HOUSING FOR A FLAMMABLE GAS DETECTOR

Abstract: A housing for a flammable gas detector comprises a housing body and a flame arrestor element (14 and 15). The housing body has an aperture (27) in which the flame arrestor element is located through which the interior of the housing body communicates with the outside. The housing body surrounding the aperture is moulded from plastics material. The portions of the housing body that form the aperture (27) are moulded around the flame arrestor element with the flame arrestor element in situ. A flammable gas detector of the type which employs a heated sensing element to oxidise any flammable gas present may be located in the housing body. Any flame front present in the housing body due to oxidation of any flammable gas is prevented from progressing to the surrounding environment by the flame arrestor element.

Description: - l - HOUSING FOR A FLAMMABLE GAS DETECTOR This invention relates to a housing for a flammable gas detector. One type of flammable gas detector employs a heated sensing element to oxidise any flammable gas present. Oxidation of flammable gases at the sensing element causes a change in electronic properties of the sensor which is detected to indicate the presence of a flammable gas. For example, a catalytic bead sensing element may comprise a coil of wire embedded m a porous bead containing a catalyst. Tne bead is heateα electrically oy passing current tnrough the wire. If flammable gas is present at the bead, t oxidises exothermally m the presence of the catalyst, causing the temperature of the beaα, and hence the wire coil, to increase. The change in resistance resulting from the change in temperature is detected to indicate tne presence of a flammable gas. Frequently two bead elements are used in a heatstone bridge arrangement with one element acting as tne sensor and the other being of similar construction but rendered inert ana acting as a control. higher output, ana nence greater sensitivity, can De ootaineα by using multiple oeaαs connecteα in series cr through the use of electronic amplification of the output signal. Altnough, for operation of tne detector, tne σas within tne detector housing must communicate with tne surrounding ambient gases, for reasons of safety, tne gas undergoing oxidation within tne nousmg must oe prevented from ιgr..t:ng any flammable gas outsiαe the nousmg. For this puroose, Known detectors include tne wall of the housing a flame arrestor tne form of a sinter element, through 'πich tne interior cf the housing communicates with the outsiαe. Tne flame arrestor snoulα proviαe as smal-. a resistance to diffusion as is consistent w th its primary requirements of strength and quenching the flame front. The housing for the sensor must be flameproof and capable of withstanding internal explosion without allowing a flame front to propagate outside the housing. Any joint or gaps m the housing must be small enough to attenuate the flame front such that it is incapable of igniting the gas outside the housing. Known housings are cast from metal. All potential flame paths require 100% inspection in order for the housing to meet the relevant safety standarαs. Not only must the flame arrestor element oe itself designed to meet the required safety standards but it must be secured to the housing in such a way as to prevent a flame path being provided at the flame arrestor element/housing interface. In order to form a satisfactory flame arresting arrangement, the housing and sinter element must be in intimate contact along the entire length of tne sinter element/housing interface. The periphery of tne sinter element, and tne internal surface of the housing over the region of contact with the sinter element, must be precisely formeα. In practice to meet trie tignt tolerances required for satisfactory performance, the cast metal housing nas to be precision macnined. In manufacturing the known flammable gas αetectors, the sinter element is fixed to tne housing in a separate operation. Tne sinter element may ioe glued to the housing, or tne housing may oe peened over t e sinter element, cr tne sinter element ray oe retaineα the housing by peenmg the housing over the edges of the sinter element, and the sinter element subsequently glued in position. The precision machining, quality assurance and fixing operations required to ensure that the known housings meets safety standards, makes them time-consuming and expensive to manufacture. According to the present invention there is provided a housing for a flammable gas detector comprising a housing body with an aperture through which tne interior of the housing body communicates with the outside, a gas permeable flame arrestor element located in the aperture, at least a portion of the housing body surrounding the aperture being moulded from plastics materials, the portions of the nousmg body that form tne aperture being moulded around tne flame arrestor element with the flame arrestor element situ whereby the flame arrestor element is fixed to the housing body. Moulαmg the body of the plastics housing around the flame arrestor element with the flame arrestor element in situ eliminates the operation of fixing the flame arrestor eieme-t into the housing. Moulding the plastics mateπa αirectly onto the periphery of the flame arrestor eiemer * e-.rainates the flame path at the interface between the arrester element and the housing. The requirements for macrmmq the housing, and fixing the sinter element t t~- "oαsmg a separate operation are eliminated reducmα f ° overall number of manufacturing operations with suoseααent savings both manufacturing time and cost. A suitable plastics material is used ::r tne nousmg body. It should exhibit hign impact strenσt", mecnanical rigidity, UV staoility and flame retardart properties over an extensive temperature ranqe . Suitable plastics might be thermoplastic, for example m eral-f-l ed PPS (polypnenv-sulphide) , PBT (polyb tylterepthaiate ) , or LCP (liquid crystal polymer such as ooly (penzoate-naptnoate) ) . Alternatively, thermosett g plastics such as DMC (dough- mouldinσ comoounα -polyester miσht be useα The porous nature of sinter elements allows any hot plastics which comes into contact with the sinter element to be wicked into the sinter element, reducing the gas permeability of the sinter element and, therefore, its effectiveness. When a sinter element which has a support ring around its periphery is used, the housing can be designed so that the hot plastics comes into contact only with the support ring and never into direct contact with the porous sinter element. If the housing is to be moulded around a sinter element which does not have a support ring, wickmg may be reduced, or eliminated, by using a sinter element which has a greater density around its periphery where the housing and the smter element will be joined, than in the middle. By making the periphery denser, the pore size is reduced and the molten plastics material cannot penetrate the sinter material so easily where it comes into contact with the nousing. Preferred embodiments of the invention will now be described in more detail witn reference to tne drawings in whicn : - Figure 1 is a side elevation of the housing; Figure 2 is a section tnrough the nousing of Figure 1 along tne line A-A showing a smter element witn a support ring; Figure 3 is an enlarged section cf tne smter element/nousmg unction of Figure 2; Figure 4 is a section cf the housing of Figure 1 along tne line A-A showing a smter element without a support ring; Figure 5 is an enlarged section of the smter element/housing junction of Figure 4; and Figure 6 shows a smter element which has increased density around its periphery. Figures 1 and 2 show a housing 10 for a flammable gas detector. The housing consists of two parts 11 and 12 which screw together to form an enclosed chamber 13. A porous smter element 14 of the type provided with a support ring 15 is mounted at one end of the housing and provides tne means by which the interior of tne housing communicates with the outside. The housing 10 accommodates the sensing elements of the flammable gas detector. A sensor retainer 16 in the form of a flanged circular disc with two axially-extendmg holes 17 locates a sensing element 18 and a reference element or control 19 within the housing. Tne elements 18 and 19 are of similar construction. Although the embodiment shown m the drawings has a single sensing element ana a single reference element, other arrangements are possible. For example tne sensor retainer may locate multiple oeaαs connected in series for tne sensing element and the reference element. The sensing element 18 consists a neater wire 20 connected oetween two lead-m conductors 21 wmcn are mounted m a supporting block 22. A dead 23 cf catalyst material distributed over a porous substrate is formed around tne heater wire. The reference element is identical to tne sensing element except that the bead is inert, either oecause the catalyst material is omitteo or oecause tne catalyst has been oelioerateiy poisoned. The lead-in conductors of the elements are connected to the outside by leads (not shown) which pass through a sleeve portion 24 of reduced diameter at the opposite end of the housing to the sinter element 14. The housing of the assembled detector is filled with an encapsulant 25 such as epoxy resin. The sensing element and the reference element are connected into opposite arms of a Wheatstone Bridge circuit (not shown) . An electric current is passed through the heater wires 20 of the elements 18 and 19 and heats the beads 23. In the absence of flammable gases, the resistances of the two elements 18 and 19 are the same and the arms of the Wheatstone bridge are balanced resulting in no voltage drop across the arms. If there is a flammable gas present in tne ambient atmosphere around the housing 10, some will diffuse through the smter element 14 into the holes 17 in wnich the sensing and reference elements 18 and 19 are located. The presence of tne heated catalyst on the bead of the sensing element 18 causes any flammable gases around it to oxidise m an exothermic reaction. This reaction neats the bead of tne sensing element 18 ano causes tne temperature cf its heating wire to increase, wnicn, m turn, causes tne resistance of the wire to increase. The increase in resistance of tne sensing element 18 compared with the resistance of the reference element 19 causes the Wheatstone Briαge to become unbalanced ana a voltage drop across the onαge is detected to indicate tne presence of the flammable gas . The construction of tne housing will now de describee in more detail. The first part 11 of tne housing 10 is made by moulding plastics material around tne support ring 15, which supports the smter element 14. One part of a die, the female die, nas a raised circular core. The sinter element is centred over the raised circular core such that the entire periphery of the support ring 15 overhangs the raised core. The male die has a smaller cross-sectional area, fitting inside the female die and locating onto and around the sinter element such that the sinter element 14 is entirely enclosed between the male die and the core of the female die keeping the plastics material from coming into contact with the smter element. Hot plastics material is injected into the die mould, and allowed to set. The plastics material which now incorporates the smter element is turned out of tne mould to form the first part 10 cf the housing as shown in Figures 2 and 3. The moulded interface 26 between the plastics first part 11 of the housing 10 and the support ring 15 avoids the creation cf any flame path between the housing and the support ring whilst retaining the properties cf the smter element, namely its gas permeability and its acuity to quench a flame front. The first part cf tne ro sing manufactured in this way has one end formed w tn an aperture 27 which is closed oy the smter eieme-t 14, the other end 29 being open and naving an externa_ screw thread 3C . The sensor retainer 16 fits insiαe tι~e iirst part 11 of the housing, locating against the inters: race of the smter element 14 and a shoulder 28 formeα c~ t~e inside of the housing. The sensor retainer noiαs tne sensing and reference elements 18 and 19 m tne reαuireα positions on the msioe of tne smter element 14. The secona part 12 of tne housing 10 is a_so moulded of plastics material One end 31 of tne part 12 has an internal screw thread 32 whicn matches tne external screw thread 3C at tre end 29 of tne first part 11 ana allows - 6 - the two parts to be screwed together to form the enclosed chamber 13. The other end 33 of the second part of the housing is formed with an integral sleeve 24 of reduced diameter to allow electrical connection of the sensing and reference elements to the electrical circuitry of the Wheatstone Bridge (not shown) . With tne sensor retainer 16 holding the elements 18 and 19 in position, in the first part of the housing, the two parts 11 and 12 are screwed togetner. Tne end of the cnamber 13 between tne retainer 16 and the sleeve 24 is then filled with an epoxy resin encapsulant 25, sealing tne interior of the noising, apart from tne holes 17 wnich accommodate the elements 18 and 19, from the environment. The housing is moulded from a plastics material which will provide satisfactory performance at the operation temperatures which the detector is likely to experience. Suitable thermoplastics include, out are not limited to, mineral filled polyphenylsulpnioe, polybutyltereptr.alate, and liquid crystal polymer sucn as poly (benzoate- naptnoatej . Alternatively tnermosettmg plastics such as dougn moulding compounα-poiyester may be used. Figures 4 and 5 show an alternative empoo mer.t of the invention, m wnich the smter element nas no support ring and tne first part 11' of the nousmg is moulded directly cnto the smter element 14' itself, so that tne smter element is in intimate contact w th the first part of the plastics nousing. In order tc prevent hot plastics of the housing from wickmg into tne smter element 1- ' α-nng the injection moulding process, tne smter element is manufactured with an increased density around its peripnery wnich results m tne centre portion cf tne smter element remaining permeable to gases, and tne peripheral portions of the s ter element being less susceptible to wickmg of the hot plastics. Figure 6 shows a smter element 14 ' which has been manufactured to provide a gas permeable central portion 35 and denser peripheral portions 36. Smter elements of this type may be manufactured m a variety of ways. Flame arrestors, in the form of smter elements, may be produced by pouring powdered metal ef a known gram size into a mould which forms one half of a die set. Tne second half of the die set is lowered and pressure is applied to the powder allowing some of the particles to bond together m a brittle oiscuit-like form Known as a green-state smter. This green smter is then fired in a furnace, the firing normally being performed m a reducing atmosphere to ensure that the powder remains in the metallic state. In the final sintered structure the metal particles are fused together giving the structure great strengtn while at the same time providing many routes for gas to pass through the smter element albeit over a distance far greater than the thicKness of the smter element. A flame front present m the interior of a detector housing may also commence along these paths. However, tne compmation of the high tnermal conductivity of the metal particles and the lengtn of the path through which the flame front must travel to reach the side of the smter element in contact with the surrounding environment ensures tnat any flame front is quenched before it reaches the surrounding environment. Tne smter element presents very little resistance to tne passage of gases. One method for producing a smter element with increased smter density around its periphery is to produce a normal, flat greer state smter element which is then placed m a second die such that an increased pressure is applied only to the periphery of the smter element oniy. The periphery tnerefore is compressed further and the powder particles become more densely packed. The smter is then fired in the normal way, using a combination of reducing furnaces. By applying sufficient pressure to the periphery during the manufacturing process it can be made so dense in the finished product that wickmg is reduced or even eliminated. The resultant smter element has a gas permeable centre section 35 and a denser periphery 36 which is less susceptible to wickmg and which may even be totally impervious to gas. In another method the initial die set can be constructed so that the powder is poured evenly into the mould and the second half of the die set has a recessed centre portion. When pressure is applied to tre second half of the die, the periphery of the smter has greater pressure applied than the centre portion. Wnen fired as above, the smter element produced has a gas permeable centre section 35 and denser peripheral sections 36 which are less susceptible to wickmg. In a third method the smter can be maαe cf two powders, a coarse powder m the centre and a fine powder toward the edge. Uniform pressure is applied tc form the smter. By the nature of the smaller gram size of fine powder a more dense and impermeable smter is made at the outside. By manufacturing a smter element witn a denser periphery, the need for a support ring is avoided, and smter elements may be manufactured more cheaply. In alternative embodiments, the smter element may be constructed by placing a greater thickness of powoered metal particles at the periphery so that in tne finished article the thickness of the element is uniform with the periphery having a greater density than the centre.

Claims: CLAIMS 1. A housing for a flammable gas detector comprising a housing body with an aperture through which the interior of the housing body communicates with the outside, a gas permeable flame arrestor element located in the aperture, at least a portion of the housing body surrounding the aperture being moulded from plastics materials, the portions of the housing body that form the aperture being moulded around the flame arrestor element with the flame arrestor element in situ whereby the flame arrestor element is fixed to the housing body. 2. A housing for a flammable gas detector according to claim 1, wherein the flame arrestor element is provided by a sinter element. 3. A housing for a flammable gas detector according to any of the preceding claims, wherein the plastics housing is manufactured by injection mould-.rq tne plastics material around the flame arrestor element 4. A housing for a flammable gas detector according to claim 3, wherein the density of the sinte. element is greater around the periphery of the smter element than m the centre. 5. A housing for a flammable gas detector according to claim 4, wherein the smter element is provided with increased density of smter around its peπpnery by a second pressing of the unfired, green-state smter. 6. A housing for a flammable gas detector according to claim 4, wherein the smter element is provided with an increased density of smter around it peripnery in a single moulding operation using a stepped or graduated mould. 7. A housing for a flammable gas detector according to claim 4, wherein the smter element is provided with an increased density of smter around its periphery by use of a fine gram powder around the periphery and a coarser grained powder in the centre. 8. A housing for a flammable gas detector according to any of the preceding claims wherein the material of the housing is thermoplastic and is mineral filled polyphenylsulphide, polybutylterepthalate or poly (benzoate-napthaoate) . 9. A housing for a flammable gas detector according to any of claims 1 to 7, wherein the plastics material of the housing is thermosettmg and is a dough moulding compound-polyester . 10. A method of forming a housing for a flammable gas detector comprising moulding at least a portion of a housing body from plastics around the periphery of a gas permeable flame arrestor element. 11. A method of forming a housing for a flammable gas detector according to claim 10, wherein the f ame arrestor element includes a smter element. 12. A method according to claim 11 including forming the smter element from powdered metal, the periphery of the smter element being compressed to form a material of greater density than at the centre. 13. A housing for a flammable gas detector according to claim 2, wherein the flame arrestor element includes a support ring, the support ring being disposed around tne smter element and the plastics material of the housing body being moulded onto the support ring.

Inventors: FOOT RUSSELL CHRISTOPHER, LEGG MARTIN CHARLES, TINDALL IAN FRANCIS, FOOT, RUSSELL, CHRISTOPHER, LEGG, MARTIN, CHARLES, TINDALL, IAN, FRANCIS
Applicants: FOOT RUSSELL CHRISTOPHER, LEGG MARTIN CHARLES, TINDALL IAN FRANCIS, ZELLWEGER ANALYTICS LTD, FOOT, RUSSELL, CHRISTOPHER, LEGG, MARTIN, CHARLES, TINDALL, IAN, FRANCIS, ZELLWEGER ANALYTICS LIMITED
Application year: 1998
Main classification: G01N33
IPC codes: G01N33