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Influence of initial pressure on spark-ignited dust explosions
Short Communiwtions Influence
of initial pressure
on spark-ignited
dust explosjons
P. R. Amyotte, B. K. Baxter and M. J. Pegg* Department of Chemical Engineering, Nova Scotia, Canada B3J 2X4 “‘Centre for Energy Studies, Technical Scotia, Canada B3J 2X4
Technical
University
of Nova Scotia, Halifax,
University
of Nova Scotia, Halifax,
Nova
An investigation was conducted on the effect of initial pressure on lycopodium dust explosions in a 26 I test chamber. Maximum explosion pressures and maximum rates of pressure rise were measured over a range of initial pressures from 0.87 bar to 2.0 bar. Spark ignition was used for all tests. For each initial pressure, the peak values of maxi’mum explosion pressure and maximum rate of pressure rise were found to occur at a fuel equivalence ratio of 4.5. When considered as a function of initial pressure, the peak explosion pressure values displayed a linear and proportional variation. (Keywords:
explosion;
pressure burst; spark)
Combustible dusts such as flour. sugar, metals. plastics and coal are produced as end- or by-products in many industries. The risk of fire or explosion in the handling of such dusts has prompted considerable research effort in the areas of explosion prevention and mitigation. The dust explosion hazard has not been eliminated, however; several recent examples are given in the text edited by Hertzberg and Cashdollar I. Two of the parameters used to categorize the explosibility of a combustible dust are the maximum explosion pressure. P,,,. and the maximum rate of pressure rise. (dP/dj),,,. The design of vessels able to withstand explosion pressures requires a knowledge of P,;,, for the dust under consideration. and (dP/dr),,,,, is commonly used in the design of vents and explosion suppression devices. Values of these characteristics have been determined for many dusts. with the tests usually conducted at an initial pressure (pressure at the time of ignition) of 1 bar. In some situations, however. it is more appropriate to specify explosion parameters at pressures other than standard or near-standard atmospheric pressure: examples include drying processes and pneumatic conveyors operated at reduced and elevated pressures, respectively. There is. thus. a need for dust explosion data at lmttal pressures different from 1 bar. but there is also a lack of such data. Recent studies of initial pressure effects on dust explosions by Walther and Schacke’ and Wiemann3
were conducted with chemical ignitors of 5 kJ and 10 kJ energy as the ignition source. While ignition energies in the kJ range are available in many industrial plants. low-energy sparks also represent an ignition source hazard for dusts. It is also known that ignition source strength can affect dust explosion data obtained in closed-vessel tests. The aim of the present work was to investigate the influence of initial pressure on P,,, and (dP/dr),,, for closed-vessel, sparkignited dust explosions.
Experimental Lycopodium. a club moss spore. was chosen as the test dust primarily because of its uniform particle diameter (approximately 30 pm). This was verified by both scanning electron microscopy and particle size analysis. The ultimate and proximate analyses of the lycopodium used are given in Table I. The apparatus for conducting the explosion tests is shown schematically in Figure I. The explosion vessel is spherical and has a volume of 26 I. Prior to
Table 1 Lycopodium analyses
Component Carbon Hydrogen Nitrogen Oxygen Ash
09504230,‘90/020261-03 @ 1990 Butterworth
Proximate
Ultimate
Sulohur Received 9 August 1989
each run, the chamber was either partially evacuated or pressurized, depending on the desired initial pressure. Dust dispersion through a ring-shaped perforated tube was achieved by an air blast from a 1 I reservoir. The dispersing air pressure was set at 16.2 bar for the lowest initial pressure studied (0.87 bar) and was increased with increasing initial pressure, up to 17.3 bar for the highest initial pressure (2.0 bar). The purpose was to maintain a constant difference between dispersion pressure and initial pressure in an attempt to ensure that the driving force for dust dispersion was the same for all initial pressure+. Ignition was by a single spark, of stored energy 16.2 J, passed between two electrodes. A fixed time delay of 200 ms between initial dust dispersion and ignition was used in all tests. Pressure development over the course of an explosion was measured by a piezoelectric transducer mounted flush with the interior of the vessel. An oscilloscope was used to record the pressure-time traces from which values were obtained. of Pln,X and (dP/dr),,,
Mass % 66.90 9.26 0.95 21.64
0.12
Component Fixed Carbon Volatile Matter Moisture
Ash
Mass % 3.90 90.90 4.07
1.13 100.00
1.13 100.00
&Co. (Publishers) Ltd
J. Loss Prev. Process lnd., 7990, Vol3, April
261
Short
Communications
Results and discussion A preliminary set of tests was conducted with quiescent methane/air mixtures consisting of 9.5 vol% methane. This was done to verify the experimental procedure, since the influence of initial pressure on stoichiometric gas-phase explosions is wellestablished5. The data displayed the expected trend of linear and propvariations of P,,, ortional and (dP/df)m;,x with initial pressure over the moderate range of initial pressures investigated (0.87-1.27 bar). For dust eiplosions, Palmer6 comments that when the initial pressure is not atmospheric it is preferable to express dust concentration as mass of dust per unit mass of air. rather than using the more common expression of mass of dust per unit volume. A suitable description of dust concentrations at rcduced and elevated pressures is. therefore, the fuel equivalence ratio. Qfuel, which is defined as the actual fuel to air mass ratio divided by the stoichiometric fuel to air mass ratio. The stoichiometric ratio for lycopodium was calculated from the ultimate analysis in Table I to be 0.1009 kgfuel/kg air. The procedure followed during a set of dust explosion tests at different lmtlal pressures was to hold &, constant by varying the actual mass of fuel in proportion to the actual mass of air. A range of fuel equivalence ratios from 1 to 5 was covered to determine the value at which the peak values of P,,, and (dY/dt),,, occurred for each initial pressure. The results from this work are shown in Figure 2. Each datum point in Figure 2 corresponds to a value of @+uel= 4.5. The optimum dust concentration is the dust loading at which the maximum values of P,,, and were observed. The linear (dP/dr),,, and proportional variations of optimum concentration, dust and Pnl,X
262
J. Loss Prev.
Process
Ind.,
7990,
(dPbhm,x with initial pressure are in agreement with the results reported by workers who used highly energetic ignition sources in studying polymer powder and benzoic acid explosions’ and brown coal dust explosions). This dependence of the peak values on initial pressof PIna, and (dP/dt),,, ure is identical to that for gases. In the present work, the explanation for this lies in the high volatile content of lycopodium (Table I) and the dust flame propagation models of Hertzberg et al. i and Dixon-Lewis er ~1.~. Once devolatilization of a sufficient number of particles occurs, the flame characteritics are dominated by the volatiles. with combustion taking place primarily in the gas phase. The most probable reason that the maximum rate of pressure rise occurs at a fuel equivalence ratio of 4.5, is that at this dust loading the rate of devolatilization is sufficient to produce a near stoichiometric concentration of combustible volatiles. In a previous Investigation. Amyotte and Peggy have commented that one of the difficulties associated with the 1.2 I vertical tube apparatus (Hartmann bomb) is that ignition may occur at different initial pressures depending on the spark ignitability of the dust being tested. The measured explosion parameters and calculated fuel equivalence ratio for dusts which are difficult to ignite may thus be different from those expected at atmospheric pressure. Figure 3 shows results from the present study to demonstrate this point. For these tests, a constant dust loading of 0.42 kg m-j (10.9 g of lycopodium in a volume of 26 I) was used, along with a lower dispersion pressure of 7.9 bar due to the relatively small amount of dust being dispersed. The apparent decrease in explosion hazard with increasing initial pressure is actually caused by the decrease in fuel equivalence ratio as the inihal pressure increases. The data in
Figure2 optimum IdP/dt),,,
Vol3,
Influence of initial pressure on dust concentration, P,,. and of lycopodium
April
Figure3 Influence of initial pressure on (d P/d t),,. of lycopodium (constant dust loading)
Figure 3 also suggest that a concentration of 0.42 kgm-” represents an optimum fuel equivalence ratio of about 4.S at an initial pressure slightly less than 0.87 bar. Although the primary purpose here was to measure P,,, and (dP/dt),,,, some information was also gained on the tlammability of lycopodium at different initial pressures. Hertzberg et al. 1~ have suggested that the following flammability limit criteria be used for dust explosions: maximum explosion pressure ratio. G=2 and ~nli,X1 (dP/df).,,,V’/’ 2 1.5 harms-I. With these criteria, it was concluded that weakly propagating explosions occurred at initial pressures of 0.87 bar and 1.27 bar and a fuel equivalence ratio of unity (corresponding to concentrations of 0.10 kg m -? and 0.15 kg m -3. respectively). The measured values of T,,,.,~ Vi13 were 2.7 and and (dpldr),,,,, 2.6 bar ms-I at 0.87 bar and 2.9 and 5.9 barmsm’ at 1.27 bar. Concentrations of 0.10 kg m-3 and 0.15 kg m-’ are, therefore, close to the apparent lean flammability limits for lycopodium at initial pressures of 0.87 bar and 1.27 bar. respectively. These results suggest that the apparent lean limit for lycopodium may vary in direct proportion to the initial pressure. However, it is noted that at initial pressures of 1.67 bar and 2.0 bar and a fuel equivalence ratio of unity (corresponding to concentrations of 0.20 kgm-” and 0.23 kgm-3. respectively). no explosions were recorded despite repeated attempts at these conditions. The implication here is that there may be an ignitability restriction for lycopodium. which is dependent on the initial pressure. These are matters for which further work is required. It is possible that clarification could be gained by conducting apparent lean limit tests at different initial pressures using a range of ignition sources from low-energy sparks to high-energy chemical ignitors. The energetic ignition source tests would also provide insight into the initial pressure dependence of the true lean flammabihty limits for lycopodium.
Case History
Conclusions Over a range of initial pressures from 0.87 bar to 2.0 bar, the maximum values of P,,, and (dP/df),,, for spark-ignited lycopodium explosions varied in a linear and proportional manner with the initial pressure. This behaviour is similar to that for gasphase combustion and is explained by burning of the devolatilization products from the high-volatile content Iycopodium. The peak values of P,,, and (dP/dr),.x occurred at a constant fuel equivalence ratio of 4.5 for all initial pressures investigated.
References I
2
3
4
Acknowledgements The authors gratefully acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada and the Union Carbide Corporation.
5
6
K. L., and Hertzberg. M.. (Eds.) in ‘Industrial Dust Explosions’, ASTM STP 9%. American Socie’ty for
Cashdollar,
Testing and Materirlls, Philadelphia, PA, USA. 1987 Walther. C. D.. and Schacker H.. 5th Intl. Symp. on Loss Prevention and Safety Promotion in the Process Industries. Cannes. France, September 1986 Wiemann. W.. in ‘Industrial Dust Explosions’. ASTM STP 958. (Eds. K. L. Cashdollar and M. Hertzberg), American Society for Testing and Materials, Philadelphia. PA. USA. 1987, pp. 33-44 Amyotte. P. R.. Chippett. S. and Pegg. M. J. Prog. Eptergy Cornbusr. Sci. 1989.. 14.293 Nagy. J.. Scilcr. E. C., Corm. J. W. and Verakis. H. C . in ‘Explosion Development in Closed Vessels’. U.S. Bur. Mines R.I. 7507. I971 Palmer, K. N.. in ‘Dust Explosions and Fires’. Chapman and Hall. London. 1973
Hertzberg, M., Zlochower, 1. A. and Cashdollar, K. L., 21st Symp. (Intl.) Cornbust., The Combustion Institute, Pittsburgh, PA, USA, 1986, pp. 325-333 Dixon-Lewis, G.. Bradley. D. and ElDin Habik. S. Archivum Comburtionis 1987.7,85 Amyotte, P. R. and Pegg, M. J. J. Loss Prev. Process Ind. lY89.2, 87
Hertzberg, M.. Cashdollar, K. L. and Zlochower, I. A., 21st Symp. (Intl.) Combust.. The Combustion Institute, Pittsburgh, PA, USA, 1986, pp. 303-313
Nomenclature (dP/dO,,x rhaximum p Inil% “mex
Ol.,l V
rate of pressure rise (bars-‘) maximum explosion pressure (bar) maximum explosion pressure ratio (P,,, divided by initial pressure) fuel equivalence ratio vessel volume (m3)
Case Histories Some old case histories In 1954 the American Oil Company published a 74 page booklet. entitled ‘IIazard of Water’. It was followed by eight more booklets. on the hazards of air, electricity. steam, furnaces, light ends. start-ups and shut-downs and air, ammonia and ammonium nitrate plants and on ‘Engineering for Safe Operation’. the last appearing in 1964. The booklets were generously illustrated with diagrams. photographs and case histories. and. although in many cases more details would have been welcome. they brought out important lessons very forcibly. I ordered scores of copies for distribution to colleagues in ICI. though at the time I was a manager. not a safety adviser. If anyone can find copies in the drawers of retired colleagues or in libraries, they should treasure them. Here are a few case histories from the booklets. A 10 inch bore underground crude oil pipeline was being emptied for repair by blowing a pig along it with compressed air. The vapour-air mixture in the pipeline exploded and destroyed a 50 km Icngth. The explosion was a classic detonation. The flame front accelerated to detonation speed. burst the pipeline. blowing out the soil above, and then started again, making another hole in the ground = 51) m further on. A photograph shows bits of the pipeline sticking up out of the ground at regular intervals. The average speed of the flame front was 90 m s-l. The source of ignition is not stated. The lesson. of is keep air and flammable course. vapour apart. (From ‘Hazard of Air’.)
A new 13000 mz tank was being hydrostatically tested for the first time. Water ran out of the uncompleted foam lines and continued to run by siphon action. It washed away part of the foundations and damaged the base of the tank. (From ‘Hazard of Water’.) A 7000 mJ spheroidal crude oil tank was emptied, filled with water and then emptied again. The overflow nozzle was below the top of the vessel, and so a pocket of hydrocarbon gas remained at the top of the vessel. Oil was also trapped in various pockets. When the water level fell, air entered the vessel. the hydrocarbons were ignited (probably by pyrophoric iron sulphide), and the vessel was destroyed. The wave of water from the spheroid overflowed the hund wall and dented the top half of a large tank in the next bund. The recommendation in ‘Hazard of Water’ is to make sure that the overflow is at the high point of the vessel. This is acceptable for pressure vessels but not for ordinary low pressure storage tanks. These will be overpressured if the liquid level rises more than 8 inches (200 mm) above the roof-wall weld. and the overflow should be at or below this level. To prevent explosions all volatile hydrocarbons should be removed by steaming before admitting air. In general, when reading old accident reports we should look critically at the recommendations. Modern practice may be to go further, hut we can still learn from the events described. An explosion that destroyed an alkylation unit was attributed to dis-
solved oxygen. According to the report, LPG contains some dissolved oxygen which may be released into the vapour space of a storage vessel. The amount is usually too small to be hazardous. However, if the level of liquid in the vessel is low and then rises, the LPG vapour condenses quickly but the oxygen dissolves slowly (if at all) and an oxygen concentration high enough for an explosion may be produced. The source of ignition was ‘probably iron sulphide’. Iron sulphide is a common contaminant in crude oil tanks, hut it is surprising to find it in an alkylation plant. However, the alkylation process is not stated; it may have used sulphuric acid. Much of the advice in the booklets is familiar but it is still useful to he reminded of it by photographs of the results of ignoring it. We all know that we should make sure, before filling a vessel with water, that it will withstand the weight of the water. ‘Safe Ups and Downs for Refinery Units’ has photographs of a group of large oil refinery vessels before and after collapse. The tube side of a condenser was pressure tested with water, after cleaning. Without venting the water, the test crew started to pressure test the shell side with steam. The rise in pressure of the water blew off the head of the exchanger which landed 15 m away, causing serious injuries and damage. (From ‘Hazard of Steam’.)
Trevor
J. Loss Prev. Process Ind., 1990, Vol3, April
Kletz
263
of initial pressure
on spark-ignited
dust explosjons
P. R. Amyotte, B. K. Baxter and M. J. Pegg* Department of Chemical Engineering, Nova Scotia, Canada B3J 2X4 “‘Centre for Energy Studies, Technical Scotia, Canada B3J 2X4
Technical
University
of Nova Scotia, Halifax,
University
of Nova Scotia, Halifax,
Nova
An investigation was conducted on the effect of initial pressure on lycopodium dust explosions in a 26 I test chamber. Maximum explosion pressures and maximum rates of pressure rise were measured over a range of initial pressures from 0.87 bar to 2.0 bar. Spark ignition was used for all tests. For each initial pressure, the peak values of maxi’mum explosion pressure and maximum rate of pressure rise were found to occur at a fuel equivalence ratio of 4.5. When considered as a function of initial pressure, the peak explosion pressure values displayed a linear and proportional variation. (Keywords:
explosion;
pressure burst; spark)
Combustible dusts such as flour. sugar, metals. plastics and coal are produced as end- or by-products in many industries. The risk of fire or explosion in the handling of such dusts has prompted considerable research effort in the areas of explosion prevention and mitigation. The dust explosion hazard has not been eliminated, however; several recent examples are given in the text edited by Hertzberg and Cashdollar I. Two of the parameters used to categorize the explosibility of a combustible dust are the maximum explosion pressure. P,,,. and the maximum rate of pressure rise. (dP/dj),,,. The design of vessels able to withstand explosion pressures requires a knowledge of P,;,, for the dust under consideration. and (dP/dr),,,,, is commonly used in the design of vents and explosion suppression devices. Values of these characteristics have been determined for many dusts. with the tests usually conducted at an initial pressure (pressure at the time of ignition) of 1 bar. In some situations, however. it is more appropriate to specify explosion parameters at pressures other than standard or near-standard atmospheric pressure: examples include drying processes and pneumatic conveyors operated at reduced and elevated pressures, respectively. There is. thus. a need for dust explosion data at lmttal pressures different from 1 bar. but there is also a lack of such data. Recent studies of initial pressure effects on dust explosions by Walther and Schacke’ and Wiemann3
were conducted with chemical ignitors of 5 kJ and 10 kJ energy as the ignition source. While ignition energies in the kJ range are available in many industrial plants. low-energy sparks also represent an ignition source hazard for dusts. It is also known that ignition source strength can affect dust explosion data obtained in closed-vessel tests. The aim of the present work was to investigate the influence of initial pressure on P,,, and (dP/dr),,, for closed-vessel, sparkignited dust explosions.
Experimental Lycopodium. a club moss spore. was chosen as the test dust primarily because of its uniform particle diameter (approximately 30 pm). This was verified by both scanning electron microscopy and particle size analysis. The ultimate and proximate analyses of the lycopodium used are given in Table I. The apparatus for conducting the explosion tests is shown schematically in Figure I. The explosion vessel is spherical and has a volume of 26 I. Prior to
Table 1 Lycopodium analyses
Component Carbon Hydrogen Nitrogen Oxygen Ash
09504230,‘90/020261-03 @ 1990 Butterworth
Proximate
Ultimate
Sulohur Received 9 August 1989
each run, the chamber was either partially evacuated or pressurized, depending on the desired initial pressure. Dust dispersion through a ring-shaped perforated tube was achieved by an air blast from a 1 I reservoir. The dispersing air pressure was set at 16.2 bar for the lowest initial pressure studied (0.87 bar) and was increased with increasing initial pressure, up to 17.3 bar for the highest initial pressure (2.0 bar). The purpose was to maintain a constant difference between dispersion pressure and initial pressure in an attempt to ensure that the driving force for dust dispersion was the same for all initial pressure+. Ignition was by a single spark, of stored energy 16.2 J, passed between two electrodes. A fixed time delay of 200 ms between initial dust dispersion and ignition was used in all tests. Pressure development over the course of an explosion was measured by a piezoelectric transducer mounted flush with the interior of the vessel. An oscilloscope was used to record the pressure-time traces from which values were obtained. of Pln,X and (dP/dr),,,
Mass % 66.90 9.26 0.95 21.64
0.12
Component Fixed Carbon Volatile Matter Moisture
Ash
Mass % 3.90 90.90 4.07
1.13 100.00
1.13 100.00
&Co. (Publishers) Ltd
J. Loss Prev. Process lnd., 7990, Vol3, April
261
Short
Communications
Results and discussion A preliminary set of tests was conducted with quiescent methane/air mixtures consisting of 9.5 vol% methane. This was done to verify the experimental procedure, since the influence of initial pressure on stoichiometric gas-phase explosions is wellestablished5. The data displayed the expected trend of linear and propvariations of P,,, ortional and (dP/df)m;,x with initial pressure over the moderate range of initial pressures investigated (0.87-1.27 bar). For dust eiplosions, Palmer6 comments that when the initial pressure is not atmospheric it is preferable to express dust concentration as mass of dust per unit mass of air. rather than using the more common expression of mass of dust per unit volume. A suitable description of dust concentrations at rcduced and elevated pressures is. therefore, the fuel equivalence ratio. Qfuel, which is defined as the actual fuel to air mass ratio divided by the stoichiometric fuel to air mass ratio. The stoichiometric ratio for lycopodium was calculated from the ultimate analysis in Table I to be 0.1009 kgfuel/kg air. The procedure followed during a set of dust explosion tests at different lmtlal pressures was to hold &, constant by varying the actual mass of fuel in proportion to the actual mass of air. A range of fuel equivalence ratios from 1 to 5 was covered to determine the value at which the peak values of P,,, and (dY/dt),,, occurred for each initial pressure. The results from this work are shown in Figure 2. Each datum point in Figure 2 corresponds to a value of @+uel= 4.5. The optimum dust concentration is the dust loading at which the maximum values of P,,, and were observed. The linear (dP/dr),,, and proportional variations of optimum concentration, dust and Pnl,X
262
J. Loss Prev.
Process
Ind.,
7990,
(dPbhm,x with initial pressure are in agreement with the results reported by workers who used highly energetic ignition sources in studying polymer powder and benzoic acid explosions’ and brown coal dust explosions). This dependence of the peak values on initial pressof PIna, and (dP/dt),,, ure is identical to that for gases. In the present work, the explanation for this lies in the high volatile content of lycopodium (Table I) and the dust flame propagation models of Hertzberg et al. i and Dixon-Lewis er ~1.~. Once devolatilization of a sufficient number of particles occurs, the flame characteritics are dominated by the volatiles. with combustion taking place primarily in the gas phase. The most probable reason that the maximum rate of pressure rise occurs at a fuel equivalence ratio of 4.5, is that at this dust loading the rate of devolatilization is sufficient to produce a near stoichiometric concentration of combustible volatiles. In a previous Investigation. Amyotte and Peggy have commented that one of the difficulties associated with the 1.2 I vertical tube apparatus (Hartmann bomb) is that ignition may occur at different initial pressures depending on the spark ignitability of the dust being tested. The measured explosion parameters and calculated fuel equivalence ratio for dusts which are difficult to ignite may thus be different from those expected at atmospheric pressure. Figure 3 shows results from the present study to demonstrate this point. For these tests, a constant dust loading of 0.42 kg m-j (10.9 g of lycopodium in a volume of 26 I) was used, along with a lower dispersion pressure of 7.9 bar due to the relatively small amount of dust being dispersed. The apparent decrease in explosion hazard with increasing initial pressure is actually caused by the decrease in fuel equivalence ratio as the inihal pressure increases. The data in
Figure2 optimum IdP/dt),,,
Vol3,
Influence of initial pressure on dust concentration, P,,. and of lycopodium
April
Figure3 Influence of initial pressure on (d P/d t),,. of lycopodium (constant dust loading)
Figure 3 also suggest that a concentration of 0.42 kgm-” represents an optimum fuel equivalence ratio of about 4.S at an initial pressure slightly less than 0.87 bar. Although the primary purpose here was to measure P,,, and (dP/dt),,,, some information was also gained on the tlammability of lycopodium at different initial pressures. Hertzberg et al. 1~ have suggested that the following flammability limit criteria be used for dust explosions: maximum explosion pressure ratio. G=2 and ~nli,X1 (dP/df).,,,V’/’ 2 1.5 harms-I. With these criteria, it was concluded that weakly propagating explosions occurred at initial pressures of 0.87 bar and 1.27 bar and a fuel equivalence ratio of unity (corresponding to concentrations of 0.10 kg m -? and 0.15 kg m -3. respectively). The measured values of T,,,.,~ Vi13 were 2.7 and and (dpldr),,,,, 2.6 bar ms-I at 0.87 bar and 2.9 and 5.9 barmsm’ at 1.27 bar. Concentrations of 0.10 kg m-3 and 0.15 kg m-’ are, therefore, close to the apparent lean flammability limits for lycopodium at initial pressures of 0.87 bar and 1.27 bar. respectively. These results suggest that the apparent lean limit for lycopodium may vary in direct proportion to the initial pressure. However, it is noted that at initial pressures of 1.67 bar and 2.0 bar and a fuel equivalence ratio of unity (corresponding to concentrations of 0.20 kgm-” and 0.23 kgm-3. respectively). no explosions were recorded despite repeated attempts at these conditions. The implication here is that there may be an ignitability restriction for lycopodium. which is dependent on the initial pressure. These are matters for which further work is required. It is possible that clarification could be gained by conducting apparent lean limit tests at different initial pressures using a range of ignition sources from low-energy sparks to high-energy chemical ignitors. The energetic ignition source tests would also provide insight into the initial pressure dependence of the true lean flammabihty limits for lycopodium.
Case History
Conclusions Over a range of initial pressures from 0.87 bar to 2.0 bar, the maximum values of P,,, and (dP/df),,, for spark-ignited lycopodium explosions varied in a linear and proportional manner with the initial pressure. This behaviour is similar to that for gasphase combustion and is explained by burning of the devolatilization products from the high-volatile content Iycopodium. The peak values of P,,, and (dP/dr),.x occurred at a constant fuel equivalence ratio of 4.5 for all initial pressures investigated.
References I
2
3
4
Acknowledgements The authors gratefully acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada and the Union Carbide Corporation.
5
6
K. L., and Hertzberg. M.. (Eds.) in ‘Industrial Dust Explosions’, ASTM STP 9%. American Socie’ty for
Cashdollar,
Testing and Materirlls, Philadelphia, PA, USA. 1987 Walther. C. D.. and Schacker H.. 5th Intl. Symp. on Loss Prevention and Safety Promotion in the Process Industries. Cannes. France, September 1986 Wiemann. W.. in ‘Industrial Dust Explosions’. ASTM STP 958. (Eds. K. L. Cashdollar and M. Hertzberg), American Society for Testing and Materials, Philadelphia. PA. USA. 1987, pp. 33-44 Amyotte. P. R.. Chippett. S. and Pegg. M. J. Prog. Eptergy Cornbusr. Sci. 1989.. 14.293 Nagy. J.. Scilcr. E. C., Corm. J. W. and Verakis. H. C . in ‘Explosion Development in Closed Vessels’. U.S. Bur. Mines R.I. 7507. I971 Palmer, K. N.. in ‘Dust Explosions and Fires’. Chapman and Hall. London. 1973
Hertzberg, M., Zlochower, 1. A. and Cashdollar, K. L., 21st Symp. (Intl.) Cornbust., The Combustion Institute, Pittsburgh, PA, USA, 1986, pp. 325-333 Dixon-Lewis, G.. Bradley. D. and ElDin Habik. S. Archivum Comburtionis 1987.7,85 Amyotte, P. R. and Pegg, M. J. J. Loss Prev. Process Ind. lY89.2, 87
Hertzberg, M.. Cashdollar, K. L. and Zlochower, I. A., 21st Symp. (Intl.) Combust.. The Combustion Institute, Pittsburgh, PA, USA, 1986, pp. 303-313
Nomenclature (dP/dO,,x rhaximum p Inil% “mex
Ol.,l V
rate of pressure rise (bars-‘) maximum explosion pressure (bar) maximum explosion pressure ratio (P,,, divided by initial pressure) fuel equivalence ratio vessel volume (m3)
Case Histories Some old case histories In 1954 the American Oil Company published a 74 page booklet. entitled ‘IIazard of Water’. It was followed by eight more booklets. on the hazards of air, electricity. steam, furnaces, light ends. start-ups and shut-downs and air, ammonia and ammonium nitrate plants and on ‘Engineering for Safe Operation’. the last appearing in 1964. The booklets were generously illustrated with diagrams. photographs and case histories. and. although in many cases more details would have been welcome. they brought out important lessons very forcibly. I ordered scores of copies for distribution to colleagues in ICI. though at the time I was a manager. not a safety adviser. If anyone can find copies in the drawers of retired colleagues or in libraries, they should treasure them. Here are a few case histories from the booklets. A 10 inch bore underground crude oil pipeline was being emptied for repair by blowing a pig along it with compressed air. The vapour-air mixture in the pipeline exploded and destroyed a 50 km Icngth. The explosion was a classic detonation. The flame front accelerated to detonation speed. burst the pipeline. blowing out the soil above, and then started again, making another hole in the ground = 51) m further on. A photograph shows bits of the pipeline sticking up out of the ground at regular intervals. The average speed of the flame front was 90 m s-l. The source of ignition is not stated. The lesson. of is keep air and flammable course. vapour apart. (From ‘Hazard of Air’.)
A new 13000 mz tank was being hydrostatically tested for the first time. Water ran out of the uncompleted foam lines and continued to run by siphon action. It washed away part of the foundations and damaged the base of the tank. (From ‘Hazard of Water’.) A 7000 mJ spheroidal crude oil tank was emptied, filled with water and then emptied again. The overflow nozzle was below the top of the vessel, and so a pocket of hydrocarbon gas remained at the top of the vessel. Oil was also trapped in various pockets. When the water level fell, air entered the vessel. the hydrocarbons were ignited (probably by pyrophoric iron sulphide), and the vessel was destroyed. The wave of water from the spheroid overflowed the hund wall and dented the top half of a large tank in the next bund. The recommendation in ‘Hazard of Water’ is to make sure that the overflow is at the high point of the vessel. This is acceptable for pressure vessels but not for ordinary low pressure storage tanks. These will be overpressured if the liquid level rises more than 8 inches (200 mm) above the roof-wall weld. and the overflow should be at or below this level. To prevent explosions all volatile hydrocarbons should be removed by steaming before admitting air. In general, when reading old accident reports we should look critically at the recommendations. Modern practice may be to go further, hut we can still learn from the events described. An explosion that destroyed an alkylation unit was attributed to dis-
solved oxygen. According to the report, LPG contains some dissolved oxygen which may be released into the vapour space of a storage vessel. The amount is usually too small to be hazardous. However, if the level of liquid in the vessel is low and then rises, the LPG vapour condenses quickly but the oxygen dissolves slowly (if at all) and an oxygen concentration high enough for an explosion may be produced. The source of ignition was ‘probably iron sulphide’. Iron sulphide is a common contaminant in crude oil tanks, hut it is surprising to find it in an alkylation plant. However, the alkylation process is not stated; it may have used sulphuric acid. Much of the advice in the booklets is familiar but it is still useful to he reminded of it by photographs of the results of ignoring it. We all know that we should make sure, before filling a vessel with water, that it will withstand the weight of the water. ‘Safe Ups and Downs for Refinery Units’ has photographs of a group of large oil refinery vessels before and after collapse. The tube side of a condenser was pressure tested with water, after cleaning. Without venting the water, the test crew started to pressure test the shell side with steam. The rise in pressure of the water blew off the head of the exchanger which landed 15 m away, causing serious injuries and damage. (From ‘Hazard of Steam’.)
Trevor
J. Loss Prev. Process Ind., 1990, Vol3, April
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