Effectiveness of dust dispersion in the 20-L Siwek chamber

Omotayo Kalejaiye a, Paul R. Amyotte a, Michael J. Pegg a,

aDepartment of Process Engineering and Applied Science, Dalhousie University, 1360 Barrington St., Halifax, Nova Scotia, Canada B3J 2X4

*

* Corresponding author. Tel.: þ1 902 494 3252; fax: þ1 902 420 7639.

E-mail address: michael.pegg@dal.ca (M.J. Pegg).

, Kenneth L. Cashdollar b

b Pittsburgh Research Laboratory, National Institute for Occupational Safety and Health, Pittsburgh, PA 15236, USA

abstract

The Siwek 20-L chamber is widely used throughout the world to evaluate the explosibility of dusts. This

research evaluated the quality of dust dispersion in the Siwek 20-L chamber using Pittsburgh coal,

Gilsonite, and purple K dusts. A Pittsburgh Research Laboratory (PRL) optical dust probe was used to

measure optical transmittance through the dust cloud at various locations within the chamber. A total of

540 tests were performed, with triplicate tests at five nominal dust concentrations and six locations.

The two standard dispersion nozzles (rebound and perforated annular nozzle) were compared. The

transmissions corresponding to the normal ignition delay period were used to: (a) determine variations

in spatial uniformity of dispersion obtained with both nozzles; (b) make comparisons between the

experimental transmission data and those calculated from theory for the three dusts; and (c) make

comparisons with transmission data measured in the PRL 20-L and Fike 1-m3 dust explosion chambers.

The uniformity of dispersion for the three dusts was similar with both nozzles, despite the differences

in nozzle geometry and mode of operation. Transmission data of the three dusts were all significantly

lower than those calculated from theory. This was discovered to be, in part, due to significant reduction

in particle size that occurred during dispersion. By measuring particle sizes before and after dispersion,

values of 60%, 50%, and 20% reduction in particle size (based on the surface-weighted mean diameter)

were obtained for Pittsburgh coal, Gilsonite, and purple K, respectively. Transmission data from the PRL

20-L, Fike 1-m3 and the Siwek 20-L chambers indicated comparable results in terms of uniformity of

dispersion. However, transmission data from the Siwek 20-L chamber were significantly lower than

those of the PRL and Fike chambers. Again, this was attributed, in part, to the significant reduction in

particle size that occurred during dispersion in the Siwek chamber. The design of the outlet (dispersion)

valve of the Siwek 20-L apparatus charge vessel was largely responsible for the particle break-up. The

contribution to particle break-up by the dispersion nozzles and the high level of turbulence in the

chamber were found to be minimal. This is a significant finding in that the dust particle size tested for

explosibility in the Siwek chamber is considerably smaller than the original dust sample.

1. Introduction

The hazards associated with combustible dusts have been

recognized for at least the past 200 years. There are five requirements for a dust explosion. The requirements for a fire are a fuel

(combustible dust), an oxidant (usually oxygen in air), and an

ignition source. The two additional requirements for an explosion

are that the dust particles must be dispersed in air at the time of

ignition and that there must be some degree of confinement.

The explosibility characteristics of dusts can be broadly classified

into two categories. The first group describes the likelihood of a dust

explosion occurring and this includes the minimum explosible

concentration (MEC), the minimum ignition energy (MIE), the

limiting oxygen concentration (LOC), and the minimum autoignition

temperature (MAIT). The second group describes the severity of

a dust explosion when it occurs and this includes the maximum

explosion pressure (Pmax) and the maximum rate of pressure rise

([dP/dt]max). Another important quantity is the deflagration index or

volume-normalized maximum rate of pressure rise. [Kst 1⁄4 (dp/

dt)max$V1/3] where V is the volume of the test chamber in m3

. Thus,

KSt (in bar$m/s) is the maximum rate of pressure rise that would be

obtained directly from a standard 1-m3 test vessel. Combustible dusts

are classified by their KSt values in increasing order of explosion

violence as follows (Bartknecht, 1989): 0 < KSt ≤ 200 corresponds to

St 1 dust explosion class; 200 < KSt ≤ 300 for St 2 dust explosion class;

and KSt > 300 for St 3 dust explosion class. Furthermore, the KSt value

of a combustible dust is an important factor that is considered in the

determination of relief areas for venting, as specified in NFPA 68
(2002). Extensive discussion of these parameters has been given in

the textbooks by Bartknecht (1989) and Eckhoff (1997).

The general procedure for the determination of explosibility

characteristics of dusts is to disperse a known mass of the dust into

the test chamber with a blast of air. For every test, the nominal dust

concentration is determined by dividing the mass of dust dispersed

by the fixed volume of the chamber. This approach is based on the

assumption that the dust cloud formed is uniform (i.e. no variation

in local dust concentration throughout the chamber volume).

However, this is an ideal case, which may be difficult, or perhaps

impossible, to achieve in reality. Clearly, the degree of dust cloud

uniformity in test chambers can affect the accuracy of the results

obtained, particularly the MEC data. Eggleston and Pryor (1967)

showed that the degree of dust cloud uniformity in the 1.2-L

Hartmann tube deviates significantly from the ideal case. Thus,

inaccurate MEC values are obtained from the Hartmann test vessel.

Recognizing the errors that can be introduced into MEC data by

highly non-uniform dust clouds in test chambers, the ASTM International standard test method E1515 (ASTM, 2005b) for the determination of minimum explosible concentrations of combustible

dusts stipulates that the test apparatus must be capable of producing

a fairly uniform dust cloud of the material. Among the available test

apparatus that have been considered as suitable for dust explosibility tests by the ASTM is the Siwek 20-L chamber (Siwek, 1977)

manufactured by Adolf Ku ̈hner AG, Birsfelden, Switzerland. For this

apparatus, two different nozzles are available for dust dispersion,

namely the rebound nozzle and the perforated annular nozzle. The

dust cloud formed in this chamber is assumed to be fairly uniform,

but this has never been documented quantitatively.

Therefore, the objectives of the present study (Kalejaiye, 2001)

were to: (i) determine the variations in spatial uniformity of the

dust cloud formed in the Siwek 20-L chamber using the rebound

nozzle and the perforated annular nozzle; (ii) assess the effectiveness of dust dispersion in the Siwek 20-L chamber, in comparison to

the Pittsburgh Research Laboratory (PRL)1

1 The Pittsburgh Research Laboratory was part of the U.S. Bureau of Mines

(USBM) before transferring to the National Institute for Occupational Safety and

Health (NIOSH) in October 1996.

20-L chamber and the

Fike2

2 Fike Corporation, Blue Springs, Missouri, USA.

1-m3 chamber as measured previously by Cashdollar and

Chatrathi (1992); and (iii) obtain a qualitative description of the

dust dispersion process in the Siwek 20-L chamber.

2. Previous studies

The difficulty of producing a perfectly uniform dust cloud in test

vessels is widely acknowledged. The high level of turbulence

required at the time of ignition, rather than dust cloud uniformity,

governs the selection of the ignition delay time, especially in the

Siwek 20-L chamber. In test vessels, the nominal dust concentration

is taken as being the representative dust concentration at the time of

ignition. Therefore, the degree of error arising from this assumption

can have an effect on the accuracy of the minimum explosible

concentrations obtained. For this reason, it is important to have an

idea of the effectiveness of dust dispersion in the test chambers at

the time of ignition, so as to evaluate the assumption of dust cloud

homogeneity and its effect (if any) on the accuracy of MEC data.

2.1. Optical dust probes

The effectiveness of dust dispersion in laboratory chambers and

experimental mines has been investigated using optical dust probes

designed at the PRL (Cashdollar, Liebman, & Conti, 1981; Liebman,

Conti, & Cashdollar, 1977). These dust probes have been used to

measure dust dispersion uniformity in an 8-L chamber (Hertzberg,

Cashdollar, & Opferman, 1979) and in the PRL 20-L chamber and Fike

1-m3 chamber (Cashdollar & Chatrathi, 1992). The probe operates on

the principle of light attenuation. The optical dust probe used in this

study measures the fraction of incident radiation that is transmitted

through the dust cloud at a wavelength of w0.95 mm. The Bouguerlaw (Middleton, 1960) relates the fraction of light transmitted

through the dust cloud (T) to the dust concentration (c). This law is

written for dust clouds with a particle size distribution as follows:

[ 3QcL T 1⁄4 exp - (1) 2rD32 

where Q 1⁄4 extinction coefficient or efficiency [-], c 1⁄4 average dust

concentration [g/m3

] across the beam path, L 1⁄4 fixed light path

length [m], r 1⁄4 density of the dust particles [g/cm3

], and

D32 1⁄4 particle surface-weighted mean diameter [mm]. The extinction coefficient/efficiency Q is a dimensionless parameter that

includes the loss of light by absorption and scattering. The equation

shows that the transmission is dependent on both the dust

concentration and the particle size. Therefore, although it can be

used to study dust dispersion uniformity, it does not measure dust

concentration directly. Fig. 1 shows a cutaway view of one half of

the PRL optical dust probe (Liebman et al., 1977).

Fig. 1. Sketch of PRL optical dust probe (from Liebman et al., 1977).

2.2. Transmission data as a measure of dust cloud uniformity

If the dust cloud is perfectly uniform, the transmission

measured at different points in the chamber at the time of ignition
should be equal. Therefore, deviations from this ideal case can be

used to quantify the degree of dust cloud homogeneity. Transmission data can also be used to make inferences about the degree

of agglomeration or de-agglomeration that occurs during dispersion. Agglomeration is the tendency of the primary dust particles to

stick together forming clusters of particles with larger diameters.

The effective particles of a dispersed dust may exist as agglomerates of fine particles rather than as the primary particles themselves. Therefore, the degree of agglomeration is another measure

of the effectiveness of dust dispersion. The degree of agglomeration

or de-agglomeration can be estimated by comparing the intrinsic

surface-weighted mean diameter D32 of the dust samples before

dispersion, to that calculated from the transmission data during

dispersion (Cashdollar et al., 1981).

Comparison of particle size analyses of the dust before and after

dispersion can also be used to confirm the inferences made from the

aforementioned procedure. In calculating D32 from transmission data,

the dust concentration (c) in Eq. (1) is the nominal dust concentration

at which the transmission data were obtained. Therefore, this method

can only be used if the dispersion is fairly uniform such that the

nominal dust concentration can be taken as a realistic measure of

the true dust concentration. The degree of agglomeration or deagglomeration (a) is then estimated as follows:

a 1⁄4 DT

32=DPSA32 (2)

in which DT

32 is calculated from transmission data and DPSA32 is

determined from the particle size analysis. A value of a 1⁄4 1 implies

that no agglomeration has occurred during dispersion. A value of

a > 1 implies that agglomeration has occurred while a < 1 implies

that de-agglomeration and/or particle break-up has occurred

during dispersion. Yet another complication is that the particle size

determined by different particle size analyzers may also vary

because of the different measurement techniques used.

2.3. The International Standardization Organization (ISO)

and Fike 1-m3 test chambers

Another severe limitation of the results (MEC, Pmax, and [dP/

dt]max) obtained from the Hartmann tube (Dorsett, Jacobson, Nagy,

& Williams, 1960) and other small test vessels at the time was that

they could not be applied to the industrial scenario where dust

handling equipment (e.g., silos, mixers, pulverizers, etc.) with large

volumes are encountered because the results do not agree with

those of the large test vessels when scaled-up. The larger test

vessels give more realistic estimates of the explosibility characteristics of dusts because their large volumes are more representative

of the industrial situation.

In 1966, Bartknecht developed a new test procedure for the

determination of the explosibility characteristics of combustible dusts

in a 1-m3 chamber (Bartknecht,1989, p.56). Results obtained with this

procedure were found to be realistic, especially in their applicability to

the industrial situation. Therefore, in 1985, the International Standardization Organization (ISO) adopted the 1-m3 test apparatus as

a standard (ISO 6184/1,1985) for the determination of the explosibility

characteristics of combustible dusts. The dispersion nozzle is a 19-mm

(internal diameter) tube formed into a perforated semicircular spray

pipe. The holes must be 4–6 mm in diameter and the number of holes

drilled in the pipe must be such that the total open area is 300 mm2

.

The spherical Fike 1-m3 vessel varies slightly from the specifications of the ISO 1-m3 test vessel because its dispersion pressure

and ignition delay time are 32 bar (g) and 550 ms compared to

20 bar(g) and 600 ms specified in the ISO standard. However, the

level of turbulence that it generates at the time of ignition (determined primarily by the initial injection pressure and ignition delay

time) is comparable to that of the ISO 1-m3 test vessel (Cashdollar &

Chatrathi, 1992).

2.4. PRL 20-L chamber

The primary objective of PRL dust explosibility research is

explosion prevention and protection in underground coal mines

and dust-related processing facilities (Cashdollar & Hertzberg,

1985). It is therefore not surprising that the main aim of the PRL in

developing a 20-L chamber was to produce accurate measurements

of the MEC of combustible dusts as well as to determine inerting

levels required for preventing dust explosions in coal mines. This is

in contrast to the Siwek 20-L chamber, which was developed

primarily for KSt and Pmax tests.

The PRL explosion chamber is a near-spherical stainless steel

vessel and the top of the chamber is hinged and opens fully

across the chamber diameter, which facilitates ease of cleaning.

Embedded in the base of the chamber is the dust reservoir in which

the dust to be tested is placed and covered with a dispersion nozzle.

For dispersion, the air from the 16-L reserve tank passes through

a solenoid valve and then disperses the dust. The dust does not pass

through the solenoid valve.

The ignition delay time, td, is 400 ms. The test apparatus is

usually equipped with two optical dust probes located at different

heights in the chamber making it the only test vessel that routinely

provides some information about the uniformity of the dust cloud

formed during dispersion. However, its level of turbulence, for the

standard dispersion procedure, is lower than that of the Siwek 20-L

chamber. For this reason, the KSt values obtained from this vessel

cannot be used to determine venting requirements.

2.5. Effectiveness of dust dispersion in the PRL 20-L

and Fike 1-m3 chambers

Cashdollar and Chatrathi (1992) have made comparisons

between the uniformity of the dust cloud formed in the PRL 20-L

chamber and Fike 1-m3 test vessel, using the PRL optical dust probes.

Tests were performed over a wide range of dust concentrations. The

transmission through dust clouds of Gilsonite, Pittsburgh coal and

anthracite was measured at various positions in both chambers.

At the start of dispersion, the transmission is 100% until attenuation begins when the dust particles reach the probe. The most

useful transmission data are those obtained at the time of ignition

(i.e., at the ignition delay time of the test apparatus). These were

obtained by averaging (due to the rapid fluctuations with time) the

transmission data over a 90-ms period prior to ignition. The

Gilsonite data showed less scatter than the Pittsburgh coal data.

The scatter in the data is probably due to variations in the

agglomerated particle size of the air-dispersed dust.

It was also observed that the 1-m3 transmission data were

somewhat lower than the 20-L chamber at low dust concentrations. A possible explanation for this is increased agglomeration in

the 20-L chamber. Furthermore, at low concentrations the dust

column is short, so air may form a rat hole and have a sputtering

type discharge. From the comparison between the transmission

data of the Fike 1-m3 and PRL 20-L chambers, Cashdollar and

Chatrathi (1992) concluded that dispersion in the Fike 1-m3

chamber was at least as good as that in the PRL 20-L chamber.

3. Experimental

3.1. Dusts

Gilsonite, purple K and Pittsburgh coal dusts were chosen for the

dispersion tests in the Siwek 20-L chamber. Gilsonite is a natural
bitumen or asphaltite that is mined in northeastern Utah, USA. It is

brittle and can be easily crushed or pulverized into a dark brown

powder and is used for making paints, varnishes and linoleum.

Transmission data for Gilsonite in the PRL 20-L and Fike 1-m3

chambers are available in the literature (Cashdollar & Chatrathi,1992).

Purple K dust is a specially coated and fluidized form of potassium bicarbonate (KHCO3) used as an extinguishing agent in fire

suppression. Although it is non-combustible, it has been used

extensively by other researchers (Cashdollar et al., 1981) for the

investigation of dispersion in test vessels using light-scattering

techniques. This is primarily because it does not agglomerate readily

and it tends not to adhere to the optical windows of dust probes.

Pittsburgh seam bituminous coal dust is often used as a ‘‘standard coal’’ in dust explosibility testing. In addition to the availability

of extensive dust explosion data, transmission data from the PRL

20-L and Fike 1-m3 chambers are also available. Particle size analyses of the three dusts were performed at the Minerals Engineering

Centre at Dalhousie University using a Malvern Instruments 2600

Series Analyzer, which operates on the principle of laser light

diffraction. The results are summarized in Table 1.

Table 1

Particle size analyses of the dusts used in the dispersion tests.

Purple K Gilsonite Pittsburgh coal

Surface-weighted mean diameter D32 (mm) 18 18 43

Mass mean diameter D43 (mm) 44 54 76

Mass median diameter Dm (mm) 36 35 66

Weight < 75 mm (%) 83 78 59

Weight < 20 mm (%) 28 33 8

Specific surface area (m2

/m3) 6 x 104 3 x 104 4 x 104

The first two

rows list the surface (D32) and mass (D43) mean diameters. The

mass median diameter (Dm) is the 50% point on the mass or volume

distribution curve. Note that the size data in Table 1 are somewhat

different than those reported in Cashdollar and Chatrathi (1992) for

the same dusts. They used a combination of sonic sieving and

Coulter Counter data for their size analyses.

3.2. Dust concentrations

Five nominal dust concentrations were tested, chosen to include

those for which transmission data were available for comparison, and

also to include the range of dust concentrations that would be

considered in a typical MEC test procedure (except for purple K since it

is a non-combustible dust). The MEC of Gilsonite and Pittsburgh coal

dust, as measured in the PRL 20-L chamber and Fike 1-m3 chamber,

are w35 g/m3 and w80 g/m3

, respectively (Cashdollar & Chatrathi,

1992). Table 2 shows the dust concentrations used in the present

work.

Table 2

Dust concentrations used in the dispersion tests.

Dust Dust Concentration (g/m3

)

Purple K 50, 100, 150, 250, 350

Gilsonite 25, 50, 100, 150, 175

Pittsburgh coal dust 50, 100, 150, 200, 250

The highest concentrations tested were limited by the necessity

of having transmission values greater than 0% at the ignition time.

3.3. Apparatus

The experimental set-up is comprised of the Siwek 20-L dust

explosion test apparatus, the PRL optical dust probe and a LabVIEWD – based data acquisition system. A schematic of the entire

system is shown in Fig. 2.

3.3.1. Siwek 20-L chamber

The Siwek 20-L dust explosion test apparatus consists of the

explosion chamber, dispersion and ignition system, pressuremeasuring system and an automated control system. The explosion

chamber is a spherical, stainless steel vessel with a capacity of 20 L

and a pressure rating of 20 bar (g), surrounded by a water jacket for

thermostatic control. At the base of the chamber is a solenoid valve

(also referred to as the inlet valve) to which a 0.6-L dust storage

chamber is connected. Main access into the chamber is through

a 94-mm diameter opening at the top. A bayonet closure, that also

holds the igniter leads, fits tightly into this opening and seals

the chamber. Two 30-mm (outer diameter) flanges are fitted on the

chamber; the first houses the pressure transducers, while the

second is a sight glass for visual observations during tests. Other

fittings attached to the explosion chamber are two valves – the

exhaust valve for venting and the vacuum valve for evacuating the

vessel prior to dispersion.

The Siwek 20-L chamber is supplied with two distinct dispersion nozzles, namely, the perforated annular nozzle (also known as

the ring nozzle) and the rebound nozzle (ASTM, E1226, 2005a).

Either of the two nozzles may be fitted into the outlet valve at the

base of the chamber. However, they vary widely in their geometry

and mode of operation. The perforated annular nozzle is ringshaped with holes uniformly distributed around the surface (see

Fig. 3). It has 112 holes, each 3 mm in diameter giving a total open

area of approximately 792 mm2

. The dust–air mixture from the

0.6-L dust storage canister is forced through the solenoid valve and

into the chamber through the holes in the annular nozzle. The

rebound nozzle (see Fig. 4), also known as the deflector plate, has

a total open area of 314 mm2

. Its mode of operation is dust

dispersion by high impact of the dust particles on the plates of the

nozzle. The dust–air mixture strikes the upper plate of the nozzle at

high velocities, rebounds off the lower plate and is directed

throughout the chamber. The standard dust dispersion time is

60 ms for the Siwek 20-L chamber.

The control system of the Siwek 20-L test unit is fully automated. It consists of a measurement and control unit (KSEP 332)

supplied by the manufacturer (Ku ̈hner AG) and PC-based data

acquisition and analysis software (KSEP 5.0d). With this system, the

entire test sequence, from the pressurization of the dust storage

chamber to the activation of the igniters, is controlled from a PC

connected to the KSEP 332.

3.3.2. PRL optical dust probe

The version of PRL optical dust probe used in the present study

was the single path length probe with air jet (Cashdollar et al., 1981;

Liebman et al., 1977). The path length (distance between the optical

windows of the LED and photo detector) is 38 mm. The light emitter

is a gallium arsenide LED that emits near-infrared radiation with

a central wavelength of 0.95 mm and a bandwidth of approximately

0.05 mm. The photo detector is a silicon photodiode, which is

connected to an operational amplifier circuit to produce a voltage

output linear with the input radiation of the photodiode. This probe

is of the same design as that used by Cashdollar and Chatrathi

(1992) in the PRL 20-L and Fike 1-m3 chambers.

To prevent adherence of dust particles to the windows of the LED

and photodiode, air exits through a 0.5-mm x 5-mm slit opening

(see Fig. 1) on the windows during a test. A solenoid valve (connected to a compressed air source at 7 bar) controls this air stream.

The dust probe was inserted into the Siwek 20-L chamber via a feedthrough in a 30-mm diameter flange, which replaced the sight glass.

The dust probe could be moved radially in the chamber, and the

feed-through was sealed with O-rings. Transmission measurements

were made at three positions along each of two perpendicular radial
axes of the chamber. The first radial axis was that which extended

from the sight glass to the geometric centre of the chamber while

the second radial axis was that which extended from the measuring

flange to the geometric centre of the chamber. For positions along

the latter, the blank flange and the measuring flange were interchanged. Fig. 5 shows a horizontal cross section of the Siwek 20-L

chamber indicating the dust probe locations. Positions 1, 2, and 3

were 60, 110, and 150 mm from the chamber wall. Positions 6, 5, and

4 were orthogonal to positions 1, 2, and 3 at similar distances from

the chamber wall.

Fig. 2. Schematic of the experimental apparatus.

3.3.3. Data acquisition system

The data acquisition and control system of the entire experimental set-up consists of the automated control system of the Siwek

20-L test unit (KSEP 332) and a LabVIEWD 5.0-based computer

program for optical dust probe data acquisition. LabVIEWD, which

stands for Laboratory Virtual Instrument Engineering Workbench, is

a programming environment in which programs are created with

graphics (Wells & Travis, 1997).

The silicon photodiode is connected to an operational amplifier

unit with the output voltage being in the range 0–4 V. Connection

to a personal computer is through a data acquisition board. This

board (National Instruments AT-MIO-16F-5) is a high performance

multifunctional analog, digital and timing input/output board with

a guaranteed maximum sampling rate of at least 200k samples/

second (National Instruments, 1994).

The amplifier and the solenoid valve (for the dust probe) were

connected to the board. The former was connected to analog

channel 0, which records the voltage signal from the amplifier unit

while the latter was connected to digital channel 0 to control the

flow of air to the optical windows of the probe. The program could

be run so as to acquire 1000 samples per second, calculate the

average and display the value as ‘‘Offset/Maximum voltage’’.

Alternatively, the program could be run to acquire data for a period

specified by the operator and display the actual sample period and

a graphical plot of the acquired voltage signal as a function of time.

Finally, provision was made for transfer of the acquired voltage

signals to a file that can be accessed through any spreadsheet

application software such as Microsoft EXCEL.

3.4. Experimental procedure

A pre-weighed amount of dust was charged to the storage

canister with the appropriate dispersion nozzle (rebound or

perforated annular nozzle) in place. The chamber was sealed with

the bayonet closure and partially evacuated to 0.4 bar (a). At this

point, the LED switch of the dust probe was turned off (equivalent

to 0% transmission) to record the offset voltage. The LED switch was

then turned on and the maximum signal (corresponding to 100%

transmission) was recorded.

The sampling rate and number of samples to be acquired were

entered on the front panel of the LabVIEWD program and the toggle

switch (Acquire) turned on. The control program of the Siwek test

unit (KSEP 5.0d) was then initiated. The dust canister was pressurized to 20 bar (g) after which the LabVIEWD program was launched

to begin data acquisition. This was done to acquire data for some

time before the start of dispersion. The outlet valve then opened to

allow the dust–air mixture in the dust storage canister into the

explosion chamber through the dispersion nozzle, thus raising the

pressure inside the chamber to 1 bar (a). The solenoid valve opened

simultaneously to allow for flow of air to the optical windows of the

dust probe. The sampling rate and number of samples acquired were

fixed at 1000 samples/second and 3000 samples, respectively. As

soon as the desired number of samples was acquired, the LabVIEW D

program displayed a dialogue box for the specification of a file name

to save the data. The toggle switch was then turned off, signaling the

end of data acquisition.

The file containing the acquired voltage signals was accessed via

Microsoft EXCEL. Then the electrical offset voltage was subtracted

from all the data. The acquired voltage signals were converted to %

transmission out as follows:

T1⁄2%]1⁄4ðVmeas=VmaxÞ$100 (3)

where Vmeas is the measured voltage with attenuation due to the

presence of dust particles and Vmax is the maximum voltage, corresponding to 100% transmission. The instant at which a drop in

transmission (from initial 100%) was first observed was taken as

point zero on the time scale. The times corresponding to the transmission data acquired prior to this point were assigned negative
Fig. 3. The perforated annular nozzle (from ASTM standard E1226).

Fig. 4. The rebound nozzle (from ASTM standard E1226).
values because they represent the data acquired before the start of

dispersion. Three replicate tests were performed for each of the

three dusts at five dust concentrations and six dust probe locations

with the two dispersion nozzles. Thus a total of 540 dispersion tests

were performed.

In some cases, particle size analyses of the dust samples prior to

and after dispersion were undertaken to obtain information about

the effect of the dispersion process on the particle size distribution

of the dusts. Therefore, for some selected dispersion tests, sufficient

time was allowed for the dust particles to settle before the bayonet

closure of the explosion chamber was opened after a test. The

settled dust was gently scooped out of the chamber after brushing

down the walls to remove any dust adhering to them. In some

cases, the outlet valve was removed so that all the settled dust was

easily collected from the base of the chamber.

A total of 24 post-dispersion particle size analyses were performed; 6 each for Gilsonite and purple K and 12 for Pittsburgh coal

dust. The dust concentrations selected were 175 g/m3 for Gilsonite,

250 and 350 g/m3 for purple K and 250 and 500 g/m3 for Pittsburgh

coal. For these dispersion tests, the rebound nozzle was used.

Fig. 5. Horizontal cross section of the Siwek 20-L chamber showing dust probe locations. A single probe was used in each test.

4. Results and discussion

4.1. Transmission-time traces

Typical transmission-time traces for 25 g/m3 Gilsonite with the

rebound nozzle at probe locations 1, 2 and 3 are shown in Fig. 6.

Fig. 6. Transmission-time traces for 25 g/m3 Gilsonite with the rebound nozzle at

probe locations 1, 2 and 3.

All

transmission-time traces showed similar trends. At the start of data

acquisition (50 ms before the start of dispersion), 100% transmission is obtained because there is no dust in the chamber and

hence no attenuation of the infrared radiation from the LED. A rapid

drop in transmission to some minimum value then follows as the

initial ‘‘lump’’ or ‘‘plug’’ of dust is ejected from the nozzle. Transmission then increases as the dust disperses throughout the

chamber, and it continues to fluctuate as the ignition delay time of

60 ms is approached.

Gilsonite is a cohesive and sticky dust that is injected into the

chamber as a dense cloud and then disperses, whereas purple K is

a specially coated and fluidized dust that flows easily and disperses

more quickly throughout the chamber. The important point to note

here is that the dusts differ in their surface properties, which can

affect their patterns of flow during dispersion. This makes it difficult

to generalize the description of the dust/air mixture pattern of flow in

the Siwek 20-L chamber from the transmission-time traces obtained

for the three dusts. Other researchers also experienced difficulties in

comparing dispersions of different dusts. As reported by Eckhoff

(1997), Schlapfer concluded that it was difficult to make meaningful

comparisons of dispersions obtained with different dusts. He

observed that dusts of different density, particle size and surface

properties are dispersed in different ways if exposed to the same air

blast conditions. In spite of these complications, some general

conclusions regarding dust dispersion in the Siwek 20-L chamber can

be deduced from the data, as described in the following sections.

4.2. Transmission at the ignition delay time

Due to rapid temporal fluctuations in the transmission data,

a 20-ms time interval centred about the 60-ms ignition delay time

used in the Siwek 20-L chamber (i.e., 50–70 ms) was selected to

determine the transmission at the time of ignition. The point zero

on the time axis of the transmission-time traces does not necessarily correspond to the exact start of dispersion but rather the time

that dust particles reach the probe as indicated by a sudden drop in
transmission from the initial value of 100%. Thus, point zero of the

time axis shifts a little to the left. Second, in an explosion test,

the igniters are activated electronically 60 ms into the dispersion

process, and the igniter flame lasts about 10 ms. Several time

intervals were considered (50–70, 50–80, 55–75 and 55–65 ms);

however, there were no significant differences in the average

transmissions (3% at the most). Hence the 50–70 ms time interval

was selected. Table 3 shows the average transmissions and standard

deviations in transmission, measured at all dust probe locations, at

the time of ignition for Gilsonite, purple K, and Pittsburgh coal,

for all dust concentrations and the two dispersion nozzles.

Table 3

Average transmissions and standard deviations in transmission at the time of

ignition for Gilsonite, purple K and Pittsburgh coal dust with the rebound (R) and

perforated annular (A) nozzles.

Gilsonite

Dust Concentration [g/m3

]

Transmission [%]

25 50 100 150 175

Dust probe location

1

2

3

4

5

6

Mean transmission

Standard deviation

R

46

46

49

32

41

54

45

8

A

48

35

32

49

47

40

42

7

R

31

32

32

29

32

32

31

1

A

35

34

34

39

35

32

35

2

R

11

20

15

15

17

12

15

3

A

19

18

19

20

18

21

19

1

R

5

9

9

6

8

4

7

2

A

8

11

9

15

9

12

11

3

R

3

4

7

5

5

2

4

2

A

4

6

6

10

10

12

8

3

Purple K

Dust Concentration [g/m3

] 50 100 150 250 350

Dust probe location

1

2

3

4

5

6

Mean transmission

Standard deviation

R

61

60

57

58

60

62

60

2

A

64

61

63

57

66

65

63

3

R

40

39

41

42

42

42

43

1

A

46

44

38

48

37

47

43

5

R

27

29

25

30

25

28

27

2

A

27

30

29

22

22

33

27

4

R

8

5

7

9

9

13

9

3

A

10

8

7

7

7

13

9

2

R

2

0

0

3

0

1

1

1

A

0

1

0

1

0

4

1

2

Pittsburgh coal

Dust Concentration [g/m3

] 50 100 150 200 250

Dust probe location

1

2

3

4

5

6

Mean transmission

Standard deviation

R

56

56

51

55

60

60

56

3

A

65

63

61

66

69

57

64

4

R

41

43

41

48

46

45

48

3

A

51

44

46

54

50

44

48

4

R

33

28

28

37

39

38

34

5

A

39

39

31

37

39

35

37

3

R

25

27

23

29

33

34

29

4

A

30

27

27

30

25

39

30

5

R

21

25

30

28

26

26

26

3

A

27

23

21

25

24

29

25

3

In the

table, each of the data values listed for a particular probe position

and dust concentration is the average of three replicate tests.

Repeatability refers to the run-to-run variations among replicate

tests under the same experimental conditions. The % deviations

of the transmission at the time of ignition obtained from each

experimental run from the average value over three replicate tests

were calculated. The transmission at the time of ignition for 98 of

the 540 experimental runs exceeded 20% deviation. However, the

majority of the deviations outside this range were obtained for high

dust concentrations of Gilsonite and purple K, where the transmission was very low. These low transmissions account for deviations as large as 50% or 100%. For example, a 1% transmission at the

ignition delay time obtained from one run gives a deviation of -50%

if the average transmission at ignition obtained over three replicate

tests was 2%. Nevertheless, 82% of the 540 experimental runs were

within a 20% deviation and it is concluded that the repeatability of

the transmission measurements was reasonable. If only the transmission data greater than 10% are considered, 90% of 414 runs were

within 20% deviation.

4.3. Variations in spatial uniformity of dispersion for

rebound and perforated annular nozzles

For a perfectly uniform dust cloud, there should be no variation

in local dust concentration throughout the chamber volume at the

time of ignition. From Bouguer’s law, Eq. (1), we know that transmission is a measure of dust concentration; hence for a perfectly

uniform dust cloud there should be no variation in the transmission

at the time of ignition obtained at different locations throughout

the chamber volume. However, the transmission is also a function

of the particle size and, therefore, the agglomeration of the dust at

different locations.

Fig. 7 shows a semi-logarithmic plot of transmission at the time

of ignition as a function of the nominal dust concentration for

Gilsonite at all six dust probe locations with the rebound and

perforated annular nozzles. Each of the plotted data points is the

average of the three runs at one position and concentration (from

Table 3). Not all of the data points at the six locations can be seen

on the graph at each concentration, due to overlap of the data.

Transmission generally decreases with increasing dust concentration in accordance with Bouguer’s law. For both nozzles, the

transmission approached zero as the dust concentration increased

to 175 g/m3

. Further increases in dust concentration would give

zero transmission at the time of ignition and it is for this reason that

higher dust concentrations were not tested.

On the semi-logarithmic plot of Fig. 7, it appears that the greatest

variation is at the highest concentration. However, on an absolute

basis (see Table 3), the widest variation in transmission for both

nozzles was obtained at the lowest dust concentration tested (i.e.

25 g/m3

), which is not surprising because as the quantity of dust in

the chamber decreases so the probability of obtaining equal portions

of dust at different locations also decreases. There do not appear to

be any significant differences between the results obtained from the

two nozzles. Variations in spatial uniformity from both nozzles are

similar as indicated by the standard deviations in transmissions

obtained at the time of ignition at the different dust probe locations

for each dust concentration (see Table 3). The low standard deviations also indicate good uniformity of dispersion with both nozzles.

However, the rebound nozzle gave somewhat lower transmission

values than those of the perforated annular nozzle at the highest

Gilsonite concentrations tested. A possible explanation for this is

that the rebound nozzle is more efficient in breaking up agglomerates of this dust. With purple K dust (Fig. 8), transmission at the time

of ignition decreased with increasing nominal dust concentration at

all six probe locations until near-zero transmissions were obtained

at a dust concentration of 350 g/m3 for both nozzles. As in Fig. 7, each

of the plotted data points is the average of the three runs at one

position and concentration. Again, there were no significant differences in the transmission data at the various locations for the

two nozzles. Furthermore, low standard deviations in transmission

(Table 3) indicated that both nozzles gave good uniformity of

dispersion. Similar results were found for Pittsburgh coal dust (Fig. 9

and Table 3). For both the purple K and Pittsburgh coal, the transmission data for the two nozzles were essentially the same.

From the results obtained for the three dusts, it can be

concluded that good uniformity of dispersion was achieved using

either the perforated annular nozzle or the rebound nozzle for

dispersion. Also, there was no significant difference in the variation

in spatial uniformity from dispersion with either nozzle, despite

their differences in geometry and mode of operation. This is

consistent with the conclusions of Siwek (1988) and Eckhoff (1977)

that both nozzles produce approximately the same degrees of dust

dispersion. Siwek’s conclusion was based on high-speed motion

pictures of dust dispersion with the two nozzles in a Plexiglas

model of the chamber that allowed for visualization of the
dispersion process, similar to the method used by Eggleston and

Pryor (1967) for the 1.2-L Hartmann tube.

Figs. 7–9 and Table 3 also show that for each set of experiments

with the three dusts there is no specific pattern in the distribution

of the transmission data across the six dust probe locations. The

inference that can be made from this is that the dispersion obtained

with the two nozzles should not affect the rate of flame propagation in the Siwek 20-L chamber.

Fig. 7. Semi-logarithmic plot of Bouguer’s law (calculated using pre- and post-dispersion D32) and experimental transmission data as a function of dust concentration for Gilsonite.

Fig. 8. Semi-logarithmic plot of Bouguer’s law (calculated using pre- and post-dispersion D32) and experimental transmission data as a function of dust concentration for purple K.

The conclusion that there are no differences in the variations in

spatial uniformity obtained with the rebound and the perforated

annular nozzle was not simply based on visual inspection of the

results shown in Table 3. It was shown statistically through an

analysis of variance (ANOVA). ANOVA is a standard procedure for

making statistical inferences about the equality of two or more

population means. In this case, the two populations are the variations

in spatial uniformity obtained with the rebound and the perforated

annular nozzles (Table 3). Details of the test procedure can be found

in, for example, Montgomery and Runger (1999). The results showed

that the computed value of the test statistic (F) was smaller than the

critical value for the rejection region (Fcritical). Therefore the null

hypotheses of equality of the rebound and perforated annular nozzle

population means for the three dusts were accepted.

4.4. Predictions from Bouguer’s law

Figs. 7–9 show semi-logarithmic plots of the theoretical and

experimental transmission data as a function of dust concentration

for Gilsonite, purple K, and Pittsburgh coal dust. Two theoretical

transmissions were calculated from Eq. (2) using an extinction

efficiency Q 1⁄4 1.5 and path length L 1⁄4 0.038 m (distance measured
between the dust probe windows), from Cashdollar et al. (1981).

The particle densities used were r 1⁄4 1.05, 2.17, and 1.34 g/cm3 for

Gilsonite, purple K and Pittsburgh coal dust, respectively. The first

prediction (shown as the solid lines in Figs. 7–9) was calculated

using particle diameters of D32 1⁄4 18, 18, and 43 mm for Gilsonite,

purple K and Pittsburgh coal dust, respectively, obtained from the

pre-dispersion particle size analyses shown in Table 1. It can be seen

from Figs. 7–9 that the experimental transmission data of the three

dusts generally follow a linear relationship as expected from Bouguer’s law. However, the experimental transmission data for the

three dusts were significantly lower than their respective theoretical relationships (shown as solid lines in Figs. 7–9).

Fig. 9. Semi-logarithmic plot of Bouguer’s law (calculated using pre- and post-dispersion D32) and experimental transmission data as a function of dust concentration for Pittsburgh

coal.

There are several possible explanations for the significant deviations of the experimental data from the theoretical lines. First, the

theoretical lines as determined from Bouguer’s law are based on

a homogeneously dispersed dust cloud scenario, in which case the

nominal dust concentration is the true or actual dust concentration.

The transmission measurements at the various locations in the

chamber showed good uniformity (Table 3), and it is difficult to see

how the concentrations at these locations could all be greater than

the nominal value. Second, the extinction efficiency (Q) is a function

of particle size, wavelength of incident radiation and the complex

refractive index of the particles. For particles larger than the wavelength of incident radiation, it varies between 1 and 2 depending on

the type of light source and detector used. Based on experimental

measurements, the extinction efficiency for the dust probes is 1.5

with an uncertainty of ±0.2 (Cashdollar et al., 1981), which is

a relatively small uncertainty. An even larger uncertainty exists with

the surface-weighted mean diameter (D32), which may vary widely

for the same dusts as a result of differences in particle size measuring

techniques. Third, the air jet that sweeps across the optical windows

of the dust probe does not keep the windows totally dust free during

dispersion. Dust coatings on the optical windows of the probe absorb

light, which will lead to a decrease in transmission.

Finally, there is the possibility of the dust particle size distribution

changing during the dispersion process. The D32 values used in

computing the theoretical transmission data (solid lines in Figs. 7–9)

were determined from particle size analyses of the dust samples

prior to dispersion (Table 1). As discussed in section 2.2, it is possible

to make inferences about the degree of agglomeration during the

dispersion process by comparing D32 before dispersion, and D32

calculated from the transmission data during dispersion. However,

agglomeration can only increase the particle size, not decrease it.

Table 4 shows that the particle sizes for all three dusts calculated from

the transmission data are much smaller than their original sizes

before dispersion.

Table 4

Comparison of pre-dispersion dust size measured by particle size analyzer and size

calculated from experimental transmission data.

Dust Dust D32 from D32 calculated

Concentration particle size from transmission

(g/m3

) analyses (mm) data (mm)

Gilsonite 25–175 18 3–5

Purple K 50–350 18 3–5

Pittsburgh coal dust 50–250 43 6–12

The values of a (i.e. the degree of agglomeration/

de-agglomeration or break-up) calculated from Eq. (3) were about 0.2

for all three dusts. These values of a are significantly less than 1,

which implies that particles break-up during dispersion. It is unlikely

that the uncertainties in comparing the theoretical and experimental

transmission data discussed earlier in this section could account for

the very low values of a obtained. Further, the manufacturer of the

Siwek 20-L test apparatus warns that the combination of the outlet

valve and the dispersion nozzle might have a grinding effect on the

dust particles leading to particle size reduction in the course of

dispersion (Adolf Ku ̈hner, 1994).Therefore, the next logical step was

to measure the particle size of the dust collected after dispersion.

4.5. Comparisons of particle size analyses before

and after dispersion

The discussion presented here is based on results obtained using

the rebound nozzle. Since transmission data for the rebound and
perforated annular nozzles are similar with both being significantly

lower than the theoretical predictions (solid lines in Figs. 7–9), the

effect of particle size reduction with both nozzles is expected to

be comparable. From particle size analyses before and after

dispersion for the three dusts there was clear evidence of particle

size reduction resulting from the process of dispersion. Several

parameters can be used to characterize the particle size of a dust.

The surface-weighted mean diameter D32, is the most appropriate

for incorporation into Bouguer’s law because light scattering is

a surface phenomenon. Therefore, estimations of the degree of

particle size reduction were based primarily on comparisons

between D32 obtained before and after dispersion. However, estimations based on Dm, the mass median diameter, and D43, the mass

mean diameter, also give comparable results.

From particle size analyses performed on four different portions

of the Gilsonite sample prior to dispersion, it was clear that intrasample variation in particle size was small. The post-dispersion

particle size analyses for six experimental runs at a concentration of

175 g/m3 also showed consistency from run-to-run. Using D32

values for comparison, the post-dispersion size was about 50% of

the original size for the Gilsonite, as listed in Table 5.

Table 5

Comparison of particle size analyses before and after dispersion for the three dusts.

Before dispersion After dispersion

Gilsonite

D32 (mm)

D43 (mm)

18

54

9

22

Purple K

D32 (mm)

D43 (mm)

18

44

15

35

Pittsburgh coal

D32 (mm)

D43 (mm)

43

76

17

40

Similarly, the

results of the particle size analyses for Pittsburgh coal dust before

and after dispersion showed consistency from run-to-run. For the

coal, there were six dispersions at 250 g/m3 and six dispersions at

500 g/m3

. Using D32 values for comparison, the post-dispersion size

was about 40% of the original size for the Pittsburgh coal, as listed in

Table 5. There was also no significant difference in the size analyses

after dispersions at the two dust concentrations.

However, in the case of purple K, there was evidence of particle

size reduction but not as significant as that obtained with Gilsonite

and Pittsburgh coal dust. The particle size analyses before dispersion showed small intra-sample variation in particle size and again

consistency was obtained in the post-dispersion particle size analyses from run-to-run. Using D32 values for comparison, the postdispersion size was about 80% of the original size, or alternatively,

the reduction in particle size was about 20%. Since Gilsonite and

purple K particle sizes are comparable in terms of fineness (Tables 1

and 5), and significant particle break-up was obtained with the

former but not the latter, it is suggested that purple K might be more

difficult to break or fragment than Gilsonite. Particle size analyses

for purple K before and after dispersion also indicated that the postdispersion values of D32 are essentially the same at the two different

dust concentrations (250 and 350 g/m3

) tested. This, along with the

coal data, suggests that the effect of dust concentration on the

extent of particle size reduction was negligible.

The next question was: what was causing the particle size

reduction – the dispersion nozzle or the outlet valve of the dispersion reservoir? To evaluate this, a dispersion test with Pittsburgh

coal was performed with the rebound dispersion nozzle removed.

The results were somewhat surprising because they showed that as

much particle size reduction occurred without the dispersion nozzle

as with the nozzle. This suggests that high velocity flow of the dust/

air mixture through the outlet valve, not the action of the dispersion

nozzle, is largely responsible for the reduction in particle size.

The outlet valve of the Siwek 20-L chamber is unique in its

design and differs from conventional ball or solenoid valves. It is

opened and closed pneumatically by means of an auxiliary piston,

which is enclosed in a valve casing attached to the opening at the

base of the chamber such that the piston of the outlet valve covers

the opening when the valve is in the closed position. When the

valve opens, the piston plunges downward to create an annular

space for the dust to flow into the chamber and returns to its

original position when the valve closes. It is the forced flow of the

dust/air mixture through the annular space existing between these

two concentric circles that leads to compaction or squashing and

grinding of the dust particles as they rub against the edges of the

piston. As a result, particle break-up occurs before the dust enters

the chamber through the dispersion nozzle. This may explain why

the extent of particle size reduction obtained with purple K was

much lower than that obtained with Gilsonite and Pittsburgh coal

dust. Purple K is a specially coated and fluidized dust that flows

easily. Therefore, friction between the dust particles and the

edges of the piston during flow is reduced and this leads to a corresponding decrease in the extent of particle break-up. Specific

details of the outlet valve are given elsewhere (Kalejaiye, 2001).

Therefore, to thoroughly evaluate the effect of particle size reduction in the Siwek 20-L chamber, pre- and post-dispersion analyses

should be made for tests with additional sizes and types of dusts

and correlated to a material frangibility parameter.

Using the post-dispersion surface-weighted mean diameters

(from Table 5) of 9, 15, and 17 mm obtained for Gilsonite, purple K

and Pittsburgh coal dust, respectively, new theoretical transmissions were calculated from Bouguer’s law. These are shown as

the dotted lines in Figs. 7–9. It can be seen that better agreement

exists between the experimental data and the post-dispersion

theoretical lines, especially for Gilsonite and Pittsburgh coal dust.

However, the experimental data are still lower than even the postdispersion theory lines.

4.6. Comparisons with transmission data from the PRL

20-L and Fike 1-m3 chambers

Figs. 10–12 are semi-logarithmic plots of experimental transmission data as a function of dust concentration for Gilsonite,

purple K, and Pittsburgh coal dust from the Siwek 20-L chamber

together with data previously reported from the PRL 20-L and Fike

1-m3 chambers (Cashdollar & Chatrathi, 1992) for the Gilsonite and

coal. The purple K data from the PRL 20-L chamber in Fig. 11 were

not previously published.

The theoretical lines displayed are those calculated for the current

work based on pre-dispersion values of D32 (i.e. the solid lines shown

in Figs. 7–9), since the particle size distributions of the Gilsonite and

Pittsburgh coal dust samples used in previous work (Cashdollar &

Chatrathi, 1992) are comparable to those used in the current work.

The PRL 20-L data for purple K in Fig. 11 were measured recently

using a sample from the same purple K that had been used for the

Siwek 20-L data. For Gilsonite (Fig. 10), the transmission data in the

PRL and Fike chambers generally followed a linear relationship as

expected from Bouguer’s law. Both showed good uniformity of

dispersion (i.e. small variations in spatial uniformity) comparable to

that obtained in the Siwek 20-L chamber. Note that the scatter in the

Siwek 20-L data is exaggerated by the semi-logarithmic plots. Similarly for purple K (Fig. 11) and Pittsburgh coal dust (Fig. 12), the

transmission data in the PRL and Fike chambers generally followed

a linear relationship. The PRL 20-L chamber data indicated good

uniformity of dispersion comparable to that of the Siwek 20-L
chamber at low dust concentrations. At the higher dust concentrations for the Pittsburgh coal, larger variations in dust probe transmission data were observed with the PRL 20-L chamber data. These

wide variations (or scatter of the data) are attributed to variations in

the agglomerated particle size of the air-dispersed dust. In particular,

PRL 20-L data shown in Fig. 11 were acquired over a period of many

years in which case there might have been variations in the actual

particle size distribution of the dusts, which could lead to variations

in the agglomerated particle sizes of the air-dispersed dust and hence

scatter in the transmission data. It should also be noted that the PRL

20-L and Fike 1-m3 data points were for individual dispersion tests

while the Siwek 20-L data points (Table 3 and Fig.12) were an average

of three tests at each concentration and location.

The Fike 1-m3 chamber transmission data (Figs. 10 and 12)

showed good uniformity of dispersion comparable to that of the

Siwek 20-L chamber. However, transmission data were obtained

using two PRL optical dust probes at fixed locations both in the

same hemisphere of the chamber and limited data points, relative

to the number of data points available from the PRL 20-L chamber.

Fig. 10. Semi-logarithmic plot of transmission (%) versus dust concentration (g/m3) data for Gilsonite from the Siwek 20-L and PRL 20-L chambers.

Fig. 11. Semi-logarithmic plot of transmission (%) versus dust concentration (g/m3) data for Pittsburgh coal dust from the Siwek 20-L, PRL 20-L and Fike 1-m3 chambers.

From Figs. 10–12 it is obvious that the Siwek 20-L transmission

data are significantly lower than the PRL and Fike transmission data

at all the dust concentrations tested. For Gilsonite (Fig. 10), the PRL
and Fike transmission data are slightly below the theoretical line,

unlike the Siwek 20-L data, where the deviations from the theoretical relationship are significant. It has been confirmed in the

preceding section of the current work that the significant reduction

in particle size obtained during dispersion in the Siwek 20-L

chamber was largely responsible for the wide deviations from the

theoretical relationship.

Fig. 12. Semi-logarithmic plot of transmission (%) versus dust concentration (g/m3) data for Pittsburgh coal dust from the Siwek 20-L, PRL 20-L and Fike 1-m3 chambers.

However, there is no clear evidence of particle size reduction in the

PRL and Fike chambers based on the transmission data. In a previous

investigation with the PRL 20-L chamber, Pittsburgh bituminous coal

was dispersed at concentrations of 400 and 600 g/m3

, and multiple

size analyses were performed for the dust before and after dispersion

(Cashdollar, 1996). The mean diameters D32 and D43 after dispersion

were 93% and 95% of their original values, respectively. This shows

that therewas almost no particle size reduction for the Pittsburgh coal

in the PRL 20-L chamber. There was evidence of particle break-up

during dispersion of TNT explosives dusts in the PRL 20-L chamber

(Cashdollar, Hertzberg, and Green, 1992; Hertzberg, Cashdollar, Zlochower, & Green, 1992), but this was for coarser sizes.

Figs. 10 and 12 show no significant differences between the PRL

20-L and Fike 1-m3 data implying that little or no particle size

reduction occurs in the Fike 1-m3 chamber during dispersion. The

level of turbulence generated in the Fike 1-m3 chamber is assumed

to be comparable to that of the ISO 1-m3 test vessel which, until

recently, was assumed to be comparable to that obtained in the

Siwek 20-L chamber. However, a recent study by Dahoe, Cant, Pegg,

and Scarlett (2001) has shown that significantly different turbulence levels occur in the two vessels at their respective ignition

delay times. The turbulence level in the Siwek chamber at 60 ms is

higher than the turbulence level in the ISO 1-m3 chamber at its

standard ignition delay of 600 ms. It is only at a later time of 200 ms

that the turbulence level in the Siwek chamber is comparable to

that in the ISO 1-m3 chamber. In addition, the dust was subjected to

a pressure of 32 bar (g) prior to dispersion in the Fike 1-m3 test

apparatus and 20 bar(g) in the Siwek 20-L chamber. The dispersion

nozzle of the Fike 1-m3 chamber is similar in design to the

perforated annular nozzle of the Siwek 20-L chamber. Surprisingly,

significant particle size reduction was obtained for Gilsonite and

Pittsburgh coal dust in the latter but not in the former. However, the

dispersion valve of the Fike 1-m3 chamber is a 19-mm pneumatically operated ball valve (Cashdollar & Chatrathi, 1992) that is

not expected to have any significant effect on the dust as it flows

through it. This strengthens the argument put forward in section

4.5 that the action of the outlet valve contributes the most to the

reduction in particle size obtained in the Siwek 20-L chamber.

Nevertheless, the effect of flow through the dispersion nozzle and

the high levels of turbulence generated are relatively minimal in

terms of particle size reduction.

5. Conclusions

This research evaluated the dust dispersion effectiveness in the

Siwek 20-L chamber, which is widely used for dust explosibility

studies. Dispersion tests were performed in the chamber with three

dusts, namely, Gilsonite, purple K and Pittsburgh coal using the

rebound and perforated annular dispersion nozzles. Transmission

data were obtained from six different locations in the chamber

using a PRL optical dust probe. Transmissions at the time of ignition

were obtained by averaging the transmission-time data over

a 20-ms interval centred about the standard ignition delay time of

the chamber (60 ms). Variations in spatial uniformity of dispersion

were determined by calculating the mean standard deviations of

the transmissions obtained at the time of ignition from the six dust

probe locations with both the rebound and perforated annular

nozzles. From these results, the following conclusions are drawn.

• Transmission generally decreased with increasing dust

concentration for the three dusts, as predicted from Bouguer’s

transmission law.

• Variations in spatial uniformity obtained with both nozzles were

found to be essentially the same for the three dusts. This statement

is consistent with the conclusions of Eckhoff (1997) and Siwek
(1988) that the rebound nozzle produces degrees of dust dispersion similar to those of the perforated annular nozzle. The low

standard deviations obtained showed that good uniformity of

dispersion was achieved for the three dusts with both nozzles.

• Comparisons were made between experimental transmission

data obtained for the three dusts in the current work and the

theoretical values calculated from Bouguer’s law, Eq. (1).

The experimental transmissions were significantly lower than

the theory for all three dusts at all the dust concentrations tested.

The surface-weighted mean particle diameters for the dispersed

dusts were calculated from the transmission data and compared

to the values obtained from the Malvern particle size analyzer

prior to dispersion. The results indicated that significant reduction in particle size had occurred during the course of dispersion.

• Particle size analyses were performed for the dusts before and

after dispersion to confirm this apparent size reduction. The

comparisons showed that the size after dispersion was about

50%, 80%, and 40% of the original size for Gilsonite, purple, and

Pittsburgh coal dust, respectively. The effect of dust concentration

on the extent of particle size reductionwas found to be negligible.

The surface-weighted mean diameters of the post-dispersion

dusts were then used to compute new theoretical lines from

Bouguer’s law. Better agreement was obtained between these

post-dispersion size values and the experimental transmission

data, especially for Gilsonite and Pittsburgh coal dust.

• Comparisons were made between the Siwek 20-L transmission

data of the current work and those obtained in the PRL 20-L

and Fike 1-m3 chambers for Gilsonite and Pittsburgh coal dust.

The variations in spatial uniformity observed from the transmission data of the Siwek, PRL and Fike chambers showed that,

in general, good and comparable uniformity of dispersion was

obtained in the three vessels. However, there was no evidence

of particle size reduction in the Fike and PRL chambers in

contrast with the Siwek chamber where significant reductions

in particle size occurred during dispersion.

• Particle size reduction in the Siwek 20-L chamber was attributed

to the unique design of its outlet (dispersion) valve and its

shearing action on the dust particles as they flow through. This

contention is strengthened by warnings from the manufacturers

of the Siwek 20-L test apparatus that the combination of the

outlet valve and the dispersion nozzle might have a grinding

effect on the dust particles, leading to a reduction in particle size.

However, the current work has shown that reduction in particle

size is largely due to the action of the outlet valve with the

contribution from the dispersion nozzle being minimal.

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