Combustion Science and Technology The Effect of

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The Effect of Vent Size on Pressure Generation in Explosions in Large L/D Vessels a

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A. ALEXIOU , H. PHYLAKTOU & G. E. ANDREWS

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Department of Fuel and Energy , Leeds University , Leeds, LS2 9JT Published online: 23 Jun 2010.

To cite this article: A. ALEXIOU , H. PHYLAKTOU & G. E. ANDREWS (1996) The Effect of Vent Size on Pressure Generation in Explosions in Large L/D Vessels, Combustion Science and Technology, 113:1, 645-652, DOI: 10.1080/00102209608935520 To link to this article: http://dx.doi.org/10.1080/00102209608935520

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Short Communication The Effect of Vent Size on Pressure Generation in Explosions in Large LID Vessels A. ALEXIOU, H. PHYLAKTOU and G. E. ANDREWS and Energy, Leeds University, Leeds LS2 9JT

Department of Fuel

Downloaded by [University of Leeds] at 07:10 30 May 2014

(Received March 1. 1996)

Abslr.cl-There is a need for more explosion relief data in large LID vessels. In this paper we report an experimental investigation of the effect of vent size on the overpressure development in a large LID, end-vented vessel. Three pressure peaks were identified: PI due to an initial elongated flame (characterislie of large LID explosions); P 2 due to turbulent combustion in the middle and end sections of Ihe vessel and P3 due to the external explosion. PI was found to be the dominant overpressure for small vent areas and 10 increase with decreasing vent size. whereas the flame speed was reduced (as well as P 2 and P,) due to reduction of un burnt gas flow Ihrough and oul of the vessel. Key words: Venting of gas explosions, influence of vent size.

INTRODUCTION Venting is one of the most common procedures used to protect industrial buildings, vessels and process installations from internal explosions. A considerable effort has been devoted in developing calculation procedures for the design of appropriate vent area for system protection. Proposed formulae for the prediction of the maximum pressure during a vented explosion are reviewed in detail by Lunn (1985) and specific recommendations are given in the NFPA standards (NFPA 68, 1988). A method used in industry is the "cubic law" based on Bartknecht (1981) and is given by Equation 1. (1)

where KG is a constant which is assumed to be representative of the explosion behavior of a gas, (dPjdt)max is the maximum rate of pressure rise and V is the volume of the vessel. This method was originally developed for dusts, but later extended also to gases, for vessels of length to diameter ratio (LjD) less than five. The majority of the venting formulae are based on data derived from tests in near cubical vessels. The pressure generation during an explosion in a large LjD vessel is quite different from one of an equal volume but small LjD vessel due to the axial flow. Phylaktou and Andrews (1991) measured fast flame speeds during the initial stages in large LjD explosions which were over ten times the flame speeds in a small LjD vessel (with near spherical flame propagation). This was attributed to an elon645


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A. ALEXIOU er al.

gated flame effect which is characteristic of flame behaviour in the initial stages of explosion in large LID vessels and which results in an increase in mass burning rate (because of the large flame area). This mechanism of initial flame acceleration is quite different from the turbulent flame propagation which may lead to detonation in long tubes (L/D> 70) for propane/air mixtures (Steen and Schampel, 1983). Current recommendations (NFPA 68, 1988) for venting of large L/D vessels are based mainly on the experimental data of Rasbash and Rogowski (1960, 1963) who carried out vented gas explosions of propane and pentane in air, in long ducts with an L/D range of 6 to 48. The size of the vent, defined by the vent coefficient, K; (equal to the ratio of the cross section area of the duct to the area of the vent) was varied from K; = I (completely open) to K; = 64. It was found that the overpressure was directly proportional to K; (or inversely proportional to the vent area). Tite et al. (1991) reported a study of vented natural gas explosions in long enclosures of square cross-section. They also found a proportional relationship between the overpressure and the vent coefficient. Bartknecht (1981) proposed that for silos up to 1000 m 3 in volume the entire roof area should always be used for explosion relief venting regardless of the volume. Additionally the relief vent areas must never fall short of that given by the standard monograms for short LID vessels i.e. the cross section of the silo defines the maximum volume that can be protected. For longer vessels such as pipe systems Bartknecht (1981) reported that effective relief venting is possible only if relief openings of sufficient size are installed at distance of I to 2 metres throughout the length of the pipe. If venting is possible only at one end, then he proposed that such pipe systems should be designed for a nominal pressure of at least 10 bar (for I atm initial system pressure). The investigations reported above are very useful as they provide the only basis for guidance for the explosion protection of large L/D systems. However, overall, the mechanism for pressure generation in these vessels is not thoroughly understood and pressure relief design is not optimized. The aim of the present work was to provide more experimental data on vented gas explosions in vessels of large LID and to explore the mechanism of pressure development and the influence of vent size. The data reported in this work is part of a much larger program investigating vented explosions in large L/D vessels with and without obstacles, currently in progress. This report concentrates on the effects of the vent area on the overpressure in a fixed geometry.

EXPERIMENTAL The tests were carried out in a cylindrical vessel of L/D = 13.6 of 76 mm in diameter constructed from 0.5 m flanged sections. A dump volume was provided in which the explosion gases could be vented safely in the laboratory. The dump volume comprised of two 0.5 m diameter vessels 1 m and 2 m long linked together with 1.5 m long "U" shaped 162 mm diameter pipe. An aluminium gate vacuum valve was used to separate the test section from the dump section. The total pressure in both sections was I atm and the gate valve was opened prior to the explosion test, thus any movement of gases or air from one section to the other was restricted. The vent


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size was varied using single hole stainless steel plates of 3.2 mm thick. A range of vent sizes from K; = 1 to K = 5 were used. The one atmosphere gas/air mixture (methane in air of 6% and 10% v/v) was formed by partial pressures and a homogeneous composition was achieved by circulating the mixture in the explosion vessel using an external recirculating pump. The mixture was then ignited with a spark flush at the end of the explosion vessel flange. The flame travel was recorded by a centerline axial array of mineral insulated exposed junction type K thermocouples. The time of flame arrival was detected as a distinct change in the gradient of the analogue output of the thermocouple and in this way the average flame speed between any two thermocouples could be calculated. The pressure variation was recorded using a series of KELLER pressure transducers mounted at different places in both explosion and dump vessel. In order to eliminate the dump vessel effect, the overpressures reported here were taken by subtracting the pressure recorded by the pressure transducer in the first dump vessel from that pressure recorded by the pressure transducer in the test section near the spark. This technique has recently tested by undertaking vented explosions in the laboratory and it proved to be valid. A fast (200 kHz) 34-channel transient data acquisition was used to record and analyze the data. Three repeat tests were performed at each condition.

RESULTS AND DISCUSSION The pressure development depends on many parameters such as permanently uncovered or covered vents, the geometry of the vessel and the properties of the gas/air mixtures. Tite et al. (1991) identified two pressure peaks; one at the time of the vent removal and one which occurred at the time of burned gas venting. Rasbash and Rogowski (1960) did not report different pressure peaks. Typical pressure time variations from the present work for K; = 1 and K; = 5 are shown in Figure 1 for a 10 % methane/air explosion. For K; = 1 two pressures peaks (P 1 & P 2) occurred before the flame exited the tube (tOUI marked on Figure I is the time at which the flame exited the tube). After the flame exit, a number of oscillatory peaks were observed, of which the highest value is marked as P3 in Figure 1. The first pressure peak (PI) was due to an elongated flame observed in large L/D vessels where the flame accelerates in the axial direction and is associated with higher combustion rate due to large flame surface area (Phylaktou and Andrews, 1991). Once the "skirt" of the elongated flame reached the tube wall the rate of production of burned gas decreased due to a reduction in flame area and the internal pressure started to fall. At this stage only unburned gas was being vented. This pressure peak was observed to be the dominant one for the higher K; vent, as shown in Figure I for K; = 1 . The turbulence of the gas flow induced by the initial fast flame resulted in flame reacceleration due to turbulent combustion and this was manifested as a second pressure peak, marked as P 2 in Figure 1. The pressure fell rapidly when the flame reached the vent marked as t o u l in Figure 1. This was due to the onset of burnt gas venting which led to higher volumetric flow through the vent as well as due to the decreased flame area inside the vessel. When the flame reached the vent, the mixture of unburned gas previously vented was ignited and a third

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A. ALEXIOU et al.

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FIGURE I Typical pressures development for K. = 1 and K. = 5, against time for 10 % CH. in air.

pressure peak, P 3' was observed. The magnitude of this pressure peak decreased with a decrease in the vent area. For K; = 5 only one pressure peak dominated the pressure trace as is shown in Figure I, and this was due to the elongated flame (P Jl.

FLAME SPEEDS Explosions in large LID totally enclosed vessels, ignited at one end, are characterized by an initial axial flame acceleration which results in high flame speeds and high rates of pressure rise, relatively early in the explosion. Phylaktou et al. (1990) and Phylaktou and Andrews (1991) using totally enclosed vessels identified this period (which was previously ignored) as being the most important in hazard consideration. Evidence from the work of Tite et al. (1991) indicated that this characteristic initial phase is still present when the vessel is vented (with the vent away from the ignition point). The present work also supports the importance of the initial phase.

649

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-r-------------------, IO%CH4 __ Kv= 1.0 -o-Kv=1.25 -6-Kv=2.0

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FIGURE 2 Flame speeds as a function of the relative distance from the spark (X/D), for 10 % CH. in air for different K values. I

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The flame speeds for different vent sizes are shown in Figure 2 for 10% methane in air. It is clear that with a reduction in the vent size the flame speed is reduced. Typically the flame speed reached a maximum value at about 5 tube diameters from ignition and then decayed to lower values until it exited the tube. The maximum value of the flame speed decreased with increasing K; (i.e. decreasing vent area). This is thought to be due to the impeding of the unburned gas flow through the vent (because of the smaller vent area); this would have resulted in an associated reduction in the flame speed. For the totally open far-end the flame continued to accelerate until it exited the tube, reaching a maximum value of 135 m/s. This setup, facilitated the free acceleration of the flame; the initial fast flame created turbulence ahead of the flame and this resulted in the acceleration of the flame by turbulent burning. The flame speeds shown in Figure 2 are higher than those measured by Rasbash and Rogowski (1960). For example for an LID = 24 circular vessel of K; = I using pentane in air (at equivalence ratio of 1.33) the maximum flame speed measured was about 65 tn]«. The smaller flame speeds reported by Rasbash and Rogowski (1960) could be due to lower flame speed resolution, since the ionization gaps used were placed at intervals of about I m along the ducts and thus they may not have resolved the maximum flame speed.

650

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Measured overpressures as a function of K,.

PRESSURE DEVELOPMENT The overpressure at the end of the fast flame period (PI) marked by the flame attachment to the wall which was taken as the time at which was a significant change in the pressure signal gradient (as shown in Fig. I ) is shown in Figure 3 as a function of K; for 10 % methane in air. It can be seen that an increase in the K; (i. e. reduction of vent size) resulted in an increase in the Pl' This increase in overpressure was due to the reduction of the outflow of unburned gas from the vessel. This overpressure increased with increasing K; and it was the dominant overpressure for values of K; > 1.25, i.e. only one major peak was observed in the tube. The variation of P 2 with the vent size is also shown in Figure 3. As was mentioned earlier this pressure peak was due to the turbulence of the gas flow induced by the initial flame acceleration, therefore an increase in K; reduced this pressure peak since it restricted the gas flow and thus prevented the development of the turbulent acceleration mechanism mentioned above. This pressure peak was not present for K;» 2. The pressure peak P 3 was significant for K v ";' 1.25. However, the magnitude of this pressure peak was reduced due to the reduction of unburnt mixture in the dump vessel. These results confirm that it is the initial fast flame event resulting in pressure

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Maximum overpressures and maximum flame speedsas function of Kvo

PI that was responsible for the events leading to P 2 and P 3' Additional work has shown that if a vent with a bursting pressure is fitted then PI is increased by the burst pressure and is the maximum overpressure. Consequently the initial flame acceleration is the most important event in vented explosion. Figure 4 shows the relationship between maximum overpressure and maximum flame speed against K; for both 6 % and 10 % methane in air. The maximum flame speed decreased while the maximum overpressure increased with increasing K u' Both effects were due to the reduction of vent gas flow with decreasing vent area. For K,:» 2 the maximum overpressure was approximately proportional to the vent coefficient (K u) and this confirms the findings of Rasbash and Rogowski (1960) and Tite et al. (1991). These overpressures were lower than those reported by Rasbash and Rogowski (1960) shown in Figure 3. The difference in overpressure between the present work and that of Rasbash and Rogowski (1960) could be due to the different gases used.

CONCLUSIONS High flame speeds were observed during vented explosion. These were found to decrease with an increase in the Kv i.e. with a reduction of the vent area, which in effect imposes a restriction to the flow of unburned gas. This work confirmed the

652

A. ALEXIOU er al.

findings of previous works that a linear dependence exists between the overpressure and the vent coefficient (Kv). The important contribution of this study was the identification of three pressure peaks during end-vented gas explosions in large LID vessels. A mechanism of generation of these pressure peaks has been proposed and some conditions under which each peak becomes dominant have been reported.


REFERENCES Bartknecht, W. (1981). Explosions Course Prevention Protection; Springer-Verlag. Lunn, G.A. (1985). Venting Gas and Dus: Explosions - A Review, An ICheM Industrial Fellowship Report, The Institute of Chemical Engineers. NFPA 68 (1988). Explosion Venting, National Fire protection Association Inc. Phylaktou, H., Andrews, G. E. and Herath, P. (1990). Fast Flame Speeds and Rates of Pressure Rise in the Initial Period of Gas Explosions in Large LID Cylindrical Enclosures. J. Loss Prevo Process Ind., 3.4,355. Phylaktou, H. and Andrews, G. E. (1991). Gas Explosions in Long Closed Vessels, Combust. Sci. and Tech.; 77, 27. . Rasbash, 0.1. and Rogowski, Z. W. (1960). Gaseous Explosion in Vented Ducts, Combusl. and Flame, 4, 301. Rasbash, D.J. and Rogowski, Z. W. (1963). Gaseous Explosions in Propane/Air Mixtures Moving in a Straight Unobstructed Duct, Second Symp. on Chemical Proc. Hazards with Special Reference 10 Plan Design, ICheM, London. Steen, A. and Shample, H. (1983). Experimental Investigation of the Run-up Distance of Gaseous Detonations in Large Pipes, IChemE Symp. Series, No. 82, p. 323. Tite, J. P., Binding, T. M. and Marshall, M. R. (1991). Explosion Reliefs for Long Vessels, Fire and Explosion Hazards, The Institute of Energy.