- Email: [email protected]
Overpressure characteristics of aluminium dust explosion vented through a relief pipe
Journal of Loss Prevention in the Process Industries 26 (2013) 676e682
Contents lists available at SciVerse ScienceDirect
Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Overpressure characteristics of aluminium dust explosion vented through a relief pipe Xing-Qing Yan, Jian-Liang Yu* School of Chemical Machinery, Dalian University of Technology, Dalian 116024, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 June 2012 Received in revised form 14 January 2013 Accepted 14 January 2013
A parametric experimental study of an aluminium dust explosion, initiated in a vessel and vented through a relief pipe, was performed. The aim is to clarify the overpressure characteristics in a vessel and relief pipe, during aluminium dust explosion venting, especially when a burn-up phenomenon occurs. For a vessel of fixed size, the influence of pipe diameter and pipe length on burn-up was discussed. Results demonstrate that burnup occurs shortly after flame only enters the initial part of the relief pipe when the original dust concentration in the vessel is at relatively high level, which is usually higher than the optimum concentration obtained from the confined vessel. When burn-up occurs, the maximal overpressure continues to increase rather than to decay along the initial part of the relief pipe. If burn-up is vigorous, a second peak on overpressure-time curve in the vessel could appear. By adding 0.1 g aluminium powders on the membrane, the second overpressure peak may even surpass the first peak. Extending pipe lengths can strengthen the overpressures around the position where burn-up occurs in the relief pipe. Reducing the pipe diameter can increase the burn-up severity in the relief pipe owing to the increased dust concentration and the pressure accumulation. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Dust explosion Ducted venting Relief pipe Overpressure Burn-up
1. Introduction Relief pipes are often recommended in the design of appropriate venting devices that reduce the overpressure resulting from hazardous internal gas and dust explosions, in order to discharge the unburned combustible materials and hot products to safe locations. However, the presence of a relief pipe can increase the explosion severity in the protected vessel, inducing a higher explosion overpressure compared to a simply vented vessel. Several studies have been carried out on the dynamics of vented gas explosions with relief pipes (Di Benedetto, Russo, & Salzano, 2008; Henneton, Ponizy, & Veyssiere, 2006; Kordylewski & Wach, 1986, 1988; McCann, Thomas, & Edwards, 1985; Ponizy & Leyer, 1999a, 1999b; Ural, 1993). The available literature showed that several phenomena could be concluded as affecting the increase of explosion overpressure: burn-up in the relief pipe (also called explosion-like combustion, or secondary explosion); frictional drag and inertia of the material column in the duct and acoustic; and Helmholtz oscillations (Ferrara, Di Benedetto, Salzano, & Russo, 2006). The burn-up phenomenon of dust vented into a relief pipe was verified in experiments by Kordylewski and Wach (1988), Lunn, Nicol, Collins, and Hubbard (1998) and Taveau (2012). Kordylewski and Wach (1988) conducted experiments on the venting of flour, dextrin and coal dust explosions, in a 22 L spherical * Corresponding author. E-mail address: [email protected] (J.-L. Yu). 0950-4230/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jlp.2013.01.003
vessel connected to a relief pipe, both ends of which were fully open. They observed secondary explosions in the relief pipe at a very early stage of the explosion in the vessel. However, they stated that, ‘its influence on the explosion course is unclear’ (Kordylewski & Wach, 1988, p. 60). Ural (1993) presented a model applicable to both gas and dust explosions, for predicting the effect of ducts with an uncovered vent connected to the primary vessel on the explosion overpressures. But the application and validation for dust explosions were not performed. Although researchers had already achieved many results on explosion-venting, connected to a relief pipe, as mentioned above, more work should still be performed using more kinds of dusts, especially on the influence of burn-up in the relief pipe with an original covered vent on the explosion venting process. The experiments in this paper were conducted for two purposes: first, to study the occurrence conditions of burn-up in the relief pipe with an originally covered vent during aluminium dust explosion venting; and second, to discuss the overpressure characteristics, both in the vessel and relief pipe when burn-up occurs. 2. Experiments 2.1. Experimental device The experimental device is represented schematically in Fig. 1. The vessel is a steel cylinder with internal diameter FC ¼ 0.068 m
X.-Q. Yan, J.-L. Yu / Journal of Loss Prevention in the Process Industries 26 (2013) 676e682
677
Fig. 3. Particle size distributions of sample with dp(50) ¼ 2 mm.
To obtain the explosion overpressure, four piezoelectric pressure transducers were arrayed along the vessel and the pipe. The first one (PC) was placed in the vessel, 66 mm lower than the pipe entrance. The second (P1) and third (P2) were placed along the pipe, 120 mm and 450 mm higher than the membrane, respectively. The fourth (P3) was placed at the downstream end of the pipe. All pressure transducers were activated at the ignition time, which was 50 ms delay after the dust was dispersed.
Fig. 1. Sketch of experimental device (not to scale). (1) One-way valve; (2) 50 mL compressed air reservoir; (3) pressure gauge; (4) solenoid valve; (5) one-way valve; (6) mushroom nozzle; (7) spark electrode; (8) vessel; (9) relief membrane; (10) relief pipe; PC, P1, P2, P3, pressure transducer.
and length LC ¼ 0.305 m, closed at the bottom end and fitted at the other with a circular pipe (internal diameter FT, length LT), whose downstream end is open to the ambient. Several steel pipes (FT ¼ 20, 25 and 40 mm) were adopted, the pipe length ranging from 0.9 m to 1.8 m. The relief diameter equals the pipe diameter. A two-layer polyethylene membrane, with a total thickness of 0.08 mm, was placed between the vessel and the pipe as the initial covered vent device in each experiment. Its bursting pressure was related to the pipe diameter, and the precise value was determined from the pressure-time history obtained in experiments. Ignition was by discharge of two high voltage spark electrodes, connected to a high-voltage transformer (HVT), placed 50 mm higher than the bottom end of the vessel. The input and output voltages of HVT were 220 V and 10,000 V, respectively. When the 10,000 V voltages were applied to the two separate electrodes, discharge occurred and ignited the aluminium dust cloud. Dust was dispersed by compressed air of 0.4 MPa into a 50 mL air reservoir, and a mushroom nozzle was welded to the bottom blank flange of the vessel (shown in Fig. 2), similar to the typical Hartmann device (Eckhoff, 2003). It has been validated in this device that if the compressed air pressure surpassed 0.4 MPa, there was no effect on the explosion severity for 2 mm aluminium dust used in the experiment.
2.2. Dust Aluminium dust, with half-content diameter 2 mm purchased from the commercial manufacturers, was used in experiments. The particle size was determined by a laser diffraction analyser, shown in Fig. 3. Chemical compositions, detected by the corresponding standard methods, are listed in Table 1. Before experiments, the aluminium sample was dried systemically for about 2 h at 50 C under vacuum conditions. The required mass of the aluminium dust in every experiment was calculated according to the original desired dust concentration in the vessel. 2.3. Experimental process Aluminium dust in a specified mass, measured by electronic balance, was firstly evenly dispersed manually on the bottom blank flange. After the components of vessel, membrane and pipe were assembled, the solenoid valve was opened by the control system. Compressed air blew through the nozzle and dispersed the dust to form a dust cloud in the vessel. After a 50 ms delay, the two electrodes discharged, igniting the powders. Meanwhile, the pressure transducers were activated and the explosion overpressures were recorded. A series of experiments was performed, corresponding to the following configurations: a) Dust explosions in the confined vessel in different aluminium concentrations;
Table 1 Chemical composition of 2 mm aluminium dust sample.
Fig. 2. Bottom blank flange and mushroom nozzle.
Activated aluminium content/%
Main impurity content/% Fe
Si
Cu
H2O
99.84
0.071
0.067
0.002
0.02
X.-Q. Yan, J.-L. Yu / Journal of Loss Prevention in the Process Industries 26 (2013) 676e682
b) Dust explosions vented directly to the ambient atmosphere (simply vented vessel), with relief diameters of 20, 25 and 40 mm in different aluminium concentrations; c) Dust explosions vented through relief pipe with diameters of 20, 25, 40 mm, and lengths of 0.9, 1.2, 1.5 and 1.8 m in different aluminium concentrations. In order to increase the occurrence probability of burn-up in the relief pipe, a series of experiments on injecting the aluminium dust twice were also conducted. After the vessel and the membrane were assembled, 0.1 g aluminium dust was placed on the membrane. The purpose was to increase the dust concentration in the relief pipe after the membrane ruptured. Experiments on injecting the aluminium dust twice corresponded to the following configuration: d) Dust explosions vented through relief pipes with diameters of 20, 25, 40 mm, and lengths of 0.9, 1.2, 1.5 and 1.8 m in different aluminium concentrations, when 0.1 g extra aluminium powders were placed on the membrane. Each experiment was performed repeatedly for reproducibility. The experiment of configurations a) and b) was normally repeated twice. However, the experiment in configurations c) and d) was conducted five or six times under the same experimental conditions. 3. Results and discussion 3.1. Characteristics of explosion overpressure in vessel and relief pipe Dust explosions in the confined vessel and in the simply vented vessel were conducted first, before doing experiments with relief pipe. The results are presented in Figs. 4 and 5. Fig. 4 illustrates the overpressure variations with time, in a confined vessel for aluminium dust concentrations ranging from 300 g m3 to 1000 g m3. Shortly after the end of 20 ms, a rapid increase is observable in the vessel, corresponding to the rapid combustion reaction of aluminium dust with oxidation. For a short period about 40e60 ms, the overpressure reaches the maximum value and then decreases slowly. Typical trends are observed from different dust concentrations. The maximum explosion overpressure achieves the maximum value at 800 g m3, followed by a decrease for either larger or smaller concentrations, in agreement
0.5
P / MPa
0.4 0.3 0.2
300g·m-3 500g·m-3
0.1
800g·m-3 1000g·m-3
0 0
50
100
150
200
250
300
t / ms Fig. 4. Overpressure variations in confined vessel for different dust concentrations.
0.3 300g·m-3 500g·m-3 800g·m-3
0.2
1000g·m-3
P / MPa
678
0.1
0 0
50
100
150
t / ms Fig. 5. Overpressure variations in the vessel vented directly to the ambient atmosphere with 25 mm relief diameter.
with results obtained in 20 L apparatus (Dufaud, Traore, Perrin, Chazelet, & Thomas, 2010). However, the maximum explosion overpressures are 0.32, 0.41, 0.43 and 0.39 MPa, corresponding to 300, 500, 800 and 1000 g m3; much lower than the values in 20 L apparatus. The reasons have been discussed by many researchers (Eckhoff, 2003). It is also observed that the time taken to reach the maximum explosion overpressure differs. The shorter times are obtained at 800 and 500 g m3, whereas the longer ones occur at 300 and 1000 g m3. From the slope of the curves in Fig. 4, it can be inferred that the maximum rate of pressure rise also follows the rule: larger ones are at 800 and 500 g m3, and smaller ones are at 300 and 1000 g m3. Fig. 5 shows the overpressure-time histories in the vessel vented directly to the ambient, with a 25 mm relief pipe. It can be seen that the bursting pressure is about 0.22e0.24 MPa (2-layer polyethylene membrane; 25 mm relief diameter). An outstanding characteristic on all curves is that the pressure begins to fall just after the membrane ruptures. Consequently, the maximum explosion overpressure in a simply vented vessel equals the bursting pressure of the membrane. The combustion time before the membrane ruptures is different for different dust concentrations: nearly 110 ms for 1000 g m3, but 70 ms for 800 g m3. The combustion time before venting (shown in Fig. 5) does not agree well with that in a confined vessel, resulting from the non-uniform dispersion and reaction of aluminium dust in the vessel. The following analysis of the characteristics of the explosion overpressure in the vessel and relief pipe is based mainly on the results obtained from the pipe FT ¼ 25 mm, LT ¼ 1.5 m, and presented in Figs. 6e11. The effects of varying the pipe length LT and diameter FT on overpressure characteristics will be discussed later. Fig. 6 shows a typical example of simultaneously recorded overpressures of PC, P1, P2 and P3, when aluminium dust concentration equals 300 g m3. For vessel-pipe assembly, PC first increases after the aluminium dust is ignited. Later, it reaches the bursting pressure (Pv ¼ 0.23 MPa) at t0 ¼ 56.52 ms. The rupture of the membrane induces the pressure to decrease rapidly. Hence the maximum explosion overpressure in the vessel is 0.23 MPa (bursting pressure). Shortly after the membrane ruptures at t1 ¼ 57.72 ms, P1 increases rapidly to the maximum value and then decreases, owing to the pressure wave passes. Soon, at the time of 57.72 ms and 59.39 ms, a similar trend can be seen in P2 and P3, respectively. Moreover, the maximum values of P1, P2 and P3 are about 0.13, 0.10 and 0.09 MPa, indicating the attenuation of pressure wave along the relief pipe.
X.-Q. Yan, J.-L. Yu / Journal of Loss Prevention in the Process Industries 26 (2013) 676e682
0.3
679
0.3
Pv
Pc
Pc
P1
P1
P2
0.2
P2
0.2
P3
P C2
P /MPa
P / MPa
P3
P C1
0.1
0.1
t2 t1
t3
t0
0 0
20
40
0
60
80
0
20
40
60
80
100
t /ms
t /ms Fig. 6. Overpressure variations in vessel (PC) and relief pipe (P1, P2, P3) (FT ¼ 25 mm, LT ¼ 1.5 m, c ¼ 300 g m3).
Fig. 8. Overpressure variations in vessel (PC) and relief pipe (P1, P2, P3) (FT ¼ 25 mm, LT ¼ 1.5 m, c ¼ 1000 g m3).
From all the experimental results with different aluminium dust concentrations, it is observed that not all the overpressuretime histories are analogous with the results shown in Fig. 6. Fig. 7 illustrates the overpressure-time histories when aluminium dust concentration equals 800 g m3. Compared to the curves in Fig. 6, the maximum value of P2 in Fig. 7 is nearly equivalent to that of P1, indicating that the pressure wave does not decay along the pipe any more. This phenomenon is more significant when aluminium dust concentration increases to 1000 g m3 (see Fig. 8). Besides the characteristics of nearly same values of the maximum values of P1 and P2, a second overpressure peak (PC2 in Fig. 8) appears during the depressurization process, although PC2 (0.16 MPa) is smaller than the first peak PC1 (0.22 MPa). The most important observation from analysis of Figs. 7 and 8 is that pressure intensification happens in the relief pipe or in vessel, during the aluminium dust explosion venting process. As might be expected, the regrowth of the explosion overpressures of PC and P2 is a result of the burn-up phenomenon, similar to that described by Ponizy and Leyer (1999a) during a gas explosion venting process. Kordylewski and Wach (1988) confirmed the burn-up phenomenon during dust explosion venting with a relief pipe when the vent is
uncovered. It has now been verified that burn-up still occurs when the vent is originally covered by the venting device (polyethylene membrane in this paper). The burn-up phenomenon in the relief pipe during the aluminium dust explosion venting can be explained as follows, in Fig. 9. Fig. 9(a) shows the flame propagating towards the pipe entrance in the confined vessel. When the membrane ruptures, pressure waves and the unburned aluminium powders enter the pipe, inducing P1 to increase rapidly. After a short period of time, the flame enters the pipe entrance (Fig. 9(b)) and is elongated in the contracted flow, owing to the sudden area change. The flame front becomes a forward coniform tip, leaving a volume of cold mixture unburned around it (Ponizy & Leyer, 1999a), which makes the heat transmit to the unburned mixture more efficiently. Consequently, the flame tends to be extinguished. If the flame is extinguished successfully, only the pressure wave generated before the membrane ruptures in the vessel is vented, damping down along the pipe, forming the overpressure-time histories shown in Fig. 6. However, if the flame is not extinguished, and the concentration of the unburned aluminium powders in the relief pipe is exactly in the range of the flammable limit, the aluminium dust will be reignited and burn-up will occur (Fig. 9(c)).
0.3 Pc P1 P2
0.2 P /MPa
P3
0.1
0 0
20
40
60
80
t /ms Fig. 7. Overpressure variations in vessel (PC) and relief pipe (P1, P2, P3) (FT ¼ 25 mm, LT ¼ 1.5 m, c ¼ 800 g m3).
Fig. 9. Burn-up phenomenon in relief pipe during dust explosion venting (a) flame propagates in the confined vessel (b) flame entering the pipe entrance (c) burn-up in relief pipe (d) influence of burn-up on overpressure in vessel and pipe.
680
X.-Q. Yan, J.-L. Yu / Journal of Loss Prevention in the Process Industries 26 (2013) 676e682
0.3 P C1 P 22
0.2
P C2
P / MPa
P 12 P 21 P 11
Pc
0.1
P 32
P1 P2 P3 P 31
0 40
50
60
70
80
90
t / ms Fig. 10. Overpressure variations in vessel (PC) and relief pipe (P1, P2, P3) when adding 0.1 g dust on membrane (FT ¼ 25 mm, LT ¼ 1.5 m, c ¼ 300 g m3).
Two kinds of results may happen as a result of the burn-up in the relief pipe: (1) Pressure intensification only occurs in the relief pipe. If the combustion reaction is moderate, or burn-up only lasts for a moment (lack of dusts or other reasons), the heat generated by the combustion reaction can only induce a slight pressure increase near the location where burn-up occurs. By comparing Figs. 6 and 7, it is noticed that only the P2 rises when burn-up happens, indicating that burn-up occurs around the position of the third pressure transducer. (2) Pressure intensification occurs both in the vessel and in the relief pipe. If the combustion reaction is vigorous, the pressure wave will block the original venting process, disturb the flow, so the unburned mixture in the corner of the vessel burns (Fig. 9(d)). The second pressure peak appears in the vessel, as shown in Fig. 8. When performing ducting experiments on dust explosion with an uncovered vent, Kordylewski and Wach (1988) reported the pressure peak resulting from burn-up (they called it ‘secondary
0.4 P C2
0.3 P / MPa
P 22
Pc
P C1
P1 P2 P3
0.2
P 12
0.1
P 32
explosion’ in their paper) preceded the main pressure wave. However, when the vent is covered, the pressure peak induced by burn-up is behind the main pressure wave. It can be inferred that the reaction is more complete in the vessel with a covered vent. When the membrane ruptures, the flame and pressure waves enter the pipe at relatively higher pressure than that in the uncovered vent. Also, the propagation of flame and pressure wave is quicker. Hence, the occurrence of the burn-up phenomenon is difficult compared with an uncovered vent. Also, the severity of burn-up becomes weaker. More details have been found from experimental results: Firstly, it is observed that the aluminium dust concentration in the vessel has a significant effect on the occurrence of burn-up. Burn-up only occurs at relatively high levels of concentrations, for example, 800 g m3 and 1000 g m3, in the experiments of FT ¼ 20 mm and LT ¼ 1.5 m. Noticing that 800 g m3 is the optimum concentration of aluminium dust in a confined vessel, this can be explained as follows: the dust concentration influences the amount of the unburned mixture vented into the relief pipe. The higher the concentration in the vessel is, the more the unburned mixture vents into the pipe, and the mixture ignites. Secondly, the occurrence of burn-up in experiments with the same experimental conditions is not deterministic, but probabilistic. It is expected that the factors influencing the occurrence of burn-up are numerous. Also, the experimental conditions, such as the uniformity level of dust deposit, dispersion and reaction, the rupture shape of the membrane, are uncontrolled. It should be emphasized that the phenomenon of burn-up in the relief pipe is definitely existent during dust explosion venting, and the probability is above 50% when the aluminium dust concentration is up to 1000 g m3. In the experiments of Kordylewski and Wach (1988), the burn-up phenomenon seemed to definitely occur, as they did not mention the probability of the burn-up. Thirdly, although P2 is greatly enhanced by burn-up in the relief pipe, the maximum explosion overpressure in the vessel still equals the bursting pressure. Experiments on further increasing the aluminium concentration in the relief pipe at the instant of the flame entering the pipe entrance were also performed, by adding 0.1 g aluminium powders on the membrane. Fig. 10 shows the overpressure variations of PC, P1, P2 and P3 when the original aluminium dust concentration is 300 g m3. For a better demonstration, the overpressure profile in the vessel during 1e40 ms is not listed. A clearly twohumped phenomenon appears on the overpressure curves, both in the vessel and in the relief pipe. Values of the overpressure peaks are listed in Table 2. It can be seen that the second peaks (P12 and P22) of P1 and P2 are both larger than their first overpressure peaks (P11 and P21), indicating that more severe pressure intensification than that in Figs. 7 and 8 occurs, and propagates owing to the re-reaction of the combustion. As seen in Fig. 10, there is a short period of time when P2 is larger than P1 and PC. It is expected that the pressure drop between PC and P2 is negative. A reverse flow is generated between relief pipe and vessel. As usual, the reason for the increased pressure in the vessel is that the disturbance and blockage to the venting process makes the unburned mixture still remaining in the vessel react. The second pressure peak in the vessel appears just after the appearance of the second peaks of P1 and P2. In this experimental condition, the
0 60
70
80
90
t / ms Fig. 11. Overpressure variations in vessel (PC) and relief pipe (P1, P2, P3) when adding 0.1 g dust on membrane (FT ¼ 25 mm, LT ¼ 1.5 m, c ¼ 800 g m3).
Table 2 Values of first and second overpressure peaks in Fig. 10/MPa. PC1
PC2
P11
P12
P21
P22
P31
P32
0.24
0.18
0.11
0.15
0.11
0.17
0.08
0.07
X.-Q. Yan, J.-L. Yu / Journal of Loss Prevention in the Process Industries 26 (2013) 676e682
second overpressure peak on PC curve (PC2) is still smaller than the first peak (PC1). When the original dust concentration is up to 800 g m3, experimental results are shown in Fig. 11. The second pressure peak PC2 (0.31 MPa) remained in the vessel is much larger than the first pressure peak (0.27 MPa, bursting pressure of membrane). The severity of the dust explosion in the vessel is enhanced greatly, because the maximum explosion overpressure changes from 0.27 MPa to 0.31 MPa. Unlike Fig. 10 mentioned above, the overpressures of P1, P2 and P3 increase directly to the second peaks, rather than having visible first peaks. In this experimental condition, the aluminium dust concentration near the pipe entrance is high enough to be ignited easily after the membrane ruptures. When the flame enters the pipe, a rapid transient combustion reaction starts violently in the pipe, accompanied by a strong pressure impulse in the pipe. Consequently, the backward and forward movement of the pressure wave and the mixture results in the obvious increase in overpressure, both in the vessel and the pipe. This kind of burn-up in the relief pipe is similar to an explosion, and was termed ‘explosion-like combustion’ by Ponizy and Leyer (1999a). 3.2. Effects of varying the pipe length LT and diameter FT on burnup A parametric study involving LT and FT was performed, in order to see whether some effects of pipe dimensions on the burn-up phenomenon could be discovered. Results are were as follows: i) The effect of pipe length on overpressures in the vessel and relief pipe is definite, as shown in Fig. 12. The curves are taken from the explosion results. For clarity and conciseness, P3 is not listed in the figure. The horizontal ordinate represents time. It can be seen that the overpressure variations for different pipe lengths are similar. The bursting pressure in every explosion is about 0.24e0.26 MPa. The maximum value of P2 is smaller than that of P1 for LT ¼ 900 mm. The maximum value of P2 gets larger with pipe length, confirming that the larger the pipe length, the more violent the burn-up. However, only at a length of 1800 mm does the severity of explosion in the vessel increase. The second overpressure peak in the vessel is larger than the original peak (bursting pressure). The reason why overpressure in the vessel only rises at LT ¼ 1800 mm is partly because of the frictional drag and inertia of the gas column in the pipe (Ponizy & Leyer, 1999a). In addition, the severity of burn-up also contributes to the increase in explosion overpressure in the vessel. When the pipe length is less than
Fig. 12. Overpressure histories in vessel (PC) and relief pipe (P1, P2) for different lengths (FT ¼ 25 mm, c ¼ 800 g m3).
681
1500 mm, heat generated by the combustion reaction in the pipe dissipates quickly. Pressure cannot be accumulated and the flame extinguishes rapidly. Thus the severity of burn-up is limited. The longer the pipe length, the slower the heat loss. The pressure wave can be accumulated and propagate easily, increasing the severity of burn-up. ii) Pipe diameter has a significant effect on the burn-up phenomenon, as shown in Fig. 13. The horizontal ordinate represents time. One notices firstly that changing the pipe diameter would change the bursting pressure of the membrane, resulting in a complicated situation when analysing the effect of diameter on burn-up phenomenon. However, it is clearly observable that the smaller diameter will increase the violence of burn-up, and hence the overpressures in the relief pipe greatly. When pipe diameter is 20 mm, P2 is much larger than P1. Increasing the pipe diameter to 25 mm and 40 mm, P2 is nearly equivalent to P1. The probable reasons are numerous: firstly, in a relief pipe with a smaller diameter, the dust concentration is relatively larger when unburned dust is vented into the pipe; secondly, the pressure accumulation generated by the combustion reaction is easier in a pipe with a smaller diameter; thirdly, the endurance period of venting is larger with smaller pipe diameters, raising the possibility of the occurrence of vigorous burn-up. It must be inferred that if the pipe diameter is too small to propagate the flame through the pipe entrance (extinguished at the pipe entrance), burnup would never happen. It seems that this problem is nonexistent with pipe diameters of 20, 25 and 40 mm. It also can be seen from Fig. 13 that changing the pipe diameter has no effect on explosion variations in the vessel. Indeed, the burnup in the relief pipe can induce a reverse flow. But if the reverse flow penetrates into a region in the vessel where no unburned mixture exists, the explosion overpressure in the vessel will not change significantly (Ponizy & Leyer, 1999a). 3.3. Further analysis and mitigation measurements on burn-up Apparently, the burn-up phenomenon may result in higher explosion overpressure, both in the vessel and in the relief pipe, rather than in a simple vented vessel, Hence, methods should be studied to prevent the occurrence of burn-up, or to reduce its severity and the subsequent secondary explosion overpressure. Henneton et al. (2006) placed a wire-net insert at the pipe entrance, to delay flame penetration into the pipe during gas explosion venting. They found it was possible to extinguish the flame penetrating into the relief pipe, and so eliminate the burn-up. Di Benedetto, Russo, and Salzano (2005) raised a mitigation method
Fig. 13. Overpressure histories in vessel (PC) and relief pipe (P1, P2) for different diameters (LT ¼ 1.5 m, c ¼ 800 g m3).
682
X.-Q. Yan, J.-L. Yu / Journal of Loss Prevention in the Process Industries 26 (2013) 676e682
of gas explosion overpressure piling, by divergent connections in the interconnected vessel. These methods may serve as a guide for the elimination and mitigation of the burn-up in a relief pipe connected to a vessel subjected to dust explosion. In addition, we conducted experiments on the suppression of pressure waves by placing aluminium silicate wool porous media on the internal wall of the relief pipe. As the experiments have not been finished, the results will not be discussed here. 4. Conclusions 1. The maximum explosion overpressure decays along the relief pipe when the burn-up phenomenon does not occur during dust explosion venting with relief pipe. However, when burnup occurs, the explosion overpressure will be intensified around the position where burn-up occurs in the relief pipe. Second overpressure peaks may appear in the overpressuretime histories, both in the vessel and in the relief pipe. 2. The aluminium dust concentration in the vessel has a significant effect on the occurrence of burn-up. Burn-up only occurs at relatively high levels of dust concentration in the vessel; usually higher than the optimum concentration in a confined vessel. The higher the dust concentration, the more easily burn-up occurs, and the more vigorous the burn-up. 3. The occurrence of the burn-up in experiments with same experimental conditions is not deterministic, but probabilistic. The burn-up phenomenon is inevitable when using a relief pipe with sudden area change, together with the unburned dust at venting instant. 4. By adding 0.1 g aluminium powders to the membrane, the severity of the dust explosion is greatly enhanced. The overpressure intensifications, both in the vessel and in the relief pipe, are so violent that second overpressure peaks are much larger than the first ones. 5. The longer the relief pipe, the more violent the burn-up. When the pipe length is less than 1200 mm in this experiment, burnup has no effect on the explosion severity in the vessel. Pipes with smaller diameters will increase the violence of the burnup greatly, until the diameter is so small that the flame will be extinguished when passing through the pipe entrance.
6. More work needs to be done on the burn-up phenomenon during dust explosion venting. The influence of particle size, bursting pressure, and ignition source position on burn-up should be studied systemically in the future.
Acknowledgements This paper was supported by the National Natural Science Foundation of China (Grant no. 50974027), and the Liaoning Province Science and Technology Project (Grant no. 2010219002).
References Di Benedetto, A., Russo, G., & Salzano, E. (2005). The mitigation of pressure piling by divergent connections. Process Safety Progress, 24, 310e314. Di Benedetto, A., Russo, G., & Salzano, E. (2008). The design of duct venting of gas explosions. Process Safety Progress, 27, 164e172. Dufaud, O., Traore, M., Perrin, L., Chazelet, S., & Thomas, D. (2010). Experimental investigation and modeling of aluminium dusts explosions in the 20 L sphere. Journal of Loss Prevention in the Process Industries, 23, 226e236. Eckhoff, R. K. (2003). Dust explosions in the process industries (3rd ed.). Oxford: Gulf Professional Publishing. Ferrara, G., Di Benedetto, A., Salzano, E., & Russo, G. (2006). CFD analysis of gas explosions vented through relief pipes. Journal of Hazardous Materials, A137, 654e665. Henneton, N., Ponizy, B., & Veyssiere, B. (2006). Control of flame transmission from a vessel to a discharge duct. Combustion Science and Technology, 178, 1803e1819. Kordylewski, W., & Wach, J. (1986). Influence of ducting on the explosion pressure. Combustion and Flame, 66, 77e79. Kordylewski, W., & Wach, J. (1988). Influence of ducting on explosion pressure: small scale experiments. Combustion and Flame, 71, 51e61. Lunn, G. A., Nicol, A. M., Collins, P. D., & Hubbard, N. R. (1998). Effects of vent ducts on the reduced pressures from explosions in dust collectors. Journal of Loss Prevention in the Process Industries, 11, 109e121. McCann, D. P. J., Thomas, G. O., & Edwards, D. H. (1985). Gas dynamics of vented explosions part I: experimental studies. Combustion and Flame, 59, 233e250. Ponizy, B., & Leyer, J. C. (1999a). Flame dynamics in a vented vessel connected to a duct: 1. Mechanism of vessel-duct interaction. Combustion and Flame, 116, 259e271. Ponizy, B., & Leyer, J. C. (1999b). Flame dynamics in a vented vessel connected to a duct: 2. Influence of ignition site, membrane rupture, and turbulence. Combustion and Flame, 116, 272e281. Taveau, J. (2012). Secondary dust explosions: how to prevent them or mitigate their effects. Process Safety Progress, 31, 36e50. Ural, E. A. (1993). A simplified method for predicting the effect of dusts connected to explosion vents. Journal of Loss Prevention in the Process Industries, 6, 3e10.
Contents lists available at SciVerse ScienceDirect
Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Overpressure characteristics of aluminium dust explosion vented through a relief pipe Xing-Qing Yan, Jian-Liang Yu* School of Chemical Machinery, Dalian University of Technology, Dalian 116024, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 June 2012 Received in revised form 14 January 2013 Accepted 14 January 2013
A parametric experimental study of an aluminium dust explosion, initiated in a vessel and vented through a relief pipe, was performed. The aim is to clarify the overpressure characteristics in a vessel and relief pipe, during aluminium dust explosion venting, especially when a burn-up phenomenon occurs. For a vessel of fixed size, the influence of pipe diameter and pipe length on burn-up was discussed. Results demonstrate that burnup occurs shortly after flame only enters the initial part of the relief pipe when the original dust concentration in the vessel is at relatively high level, which is usually higher than the optimum concentration obtained from the confined vessel. When burn-up occurs, the maximal overpressure continues to increase rather than to decay along the initial part of the relief pipe. If burn-up is vigorous, a second peak on overpressure-time curve in the vessel could appear. By adding 0.1 g aluminium powders on the membrane, the second overpressure peak may even surpass the first peak. Extending pipe lengths can strengthen the overpressures around the position where burn-up occurs in the relief pipe. Reducing the pipe diameter can increase the burn-up severity in the relief pipe owing to the increased dust concentration and the pressure accumulation. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Dust explosion Ducted venting Relief pipe Overpressure Burn-up
1. Introduction Relief pipes are often recommended in the design of appropriate venting devices that reduce the overpressure resulting from hazardous internal gas and dust explosions, in order to discharge the unburned combustible materials and hot products to safe locations. However, the presence of a relief pipe can increase the explosion severity in the protected vessel, inducing a higher explosion overpressure compared to a simply vented vessel. Several studies have been carried out on the dynamics of vented gas explosions with relief pipes (Di Benedetto, Russo, & Salzano, 2008; Henneton, Ponizy, & Veyssiere, 2006; Kordylewski & Wach, 1986, 1988; McCann, Thomas, & Edwards, 1985; Ponizy & Leyer, 1999a, 1999b; Ural, 1993). The available literature showed that several phenomena could be concluded as affecting the increase of explosion overpressure: burn-up in the relief pipe (also called explosion-like combustion, or secondary explosion); frictional drag and inertia of the material column in the duct and acoustic; and Helmholtz oscillations (Ferrara, Di Benedetto, Salzano, & Russo, 2006). The burn-up phenomenon of dust vented into a relief pipe was verified in experiments by Kordylewski and Wach (1988), Lunn, Nicol, Collins, and Hubbard (1998) and Taveau (2012). Kordylewski and Wach (1988) conducted experiments on the venting of flour, dextrin and coal dust explosions, in a 22 L spherical * Corresponding author. E-mail address: [email protected] (J.-L. Yu). 0950-4230/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jlp.2013.01.003
vessel connected to a relief pipe, both ends of which were fully open. They observed secondary explosions in the relief pipe at a very early stage of the explosion in the vessel. However, they stated that, ‘its influence on the explosion course is unclear’ (Kordylewski & Wach, 1988, p. 60). Ural (1993) presented a model applicable to both gas and dust explosions, for predicting the effect of ducts with an uncovered vent connected to the primary vessel on the explosion overpressures. But the application and validation for dust explosions were not performed. Although researchers had already achieved many results on explosion-venting, connected to a relief pipe, as mentioned above, more work should still be performed using more kinds of dusts, especially on the influence of burn-up in the relief pipe with an original covered vent on the explosion venting process. The experiments in this paper were conducted for two purposes: first, to study the occurrence conditions of burn-up in the relief pipe with an originally covered vent during aluminium dust explosion venting; and second, to discuss the overpressure characteristics, both in the vessel and relief pipe when burn-up occurs. 2. Experiments 2.1. Experimental device The experimental device is represented schematically in Fig. 1. The vessel is a steel cylinder with internal diameter FC ¼ 0.068 m
X.-Q. Yan, J.-L. Yu / Journal of Loss Prevention in the Process Industries 26 (2013) 676e682
677
Fig. 3. Particle size distributions of sample with dp(50) ¼ 2 mm.
To obtain the explosion overpressure, four piezoelectric pressure transducers were arrayed along the vessel and the pipe. The first one (PC) was placed in the vessel, 66 mm lower than the pipe entrance. The second (P1) and third (P2) were placed along the pipe, 120 mm and 450 mm higher than the membrane, respectively. The fourth (P3) was placed at the downstream end of the pipe. All pressure transducers were activated at the ignition time, which was 50 ms delay after the dust was dispersed.
Fig. 1. Sketch of experimental device (not to scale). (1) One-way valve; (2) 50 mL compressed air reservoir; (3) pressure gauge; (4) solenoid valve; (5) one-way valve; (6) mushroom nozzle; (7) spark electrode; (8) vessel; (9) relief membrane; (10) relief pipe; PC, P1, P2, P3, pressure transducer.
and length LC ¼ 0.305 m, closed at the bottom end and fitted at the other with a circular pipe (internal diameter FT, length LT), whose downstream end is open to the ambient. Several steel pipes (FT ¼ 20, 25 and 40 mm) were adopted, the pipe length ranging from 0.9 m to 1.8 m. The relief diameter equals the pipe diameter. A two-layer polyethylene membrane, with a total thickness of 0.08 mm, was placed between the vessel and the pipe as the initial covered vent device in each experiment. Its bursting pressure was related to the pipe diameter, and the precise value was determined from the pressure-time history obtained in experiments. Ignition was by discharge of two high voltage spark electrodes, connected to a high-voltage transformer (HVT), placed 50 mm higher than the bottom end of the vessel. The input and output voltages of HVT were 220 V and 10,000 V, respectively. When the 10,000 V voltages were applied to the two separate electrodes, discharge occurred and ignited the aluminium dust cloud. Dust was dispersed by compressed air of 0.4 MPa into a 50 mL air reservoir, and a mushroom nozzle was welded to the bottom blank flange of the vessel (shown in Fig. 2), similar to the typical Hartmann device (Eckhoff, 2003). It has been validated in this device that if the compressed air pressure surpassed 0.4 MPa, there was no effect on the explosion severity for 2 mm aluminium dust used in the experiment.
2.2. Dust Aluminium dust, with half-content diameter 2 mm purchased from the commercial manufacturers, was used in experiments. The particle size was determined by a laser diffraction analyser, shown in Fig. 3. Chemical compositions, detected by the corresponding standard methods, are listed in Table 1. Before experiments, the aluminium sample was dried systemically for about 2 h at 50 C under vacuum conditions. The required mass of the aluminium dust in every experiment was calculated according to the original desired dust concentration in the vessel. 2.3. Experimental process Aluminium dust in a specified mass, measured by electronic balance, was firstly evenly dispersed manually on the bottom blank flange. After the components of vessel, membrane and pipe were assembled, the solenoid valve was opened by the control system. Compressed air blew through the nozzle and dispersed the dust to form a dust cloud in the vessel. After a 50 ms delay, the two electrodes discharged, igniting the powders. Meanwhile, the pressure transducers were activated and the explosion overpressures were recorded. A series of experiments was performed, corresponding to the following configurations: a) Dust explosions in the confined vessel in different aluminium concentrations;
Table 1 Chemical composition of 2 mm aluminium dust sample.
Fig. 2. Bottom blank flange and mushroom nozzle.
Activated aluminium content/%
Main impurity content/% Fe
Si
Cu
H2O
99.84
0.071
0.067
0.002
0.02
X.-Q. Yan, J.-L. Yu / Journal of Loss Prevention in the Process Industries 26 (2013) 676e682
b) Dust explosions vented directly to the ambient atmosphere (simply vented vessel), with relief diameters of 20, 25 and 40 mm in different aluminium concentrations; c) Dust explosions vented through relief pipe with diameters of 20, 25, 40 mm, and lengths of 0.9, 1.2, 1.5 and 1.8 m in different aluminium concentrations. In order to increase the occurrence probability of burn-up in the relief pipe, a series of experiments on injecting the aluminium dust twice were also conducted. After the vessel and the membrane were assembled, 0.1 g aluminium dust was placed on the membrane. The purpose was to increase the dust concentration in the relief pipe after the membrane ruptured. Experiments on injecting the aluminium dust twice corresponded to the following configuration: d) Dust explosions vented through relief pipes with diameters of 20, 25, 40 mm, and lengths of 0.9, 1.2, 1.5 and 1.8 m in different aluminium concentrations, when 0.1 g extra aluminium powders were placed on the membrane. Each experiment was performed repeatedly for reproducibility. The experiment of configurations a) and b) was normally repeated twice. However, the experiment in configurations c) and d) was conducted five or six times under the same experimental conditions. 3. Results and discussion 3.1. Characteristics of explosion overpressure in vessel and relief pipe Dust explosions in the confined vessel and in the simply vented vessel were conducted first, before doing experiments with relief pipe. The results are presented in Figs. 4 and 5. Fig. 4 illustrates the overpressure variations with time, in a confined vessel for aluminium dust concentrations ranging from 300 g m3 to 1000 g m3. Shortly after the end of 20 ms, a rapid increase is observable in the vessel, corresponding to the rapid combustion reaction of aluminium dust with oxidation. For a short period about 40e60 ms, the overpressure reaches the maximum value and then decreases slowly. Typical trends are observed from different dust concentrations. The maximum explosion overpressure achieves the maximum value at 800 g m3, followed by a decrease for either larger or smaller concentrations, in agreement
0.5
P / MPa
0.4 0.3 0.2
300g·m-3 500g·m-3
0.1
800g·m-3 1000g·m-3
0 0
50
100
150
200
250
300
t / ms Fig. 4. Overpressure variations in confined vessel for different dust concentrations.
0.3 300g·m-3 500g·m-3 800g·m-3
0.2
1000g·m-3
P / MPa
678
0.1
0 0
50
100
150
t / ms Fig. 5. Overpressure variations in the vessel vented directly to the ambient atmosphere with 25 mm relief diameter.
with results obtained in 20 L apparatus (Dufaud, Traore, Perrin, Chazelet, & Thomas, 2010). However, the maximum explosion overpressures are 0.32, 0.41, 0.43 and 0.39 MPa, corresponding to 300, 500, 800 and 1000 g m3; much lower than the values in 20 L apparatus. The reasons have been discussed by many researchers (Eckhoff, 2003). It is also observed that the time taken to reach the maximum explosion overpressure differs. The shorter times are obtained at 800 and 500 g m3, whereas the longer ones occur at 300 and 1000 g m3. From the slope of the curves in Fig. 4, it can be inferred that the maximum rate of pressure rise also follows the rule: larger ones are at 800 and 500 g m3, and smaller ones are at 300 and 1000 g m3. Fig. 5 shows the overpressure-time histories in the vessel vented directly to the ambient, with a 25 mm relief pipe. It can be seen that the bursting pressure is about 0.22e0.24 MPa (2-layer polyethylene membrane; 25 mm relief diameter). An outstanding characteristic on all curves is that the pressure begins to fall just after the membrane ruptures. Consequently, the maximum explosion overpressure in a simply vented vessel equals the bursting pressure of the membrane. The combustion time before the membrane ruptures is different for different dust concentrations: nearly 110 ms for 1000 g m3, but 70 ms for 800 g m3. The combustion time before venting (shown in Fig. 5) does not agree well with that in a confined vessel, resulting from the non-uniform dispersion and reaction of aluminium dust in the vessel. The following analysis of the characteristics of the explosion overpressure in the vessel and relief pipe is based mainly on the results obtained from the pipe FT ¼ 25 mm, LT ¼ 1.5 m, and presented in Figs. 6e11. The effects of varying the pipe length LT and diameter FT on overpressure characteristics will be discussed later. Fig. 6 shows a typical example of simultaneously recorded overpressures of PC, P1, P2 and P3, when aluminium dust concentration equals 300 g m3. For vessel-pipe assembly, PC first increases after the aluminium dust is ignited. Later, it reaches the bursting pressure (Pv ¼ 0.23 MPa) at t0 ¼ 56.52 ms. The rupture of the membrane induces the pressure to decrease rapidly. Hence the maximum explosion overpressure in the vessel is 0.23 MPa (bursting pressure). Shortly after the membrane ruptures at t1 ¼ 57.72 ms, P1 increases rapidly to the maximum value and then decreases, owing to the pressure wave passes. Soon, at the time of 57.72 ms and 59.39 ms, a similar trend can be seen in P2 and P3, respectively. Moreover, the maximum values of P1, P2 and P3 are about 0.13, 0.10 and 0.09 MPa, indicating the attenuation of pressure wave along the relief pipe.
X.-Q. Yan, J.-L. Yu / Journal of Loss Prevention in the Process Industries 26 (2013) 676e682
0.3
679
0.3
Pv
Pc
Pc
P1
P1
P2
0.2
P2
0.2
P3
P C2
P /MPa
P / MPa
P3
P C1
0.1
0.1
t2 t1
t3
t0
0 0
20
40
0
60
80
0
20
40
60
80
100
t /ms
t /ms Fig. 6. Overpressure variations in vessel (PC) and relief pipe (P1, P2, P3) (FT ¼ 25 mm, LT ¼ 1.5 m, c ¼ 300 g m3).
Fig. 8. Overpressure variations in vessel (PC) and relief pipe (P1, P2, P3) (FT ¼ 25 mm, LT ¼ 1.5 m, c ¼ 1000 g m3).
From all the experimental results with different aluminium dust concentrations, it is observed that not all the overpressuretime histories are analogous with the results shown in Fig. 6. Fig. 7 illustrates the overpressure-time histories when aluminium dust concentration equals 800 g m3. Compared to the curves in Fig. 6, the maximum value of P2 in Fig. 7 is nearly equivalent to that of P1, indicating that the pressure wave does not decay along the pipe any more. This phenomenon is more significant when aluminium dust concentration increases to 1000 g m3 (see Fig. 8). Besides the characteristics of nearly same values of the maximum values of P1 and P2, a second overpressure peak (PC2 in Fig. 8) appears during the depressurization process, although PC2 (0.16 MPa) is smaller than the first peak PC1 (0.22 MPa). The most important observation from analysis of Figs. 7 and 8 is that pressure intensification happens in the relief pipe or in vessel, during the aluminium dust explosion venting process. As might be expected, the regrowth of the explosion overpressures of PC and P2 is a result of the burn-up phenomenon, similar to that described by Ponizy and Leyer (1999a) during a gas explosion venting process. Kordylewski and Wach (1988) confirmed the burn-up phenomenon during dust explosion venting with a relief pipe when the vent is
uncovered. It has now been verified that burn-up still occurs when the vent is originally covered by the venting device (polyethylene membrane in this paper). The burn-up phenomenon in the relief pipe during the aluminium dust explosion venting can be explained as follows, in Fig. 9. Fig. 9(a) shows the flame propagating towards the pipe entrance in the confined vessel. When the membrane ruptures, pressure waves and the unburned aluminium powders enter the pipe, inducing P1 to increase rapidly. After a short period of time, the flame enters the pipe entrance (Fig. 9(b)) and is elongated in the contracted flow, owing to the sudden area change. The flame front becomes a forward coniform tip, leaving a volume of cold mixture unburned around it (Ponizy & Leyer, 1999a), which makes the heat transmit to the unburned mixture more efficiently. Consequently, the flame tends to be extinguished. If the flame is extinguished successfully, only the pressure wave generated before the membrane ruptures in the vessel is vented, damping down along the pipe, forming the overpressure-time histories shown in Fig. 6. However, if the flame is not extinguished, and the concentration of the unburned aluminium powders in the relief pipe is exactly in the range of the flammable limit, the aluminium dust will be reignited and burn-up will occur (Fig. 9(c)).
0.3 Pc P1 P2
0.2 P /MPa
P3
0.1
0 0
20
40
60
80
t /ms Fig. 7. Overpressure variations in vessel (PC) and relief pipe (P1, P2, P3) (FT ¼ 25 mm, LT ¼ 1.5 m, c ¼ 800 g m3).
Fig. 9. Burn-up phenomenon in relief pipe during dust explosion venting (a) flame propagates in the confined vessel (b) flame entering the pipe entrance (c) burn-up in relief pipe (d) influence of burn-up on overpressure in vessel and pipe.
680
X.-Q. Yan, J.-L. Yu / Journal of Loss Prevention in the Process Industries 26 (2013) 676e682
0.3 P C1 P 22
0.2
P C2
P / MPa
P 12 P 21 P 11
Pc
0.1
P 32
P1 P2 P3 P 31
0 40
50
60
70
80
90
t / ms Fig. 10. Overpressure variations in vessel (PC) and relief pipe (P1, P2, P3) when adding 0.1 g dust on membrane (FT ¼ 25 mm, LT ¼ 1.5 m, c ¼ 300 g m3).
Two kinds of results may happen as a result of the burn-up in the relief pipe: (1) Pressure intensification only occurs in the relief pipe. If the combustion reaction is moderate, or burn-up only lasts for a moment (lack of dusts or other reasons), the heat generated by the combustion reaction can only induce a slight pressure increase near the location where burn-up occurs. By comparing Figs. 6 and 7, it is noticed that only the P2 rises when burn-up happens, indicating that burn-up occurs around the position of the third pressure transducer. (2) Pressure intensification occurs both in the vessel and in the relief pipe. If the combustion reaction is vigorous, the pressure wave will block the original venting process, disturb the flow, so the unburned mixture in the corner of the vessel burns (Fig. 9(d)). The second pressure peak appears in the vessel, as shown in Fig. 8. When performing ducting experiments on dust explosion with an uncovered vent, Kordylewski and Wach (1988) reported the pressure peak resulting from burn-up (they called it ‘secondary
0.4 P C2
0.3 P / MPa
P 22
Pc
P C1
P1 P2 P3
0.2
P 12
0.1
P 32
explosion’ in their paper) preceded the main pressure wave. However, when the vent is covered, the pressure peak induced by burn-up is behind the main pressure wave. It can be inferred that the reaction is more complete in the vessel with a covered vent. When the membrane ruptures, the flame and pressure waves enter the pipe at relatively higher pressure than that in the uncovered vent. Also, the propagation of flame and pressure wave is quicker. Hence, the occurrence of the burn-up phenomenon is difficult compared with an uncovered vent. Also, the severity of burn-up becomes weaker. More details have been found from experimental results: Firstly, it is observed that the aluminium dust concentration in the vessel has a significant effect on the occurrence of burn-up. Burn-up only occurs at relatively high levels of concentrations, for example, 800 g m3 and 1000 g m3, in the experiments of FT ¼ 20 mm and LT ¼ 1.5 m. Noticing that 800 g m3 is the optimum concentration of aluminium dust in a confined vessel, this can be explained as follows: the dust concentration influences the amount of the unburned mixture vented into the relief pipe. The higher the concentration in the vessel is, the more the unburned mixture vents into the pipe, and the mixture ignites. Secondly, the occurrence of burn-up in experiments with the same experimental conditions is not deterministic, but probabilistic. It is expected that the factors influencing the occurrence of burn-up are numerous. Also, the experimental conditions, such as the uniformity level of dust deposit, dispersion and reaction, the rupture shape of the membrane, are uncontrolled. It should be emphasized that the phenomenon of burn-up in the relief pipe is definitely existent during dust explosion venting, and the probability is above 50% when the aluminium dust concentration is up to 1000 g m3. In the experiments of Kordylewski and Wach (1988), the burn-up phenomenon seemed to definitely occur, as they did not mention the probability of the burn-up. Thirdly, although P2 is greatly enhanced by burn-up in the relief pipe, the maximum explosion overpressure in the vessel still equals the bursting pressure. Experiments on further increasing the aluminium concentration in the relief pipe at the instant of the flame entering the pipe entrance were also performed, by adding 0.1 g aluminium powders on the membrane. Fig. 10 shows the overpressure variations of PC, P1, P2 and P3 when the original aluminium dust concentration is 300 g m3. For a better demonstration, the overpressure profile in the vessel during 1e40 ms is not listed. A clearly twohumped phenomenon appears on the overpressure curves, both in the vessel and in the relief pipe. Values of the overpressure peaks are listed in Table 2. It can be seen that the second peaks (P12 and P22) of P1 and P2 are both larger than their first overpressure peaks (P11 and P21), indicating that more severe pressure intensification than that in Figs. 7 and 8 occurs, and propagates owing to the re-reaction of the combustion. As seen in Fig. 10, there is a short period of time when P2 is larger than P1 and PC. It is expected that the pressure drop between PC and P2 is negative. A reverse flow is generated between relief pipe and vessel. As usual, the reason for the increased pressure in the vessel is that the disturbance and blockage to the venting process makes the unburned mixture still remaining in the vessel react. The second pressure peak in the vessel appears just after the appearance of the second peaks of P1 and P2. In this experimental condition, the
0 60
70
80
90
t / ms Fig. 11. Overpressure variations in vessel (PC) and relief pipe (P1, P2, P3) when adding 0.1 g dust on membrane (FT ¼ 25 mm, LT ¼ 1.5 m, c ¼ 800 g m3).
Table 2 Values of first and second overpressure peaks in Fig. 10/MPa. PC1
PC2
P11
P12
P21
P22
P31
P32
0.24
0.18
0.11
0.15
0.11
0.17
0.08
0.07
X.-Q. Yan, J.-L. Yu / Journal of Loss Prevention in the Process Industries 26 (2013) 676e682
second overpressure peak on PC curve (PC2) is still smaller than the first peak (PC1). When the original dust concentration is up to 800 g m3, experimental results are shown in Fig. 11. The second pressure peak PC2 (0.31 MPa) remained in the vessel is much larger than the first pressure peak (0.27 MPa, bursting pressure of membrane). The severity of the dust explosion in the vessel is enhanced greatly, because the maximum explosion overpressure changes from 0.27 MPa to 0.31 MPa. Unlike Fig. 10 mentioned above, the overpressures of P1, P2 and P3 increase directly to the second peaks, rather than having visible first peaks. In this experimental condition, the aluminium dust concentration near the pipe entrance is high enough to be ignited easily after the membrane ruptures. When the flame enters the pipe, a rapid transient combustion reaction starts violently in the pipe, accompanied by a strong pressure impulse in the pipe. Consequently, the backward and forward movement of the pressure wave and the mixture results in the obvious increase in overpressure, both in the vessel and the pipe. This kind of burn-up in the relief pipe is similar to an explosion, and was termed ‘explosion-like combustion’ by Ponizy and Leyer (1999a). 3.2. Effects of varying the pipe length LT and diameter FT on burnup A parametric study involving LT and FT was performed, in order to see whether some effects of pipe dimensions on the burn-up phenomenon could be discovered. Results are were as follows: i) The effect of pipe length on overpressures in the vessel and relief pipe is definite, as shown in Fig. 12. The curves are taken from the explosion results. For clarity and conciseness, P3 is not listed in the figure. The horizontal ordinate represents time. It can be seen that the overpressure variations for different pipe lengths are similar. The bursting pressure in every explosion is about 0.24e0.26 MPa. The maximum value of P2 is smaller than that of P1 for LT ¼ 900 mm. The maximum value of P2 gets larger with pipe length, confirming that the larger the pipe length, the more violent the burn-up. However, only at a length of 1800 mm does the severity of explosion in the vessel increase. The second overpressure peak in the vessel is larger than the original peak (bursting pressure). The reason why overpressure in the vessel only rises at LT ¼ 1800 mm is partly because of the frictional drag and inertia of the gas column in the pipe (Ponizy & Leyer, 1999a). In addition, the severity of burn-up also contributes to the increase in explosion overpressure in the vessel. When the pipe length is less than
Fig. 12. Overpressure histories in vessel (PC) and relief pipe (P1, P2) for different lengths (FT ¼ 25 mm, c ¼ 800 g m3).
681
1500 mm, heat generated by the combustion reaction in the pipe dissipates quickly. Pressure cannot be accumulated and the flame extinguishes rapidly. Thus the severity of burn-up is limited. The longer the pipe length, the slower the heat loss. The pressure wave can be accumulated and propagate easily, increasing the severity of burn-up. ii) Pipe diameter has a significant effect on the burn-up phenomenon, as shown in Fig. 13. The horizontal ordinate represents time. One notices firstly that changing the pipe diameter would change the bursting pressure of the membrane, resulting in a complicated situation when analysing the effect of diameter on burn-up phenomenon. However, it is clearly observable that the smaller diameter will increase the violence of burn-up, and hence the overpressures in the relief pipe greatly. When pipe diameter is 20 mm, P2 is much larger than P1. Increasing the pipe diameter to 25 mm and 40 mm, P2 is nearly equivalent to P1. The probable reasons are numerous: firstly, in a relief pipe with a smaller diameter, the dust concentration is relatively larger when unburned dust is vented into the pipe; secondly, the pressure accumulation generated by the combustion reaction is easier in a pipe with a smaller diameter; thirdly, the endurance period of venting is larger with smaller pipe diameters, raising the possibility of the occurrence of vigorous burn-up. It must be inferred that if the pipe diameter is too small to propagate the flame through the pipe entrance (extinguished at the pipe entrance), burnup would never happen. It seems that this problem is nonexistent with pipe diameters of 20, 25 and 40 mm. It also can be seen from Fig. 13 that changing the pipe diameter has no effect on explosion variations in the vessel. Indeed, the burnup in the relief pipe can induce a reverse flow. But if the reverse flow penetrates into a region in the vessel where no unburned mixture exists, the explosion overpressure in the vessel will not change significantly (Ponizy & Leyer, 1999a). 3.3. Further analysis and mitigation measurements on burn-up Apparently, the burn-up phenomenon may result in higher explosion overpressure, both in the vessel and in the relief pipe, rather than in a simple vented vessel, Hence, methods should be studied to prevent the occurrence of burn-up, or to reduce its severity and the subsequent secondary explosion overpressure. Henneton et al. (2006) placed a wire-net insert at the pipe entrance, to delay flame penetration into the pipe during gas explosion venting. They found it was possible to extinguish the flame penetrating into the relief pipe, and so eliminate the burn-up. Di Benedetto, Russo, and Salzano (2005) raised a mitigation method
Fig. 13. Overpressure histories in vessel (PC) and relief pipe (P1, P2) for different diameters (LT ¼ 1.5 m, c ¼ 800 g m3).
682
X.-Q. Yan, J.-L. Yu / Journal of Loss Prevention in the Process Industries 26 (2013) 676e682
of gas explosion overpressure piling, by divergent connections in the interconnected vessel. These methods may serve as a guide for the elimination and mitigation of the burn-up in a relief pipe connected to a vessel subjected to dust explosion. In addition, we conducted experiments on the suppression of pressure waves by placing aluminium silicate wool porous media on the internal wall of the relief pipe. As the experiments have not been finished, the results will not be discussed here. 4. Conclusions 1. The maximum explosion overpressure decays along the relief pipe when the burn-up phenomenon does not occur during dust explosion venting with relief pipe. However, when burnup occurs, the explosion overpressure will be intensified around the position where burn-up occurs in the relief pipe. Second overpressure peaks may appear in the overpressuretime histories, both in the vessel and in the relief pipe. 2. The aluminium dust concentration in the vessel has a significant effect on the occurrence of burn-up. Burn-up only occurs at relatively high levels of dust concentration in the vessel; usually higher than the optimum concentration in a confined vessel. The higher the dust concentration, the more easily burn-up occurs, and the more vigorous the burn-up. 3. The occurrence of the burn-up in experiments with same experimental conditions is not deterministic, but probabilistic. The burn-up phenomenon is inevitable when using a relief pipe with sudden area change, together with the unburned dust at venting instant. 4. By adding 0.1 g aluminium powders to the membrane, the severity of the dust explosion is greatly enhanced. The overpressure intensifications, both in the vessel and in the relief pipe, are so violent that second overpressure peaks are much larger than the first ones. 5. The longer the relief pipe, the more violent the burn-up. When the pipe length is less than 1200 mm in this experiment, burnup has no effect on the explosion severity in the vessel. Pipes with smaller diameters will increase the violence of the burnup greatly, until the diameter is so small that the flame will be extinguished when passing through the pipe entrance.
6. More work needs to be done on the burn-up phenomenon during dust explosion venting. The influence of particle size, bursting pressure, and ignition source position on burn-up should be studied systemically in the future.
Acknowledgements This paper was supported by the National Natural Science Foundation of China (Grant no. 50974027), and the Liaoning Province Science and Technology Project (Grant no. 2010219002).
References Di Benedetto, A., Russo, G., & Salzano, E. (2005). The mitigation of pressure piling by divergent connections. Process Safety Progress, 24, 310e314. Di Benedetto, A., Russo, G., & Salzano, E. (2008). The design of duct venting of gas explosions. Process Safety Progress, 27, 164e172. Dufaud, O., Traore, M., Perrin, L., Chazelet, S., & Thomas, D. (2010). Experimental investigation and modeling of aluminium dusts explosions in the 20 L sphere. Journal of Loss Prevention in the Process Industries, 23, 226e236. Eckhoff, R. K. (2003). Dust explosions in the process industries (3rd ed.). Oxford: Gulf Professional Publishing. Ferrara, G., Di Benedetto, A., Salzano, E., & Russo, G. (2006). CFD analysis of gas explosions vented through relief pipes. Journal of Hazardous Materials, A137, 654e665. Henneton, N., Ponizy, B., & Veyssiere, B. (2006). Control of flame transmission from a vessel to a discharge duct. Combustion Science and Technology, 178, 1803e1819. Kordylewski, W., & Wach, J. (1986). Influence of ducting on the explosion pressure. Combustion and Flame, 66, 77e79. Kordylewski, W., & Wach, J. (1988). Influence of ducting on explosion pressure: small scale experiments. Combustion and Flame, 71, 51e61. Lunn, G. A., Nicol, A. M., Collins, P. D., & Hubbard, N. R. (1998). Effects of vent ducts on the reduced pressures from explosions in dust collectors. Journal of Loss Prevention in the Process Industries, 11, 109e121. McCann, D. P. J., Thomas, G. O., & Edwards, D. H. (1985). Gas dynamics of vented explosions part I: experimental studies. Combustion and Flame, 59, 233e250. Ponizy, B., & Leyer, J. C. (1999a). Flame dynamics in a vented vessel connected to a duct: 1. Mechanism of vessel-duct interaction. Combustion and Flame, 116, 259e271. Ponizy, B., & Leyer, J. C. (1999b). Flame dynamics in a vented vessel connected to a duct: 2. Influence of ignition site, membrane rupture, and turbulence. Combustion and Flame, 116, 272e281. Taveau, J. (2012). Secondary dust explosions: how to prevent them or mitigate their effects. Process Safety Progress, 31, 36e50. Ural, E. A. (1993). A simplified method for predicting the effect of dusts connected to explosion vents. Journal of Loss Prevention in the Process Industries, 6, 3e10.