Fire Safety Journal, 12 (1987) 153 - 164 153

Initiation of G r a i n D u s t E x p l o s i o n s b y H e a t G e n e r a t e d d u r i n g Single Impact b e t w e e n S o l i d B o d i e s

GEIR H. PEDERSEN and ROLF K. ECKHOFF

Chr. Michelsen Institute, Department of Science and Technology, 5036 Fantoft, Bergen (Norway)

(Received March 3, 1987; in final form May 25, 1987)

SUMMARY

Several independent investigations o f past dust explosions in the grain, feed and flour industries in the U.S.A. and Europe arrive at the conclusion that 'friction sparks' may have been the initiator o f a substantial fraction, up to 50%, o f all the explosions recorded. How- ever, this conclusion has to a large ex ten t been based on indirect evidence, and essential de- tails o f the suspected ignition process most of ten remained unconfirmed.

In the present investigation a comprehen- sive series o f single-impact ignition experi- ments were carried out, by generating explo- sible dust clouds o f dried maize starch, in the region o f tangential impact between a moving body and a stagnant horizontal anvil.

The net energy o f the impact was defined as the loss o f kinetic energy o f the moving body during impact. The influence o f the impact energy on the frequency o f ignition was determined. I t was found that much higher energies were required for ignition at high velocities o f impact than at low ones. The reason is that any moving object colliding with a solid surface, reduces the ignition sensi- tivity o f the dust cloud in the vicinity o f the impact po in t by inducing turbulence.

Details o f the impact and subsequent igni- tion o f dust clouds were studied using a high- speed movie camera. The temperatures o f the metal sparks generated from a range o f differ- ent metals were measured by a four-wave- length optical pyrometer. Typical spark tem- peratures were ~ 1500 - 2700 °C for mild steel and ~ 2000 - 2900 °C for titanium.

The overall conclusion from the present investigation is as follows: steel sparks pro- duced by single impacts o f net energies o f up to 20 J, between steel and concrete, steel or rusty steel, are unlikely to ignite clouds o f

dried maize starch. In the case o f maize starch o f 10% moisture, ignition is unlikely even with titanium sparks. Maize starch is amongst the easiest ignitable dusts encountered in the grain, feed and flour industry. Also, most dusts found in practice will contain some moisture. Therefore, it seems unlikely that dust explosions involving dusts o f grain, feed or f lour can be initiated by heat from acciden- tal single impacts between tramp metal, and anvils o f metal, corroded metal, stone or con- crete, unless the net impact energies are much higher than 20 J.

1. BACKGROUND AND MOTIVATION FOR THE PRESENT WORK

Whether or not metal sparks or hot spots from single accidental impacts between solid bodies can initiate dust explosions, has re- mained a controversial issue for a long time. Several at temps [1 - 4] have been made at resolving the puzzle with reference to the grain, feed and flour industry by analysing past accidents with the objective to identify the ignition sources. A summary is given in Table 1.

As can be seen, 'friction sparks' are claimed to play a significant part. If it is further taken into account that it is of ten tacit ly implied that a substantial part of the 'unknowns ' may have been initiated by some untraceable sources such as metal sparks and electrostatic discharges, the friction spark becomes the most suspect of all the potential ignition sources.

The situations in which metal sparks can be generated in an industrial process plant fall into two main categories. The first is grinding and cutting operations, by which continuous, dense showers of sparks are produced. The second is single accidental impacts.

0379-7112/87/$3.50 © Elsevier Sequoia/Printed in The Netherlands

154

TABLE ]

Percentage of dust explosions in the grain, feed and food industry assumed t,) be initiated i)y 'friction sparks" or unknown sources

Ref. Number of % ignited by % unknown ';;~ friction Period explosions friction sparks sparks

+ unknown

1 535 20 46 66 1860-1973 1 128 17 27 44 1949-1973 2 91 54 18 72 1941 -1945 3 137 9 62 71 1958 -1975 4 83 28 5 30-35 1965 -1980

The ability of metal sparks or hot spots from grinding and cutting to ignite dust clouds has been demonstrated by several researchers. The experiments by Leuschke and Zehr [5], published in 1962, are probably the first ones in which dust clouds were ignited by grinding wheel metal sparks. However, no clouds of organic dusts ignited. Zuzuki et al. [6] ignited different coal dusts using both sparks and hot spots from a piece of steel in contact with a grinding wheel rotating at 23 - 47 m/s peri- pheral velocity, Later Allen and Calcote [7] conducted similar experiments in which metal sparks were generated by pressing a steel rod against a rotating grinding wheel. By retarding and focusing the spark stream, it was possible to ignite clouds of natural organic dusts such as maize starch and wheat grain dusts. An- other recent, and very comprehensive study is Ritter's [8] investigation. This work confirm- ed that metal sparks generated by grinding, rubbing and repeated impacts can ignite clouds of several dusts, including cellulose and lycopodium.

The second situation, in which metal sparks may be generated in industry, is accidental single impacts between solid bodies. In this case relevant evidence of the ignition poten- tial has been lacking. However, as long as the necessary conditions for initiation of dust explosions by such impacts remain unidenti- fied, one is forced to maintain the hypothesis that such sparks may be hazardous in general. This in turn forces industry to take precau- tions that may be superfluous, and causes fear that may be unnecessary.

This unsatisfactory state of affairs created the motivation for the present work which has been reported in greater detail elsewhere

[9, 10]. The specific objective has been to answer the following question: under what circumstances of impact energy, impacting velocity and type of moving object and anvil, can single impacts between solid bodies ini- tiate a dust explosion in a cloud of grain dust in air?

2. EXPERIMENTAL APPARATUS

2.1. Impact generator After having considered various principles

for constructing a single-impact generator suitable for laboratory work, it was decided to build the apparatus shown in Fig. 1. The length of the rigid, spring-loaded arm, the 'hammer' , from the centre of rotation to the tip of the test object, can be varied by means of an adjustment screw. The condition for obtaining impact is that the arm is slightly longer than the perpendicular distance from the arm axis to the anvil. By careful adjust- ment of this small difference it is possible to generate impacts of different impact energies without changing the impact velocity. The cylindrical tip of the test objects that impact- ed on the anvil material had a reduced diam- eter of 4 mm. Details of the end of the arm are shown in Fig. 2. Springs of different strengths were used for generating a range of impacting velocities of approximately 8 - 25 m/s. Fine adjustment of the velocity was pos- sible by adjusting the spring tension.

When performing an impact test, the arm is first twisted to the start position, whereby the helical tension spring is stretched. In this position the arm is locked by a pneumatically operated device that can be released electri- cally. After having been released, the arm

SPRING DRIVEN iMPACTING ARM "----~ INTER- CHANGEABLE METAL OBJECT

DUST ~ CLOUD

....~.

30 cm

Fig. 1. Sketch of the test rig.

.,,...I.- Atugm'hum profde - - 3 0 - 30,, 2

i

; / ~ljustment screw M16 ~ 1

Steel msert

/ ~ ~ Lock,rig nut

~l ~ Con~c~ tocWmcJ nut

Test object

Fig. 2. Detail of the arm of the test rig.

accelerates until it reaches its maximum an- gular velocity close to the 'six o'clock' posi- tion. In this position the spring exerts no force upon the arm and the velocity remains con- stant for a short period of time. At this point, impact occurs between the test object at the tip of the arm and the plane, horizontal anvil. The anvil is fixed in a strong vice mounted rigidly on the test rig. After impact the arm

155

will be further retarded by the spring, and subsequently arrested by an arm catcher.

2.2. Dust dispersion system Considerable work was carried out to devel-

op a suitable technique for generating the dust clouds to be used in the experiments. A dis- persion system based on a modified version of the dispersion cup of the Hartmann bomb [11] was adopted. The dispersion cup was covered by a cap in order to deflect the dust cloud being expelled from the cup, from the vertical to the horizontal direction. From the dispersion cup, the dust cloud flowed into a rectangular channel consisting of two parallel side walls and a bo t tom plate. The height of the side walls was 20 cm, the length 50 cm, and the spacing between them 12 cm. Hence, the volume of the dust cloud was approxi- mately 12 litres. The front side wall was made of transparent plastic, which made it possible to photograph the impacts and the ignitions.

In order to establish the combination of quant i ty of dust, dispersion pressure and delay time, that gave the opt imum conditions for ignition, the minimum electrical ignition ener- gy of the dust clouds generated in the 12-1 channel, was determined by means of cali- brated electric sparks. The various parameters were adjusted until an optimal combination that gave dust clouds of the lowest minimum ignition energy had been found.

2. 3. Dust concentration probes The concentration of the optimal dust

clouds as a funct ion of t ime and space was measured using a light at tenuat ion probe con- structed by The Chr. Michelsen Institute (CMI) on the basis of the work by the US Bureau of Mines [12]. The probe, which is shown in Fig. 3, is described in detail else- where [13]. By repeating the dust-cloud gen- eration process with the dust concentration

Fig. 3. Light attenuation dust concentration probe.

156

probe in various locations, the concentration development of the dust cloud with time, at any desired location, could be determined.

2.4. Timing system A microprocessor-based 4-channel timer

was used to control the system. The timer controlled the triggering of the impacting arm, dispersion of the dust, as well as high-speed movie camera and other photographic equip- ment.

2.5. Equipment for measuring the velocity of the arm and the impact energy

The kinetic energy of the arm at any instant was calculated from its mass distribution and instantaneous velocity. By measuring the velo- city of the arm immediately before and after the impact, i.e., immediately before and after '6 o'clock' position, the kinetic energy lost in the impact could be calculated. Two sets of reflective opto-switches were used for this purpose. The opto-switches were mounted on the wall of the test rig, with narrow clearance to the passing arm. A small mirror was fixed to the arm, and an electrical signal was gener- ated when the mirror passed the opto-switches.

2.6. Photographic equipment Different kinds of photographic equipment,

including ordinary single-frame cameras, video camera, 16 mm movie camera and 16 mm high-speed movie camera, were used. The high- speed camera (Hycam 4043020) was primarily used to make black and white negative film recordings at 1000 and 2000 frames per sec- ond. The films were developed and examined at CMI immediately after each experiment.

2.7. Temperature measurements by a four- wavelength optical pyrometer

In order to measure the temperature of the sparks, a four-wavelength optical pyrometer was used [14] (see 'Acknowledgements'). The method is based on the determination of the relative magnitude of the radiation inten- sities from the investigated solid body at four different wavelengths. The intensity ratios are compared with the radiation intensity ratios from black bodies of various temperatures. By identifying the black body temperature that gives the best fit, a best estimate of the 'colour temperature' of the solid body is obtained. The 'colour temperature' is close to the real

temperature of the body ff the spectral emis- sivity is approximately constant in the range of the spectrum of wavelengths used in the measurement.

2.8. Determination of minimum electric spark energies for ignition of dust clouds

These energies were determined by means of the method described by Eckhoff [15]. This method, which is based on using electric sparks of known energies (integration of power-versus-time curves), is essentially iden- tical with the new method proposed recently in W. Germany [16].

3. DUSTS USED IN THE EXPERIMENTS

Since maize starch is one of the agricultural materials that have often been involved in dust explosions in the U.S.A. and is one of the most sensitive to ignition, it had been de- cided to use this material in the experiments. Native maize starch supplied from Amylum, Aalst, Belgium, was used. On receipt the starch contained 10 - 11% moisture, which increased the minimum ignition energy con- siderably compared with that of dried starch. In order to ensure maximum sensitivity to ignition, the starch used in the majority of the present experiments was first dried for four hours at about 105 °C to remove the moisture.

A few other dusts of different minimum electric ignition energies (see Table 2), were used in order to investigate how the ignition probability in impact ignition tests ~}aried with the minimum electric spark ignition energy.

4. CONCENTRATION DISTRIBUTION AND ELECTRIC SPARK IGNITION SENSITIVITY OF THE EXPERIMENTAL DUST CLOUDS

4.1. Dust cloud concentration distribution The first phase of the experiments concen-

trated on formation of a suitable experimen- tal dust cloud of sufficient volume and low minimum ignition energy. This was achieved by dispersing 8 g of dried maize starch at 2 bar overpressure in the compressed air line. With 8 g of dust, the theoretical average dust con- centration of the cloud in the 12-1 channel was

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TABLE 2

M i n i m u m igni t ion energies and part ic le sizes of the dusts used in the e x p e r i m e n t s

Dust Min. ign. energy Part ic le (e lectr ic sparks) d i ame te r (m J) (/Jm)

Maize s ta rch , < 4.5 dried

Maize s ta rch , 27 - 36 10 - 11% mois tu re

L y c o p o d i u m < 4.5

Barley p ro te in ] 3

Barley s tarch 18 - 22

Barley f ibre 47 - 59

Wheat dus t 36 - 47

~5 - 20

~5 - 2 0

~ 3 0

DUSI D|SP::RSIO!~I CUe'

DU5~ C l ~ D

I 2 3 4 + + + +

t ANVIL

Fig. 4. Al te rna t ive loca t ions o f t he dus t c o n c e n t r a t i o n p robe dur ing c o n c e n t r a t i o n m e a s u r e m e n t s in t he ex- plos ion channe l .

667 mg/1. However, the dust concentration was not uniform. Therefore, the dust concen- tration development had to be measured as functions o f both t ime and location, using the optical dust concentration probe mounted in different locations of the cloud. The various positions of the dust concentration probe dur- ing these measurements are shown in Fig. 4. The results from the measurements are given in Fig. 5.

The dust concentrat ion measurements showed that at 120 - 300 ms after opening the solenoid valve, the dust concentration was most uniform. At this moment the dust con- centration varied from approximately 400 to 800 g/m 3. For this reason the timing system was always adjusted so the impact occurred at this point in time.

4.2. Ignition sensitivity for quiescent and turbulent dust clouds

The minimum ignition energies of the du.st clouds in the 12-1 channel were measured by

1oo I

g zoo I

'88o i o

i (

0.5 i , t , t • , i , i , ,

lo 1.5 ;~.0 TIME (r,.)

Fig. 5. Dust c o n c e n t r a t i o n d e v e l o p m e n t as a f unc t i on of t ime at loca t ions 1 to 4 (see Fig. 4). S m o o t h e d average curves based on th ree or more repea ted mea- s u r e m e n t s at each loca t ion .

means of calibrated electric sparks. The elec- trodes were mounted horizontally a fraction of a miUimetre above the surface of the plain anvil, at the same location as a hot spot would be expected to occur and metal sparks be gen- erated in an impact test. The electric spark was passed 120 - 300 ms after starting the dust dispersion. Both dried maize starch and lycopodium ignited with 100% ignition prob- ability even at spark energies as low as 4.5 mJ.

However, any object travelling through a dust cloud at a high velocity will create tur- bulence in its vicinity. This is also the case in the test rig when the arm passes through the dust cloud. It would be expected, therefore, to be more difficult to ignite the dust cloud when the arm is passing than when the cloud is comparatively quiescent.

The influence of the moving arm on the ignition sensitivity of the dust cloud in the vicinity of the point of impact was studied by means of electric sparks of known energies. The spark electrodes were again mounted at the same point as a hot spot would be expect- ed to occur in case of an impact test. However, in these tests the arm used to generate impact sparks was passed through the dust cloud as it would be in an impact test, only with a small clearance to the electric spark electrodes. The electric spark was discharged 0.3 - I0 ms after the moving arm had passed the '6 o 'clock' position. Three different peripheral velocities of the arm were used, viz., 9.5, 15 and 22 m/s.

Results showing probabilities of ignition of clouds of lycopodium as functions of electric spark energy at different arm velocities and delay times, are summarized in Fig. 6. When

158

1.0

0.9

O.B

0.'/

~ 0.6

E, o.s >,-

O.l.

g o.~ c )

R~ 0.2

0.1

I 5m/,~

0.1 I 10 100 1000

SPARK ENERGY , mJ

Fig. 6. Effec t of impac t veloci ty on igni t ion sensi- t ivi ty of l ycopod ium. Delay be tween impac t and trig- gering o f the electr ic spark is 0.3 - 10 ms. Envelopes enclose the e x p e r i m e n t a l points .

the peripheral velocity of the arm was 22 m/s, the electric spark energy necessary to ignite the turbulent dust cloud was two orders of magnitude higher than the 4.5 mJ necessary to ignite the quiescent dust cloud. At 9.5 m/s the electric spark energy required for ignition was approximately one order of magnitude higher. This relative ranking of the ignition sensitivities of the clouds only applies when ignition occurs close to the point of impact. For spark~luster ignition away from the impact point, it would be necessary to perform separate electric spark ignition tests with the spark gap at the location where ignition takes place, and with the appropriate delay between passing the impacting arm and firing the elec- tric spark.

5. T E M P E R A T U R E S OF M E T A L S P A R K S FROM

M E C H A N I C A L IMPACTS A N D G R I N D I N G

Temperatures of single-impact sparks from ordinary mild steel (St37), alloy steel and tita- nium were measured by means of the four- wave optical pyrometer. The anvil materials used were rusty steel and concrete. For comparison, temperatures of sparks from mild steel in contact with a rotating grinding wheel were also measured. In the case of the single impacts the temperatures were measured on sparks 40 ram 'downstream' of the impact point and 10 mm above the anvil surface.

Temperatures of t i tanium sparks were also measured 140 mm and 240 mm downstream of the impact point. The arm velocity just prior to impact was 24 m/s and the net impact energy about 20 J in all the experiments. In the case of grinding-wheel sparks, tempera- tures were measured 40 mm downstream of the contact point between the steel rod and the grinding wheel. The peripheral velocity of the grinding wheel was 29 m/s.

The results are summarized in Table 3. Ap- proximately 10 tests were carried out for each combination of test object and anvil material, using different combinations of detecting wavelengths in the pyrometer.

T A B L E 3

Resul ts f rom the spark t e m p e r a t u r e m e a s u r e m e n t s by the four -wavelength p y r o m e t e r

Test ob jec t Anvil mater ia l Spark t empe ra tu r e s C~c)

T i t a n i u m Rus ty steel 2030 - 2900

Mild steel, S t37 Rus ty steel 1730 - 2380

Alloy steel Rus ty steel 1530 - 1980

Mild steel, S t37 Concre te 2530 - 2730

Mild steel, S t37 Ro ta t ing 1600 - 1730 gr inding wheel

The results show that the highest tempera- tures were measured on sparks produced from impacts of t i tanium against rusty steel, and mild steel against concrete. The temperatures of t i tanium sparks at 140 - 240 mm down- stream of the impact point were generally higher than at 40 mm. There was a clear ten- dency of increasing temperature with increas- ing length of travel, i.e., increasing burning time.

The measured temperatures of sparks from mild steel against concrete were also high. At 40 mm from the point of impact, some steel spark temperatures were even higher than some ti tanium spark temperatures. However, the boiling point of t i tanium is higher than that of iron, indicating that on average the ti tanium sparks should attain higher tempera- tures than sparks from steel. This was also observed.

The results from the tests with mild steel as spark-producing material showed a large

variation in the spark temperatures, depend- ing upon the anvil material. The lowest spark temperatures were obtained in the tests with the rotating grinding wheel. The observed temperatures of 1600 - 1730 °C were very close to those for mild steels measured by Hardt [17 ], and also in agreement with Wahl's [18] t w o ~ o l o u r pyrometer measurement of temperatures of steel sparks from grinding wheels.

The temperature measurements showed that impact sparks from mild steel attained higher temperatures when concrete was used as wall material than when rusty steel was used. This may in part be due to the heat conductivity of concrete being much lower than that of steel, causing more of the fric- tional heat generated during an impact to be retained in the particles. This may provide more favourable initial conditions for com- plete combust ion of the particles and hence contr ibute indirectly to a significantly higher spark temperature.

The observation that the temperatures of single-impact steel sparks were generally considerably higher than of steel grinding- wheel sparks, is interesting. One reason for this effect could be the substantial difference in the contact pressure between metal and anvil during spark formation (see next Sec- tion). A higher contact pressure could lead to transfer of more heat to the particles during their formation. Although the maximum pos- sible difference between the amounts of heat transferred to a particle is small compared with the maximum possible combust ion heat, it could significantly influence the extent to which the particle will oxidize completely. This, in turn, could result in significantly different spark temperatures.

6. ESTIMATION OF THE IMPACT PRESSURE

The sparks from single impacts be tween carbon steel and concrete or rusty steel, gen- erated in the present investigation, attained higher temperatures than the grinding wheel sparks. A possible explanation could be that the maximum compression pressure in the impact apparatus was much higher than the pressure of the metal against the grinding wheel. Ritter [8] found that the spark tem- perature increases with increasing contact pressure to a certain point where the spark

159

temperature becomes constant and indepen- dent of the pressure. A similar observation was made by Allen and Calcote [7] who found that low pressure sparks were relatively small and irregular in shape. The irregular shape indicated that the sparks had not reached the melting point. The high pressure sparks were, however, spherical and coated with oxide, which means that the particles had melted and burnt.

In the present investigation, the maximum contact pressure was estimated by measuring the static force exerted on the anvil by the tip of the impacting arm, when the arm was left in the vertical position. The measurement was accomplished by mounting a mechanical load cell between the tip of the arm and the anvil. The force was distributed over a circular area of diameter 4 ram. The static pressure at the contact area as a function of excess in arm length is given in Fig. 7.

Pressure. [MPa)

200

100

025 050 075 100 T [mml

Excess Am Length

Fig. 7. Stat ic pressure at the contact area between the test object and the wal l mater ia l as a func t ion of di f - ference between arm length and perpendicular dis- tanee f rom arm axis to anvi l plane.

The dynamic pressures during impacts may be somewhat different from those measured under static conditions. It seems reasonable to assume, however, that the dynamic peak pres- sures in the present investigation were consid- erably higher than the contact pressures re- proted by Ritter [8] (10 - 15 MPa) and Allen and Calcote [7] {about 1 MPa). It is worth mentioning that the yield strengths of carbon and low-alloy steels are approximately 25 MPa.

7. INFLUENCE OF IMPACT ENERGY AND

IMPACT VELOCITY ON IGNITION FREQUENCY

The influence of the net impact energy and the peripheral velocity of the arm on the fre-

160

quency of ignition of maize starch by tita- nium sparks, was studied. The peripheral velocity of the arm was varied using different springs, whereas the net impact energy was varied by adjusting the excess arm length.

The results are given in Fig. 8. The frequen- cy of ignition for each combination of arm velocity and impact energy is based on obser- vations from 10 or 20 tests. The scatter (+1 std. dev.) is indicated. At constant velocity of approach, the frequency of ignition decreases systematically with decreasing impact energy, as would be expected. Low-energy impacts produce fewer sparks than high-energy im- pacts, and if the number of sparks is low, the ignition probability is also low.

10

0 9

0 8 ,

0 7

c 0 6

o

c, ~ 0 ~

0 3

02

0 t

~ m / s

I i i i i i i t i i

2 z. 6 8 10 12 ~z. 16 18 ~0

Impact e n e r g y

Fig. 8. F r e q u e n c y of igni t ion of c louds of maize s ta rch as ,~ f u n c t i o n of impac t energy at 16 m / s and 24 m/s, for impaet~ be t w een t i t a n i u m and rus ty steel.

The impacts of similar net energies, but of different velocities of approach, i.e., different turbulence levels, gave decreasing frequency of ignition with increasing turbulence. This is in agreement with the results from the experi- ments where the minimum electric spark ener- gy for ignition was determined when the arm was moving through the dust cloud at the moment of spark discharge (Section 4.2).

8. HIGH-SPEED FILMI N G

The 1000 - 2000 frames per second high- speed films confirmed that impacts with mild steel against rusty steel or concrete produced

very few sparks compared with t i tanium im- pacts. Typical numbers of sparks produced from mild steel were 5 - 50. Immediately after impact, no or very few sparks were visible. But 3 - 5 ms after impact, some sparks became visible, and after 10 - 20 ms the number of sparks visible on the f i l l was at maximum. Impacts with steel never generated a sufficient number of sparks to ignite clouds of dried maize starch.

The films of ignition by sparks from tita- nium against rusty steel showed that the dust clouds could be ignited in two different ways. (Reference 9 contains reproductions of some of the film pictures.) First, ignition frequently occurred very close to the point of impact, immediately (i.e., 0 - 5 ms) after the impact had taken place. At that moment an extremely luminous hemispherical 'pocket ' close to the impact point, with a radius of a few centi- meters and containing 100 - 1000 sparks, was observed. In addition to the great number of discrete particles, a ' thermite ' flash probably also contributed to the formation of the lu- minous pocket. Impacts of t i tanium against concrete also generated high spark densities close to the point of impact, but not the addi- tional continuous, luminous phase as was seen with impacts against rust. Flame propagation started somewhere in the region of the 'lumi- nous pocket ' , but could neither be referred specifically to one single metal particle nor to the hot spot generated at the point of impact. All the high-speed films of ignition of maize starch by t i tanium impacts against rusty steel showed that ignition and flame propagation started in this region.

The second mode of ignition was only ob- served in some of the experiments with lyco- podium. In this case ignition occurred 10- 50 ms after impact at a distance of 10 - 30 cm downstream of the point of impact. In this region the number of sparks per unit volume of dust cloud was considerably lower than in the 'luminous pocket ' close to the point of impact. But still it was not possible to trace the ignition back to any single metal spark. 'Explosion' and fragmentation of single metal sparks were observed frequently, but never seemed to cause ignition of the dust cloud.

The high-speed films suggest that a cluster of a large number of sparks is necessary to cause ignition of a cloud of grain dust. Im- pacts with t i tanium as spark-producing

material generated such conditions, and usual- ly resulted in ignition.

9. RESULTS FROM IGNITION TRIALS WITH DIFFERENT DUST CLOUDS AND IMPACTING MATERIALS

A series o f impact tests with various mate- rials was carried ou t in clouds of dried maize starch in order to identify combinations of impacting materials that would cause ignition. The anvil materials used comprised typical construct ion steels with either fresh or rusty surfaces, and different concrete ~mples . Sup- plementary tests were conducted with anvils of different minerals and grinding wheels.

The steel alloys used in the experiments were obtained f rom the CMI workshop. Pre- cise names or composit ions of the alloys were no t available, but they are likely to be repre- sentative of the range of steels in common use. The combinations of steel test objects and anvil materials investigated, are given in Table 4.

TABLE 4

Combinations of steel test objects and anvil materials used in the experiments

Mild steel against -- mild steel (fresh surface or rusty surface) -- concrete (incl. two samples supplied by NGFA) - - minerals (granite, gneiss, s'andstone, quartz) - - grinding wheel

Acid resistant steel, alloyed steel, hardened steel or stainless steel against

-- mild steel -- concrete -- granite -- grinding wheel

The peripheral velocity of the arm in the test rig just before impact was 24 m/s. The calculated average impact energy was about 20 J. After each test, the anvil was displaced slightly, so that every impact took place on fresh anvil surface.

The experiments revealed tha t neither mild steel against concrete, nor any o ther combina- tions o f steel objects and anvil materials, pro- duced any sign of ignition of the dried maize starch cloud.

From in t roductory tests with concrete as anvil material, it was found that the nature of the surface was of decisive importance for

161

the spark format ion process. Impacts by car- bon steel against a 'fresh' surface containing a thin layer of cement, produced no visible sparks at all. However, if the cement layer was removed, so that the impact took place on the gravel or crushed stone fract ion of the concrete, visible sparks were produced. For this reason experiments using concrete as anvil material were always carried out after removal of the cement layer.

Ignition experiments with impacts between t i tanium and several anvil materials were also carried out . The combinat ions of materials used, and the results f rom the tests, are given in Table 5. Dust clouds of dried maize starch were used in all tests.

TABLE 5

Results from the experiments with impacts between titanium and different anvil materials

Titanium against Ignition ?

-- rusty, mild steel Yes -- concrete Yes -- sandstone Yes - - granite, gneiss, quartz No - - grinding wheel No

The experiments showed that visible sparks were produced f rom all combinations of im- pacts between t i tanium and anvil materials. The number of sparks produced depended upon the surface tex ture of the anvil material. A rough surface produced more sparks than a smooth surface, and ignitions were only ob- tained f rom impacts against anvil materials having a relatively rough surface.

The importance of the surface properties of the anvil material was emphasized during the experiments with t i tanium against rusty steel. Surfaces apparently covered by the same kind of rust, would in some cases pro- duce large numbers of powerful sparks tha t ignited the dust cloud, whereas in o ther cases considerably fewer sparks were produced and no ignition occurred.

A few experiments with zirconium as spark- producing material were also carried out. Rusty steel was used as anvil material. The impacts produced more sparks than t i tanium against rusty steel, and ignition of clouds of dried maize starch were easily obtained.

162

It is worth mentioning that at the very be- ginning of the present investigation it was attempted to generate impact sparks by strik- ing a test object of standard quality alumi- nium against a rusty steel anvil. However, no metal sparks or any other luminous thermal reaction could be observed, only a thin smear of aluminium was deposited on top of the rust at the point of impact. This is contrary to the frequent assumption that impacts of aluminium against rust will generate energetic thermite sparks. It should be added, though, that thermite sparks may be generated if such a smear of aluminium on rusty steel is subse- quently struck by some other object.

In order to produce more steel sparks than those being produced by single impacts in the present test rig, a strong rotating cutting disk was pressed against a piece of low carbon steel. A continuous powerful shower of steel sparks was generated, but dust clouds of lycopodium and dried maize starch exposed to this heavy spark shower did not ignite. The reason for this has not been fully disclosed. It should be mentioned, though, that the number of sparks per unit volume of cloud was considerably lower than in the ' luminous pocket ' generated by t i tanium impacts.

Finally, the ignition sensitivity of some further agricultural dusts were compared with that of dried maize starch and lycopodium clouds, using sparks from impacts between titanium and rusty steel. The peripheral velo- city of the arm was 24 m/s and the net fric- tional impact energy approximately 20 J. The minimum electric ignition energy, Emin of the dusts were measured by calibrated electric sparks in the Hartmann tube. The results are given in Table 6.

The results show that the impact sparks were only able to ignite the dust clouds of the lowest minimum electric ignition energies. Apart from dried maize starch and lycopo- dium, ignition was only obtained with one dust, with Emin equal to ~ 13 mJ. Maize starch with 10 - 11% moisture and Emin 27 - 36 mJ did not ignite. This fairly consistent correla- tion of minimum electric spark ignition ener- gies and the frequency of ignition by impact sparks should probably not be generalized beyond the group of natural organic sub- stances to which all the dusts tested belong. In general, the relationships are likely to be more complex [8].

TABLE 6

Results from the ignition tests of dust clouds of dif- ferent 'minimum ignition energy'

Dust Minimum Frequency electric of ignition ignition in impact energy (mJ) tests (%)

Maize starch, dried .~. 4.5 100 Lycopodium .: 4.5 100 Barley protein 13 l 0 Barley starch 18 - 22 0 Maize starch, 27 - 36 0

10 - 11% moisture Barley fibre ,17 - 59 0

10. CONCLUSIONS

(1) Metal sparks or hot spots generated in single impacts between moving steel objects and anvils of steel, rusty steel, concrete or minerals failed to ignite clouds of dried maize starch in air over the entire range of experi- mental conditions. (Net impact energies of up to 20 J and velocities of approach from 10 m/s tp 25 m/s.) Clouds of dried maize starch have a minimum electric spark ignition energy of less than 5 mJ, and are probably among the most ignition-sensitive dusts encountered in the grain, feed and flour industries.

(2) However, clouds of dried maize starch could be ignited by single impacts of t i tanium on an anvil of either rusty steel, concrete or sandstone, but if the maize starch contained about 10% moisture, it could not be ignited even by t i tanium impacts.

(3) Single impacts with steel as spark- producing material produced a very low num- ber of sparks as compared to the number pro- duced by t i tanium under the same impact con- ditions. The temperatures of individual steel sparks, however, could reach the same level as those of t i tanium sparks (~2500 °C). How- ever, the temperatures of grinding-wheel steel sparks ate considerably lower, of the order of 1500 - 2000 °C.

(4) Impacts of standard quality aluminium against rusty steel did not generate any sparks or any other luminous reaction at all, only a thin smear of aluminium on top of the rust. Impacts with harder aluminium-containing alloys were not investigated.

(5) In most cases, ignition by t i tanium sparks was observed very close to the point of impact. However, ignition was occasionally observed 10 - 30 cm downstream of the im- pact point. Ignition by one single metal spark was never observed. A fairly dense cluster of sparks seemed to be necessary for igniting the clouds of maize starch.

(6) Any moving object in the dust cloud reduces the ignition sensitivity of the cloud in the vicinity of the object by inducing turbu- lence. Experiments with t i tanium against rusty steel showed that at a given net impact energy, the ignition frequency dropped when the im- pact velocity increased. Thus, at a given net impact energy, objects generating low turbu- lence represent a greater ignition hazard than objects generating high turbulence.

(7) At a given impact velocity, the frequen- cy of ignition by t i tanium sparks decreased systematically with the impact energy, as would be expected.

(8) The microscopic nature of the anvil sur- face is decisive for the spark formation pro- cess. For example, impacts against the gravel or crushed stone fraction of concrete pro- duced considerably more sparks than impacts against a fresh concrete surface covered with cement.

(9) The overall practical conclusion of the present investigation is that up to net impact energies of 20 J, tangential accidental single impacts between various types of steel, and between steel and rusty steel or concrete, are unable to ignite clouds of grain and feed dust, or flour, even if the dusts are dry. Impacts of standard quality aluminium against rusty steel did not even generate any visible sparks. In the case of t i tanium or zirconium, the sparks produced may initiate explosions in clouds of dried dusts, but not in clouds of dust contain- ing 10% moisture or more.

ACKNOWLEDGEMENTS

CMI wishes to express its sincere gratitude to the National Grain and Feed Association (NGFA), Washington, DC, U.S.A., for the substantial financial support rendering this work possible, and for unfailing constructive advice and support throughout . This paper is a condensed version of a comprehensive report prepared by CMI for NGFA [9].

163

During his stay at CMI in the autumn of 1985, Dr. R. Klemens, Technical University of Warsaw, gave invaluable help, both theo- retically and experimentally, with the mea- surement of metal spark temperatures by a four-wavelength optical pyrometer borrowed from TU of Warsaw.

REFERENCES

1 H. de Goijer, Th. M. Groothhuizen and M. E. Reinders, Literature Investigation into the Dust Explosion Danger in Industries Storing and Pro- cessing Cereals and Flour, Report from Tech- nological Laboratory TNO, October, 1975.

2 D. J. Rasbash, Fires Involving Dusts, Technical Booklet No. 5, Fire Protection Association, London, 1949.

3 M. Verkade and P. Chiotti, Literature Survey of Dust Explosions in Grain Handling Facilities: Causes and Prevention, Energy and Mineral Re- sources Institute, Iowa State University, Ames, IA, 1976.

4 H. Beck and A. Jeske, Documentation Staub- explosionen, Analyse und Einzelfallderstellung, Report No. 4/82, Berufsgenossenschaftliches Institut fiir Arbeitssicherheit, St. Augustin, F.R.G., 1982.

5 G. Leuschke and J. Zehr, Ziindung yon Staub- lagerungen und Staub/LuftGemischen dutch me- chanisch erzeugte Funken, Arbeitsschutz, 6 (1962) 146.

6 T. Zuzuki, S. Takaoka and S. Fujii, The ignition of coal dust by rubbing, frictional heat and sparks, Proc. Restricted Int. Conf. of Directors of Safety in Mines Research, July 1965, Safety in Mines Research Establishment, Sheffield, England.

7 J. Allen and H. F. Calcote, Grain Dust Ignition by Friction Sparks, SMS-81-049, National Grain and Feed Association, Washington, DC 20005, 1981.

8 K. Ritter, Die Ziindwirksamkeit mechanisch erzeugter Funken gegeniiber Gas/Luft- und Staub/Luft-Gemischen, Dr.-Ing. Dissertation, Uni- versit/it Fridericiana, Karlsruhe, 1984.

9 G. H. Pedersen, Initiation of Dust Explosions by Heat Generated during Single Impact between Solid Bodies, Fire and Explosion Research Re- port No. SMS-86-055, National Grain and Feed Association, Washington, DC, 1986. (Prepared by Chr. Michelsen Institute, Report No 833310-2, December 1985.)

10 G. H. Pedersen and R. K. Eckhoff, Initiation of dust explosions by heat generated during single impact between solid bodies, Proc. 2nd Int. Colloquium in Dust Explosions, Jadwisin, Poland, November, 1986, Report No. 863351-1, CMI, 1986.

11 R. K. Eckhoff, Use of (alP/dr)max from closed- bomb tests for predicting violence of accidental dust explosions in industrial plants, Fire Safety J., 8 (1984/85) 159. (Paper presented at the First Int. Colloquium on Explosibility of Industrial

164

Dusts in Baranow, Poland, November 8 - 10, 1984. 12 K. L. CashdoUar, I. Liebman and R. S. Conti,

Three Bureau o f Mines Optical Dust Probes, Re- port o f Investigations No. 8542, U.S. Bureau of Mines, 1981.

13 R. K. Eckhoff, K. Fuhre and G. tt. Pedersen, Vented Maize Starch Explosions in a 236 m 3 Experimental Silo, Fire and Explosions Research Report No. ESV-86-070, National Grain and Feed Association, Washington, DC, 1986. (Pre- pared by Chr. Michelsen Institute, Report No. 843307-2, December, 1985.)

14 K. L. Cashdollar and M. Hertzberg, Infrared Pyrometers for measuring dust explosion temper- atures, Optical Eng., 21 (1) {1982) 82 - 86.

15 R. K. Eckhoff, Towards absolute minimum igni- tion energies for dust clouds?, Combust. Flame, 24 (1975) 53 - 64.

16 W. Berthold, Bestimmung der Mindestziindenergie yon Staub/Luft-Gemischen, Progress Report No. 134, VDI-Verlag, Diisseldorf, 1987.

17 L. Hardt, Temperaturme~ungen an Schleiffun- ken, Arbeitsschutz, 15 (1954) 430 - 436.

18 H. Wahl, Z. Angew. Phys., 12 {1960) 60 - 62.

Erratum

A simplified characterization of upholstered furniture heat release rates, by V. Babrauskas and W. D. Walton, published in Fire Safety Journal, 11 (1986) 181 - 192.

In eqn. (4), [style factor] = 1.0 for plain primarily rectilinear construction, not 1.5

as p r i n t e d on p. 1 8 9 at t he t o p o f c o l u m n

2.