to all companies that handle potentially combustible dust. Keith D. Robinson, P.E. announces that he will be giving a presentation that will discuss the factors that cause dust explosions and give an overview of the new NFPA standards. The intended audience is Environment Health and Safety (EH&S) personnel and facility managers. However, anyone interested in this topic may attend.

Space is limited. Please call or email to reserve a spot.

Time & Date

July 19, 2017

11 AM

Location

Library 21c: Ent Conference Center

1175 Chapel Hill Drive
Colorado Springs, CO 80920

This event is not endorsed by or affiliated with the Pikes Peak Library District.

Contact

To learn more about this presentation or to reserve your spot, please contact

Keith D. Robinson, P.E.

4966 Daybreak Cir

Colorado Springs, CO 80917

303-746-8904

dust@kdrobinsonpe.com