Printer Friendly

Bridging the gap: explosion testing to explosion protection.

A generalized hazards analysis approach for dust explosions is recommended

An explosion occurs when energy is released over a sufficiently small time and in a sufficiently small volume so as to generate a pressure wave of finite amplitude that travels away from the source. For a dust explosion to occur, a fuel and an oxidizer must exist in proper proportions inside a confined or partially confined space along with an ignition source of sufficient strength, Fig. 1. A dust explosion increases pressure inside the confining vessel in an extremely short time. As a result, unless the confining vessel has been built to contain the pressure, the pressure due to the explosion causes failure of the vessel. In the worst case, the explosion causes violent damage to the vessel and its surroundings.

An engineer involved in handling of granulated solids or powders must evaluate the potential of dust explosions in his facility. Furthermore, the engineer must know the options available to deal with the potential of dust explosions. The available options can be categorized into either explosion prevention or explosion protection. Explosion prevention techniques try to minimize the occurrence of explosions by manipulating one or more of the elements leading to an explosion. Minimizing the existence of ignition sources and removal of the oxidizer (inerting) are commonly used prevention techniques.

Explosion protection techniques try to minimize or eliminate the damage caused by an explosion. Venting of enclosures and use of suppression systems are examples of minimizing damage by limiting the increase of pressures generated during an explosion.

The purpose of this article is to describe some of the commonly used dust explosion tests, and dust explosion protection techniques. Information necessary for the design of explosion protection methods is discussed from the perspective of currently used design methods and from the perspective of design methods under development.

Hazard assessment

An engineer responsible for a dust handling system must address the dust explosion potential. The first question to be answered is: "Is the dust being handled explosible?" This question can be answered by checking the literature for explosibility data. Literature data may clearly show that the dust being handled is explosible or the dust being handled is not explosible. However, in the event the data are ambiguous or non-existent, the next step would be to conduct a classification test.

In the classification test, the dust is subjected to various ignition sources to determine the explosibility. To obtain unambiguous results, Bartknecht suggests the use of chemical ignitors (E = 10 KJ), 20 to 50 g of guncotton, and the flame of a welding torch. If a dust is found to be non-explosible when subjected to these severe ignition sources, the probability of a dust explosion occurring during normal handling can be assumed to be small. Careful attention should be given to the entire dust handling process before deciding the dust does not provide an explosion hazard.

After the classification test, if the dust has been determined to be explosible, the next step is to determine the explosion parameters of the dust. It is usually good practice to establish the explosion parameters via standard tests instead of relying on literature data because explosion parameters are a function of:

* physical and chemical properties of the dust (particle size distribution and chemical composition);

* initial pressure and temperature;

* concentration, homogeneity and turbulence;

* geometry of test vessel;

* type, energy and location of ignition source.
TABLE 1. Explosion parameters of industrial dusts.
 |P.sub.max~ |K.sub.St~
Material (bar ga) (bar m/s)
Cornstarch 10.3 202
Coal 9.2 129
Lactose 7.7 81
Magnesium 17.5 508

If literature data are to be relied upon for design of protection or prevention systems, it will be important to determine the applicability of the data to the dust under consideration. Because explosion parameters vary as a function of particle size, the particle size of literature data must be checked against the particle size of the dust under consideration prior to using literature data to design protection or prevention systems. The assessment of explosion data from the literature must go beyond a comparison of the chemical name.

Dust explosibility testing

The primary reason for the destructive nature of dust explosions is the generation of high pressures at a rapid rate. The current standard test for the quantification of dust explosion pressures is entitled Standard Test Method for Pressure and Rate of Pressure Rise for Combustible Dusts. This test method is under the jurisdiction of ASTM Committee E-27 on Hazard Potential of Chemicals and is the direct responsibility of Subcommittee E27.05 on Dusts (see ASTM E1226-88).
TABLE 2. Minimum explosible concentration.
Material (g/|m.sup.3~)
Rice Starch 60
Coke 125
Calcium Acetate 500
Magnesium 30

A typical pressure time curve from a dust explosion in a 20-L chamber is shown in Fig. 2. A schematic of the 20-L apparatus used for measuring dust explosion parameters is shown in Fig. 3. |P.sub.ex~ is the maximum pressure observed during the course of the explosion. |(dp/dt).sub.ex~ is the maximum rate of pressure rise during the explosion. |P.sub.ex~, and |(dp/dt).sub.ex~ vary as a function of several parameters.

In the standard test method, parameters such as homogeneity and turbulence, the type of ignition energy and location of the ignition source, vessel geometry and initial conditions are kept constant. |P.sub.ex~ and |(dp/dt).sub.ex~ are measured in a series of tests over a large range of dust concentrations. |P.sub.max~, the maximum explosion pressure, and |(dp/dt).sub.max~, the maximum rate of pressure rise are determined from this series of tests.

In determining |P.sub.max~ and |(dp/dt).sub.max~, the standard also recommends criteria for particle size and moisture content of the dust. A criteria for standardization and calibration is also presented. An additional explosion parameter, |K.sub.St~, can be determined from having conducted a series of tests. |K.sub.St~ is the maximum rate of rise normalized to a 1|m.sup.3~ volume.

|K.sub.St~ = |(dp/dt).sub.max~ |V.sup.1/3~

V is the volume of the explosion chamber in which the tests were conducted.

In addition to the maximum explosion pressure and the maximum explosion pressure rate of rise, the explosible behavior of dusts can be described by the minimum explosible concentration. The minimum explosible concentration (MEC) is the concentration of dust in air necessary to allow the propagation of a deflagration wave. The minimum explosible concentration is also referred to as the lean flammability limit. It is observed that as dust concentration is lowered, |P.sub.ex~ and |(dp/dt).sub.ex~ are also lowered.

At the minimum explosible concentration, the effect of the ignition source must be taken into consideration. Figure 2 shows a typical pressure vs. time curve produced by the standard ignition source. The effect of the ignition source is taken into account by observing the relationship between dust concentration and ||P.sub.ex~ - |P.sub.lg~~. The criteria for MEC is ||P.sub.ex~ - |P.sub.lg~~ = 2 for dust explosions occurring at normal or ambient pressures. The MEC's of various industrial dusts are shown in Table 2.

A typical dust system consists of a solid fuel and a gaseous oxidizer. The most common gaseous oxidizer is oxygen and it is present in the dust system along with nitrogen. Because most solid fuels are inert to nitrogen, the increase of nitrogen in a dust system will result in a variation of |P.sub.ex~ and |(dp/dt).sub.ex~. As a result, one of the dust explosion parameters of interest is the maximum oxygen concentration to prevent a dust explosion.

The maximum oxygen concentration (MOC) is the highest percent of oxygen in an oxygen-nitrogen atmosphere that will prevent the propagation of a deflagration wave in a closed vessel. In the MEC test, the fuel is decreased to determine the limits of dust explosibility. In the MOC test, the oxidizer is decreased to determine the limits of dust explosibility. In practice, the effect of decreasing oxygen concentration on |P.sub.ex~ and |(dp/dt).sub.ex~ is experimentally observed. A series of tests are conducted until an oxygen concentration is found at which ||P.sub.ex~ - |P.sub.lg~~ = 0.

As described earlier, Fig. 1, one of the critical elements of a dust explosion is an ignition source of sufficient strength. It follows that one of the techniques of preventing explosions is to eliminate or minimize ignition sources. Two common ignition sources are hot surfaces and electrostatic sparks.

The relative ease of ignition by an electrostatic spark discharge can be observed by determining the minimum ignition energy of a dust/air mixture. The minimum ignition energy (MIE) of a combustible dust is the lowest energy which is capable of igniting the dust/air mixture at ambient pressures and temperatures. Typically the energy is stored on capacitors and discharged across a spark gap. The energy of the spark is calculated on the basis of:

E = 1/2 C|V.sup.2~

Where, E is energy, C is capacitance and V is voltage. The minimum ignition energies of various industrial dusts are shown in Table 4.
TABLE 3. Maximum allowable oxygen concentration.
Material (% |O.sup.2~)
Cornstarch 8
Coal 15.8
Phthalic Anhydride 11.9
Magnesium 0
TABLE 4. Minimum ignition energies.
Material MIE (ms)
Cornstarch 30
Lycopodium 22
Methyl Cellulose 40
Aluminum 10

When a dust cloud is exposed to hot air in an enclosure, the cloud will autoignite. The minimum temperature at which a dust will autoignite when exposed to air heated in a furnace is the minimum autoignition temperature. Experimentally, a furnace is heated to a temperature and a dust is blown into the furnace to determine whether it will ignite. A series of tests are conducted until a minimum autoignition temperature (MAIT) is determined to a specified accuracy (typically |+ or -~ 10 |degrees~ C).

Explosion protection

Three of the standard tests described -- MEC, MIE, and MAIT -- provide information regarding the relative sensitivity of the dusts and provide the engineer an indication of the probability of an explosion occurring. As the probability of explosion increases, the measures taken to prevent the explosion should be more exhaustive. The MOC test provides the necessary information for the design of an inerting system. Information from the pressure and rate of pressure rise test can be used in the design of explosion venting, explosion suppression, and explosion isolation.


The 1988 edition of NFPA 68 Venting of Deflagrations contains guidelines for the design and use of vents as an explosion protection system. Since the weakest part of a vessel or enclosure will be the first to rupture due to the pressures generated by a deflagration (explosion), a vent is designed and placed on the vessel such that the vent is the first part of the vessel to rupture.

A deflagration vent minimizes the structural and mechanical damage caused by explosion pressures. Venting also allows the flow of combustion gases and unburnt material to be directed away from personnel areas to a safe exterior location.

The design of a venting system must take into consideration the dimensions, volume, and construction of the enclosure and the deflagration properties of a combustible material. In order to use NFPA 68 it is necessary to know the |K.sub.St~ of the dust.

The primary design variable is the vent area and it is determined from the desired reduced pressure, the vessel volume and the static burst pressure of the vent. The reduced pressure, |, is the maximum pressure developed during the venting of a deflagration and is usually much lower than the maximum explosion pressure.

Venting systems are designed such that enough vent area is provided to keep | below the maximum allowable working pressure of the vessel being protected. The relief of combustion gases sometimes requires a vent duct which may decrease the efficiency of the vent. As a result, the vent area may have to be increased.

One disadvantage of venting is that the combustion reaction has not been extinguished. When a vent opens, it will exhaust both hot gases and unburnt material. The unburnt material will likely continue to burn in the exhaust area and create a secondary hazard.

Venting does relieve pressure and decreases damage in the primary vessel. But, if connecting piping contains combustible materials, the flame is likely to propagate through this piping into other vessels or other locations. Venting is not suitable for environmentally toxic materials or materials that have toxic combustion products.


Explosion suppression is a protection method in which the explosion is detected in its early stages and a suppressant material is introduced to prevent the continuation of the combustion reaction. Some common suppressant materials are: water, halogenated hydrocarbons, ammonium phosphate, sodium bicarbonate, and sodium chloride.

Suppressant agents prevent the propagation of the combustion reaction by:

* Absorbing the energy produced by the combustion reaction and in essence "cooling down" the combustion volume;

* Inhibiting the critical chain reactions by which the combustion reaction propagates;

* Creating a combustible concentration below the lower flammable limit.

All three aspects contribute to the effectiveness of a suppressant agent once the agent has been delivered into the combustion volume must begin within a few milliseconds after the detection of an explosion and, therefore, delivery speed is a critical element of a suppression system. Suppressant systems must be able to detect the early stages of an explosion and then quickly deliver sufficient quantities of suppressant agent to keep the combustion pressure below the safe working pressure of the vessel.

A pressure detector is used in conjunction with a control panel to provide an electrical signal in the event the vessel pressure exceeds a preset level. The pre-set detection pressure takes into account the normal pressure perturbations in the vessel. The pressure detector is set such that the incipient stage of an explosion is detected.

Once the incipient explosion pressure has been detected, a signal from the control panel is used to initiate the release of the suppressant agent from a suppressant cylinder. The suppressant agent cloud will then interact with the growing "fireball" (combustion zone) and prevent the propagation of the combustion reaction.

Explosion suppression systems must be designed for each individual application. The choice of suppressant agent, the number of suppressant containers necessary, charge pressure of the containers, location of pressure detectors, set-point of pressure detectors and interaction with other fire and explosion protection systems are some of the factors that must be considered.

Vendors typically have a model that will predict the pressure due to suppression on the basis of protection system parameters and deflagration properties such as |K.sub.St~.

The advantages of explosion suppression systems are the ability to limit pressures due to explosion and the ability to prevent fire or explosion propagation into other vessels. Explosion suppression does not require release of vessel contents or combustion products into the surrounding atmosphere. In comparison to explosion venting, explosion suppression is relatively expensive. Suppression systems require a more active maintenance than explosion vents. The design methodology for venting exists in the form of NFPA 68, whereas, the design methodology for suppression is not as well published; testing of suppression systems is also recommended for non-standard applications.


In a process system with several interconnected vessels that handle combustible materials, it may be necessary to protect or isolate the vessels from each other. An explosion occurring in a first vessel will propagate through piping and initiate an explosion in a connected second vessel. The effects of the explosion -- maximum explosion pressure and maximum explosion pressure rate of rise -- are greater in the second and subsequent vessels.
TABLE 5. Minimum autoignition temperature.
Material MAIT (|degrees~ C)
Cornstarch 390
Coal 610
Urea 900
Aluminum 420
Zirconium 20

The effect of explosions increase in magnitude with increasing initial pressure. In interconnected vessels, the first explosion increases the pressure in the entire system and secondary explosions occur at higher initial pressures and result in higher explosion effects. This phenomena of "pressure piling" can be protected against by the use of explosion isolation systems. Explosion isolation systems can be separated into mechanical systems and extinguishing systems.

The explosion isolation valve and the backflash interrupter (BFI) are mechanical isolation systems. Both are designed to be a part of the piping system between vessels and to prevent propagation of the deflagration through the pipe. The explosion isolation valve is a responsive system which will rapidly close in the event of an explosion. An explosion detector and a control panel are integral parts of the system.

Upon detection, the control panel fires an electrical Squib. The shock wave from the Squib opens a rupture disc which, in turn, allows the release of pressurized nitrogen. The release of nitrogen pushes a piston and the valve closes. The closed valve prevents propagation of the deflagration wave into other vessels. Closure times of the valves range from 20 ms to 100 ms for 4 to 24-in. valves.

The BFI is a passive mechanical isolation device that is currently under development and that does not require a detection or control system. In the event of an explosion, the dome on the BFI is opened by the pressure from the explosion and propagation of the deflagration wave is interrupted by release into an open area.

Extinguishing isolation systems are comprised of a detector, a control system and a suppressant release container. Similar to explosion suppression systems and explosion isolation valve systems, the pressure due to an explosion is detected and the control panel then initiates the protection system.

Extinguishing isolation systems are placed on interconnecting piping, and the same extinguished extinguishing agents (suppressant agents) listed previously can be used. The residence time of the extinguishing agents in the piping system can be of concern in systems with a high flow rate. The extinguishing agents are intended to provide a chemical barrier through which the deflagration cannot propagate. Normally, the release time of the extinguishing agents can be controlled to provide a chemical barrier with adequate residence time. The choice of agent, location of system, and other system parameters must be designed for each application. The primary deflagration property of interest in isolation is flame speed in a pipe.

Flame speed is reported by Bartknecht to correlate with |K.sub.St~ as follows:

|K.sub.St~ |is less than or equal to~ bar|center dot~m/s : Vex |is less than or equal to~ 300 m/s 201 |is less than or equal to~ |K.sub.St~ |is less than or equal to~ 300 bar|center dot~m/s : 301 |is less than or equal to~ Vex |is less than or equal to~ 400 m/s

|K.sub.St~ |is greater than~ 300 bar|center dot~m/s : Vex |is greater than~ 400 m/s

Because isolation systems are relatively new in comparison to venting and suppression, their design is still being developed.


Through various standard dust explosibility tests exist, a majority of the tests provide information to support explosion prevention efforts. The primary test of interest in trying to design explosion protection is the measurement of pressure and rate of pressure rise. It has direct applicability to the design of explosion venting and explosion suppression and indirectly can support explosion isolation design calculations. A further development of dust explosion science must occur prior to the development of standard laboratory tests that would provide more directly useful information for explosion protection design methods.

Kris Chatrathi is with the Fike Corp., Blue Springs, MO.
COPYRIGHT 1993 Chemical Institute of Canada
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:dust explosions assessment
Author:Chatrathi, Kris
Publication:Canadian Chemical News
Date:Jun 1, 1993
Previous Article:Fastfonts for DOS.
Next Article:Towards reducing major industrial accidents.

Terms of use | Copyright © 2016 Farlex, Inc. | Feedback | For webmasters