A METHOD AND APPARATUS FOR SUPPRESSING THE PROPAGATION OF EXPLOSIONS DURING PASSAGE OF PARTICULATES OR POWDERED MATERIALS

Abstract: The present invention relates to a method and apparatus for suppressing the propagation of explosions in the flow of a powdered material/air mixture comprising the step of introducing a swirl component to the flow of the powdered material/air mixture about the axis of flow to cause segregation of the powdered material such that the resulting distribution of powdered material will be more concentrated within a given area of the cross-section of flow thus restricting access of a burning particle from an upstream explosion to the oxygen in the air.

Description: A METHOD AND APPARATUS FOR SUPPRESSING THE PROPAGATION OF EXPLOSIONS DURING PASSAGE OF PARTICULATES OR POWDERED MATERIALS The present invention relates to a method and apparatus for suppressing the propagation of explosions during passage of particulates or powdered materials, in particular, the passage of pulverised fuel/air mixtures in pipelines. A problem often encountered in industries which handle fine powders under pneumatic conditions is that the propagation of deflagrations within a system can proceed almost unchecked along the passage or pipeline which carries the powder. Thus an explosion can develop and present a substantial hazard during periods of operation. It is, therefore, considered desirable to provide a method of curtailing the development of such explosions. According to the present invention there is provided a method for suppressing the propagation of explosions in the flow of a powdered material/air mixture comprising the step of introducing a swirl component to the flow of the powdered material/air mixture about the axis of flow to cause segregation of the powdered material such that the resulting distribution of powdered material will be more concentrated within a given area of the cross-section of flow thus restricting access of a burning particle from an upstream explosion to the oxygen in the air. Preferably, segregation of the powdered material is carried out by placing a diametrical flow divider within a cylindrical duct, which duct leads from the potential explosion chamber, the flow divider being helical in shape. Preferably, the edge of the helical flow divider at the entry of the flow is set at zero incidence to the flow in order to minimise the separation effects of the divider at entry. Preferably, the entrance to the flow divider is located at a predetermined distance from the entrance to the duct depending on the length and diameter of duct, flow velocity and powdered material/air mixture concentration and explosibility. Preferably, the length of the duct is substantially 42m the diameter substantially 0.6m, the flow velocity substantially 20ms -1 and the powdered material/air mixture concentration lies within the range 0.1 to 0.6 kgm" 3 and the entrance to the flow divider is located at substantially 18.5m from the entrance to the duct. Preferably, the flow divider is constructed to twist the flow through 180° over a distance of 2m of duct length. Preferably, the leading edge of the flow divider is set at zero incidence to the flow at entry being gradually turned into a regular 30° helix over a transition length of substantially 0.53m. Preferably, the method of the present invention is used in an arrangement which carries a pulverised fuel/air mixture. The present invention further provides a device for suppressing the propagation of explosions in the flow of a powdered material/air mixture which introduces swirl about the axis of flow of the powdered material/air mixture to cause segregation of the powdered material such that the resulting distribution of powdered material is concentrated within a given area of the flow cross-section thus restricting access of a burning particle from an upstream explosion to the oxygen in the air. Preferably, the device comprises a diametrical flow divider placed within a duct which duct leads from the potential explosion chamber, the flow divider being helical in shape. A preferred embodiment of the present invention will now be described in detail, by way of example only, with reference to the accompanying drawings, of which: Fig. 1 is a perspective view of a flow divider or segregation device according to a preferred embodiment of the present invention; Fig. 2 is a perspective view of an arrangement used for testing the method and apparatus of the present invention; Fig. 3 depicts the variation of maximum pressure . ith pulverised fuel/air concentrations for different settings of the segregation device of Fig. 1; Fig. 4 depicts the pressure and flame distance record for a given setting of the device of Fig. 1. Fig. 1 is a perspective view of a segregation device suitable for suppressing the propagation of explosions in the flow of a pulverised fuel/air mixture. The device comprises a helical flow divider 1 which is located in the flow of the mixture indicated by arrows A. The flow divider 1 will typically be positioned within a duct 2 which carries the mixture. As can be seen in Fig. 1, the leading edge 3 of the flow divider 1 is at zero incidence to the flow of the mixture to ensure that the separation effects upon entry are minimised. In this embodiment the leading edge 3 turns into a regular 30° helix 4 which rotates the flow through 180° over the length of the flow divider. In a duct 2 of diameter 0.6m and length 2m, the transition length for the leading edge 3 to turn into the helix 4 would be substantially 0.53m. In use, a flow passing through the duct 2 in direction A will have a swirl induced around the axis of flow which will cause segregation of the pulverised fuel particles and concentrate their distribution within a given area of the flow cross-section. Such a concentration of pulverised fuel particles will restrict access of a burning particle from an upstream explosion to the oxygen in the air thus suppressing or interrupting the development of a propagating deflagration. Fig. 2 shows an arrangement within which a device such as that depicted in Fig. 1 was tested for * its ability to suppress the propagation of an explosion. The arrangement generally comprises a motor 5, connected to a centrifugal blower 6, a pressure venting chamber 7 and a flow straightener 8. There is a flow and temperature measurement section 9. The pulverised fuel is stored in a warehouse 10 and delivered to building 11 which contains pulverised fuel and feed-rate control equipment. Expansion pieces 12 and 13 are located on either side of the building 11 to allow expansion of the ducting 2. The explosion chamber 14 is located between the building 11 and a length of test ducting 15 within which a flow divider such as that in Fig. 1 is placed. A safety vent 16 is provided at the entry to the explosion chamber 14. Explosion tests were carried out with the device of Fig. 1 installed at different locations along the test ducting 15 which was typically 42m in length and 0.6m in diameter. The explosion chamber 14 had a volume of substantially 20m 3 . The pulverised fuel/air mixtures used had a flow velocity of substantially 20ms" 1 and various concentrations were ignited in the explosion chamber 14 by a flame-jet. The tests employed NCB type 502 coal of two representative pulverised fuel duct grades, nominally 250 grade (78% < 71μm) and 190 grade (66% < 71μm) . The leading edge 3 of the flow divider 1 was positioned vertically within the test ducting 15, first at 18.5m and then at 33.5m from the entry to the test ducting 15. In the tests, positioning the flow divider 1 at 33.5m failed to stop strongly propagating incident deflagrations from continuing their development over the remaining downstream ducting and high maximum pressures were recorded as can be seen in Fig. 3. It is thought that with the flow divider at the 33.5m location the amplitude of the incident pressure wave probably re-disperses the concentrated pulverised fuel ahead of the flame. In contrast, however, with the flow divider at the 18.5m location no increase in flame velocity or pressure wave amplitude occurred downstream as can be seen from Fig. 4. Fig. 3 is a graph of p max V pulverised fuel (PF)/air concentrations for given settings of the flow divider. The table below explains the symbols used on the graph. As can clearly be seen, whereas the P m χ value for a setting of 33.5m is as high as 15 bar, the P ma values for settings of 18.5m are ≤ 5 bar. Tests were also carried out without using a flow divider. The approximate P max envelope for the unobstructed duct is indicated on the graph by broken lines B. The tests yielded an array of peak explosion pressure Pma values within the envelope with a maximum value of 33.3 bar at 0.27 kgm -3 pulverised fuel/air concentration. In contrast, the Pmax envelopes when a flow divider was used for a similar coal type and size grade are indicated by broken lines C and D. Fig. 4 depicts graphically the pressure and flame distance - time record for a coal type 502 size grade 250, concentration 0.26 kgm -3 , velocity 20ms" 1 when the flow divider is located at the 18.5m setting. As can be seen there was no increase in flame velocity or pressure wave amplitude downstream of the flow divider. Beyond point E the flame front trajectory showed a constant flame velocity. The present invention, therefore, is capable of curtailing the development of propagating pulverised fuel explosions which might otherwise reach damaging intensities and will limit the maximum pressure development to a level that is easily contained, provided that the flow divider or segregating device is installed suitably close to a potential explosion chamber such as a coal mill or classifier. The present invention is designed to produce segregation of particles only when there is a high velocity transient flow of typically 400ms- 1 which is often experienced during passage of an explosion wave. At normal pulverised fuel conveying velocities which are typically one order of magnitude less, the flow resistance and swirl produced will be substantially lower.

Claims: CLAIMS : 1. A method for suppressing the propagation of explosions in the flow of a powdered material/air mixture comprising the step of introducing a swirl component to the flow of the powdered material/air mixture about the axis of flow to cause segregation of the powdered material such that the resulting distribution of powdered material will be more concentrated within a given area of the cross-section of flow thus restricting access of a burning particle from an upstream explosion to the oxygen in the air. 2. A method as claimed in Claim 1 wherein segregation of the powdered material is carried out by placing a diametrical flow divider within a cylindrical duct, which duct leads from the potential explosion chamber, the flow divider being helical in shape. 3. A method as claimed in Claim 2 wherein the edge of the helical flow divider at the entry of the flow is set at zero incidence to the flow in order to minimise the separation effects of the divider at entry. 4. A method as claimed in Claim 2 or Claim 3 wherein the entrance to the flow divider is located at a predetermined distance from the entrance to the duct depending on the length and diameter of duct, flow velocity and powdered material/air mixture concentration and explosibility. 5. A method as claimed in Claim 4 wherein the length of the duct is substantially 42m, the diameter substantially 0.6m, the flow velocity substantially 20ms -1 and the powdered material/air mixture concentration lies within the range 0.1 to 0.6 kgm -3 and the entrance to the flow divider is located at substantially 18.5m from the entrance to the duct. 6. A method as claimed in Claim 5 wherein the flow divider is constructed to twist the flow through 180° over a distance of 2m of duct length. 7. A method as claimed in Claim 6 wherein the leading edge of the flow divider is set at zero incidence to the flow at entry being gradually turned into a regular 30° helix over a transition length of substantially 0.53m. 8. A method as claimed in any preceding claim for use in an arrangement which carries a pulverised fuel/air mixture. 9. A device for suppressing the propagation of explosions in the flow of a powdered material/air mixture which introduces swirl about the axis of flow of the powdered material/air mixture to cause segregation of the powdered material such that the resulting distribution of powdered material is concentrated within a given area of the flow cross-section thus restricting access of a burning particle from an upstream explosion to the oxygen in the air. 10. A device as claimed in Claim 9 comprising a diametrical flow divider placed within a duct which duct leads from the potential explosion chamber, the flow divider being helical in shape. - lO - 11. A device as claimed in Claim 10 in which the leading edge of the helical flow divider at the entry of flow is set at zero incidence to the flow in order to minimise the separation effects of the divider at entry. 12. A device as claimed in Claims 10 or 11 wherein the flow divider is constructed to twist the flow through 180° over a given distance. 13. A device as claimed in any of Claims 10 to 12 wherein the leading edge of the flow divider is set at zero incidence to the flow at entry being gradually turned into a regular 30° helix over a transition length of substantially 0.53m. 14. A device as claimed in any of Claims 9 to 13 for use in an arrangement which carries a pulverised fuel/air mixture. 15. A method for suppressing the propagation of explosions in the flow of a powdered material/air mixture substantially as herein described with reference to the accompanying drawings. 16. A device for suppressing the progagation of explosions in the flow of a powdered material/air mixture substantially as herein described and as illustrated in Fig. 1.

Inventors: GARDNER BRIAN ROBIN, GARDNER, BRIAN, ROBIN
Applicants: NAT POWER PLC, NATIONAL POWER PLC
Application year: 1991
Main classification: A62C4
IPC codes: A62C4