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Measurements of turbulence intensity in the standard 1 m3 vessel
Journal of Loss Prevention in the Process Industries 40 (2016) 180e187
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Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Measurements of turbulence intensity in the standard 1 m3 vessel Zdzislaw Dyduch*, Adrian Toman, Wojciech Adamus w, ul. Podleska 72, Poland Experimental Mine “Barbara” of Central Mining Institute, PL-43-190, Mikoło
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 July 2015 Received in revised form 20 December 2015 Accepted 21 December 2015 Available online 24 December 2015
The standard 1 m3 vessel is an apparatus recommended for assessment of dust explosion indices by ISO, European and American Standards. Unlike the standard 20-l sphere, only few authors reported measurements of turbulence intensity in 1 m3 vessel. The paper presents results of such measurements performed in 1 m3 vessel built and used at Experimental Mine Barbara of Central Mining Institute, Poland. Transient flow velocity in the vessel generated by an air injection from the dust dispersion system was measured with use of Bi-Directional Velocity Probe developed by McCaffrey&Heskestad. The velocity was measured in several points along the vessel's axes and in two directions at each point. From the measurements the root-mean-square of instantaneous velocity u'rms and the characteristic length scale of turbulence were calculated. The results confirm discrepancy between turbulence intensity in the time periods characteristic for 1 m3 vessel and 20-l sphere, reported previously by other authors. The analysis of possible reasons of the discrepancy identifies the dust dispersion system as the main source of ambiguity, particularly the design and operation of the fast acting valve. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Explosion parameters Turbulence intensity Bi-Directional Velocity Probe Standard test methods Turbulence measurement
1. Introduction Maximum explosion pressure and maximum rate of pressure rise are the most important explosion severity parameters used in the process safety assessment of dust explosion hazard in industry. Subsequently these parameters are essential for adequate design of protective methods to assure proper control of the dust explosion (Eckhoff, 2003). Standards either in Europe (EN 14034-1, 2011; EN 14034-2, 2011) or in the United States (ASTM E1226-12a, 2012) or international (ISO 6184/1, 1985) define the test methodology. The recommended size of the test vessel is 1 m3 but other experimental vessels are allowed. The smaller volume test chamber requires less sample mass per test, it is simpler in operation and therefore allows higher number of test performed. The most widely used is the 20-l sphere manufactured by Kühner AG. However the explosion severity parameters, especially the KSt max, not always agree with each other when measured in these two vessels (Proust et al., 2007). Amongst factors that contribute to the difference is different turbulence intensity created inside the test vessel after dispersion of a test sample and different rate of the turbulence decay. The turbulence intensity measured in 20-l explosion sphere
* Corresponding author. E-mail addresses: [email protected] (Z. Dyduch), [email protected] (A. Toman), w. [email protected] (W. Adamus). http://dx.doi.org/10.1016/j.jlp.2015.12.019 0950-4230/© 2016 Elsevier Ltd. All rights reserved.
by Dahoe (2000) and Pu & Jarosinski (1991) overlap very well. The ignition starts at 60 ms after the injection. At that time the turbulence intensity quantified by root-mean-square of velocity fluctuation is approximately 3 m/s. In 200 ms, this is a time when an explosion usually is completed, the turbulence intensity decays to approximately 0.5 m/s with Rebound Nozzle used for dispersion. Other types of the dispersion nozzles generate different levels of the initial turbulence intensity. In 1 m3 test vessel the mixture is ignited at 600 ms. The turbulence intensity measured by van der Wel (1993) at that time is 0.5 m/s. Measurements reported by Proust et al. (2007) in his 1 m3 vessel show fast turbulence decay after injection, however at the time of typical explosion duration, between 600 and 1000 ms, the turbulence intensity levels off at the value of 2 m/s. Hauert et al. (1994) measured both horizontal and vertical components and obtained the values of 1.2 and 5.36 m/s, respectively, after 600 ms. The difference between the measured turbulence intensities in 1 m3 vessel might be due to differences in engineering factors like shape of the test vessel, dispersion system or dispersion nozzle used. The paper presents results of turbulence intensity measurement performed in 1 m3 vessel built and used at Experimental Mine Barbara of Central Mining Institute, Poland. The vessel fulfils the requirements imposed by the European Standards EN 14034. Instantaneous velocity in the vessel generated by an air injection from the dust dispersion system was measured with use of BiDirectional Velocity Probe BDVP developed by McCaffrey &
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Heskestad (1976). The velocity was measured in several points along two vessel's axes, in two directions at each point. From the measurements the root-mean-square of velocity fluctuation u'rms and the characteristic length scale of turbulence were calculated. 2. Experimental setup Transient flow velocity generated by air outflow from the dust dispersion system was measured inside the standard 1 m3 vessel. The vessel, together with the dispersion system, meets the requirements of the European Standards EN 14034. The only difference was the dispersion nozzle. The Standards recommends Perforated Dispersion Ring as a first choice, allows however the usage of Rebound Nozzle for dusts difficult to disperse. Nowadays Rebound Nozzle probably is used most often both in the standard 20-l sphere and 1 m3 vessel. This type of nozzle was used in all tests described in the paper. In Fig. 1 an outline of the whole apparatus is presented. 2.1. 1 m3 vessel The vessel consists of cylindrical part closed at both ends with ellipsoidal bowls. The shape of the vessel is nearly spherical (L/ D z 1). Several ports at the vessel's wall are used to mount required equipment. At the wall a dust dispersion system is mounted. As the Standards EN 14034 recommend the usage of two dispersion systems for tests with dusts of small bulk densities measurements with two systems were also performed. The inlets of the dispersion systems, ended with the dispersion nozzles, are located at the opposite sides in the middle of the cylindrical part, at half of the vessel's height. From the top the electrodes for the ignition system are introduced such a way that the ignition source can be located at the vessel's centre. In order to measure the pressure two piezoresistive pressure transducer are installed at the opposite sides of the vessel. One of the ports is used as an outlet of the combustion products.
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valve, connecting tube and dispersion nozzle. The canister has a volume of 5.4 l and form factor 3:1, as required by the Standards. Its lower part is cone-shape to facilitate dust outflow. At the canister's bottom a fast acting valve was installed. Instead of the valve types recommended by the Standards EN 14034 a valve similar to that in Kühner's 20-l sphere was used. Rather than closing the outlet of the canister the valve's piston closed the inlet to the vessel e the piston moved horizontally. Fig. 2 presents two versions of the valve. In all tests described in this paper the first version of the valve (Red valve in Fig. 2a) was used. Later a new version of the valve (Blue valve in Fig. 2b) was prepared. The only difference between the Red and the Blue valve is the size of their housing: it is larger in case of Blue valve resulting in larger gap between the piston and the housing wall. The length of the tube connecting the fast acting valve with the dispersion nozzle was 210 mm with inner diameter 24 mm. As a disperser so-called Rebound Nozzle was used. The nozzle was built following the precise drawing included in the Standards EN 14034 (Fig. B.2 in the Standard). As described in the ISO and European Standards prior to the dispersion the canister was filled with pressurized air of 20 bar g. Partial air evacuation from the vessel is not required by the Standards. The air injection was initiated by opening the fast acting valve. The valve stayed opened for 600 ms. Then it closed the inlet to the vessel. Performance of the dispersion system was checked by the measurement of the pressure in the dust canister during dispersion. In Fig. 3 pressure drop in the container during the dispersion of pure air is shown together with the reference curves from the Standards EN 14034. Generally, the shape of the pressure-time curve is consistent with requirements of the Standards for an electro pneumatic valve. After about 500 ms from initiation pressure in the container drops to atmospheric. In comparison to the fast acting ball valve the discharge starts earlier but the rate of discharge is slower. On the other hand the rate of discharge is similar to that of the blasting cap activated valve. The atmospheric pressure in the container is attained practically at the same moment.
2.2. Dispersion system The dispersion system consisted of a dust canister, fast acting
Fig. 1. The setup of 1 m3 vessel at the EM “Barbara” (BDVP's sensing head not to scale).
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Fig. 2. Two versions of the fast acting valve.
25
Current measurements EN blasting cap valve EN ball valve
20
p, bar g
15
its front and rear surfaces. The pressure difference is measured using a differential pressure transducer. By use of the well-known relation for pitot-static probes, the flow velocity is inferred from the local differential pressure and gas density. The BDVP was calibrated with pressure pulses of known heights. In the range 8e68 mbar the voltage signal of the probe was a linear function of the pressure. 2.4. Experimental setup
10
5
0 0
200
400
600
800
1000
1200
t, ms Fig. 3. Air discharge from the dust container e measured and recommended.
2.3. Bi-Directional Velocity Probe The Bi-Directional Velocity Probe is an impact probe similar to the pitot-static probe. The probe (Fig. 4) was constructed based on the design of McCaffrey and Heskestad (1976). However the original design was slightly modified. Unlike in the original probe the differential pressure transducer was located directly inside the sensing head and used to divide the head into two zones. The size of the sensing head is 20 mm in diameter with 40 mm length. The probe obstructs the flow and a pressure differential exists between
Most of previous measurements of instantaneous velocity in 20l sphere and larger vessels (Dahoe, 2000; Tamanini, 1998; Pu & Jarosinski, 1991; van der Wel, 1993) were performed with air discharge, without dust, from the dust canister. Tamanini (1998) made limited measurements with dust charge in the injection flow concluding that the major effect of the dust in the injection charge is to delay the flow development leading to higher turbulence during the time of interest for the explosion tests. Hauert et al. (1994) measured transient flow in the ISO 1 m3 vessel with dust concentration form 30e120 g/m3 but he did not report results for pure air. Di Benedetto et al. (2013) developed a CFD model and simulated turbulent flow in 20-l sphere caused by dispersion of air and air with dust. They found that rms velocity slightly differs in the first 30 ms and then is practically the same in both cases. Bearing those results in mind it was chosen to measure flow generated by discharges of pure air as a starting point of the measurements. To measure transient flow velocity the BDVP was introduced into the vessel either from the bottom (d-port) or through a side wall (w-port) with the sensing head positioned at different points inside the vessel. At each point the probe was oriented in two directions. For each position and orientation the measurements were repeated several times. To describe the conditions of the measurements the following notation was applied (Fig. 5a and b). A capital letter denotes the orientation of the sensing head (N, W or D). The following number describes the distance between the head and the vessel's wall. The small letter indicates the port at the wall through which the probe was introduced into the vessel (either w or d). In Fig. 5c and d photos of BDVP introduced from the side wall (w-port) and the bottom of the vessel (d-port) is shown. 3. Results and discussion
Fig. 4. Bi-Directional Velocity Probe.
Measurement of instantaneous velocity u at each measuring point and two directions was repeated several times. That enabled
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Fig. 5. Layout of the instantaneous velocity measurements (in a & b the sensing head not to scale): a e top view (probe at the position N … d), b e side view (probe at two positions: N55d and N78d), c e BDVP introduced through side wall (w-port), d e BDVP introduced through bottom (d-port).
to assess an error of the measurement of the velocity u and all other information derived from u. Using Reynolds decomposition the mean flow and velocity fluctuations u’ were calculated. A time window of 10 ms was applied. The instantaneous velocity was decomposed using discreet wavelet transformation (DWT). DWT is time-scale representation technique which transforms input signal to a two dimensional function of time and scale. Hard thresholding function with universal threshold method described by Donoho & Johnstone (1994) was used to remove coefficients representing low frequency velocity changes. Out of several existing methods of the signal decomposition that one was chosen because it is practically “user independent”. While in other methods the threshold frequency should be chosen here it is obtained by an assessment of the velocity fluctuation energy.
seemed useful to make at least qualitative comparison. In Fig. 6 the results of u'rms measurement are presented together with the results of Dahoe (2000). Both measurements yield comparable results but in our case high intensity of turbulence lasts longer. Later the turbulence decay is very similar. That can be shown by shifting our data back in time. A 16 ms shift results in excellent agreement between both sets of data. A larger discrepancy appear at the early stage of the dispersion when the air density is significantly smaller than atmospheric. The shift was chosen arbitrary by visually fitting the set of our data to that of Dahoe, so it
3.1. Velocity fluctuations in the standard 20-l sphere Before the actual measurements of flow velocity in 1 m3 vessel the BDVP performance was verified. The probe was used to measure instantaneous flow velocity in the standard 20-l sphere. The BDVP does not seem to be a suitable tool for measurements of the flow induced by dispersion process in the sphere. Unlike PIV that is based on optical measurements the use of BDVP requires the utilisation of instantaneous gas density. That density changes dynamically during the dispersion in 20-l sphere. Also the size of the probe, in comparison to the sphere size, is more significant than it is in the case of measurements in the vessel of 1 m3 volume. Therefore a smaller part of eddies can be resolved and the flow interference caused by BDVP should be somewhat more important. Nevertheless, as data of good quality exists for the 20-l sphere, it
Fig. 6. Root-mean-square of velocity fluctuation in 20-l sphere.
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is rather a rough approximation. The reason of longer high turbulence period is the operation of the valve of the dust dispersion system. In Dahoe's tests the dispersion system was slightly modified to improve the valve performance (Dahoe, 2014). Therefore in his apparatus the valve operated faster. In spite of the time discrepancy the performance of the BDVP in measurements in 20-l sphere is surprisingly accurate. 3.2. Root-mean-square of velocity fluctuation in 1 m3 vessel In 1 m3 vessel the instantaneous velocity was measured at two points with the BDVP introduced from the bottom of the vessel (bport) and four points with introduction through w-port. At each point the probe was oriented at two perpendicular directions. At the midpoint (N55d & W55d) also the velocity generated by two dispersion systems was measured. The measurements are summarized in Fig. 7. Each point of the plots is the mean value of about 10 measurements.
The shapes of all plots in Fig. 7 are similar. The largest differences in value of u0 rms, about 10 m/s, appear when turbulence intensity is the highest. Unexpectedly, even in that period u'rms does not differ significantly when two dispersion systems were used. Later, when turbulence decays, all data approaches the same shape. Starting from about 600 ms, the moment of ignition, all plots follow practically the same tendency. The only exception is the measurement N12w, close to the vessel's wall. As expected, turbulence decays faster in that point. The measurements of u0 rms indicate that starting from about 600 ms turbulence in the standard 1 m3 vessel is isotropic and homogeneous. All plots presented in Fig. 7 are summarized in Fig. 8. Each point of the plot is an average value over all measurements done at a given moment of time. The size of the error bars confirms the similarity of turbulence intensity at all measuring points. From the viewpoint of dust explosion experiments the most important is the time range between 600 and 1000 ms. This is typical time of a dust explosion development in 1 m3 vessel. That
Fig. 7. Root-mean-square of fluctuation velocity in 1 m3 vessel.
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3.3. Length scale of turbulence Another parameter that describes turbulence is a characteristic length scale lt. To calculate the length scale more comprehensive information on velocity fluctuation than just measurements in a few points are required. A method to overcome that requirements is to find the time scale of the turbulence and, together with the relation u0 rms vs. time, use it to calculate the length scale. That method requires certain assumptions on turbulence flow structure. Application of the method to our data resulted in the length scales much smaller than the size of BDVP's sensing head so it would be difficult to justify such results. Another method, described and used by Tamanini (1998), is based on the assumption that the fluctuation velocity field is isotropic. That enables to write turbulence kinetic energy in the form:
Fig. 8. Space/direction averaged root-mean-square of fluctuation velocity in 1 m3 vessel.
3 k ¼ u0rms 2 2
(1)
Characteristic length scale of turbulence may then by expressed as:
part of the plot in Fig. 8 is presented in Fig. 9 together with the fit to the experimental points. Also included are results from the standard 20-l sphere (Dahoe et al., 2001a) for the time period characteristic for the explosions in that vessel. In the time periods characteristic for both vessels the values of u0 rms are larger in 20-l sphere. Moreover in the smaller vessel turbulence decays faster. Typically maximum rate of pressure rise in the standard 20-l sphere is reached at about 90 ms. The corresponding value of u0 rms is then 1.5 m/s. In 1 m3 maximum rate of pressure rise usually occurs between 700 and 800 ms. At that time the value of u0 rms changes from 0.8 to 0.7 m/s. If the same tendency as the one obtained by Dahoe et al. (2001a) for 20-l sphere (u0 rms slightly smaller for Perforated Dispersion Ring than for Rebound Nozzle) is hold for 1 m3 vessel, then that result corresponds well with results of van der Wel (1993). He measured the flow generated in 1 m3 vessel by Perforated Dispersion Ring and obtained u0 rms at 600 ms equal 0.5 m/s.
lt ¼ Cm3=4
k3=2 ε
(2)
where ε is dissipation of turbulence kinetic energy ε ¼ dk and the dt value of the constant is usually taken Cm ¼ 0.09. By substitution of Eq. (1) and the fitted relation u0 rms(t) (see Fig. 9) into Eq. (2) the following expression is obtained:
lt ¼ 0:043
0:41 t t0
(3)
Characteristic length scale of turbulence in 1 m3 vessel in the time between 600 and 1000 ms calculated from Eq. (3) decreases from 43 mm to 35 mm. From Dahoe's fit of u0 rms(t) for 20-l sphere in the time period 60e200 ms the length scale of turbulence calculated the same way decreases from 3.9 mm to 3.4 mm. On the other hand the length scale calculated by Dahoe from the time scale decreases much faster, from 12 mm to 1.2 mm in the same time period. 4. Discussion
Fig. 9. Comparison of u0 rms in 1 m3 and 20-l vessels in the time ranges when an explosion takes place in both vessels.
The standard 1 m3 vessel is an experimental apparatus recommended by ISO and European Standards for measurement of explosion severity parameters, among others KSt max value. The Standards are based on original results of Bartknecht (1989) and Siwek (1988). Also ASTM mentions that in case of questionable results a 1 m3 vessel test should be run. The values of the explosion parameters, pmax and KSt max, obtained following the Standards are used to properly select protective systems for industrial installations: vent sizing, selection and configuration of suppression systems, etc. Therefore the agreement of results obtained in any other vessel with original Bartknecht's data is of prime importance. Alternative test equipment, including Kühner's 20-l sphere, may be used providing that it has been proven that they give results that do not differ by more than 10% from those from the standard vessel. The results obtained in the vessel built according the instructions included in the Standards confirm discrepancy between turbulence intensity in the time periods characteristic for 1 m3 vessel and 20-l sphere reported previously by van der Wel and Dahoe. The values of u0 rms are significantly higher in the latter vessel. Therefore it seems unlikely that KSt max values obtained in both vessels conform. And in fact the values of KSt max obtained in
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1 m3 vessel are significantly smaller than those from 20-l sphere. For niacin used in Calibration round robin test CaRo13 the assessment of in our 1 m3 vessel was 8.7 bar and 173 m bar s1 respectively while the related reference values are 8.2 bar and 253 m bar s1. The source of the difference is most probably the significant difference in the turbulence intensity as turbulence strongly influences the value of KSt max. That statement also confirms good agreement of maximum pressures pmax, the parameter that is known to weakly depend on turbulence. On the other hand Bartknecht (1989) and Siwek (1988) presented research which showed that KSt max found with use of 20-l sphere is equivalent to that measured in the standard 1 m3 vessel (after applying certain corrections for the influence of ignition source or difference in heat losses). Searching for the reason of the inconsistency between the equivalence and the discrepancy mentioned earlier it is useful to look closer at the design of both vessels. The design of the standard 20-l sphere is well documented and has not been changed significantly in many years. In the newer version Perforated Dispersion Ring was replaced with Reboud Nozzle. This is probably the most significant modification that could influence the turbulence intensity. In contrast, the available descriptions of 1 m3 vessel provided by ISO and European Standards are definitely insufficiently precise. The part of the apparatus that requires better definition is certainly dispersion system e the equipment that establishes preignition conditions of the dust/air mixture. For the dispersion nozzle both Standards recommend Perforated Dispersion Ring as a first choice. In our vessel we use so-called Reboud Nozzle. Nowadays this is the nozzle probably most often used. The nozzle allows proper dispersion of wider range of dust without the problem of blocking up. The measurements made by Dahoe (2000) in 20-l sphere suggest that replacement of Perforated Dispersion Ring with Reboud Nozzle should rather increase the turbulence intensity and results in larger values of KSt max. It is unlikely that the dust canister influences the turbulence as long as it ensures smooth supply of the dust to the other parts downstream the system and its volume meets the requirements of the Standards. The European Standard requires additionally the form factor of the canister 3:1 and the diameter of its outlet DN25. Those requirements are met in our system. The parts of the dispersion system that definitely influence the turbulence are the connecting tube between the fast acting valve and the dispersion nozzle, and the valve itself. Very little information on the tube and the way it should be connected with the valve is included in the ISO Standard. Only the diameter of ¾in is recommended. The European Standard additionally limits the tube length to 350 mm and presents two ways the tube may be connected with the valve: either the tube is mounted to the valve's side wall (the blasting cup activated valve) or it should be in a line with the canister's outlet and further form an elbow with curvature R75. In both cases the valve is connected directly to the dust container's outlet and closes the outlet until the dispersion begins. In our dispersion system we applied the design described in part 2.2. Diameter of our connecting tube is a bit larger (DN24) and its length is 210 mm so it basically meets the requirements of the Standards. It may be expected that the larger diameter of the tube will rather increase the turbulence intensity than cause its decrease. The most questionable part of the dispersion system is the fast acting valve. The blasting cup activated valve recommended as a first choice by ISO and European Standards is impractical for everyday use. Setting the valve after its operation is labour- and time-consuming. Also, the moving part of the valve has limited lifetime. For an alternative type, a ball valve with an electro pneumatic drive, the opening time <100 ms is a very demanding requirements. Furthermore, its operation in dusty environment
would result in degradation of its performance in short time. The valve in our system does not have the disadvantages mentioned. Even so presented results indicates that the valve is probably a bottleneck for the air flow in the dispersion system and influences or even controls the turbulence intensity in the vessel. To confirm that observation the valve was redesigned. In the new valve the gap between the valve's piston and the housing has been enlarged (compare Red in Fig. 2a and Blue in Fig. 2b). That allows an easier flow through the valve. Preliminary results indicate that KSt max values increase and confirm that this is the right direction for improvement. Another option would be a rearrangement of the valve in such a way that instead of closing the inlet to the vessel it will close the canister's outlet e the arrangement recommended by the Standards. The results presented suggest that differences in the turbulence intensity is responsible for the discrepancy in the measured KSt max values in different apparatus. Therefore the question arises what is the correct intensity of the turbulence in the standard 1 m3 vessel? The simplest answer the same as in the original vessel is not useful because, as far as we know, that information is not available. Other possibility is the agreement with turbulence in the 20-l sphere but also in that case one may meet certain difficulties. In the sphere turbulence decays fast (see Fig. 9) and the choice of the reference intensity is biased against the time elapsed from beginning of the dispersion to the moment (dp/dt)max, and KSt, is reached. Usually, it is about 90 ms but in some cases, e.g. coal dusts, the time may differ significantly. A possible way out is a choice of a set of reference dusts with the values of explosion parameters known from original 1 m3 vessel. Such values are given for Lycopodium in the ASTM E 1226-05 Standard. Once the correctness of the vessel will have been proven by tests of reference dusts it will be possible to use the turbulence intensity measured in such a vessel as a reference. In most cases turbulence intensity, both in 20-l sphere and 1 m3 vessel, was measured when pure air was discharged by the dispersion system. In actual dust explosion tests dust with air is injected into the vessel. Dahoe et al. (2001b) measured how the turbulence decay in the standard 20-l sphere is modified by the presence of a dust load in the injected air. They injected various quantities of corn-starch, up to the nominal dust concentration of 625 g /m3. They concluded that turbulent fluctuations of the gas phase behave more or less independently of the presence of a solid phase. They only observed a larger scatter of the experimental points. That result was attributed to much larger response time of the corn-starch particles than the time scale of the gas-scale fluctuations and because of that small kinetic energy transfer between the phases. Hauert et al. (1994) measured root mean square of the velocity fluctuations at geometric centre of the vessel when cornstarch was injected through the Perforated Dispersion Ring. However he did not presented results for air injection. Limited information was obtained by Tamanini (1998) in tests in large 63.7 m3 room-like structure. He concluded that the major effect of the dust in the injected charge is to delay the flow development leading to higher turbulence during the time of interest for the explosion tests. Also a question arises if the way the flow is modified by dust and the magnitude of the modifications depend on a vessel geometry and volume. Tests that would clarify those questions are planned in near future. Finally, another factor that should be taken into account is the flow/explosion flame front interaction. As far as we know no data exists that can provide information on that in case of dust explosion. 5. Conclusions The Bi-Directional Velocity Probe BDVP was used to measure
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instantaneous velocity of the flow generated by air impulse from the dispersion system in the standard 1 m3 vessel. The probe proved to be able to provide useful information on flow velocity even in 20-l sphere. The results of measurements performed in 1 m3 vessel suggest that during the time of interest for the explosion tests turbulence in the vessel is isotropic and homogeneous. Presented results confirm discrepancy between turbulence intensity in the time periods characteristic for 1 m3 vessel and 20-l sphere, reported previously by other authors. The values of u'rms are significantly higher in the latter vessel. Therefore it seems unlikely that KSt max values obtained in both vessels conform. Explosibility tests performed in 1 m3 vessel confirmed those predictions. An analysis of the possible reason of the discrepancy led to the conclusion that the part of the dispersion system responsible for that is the fast acting valve. Preliminary tests with the new, redesigned valve seems to confirm that conclusion. Presented results indicate that more precise description of the design of the standard 1 m3 vessel, its dispersion system is required than those given in ISO and European Standards, particularly the design and operation of the fast acting valve. Acknowledgements The authors gratefully acknowledge the financial support of the National Centre for Research and Development by Research Project No. O ROB 0005 01/2011/01 Technologies of explosion protection for storage places of bulk materials. References ASTM E 1226e12a, 2012. Standard Test Method for Explosibility of Dust Clouds. Bartknecht, W., 1989. Dust Explosions: Course, Prevention, Protection. SpringerVerlag, Berlin.
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Dahoe, A.F., 2000. Dust Explosions: a Study of Flame Propagation. Ph.D. Thesis. Delft University of Technology. Dahoe, A.F., Cant, R.S., Scarlett, B., 2001a. On the decay of turbulence in the 20-Liter explosion sphere. Flow Turbul. Combust. 64, 159e184. Dahoe, a.f., van der Nat, K., Braithwaite, M., Scarlett, B., 2001b. On the sensitivity of the maximum explosion pressure of a dust deflagration to turbulence. KONA Powder Part. J. 19, 178e196. Dahoe, A.F., 2014. Private Communication. Di Benedetto, A., Russo, P., Sanchirio, R., Di Sarli, V., 2013. CFD simulations of turbulent fluid flow and dust dispersion in the 20 liter explosion vessel. AIChE J. 59 (7), 2485e2496. Donoho, D.L., Johnstone, I.M., 1994. Ideal spatial adaptation by wavelet shrinkage. Biometrika 81. Eckhoff, R.K., 2003. Dust Explosions in the Process Industries, third ed. Gulf Professional Publishing, Amsterdam. EN 14034e1:2006þA1, 2011. Determination of Explosion Characteristics of Dust Clouds e Part 1: Determination of the Maximum Explosion Pressure Pmax of Dust Clouds. EN 14034e2:2006þA1, 2011. Determination of Explosion Characteristics of Dust Clouds e Part 2: Determination of the Maximum Rate of Explosion Pressure Rise (Dp/dt)max of Dust. Clouds. Hauert, F., Vogl, A., Radant, S., 1994. Measurement of turbulence and dust concentration in silos and vessels. In: Proceedings of the 6th International Colloquium on Dust Explosions, Shenyang. ISO 6184/1, 1985. Explosion Protection Systems e Part 1: Determination of Explosion Indices of Combustible Dusts in Air. McCaffrey, B.J., Heskestad, G., 1976. A robust bidirectional low-velocity probe for flame and fire application. Combust. Flame 26, 125e127. Proust, Ch, Accorsi, A., Dupont, L., 2007. Measuring the violence of dust explosions with the “20 l sphere” and with the standard “ISO 1 m3 vessel”. Systematic comparison and analysis of the discrepancies. J. Loss Prev. Process Ind. 20, 599e606. Pu, Y.K., Jarosinski, J., 1991. Turbulence effects on dust explosions in the 20-Litre spherical vessel. In: Proceedings of the 23rd Symposium (International) on Combustion. The Combustion Institute, pp. 843e849. Siwek, R., 1988. Reliable determination of the safety characteristics in 20-l apparatus. In: Proceedings of the Flammable Dust Explosion Conference; St. Louis: MO, pp. 529e573. Tamanini, F., 1998. The role of turbulence in dust explosions. J. Loss Prev. Process Ind. 11, 1e10. van der Wel, P.G.J, 1993. Ignition and Propagation of Dust Explosions. Ph.D. Thesis. Delft University of Technology.
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Measurements of turbulence intensity in the standard 1 m3 vessel Zdzislaw Dyduch*, Adrian Toman, Wojciech Adamus w, ul. Podleska 72, Poland Experimental Mine “Barbara” of Central Mining Institute, PL-43-190, Mikoło
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 July 2015 Received in revised form 20 December 2015 Accepted 21 December 2015 Available online 24 December 2015
The standard 1 m3 vessel is an apparatus recommended for assessment of dust explosion indices by ISO, European and American Standards. Unlike the standard 20-l sphere, only few authors reported measurements of turbulence intensity in 1 m3 vessel. The paper presents results of such measurements performed in 1 m3 vessel built and used at Experimental Mine Barbara of Central Mining Institute, Poland. Transient flow velocity in the vessel generated by an air injection from the dust dispersion system was measured with use of Bi-Directional Velocity Probe developed by McCaffrey&Heskestad. The velocity was measured in several points along the vessel's axes and in two directions at each point. From the measurements the root-mean-square of instantaneous velocity u'rms and the characteristic length scale of turbulence were calculated. The results confirm discrepancy between turbulence intensity in the time periods characteristic for 1 m3 vessel and 20-l sphere, reported previously by other authors. The analysis of possible reasons of the discrepancy identifies the dust dispersion system as the main source of ambiguity, particularly the design and operation of the fast acting valve. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Explosion parameters Turbulence intensity Bi-Directional Velocity Probe Standard test methods Turbulence measurement
1. Introduction Maximum explosion pressure and maximum rate of pressure rise are the most important explosion severity parameters used in the process safety assessment of dust explosion hazard in industry. Subsequently these parameters are essential for adequate design of protective methods to assure proper control of the dust explosion (Eckhoff, 2003). Standards either in Europe (EN 14034-1, 2011; EN 14034-2, 2011) or in the United States (ASTM E1226-12a, 2012) or international (ISO 6184/1, 1985) define the test methodology. The recommended size of the test vessel is 1 m3 but other experimental vessels are allowed. The smaller volume test chamber requires less sample mass per test, it is simpler in operation and therefore allows higher number of test performed. The most widely used is the 20-l sphere manufactured by Kühner AG. However the explosion severity parameters, especially the KSt max, not always agree with each other when measured in these two vessels (Proust et al., 2007). Amongst factors that contribute to the difference is different turbulence intensity created inside the test vessel after dispersion of a test sample and different rate of the turbulence decay. The turbulence intensity measured in 20-l explosion sphere
* Corresponding author. E-mail addresses: [email protected] (Z. Dyduch), [email protected] (A. Toman), w. [email protected] (W. Adamus). http://dx.doi.org/10.1016/j.jlp.2015.12.019 0950-4230/© 2016 Elsevier Ltd. All rights reserved.
by Dahoe (2000) and Pu & Jarosinski (1991) overlap very well. The ignition starts at 60 ms after the injection. At that time the turbulence intensity quantified by root-mean-square of velocity fluctuation is approximately 3 m/s. In 200 ms, this is a time when an explosion usually is completed, the turbulence intensity decays to approximately 0.5 m/s with Rebound Nozzle used for dispersion. Other types of the dispersion nozzles generate different levels of the initial turbulence intensity. In 1 m3 test vessel the mixture is ignited at 600 ms. The turbulence intensity measured by van der Wel (1993) at that time is 0.5 m/s. Measurements reported by Proust et al. (2007) in his 1 m3 vessel show fast turbulence decay after injection, however at the time of typical explosion duration, between 600 and 1000 ms, the turbulence intensity levels off at the value of 2 m/s. Hauert et al. (1994) measured both horizontal and vertical components and obtained the values of 1.2 and 5.36 m/s, respectively, after 600 ms. The difference between the measured turbulence intensities in 1 m3 vessel might be due to differences in engineering factors like shape of the test vessel, dispersion system or dispersion nozzle used. The paper presents results of turbulence intensity measurement performed in 1 m3 vessel built and used at Experimental Mine Barbara of Central Mining Institute, Poland. The vessel fulfils the requirements imposed by the European Standards EN 14034. Instantaneous velocity in the vessel generated by an air injection from the dust dispersion system was measured with use of BiDirectional Velocity Probe BDVP developed by McCaffrey &
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Heskestad (1976). The velocity was measured in several points along two vessel's axes, in two directions at each point. From the measurements the root-mean-square of velocity fluctuation u'rms and the characteristic length scale of turbulence were calculated. 2. Experimental setup Transient flow velocity generated by air outflow from the dust dispersion system was measured inside the standard 1 m3 vessel. The vessel, together with the dispersion system, meets the requirements of the European Standards EN 14034. The only difference was the dispersion nozzle. The Standards recommends Perforated Dispersion Ring as a first choice, allows however the usage of Rebound Nozzle for dusts difficult to disperse. Nowadays Rebound Nozzle probably is used most often both in the standard 20-l sphere and 1 m3 vessel. This type of nozzle was used in all tests described in the paper. In Fig. 1 an outline of the whole apparatus is presented. 2.1. 1 m3 vessel The vessel consists of cylindrical part closed at both ends with ellipsoidal bowls. The shape of the vessel is nearly spherical (L/ D z 1). Several ports at the vessel's wall are used to mount required equipment. At the wall a dust dispersion system is mounted. As the Standards EN 14034 recommend the usage of two dispersion systems for tests with dusts of small bulk densities measurements with two systems were also performed. The inlets of the dispersion systems, ended with the dispersion nozzles, are located at the opposite sides in the middle of the cylindrical part, at half of the vessel's height. From the top the electrodes for the ignition system are introduced such a way that the ignition source can be located at the vessel's centre. In order to measure the pressure two piezoresistive pressure transducer are installed at the opposite sides of the vessel. One of the ports is used as an outlet of the combustion products.
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valve, connecting tube and dispersion nozzle. The canister has a volume of 5.4 l and form factor 3:1, as required by the Standards. Its lower part is cone-shape to facilitate dust outflow. At the canister's bottom a fast acting valve was installed. Instead of the valve types recommended by the Standards EN 14034 a valve similar to that in Kühner's 20-l sphere was used. Rather than closing the outlet of the canister the valve's piston closed the inlet to the vessel e the piston moved horizontally. Fig. 2 presents two versions of the valve. In all tests described in this paper the first version of the valve (Red valve in Fig. 2a) was used. Later a new version of the valve (Blue valve in Fig. 2b) was prepared. The only difference between the Red and the Blue valve is the size of their housing: it is larger in case of Blue valve resulting in larger gap between the piston and the housing wall. The length of the tube connecting the fast acting valve with the dispersion nozzle was 210 mm with inner diameter 24 mm. As a disperser so-called Rebound Nozzle was used. The nozzle was built following the precise drawing included in the Standards EN 14034 (Fig. B.2 in the Standard). As described in the ISO and European Standards prior to the dispersion the canister was filled with pressurized air of 20 bar g. Partial air evacuation from the vessel is not required by the Standards. The air injection was initiated by opening the fast acting valve. The valve stayed opened for 600 ms. Then it closed the inlet to the vessel. Performance of the dispersion system was checked by the measurement of the pressure in the dust canister during dispersion. In Fig. 3 pressure drop in the container during the dispersion of pure air is shown together with the reference curves from the Standards EN 14034. Generally, the shape of the pressure-time curve is consistent with requirements of the Standards for an electro pneumatic valve. After about 500 ms from initiation pressure in the container drops to atmospheric. In comparison to the fast acting ball valve the discharge starts earlier but the rate of discharge is slower. On the other hand the rate of discharge is similar to that of the blasting cap activated valve. The atmospheric pressure in the container is attained practically at the same moment.
2.2. Dispersion system The dispersion system consisted of a dust canister, fast acting
Fig. 1. The setup of 1 m3 vessel at the EM “Barbara” (BDVP's sensing head not to scale).
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Fig. 2. Two versions of the fast acting valve.
25
Current measurements EN blasting cap valve EN ball valve
20
p, bar g
15
its front and rear surfaces. The pressure difference is measured using a differential pressure transducer. By use of the well-known relation for pitot-static probes, the flow velocity is inferred from the local differential pressure and gas density. The BDVP was calibrated with pressure pulses of known heights. In the range 8e68 mbar the voltage signal of the probe was a linear function of the pressure. 2.4. Experimental setup
10
5
0 0
200
400
600
800
1000
1200
t, ms Fig. 3. Air discharge from the dust container e measured and recommended.
2.3. Bi-Directional Velocity Probe The Bi-Directional Velocity Probe is an impact probe similar to the pitot-static probe. The probe (Fig. 4) was constructed based on the design of McCaffrey and Heskestad (1976). However the original design was slightly modified. Unlike in the original probe the differential pressure transducer was located directly inside the sensing head and used to divide the head into two zones. The size of the sensing head is 20 mm in diameter with 40 mm length. The probe obstructs the flow and a pressure differential exists between
Most of previous measurements of instantaneous velocity in 20l sphere and larger vessels (Dahoe, 2000; Tamanini, 1998; Pu & Jarosinski, 1991; van der Wel, 1993) were performed with air discharge, without dust, from the dust canister. Tamanini (1998) made limited measurements with dust charge in the injection flow concluding that the major effect of the dust in the injection charge is to delay the flow development leading to higher turbulence during the time of interest for the explosion tests. Hauert et al. (1994) measured transient flow in the ISO 1 m3 vessel with dust concentration form 30e120 g/m3 but he did not report results for pure air. Di Benedetto et al. (2013) developed a CFD model and simulated turbulent flow in 20-l sphere caused by dispersion of air and air with dust. They found that rms velocity slightly differs in the first 30 ms and then is practically the same in both cases. Bearing those results in mind it was chosen to measure flow generated by discharges of pure air as a starting point of the measurements. To measure transient flow velocity the BDVP was introduced into the vessel either from the bottom (d-port) or through a side wall (w-port) with the sensing head positioned at different points inside the vessel. At each point the probe was oriented in two directions. For each position and orientation the measurements were repeated several times. To describe the conditions of the measurements the following notation was applied (Fig. 5a and b). A capital letter denotes the orientation of the sensing head (N, W or D). The following number describes the distance between the head and the vessel's wall. The small letter indicates the port at the wall through which the probe was introduced into the vessel (either w or d). In Fig. 5c and d photos of BDVP introduced from the side wall (w-port) and the bottom of the vessel (d-port) is shown. 3. Results and discussion
Fig. 4. Bi-Directional Velocity Probe.
Measurement of instantaneous velocity u at each measuring point and two directions was repeated several times. That enabled
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Fig. 5. Layout of the instantaneous velocity measurements (in a & b the sensing head not to scale): a e top view (probe at the position N … d), b e side view (probe at two positions: N55d and N78d), c e BDVP introduced through side wall (w-port), d e BDVP introduced through bottom (d-port).
to assess an error of the measurement of the velocity u and all other information derived from u. Using Reynolds decomposition the mean flow and velocity fluctuations u’ were calculated. A time window of 10 ms was applied. The instantaneous velocity was decomposed using discreet wavelet transformation (DWT). DWT is time-scale representation technique which transforms input signal to a two dimensional function of time and scale. Hard thresholding function with universal threshold method described by Donoho & Johnstone (1994) was used to remove coefficients representing low frequency velocity changes. Out of several existing methods of the signal decomposition that one was chosen because it is practically “user independent”. While in other methods the threshold frequency should be chosen here it is obtained by an assessment of the velocity fluctuation energy.
seemed useful to make at least qualitative comparison. In Fig. 6 the results of u'rms measurement are presented together with the results of Dahoe (2000). Both measurements yield comparable results but in our case high intensity of turbulence lasts longer. Later the turbulence decay is very similar. That can be shown by shifting our data back in time. A 16 ms shift results in excellent agreement between both sets of data. A larger discrepancy appear at the early stage of the dispersion when the air density is significantly smaller than atmospheric. The shift was chosen arbitrary by visually fitting the set of our data to that of Dahoe, so it
3.1. Velocity fluctuations in the standard 20-l sphere Before the actual measurements of flow velocity in 1 m3 vessel the BDVP performance was verified. The probe was used to measure instantaneous flow velocity in the standard 20-l sphere. The BDVP does not seem to be a suitable tool for measurements of the flow induced by dispersion process in the sphere. Unlike PIV that is based on optical measurements the use of BDVP requires the utilisation of instantaneous gas density. That density changes dynamically during the dispersion in 20-l sphere. Also the size of the probe, in comparison to the sphere size, is more significant than it is in the case of measurements in the vessel of 1 m3 volume. Therefore a smaller part of eddies can be resolved and the flow interference caused by BDVP should be somewhat more important. Nevertheless, as data of good quality exists for the 20-l sphere, it
Fig. 6. Root-mean-square of velocity fluctuation in 20-l sphere.
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is rather a rough approximation. The reason of longer high turbulence period is the operation of the valve of the dust dispersion system. In Dahoe's tests the dispersion system was slightly modified to improve the valve performance (Dahoe, 2014). Therefore in his apparatus the valve operated faster. In spite of the time discrepancy the performance of the BDVP in measurements in 20-l sphere is surprisingly accurate. 3.2. Root-mean-square of velocity fluctuation in 1 m3 vessel In 1 m3 vessel the instantaneous velocity was measured at two points with the BDVP introduced from the bottom of the vessel (bport) and four points with introduction through w-port. At each point the probe was oriented at two perpendicular directions. At the midpoint (N55d & W55d) also the velocity generated by two dispersion systems was measured. The measurements are summarized in Fig. 7. Each point of the plots is the mean value of about 10 measurements.
The shapes of all plots in Fig. 7 are similar. The largest differences in value of u0 rms, about 10 m/s, appear when turbulence intensity is the highest. Unexpectedly, even in that period u'rms does not differ significantly when two dispersion systems were used. Later, when turbulence decays, all data approaches the same shape. Starting from about 600 ms, the moment of ignition, all plots follow practically the same tendency. The only exception is the measurement N12w, close to the vessel's wall. As expected, turbulence decays faster in that point. The measurements of u0 rms indicate that starting from about 600 ms turbulence in the standard 1 m3 vessel is isotropic and homogeneous. All plots presented in Fig. 7 are summarized in Fig. 8. Each point of the plot is an average value over all measurements done at a given moment of time. The size of the error bars confirms the similarity of turbulence intensity at all measuring points. From the viewpoint of dust explosion experiments the most important is the time range between 600 and 1000 ms. This is typical time of a dust explosion development in 1 m3 vessel. That
Fig. 7. Root-mean-square of fluctuation velocity in 1 m3 vessel.
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3.3. Length scale of turbulence Another parameter that describes turbulence is a characteristic length scale lt. To calculate the length scale more comprehensive information on velocity fluctuation than just measurements in a few points are required. A method to overcome that requirements is to find the time scale of the turbulence and, together with the relation u0 rms vs. time, use it to calculate the length scale. That method requires certain assumptions on turbulence flow structure. Application of the method to our data resulted in the length scales much smaller than the size of BDVP's sensing head so it would be difficult to justify such results. Another method, described and used by Tamanini (1998), is based on the assumption that the fluctuation velocity field is isotropic. That enables to write turbulence kinetic energy in the form:
Fig. 8. Space/direction averaged root-mean-square of fluctuation velocity in 1 m3 vessel.
3 k ¼ u0rms 2 2
(1)
Characteristic length scale of turbulence may then by expressed as:
part of the plot in Fig. 8 is presented in Fig. 9 together with the fit to the experimental points. Also included are results from the standard 20-l sphere (Dahoe et al., 2001a) for the time period characteristic for the explosions in that vessel. In the time periods characteristic for both vessels the values of u0 rms are larger in 20-l sphere. Moreover in the smaller vessel turbulence decays faster. Typically maximum rate of pressure rise in the standard 20-l sphere is reached at about 90 ms. The corresponding value of u0 rms is then 1.5 m/s. In 1 m3 maximum rate of pressure rise usually occurs between 700 and 800 ms. At that time the value of u0 rms changes from 0.8 to 0.7 m/s. If the same tendency as the one obtained by Dahoe et al. (2001a) for 20-l sphere (u0 rms slightly smaller for Perforated Dispersion Ring than for Rebound Nozzle) is hold for 1 m3 vessel, then that result corresponds well with results of van der Wel (1993). He measured the flow generated in 1 m3 vessel by Perforated Dispersion Ring and obtained u0 rms at 600 ms equal 0.5 m/s.
lt ¼ Cm3=4
k3=2 ε
(2)
where ε is dissipation of turbulence kinetic energy ε ¼ dk and the dt value of the constant is usually taken Cm ¼ 0.09. By substitution of Eq. (1) and the fitted relation u0 rms(t) (see Fig. 9) into Eq. (2) the following expression is obtained:
lt ¼ 0:043
0:41 t t0
(3)
Characteristic length scale of turbulence in 1 m3 vessel in the time between 600 and 1000 ms calculated from Eq. (3) decreases from 43 mm to 35 mm. From Dahoe's fit of u0 rms(t) for 20-l sphere in the time period 60e200 ms the length scale of turbulence calculated the same way decreases from 3.9 mm to 3.4 mm. On the other hand the length scale calculated by Dahoe from the time scale decreases much faster, from 12 mm to 1.2 mm in the same time period. 4. Discussion
Fig. 9. Comparison of u0 rms in 1 m3 and 20-l vessels in the time ranges when an explosion takes place in both vessels.
The standard 1 m3 vessel is an experimental apparatus recommended by ISO and European Standards for measurement of explosion severity parameters, among others KSt max value. The Standards are based on original results of Bartknecht (1989) and Siwek (1988). Also ASTM mentions that in case of questionable results a 1 m3 vessel test should be run. The values of the explosion parameters, pmax and KSt max, obtained following the Standards are used to properly select protective systems for industrial installations: vent sizing, selection and configuration of suppression systems, etc. Therefore the agreement of results obtained in any other vessel with original Bartknecht's data is of prime importance. Alternative test equipment, including Kühner's 20-l sphere, may be used providing that it has been proven that they give results that do not differ by more than 10% from those from the standard vessel. The results obtained in the vessel built according the instructions included in the Standards confirm discrepancy between turbulence intensity in the time periods characteristic for 1 m3 vessel and 20-l sphere reported previously by van der Wel and Dahoe. The values of u0 rms are significantly higher in the latter vessel. Therefore it seems unlikely that KSt max values obtained in both vessels conform. And in fact the values of KSt max obtained in
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1 m3 vessel are significantly smaller than those from 20-l sphere. For niacin used in Calibration round robin test CaRo13 the assessment of in our 1 m3 vessel was 8.7 bar and 173 m bar s1 respectively while the related reference values are 8.2 bar and 253 m bar s1. The source of the difference is most probably the significant difference in the turbulence intensity as turbulence strongly influences the value of KSt max. That statement also confirms good agreement of maximum pressures pmax, the parameter that is known to weakly depend on turbulence. On the other hand Bartknecht (1989) and Siwek (1988) presented research which showed that KSt max found with use of 20-l sphere is equivalent to that measured in the standard 1 m3 vessel (after applying certain corrections for the influence of ignition source or difference in heat losses). Searching for the reason of the inconsistency between the equivalence and the discrepancy mentioned earlier it is useful to look closer at the design of both vessels. The design of the standard 20-l sphere is well documented and has not been changed significantly in many years. In the newer version Perforated Dispersion Ring was replaced with Reboud Nozzle. This is probably the most significant modification that could influence the turbulence intensity. In contrast, the available descriptions of 1 m3 vessel provided by ISO and European Standards are definitely insufficiently precise. The part of the apparatus that requires better definition is certainly dispersion system e the equipment that establishes preignition conditions of the dust/air mixture. For the dispersion nozzle both Standards recommend Perforated Dispersion Ring as a first choice. In our vessel we use so-called Reboud Nozzle. Nowadays this is the nozzle probably most often used. The nozzle allows proper dispersion of wider range of dust without the problem of blocking up. The measurements made by Dahoe (2000) in 20-l sphere suggest that replacement of Perforated Dispersion Ring with Reboud Nozzle should rather increase the turbulence intensity and results in larger values of KSt max. It is unlikely that the dust canister influences the turbulence as long as it ensures smooth supply of the dust to the other parts downstream the system and its volume meets the requirements of the Standards. The European Standard requires additionally the form factor of the canister 3:1 and the diameter of its outlet DN25. Those requirements are met in our system. The parts of the dispersion system that definitely influence the turbulence are the connecting tube between the fast acting valve and the dispersion nozzle, and the valve itself. Very little information on the tube and the way it should be connected with the valve is included in the ISO Standard. Only the diameter of ¾in is recommended. The European Standard additionally limits the tube length to 350 mm and presents two ways the tube may be connected with the valve: either the tube is mounted to the valve's side wall (the blasting cup activated valve) or it should be in a line with the canister's outlet and further form an elbow with curvature R75. In both cases the valve is connected directly to the dust container's outlet and closes the outlet until the dispersion begins. In our dispersion system we applied the design described in part 2.2. Diameter of our connecting tube is a bit larger (DN24) and its length is 210 mm so it basically meets the requirements of the Standards. It may be expected that the larger diameter of the tube will rather increase the turbulence intensity than cause its decrease. The most questionable part of the dispersion system is the fast acting valve. The blasting cup activated valve recommended as a first choice by ISO and European Standards is impractical for everyday use. Setting the valve after its operation is labour- and time-consuming. Also, the moving part of the valve has limited lifetime. For an alternative type, a ball valve with an electro pneumatic drive, the opening time <100 ms is a very demanding requirements. Furthermore, its operation in dusty environment
would result in degradation of its performance in short time. The valve in our system does not have the disadvantages mentioned. Even so presented results indicates that the valve is probably a bottleneck for the air flow in the dispersion system and influences or even controls the turbulence intensity in the vessel. To confirm that observation the valve was redesigned. In the new valve the gap between the valve's piston and the housing has been enlarged (compare Red in Fig. 2a and Blue in Fig. 2b). That allows an easier flow through the valve. Preliminary results indicate that KSt max values increase and confirm that this is the right direction for improvement. Another option would be a rearrangement of the valve in such a way that instead of closing the inlet to the vessel it will close the canister's outlet e the arrangement recommended by the Standards. The results presented suggest that differences in the turbulence intensity is responsible for the discrepancy in the measured KSt max values in different apparatus. Therefore the question arises what is the correct intensity of the turbulence in the standard 1 m3 vessel? The simplest answer the same as in the original vessel is not useful because, as far as we know, that information is not available. Other possibility is the agreement with turbulence in the 20-l sphere but also in that case one may meet certain difficulties. In the sphere turbulence decays fast (see Fig. 9) and the choice of the reference intensity is biased against the time elapsed from beginning of the dispersion to the moment (dp/dt)max, and KSt, is reached. Usually, it is about 90 ms but in some cases, e.g. coal dusts, the time may differ significantly. A possible way out is a choice of a set of reference dusts with the values of explosion parameters known from original 1 m3 vessel. Such values are given for Lycopodium in the ASTM E 1226-05 Standard. Once the correctness of the vessel will have been proven by tests of reference dusts it will be possible to use the turbulence intensity measured in such a vessel as a reference. In most cases turbulence intensity, both in 20-l sphere and 1 m3 vessel, was measured when pure air was discharged by the dispersion system. In actual dust explosion tests dust with air is injected into the vessel. Dahoe et al. (2001b) measured how the turbulence decay in the standard 20-l sphere is modified by the presence of a dust load in the injected air. They injected various quantities of corn-starch, up to the nominal dust concentration of 625 g /m3. They concluded that turbulent fluctuations of the gas phase behave more or less independently of the presence of a solid phase. They only observed a larger scatter of the experimental points. That result was attributed to much larger response time of the corn-starch particles than the time scale of the gas-scale fluctuations and because of that small kinetic energy transfer between the phases. Hauert et al. (1994) measured root mean square of the velocity fluctuations at geometric centre of the vessel when cornstarch was injected through the Perforated Dispersion Ring. However he did not presented results for air injection. Limited information was obtained by Tamanini (1998) in tests in large 63.7 m3 room-like structure. He concluded that the major effect of the dust in the injected charge is to delay the flow development leading to higher turbulence during the time of interest for the explosion tests. Also a question arises if the way the flow is modified by dust and the magnitude of the modifications depend on a vessel geometry and volume. Tests that would clarify those questions are planned in near future. Finally, another factor that should be taken into account is the flow/explosion flame front interaction. As far as we know no data exists that can provide information on that in case of dust explosion. 5. Conclusions The Bi-Directional Velocity Probe BDVP was used to measure
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instantaneous velocity of the flow generated by air impulse from the dispersion system in the standard 1 m3 vessel. The probe proved to be able to provide useful information on flow velocity even in 20-l sphere. The results of measurements performed in 1 m3 vessel suggest that during the time of interest for the explosion tests turbulence in the vessel is isotropic and homogeneous. Presented results confirm discrepancy between turbulence intensity in the time periods characteristic for 1 m3 vessel and 20-l sphere, reported previously by other authors. The values of u'rms are significantly higher in the latter vessel. Therefore it seems unlikely that KSt max values obtained in both vessels conform. Explosibility tests performed in 1 m3 vessel confirmed those predictions. An analysis of the possible reason of the discrepancy led to the conclusion that the part of the dispersion system responsible for that is the fast acting valve. Preliminary tests with the new, redesigned valve seems to confirm that conclusion. Presented results indicate that more precise description of the design of the standard 1 m3 vessel, its dispersion system is required than those given in ISO and European Standards, particularly the design and operation of the fast acting valve. Acknowledgements The authors gratefully acknowledge the financial support of the National Centre for Research and Development by Research Project No. O ROB 0005 01/2011/01 Technologies of explosion protection for storage places of bulk materials. References ASTM E 1226e12a, 2012. Standard Test Method for Explosibility of Dust Clouds. Bartknecht, W., 1989. Dust Explosions: Course, Prevention, Protection. SpringerVerlag, Berlin.
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Dahoe, A.F., 2000. Dust Explosions: a Study of Flame Propagation. Ph.D. Thesis. Delft University of Technology. Dahoe, A.F., Cant, R.S., Scarlett, B., 2001a. On the decay of turbulence in the 20-Liter explosion sphere. Flow Turbul. Combust. 64, 159e184. Dahoe, a.f., van der Nat, K., Braithwaite, M., Scarlett, B., 2001b. On the sensitivity of the maximum explosion pressure of a dust deflagration to turbulence. KONA Powder Part. J. 19, 178e196. Dahoe, A.F., 2014. Private Communication. Di Benedetto, A., Russo, P., Sanchirio, R., Di Sarli, V., 2013. CFD simulations of turbulent fluid flow and dust dispersion in the 20 liter explosion vessel. AIChE J. 59 (7), 2485e2496. Donoho, D.L., Johnstone, I.M., 1994. Ideal spatial adaptation by wavelet shrinkage. Biometrika 81. Eckhoff, R.K., 2003. Dust Explosions in the Process Industries, third ed. Gulf Professional Publishing, Amsterdam. EN 14034e1:2006þA1, 2011. Determination of Explosion Characteristics of Dust Clouds e Part 1: Determination of the Maximum Explosion Pressure Pmax of Dust Clouds. EN 14034e2:2006þA1, 2011. Determination of Explosion Characteristics of Dust Clouds e Part 2: Determination of the Maximum Rate of Explosion Pressure Rise (Dp/dt)max of Dust. Clouds. Hauert, F., Vogl, A., Radant, S., 1994. Measurement of turbulence and dust concentration in silos and vessels. In: Proceedings of the 6th International Colloquium on Dust Explosions, Shenyang. ISO 6184/1, 1985. Explosion Protection Systems e Part 1: Determination of Explosion Indices of Combustible Dusts in Air. McCaffrey, B.J., Heskestad, G., 1976. A robust bidirectional low-velocity probe for flame and fire application. Combust. Flame 26, 125e127. Proust, Ch, Accorsi, A., Dupont, L., 2007. Measuring the violence of dust explosions with the “20 l sphere” and with the standard “ISO 1 m3 vessel”. Systematic comparison and analysis of the discrepancies. J. Loss Prev. Process Ind. 20, 599e606. Pu, Y.K., Jarosinski, J., 1991. Turbulence effects on dust explosions in the 20-Litre spherical vessel. In: Proceedings of the 23rd Symposium (International) on Combustion. The Combustion Institute, pp. 843e849. Siwek, R., 1988. Reliable determination of the safety characteristics in 20-l apparatus. In: Proceedings of the Flammable Dust Explosion Conference; St. Louis: MO, pp. 529e573. Tamanini, F., 1998. The role of turbulence in dust explosions. J. Loss Prev. Process Ind. 11, 1e10. van der Wel, P.G.J, 1993. Ignition and Propagation of Dust Explosions. Ph.D. Thesis. Delft University of Technology.