Deflagration Index aka(Kst)

•Kst provides the best “single number” estimate of the anticipated behavior of a dust deflagration.

•Combustible dusts are classified by the numerical value of Kst.

Dust Enclosures for 

Combustible Dusts

Many times the terms for the enclosure can be just as confusing as the terms for the dust characteristics.  Below I will give some basic definitions of the most common terms.

Vented Explosion Pressure (Pred)

-The maximum pressure developed in a vented enclosure during a vented deflagration

Enclosure Strength (Pes)

-Up to two-thirds the ultimate strength for low-strength enclosures; for high strength enclosures the enclosure design pressure sufficient to resist Pred.

Ultimate Strength

-The pressure that results in the failure of the weakest structural component of an enclosure.

Static Activation Pressure (Pstat)

-Pressure that activates a vent closure when the pressure is increased slowly (with a rate of pressure rise less than 0.1 bar/min = 1.5 psi/min).

Air–Material Separator (AMS)

-A collector designed to separate the conveying air from the material being conveyed.

Air-Moving Device (AMD)

-A power-driven fan, blower, or other device that establishes an airflow by moving a given volume of air per unit time.

Enclosure

-A confined or partially confined volume.

Combustible Dust

When it comes to combustible dusts there are many terms common to the industry that are probably not so common to you.  I will try to briefly define what some of these terms.  

Dust Class (ST)
-ST 0, ST 1, ST 2, ST 3

Volume –Normalized Rate of Pressure increase (Kst)
-The deflagration index of a dust cloud.

Rate of Pressure Rise (dP/dt)
-The increase in pressure divided by the time interval necessary for that increase to occur.

Maximum Rate of Pressure Rise [(dP/dt)max]
-The slope of the steepest part of the pressure-versus-time curve recorded during deflagration in a closed vessel.

Maximum Pressure (Pmax)
-The maximum pressure developed in a contained deflagration of an optimum mixture.

Minimum Explosive Concentration (MEC)
-The minimum concentration of a combustible dust cloud that is capable of propagating a deflagration through a uniform mixture of the dust and air under the specified conditions of test.

Minimum Ignition Energy (MIE)
-The minimum amount of energy released at a point in a combustible mixture that causes flame propagation away from the point, under specified test conditions.

Minimum Ignition Temperature (MIT)
-Minimum ignition temperature of a hot surface capable of causing ignition of dust in suspension.

Layer ignition temperature or Auto Ignition Temperature (AIT)
-Temperature at which a dust layer ignites on a hot surface (1/2” thickness test base)

Limiting Oxygen Concentration (LOC)
-To determine the highest oxygen concentration at which ignition of a dispersed dust at ambient temperature and pressure is not possible.

Electrical Volume Resistivity (Rv)
-The measure of how easily electrical charge can flow through the particulate.

Charge Relaxation Time
-The time it take for a charge to decrease to 0.37 x it’s initial value.

Powder Chargeability 
-The charge per unit mass that can be developed on a specific material during processing

Burn Rate 
-The flammability and degree of combustibility of metal powder/dust

Thermal Stability
-Process risk at higher operating temperatures and storage conditions.

Water Reactivity
-A material that chemically reacts with water, producing another compound that can represent a different set of fire protection concerns

Water-Incompatible
-A material that does not chemically react with water, but which undergoes a change of phase or state upon mixture with water that renders it permanently changed or incompatible with the remainder of the process

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AHJ

Authority Having Jurisdiction or AHJ

The AHJ as they are often referred to have a large portion of the control around what is and is not acceptable for your dust collection system.  The NFPA today serves as a set of guidelines for people to follow but leave quite a bit to interpretation.  

My recommendation would be to involve your local AHJ in some part of the process to ensure you are working toward a agreeable and safe solution for your employees and facility.  Examples of who your AHJ may be:

Insurance underwriter

Local Fire Inspector 

Building Inspector

In most instances the person and the level of understanding will vary from city to city and makes it very difficult to formalize a standard that can be adopted nationwide.  Through this blog i will tackle some of these topics and discuss my interpretation of these guidelines.

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NFPA Dust Collection Blog Objective

Welcome to my blog page, this is my first attempt at anything like this so be patient and I plan to tackle some very common questions on NFPA.

Objective:

1. Discuss NFPA regulations in regards to dust collection
2. Answer questions that may be submitted
3. Develop some simple illustrations to help simplify this process.

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