Dust Explosions in the Process Industries Second Edition phần 8 docx

Sizing of dust explosion vents 453 Similar cyclone explosion experiments were conducted in Japan more recently by Hayashi and Matsuda (1988). Their apparatus is illustrated in Figure 6.13. The volume of the cyclone vessel was 0.32 m3, its total height 1.8 m and the diameter of the upper cylindrical part 0.6 m. Dust clouds were blown into the cyclone through a 150 mm diameter duct. The desired dust concentration was acquired by independent control of the air flow through the duct (suction fan at downstream end of system), and the dust feeding rate into the air flow. The dust trapped in the cyclone dropped into a 0.15 m3 dust collecting chamber bolted to the bottom outlet. The exhaust duct of 0.032 m2 cross section and 3 m length ended in a 0.73 m3 cubical quenching box fitted with two vents of 0.3 m2 and 0.1 m2 respectively. The venting of the cyclone itself was through the 0.032 m2 Figure 6.1 3 industrial conditions (From Hayashi and Matsuda, 1988) Experimental cyclone plant for studying dust explosion development under realistic exhaust duct and the almost 10 m long 0.008 m2 dust feeding duct. During explosion experiments two water spraying nozzles for flame quenching were in operation in the exhaust duct in order to protect the fan just outside the quenching box. The ignition source was a 5 kJ chemical igniter located in the dust feeding duct about 2 m upstream of the cyclone. Two different polymer dusts were used in the experiments, namely an ABS resin dust of median particle size 180 pm, and an ethylene-vinyl acetate copolymer dust (EVA) of median particle size 40 pm. In addition to the realistic ‘dynamic’ explosion experiments, Hayashi and Matsuda (1988) conducted a series of experiments with the same two dusts, using an artificial ‘static’ 454 Dust Explosions in the Process industries dust cloud generation method, very similar to that used in the experiments being the basis of the VDI 3673 (1979 edition). As illustrated in Figure 6.14, the dust feeding duct was then blocked at the entrance to the cyclone, which reduced the effective vent area slightly, to 0.032 m2. Figure 6.1 4 0.32 m3 cyclone modified for gen- eration of dust clouds by high-pressure injection through perforated dust dispersion tubes (From Hayashi and Matsuda, 1988) A system of two pressurized dust reservoirs and perforated tube dispersion nozzles were employed for generating the dust clouds. The 5 kJ ignition source was located inside the cyclone, half way up on the axis (indicated by X2). The ignition source was activated about 100 ms after onset of dust dispersion. Envelopes embracing the results of both series of experiments are given in Figure 6.15. As can be seen, the artificial ‘static’ method of dust dispersion gave considerably higher maximum explosion pressures in the cyclone, than the realistic ‘dynamic’ method. This is in accordance with the results of the earlier realistic cyclone experiments of Tonkin and Berlemont (1972). It is of interest to compare the ‘static’ results in Figure 6.15 with predictions by VDI 3673 (1979 edition). A slight extrapolation of the nomographs to 0.32 m2 vent area, assuming St 1 dusts, gives an expected maximum overpressure of about 2.5 bar(g), which is of the same order as the highest pressures of 1.5 bar(g) measured for Sizing of dust explosion vents 455 Figure 6.1 5 Results from vented dust explosions in a 0.32 rn3 cyclone using two different polymer dusts and two different methods of dust cloud generation. 0.03-0.04m2 open vents with ducts. Data from Hayashi and Matsuda (I 988) (From Eckhoff, 1990) the artificial ‘static’ dust dispersion method, and much higher than the pressures measured in the realistic experiments. The NFPA 68 (1988 edition) includes an alternative nomograph which covers all St 1 dusts that do not yield higher P,, in standard closed bomb tests than 9 bar(g). This nomograph gives much lower Pred values than the standard nomograph, in particular for small volumes. In the case of the 0.32 m3 cyclone with a 0.032 m2 vent, the alternative nomograph gives Pred equal to 0.50 bar(g), which in fact is close to the realistic experimental values. This alternative nomograph originates from Bartknecht (1987), and represents a considerable liberalization, by a factor of two or so, of the vent area requirements for most St 1 and St 2 dusts. However, the scientific and technical basis for this liberalization does not seem to have been fully disclosed in the open literature. 6.2.5 REALISTIC EXPERIMENTS IN BAG FILTERS 6.2.5.1 Vented explosions in a 6.7 m3 industrial bag filter unit in UK Lunn and Cairns (1985) reported on a series of dust explosion experiments in a 6.7 m3 industrial bag filter unit. The experiments were conducted during normal operation of the filter, which was of the pulsed-air, self-cleaning type. Four different dusts were used, and their Ks, values were determined according to IS0 (1985) (see Chapter 7). The ignition source was located in the hopper below the filter bag section. In the experiments of main interest here, the vent was in the roof of the filter housing. Hence, in order to get to the 456 Dust Explosions in the Process Industries vent, the flame had to propagate all the way up from the hopper and through the congested filter bag section. The results from the experiments are summarized in Figure 6.16, together with the corresponding VDI 3673 (1979 edition) predictions. Figure 6.16 first shows that the Pred in the actual filter explosions were mostly considerably lower than the corresponding VDI 3673 predictions and close to the theoretical minimum value 0.1 bar(g) at which the vent cover ruptured. Secondly, there is no sensible correlation between the VDI 3673 ranking of expected pressures according to the Ks, values, and the ranking actually found. Figure 6.16 Maximum explosion pressures Prd measured in dust explosions in an industrial 6.7 m3 bag filter unit in normal operation. P,,, = 0.1 bar(g). Data from Lunn and Cairns (1985). Comparison with VDI 3673 (7 979 edition) (From Eckhoff, 1990) Lunn and Cairns (1985) also reported on a series of dust explosion experiments in a generously vented 8.6 m3 empty horizontal cylindrical vessel of LID = 6. The same dusts were used as in the filter experiments, but the dust clouds were generated ‘artificially’ by injection from pressurized reservoirs as in the standard VDI 3673 method. In spite of the similarity between the dust dispersion method used and the VDI 3673 dispersion method, there was no correlation between Pred and Ksr. 6.2.5.2 Vented explosions in a 5.8 m3 bag filter in Norway These experiments were reported in detail by Eckhoff, Alfert and Fuhre (1989). A perspective drawing of the experimental filter is shown in Figure 6.17 and a photograph of a vented maize starch explosion in the filter in Figure 6.18. Dust explosions were initiated in the filter during normal operation. A practical worst-case situation was realized by blowing dust suspensions of the most explosible concentration into the filter at 35 m/s and igniting the cloud in the filter during injection. Four dusts were used, namely, maize starch and peat dust, both having Ksr = 115 bar m/s, and polypropylene and silicon dusts, both having Kst = 125 bar m/s. Considerable effort was made to identify worst-case conditions of dust concentration, and ignition-timing. At these conditions, experimental correlations of vent area and Pred were determined for each dust. Sizing of dust explosion vents 457 Figure 6.17 5.8 m3 experimental bag filter in Norway (from Eckhoff, Alfert and Fuhre, 19891 Figure 6.18 Maize starch explosion in 5.8 m3 experimental bag filter unit in Norway. Vent area 0.16 m2. Static opening pressure of vent cover 0.10 bar(@. Maximum explosion pressure 0.15 bar@). for a much clearer picture see colour plate 8 458 Dust Explosions in the Process Industries As shown in Figure 6.19, the peat dust gave significantly lower explosion pressures than those predicted by VDI 3673 (1979), even if the predictions were based on the volume of the dusty filter section (3.8 m3) only. Figure 6.19 Results from vented peat dust explosions in a 5.8 m3 filter at P,,, = 0.1 bar(@. Comparison with VDI 3673 (I 979 edition) and vent sizing method used in Norway (Eckhoff (1 988)). Injected dust concentration 600 g/m3. e = dusty section of filter, 0 = clean section of filter (From Eckhoff, 1990) Figure 6.20 summarizes the results for all the four dusts. As can be seen, the explosion pressures measured were generally considerably lower than those predicted by VDI 3673 (1979 edition) for all the four dusts as long as the ignition source was a nitrocellulose flame. However, the singular result obtained for silicon dust ignited by a silicon dust flame emphasizes the different nature of initiation and propagation of metal dust flames, as compared with flames of organic dusts. (See discussion by Eckhoff, Alfert and Fuhre (1989), and Chapter 4.) As illustrated by Figure 6.19, Pred scattered considerably, even when the nominal experimental conditions were identical. This again illustrates the risk-analytical aspect of the vent sizing problem (see Section 6.6). Figure 6.19 suggests that VDI 3673 is quite conservative, whereas the method used in Norway is quite liberal, in agreement with the picture in Figure 6.3. In Figures 6.20 and 6.21 the 5.8 rn3 filter results for all four dusts are plotted as functions of Ksr from 1 m3 IS0 standard tests, and (dPldt),,, from Hartmann bomb tests. (See Chapter 7.) Predictions by various vent sizing methods have also been included for comparison. The data in Figure 6.20 show poor correlation between the maximum explosion pressures measured in the filter at a given vent area, and the maximum rates of pressure rise determined in standard laboratory tests. Although the Kst values of the four dusts were very similar, ranging from 115 to 125 bar ds, the Pred (nitrocellulose flame ignition) for the four dusts varied by a factor of two to three. In the case of the Hartmann bomb Figure 6.21 indicates a weak positive correlation between Pred and (dPldt),,, for nitrocellulose ignition, but it is by no means convincing. Figure 6.21 also gives the corresponding correlations predicted by three different vent sizing methods based on Hartmann bomb tests. Both the Swedish and the Norwegian methods are quite liberal. The Rust method oversizes the vents for the organic dusts excessively for (dPldt),,, > 150 bark There is, however, fair agreement with the data for silicon dust ignited by a silicon dust flame. Sizing of dust explosion vents 459 Figure 6.20 Maximum explosion pressures for four dusts in a vented 5.8 m3 filter at two vent areas, as functions of KS, determined by the 20 litre Siwek sphere. = 0.2 m2 vent area 0 = 0.3 m2 vent area + = silicon dust flame ignition of silicon dust P,,,, = 0.1 bar(@ Comparison with VDI (1979 edition) predictions for 3.8 m3 volume (dusty section of filter) (From Eckhofc 1990) nitrocellulose flame ignition I Figure 6.21 Maximum explosion pressures for four different dusts in a vented 5.8 m3 filter at two vent areas, as functions of (dP/dt),,, determined by the Hartmann bomb. = 0.2 m2 vent area 0 = 0.3 m2 vent area + = silicon dust flame ignition of silicon dust P,,, = 0.1 bar@) Cornparison with maximum explosion pressures pre- dicted for 3.8 m3 volume (dusty section of filter) by three different methods (From Eckhoff, 1990) nitrocellulose flame ignition I The use of closed-bomb tests for predicting the violence of accidental dust explosions in industrial plants was discussed by Eckhoff (1984/85). (See also Chapter 7.) 6.2.6 OTHER LARGE-SCALE EXPERIMENTS RELEVANT TO INDUSTRIAL PRACTICE Some quite early work that is still of considerable interest and practical value deserves attention. The pioneering work of Greenwald and Wheeler (1925) on venting of coal dust explosions in long galleries is discussed in Section 4.4.7 in Chapter 4. 460 Dust Explosions in the Process Industries A set of results from the comprehensive investigation by Brown and Hanson (1933) on venting of dust explosions in volumes typical of the process industry were reproduced in Figure 6.1. The paper by Brown and Hanson describes a number of interesting observations and considerations including the effect of the location and distribution of the vents and the influence of the size and type of ignition source. Brown (1951) studied the venting of dust explosions in a 1.2 m diameter, 17 m long horizontal tube with and without internal obstructions. The tube was either closed at one end and vented at the other, or vents were provided at both ends. In some experiments an additional vent was also provided in the tube wall midway between the two ends. The location of the ignition point was varied. Brown and Wilde (1955) extended the work of Brown (1951) by investigating the performance of a special hinged vent cover design on the explosion pressure development in a 0.76 m diameter, 15 m long tube with one or more vents at the tube ends and/or in the tube wall. Pineau, Giltaire and Dangreaux (1974, 1976), using geometrically similar vented vessels of LID about 3.5 and volumes 1,10 and 100 m3, investigated the validity of the vent area scaling law A2 = AI (V21V1)2/3. They concluded that this law, which implies geometrical similarity even of vent areas, was not fully supported by the experiments. However, as long as the dust clouds were generated in similar ways in all three vessel sizes, and the ignition points were at the vessel centres, the experiments were in agreement with the law A2 = A1 (V2/V1)0.52. Pineau, Giltaire and Dangreaux (1978) presented a series of experimentally based correlations for various dusts between vent area and vessel volume for open and covered vents, with and without vent ducts. Both bursting membranes and spring-loaded and hinged vent covers were used in the experiments. Zeeuwen and van Laar (1985) and van Wingerden and Pasman (1988) studied the influence of the initial size of the exploding dust cloud in a given vented enclosure, on the maximum pressure developed during the vented explosion. The investigation showed that the pressure rise caused by the explosion of a dust cloud filling only part of a vented enclosure is higher than would perhaps be intuitively expected. Even if the dust cloud is considerably smaller than the enclosure volume, it is usually necessary to size the vent as if the entire volume of the enclosure were filled with explosible cloud. Gerhold and Hattwig (1989) studied the pressure development during dust explosions in a vented steel silo of rectangular cross section. The length-to-equivalent-diameter ratio could be varied between two and six. The explosion pressure and flame front propagation histories were measured using a measurement system similar to that illustrated in Figure 6.6. The influence of the key parameters of industrial pneumatic dust injection systems on the explosion development was investigated, in particular injection pipe diameter, air flow and dust-to-air ratio. The general conclusion was that the maximum pressures generated with realistic pneumatic injection were substantially lower than those predicted by the VDI 3673 (1979 edition) guideline. Sizing of dust explosion vents 46 1 6.3 VENT SIZING PROCEDURES FOR THE PRESENT AND NEAR FUTURE 6.3.1 BASIC APPROACH AND LIMITATIONS As shown in Section 6.2, realistic vented dust explosion experiments, mostly conducted during the 1980s, have demonstrated that none of the vent sizing codes in use up to 1990 are fully adequate. It is proposed, therefore, that for the present and near future, sizing of dust explosion vents be primarily based on the total evidence from realistic experiments that is available at any time. The following suggestions presuppose that the initial pressure in the enclosure to be vented is atmospheric. Furthermore, the vent covers must open completely within times comparable to the opening times of standard calibrated rupture diaphragms. In the case of heavier, and reversible, vent covers such as hinged doors with counterweights, or spring-loaded covers, additional considerations are required. The same applies to the use of vent ducts and the new, promising vent closure concept that relieves the pressure, but retains the dust and flame, thus rendering vent ducts superfluous. (See Section 1.4.6 in Chapter 1.) 6.3.2 LARGE EMPTY ENCLOSURES OF VD < 4 As shown in Figure 6.3, a large empty enclosure of volume 500 m3 and LID = 4, in the absence of excessive dust cloud turbulence, requires considerably smaller vents than those specified by VDI 3673 (1979 edition) or NFPA 68 (1988 edition). This also applies to the more liberal St 1 nomograph for constant-volume pressures P,,, < 9 bar(g), proposed by Bartknecht (1987). (Not included in Figure 6.3.) As shown in Figure 6.12, even more dramatic reductions in vent area requirements were found in a 250 m3 spherical vessel. In this case the vent area actually needed was only one-eighth of that specified by VDI 3673 (1979 edition). When sizing vents for large enclosures of LID d 4, the exact vent area reduction factor as compared with VDI 3673 (1979 edition), has to be decided in each case, but it should certainly not be greater than 0.5. In some cases it may be as small as 0.2 to 0.1. The new edition of VDI 3673 (draft probably 1991) is likely to take this into account. 6.3.3 LARGE, SLENDER ENCLOSURES (SILOS) OF VD > 4 The only investigation of vented dust explosions in vertical silos of LID > 4 and volumes > 100 m3 that has been traced, is that described in Section 6.2.2. The strong influence of the location of the ignition source on the explosion violence, as illustrated in 462 Dust Explosions in the Process industries Figure 6.9, is a major problem. It is necessary, in each specific case, to analyse carefully what kind of ignition sources are likely to occur, and at what locations within the silo volume ignition has a significant probability (Eckhoff (1987)). For example, if the explosion in the silo cell can be assumed to be a secondary event, initiated by an explosion elsewhere in the plant, ignition will probably occur in the upper part of the silo by flame transmission through dust extraction ducts or other openings near the silo top. In this case a vent of moderate size will serve the purpose even if LID of the silo is large. However, the analysis might reveal that ignition in the lower part of the silo is also probable, for example because the dust has a great tendency to burn or smoulder. In this case even the entire silo roof may in some situations be insufficient for venting, and more sophisticated measures may have to be taken in order to control possible dust explosions in the silo. 6.3.4 SMALLER, SLENDER ENCLOSURES OF VD > 4 The data of Bartknecht (1988) and Radandt (1985, 1989) from experiments in the 20 m3 silo constitute one useful reference point. Further data for a 8.7 m3 vessel of LID = 6 is found in the paper by Lunn and Cairns (1985). However, it is necessary to pay adequate attention to the way in which the dust clouds are generated in the various experiments and select experimental conditions that are as close as possible to the conditions prevailing in the actual industrial enclosure (see Figure 6.11). Depending on the way in which the dust cloud is generated in practice, vent area reduction factors, with reference to VDI 3673 (1979), may vary between 1.0 and 0.1. 6.3.5 INTERMEDIATE (10-25 m3) ENCLOSURES OF SMALL VD The experimental basis is that of the VDI 3673 guideline (1979 edition) with highly homogeneous, well-dispersed and turbulent dust clouds, and the more recent results for much less homogeneous and less well-dispersed clouds (Figure 6.12). The vent area requirements identified by these two sets of experiments differ by a factor of up to 5. Adequate vent sizing therefore requires that the conditions of turbulence, dust dispersion and level and homogeneity of dust concentration for the actual enclosure be evaluated in each specific case. 6.3.6 CYCLONES Two realistic investigations have been traced (Tonkin and Berlemont (1972) and Hayashi and Matsuda (1988)), and both suggest a significant vent area reduction in relation to VDI 3673 (1979 edition). The early investigation by Tonkin and Berlemont using a cyclone of 1.2 m3, indicates an area reduction factor of 0.2. The more recent investigation by Hayashi and Matsuda, using a smaller cyclone of 0.32 m3, indicates a factor of about 0.5. [...]... representative of the dust or powder in the industrial process of concern, even the most perfect pair of test method and theory (Figure 7.1) will yield misleading assessments of the real industrial hazard  486 Dust Explosions in the Process industries It is useful to distinguish between two different levels of sampling The first, and often most crucial, is the initial collection of dust in the industrial plant... the value, in relation to the dust cloud state in the actual industrial situation to be simulated, must be evaluated The second part of the venting theories, describing the flow out of the vent, is generally based on the classical, well-established theory for flow of gases through orifices 4 68 Dust Explosions in the Process Industries A third common feature of existing theories is the use of the fact... that the state of the dust cloud in the closed-bomb test used for predicting the explosion violence corresponds to the state of the dust cloud in the vented explosion of concern 6.5.1 0 CONCLUDING REMARK In all the theories outlined above, the modelling of the burning rate of the dust cloud is incomplete The situation may be improved by making use of systematic correlations of burning rates and initial... UK, whereas Burgoyne (19 78) related the results of the various test methods to means of preventing and mitigating the industrial hazard  482 Dust Explosions in the Process industries In Germany, Selle (1957) gave an account of the quite extensive work on dust explosion testing that was carried out, in particular at the Bundesanstalt fur Materialpriifung (BAM) in Berlin, in the first half of this century... progress in gas and dust explosion modelling will soon result in comprehensive theories and computer simulation codes for conventional venting configurations in the process industry However, in the meantime several less comprehensive, more approximate theories are in use, in which it is assumed that the burning of the dust cloud and the flow out of the vent can be regarded as independent processes In all the. .. processes In all the theories traced, it is assumed that the burning rate of the dust cloud in the vented enclosure can in some way or other be derived from the burning rate of the same dust in a standard closed-bomb test The theories vary somewhat in the way in which this derivation is performed, but in general none of the existing venting theories seem to handle the complex burning rate problem satisfactorily... all the gas in the vessel being to the left of the surface, and sufficiently apart from the vent for the gas velocity through the surface to be negligible They then formulated a macroscopic energy balance equation for the flow system describing the venting process, assuming that all the pressure and heat energy was located to the left of the x - x line in Figure 6.24, and all the kinetic energy to the. .. knowledge about the actual industrial process and plant, constitute the existing basis for assessing the ‘worst credible explosion’ In the future, systematic studies of different selected representative scenarios can probably be conducted by using comprehensive computer simulation models 4 78 Dust Explosions in the Process Industries REFERENCES Anthony, E L (1977119 78) The Use of Venting Formulae in the Design... containers, each of 0.33 m3 capacity and 8. 3 bar(g) initial pressure and being connected to four perforated dust dispersion nozzles Two nozzle sets, i.e eight nozzles, were mounted on each of two opposite walls inside the chamber The dust was placed in four canisters, one for each of the pressurized air containers, located in the lines between the pressurized 466 Dust Explosions in the Process Industries. .. rise closer to industrial reality 470 Dust Explosions in the Process industries In a subsequent investigation, Heinrich and Kowall (1972) discussed the influence on Pred replacing the point ignition source normally used in the large-scale experiments, by of a turbulent flame jet Whereas flame-jet ignition caused a considerable increase of (dPldt),,, in closed vessel experiments, the increase of Pred . placed in four canisters, one for each of the pressurized air containers, located in the lines between the pressurized 466 Dust Explosions in the Process Industries containers and the dispersion. experiments with the same two dusts, using an artificial ‘static’ 454 Dust Explosions in the Process industries dust cloud generation method, very similar to that used in the experiments being the basis. venting of coal dust explosions in long galleries is discussed in Section 4.4.7 in Chapter 4. 460 Dust Explosions in the Process Industries A set of results from the comprehensive investigation