Monday, July 24, 2017

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 (KSt): 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 (PMax): 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.

Test Method
Dust deflagration index
Measures how fast the pressure rises within an enclosed vessel. Used to design relief vents.
ASTM E 1226
Maximum explosion pressure generated in the test chamber
Used to design enclosures and predict the severity of the consequence.
ASTM E 1226
Minimum Ignition energy
Predicts the ease and likelihood of ignition of a dispersed dust cloud.
ASTM E 2019
Minimum explosible concentration
Measures the minimum amount of dust, dispersed in air, required to spread an explosion.
ASTM E 1515

All of the previous information is available from a wide variety of sources. My primary source for this material is the book Guidelines for Combustible Dust Hazard Analysis by the Center for Chemical Process Safety of the American Institute of Chemical Engineers. The U S Chemical Safety Board Combustible Dust Hazard Study Report 2006-H-1 also presents this information.

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