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Analysis of an LPG Accidental Release
0957–5820/04/$30.00+0.00 # 2004 Institution of Chemical Engineers Trans IChemE, Part B, March 2004 Process Safety and Environmental Protection, 82(B2): 128–131
www.ingentaselect.com=titles=09575820.htm
ANALYSIS OF AN LPG ACCIDENTAL RELEASE M. DEMICHELA*, N. PICCININI and A. POGGIO SAfeR, Centro Studi su Sicurezza, Affidabilita` e Rischi, Politecnico di Torino, Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Torino, Italy
A
n industrial LPG storage accident due to the release of propane from a tanker is described in this paper and the sequence of events that led to the collapse of a storage tank is examined using simulation software. A model taking both released flows into account as a radiant thermal contribution in the presence of flames is presented. The progression of the accident is assessed through digitalization and processing of the film of the fireball produced by about 500 L of LPG, whose diameter was calculated as approximately 50 m. Keywords: LPG release; accident analysis.
INTRODUCTION
Maintenance procedures showed that the valve operating mechanism could be dismounted if the tank was loaded, due to the plug sealing. Thus the operator started to dismantle the valve’s operating mechanism with the aim of repairing it. The tanker was full and still connected to the hoses. The operator was not able to verify the valve sealing, because the on-board instrumentation only indicated the flow direction, while it was not suitable for verifying the pressure after the valve. When the last bolt was removed, the strong pressure inside the tank blew the mechanism out of the body of the valve, through which a jet of propane escaped and coldburned the worker’s face and hands. Part of the liquid propane discharged through the suction pipe and the valve underwent isenthalpic flash evaporation. The remainder formed an unconfined pool that also began to evaporate. These vapours formed a low, dense cloud whose propagation was influenced by buildings, in particular the bottling shop. The cloud initially spread over the loading= unloading yard and then passed along the NW branch of the factory in the direction of its offices and forecourt. The fire brigade was called and two fire engines arrived a few minutes later and stopped in the forecourt to try to disperse the cloud with nebulized water. About 20 min after the release had started, however, the explosive mixture that had been formed was ignited. The explosion destroyed the two fire engines and brought down part of the office building. The cloud of propane gave rise to a deflagration that struck the firemen and the employees who had intervened, and set the release of propane on fire. The pool started a ‘pool fire’, while the jet from the valve produced a jet fire at the bottom of the tank (shown as ‘jet fire bottom’ in Figures 2 and 3). The flames licked the back of the tanker and heated the large quantity of propane still present in its tank. The internal pressure rose and strained the shell of the tanker, which was now weakened by the very great heat.
Investigation of real accidents discloses information that can be used to improve the safety of similar installations. The complexity of the events involved, in fact, often means that a priori assessments of the consequences of an accident, including any domino effects, make use of over-simplified assumptions that cannot be verified (Baker, 1983). This paper describes the application of widely used models (‘EFFECTS’, TNO) to the information obtained from an accident that occurred at Paese (Treviso) on 15 March 1996, in which two persons were killed, five firemen were seriously injured and windows were broken within a range of about 300 m (VVF, 1996). DESCRIPTION OF THE ACCIDENT At 7 a.m., a 50 m3 road tanker carrying 18 tonnes of propane entered the depot. There were five railway tankers holding a total of 170 tonnes of propane in the vicinity, together with six 15 m3 tank trucks, another 50 m3 road tanker and storage tanks holding about 800 tonnes of LPG (Figure 1). At the transfer station the operator connected the hoses to the tank and began to discharge its contents. This, however, did not take place correctly, as the flow indicator on the plant showed that the outflow rate was low. The cause of the defect was traced to the insufficient opening of the liquid phase valve. The valve plug was stuck by a screw nut left in the pipe during previous maintenance intervention. This reduced the outflow and prevented the valve from completely closing and sealing the tank content. *Correspondence to: Dr M. Demichela, Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Corso Duca Degli Abruzzi, 24, 10129 Torino, Italy. E-mail: [email protected]
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Figure 1. Depot layout. Figure 3. Collapse of the first tank, with its two jet fires.
After about 15 min, part of the tank yielded and the upper part of the shell ceded at the welds between the cylindrical plating and the hemispherical base. The high internal pressure determined a very fast outflow that became a jet fire (shown as ‘jet fire top’ in Figures 2 and 3). The deflagration of the cloud may also have damaged the connections of one of the transfer stations, resulting in another jet fire that involved one of the 15 m3 tank truck holding about 0.8 tonnes of product and was completely destroyed by a BLEVE, resulting in the formation of a fireball (Figure 4). The windows of most of the surrounding houses were smashed and pieces of metal from the tank were hurled as far as 600 m. The firemen extinguished the many fires that had broken out in the factory and went on cooling the railway trucks and the undamaged tank trucks until the jet fire from the first tanker involved burnt itself out. ANALYSIS OF THE EVENTS The events leading to the yielding of the first tank were examined using the simulation models contained in TNO’s
Figure 2. Accident sequence.
EFFECTS Version 2.1 (TNO Department of Industrial Safety, 1996). Key variables evolution was studied over time through their step-by-step discretization. Mass and energy balance and liquid phase=vapour phase equilibrium equations as a function of temperature were applied for each step, as indicated in Figure 5. The solutions of these equations showed the temperature and pressure inside the tank. The mass discharged between one step and the next was deducted at each step. The propane flow has been modelled as a two phase release. The outflow as a function of the pressure in the tank was calculated for release via the 38 mm diameter opening left in the valve body after the removal of the operating mechanism, discharging in a 3 inch diameter pipe. The equivalent length considered took account of the load losses in the valve. The evaporation by isenthalpic flash involved about 30% of the flow. The evaporation rate of the pool formed by the remainder was also determined. The total mass vaporized and still present in the pool was thus calculated for each step. When the explosion took place, about 20 min after the release started, the discharged mass amounted to about 4200 kg, namely about 1800 kg in the cloud and a little less than 2400 kg in the approximately 16 m diameter pool.
Figure 4. BLEVE and explosion of the 15 m3 tank.
Trans IChemE, Part B, Process Safety and Environmental Protection, 2004, 82(B2): 128–131
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Figure 5. Simulation of the yielding of tank A.
Figure 6. Changes in the dimensions of the fireball.
The energy balance of the tank after the explosion must take account of the heat irradiated by the pool fire and the jet fire. The simulation showed that the propane in the pool was all burnt within about a minute and a half. Heating of the propane in the tank was thus mainly due to the jet fire, whose intensity was also magnified by the faster outflow caused by the increase in pressure, resulting in a loop that raised the pressure itself even faster. The tank had previously been tested at an internal pressure of 25 bar.
A study of the same accident by Dattilo et al. (1998), however, has shown that its loss of strength due to overheating meant that it probably yielded when the pressure rose to 17 bar. The simulation indicated that this value was reached after a little more than 15 min and when the tank still held about 8700 kg of propane. Collapse of the mantle produced the equivalent of a 50 cm diameter orifice through which the overpressure in the tank forced out the propane at more than 460 kg s 1. Some 30 s
Trans IChemE, Part B, Process Safety and Environmental Protection, 2004, 82(B2): 128–131
ANALYSIS OF AN LPG ACCIDENTAL RELEASE later, the pressure dropped to about that of the atmosphere and there were less than 2800 kg of propane still in the tank. These deductions are plotted in Figure 5(b–d). The initial release is not accompanied by any marked change in the pressure and temperature inside the tank, whereas the heat irradiated by the jet fire and the pool fire quickly increased both parameters and the outflow rate. Yielding of the tank was followed by an abrupt drop in both the pressure and the amount of propane in the tank, owing to its very fast discharge through the tear in the mantle. PROGRESSION OF THE FIREBALL Reconstruction of the events related to the BLEVE of the second road tanker is impossible because its contents and the way in which its pressure was increased are unknown. The fireball caused by the collapse of the second tank was partly included in a film of the events that followed the yielding of the first tank taken from a helicopter. Converting the film into digital form and processing the single pictures obtained from each of its frames allowed examination of its progression. The dimensions of the fireball were determined by using the length of a rail tanker (about 15 m; Figure 6) as a yardstick. Their changes over time were determined with reference to the 40 ms interval between one frame and the next. The results of the processing of the pictures are shown in Figure 6. The diameter of the fireball itself and its stem were constant at 50 m and about 17 m, whereas the height of its base from the ground increased from 30 to 45–50 m in just over 1 s. All these calculations are obviously approximate deductions from uncertain dimensions. CONCLUSIONS Looking for the accident causes, two main aspects have been identified: the erroneous maintenance procedure, that did not prevent LPG leakage, was conducted in an unsafe location; the plant instrumentation was inadequate, since it
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consisted only of a flow meter without any indication of pressure and thus it did not allow verification of the valve seal. Also the emergency procedures showed some lack of organization and equipment. If the fire engines were equipped to keep a greater safety distance from the tanks involved in the fire, their destruction could have been avoided. The sequence of events that burst the first tank is evident from this analysis of the way its internal temperature and pressure were influenced by what was happening outside. The outflow rate from the discharge orifice was governed by the inside pressure and this in turn was determined by the temperature of the propane. Deflagration of the cloud started a pool fire and a jet fire, provoking a flow of irradiated heat dependent on the release rate. The reciprocal links between the variables, however, resulted in a non-linear progression, whereas EFFECTS and other simulation models are designed for fixed accident scenarios. When the conditions vary, perhaps due to the formation of a loop as in this case, complete simulation requires deeper analysis of the time domain and the elaboration of an ad hoc model, as in Shebeko et al. (2000) and Simpson (2003). REFERENCES Baker, W.E., 1983, Explosion Hazards and Evaluation (Elsevier, Amsterdam). Dattilo, F., Rosa, L., Tiberio, A., Cusin, C. and Andriotto, E., 1998, Metodologie e analisi di rischio—studio della dinamica di un incidente, in Proceedings of VGR 98, Pisa. Shebeko, Yu.N., Bolodian, I.A., Filippov, V.N., Navzenya, V.Yu., Kostyuhin, A.K., Tokarev, P.M. and Zamishevski, E.D., 2000, A study of the behaviour of a protected vessel containing LPG during pool fire engulfment, J Hazard Mater, A77: 43–56. Simpson, L.L., 2003, Fire exposure of liquid-filled vessel, Proc Safety Prog, 22(1): 27–32. TNO Department of Industrial Safety, 1996, EFFECTS Version 2.1 Manual (TNO, Apeldoorn). VVF, 1996, VHS Film of the Accident (Treviso). The manuscript was received 26 June 2003 and accepted for publication after revision 18 November 2003.
Trans IChemE, Part B, Process Safety and Environmental Protection, 2004, 82(B2): 128–131
www.ingentaselect.com=titles=09575820.htm
ANALYSIS OF AN LPG ACCIDENTAL RELEASE M. DEMICHELA*, N. PICCININI and A. POGGIO SAfeR, Centro Studi su Sicurezza, Affidabilita` e Rischi, Politecnico di Torino, Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Torino, Italy
A
n industrial LPG storage accident due to the release of propane from a tanker is described in this paper and the sequence of events that led to the collapse of a storage tank is examined using simulation software. A model taking both released flows into account as a radiant thermal contribution in the presence of flames is presented. The progression of the accident is assessed through digitalization and processing of the film of the fireball produced by about 500 L of LPG, whose diameter was calculated as approximately 50 m. Keywords: LPG release; accident analysis.
INTRODUCTION
Maintenance procedures showed that the valve operating mechanism could be dismounted if the tank was loaded, due to the plug sealing. Thus the operator started to dismantle the valve’s operating mechanism with the aim of repairing it. The tanker was full and still connected to the hoses. The operator was not able to verify the valve sealing, because the on-board instrumentation only indicated the flow direction, while it was not suitable for verifying the pressure after the valve. When the last bolt was removed, the strong pressure inside the tank blew the mechanism out of the body of the valve, through which a jet of propane escaped and coldburned the worker’s face and hands. Part of the liquid propane discharged through the suction pipe and the valve underwent isenthalpic flash evaporation. The remainder formed an unconfined pool that also began to evaporate. These vapours formed a low, dense cloud whose propagation was influenced by buildings, in particular the bottling shop. The cloud initially spread over the loading= unloading yard and then passed along the NW branch of the factory in the direction of its offices and forecourt. The fire brigade was called and two fire engines arrived a few minutes later and stopped in the forecourt to try to disperse the cloud with nebulized water. About 20 min after the release had started, however, the explosive mixture that had been formed was ignited. The explosion destroyed the two fire engines and brought down part of the office building. The cloud of propane gave rise to a deflagration that struck the firemen and the employees who had intervened, and set the release of propane on fire. The pool started a ‘pool fire’, while the jet from the valve produced a jet fire at the bottom of the tank (shown as ‘jet fire bottom’ in Figures 2 and 3). The flames licked the back of the tanker and heated the large quantity of propane still present in its tank. The internal pressure rose and strained the shell of the tanker, which was now weakened by the very great heat.
Investigation of real accidents discloses information that can be used to improve the safety of similar installations. The complexity of the events involved, in fact, often means that a priori assessments of the consequences of an accident, including any domino effects, make use of over-simplified assumptions that cannot be verified (Baker, 1983). This paper describes the application of widely used models (‘EFFECTS’, TNO) to the information obtained from an accident that occurred at Paese (Treviso) on 15 March 1996, in which two persons were killed, five firemen were seriously injured and windows were broken within a range of about 300 m (VVF, 1996). DESCRIPTION OF THE ACCIDENT At 7 a.m., a 50 m3 road tanker carrying 18 tonnes of propane entered the depot. There were five railway tankers holding a total of 170 tonnes of propane in the vicinity, together with six 15 m3 tank trucks, another 50 m3 road tanker and storage tanks holding about 800 tonnes of LPG (Figure 1). At the transfer station the operator connected the hoses to the tank and began to discharge its contents. This, however, did not take place correctly, as the flow indicator on the plant showed that the outflow rate was low. The cause of the defect was traced to the insufficient opening of the liquid phase valve. The valve plug was stuck by a screw nut left in the pipe during previous maintenance intervention. This reduced the outflow and prevented the valve from completely closing and sealing the tank content. *Correspondence to: Dr M. Demichela, Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Corso Duca Degli Abruzzi, 24, 10129 Torino, Italy. E-mail: [email protected]
128
ANALYSIS OF AN LPG ACCIDENTAL RELEASE
129
Figure 1. Depot layout. Figure 3. Collapse of the first tank, with its two jet fires.
After about 15 min, part of the tank yielded and the upper part of the shell ceded at the welds between the cylindrical plating and the hemispherical base. The high internal pressure determined a very fast outflow that became a jet fire (shown as ‘jet fire top’ in Figures 2 and 3). The deflagration of the cloud may also have damaged the connections of one of the transfer stations, resulting in another jet fire that involved one of the 15 m3 tank truck holding about 0.8 tonnes of product and was completely destroyed by a BLEVE, resulting in the formation of a fireball (Figure 4). The windows of most of the surrounding houses were smashed and pieces of metal from the tank were hurled as far as 600 m. The firemen extinguished the many fires that had broken out in the factory and went on cooling the railway trucks and the undamaged tank trucks until the jet fire from the first tanker involved burnt itself out. ANALYSIS OF THE EVENTS The events leading to the yielding of the first tank were examined using the simulation models contained in TNO’s
Figure 2. Accident sequence.
EFFECTS Version 2.1 (TNO Department of Industrial Safety, 1996). Key variables evolution was studied over time through their step-by-step discretization. Mass and energy balance and liquid phase=vapour phase equilibrium equations as a function of temperature were applied for each step, as indicated in Figure 5. The solutions of these equations showed the temperature and pressure inside the tank. The mass discharged between one step and the next was deducted at each step. The propane flow has been modelled as a two phase release. The outflow as a function of the pressure in the tank was calculated for release via the 38 mm diameter opening left in the valve body after the removal of the operating mechanism, discharging in a 3 inch diameter pipe. The equivalent length considered took account of the load losses in the valve. The evaporation by isenthalpic flash involved about 30% of the flow. The evaporation rate of the pool formed by the remainder was also determined. The total mass vaporized and still present in the pool was thus calculated for each step. When the explosion took place, about 20 min after the release started, the discharged mass amounted to about 4200 kg, namely about 1800 kg in the cloud and a little less than 2400 kg in the approximately 16 m diameter pool.
Figure 4. BLEVE and explosion of the 15 m3 tank.
Trans IChemE, Part B, Process Safety and Environmental Protection, 2004, 82(B2): 128–131
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DEMICHELA et al.
Figure 5. Simulation of the yielding of tank A.
Figure 6. Changes in the dimensions of the fireball.
The energy balance of the tank after the explosion must take account of the heat irradiated by the pool fire and the jet fire. The simulation showed that the propane in the pool was all burnt within about a minute and a half. Heating of the propane in the tank was thus mainly due to the jet fire, whose intensity was also magnified by the faster outflow caused by the increase in pressure, resulting in a loop that raised the pressure itself even faster. The tank had previously been tested at an internal pressure of 25 bar.
A study of the same accident by Dattilo et al. (1998), however, has shown that its loss of strength due to overheating meant that it probably yielded when the pressure rose to 17 bar. The simulation indicated that this value was reached after a little more than 15 min and when the tank still held about 8700 kg of propane. Collapse of the mantle produced the equivalent of a 50 cm diameter orifice through which the overpressure in the tank forced out the propane at more than 460 kg s 1. Some 30 s
Trans IChemE, Part B, Process Safety and Environmental Protection, 2004, 82(B2): 128–131
ANALYSIS OF AN LPG ACCIDENTAL RELEASE later, the pressure dropped to about that of the atmosphere and there were less than 2800 kg of propane still in the tank. These deductions are plotted in Figure 5(b–d). The initial release is not accompanied by any marked change in the pressure and temperature inside the tank, whereas the heat irradiated by the jet fire and the pool fire quickly increased both parameters and the outflow rate. Yielding of the tank was followed by an abrupt drop in both the pressure and the amount of propane in the tank, owing to its very fast discharge through the tear in the mantle. PROGRESSION OF THE FIREBALL Reconstruction of the events related to the BLEVE of the second road tanker is impossible because its contents and the way in which its pressure was increased are unknown. The fireball caused by the collapse of the second tank was partly included in a film of the events that followed the yielding of the first tank taken from a helicopter. Converting the film into digital form and processing the single pictures obtained from each of its frames allowed examination of its progression. The dimensions of the fireball were determined by using the length of a rail tanker (about 15 m; Figure 6) as a yardstick. Their changes over time were determined with reference to the 40 ms interval between one frame and the next. The results of the processing of the pictures are shown in Figure 6. The diameter of the fireball itself and its stem were constant at 50 m and about 17 m, whereas the height of its base from the ground increased from 30 to 45–50 m in just over 1 s. All these calculations are obviously approximate deductions from uncertain dimensions. CONCLUSIONS Looking for the accident causes, two main aspects have been identified: the erroneous maintenance procedure, that did not prevent LPG leakage, was conducted in an unsafe location; the plant instrumentation was inadequate, since it
131
consisted only of a flow meter without any indication of pressure and thus it did not allow verification of the valve seal. Also the emergency procedures showed some lack of organization and equipment. If the fire engines were equipped to keep a greater safety distance from the tanks involved in the fire, their destruction could have been avoided. The sequence of events that burst the first tank is evident from this analysis of the way its internal temperature and pressure were influenced by what was happening outside. The outflow rate from the discharge orifice was governed by the inside pressure and this in turn was determined by the temperature of the propane. Deflagration of the cloud started a pool fire and a jet fire, provoking a flow of irradiated heat dependent on the release rate. The reciprocal links between the variables, however, resulted in a non-linear progression, whereas EFFECTS and other simulation models are designed for fixed accident scenarios. When the conditions vary, perhaps due to the formation of a loop as in this case, complete simulation requires deeper analysis of the time domain and the elaboration of an ad hoc model, as in Shebeko et al. (2000) and Simpson (2003). REFERENCES Baker, W.E., 1983, Explosion Hazards and Evaluation (Elsevier, Amsterdam). Dattilo, F., Rosa, L., Tiberio, A., Cusin, C. and Andriotto, E., 1998, Metodologie e analisi di rischio—studio della dinamica di un incidente, in Proceedings of VGR 98, Pisa. Shebeko, Yu.N., Bolodian, I.A., Filippov, V.N., Navzenya, V.Yu., Kostyuhin, A.K., Tokarev, P.M. and Zamishevski, E.D., 2000, A study of the behaviour of a protected vessel containing LPG during pool fire engulfment, J Hazard Mater, A77: 43–56. Simpson, L.L., 2003, Fire exposure of liquid-filled vessel, Proc Safety Prog, 22(1): 27–32. TNO Department of Industrial Safety, 1996, EFFECTS Version 2.1 Manual (TNO, Apeldoorn). VVF, 1996, VHS Film of the Accident (Treviso). The manuscript was received 26 June 2003 and accepted for publication after revision 18 November 2003.
Trans IChemE, Part B, Process Safety and Environmental Protection, 2004, 82(B2): 128–131