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Mobile gas cylinders in fire: Consequences in case of failure
Fire Safety Journal xxx (xxxx) xxx–xxx
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Mobile gas cylinders in fire: Consequences in case of failure ⁎
Rico Tschirschwitz , Daniel Krentel, Martin Kluge, Enis Askar, Karim Habib, Harald Kohlhoff, Patrick P. Neumann, Sven-Uwe Storm, Michael Rudolph, André Schoppa, Mariusz Szczepaniak Bundesanstalt für Materialforschung und –prüfung (BAM), Berlin, Deutschland Unter den Eichen 87 12205, Berlin
A R T I C L E I N F O
A BS T RAC T
Keywords: Explosion Gas cylinders Consequences
Commercial, off-the shelf propane cylinders are subjected to high safety regulations. Furthermore, those cylinders are equipped with safety devices like pressure relief valves (PRV). Despite these regulations and safety measures, a failure of the container is possible if exposed to an intense fire. The result of this is severe hazard for users, rescue forces and infrastructure. Within the framework of a destructive test series, 15 identical propane cylinders, without pressure relief devices, were exposed to an intensive fire in horizontal position until failure. Each cylinder was filled with a mass of m =11 kg of liquid propane. Three different fire sources were used (wood, petrol, propane). The experiments revealed the failure of all cylinders in a time period t < 155 s. The failure lead to a fragmentation into several major parts with throwing distances of up to l =262 m. In all trials, the temperature of the cylinder wall (top, side, bottom), of the liquid phase inside and of the surrounding fire (top, side, bottom) was recorded. In addition, the inner cylinder pressure and the induced overpressure of the blast wave after the failure were recorded. Overpressures of up to p=0.27 bar were recorded close to the cylinder (l =5 m). All tests were documented by video from several positions (general view, close-up, high-speed 5000 fps). This test series creates the basis for further experimental studies in the field of alternative fuels for vehicles. The aim of this test series is to assess and analyse the consequences of the failure of gas vessels (for LPG, CNG, CGH2) in the aftermath of severe incidents.
1. Introduction Mobile propane cylinders are in wide spread use in commercial (cooking), industrial (forklift), and private (barbecue grills, campers) occupancies. Therefore, it is not uncommon that in case of a fire (e.g. in garages, houses, workshops, campers, production halls), propane cylinders are involved. As heated, liquid propane vaporizes and increases the cylinder pressure. Typically overpressure of the cylinder is prevented by safety devices like a pressure relief valve (PRV). However, if these devices fail, the pressure could reach the burst limit of the cylinder causing catastrophic failure. The released propane vapour, mixed with air, could ignite abruptly causing a hazard for humans and infrastructure (e.g. thrown fragments, overpressure, heat radiation). These scenarios are not unrealistic, as evidenced by previous incidents with exploding propane cylinders resulted in severe damage (e.g. [1,2]), serious injuries and even fatalities. In the late 1980s, a passer-by was killed by a fragment of a propane cylinder at a distance of l =200 m in Düsseldorf (Germany). The propane cylinder was heated by a burning asphalt cooker on a roof top [3]. A special hazard arises for firefighters in action. It is not always possible to anticipate the danger of propane cylinders present during a ⁎
fire. Two fire fighters were seriously injured by an exploding domestic 13 kg propane cylinder during response to a house fire in Marseille (France) in 2015. Due to the explosion one of them was thrown out of a window [4]. Another tragic incident happened in 1983 in Berlin. A firefighter working on the street was killed when a propane cylinder exploded on a neighbouring burning hotel roof [5]. These examples demonstrate that the knowledge of the special hazards and consequences in case of the failure of propane cylinders is essential, especially for firefighters and other rescue forces. Regulations and guidelines with information for fire fighters already exist in Germany. For example, the German Fire Service Regulation 500 describes very extensively and clearly the tactics and safety measure necessary for rescue forces responding to incidents with CBRN hazards (chemical, biological, radiological and nuclear). They consist of comprehensive instructions for tanks with compressed and liquefied gases which are inflammable. The focus lies on large tank wagons, tank trucks and industrial tanks, resulting in comparably large safety distances: The danger zone is set to a radius of l =300 m, the shut-off zone to a radius of l =1000 m [6]. Another recommendation is given by the German Fire Protection Association (ger.: vfdb). This consultative document about hazards of LPG (liquefied petroleum gas, with propane as main component)
Corresponding author.
http://dx.doi.org/10.1016/j.firesaf.2017.05.006 Received 10 February 2017; Received in revised form 3 May 2017; Accepted 4 May 2017 0379-7112/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Tschirschwitz, R., Fire Safety Journal (2017), http://dx.doi.org/10.1016/j.firesaf.2017.05.006
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Greek
Nomenclature A d f h l m ṁ p T t V
ρ Δ
area (m2) diameter (m) frequency (s−1,Hz) height (m) length (m) mass (kg) mass flow (kg/s) pressure (Pa, bar) temperature (absolute: °C, difference: K) time (s) volume (m3)
density (kg/m3) difference
Subscripts a b c h s v 0
ambient burst calculation test starting vapour ambient conditions
option. In another work, different gas cylinders (oxygen, acetylene, hydrogen, propane) were exposed to wood fire. If it is statutory, the cylinders were equipped with a PRV. The focus of these experiments was to investigate the consequence to building structures in case of cylinder failure [14]. Another article described a test series, where some conventional 5 kg and 11 kg LPG cylinders were tested with a gas burner. The throwing distance of fragments was reported to be up to l =300 m [15]. Another paper deals with camping gas cartridges (m =0.44 kg mixture propane/butane) in a fire. Three of these small cylinders have been tested with the fire of a barbecue grill. All three cartridges failed in a BLEVE (boiling liquid expanding vapour explosion) [16]. All the previous works presented [13–16] comprise only of single experiments (e.g. different fire methods and burner configurations, different fill levels of fuel). Thus, valid conclusions about the consequences of the failure of the cylinders are hardly possible.
divides the operational guidelines according to the size of the tank involved in the incident (ordinary cylinders used at home, car tank, tank for house heating, tank wagon, tank truck, tank ship). The danger zone is set to a radius of l =50 m and the shut-off zone to a radius of l =100 m for an ordinary, privately used propane cylinder [7]. These two regulations give an insight into the difficulties which rescue forces have to face concerning their specific risk assessment when responding to incidents with propane cylinders. 2. Current state of regulations and research The design and construction of transportable refillable welded steel LPG cylinders with a capacity of V =(0.5–150) dm3 is regulated by the European standard EN 1442. This standard defines the calculation pressure (pc) as the pressure of the medium in the cylinder at a temperature of T=65 °C minus p=1 bar [8]. Thus, the calculation pressure for propane is pc =22.25 bar (pv =23.25 bar at T=65 °C [9]). The burst pressure (pb) of the cylinder, also defined by the EN 1442, must be at least 2.25 times the calculation pressure. Hence, a propane cylinder according to EN 1442 must have a burst pressure of at least pb =50.1 bar. The burst test must be carried out under hydraulic conditions [8]. In order to avoid a bursting cylinder, safety devices like pressure relief valves (PRV) can be installed. The cylinder valves are described in the standard ISO 15995. According to this regulation, a PRV is an optional component [10]. In Germany, however, it is statutory. The requirements on the valves are described in the European standard EN 13953 [11]. Depending on the specific incident conditions (type of fire, local/global fire impact, wind, etc.), the examples described above [1–3,5] indicate that severe accidents are nevertheless possible. In the field of motor vehicle approval, a standardised test method for fire safety of fuel tanks exists. For LPG tanks it is described in regulation No. 67 of the Economic Commission for Europe of the United Nations (UN/ECE). According to this regulation, a filled cylinder must be installed centrally at a distance of l =0.1 m over a fire source and exposed to a flame temperature of at least T=590 °C. The test is considered successful if the PRV opens and the pressure decrease results in the cylinder in a safe condition [12]. Previous scientific work investigated the behaviour of mobile gas cylinders in fire conditions. In a test series, six propane cylinders (m =15.2 kg liquid propane) were exposed to fire, three made of aluminium, three made of steel. The six bottles were tested with three different burner configurations: one, two and three nozzles. The cylinders were equipped with a PRV. In the tests with the steel cylinders, the PRV opened as prescribed and the cylinders did not fail after at least t=40 min. However, the three aluminium cylinders failed within a time of t < 10 min [13]. The focus of these experiments was set on the correct operation of the PRV and not on the consequences of a potential failure. In this context, the failure of the cylinder is only one
2.1. Preliminary investigations of this work As previously seen in the presented scientific works, different exposure methods have been used: burner with liquid propane [13], wood fire [14], “conventional household propane burner” [15] and a barbecue grill [16]. For the standardised test for LPG tanks in motor vehicles, the bonfire test is not described in detail. It is only specified as “fire source”, and any fuel may be used (Annex 10, section 2.6.3 [12]). Only the impact of the fire is described in detail, resulting in a flame temperature near the tank of at least T=590 °C within t=5 min (Annex 10, section 2.6.5 [12]). However, the specific type of the fire source has an important impact on the test results. In preliminary investigations, three different fire methods have been quantified [17]. The following types of fire exposures have been validated:
• • •
Wood fire, stack similar to the conditions described in the UN 6(c) test [18] Petrol pool fire, V =0.1 m3 petrol, covered area A =(1.5×1.5) m Propane gas fire, 4×5 nozzles, propane mass flow per nozzle ṁ =180 g/min
For this test series, a cylinder dummy (V =0.094 m3) with circulating water was positioned above the three different fire sources. The experimental setup is shown schematically in Fig. 1. The temperature was measured at the water inlet and the water outlet. Using the temperature difference between the inlet and outlet and the water mass flow, the unsteady heat flux into the cylinder dummy could be calculated. With each exposure method (wood, petrol, propane), three identical tests were conducted. Typical results for one trial for each fuel type are 2
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The cylinders were mounted in the horizontal position, with the tube adapter sideward. To get information about the temperature distribution on the cylinder mantle, three thermocouples (type K, d =1.5 mm) were attached to the surface. These three measuring points were positioned at the middle extent of the cylinder, one at the top (Fig. 3: TIR 101), one at the middle (Fig. 3: TIR 102) and one at the bottom, next to the fire source (Fig. 3: TIR 103). A small metal sheet was welded over the top of the thermocouple to protect the junction (cf. Fig. 4, yellow arrow). To measure the flame temperature of each fire source, three thermocouples (type K, d =3.0 mm) were mounted at a radial distance of l =25 mm to the measuring points for the temperature on the cylinder surface (cf. Fig. 4, red arrows). In Fig. 3, these measuring points for the flame temperature are denoted with TIR 104−106. Small metal cylinders were fixed to the top of these thermocouples to protect the junction (visible on Fig. 4). A thermocouple (type K, d =1.5) was integrated into each cylinder before filling. These thermocouples were bent in such a direction that they measured the liquid phase (cf. Fig. 3 TIR 107). A 6 m long, ¼” tube was mounted at the tube adapter. At the end of this pipe, a piezoresistive pressure transducer (full scale: p=100 bar, precision: 0.5% full scale) was installed (cf. Fig. 3 PIR 201). A thermocouple (type K, d =1.5 mm) was used to monitor the temperature in the tube near the pressure transducer (cf. Fig. 3 TIR 108) to ensure the transducer remained within its operating range. To document the consequences of the propagating pressure wave in case of a failure the overpressure was measured at distances of l =(5; 7; 9) m from the centre of the cylinder using piezo-resistive pressure transducer (full scale: p=2 bar, precision: 0.25% full scale, cf. Fig. 3 PIR 202-204). The membrane of these sensors (d =15 mm) was aligned vertically to the direction of movement of a potentially reaction or pressure front. Each trial was documented from five camera positions. Two action cameras (f =60 fps) were positioned next to the fire. One was located at a distance of l =7 m away from the cylinder, the other at a distance of l =9 m in the opposite direction. For the overview of the entire test, a Full-HD-Camera (f =50 fps) was positioned at a distance of l =200 m. To get detailed information about the type of cylinder failure, a highspeed camera (f =5000 fps) with an 800 mm lens was positioned at a distance of l =200 m focused directly on the side of the cylinder. Moreover, in most tests an unmanned aerial vehicle (UAV) with a 4Kcamera was used to provide aerial photography. Two AD data acquisition systems were used. The first one was combined with a thermocouple input signal conditioner to acquire the temperature values (cf. Fig. 3 TIR 101−108). The sample rate in this
Fig. 1. Schematic experimental setup for the determination of the heat flux onto a cylinder dummy.
shown in Fig. 2. Evidently, a wood fire generates a maximum heat flux twice as high as the petrol pool fire, but with a higher variance. Pool fires and a gas fires have a defined start and end point (begin: ignition, end: fuel is empty or fuel flux is shut off), whereas the wood fire starts with an insipient phase prior to a long, intensive blaze with significant heat flux, prior to burning out. 2.2. Objective of the following investigation As described above, accidents occur repeatedly with mobile gas cylinders involved in a fire. Hence, it is necessary to gain comprehensive knowledge of the consequences of a cylinder failure to protect responding rescue forces. Most of the scientific work done so far comprised only single experiments and had a different focus. Fifteen (15) identical propane cylinders, each containing m =11 kg of liquid propane, were exposed to fire. In order to further investigate the influence of the exposure method, five cylinders were tested with each of the three types of fuels (wood, petrol, propane). 3. Experimental setup 3.1. Preparation of cylinders A series of 15 identical, off-the-shelf LPG cylinders were used for the experiments. The cylinders were stress relieved and made of two large pieces with one weld seam, circular at the surface between the top and the bottom (e.g. EN 1442, picture 5 [6]). The bottles were marked according to the EN 14894 [19] with a capacity of V =27.2 dm³. The test pressure was stated as ph =30 bar. According to EN 1442, the cylinders used must have a burst pressure of at least pb =50.1 bar (cf. Current state of regulations and research). The cylinder valves were equipped with an integrated PRV. For the test series, these valves were dismounted and replaced with a ¼” tube adapter and a needle valve. Consequently, the cylinders did not have any safety devices which counteracted a possible pressure increase caused by the heat flux. The gas vessels were filled by weight with m =11 kg of liquid propane (purity: ≥95%). Therefore, the volume of the cylinder was filled up to 81.3% (ρ=0.501 kg/dm³ for liquid propane at T=20 °C [9]). 3.2. Measurement equipment and instrumentation Eight temperature signals and four pressure signals were recorded during all experiments. Each test was documented with five cameras to get information about the throwing direction of the fragments, the type and details of the failure of the cylinder and the time until the failure. The whole measurement setup is shown in Fig. 3.
Fig. 2. Effective heat flux into the cylinder dummy with circulating water as a function of the time (reference area: complete dummy surface), exemplary for the three investigated fire types.
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Fig. 3. Schematic diagram of measurement setup for the fire tests.
to the largest available tanks for regular large vehicles. For the third fire exposure, a rack consisting of four rows, each equipped with five burner nozzles, was used (cf. Fig. 5, right side). Each single propane nozzle had a fuel mass flow of ṁ =180 g/min (liquid propane). The cylinder was positioned hanging on a framework at a distance of h =0.2 m above the propane nozzles.
4. Results 4.1. General description An example of the cylinder failure (propane cylinder no. 15; test pc15) is shown in the sequence in Fig. 6. For each individual trial, the consequences of the exposure onto the cylinder were in principle the same. After ignition, the flame temperature (cf. Fig. 3 TIR 104 – 106) and the temperatures of the cylinder surface (cf. Fig. 3 TIR 101 – 103) increase. As the temperature of the liquid phase (cf. Fig. 3 TIR 107) increases the liquid propane vaporizes and the cylinder pressure begins to rise (cf. Fig. 3 PIR 201). Once the pressure reaches the specific bursting pressure of the cylinder it ruptures (cf. Fig. 6, no. 2) and the remaining amount of liquid propane vaporizes abruptly (cf. Fig. 6, no. 3). After the release of the propane, the vapour mixes with oxygen in the ambient air and ignites promptly (cf. Fig. 6, no. 4). The result is a fire ball of d > 15 m, as can be seen in pictures 5–6 in Fig. 6 and Fig. 7 (test pc06). All 15 propane cylinders subjected to the fire exposure failed as described above. The time until failure for each test is provided in Table 1. For the wood fires, it is difficult to define a starting point due to the delay caused by the flame spread at the beginning. However, propane fires as well as petrol fires promptly provide a homogeneous flame over the entire area. To normalize the starting point for each test, a temperature increase of ΔT=5 K over ambient, measured at TIR 106 (cf. Fig. 3), was defined as the start of the test. The time to failure of the
Fig. 4. Temperature measuring points on the cylinder surface (yellow arrows, cf. Fig. 3 TIR 101 – 103) and for the flame temperature (red arrows, cf. Fig. 3 TIR 104 – 106).
case was at f =100 Hz. The pressure values were recorded with the second AD data acquisition system and a sample rate of f =1000 Hz. 3.3. Exposure fires Cylinders were exposed to a fire from three combustible materials (wood, petrol and propane, cf. Preliminary investigation). The three exposure sources are shown in Fig. 5. The stack of wood was prepared according to the UN 6(c) test [18] (cf. Fig. 5, left side). It has been shown in preliminary investigations that the half height of the wood stack (h =0.5 m) is sufficient to assure the failure of the gas cylinder. The propane cylinder was placed on a rack, directly above the wooden slats. A square pan (A =(1.5×1.5) m) with a rim of h =0.1 m was used for the petrol pool fire (cf. Fig. 5, middle). The propane cylinder was positioned on two small racks at a height of h =0.25 m over the pool surface. A petrol volume of V =100 dm³ was used as fuel; this amount is sufficient to assure a burning period of 8–10 min and is comparable
Fig. 5. Setup for the three firing methods: wood fire (left), square pan for pool fire (middle) and rack with burner nozzles (right).
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Fig. 6. Frame sequence of the process during cylinder failure, example: test pc15, exposure: propane fire, camera: action camera, l =7 m in front.
4.2. State of the cylinder at time of failure At the point of failure, two different types of measurement values were of special interest for this campaign: the state (pressure and temperature) inside the cylinder and the temperatures on the cylinder surface. One example for the cylinder pressure (cf. Fig. 3 PIR 201) and the liquid phase temperature (cf. Fig. 3 TIR 107) is shown in Fig. 8. The cylinder pressure at the time of failure is hereinafter referred to as burst pressure. For pc06 in Fig. 8, the burst pressure is pb =86.2 bar and the temperature of the liquid phase at the time of failure is Tb =99.3 °C. The values for the burst pressure, the peak pressure gradient of the cylinder pressure and the liquid phase temperature at the time of failure are shown in Table 2. For all tests, the burst pressure is in the range of pb =(70.7 – 98.2) bar. The burst pressures for the tests with the propane gas fires are slightly higher than for the other tests. The liquid phase temperature at the burst time is in a range of T=(71.8– 111.4) °C. For the tests with the wood fire, the average of these temperatures is lower than for the other firing methods (ΔT≈13 K). Table 3 shows the temperatures measured on the cylinder surface (top, middle, bottom) and the corresponding flame temperatures at a distance of l =25 mm, at the time of failure. Overall, the surface temperatures at the bottom and at the middle were about T=(150−250) °C. However, the temperatures on the top surface were quite higher, in
Fig. 7. Resulting fireball after failure, example: pc06, exposure: petrol pool fire, camera: Canon EOS-1, l =230 m in front.
cylinder for all tests was in the range of t=(70 − 152) s. Due to the high variance, no specific correlation between the exposure fire and time to cylinder failure (cf. Table 1, column 4: average ± standard deviation for each exposure fire) could be determined. Note, the distance between the propane cylinder and the petrol pool was approximately half the distance used during the quantification of the fire exposures, so an increased fire impact was expected.
Table 1 Time to failure for 15 cylinders exposed to fire. Test
Exposure Fire
Time to failure [s]a
Average ± stdev for each exposure fire [s]
pc01 pc02 pc03 pc04 pc05
Wood fire
152 123 101 97 100
115 ± 23
pc06 pc07 pc08 pc09 pc10
Petrol pool fire
148 138 70 108 138
120 ± 32
pc11 pc12 pc13 pc14 pc15
Propane gas fire
107 142 143 146 115
131 ± 18
a
tb-ts; ts is the time at ΔT= T(TIR 106) – Ta =5 K.
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Table 4 Number of fragments and regarding throwing distances of main fragments. Test
Number of fragments
Throwing distance [m]
pc01 pc02 pc03 pc04 pc05 pc06 pc07 pc08 pc09 pc10 pc11 pc12 pc13 pc14 pc15
3 3 5 2 2 5 4 2 4 4 6 5 5 7 3
4 54 0 0 0 42 10 10 62 56 9 1 31 0 0.3
11 118 0 0 0 58 60 16 76 77 10 4 86 0.8 1
61 154 16
48
70
82 154
109 188
174
83 81 10 21 120 1.8 7
115 99 15 70 145 1.9
35 262 238 4
59
6
6
Fig. 8. Cylinder pressure and liquid phase temperature, example: pc06, exposure: petrol pool fire (cf. Fig. 7).
the range of T=(250−470) °C. At the bottom and middle, the flame temperature is mostly in the range of T=(400 −700) °C. In contrast to the situation of the surface temperatures, the flame temperature at the top is lower than at the bottom and middle, about T=(300 −600) °C.
Table 2 Burst pressure, pressure gradient and liquid phase temperature at the time of failure, (full scale: p=100 bar pressure transducer, accuracy 0.5% full scale). Test
pc01 pc02 pc03 pc04 pc05 pc06 pc07 pc08 pc09 pc10 pc11 pc12 pc13 pc14 pc15
pb [bar]
80.0 86.5 79.7 75.7 74.6 86.2 82.1 70.7 86.9 80.4 90.9 81.1 86.8 98.2 86.4
Average each exposure fire [bar]
Pressure gradient dp/dt [bar/s]
Tb liquid phase at time of failure [°C]
Average each exposure fire [°C]
79.3
0.39 0.56 0.62 0.64 0.61 0.50 0.51 0.84 0.66 0.48 0.69 0.51 0.52 0.59 0.66
90.7 93.5 83.3 71.8 83.6 99.3 111.4 74.9 97.0 106.6 102.8 90.8 fault 97.4 fault
84.6
81.4
88.7
4.3. Fragmentation Directly after a test, the fragments were georeferenced to gain information about the throwing distance and direction of the fragments. Table 4 provides the information about the cylinder pieces that were found (90% of mass); all fragments with a significant mass were considered in this overview. Note, the throwing distances depend on the surrounding obstacles, like the earth wall at the blast area of the test site (cf. Fig. 5, left side) and the protection concrete elements at the fire test bench (cf. Fig. 6). Every cylinder ruptured into several fragments (two up to seven). In tests pc04 and pc05, the fragments remained on the rack and did not leave the fire site. In six tests, throwing distances of l > 100 m were detected. Noticeable is the position of the fragmentation of test pc13. The main fragment was thrown over a distance of l =238 m (cf. Fig. 9).
97.8
97.0
4.4. Overpressure effect The overpressure caused by the cylinder failure was measured at distances of l =(5; 7; 9) m. The maximum overpressure is provided in Table 5. The observed overpressure at a distance of l =5 m was in the range of about p=(0.05 – 0.15) bar for most of the tests. The maximum measured overpressure, during test pc10, was p=0.27 bar. As the distance of the pressure sensor to the centre of the failure increased, the pressure peak decreased significantly.
Table 3 Temperature measurements at time of failure, nomenclature according Fig. 3, thermocouple type K, class 1, accuracy according EN 60584 [20]. Test
T cylinder top [°C] TIR 101
T flame top [°C] TIR 104
T cylinder middle [°C] TIR 102
T flame middle [°C] TIR 105
T cylinder bottom [°C] TIR 103
T flame bottom [°C] TIR 106
pc01 pc02 pc03 pc04 pc05 pc06 pc07 pc08 pc09 pc10 pc11 pc12 pc13 pc14 pc15 average
328 353 407 464 412 305 251 419 293 270 fault 430 370 336 381 359
400 519 441 352 653 364 266 507 378 311 388 559 485 480 502 440
152 186 171 155 157 170 216 174 169 230 192 160 161 167 150 174
576 634 504 487 531 524 366 543 467 708 685 693 687 652 660 581
120 146 146 152 139 156 335 138 455 204 223 137 173 160 135 188
324 576 497 577 455 545 605 529 595 642 605 487 551 587 585 544
Fig. 9. Main fragment of a propane cylinder after failure, during the ballistic flight period, test: pc13.
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The much larger safety zones defined in [6] for large tanks have not been exceeded during the presented experiments. Of course several additional aspects (tactics, equipment etc.) should be considered for a potential future revision of the recommended safety distances.
Table 5 Maximum overpressure at a distance of l =(5; 7; 9) m, nomenclature according to Fig. 3 (full scale: p=2 bar pressure transducer, accuracy 0.25% full scale). Test
pc01 pc02 pc03 pc04 pc05 pc06 pc07 pc08 pc09 pc10 pc11 pc12 pc13 pc14 pc15
Max. overpressure l =5 m [bar] PIR 202
Max. overpressure l =7 m [bar] PIR 202
Max. overpressure l =9 m [bar] PIR 202
0.05 0.10 0.05 0.07 0.09 0.12 0.15 0.09 0.15 0.27 0.14 0.11 0.10 0.12 0.10
0.04 0.09 0.04 0.05 0.07 0.10 0.11 0.07 0.11 0.18 0.11 0.08 0.08 0.10 0.07
0.04 0.06 0.04 0.04 0.06 0.08 0.09 0.05 0.10 fault 0.06 0.05 0.05 0.07 0.05
6. Conclusion In the presented test campaign, 15 off-the-shelf propane gas cylinders were tested with three different exposure fires. All three fire types, which have been proved to be suitable for testing gas cylinders, resulted in cylinder failure. The burst pressure, the liquid phase temperature and the time period until structural failure of the cylinder displayed small variability. Observed throwing distances of the main fragments up to 262 m clearly exceeded values reported in the literature. The data recorded during the experiments (fragment position, temperatures, burst pressure, pressure wave overpressure, etc.) represent a suitable aid to assess potential consequences of a commercial propane cylinder failure. The propane/air-fire source appeared to provide the best control of the exposure fire. Nevertheless, this fire source requires a more complex setup and infrastructure and results in an implied risk for collateral damage to the experimental infrastructure. The wood and the petrol pool fires are more suitable for a simplified experimental setup. However, neither of these last-mentioned exposure method can be stopped or adjusted once it has been initiated. The observed fragmentation, including the number, type of fragments and throwing distances, show the large variety in the mode of cylinder failure. The chosen number of five identical tests generated a set of clear meaningful data in comparison to single tests. Thus this preliminary investigation was indispensable to validate the test methods and measurement instrumentation for future test campaigns with composite vessels for vehicles powered by alternative fuels like LPG (liquefied petroleum gas), CNG (compressed natural gas) or CGH2 (compressed gas hydrogen).
5. Discussion According to EN 1442, the minimum burst pressure for the propane cylinders used in this program is stated to be pb =50.1 bar, (cf. Current state of regulations and research). All burst pressures measured during the tests were considerably higher (cf. Table 2, pb > 70 bar), some twice as prescribed (test pc14). The exposure tests have shown that the temperature of the cylinder top surface is significantly higher than at the middle and the bottom. This is a result of the aggregate states of the propane at the different levels within the cylinder. At the bottom and at the middle, the liquid phase is effective in cooling the cylinder surface. Also a large amount of the energy, which has transferred into the cylinder, is necessary for the vaporization of the liquid propane. In contrast, the gaseous phase at the upper part has a much lower heat capacity compared to the liquid phase. Therefore, the gaseous phase heats up much faster, the cooling effect on the upper surface of the cylinder is less effective. All experiments have demonstrated that a propane bottle, without an operational pressure relief device, fails in the event of an intense fire exposure within a time of t < 3 min. Operational safety devices can prevent this event, or generate a significant delay. To evaluate the safety of responding rescue forces, their arrival time and standard procedures performed within the first minutes of the response must be considered. As an example, in 2015, the metropolitan fire service of Berlin had an average response time of t=9.4 min (number of fire incidents: 7165) [21]. However, the time of cylinder failure may overlap with the arrival of the fire brigade or other rescue responders, or at an arbitrary time after arrival. Cylinders far from the point of origin may have indirect heat impact and slower heating, or become involved as the fire spreads. Maximum overpressure data as a function of distance from the burst site can be taken to estimate possible structural damage near the event. According to [22], in the close-up range, severe injuries to humans are possible (knocking over of persons, perforated eardrum). Also structural damages, like bursting of windows and destruction of brickwork are possible within a wide range. Moreover, the measured overpressures represent the lower end of the possible overpressure generated by the burst. The shock wave pressure would be much higher in case of the incident taking place in a confined or partly confined scenario, leading to a drastic increase in the severity of damages to structures and injuries and also an increase of the danger zone. Observations of the throwing distance of the cylinder fragments conclude that a significant number of pieces exceed the extent of the proposed danger zone (radius l =50 m) and the shut-off zone (radius of l =100 m) defined in the recommendation for German firefighters [7].
References [1] M. Kerzel, Gasflaschen explodieren auf Campingplatz, Göttinger Tageblatt, Verlagsgesellschaft Madsack GmbH & Co. KG, Hannover, 23.12, 2012. [2] M. Frietsch, Feuer in Kleingartenanlage: Zwei Hütten in Bötzinger Nachtwaid zerstört / Gasflasche detoniert / Verdacht auf Brandstiftung. Badische Zeitung, Badisches Pressehaus GmbH & Co. KG, Freiburg i. Br., 14. April, 2016. [3] J. Leineweber, Einsatz 2-46-1, 3-11-1,…, …überhitzte Acetylenflasche nach Brand…, Feuermelder – Zeitschrift der Feuerwehr Düsseldorf, vol.51, pp. 8–16. [4] A. Vergnenegre, Incendie à Marseille: six blessés dont cinq marins-pompiers, Fr. 3, Fr. Télévisions, Paris 05 (2015) 02. [5] Feuerwehrmann durch eine Gasflasche tödlich verletzt - Bitumen hatte Dach eines Charlottenburger Rohbaus in Brand gesetzt, Der Tagesspiegel, Berlin, 14.06.1983, p. 17. [6] Ausschuss Feuerwehrangelegenheiten, Katastrophenschutz und zivile Verteidigung (AFKzV) (01)Feuerwehr-Dienstvorschrift FwDV 500 Einheiten im ABC – Einsatz“ (2012), 2012. [7] Vereinigung zur Förderung des Deutschen Brandschutzes e. V.(vfdb), Merkblatt Empfehlung für den Feuerwehreinsatz bei Gefahr durch Flüssiggas, ed, 11/, 2013. [8] DIN EN 1442:2008-04, LPG equipment and accessories – Transportable refillable welded steel cylinders for LPG – Design and construction (includes Amendment A1) (2008). [9] C. Yaws, Chemical Properties Handbook: Physical, Thermodynamics, Environmental Transport, Safety & Health Related Properties for Organic & , McGraw-Hill Education, New York, 1999 (ISBN 0070734011). [10] ISO 15995, Gas cylinders — Specifications and testing of LPG cylinder valves — Manually operated. (2006). [11] DIN EN 13953, (05)LPG equipment and accessories – Pressure relief valves for transportable refillable cylinders for Liquefied Petroleum Gas (LPG) (2015). [12] Regulation No 67 of the Economic Commission for Europe of the United Nations (UN/ECE) — Uniform provisions concerning: i. Approval of specific equipment of motor vehicles using liquefied petroleum gases in their propulsion system; and II. Approval of a vehicle fitted with specific equipment for the use of liquefied petroleum gases in its propulsion system with regard to the installation of such equipment, Economic Commission for Europe of the United Nations (UN/ECE), Brussels, 2007. [13] A.M. Birk, J.D.J. VanderSteen, The survivability of steel and aluminum 33.5 pound propane cylinders in fire, Process Saf. Progress. 22 (2003) 129–135. http:// dx.doi.org/10.1002/prs.680220209.
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R. Tschirschwitz et al. [14] J. Hora, J. Karl, O. Suchý, Pressure cylinders under fire condition, Perspect. Sci. 7 (2015) 208–221. http://dx.doi.org/10.1016/j.pisc.2015.11.035. [15] J. Stawczyk, Experimental evaluation of LPG tank explosion hazards, J. Hazard. Mater. 96 (2002) 189–200. http://dx.doi.org/10.1016/S0304-3894(02)00198-X. [16] N. Davison, M.R. Edwards, Effects of fire on small commercial gas cylinders, Eng. Fail. Anal. 15 (2008) 1000–1008. http://dx.doi.org/10.1016/j.engfailanal.2007.12.003. [17] D. Krentel, R. Tschirschwitz, M. Kluge, E. Askar, K. Habib, H. Kohlhoff, G. Mair, P.P. Neumann, M. Rudolph, A. Schoppa, S.-U. Storm, M. Szczepaniak, Auswirkungen von unfallbedingtem Behälterversagen bei alternativen PKWAntrieben, Teil 1: Probl., Stand der Tech. und Voruntersuchungen, Tech. Sicherh. 6
(2016) 39–46. [18] United Nations, Recommendations on the Transport of Dangerous Goods. Manual of Tests and Criteria 5, Geneva, New York, 2009, p. 456. [19] DIN EN 14894, (06)LPG equipment and accessories – Cylinder and drum marking (2013). [20] DIN EN 60584, (10)Thermocouples - Part 2: Toler. (1994). [21] Berliner Feuerwehr, (Berlin)Jahresbericht 2015 (2016) (Berlin). [22] Bericht Umweltbundesamt, Ermittlung und Berechnung von Störfallablaufszenarien nach Maßgabe der 3. Störfallverwaltungsvorschrift, Forschungs- und Entwicklungsvorhaben 204 (2000), 2000, p. 428.
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Fire Safety Journal journal homepage: www.elsevier.com/locate/firesaf
Mobile gas cylinders in fire: Consequences in case of failure ⁎
Rico Tschirschwitz , Daniel Krentel, Martin Kluge, Enis Askar, Karim Habib, Harald Kohlhoff, Patrick P. Neumann, Sven-Uwe Storm, Michael Rudolph, André Schoppa, Mariusz Szczepaniak Bundesanstalt für Materialforschung und –prüfung (BAM), Berlin, Deutschland Unter den Eichen 87 12205, Berlin
A R T I C L E I N F O
A BS T RAC T
Keywords: Explosion Gas cylinders Consequences
Commercial, off-the shelf propane cylinders are subjected to high safety regulations. Furthermore, those cylinders are equipped with safety devices like pressure relief valves (PRV). Despite these regulations and safety measures, a failure of the container is possible if exposed to an intense fire. The result of this is severe hazard for users, rescue forces and infrastructure. Within the framework of a destructive test series, 15 identical propane cylinders, without pressure relief devices, were exposed to an intensive fire in horizontal position until failure. Each cylinder was filled with a mass of m =11 kg of liquid propane. Three different fire sources were used (wood, petrol, propane). The experiments revealed the failure of all cylinders in a time period t < 155 s. The failure lead to a fragmentation into several major parts with throwing distances of up to l =262 m. In all trials, the temperature of the cylinder wall (top, side, bottom), of the liquid phase inside and of the surrounding fire (top, side, bottom) was recorded. In addition, the inner cylinder pressure and the induced overpressure of the blast wave after the failure were recorded. Overpressures of up to p=0.27 bar were recorded close to the cylinder (l =5 m). All tests were documented by video from several positions (general view, close-up, high-speed 5000 fps). This test series creates the basis for further experimental studies in the field of alternative fuels for vehicles. The aim of this test series is to assess and analyse the consequences of the failure of gas vessels (for LPG, CNG, CGH2) in the aftermath of severe incidents.
1. Introduction Mobile propane cylinders are in wide spread use in commercial (cooking), industrial (forklift), and private (barbecue grills, campers) occupancies. Therefore, it is not uncommon that in case of a fire (e.g. in garages, houses, workshops, campers, production halls), propane cylinders are involved. As heated, liquid propane vaporizes and increases the cylinder pressure. Typically overpressure of the cylinder is prevented by safety devices like a pressure relief valve (PRV). However, if these devices fail, the pressure could reach the burst limit of the cylinder causing catastrophic failure. The released propane vapour, mixed with air, could ignite abruptly causing a hazard for humans and infrastructure (e.g. thrown fragments, overpressure, heat radiation). These scenarios are not unrealistic, as evidenced by previous incidents with exploding propane cylinders resulted in severe damage (e.g. [1,2]), serious injuries and even fatalities. In the late 1980s, a passer-by was killed by a fragment of a propane cylinder at a distance of l =200 m in Düsseldorf (Germany). The propane cylinder was heated by a burning asphalt cooker on a roof top [3]. A special hazard arises for firefighters in action. It is not always possible to anticipate the danger of propane cylinders present during a ⁎
fire. Two fire fighters were seriously injured by an exploding domestic 13 kg propane cylinder during response to a house fire in Marseille (France) in 2015. Due to the explosion one of them was thrown out of a window [4]. Another tragic incident happened in 1983 in Berlin. A firefighter working on the street was killed when a propane cylinder exploded on a neighbouring burning hotel roof [5]. These examples demonstrate that the knowledge of the special hazards and consequences in case of the failure of propane cylinders is essential, especially for firefighters and other rescue forces. Regulations and guidelines with information for fire fighters already exist in Germany. For example, the German Fire Service Regulation 500 describes very extensively and clearly the tactics and safety measure necessary for rescue forces responding to incidents with CBRN hazards (chemical, biological, radiological and nuclear). They consist of comprehensive instructions for tanks with compressed and liquefied gases which are inflammable. The focus lies on large tank wagons, tank trucks and industrial tanks, resulting in comparably large safety distances: The danger zone is set to a radius of l =300 m, the shut-off zone to a radius of l =1000 m [6]. Another recommendation is given by the German Fire Protection Association (ger.: vfdb). This consultative document about hazards of LPG (liquefied petroleum gas, with propane as main component)
Corresponding author.
http://dx.doi.org/10.1016/j.firesaf.2017.05.006 Received 10 February 2017; Received in revised form 3 May 2017; Accepted 4 May 2017 0379-7112/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Tschirschwitz, R., Fire Safety Journal (2017), http://dx.doi.org/10.1016/j.firesaf.2017.05.006
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Greek
Nomenclature A d f h l m ṁ p T t V
ρ Δ
area (m2) diameter (m) frequency (s−1,Hz) height (m) length (m) mass (kg) mass flow (kg/s) pressure (Pa, bar) temperature (absolute: °C, difference: K) time (s) volume (m3)
density (kg/m3) difference
Subscripts a b c h s v 0
ambient burst calculation test starting vapour ambient conditions
option. In another work, different gas cylinders (oxygen, acetylene, hydrogen, propane) were exposed to wood fire. If it is statutory, the cylinders were equipped with a PRV. The focus of these experiments was to investigate the consequence to building structures in case of cylinder failure [14]. Another article described a test series, where some conventional 5 kg and 11 kg LPG cylinders were tested with a gas burner. The throwing distance of fragments was reported to be up to l =300 m [15]. Another paper deals with camping gas cartridges (m =0.44 kg mixture propane/butane) in a fire. Three of these small cylinders have been tested with the fire of a barbecue grill. All three cartridges failed in a BLEVE (boiling liquid expanding vapour explosion) [16]. All the previous works presented [13–16] comprise only of single experiments (e.g. different fire methods and burner configurations, different fill levels of fuel). Thus, valid conclusions about the consequences of the failure of the cylinders are hardly possible.
divides the operational guidelines according to the size of the tank involved in the incident (ordinary cylinders used at home, car tank, tank for house heating, tank wagon, tank truck, tank ship). The danger zone is set to a radius of l =50 m and the shut-off zone to a radius of l =100 m for an ordinary, privately used propane cylinder [7]. These two regulations give an insight into the difficulties which rescue forces have to face concerning their specific risk assessment when responding to incidents with propane cylinders. 2. Current state of regulations and research The design and construction of transportable refillable welded steel LPG cylinders with a capacity of V =(0.5–150) dm3 is regulated by the European standard EN 1442. This standard defines the calculation pressure (pc) as the pressure of the medium in the cylinder at a temperature of T=65 °C minus p=1 bar [8]. Thus, the calculation pressure for propane is pc =22.25 bar (pv =23.25 bar at T=65 °C [9]). The burst pressure (pb) of the cylinder, also defined by the EN 1442, must be at least 2.25 times the calculation pressure. Hence, a propane cylinder according to EN 1442 must have a burst pressure of at least pb =50.1 bar. The burst test must be carried out under hydraulic conditions [8]. In order to avoid a bursting cylinder, safety devices like pressure relief valves (PRV) can be installed. The cylinder valves are described in the standard ISO 15995. According to this regulation, a PRV is an optional component [10]. In Germany, however, it is statutory. The requirements on the valves are described in the European standard EN 13953 [11]. Depending on the specific incident conditions (type of fire, local/global fire impact, wind, etc.), the examples described above [1–3,5] indicate that severe accidents are nevertheless possible. In the field of motor vehicle approval, a standardised test method for fire safety of fuel tanks exists. For LPG tanks it is described in regulation No. 67 of the Economic Commission for Europe of the United Nations (UN/ECE). According to this regulation, a filled cylinder must be installed centrally at a distance of l =0.1 m over a fire source and exposed to a flame temperature of at least T=590 °C. The test is considered successful if the PRV opens and the pressure decrease results in the cylinder in a safe condition [12]. Previous scientific work investigated the behaviour of mobile gas cylinders in fire conditions. In a test series, six propane cylinders (m =15.2 kg liquid propane) were exposed to fire, three made of aluminium, three made of steel. The six bottles were tested with three different burner configurations: one, two and three nozzles. The cylinders were equipped with a PRV. In the tests with the steel cylinders, the PRV opened as prescribed and the cylinders did not fail after at least t=40 min. However, the three aluminium cylinders failed within a time of t < 10 min [13]. The focus of these experiments was set on the correct operation of the PRV and not on the consequences of a potential failure. In this context, the failure of the cylinder is only one
2.1. Preliminary investigations of this work As previously seen in the presented scientific works, different exposure methods have been used: burner with liquid propane [13], wood fire [14], “conventional household propane burner” [15] and a barbecue grill [16]. For the standardised test for LPG tanks in motor vehicles, the bonfire test is not described in detail. It is only specified as “fire source”, and any fuel may be used (Annex 10, section 2.6.3 [12]). Only the impact of the fire is described in detail, resulting in a flame temperature near the tank of at least T=590 °C within t=5 min (Annex 10, section 2.6.5 [12]). However, the specific type of the fire source has an important impact on the test results. In preliminary investigations, three different fire methods have been quantified [17]. The following types of fire exposures have been validated:
• • •
Wood fire, stack similar to the conditions described in the UN 6(c) test [18] Petrol pool fire, V =0.1 m3 petrol, covered area A =(1.5×1.5) m Propane gas fire, 4×5 nozzles, propane mass flow per nozzle ṁ =180 g/min
For this test series, a cylinder dummy (V =0.094 m3) with circulating water was positioned above the three different fire sources. The experimental setup is shown schematically in Fig. 1. The temperature was measured at the water inlet and the water outlet. Using the temperature difference between the inlet and outlet and the water mass flow, the unsteady heat flux into the cylinder dummy could be calculated. With each exposure method (wood, petrol, propane), three identical tests were conducted. Typical results for one trial for each fuel type are 2
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The cylinders were mounted in the horizontal position, with the tube adapter sideward. To get information about the temperature distribution on the cylinder mantle, three thermocouples (type K, d =1.5 mm) were attached to the surface. These three measuring points were positioned at the middle extent of the cylinder, one at the top (Fig. 3: TIR 101), one at the middle (Fig. 3: TIR 102) and one at the bottom, next to the fire source (Fig. 3: TIR 103). A small metal sheet was welded over the top of the thermocouple to protect the junction (cf. Fig. 4, yellow arrow). To measure the flame temperature of each fire source, three thermocouples (type K, d =3.0 mm) were mounted at a radial distance of l =25 mm to the measuring points for the temperature on the cylinder surface (cf. Fig. 4, red arrows). In Fig. 3, these measuring points for the flame temperature are denoted with TIR 104−106. Small metal cylinders were fixed to the top of these thermocouples to protect the junction (visible on Fig. 4). A thermocouple (type K, d =1.5) was integrated into each cylinder before filling. These thermocouples were bent in such a direction that they measured the liquid phase (cf. Fig. 3 TIR 107). A 6 m long, ¼” tube was mounted at the tube adapter. At the end of this pipe, a piezoresistive pressure transducer (full scale: p=100 bar, precision: 0.5% full scale) was installed (cf. Fig. 3 PIR 201). A thermocouple (type K, d =1.5 mm) was used to monitor the temperature in the tube near the pressure transducer (cf. Fig. 3 TIR 108) to ensure the transducer remained within its operating range. To document the consequences of the propagating pressure wave in case of a failure the overpressure was measured at distances of l =(5; 7; 9) m from the centre of the cylinder using piezo-resistive pressure transducer (full scale: p=2 bar, precision: 0.25% full scale, cf. Fig. 3 PIR 202-204). The membrane of these sensors (d =15 mm) was aligned vertically to the direction of movement of a potentially reaction or pressure front. Each trial was documented from five camera positions. Two action cameras (f =60 fps) were positioned next to the fire. One was located at a distance of l =7 m away from the cylinder, the other at a distance of l =9 m in the opposite direction. For the overview of the entire test, a Full-HD-Camera (f =50 fps) was positioned at a distance of l =200 m. To get detailed information about the type of cylinder failure, a highspeed camera (f =5000 fps) with an 800 mm lens was positioned at a distance of l =200 m focused directly on the side of the cylinder. Moreover, in most tests an unmanned aerial vehicle (UAV) with a 4Kcamera was used to provide aerial photography. Two AD data acquisition systems were used. The first one was combined with a thermocouple input signal conditioner to acquire the temperature values (cf. Fig. 3 TIR 101−108). The sample rate in this
Fig. 1. Schematic experimental setup for the determination of the heat flux onto a cylinder dummy.
shown in Fig. 2. Evidently, a wood fire generates a maximum heat flux twice as high as the petrol pool fire, but with a higher variance. Pool fires and a gas fires have a defined start and end point (begin: ignition, end: fuel is empty or fuel flux is shut off), whereas the wood fire starts with an insipient phase prior to a long, intensive blaze with significant heat flux, prior to burning out. 2.2. Objective of the following investigation As described above, accidents occur repeatedly with mobile gas cylinders involved in a fire. Hence, it is necessary to gain comprehensive knowledge of the consequences of a cylinder failure to protect responding rescue forces. Most of the scientific work done so far comprised only single experiments and had a different focus. Fifteen (15) identical propane cylinders, each containing m =11 kg of liquid propane, were exposed to fire. In order to further investigate the influence of the exposure method, five cylinders were tested with each of the three types of fuels (wood, petrol, propane). 3. Experimental setup 3.1. Preparation of cylinders A series of 15 identical, off-the-shelf LPG cylinders were used for the experiments. The cylinders were stress relieved and made of two large pieces with one weld seam, circular at the surface between the top and the bottom (e.g. EN 1442, picture 5 [6]). The bottles were marked according to the EN 14894 [19] with a capacity of V =27.2 dm³. The test pressure was stated as ph =30 bar. According to EN 1442, the cylinders used must have a burst pressure of at least pb =50.1 bar (cf. Current state of regulations and research). The cylinder valves were equipped with an integrated PRV. For the test series, these valves were dismounted and replaced with a ¼” tube adapter and a needle valve. Consequently, the cylinders did not have any safety devices which counteracted a possible pressure increase caused by the heat flux. The gas vessels were filled by weight with m =11 kg of liquid propane (purity: ≥95%). Therefore, the volume of the cylinder was filled up to 81.3% (ρ=0.501 kg/dm³ for liquid propane at T=20 °C [9]). 3.2. Measurement equipment and instrumentation Eight temperature signals and four pressure signals were recorded during all experiments. Each test was documented with five cameras to get information about the throwing direction of the fragments, the type and details of the failure of the cylinder and the time until the failure. The whole measurement setup is shown in Fig. 3.
Fig. 2. Effective heat flux into the cylinder dummy with circulating water as a function of the time (reference area: complete dummy surface), exemplary for the three investigated fire types.
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Fig. 3. Schematic diagram of measurement setup for the fire tests.
to the largest available tanks for regular large vehicles. For the third fire exposure, a rack consisting of four rows, each equipped with five burner nozzles, was used (cf. Fig. 5, right side). Each single propane nozzle had a fuel mass flow of ṁ =180 g/min (liquid propane). The cylinder was positioned hanging on a framework at a distance of h =0.2 m above the propane nozzles.
4. Results 4.1. General description An example of the cylinder failure (propane cylinder no. 15; test pc15) is shown in the sequence in Fig. 6. For each individual trial, the consequences of the exposure onto the cylinder were in principle the same. After ignition, the flame temperature (cf. Fig. 3 TIR 104 – 106) and the temperatures of the cylinder surface (cf. Fig. 3 TIR 101 – 103) increase. As the temperature of the liquid phase (cf. Fig. 3 TIR 107) increases the liquid propane vaporizes and the cylinder pressure begins to rise (cf. Fig. 3 PIR 201). Once the pressure reaches the specific bursting pressure of the cylinder it ruptures (cf. Fig. 6, no. 2) and the remaining amount of liquid propane vaporizes abruptly (cf. Fig. 6, no. 3). After the release of the propane, the vapour mixes with oxygen in the ambient air and ignites promptly (cf. Fig. 6, no. 4). The result is a fire ball of d > 15 m, as can be seen in pictures 5–6 in Fig. 6 and Fig. 7 (test pc06). All 15 propane cylinders subjected to the fire exposure failed as described above. The time until failure for each test is provided in Table 1. For the wood fires, it is difficult to define a starting point due to the delay caused by the flame spread at the beginning. However, propane fires as well as petrol fires promptly provide a homogeneous flame over the entire area. To normalize the starting point for each test, a temperature increase of ΔT=5 K over ambient, measured at TIR 106 (cf. Fig. 3), was defined as the start of the test. The time to failure of the
Fig. 4. Temperature measuring points on the cylinder surface (yellow arrows, cf. Fig. 3 TIR 101 – 103) and for the flame temperature (red arrows, cf. Fig. 3 TIR 104 – 106).
case was at f =100 Hz. The pressure values were recorded with the second AD data acquisition system and a sample rate of f =1000 Hz. 3.3. Exposure fires Cylinders were exposed to a fire from three combustible materials (wood, petrol and propane, cf. Preliminary investigation). The three exposure sources are shown in Fig. 5. The stack of wood was prepared according to the UN 6(c) test [18] (cf. Fig. 5, left side). It has been shown in preliminary investigations that the half height of the wood stack (h =0.5 m) is sufficient to assure the failure of the gas cylinder. The propane cylinder was placed on a rack, directly above the wooden slats. A square pan (A =(1.5×1.5) m) with a rim of h =0.1 m was used for the petrol pool fire (cf. Fig. 5, middle). The propane cylinder was positioned on two small racks at a height of h =0.25 m over the pool surface. A petrol volume of V =100 dm³ was used as fuel; this amount is sufficient to assure a burning period of 8–10 min and is comparable
Fig. 5. Setup for the three firing methods: wood fire (left), square pan for pool fire (middle) and rack with burner nozzles (right).
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Fig. 6. Frame sequence of the process during cylinder failure, example: test pc15, exposure: propane fire, camera: action camera, l =7 m in front.
4.2. State of the cylinder at time of failure At the point of failure, two different types of measurement values were of special interest for this campaign: the state (pressure and temperature) inside the cylinder and the temperatures on the cylinder surface. One example for the cylinder pressure (cf. Fig. 3 PIR 201) and the liquid phase temperature (cf. Fig. 3 TIR 107) is shown in Fig. 8. The cylinder pressure at the time of failure is hereinafter referred to as burst pressure. For pc06 in Fig. 8, the burst pressure is pb =86.2 bar and the temperature of the liquid phase at the time of failure is Tb =99.3 °C. The values for the burst pressure, the peak pressure gradient of the cylinder pressure and the liquid phase temperature at the time of failure are shown in Table 2. For all tests, the burst pressure is in the range of pb =(70.7 – 98.2) bar. The burst pressures for the tests with the propane gas fires are slightly higher than for the other tests. The liquid phase temperature at the burst time is in a range of T=(71.8– 111.4) °C. For the tests with the wood fire, the average of these temperatures is lower than for the other firing methods (ΔT≈13 K). Table 3 shows the temperatures measured on the cylinder surface (top, middle, bottom) and the corresponding flame temperatures at a distance of l =25 mm, at the time of failure. Overall, the surface temperatures at the bottom and at the middle were about T=(150−250) °C. However, the temperatures on the top surface were quite higher, in
Fig. 7. Resulting fireball after failure, example: pc06, exposure: petrol pool fire, camera: Canon EOS-1, l =230 m in front.
cylinder for all tests was in the range of t=(70 − 152) s. Due to the high variance, no specific correlation between the exposure fire and time to cylinder failure (cf. Table 1, column 4: average ± standard deviation for each exposure fire) could be determined. Note, the distance between the propane cylinder and the petrol pool was approximately half the distance used during the quantification of the fire exposures, so an increased fire impact was expected.
Table 1 Time to failure for 15 cylinders exposed to fire. Test
Exposure Fire
Time to failure [s]a
Average ± stdev for each exposure fire [s]
pc01 pc02 pc03 pc04 pc05
Wood fire
152 123 101 97 100
115 ± 23
pc06 pc07 pc08 pc09 pc10
Petrol pool fire
148 138 70 108 138
120 ± 32
pc11 pc12 pc13 pc14 pc15
Propane gas fire
107 142 143 146 115
131 ± 18
a
tb-ts; ts is the time at ΔT= T(TIR 106) – Ta =5 K.
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Table 4 Number of fragments and regarding throwing distances of main fragments. Test
Number of fragments
Throwing distance [m]
pc01 pc02 pc03 pc04 pc05 pc06 pc07 pc08 pc09 pc10 pc11 pc12 pc13 pc14 pc15
3 3 5 2 2 5 4 2 4 4 6 5 5 7 3
4 54 0 0 0 42 10 10 62 56 9 1 31 0 0.3
11 118 0 0 0 58 60 16 76 77 10 4 86 0.8 1
61 154 16
48
70
82 154
109 188
174
83 81 10 21 120 1.8 7
115 99 15 70 145 1.9
35 262 238 4
59
6
6
Fig. 8. Cylinder pressure and liquid phase temperature, example: pc06, exposure: petrol pool fire (cf. Fig. 7).
the range of T=(250−470) °C. At the bottom and middle, the flame temperature is mostly in the range of T=(400 −700) °C. In contrast to the situation of the surface temperatures, the flame temperature at the top is lower than at the bottom and middle, about T=(300 −600) °C.
Table 2 Burst pressure, pressure gradient and liquid phase temperature at the time of failure, (full scale: p=100 bar pressure transducer, accuracy 0.5% full scale). Test
pc01 pc02 pc03 pc04 pc05 pc06 pc07 pc08 pc09 pc10 pc11 pc12 pc13 pc14 pc15
pb [bar]
80.0 86.5 79.7 75.7 74.6 86.2 82.1 70.7 86.9 80.4 90.9 81.1 86.8 98.2 86.4
Average each exposure fire [bar]
Pressure gradient dp/dt [bar/s]
Tb liquid phase at time of failure [°C]
Average each exposure fire [°C]
79.3
0.39 0.56 0.62 0.64 0.61 0.50 0.51 0.84 0.66 0.48 0.69 0.51 0.52 0.59 0.66
90.7 93.5 83.3 71.8 83.6 99.3 111.4 74.9 97.0 106.6 102.8 90.8 fault 97.4 fault
84.6
81.4
88.7
4.3. Fragmentation Directly after a test, the fragments were georeferenced to gain information about the throwing distance and direction of the fragments. Table 4 provides the information about the cylinder pieces that were found (90% of mass); all fragments with a significant mass were considered in this overview. Note, the throwing distances depend on the surrounding obstacles, like the earth wall at the blast area of the test site (cf. Fig. 5, left side) and the protection concrete elements at the fire test bench (cf. Fig. 6). Every cylinder ruptured into several fragments (two up to seven). In tests pc04 and pc05, the fragments remained on the rack and did not leave the fire site. In six tests, throwing distances of l > 100 m were detected. Noticeable is the position of the fragmentation of test pc13. The main fragment was thrown over a distance of l =238 m (cf. Fig. 9).
97.8
97.0
4.4. Overpressure effect The overpressure caused by the cylinder failure was measured at distances of l =(5; 7; 9) m. The maximum overpressure is provided in Table 5. The observed overpressure at a distance of l =5 m was in the range of about p=(0.05 – 0.15) bar for most of the tests. The maximum measured overpressure, during test pc10, was p=0.27 bar. As the distance of the pressure sensor to the centre of the failure increased, the pressure peak decreased significantly.
Table 3 Temperature measurements at time of failure, nomenclature according Fig. 3, thermocouple type K, class 1, accuracy according EN 60584 [20]. Test
T cylinder top [°C] TIR 101
T flame top [°C] TIR 104
T cylinder middle [°C] TIR 102
T flame middle [°C] TIR 105
T cylinder bottom [°C] TIR 103
T flame bottom [°C] TIR 106
pc01 pc02 pc03 pc04 pc05 pc06 pc07 pc08 pc09 pc10 pc11 pc12 pc13 pc14 pc15 average
328 353 407 464 412 305 251 419 293 270 fault 430 370 336 381 359
400 519 441 352 653 364 266 507 378 311 388 559 485 480 502 440
152 186 171 155 157 170 216 174 169 230 192 160 161 167 150 174
576 634 504 487 531 524 366 543 467 708 685 693 687 652 660 581
120 146 146 152 139 156 335 138 455 204 223 137 173 160 135 188
324 576 497 577 455 545 605 529 595 642 605 487 551 587 585 544
Fig. 9. Main fragment of a propane cylinder after failure, during the ballistic flight period, test: pc13.
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The much larger safety zones defined in [6] for large tanks have not been exceeded during the presented experiments. Of course several additional aspects (tactics, equipment etc.) should be considered for a potential future revision of the recommended safety distances.
Table 5 Maximum overpressure at a distance of l =(5; 7; 9) m, nomenclature according to Fig. 3 (full scale: p=2 bar pressure transducer, accuracy 0.25% full scale). Test
pc01 pc02 pc03 pc04 pc05 pc06 pc07 pc08 pc09 pc10 pc11 pc12 pc13 pc14 pc15
Max. overpressure l =5 m [bar] PIR 202
Max. overpressure l =7 m [bar] PIR 202
Max. overpressure l =9 m [bar] PIR 202
0.05 0.10 0.05 0.07 0.09 0.12 0.15 0.09 0.15 0.27 0.14 0.11 0.10 0.12 0.10
0.04 0.09 0.04 0.05 0.07 0.10 0.11 0.07 0.11 0.18 0.11 0.08 0.08 0.10 0.07
0.04 0.06 0.04 0.04 0.06 0.08 0.09 0.05 0.10 fault 0.06 0.05 0.05 0.07 0.05
6. Conclusion In the presented test campaign, 15 off-the-shelf propane gas cylinders were tested with three different exposure fires. All three fire types, which have been proved to be suitable for testing gas cylinders, resulted in cylinder failure. The burst pressure, the liquid phase temperature and the time period until structural failure of the cylinder displayed small variability. Observed throwing distances of the main fragments up to 262 m clearly exceeded values reported in the literature. The data recorded during the experiments (fragment position, temperatures, burst pressure, pressure wave overpressure, etc.) represent a suitable aid to assess potential consequences of a commercial propane cylinder failure. The propane/air-fire source appeared to provide the best control of the exposure fire. Nevertheless, this fire source requires a more complex setup and infrastructure and results in an implied risk for collateral damage to the experimental infrastructure. The wood and the petrol pool fires are more suitable for a simplified experimental setup. However, neither of these last-mentioned exposure method can be stopped or adjusted once it has been initiated. The observed fragmentation, including the number, type of fragments and throwing distances, show the large variety in the mode of cylinder failure. The chosen number of five identical tests generated a set of clear meaningful data in comparison to single tests. Thus this preliminary investigation was indispensable to validate the test methods and measurement instrumentation for future test campaigns with composite vessels for vehicles powered by alternative fuels like LPG (liquefied petroleum gas), CNG (compressed natural gas) or CGH2 (compressed gas hydrogen).
5. Discussion According to EN 1442, the minimum burst pressure for the propane cylinders used in this program is stated to be pb =50.1 bar, (cf. Current state of regulations and research). All burst pressures measured during the tests were considerably higher (cf. Table 2, pb > 70 bar), some twice as prescribed (test pc14). The exposure tests have shown that the temperature of the cylinder top surface is significantly higher than at the middle and the bottom. This is a result of the aggregate states of the propane at the different levels within the cylinder. At the bottom and at the middle, the liquid phase is effective in cooling the cylinder surface. Also a large amount of the energy, which has transferred into the cylinder, is necessary for the vaporization of the liquid propane. In contrast, the gaseous phase at the upper part has a much lower heat capacity compared to the liquid phase. Therefore, the gaseous phase heats up much faster, the cooling effect on the upper surface of the cylinder is less effective. All experiments have demonstrated that a propane bottle, without an operational pressure relief device, fails in the event of an intense fire exposure within a time of t < 3 min. Operational safety devices can prevent this event, or generate a significant delay. To evaluate the safety of responding rescue forces, their arrival time and standard procedures performed within the first minutes of the response must be considered. As an example, in 2015, the metropolitan fire service of Berlin had an average response time of t=9.4 min (number of fire incidents: 7165) [21]. However, the time of cylinder failure may overlap with the arrival of the fire brigade or other rescue responders, or at an arbitrary time after arrival. Cylinders far from the point of origin may have indirect heat impact and slower heating, or become involved as the fire spreads. Maximum overpressure data as a function of distance from the burst site can be taken to estimate possible structural damage near the event. According to [22], in the close-up range, severe injuries to humans are possible (knocking over of persons, perforated eardrum). Also structural damages, like bursting of windows and destruction of brickwork are possible within a wide range. Moreover, the measured overpressures represent the lower end of the possible overpressure generated by the burst. The shock wave pressure would be much higher in case of the incident taking place in a confined or partly confined scenario, leading to a drastic increase in the severity of damages to structures and injuries and also an increase of the danger zone. Observations of the throwing distance of the cylinder fragments conclude that a significant number of pieces exceed the extent of the proposed danger zone (radius l =50 m) and the shut-off zone (radius of l =100 m) defined in the recommendation for German firefighters [7].
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