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.