The Dust Sampling Plan

After deciding to carry out combustible dust tests the first thing you should do is to gather dust sample(s) and sent them to the lab, right? Actually, no. NFPA 652 requires that a written sampling plan be developed with a rational reason for why the sample locations were chosen. So what is so important about a written sampling plan?

The purpose of the combustible dust tests is to determine the hazards presented by the dust that is actually in your facility and in your process. The actual hazard presented by YOUR dust may be substantially different than in another facility handling the same type of dust but that has a different process. For example, I have been in a facility that handles grain where the grain goes directly from a storage bin into a process. In this case a sample of grain from the storage bin taken from the storage bin accurately reflected the dust in the system. At the same time a different facility across town used the same grain (it actually came from the same source) but their process included a hammer mill between the storage bin and the process. In this case a sample of grain taken from the storage bin does not represent the hazard in the system because the fugitive dust emanating from the hammer mill and the product stream to the process were much smaller than the dust found in the storage bin and had a much higher explosibility. (See previous entry.) While the grain in both facilities started out the same, the hammer mill process changed hazards associated with the dust. The end result was that the two facilities were very different.

This simple example helps demonstrate why a written plan is required. A written plan requires time to be taken to evaluate and record the reasons why a sample accurately reflects the hazards in a facility. A written plan will help deflect/answer questions if an audit of system or safety documentation were to occur. A non-existent or slipshod sampling plan will leave many questions unanswered and leave the door wide open for further inquiries. On the other hand, a well thought-out and written plan will provide answers to many, if not all, of the auditor’s questions.

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.

NFPA 652: Standard on Fundamentals of Combustible Dust

NFPA serves as the foundational NFPA document to guide the control of the hazards associated with combustible dust. One of the key items introduced by this standard is that 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. If the dust is not combustible, then the owner/operator has complied with the standard (and probably the OSHA General Duty Clause) and the formal documentation process concern combustible dust is over. (Please note that the everyday aspects of safety still apply: housekeeping as an example. You can’t leave piles of dust around even if it can’t explode.) If the dust IS combustible, then further tests are required. In addition, the owner/operator must carry out a formal Dust Hazard Analysis (DHA).

What is a DHA? In the simplest sense the DHA is a formal process whose output is a document which describes/assesses the hazards present in a facility, describes how the hazards will be controlled, eliminated, or mitigated, and describes what management systems will be implemented to ensure that the hazards continue to be controlled in the future. The hazard assessment must be backed up by results from combustible dusts tests. The management portion must include Standard Operating Procedures for items such as housekeeping, preventative maintenance, and training.

The overall goal of the DHA is that the owner/operator of a facility performs a legitimate and thorough evaluation and not just a cursory look at a system. A thorough evaluation will find and control a lot of simple, easily fixed risks (the “low hanging fruit”) which typically increases the safety of employees operating a process or working in a facility. 

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).

What is Fire?

We have all seen a fire. But what is it really? In simplistic terms, fire is a rapid reaction between a material, i.e. the fuel, and any oxygen in the environment surrounding that fuel, i.e. the oxidizer. This is the same reaction which creates rust on iron and steel surfaces, although the rate of the “rusting” reaction is many orders of magnitude slower than the “burning” reaction. While I suppose it would be technically correct to say “We melted marshmallows over a camp-rust” or “There was a forest-rust that destroyed 4,000 acres” it just doesn’t sound correct. We understand the word “fire” to mean an energetic and rapid release of hot gases and visible flames and the word “rust” to mean a relatively slow process. Although in wet, humid environments the rusting can take place in a very short time.

The flames we see during a fire are the visible release of energy from the oxidation reaction that is occurring on the surface of the fuel. In most cases the flames are visible but there are some fuels that emit invisible flames. I can remember seeing video footage of the pit area of a car race where all the pit members started to act excitedly: they were jumping up and down, frantically waving their arms, running around. It was just general chaos. What I didn’t realize was that the fuel they were using did not have visible flames. Those pit crew members were on fire! (This occurred in the 1981 Indianapolis 500. Rick Mears was the driver and he received 1^st and 2^nd degree burns. His crew chief received much worse burns.) There were no visible signs of the rapid oxidation reaction that was taking place. This incident led to a change in fuel so that visible flames could be seen if the fuel was burning.

As a first step look at the basic oxidation reaction for methane (CH₄):

Two sides of the fire triangle are represented in this equation: fuel and oxidizer. The left side of this equation is before the reaction takes place and there is a fuel (CH₄) and an oxidizer (O₂) present. If you have a methane and oxygen mixture and an ignition source is present, the reaction occurs. After the reaction (the arrow), there are two different substances present: carbon dioxide and water. The reaction also releases a lot of heat.

The oxidization reaction for rusting iron (Fe) is:

Like the reaction equation for methane, the left side of the equation contains a fuel (Fe) and an oxidizer (O₂). The right side contains the products of combustion. Besides the fact that one fuel is a gas and one is a solid, the primary differences between these two reactions are:

1. The activation energy for the iron oxidation (rusting) is much lower than that required to start the burning process for methane.
2. The amount of heat released by the rusting iron is less than heat released by the burning methane.
3. The rate of the methane oxidation is much faster than the iron oxidation.

So fire is basically the same reaction as the rusting of those old garden tools stored in the shed. It occurs many times faster than the rusting process but it is the same reaction: the oxidation of a material.

Explosion Kills 4 in Cambria, WI

On Wednesday May 31, 2017 a powerful explosion destroyed much of a corn mill located in Cambria, Wisconsin. According to Fox 6 Now three workers were killed in the blast with the fourth victim succumbing to his injuries on June 6th, 2017. Along with the four fatalities, eleven others were injured in the explosion. An investigation into the cause of the blast is underway but the cause of the explosion is unknown at this time. However one potential cause that is being investigated is a dust explosion.

Dust explosion are typically caused when a cloud of combustible dust reaches a critical concentration and is ignited by some energy source. Once ignited, the dust cloud rapidly burns and, if it occurs in an enclosed space, results in a rapid pressure increase and an explosion. Large amounts of combustible dust are present in many workplaces, especially in facilities that handle grain, but the risk is frequently underestimated. According to NFPA 652 it is the responsibility of the owner/operator of a facility to conduct a Dust Hazard Analysis (DHA) to determine the risks present in their facility and to develop and implement a plan to manage those risks. This plan also needs to include training for all employees and contractors so that they can recognize the risks created by combustible dust.

No matter what caused the explosion in Wisconsin, it should remind all of us that risks are present in our workplaces. Safety is not just the first agenda item in a meeting: it needs to be part of our everyday procedures and every task we perform. All of us, from the CEO on down to the newest untrained employee, need to be on the lookout for the hazards around us. This is especially true if the hazard is a combustible dust. Because if a dust explosion occurs, it will likely have devastating consequences.