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WO2006111765A1 - Detonation flame arrester - Google Patents
Flame Arrestor Animation. Featured Products. Enardo Detonation Flame Arrestors. Series 8 High Pressure Deflagration Arrestor. Series 7 In-Line Flame Arrestor. A range of initial pressures up to Evans et a!. In fact, experiments have shown that the re-intension or re-initiation process of detonation waves occurs downstream of the acoustic absorbing walled section in the pipe, including the onset of an overdriven detonation at some distance away from the exit of the acoustically absorbent section.
Other aspects of the present invention seek to provide a detonation flame arrester which can operate at relatively high initial pressures and can withstand high detonation pressures and velocities. Further aspects of the present invention seek to provide a detonation flame arrester which is substantially shorter than existing arresters especially with larger nominal pipe diameters.
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Aspects of the invention seek to provide a detonation flame arrester which does not require an expanded section, i. According to a first aspect of the present invention, there is provided a detonation flame arrester comprising at least one detonation arresting element and at least one serially- disposed deflagration arresting element, the detonation arresting element comprising a plurality of generally parallel channels, characterised in that said channels are not interconnected and in that each channel has a characteristic transverse dimension of 0.
The characteristic transverse dimension can be the cross-sectional size of a passageway through a tube for example. It can be the equivalent circular diameter or the hydraulic diameter, or the pore dimension. Advantages of the arrester are that it serves to isolate detonation in the gas and efficiently removes heat from the flame front. Preferably at least the internal walls of each channel are substantially smooth. It is believed that the smooth nature of the walls will cause less compression effects on gas i.
On the other hand, the severely pre-compressed gas due to porous walls is more susceptible to reinitiation of a detonation. In preferred arrangements the length of the detonation arresting element is substantially greater than that of the deflagration arresting element. In preferred arrangements the factor is at least two, and in some preferred arrangements the factor is at least ten.
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In general the length of the detonation arresting element is adjustable to optimised dimensions for the length if the deflagration arresting element. Because of the relatively small length of the deflagration element, it does not produce a high pressure drop despite its smaller apertures. For similar reasons, an advantageous arrangement is obtained in an arrester comprising a deflagration arresting element disposed between two detonation arresting elements.
Such an arrester has the particular advantage of providing a compact, all-purpose arrester in a single unit which can be used in different applications for different gases. Because it can be produced in large quantities to benefit from the economy of scale, it can still be specified in many locations, even if its performance is higher than required, as it provides an additional safety factor. According to a second aspect of the present invention, there is provided a detonation flame arrester comprising at least one detonation arresting element and at least one serially- disposed deflagration arresting element, the detonation arresting element comprising a plurality of generally parallel channels, characterised in that the walls of said channels are non-porous and in that each channel has a characteristic transverse dimension of 0.
Such non-porous walls are solid and impermeable to gases. According to a third aspect of the present invention, there is provided a detonation flame arrester comprising at least one detonation arresting element and at least one serially- disposed deflagration arresting element, the detonation arresting element comprising a plurality of generally parallel channels, characterised in that the walls of said channels are of an acoustically reflective material and in that each channel has a characteristic transverse dimension of 0.
According to a fourth aspect of the present invention, there is provided a detonation arrester comprising a plurality of generally parallel channels, characterised in that said channels are not interconnected and in that each channel has a characteristic transverse dimension of 0. Such an arrester is suitable for retro-fitting in a situation where a deflagration arrester element is already installed. The gas is usually a mixture of individual gases. The length of the detonation arresting element in preferred embodiments is at least ten times the length of the deflagration arresting element, especially if the deflagration arresting element is of sintered gauze laminate.
However, similar length detonation arresting elements, but at least twice the length of the deflagration arresting element, may be employed, especially when the deflagration arresting element consists of crimped ribbon. In one preferred embodiment, some or all of the elements are arranged in a radially enlarged portion of a pipeline. Such an arrangement reduces the pressure drop in the pipeline.
In another embodiment, all of the components are arranged in a part of the pipeline which has the same diameter as the adjacent pipeline. Such an arrangement can save space around the pipeline, avoid the need to introduce bends in the pipeline, and facilitate retrofitting in suitable circumstances. Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, of which:. Figure 1 is a schematic side view of an arrester in accordance with a first embodiment of the present invention;.
Figure 2 is a cross-sectional view of part of a first arrester component i. Figure 3 is a cross-sectional view of part of a second arrester component i. Figures are schematic sectional side views of second, third, fourth and fifth embodiments, respectively, of the present invention;. Figure 8 is a side sectional view of an arrester in accordance with a sixth embodiment of the present invention;.
Figure 9a is a left-hand end view of the main section of the arrester of Figure 8 showing a first component thereof; Figure 9b is a side sectional view of the main section of the arrester of Figure 8;. Figure 9c is a right-hand end view of the main section of the arrester of Figure 8 showing a second component thereof; and. Figure 10 is a side sectional view of an arrester in accordance with a seventh embodiment of the present invention. Referring to the drawings, Figure 1 shows a detonation flame arrester 10 in accordance with a first embodiment.
The arrester is connected in series between adjacent lengths of a gas pipeline 11 having a diameter 'd'. The arrester is located in a widened section 17 of the pipeline having a diameter D which is typically twice 'd'. Section 17 is connected to each adjacent length of pipeline by means of a respective tapering portion 27 of axial length b and defining an angle of relative to the axial direction. The arrester comprises a first component 12 comprising a matrix of non-connected tubular passages A cross section of these passages 15 is shown in Figure In the example the passages 14 are shown to tessellate the cross section.
The apertures of the array of tubes are larger than those used in conventional flame arresters. The length "f" of the first component is of the order of 10 cm. Component 12 serves to damp shock waves associated with detonations travelling down pipeline Located immediately downstream of component 12 of the direction of gas flow indicated by arrow 18 is a second component The porous medium 24 of component 23 may take the form of a matrix of tortuous connected pathways or non-connected pathways, as shown in Figure 3.
The effective diameters of these pores are typically in the range 0. Component 23 serves to quench flames travelling from component Thus component 12 is longer than or similar to the corresponding conventional component, but component 23 is much shorter. When designing a particular arrester 10, the characteristic transverse dimension "a" of the tubes 15 corresponding to the diameter of a circular tube is selected so that a detonation cannot propagate therethrough.
It depends on a number of factors, including the nature of the gas system in pipeline 11, the gas velocity and the gas pressure and should also include a safety margin. For stoichiometric fuel-air mixtures at atmospheric pressure, there is a minimum transverse detonation cell size "s" of the explosive mixture, see Table 2.
Some data for four typical gases in air are shown in Table 2, with the gases ranked in order of increasing difficulty with respect to attenuating the shock wave. One example of the dimension "a" is shown in Table 2 for each gas in air. The dimension "a" is a significant parameter and has an upper limit of s.
The length "f" of component 12 should be sufficiently large to dissipate the shock wave before the porous medium One example of "f" is shown in Table 2 for each gas. For smaller values of "a", a shorter length "f" is required to attenuate a shock wave.
The value of the length "f" is, in principle, independent of the arrester size represented by the nominal bore of the pipe connection 'd'. Therefore, for larger arresters the overall dimensions of the new design will be smaller than for conventional units as the length of these tends to increase as 'd' increases. Examples, in terms of the diameter 'd' of the pipeline, are given in Table 2 for the length 'b' of the tapering section 27 and the distance 'c' between the wider end of section 27 and the centre line of the second component In preferred embodiments, tubes 15 have a wall thickness in the range of 0.
The above dimensions give only a general indication based on various assumptions, e. Due to the uncertainty of the viscosity of the gas in the combustion zone at the outer edge of the boundary layer, various dimensions and especially damping length "f" should be determined by experiments.
In actual applications, dimensions "a" and "f" should be optimised to increase the quenching efficiency and make the device more compact. In use of the arrester 10, a detonation-produced pressure or shock wave travelling in the direction of arrow 18 encounters first component In view of the above-described parameters, this prevents the detonation from reaching the second component Only the deflagration reaction front reaches the second component 23, and is extinguished in the medium.
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Arresters according to the present invention can be used for gas-air and gas-oxygen mixtures. The above-described arrester has a number of advantages. Firstly, the flow resistance across the composite system is less than that of a conventional detonation flame arrester containing porous media. This is based on the realisation that there is no need to be restricted by reliance on MESG criteria for detonation.
Thus there is a smaller pressure drop across the device. At first glance, this use of wider apertures appears to be counterintuitive but is backed by detonation physics indeed. As a result the arrester 10 has a certain degree of design freedom, in that the diameter D of section 17 can be reduced since there is less of a pressure drop to be compensated and detonation waves can be attenuated by component Another advantage is that the weight and cost of the composite media is less than that normally used in conventional arresters.
On large systems, this has a significant advantage for installations at elevated positions. Tests have shown that the above arresters in accordance with the present invention can operate at substantially higher initial pressures e. The theoretical basis that underpins the invention described in the patent of Lee and Strehlow is not well defined. In one embodiment of the patent they describe a configuration in which "the absorbent may be disposed in a porous walled tube bundle arrangement which is inserted in the pipe such that the axes of the tubes are parallel to the centre of the pipe.
On the other hand, since the channel walls of embodiments in accordance with the present invention do not have connections between the channels, such linkage is prevented. In addition the channel walls of preferred embodiments are substantially smooth and it is believed that the gas in the channels is less compressed i. In another embodiment of their invention, Lee and Strehlow describe an arrangement in which the walls of the pipe are lined with an acoustically absorbent material.
The walls are impermeable and therefore the mechanism described above cannot apply to this case. The mechanism on which this embodiment of their invention may rely is attenuation of the transverse waves in or by the acoustically absorbent material. However, in more recent work according to Radulescu and Lee , "conclusive proof of the important role of the transverse waves on the propagation mechanism of detonations is still lacking". The paper also indicates that for the system with a regular cellular structure with weaker transverse waves, the detonation transverse waves do not play a significant role in detonation propagation mechanism, i.
More significantly and importantly, experiments including Lee's work show that rapid attenuation of the detonation waves due to acoustically absorbent porous walls is limited to relatively low initial pressures. On the other hand, at higher initial pressures, the porous walled tubes can cause much higher hydraulic resistance and more severe pre-compression effects on gas.
The re-initiation detonation lengths decrease with the increase of the initial pressure. Furthermore, the distance 2D required by Lee and Strehlow is not allowed in embodiments of the present invention because this distance will cause re-generation of detonation upon exiting the damping section and the initial C-J detonation velocity will be recovered.
Various modifications may be made to the previously-described arrangements. The cross- sections of the tubes or passageways within component 12 may have any desired shape, in particular exact or approximate triangles, squares, rectangular parallelograms, honeycombs, other polygons, circles or other curved outlines.
Besides crimped ribbon or sintered gauze laminate, the passageways within component 23 can be of knitted mesh, enclosed tubes, randomly packed particles of a fill medium, solid rod elements with passageways therebetween, or parallel plate elements with slits there between.
A metal foam member can be used to provide an additional heat transfer surface to deal with deflagration. Since component 12 is required only to attenuate shock waves and quench the detonation, it can be made of materials other than steel, the design of which must be able to withstand the radial compressive load resulting from the shock wave. Alternative materials may include other metals and alloys, carbon and other composites, polymers and other plastics, glass and ceramics. This enables weight and cost to be saved, particularly as this is the larger of the two components.
These materials are provided in solid wall form, but the surface may be treated with coatings of various forms to provide resistance to chemical attack and withstand mechanical loading due to shock wave and also to provide optimal surface conditions. In addition, the component may be formed using any of the following manufacturing processes: fabrication e. In alternative or additional modifications of the detonation arresting component 12, the detonation arresting element can be formed by two or more parts, each having same or different apertures, and some or all of the channels may be inclined to the central longitudinal axis of the arrester.
To protect the front of component 12 from damage by the direct impact of a shock wave, it can be provided with a thin piece of crimped metal ribbon, perforated plate, wire grid or wire mesh. Four prototype 50 mm nominal bore unstable detonation arresters have been tested. These prototype devices were of different configurations with different combinations of elements, in which detonation attenuation elements had different apertures and damping lengths of honeycomb cores.
Detonation and deflagration flame arresters - Valvulas Nacional
In general, the testing results demonstrated that the detonation waves were effectively attenuated by the detonation arresting elements and indeed became deflagration. Both detonation arresters, bi-directional and uni-directional, have been successfully tested to stop flame transmission into the protected side for gas group IIB3 6. In tests, the bi-directional arrester according to the present invention, as shown in Figure 10 which comprises detonation arresting elements of honeycomb cores and a deflagration arresting element of sintered gauze laminate, successfully prevented flame transmission into the protected side in any deflagration and unstable detonation tests for gas group IIB3 6.
On the other hand, the detonation arrester significantly reduces the pressure drops over the arrester, that is. It is worth mentioning here a phenomenon known as "pressure piling". As a shock or combustion wave travels down a pipe in which there is a flow restriction such as a flame arrester , the unburned gas immediately upstream of the restriction is subjected to increased pressure. So, although the system pressure in the pipe immediately prior to ignition may be slightly more than atmospheric pressure e. The amount of energy released during the detonation is related to the gas pressure, and further this relationship is not linear.
So the force of the shock wave can be very significantly higher at the arrester inlet if the effect of pressure piling is significant, and could cause the arrester to transmit a flame resulting in catastrophe. Accordingly, it is a significant benefit to have a device in which the pressure drop across the unit is as small as possible to minimise the effect of the pressure piling. This is achieved in the present invention by means of the larger aperture channels used to attenuate the detonation and the relatively low flow resistance associated with the deflagration element when compared with conventional devices.
The construction of the arrester is flexible and it may be designed to suit duties with any gas group — data on the detonation cell width is well documented for all the principal gases. The construction opens up the possibility of designing an "all-purpose" arrester for each gas group identified in EN This results in a single product for each gas group to deal with unstable and stable detonations and also deflagrations instead of the three separate products that exist for such duties.
The design may be adapted to pre- volume applications - i. The arrester may be constructed of materials that enable it to be used in corrosive environments. It is easier to clean and cheaper to maintain, and the manufacturing process is simpler and manufacturing tolerances are less problematic in terms of process control. In addition the arrester may be retrofitted to existing deflagration arresters.
If desired, components 12 and 23 within section 17 need not be in intimate contact. In the arrester 40 of Figure 4, for example, another second component 23' is located downstream of, and spaced from component This provides an additional safety factor. In the arrester 50 of Figure 5, a single component 23 is sandwiched between two first components 12, 12'. This forms a bi-directional arrester which can handle gas flows, and explosions, in either direction. In a modification, one or both components 12, 12' may be spaced from component 23 if desired.
In another modification additional component pairs may be added to the sandwich. In the arrester 60 of Figure 6, the first component 12" is arranged in a section of pipeline 11 of the nominal pipe diameter d, with the flame-quenching component 23 remaining in widened section The dimension "a" and the length "f are determined according to the same criteria as for the embodiment of Figure 1.
Component 12" may partly extend into the widened section In the arrester 70 of Figure 7, the widened section 17 is dispensed with completely and both components 12 and 23 are provided in a section of pipeline 11 or nominal diameter. The dimension "a" and the length "f" are again determined according to the same criteria as for the embodiment of Figure 1. An advantage of this embodiment is that no alteration of the diameter of pipeline 11 is necessary, which means that no extra space is required.
This allows the arrester 70 to be readily retro-fitted to an existing pipeline if required. A sixth embodiment of the present invention is shown to scale in Figures 8 and 9. An arrester 80 comprises a first component 12 and a second component 23 arranged to be connected to a pipeline 11 by flange members 81 to 84 and tapering sections The individual tubes 87 of component 12 have an outside diameter of 6 mm and an inside diameter of 5 mm.
The components 12 and 23 are located directly adjacent to each other within a housing 88, having fixing tabs A seventh embodiment of the present invention is shown in Figure An arrester 90 located in a gas flow 18 comprises an expansion section 91 the purpose of which is to allow the arrester element to have a diameter D which is larger than the inlet pipe 97 of diameter d to which it is attached.
This allows the pressure drop across the system to be reduced to acceptable levels. The arrester further comprises an element housing 92, which is effectively a straight length of pipe, containing a first detonation wave attenuation element 93 designed to modulate the shock and reduce the flame speed from supersonic velocities to subsonic velocities before it enters the deflagration element.
The arrester further comprises a deflagration arrester element 94 which is designed to prevent flame transmission by means of heat transfer from the flame front to the quenching element and support structure or by removing reactive intermediates e. There is further provided a second detonation wave attenuation device 95 of the same or different construction as element 93 to form a bi-directional arrester.
The various components are held in place by a housing An arrester based on Figure 10 has successfully passed the flame transmission tests under unstable detonation and deflagration conditions. The embodiment of Figure 10 may be modified in various ways. This can be achieved because element 93 can effectively attenuate the detonation waves and further because of the preferential pressure drops that can be achieved across this device compared with other products available in the field. In the case where the element has the same diameter as the pipework system, there is no need for the expansion and reduction sections 91 and These assemblies may be replaced by a single flange suitable for the design pressure in the pipework itself.
The device as described is bi-directional but may be made a uni-directional arrester simply by removing the second attenuation element 95 and one set of support bars This has the advantage of reducing size, weight, cost and pressure drop through the finished unit. It does however require the direction of gas flow to be clearly marked on the unit to avoid human errors in installation. The support bars serve as spacing elements. They may be made of wire gauzes, wire grids, wire meshes or other suitable material.