Abstract:
Valves for controlling the release of a substance, include a housing having an inlet for connection to a source of the substance, an outlet, and a passage extending therebetween. The passage is enclosed by a ceramic disc and means are provided to apply an electrical pulse to the disc to break the disc and so connect the inlet to the outlet. The methods include containing the substance in a container, connecting the container to a valve; and applying an electrical pulse to the disc to break the disc and connect the inlet to the outlet. The valves and methods are particularly suited, but not limited, to the control of substances such as pressurised fire extinguishing media.

Description:
FIELD OF THE INVENTION 
   The invention relates to devices and methods for controlling the release of a substance. It is particularly suited, but not limited, to the control of substances such as pressurised fire extinguishing media. 
   A known device for controlling the release of a substance comprises a housing having an inlet for connection to a source of the substance, an outlet and a passage extending therebetween, the passage being closed by, for example, a frangible metal disc which may be disrupted by electro-mechanical or chemical (pyrotechnic or explosive) means to connect the inlet to the outlet and release the substance. 
   SUMMARY OF THE INVENTION 
   According to a first aspect of the invention there is provided a device for controlling release of a substance comprising a housing having an inlet for connection to a source of the substance, an outlet and a passage extending therebetween, the passage being closed by a ceramic member and means being provided to apply an electrical pulse to the member to break the member and so connect the inlet to the outlet. 
   According to a second aspect of the invention there is also provided a method for controlling the release of a substance, comprising the steps of containing the substance in a container, connecting the container to a device comprising a housing having an inlet for connection to the source of the substance, an outlet and a passage extending therebetween, the passage being closed by a ceramic member, and applying an electrical pulse to the ceramic member to break the member and so connect the inlet to the outlet. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS  
     Apparatus and methods for controlling the release of a substance in the form of pressurised fire extinguishing media according to the invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
       FIG. 1A  shows a schematic elevation of a valve for controlling the release of pressurised fire extinguishing media connected between a source of the fire extinguishing media and an outlet, in which the passage between the source and outlet is closed by a first form of ceramic disc; 
       FIG. 1B  is a similar view to  FIG. 1A  but showing an electrical circuit, a control system and a switch for controlling the opening of the device; 
       FIG. 1C  is a similar view to  FIGS. 1A and 1B  but showing the ceramic member broken; 
       FIG. 2A  is a similar view to  FIG. 1A  but showing a second form of ceramic member; 
       FIG. 2B  is a similar view to  FIG. 1B  but showing the second form of ceramic member; 
       FIG. 2C  is a similar view to  FIG. 1C  but showing the second form of ceramic member broken; 
       FIG. 3A  is a similar view to  FIG. 1A  but showing a third form of ceramic member; 
       FIG. 3B  is a similar view to  FIG. 1B  but showing the third form of ceramic member; 
       FIG. 3C  is a similar view to  FIG. 3A  but showing the structure of the third form of ceramic member in more detail; 
       FIG. 3D  is a similar view to  FIG. 1C  but showing the third form of ceramic member broken; and 
       FIG. 4  is a perspective view of the first or second or third ceramic member including a circumferential groove. 
   

   In the drawings, like elements are generally designated with the same reference numeral. 
   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1A , the valve  1  comprises a housing (not shown) made of metal or any other suitable material having an inlet  3 , an outlet  5  and a passage  7  extending between the inlet  3  and the outlet  5 . The inlet  3  is connected to a source  4  of pressurised fire extinguishing media and the outlet  5  to a delivery passage  6 . The passage  7  is closed by a first form of ceramic disc  9 . In this embodiment, the ceramic disc  9  comprises a single ceramic material having a low dielectric strength and described in more detail below. Referring to  FIG. 1B , the ceramic disc  9  is connected to a source  10  of electrical current  11  via a controller  12  actuated by a switch or other means  13 . 
   In use, the valve  1  is activated by operation of the switch or other means  13  which causes the controller to apply an electrical pulse to the disc  9 . The electrical pulse causes a voltage over-stress or an electrical current over-stress in the ceramic disc  9  that leads to oxide breakdown within the ceramic material of the ceramic disc  9 , resulting in cracks which propagate under pressure through the disc  9 . This mechanism leads to the breakage of the disc  9 , as depicted in  FIG. 1C , thereby connecting the inlet  5  to the outlet  7  and allowing the release of the pressurised fire extinguishing media. 
   For optimisation of the breakage mechanism according to this first embodiment, the dielectric strength of the ceramic material forming the disc  9  is preferably equal to or lower than 10 5  Vm −1 . For disc breakage resulting from voltage over-stress, the ceramic material is preferably one of a ZrO 2 /MgO composite, MgO, MgAl 2 O 4 , Al 2 O 3  or Y 2 O 3 . For disc breakage resulting from electrical current over-stress the ceramic material is preferably one of LaCrO 3 , LaCoO 3 , La 2 NiO 4  or NiMn 2 O 4 . 
     FIGS. 2A ,  2 B and  2 C show a second embodiment of the invention. The valve  1  of  FIG. 2A  is generally the same as the valve  1  of  FIG. 1A , with like elements being given the same reference numerals, and is not described in detail. In the second embodiment, there is a second form of ceramic disc  9  comprising first and second ceramic materials  15 ,  17  arranged to form first and second distinct layers, the first ceramic material  15  forming the first layer of the disc  9  (situated closer to the inlet  3 ) and having a relatively high thermal expansion coefficient and the second ceramic material  17  forming the second layer of the disc  9  (situated closer to the outlet  5 ) and having a relatively low thermal expansion coefficient. 
   In this second embodiment, application of the electrical pulse on operation of the switch  13  (see  FIG. 2B ) causes breakage of the ceramic disc  9  by thermal expansion. The first ceramic material  15  (having a relatively high thermal expansion coefficient) will expand to a greater extent than the second ceramic material  17  (having a relatively low thermal expansion coefficient). This mechanism leads to the breakage of the disc  9 , as depicted in  FIG. 2C , thereby connecting the inlet  9  to the outlet  5  and allowing the release of the fire extinguishing media. 
   In order to cause the disc to heat upon the application of an electrical pulse, one of the two ceramic materials  15 ,  17  must also have a low electrical resistivity. Low electrical resistivity is advantageously provided by the second ceramic material  17 . Additionally, the surface of the disc  9  is advantageously metallised so that the electrical current circulation is uniform, thereby avoiding breakdown and favouring heat transfer between the two ceramic materials  15 ,  17 . The surface of the disc  9  may be metallised by physical vapour deposition (PVD) or chemical vapour deposition (CVD), and the metal used is preferably aluminium based or nickel based. 
   For optimisation of the breakage mechanism according to this second embodiment, the difference between the thermal expansion coefficients of the ceramic materials  15 ,  17  is of the order of 5%. If the difference between the thermal expansion coefficients of the ceramic materials  15 ,  17  is too great, the disc  9  may unexpectedly break when the ambient temperature is increased. The relatively high thermal expansion coefficient first ceramic material  15  is preferably one of Al 2 O 3 , a ZrO 2 /Y 2 O 3  composite, a ZrO 2 MgO composite or MgO. The second ceramic material  17  forming the disc  9  and having a relatively low thermal expansion coefficient and electrical resistivity equal to or lower than 2.10 2  Ohm.m is preferably one of La 2 NiO 4 , ZnO, LaCrO 3 , LaCoO 3 , or NiMn 2 O 4 . 
   Referring next to  FIGS. 3A ,  3 B and  3 C the valve  1  of these figures is generally the same as the valve  1  of  FIGS. 1A to 1C  and  2 A to  2 C, with like elements being given the same reference numerals, and is not described in detail. In the third embodiment, the third form of the ceramic disc  9  comprises a composite of a ceramic material  19 , having a high thermal conductivity, and carbon. On activation of the valve  1  by operation of the switch  13 , an electrical pulse is applied to the ceramic disc  9 , which breaks by composite combustion. The electrical pulse heats the disc  9  leading to the combustion of the carbon within the disc  9  to cause vacancies  21  within the disc  9  (see  FIG. 3C ). The carbon combustion weakens the disc  9  which then breaks down due to the pressure exerted on the disc  9  by the fire extinguishing media (see  FIG. 3D ), thereby connecting the inlet  3  to the outlet  5  and allowing the release of the fire extinguishing media. 
   For optimisation of the breakage mechanism according to this third embodiment, the electrical resistivity of the ceramic material  19  comprising the disc  9  is preferably equal to or lower than 2.10 2  Ohm.m. The ceramic material  19  is preferably one of LaCrO 3 , SiC or La 2 NiO 4 . The carbon included in the disc  9  is preferably graphitic carbon. 
   In any one of the embodiments described herein with reference to  FIGS. 1A to 1C ,  FIGS. 2A to 2C  or  FIGS. 3A to 3D , and in order to advantageously facilitate the breakage of the disc  9 , the disc  9  may be formed with opposed first and second faces  23 ,  25  and with a groove  27  on the second face  25 . This is shown in  FIG. 4 , in which the groove  27  is circumferential to the disc  9 , the groove  27  being an annular groove  27  concentric with the disc  9  and located towards the periphery of the disc  9 . The failure mechanism for the disc  9  in this case is the propagation of cracks and fracture from the groove  27  such that the centre of the disc  9  is removed to connect the inlet to the outlet and release the extinguishing media. In use, the ceramic disc  9  is mounted with the second face  25  facing the outlet  5 . It has been found that less power is required to cause breakage of a disc  9  with such a groove  27  when compared to a disc  9  without such a groove  27 . In one test, the electric field was found to be approximately three times stronger around the groove  27 , as compared to a disc  9  without a groove  27 , leading to a higher stress. The presence of the groove  27  also leads to cleaner and more controlled breakage of the disc  9 . Although a groove  27  is described to facilitate breakage of the disc  9 , other means may be provided for this purpose. For example, some other form of surface weakness may be used. 
   It will of course be appreciated by one skilled in the art that any suitable alternative ceramic material, combination of ceramic materials or ceramic composite could be used in accordance with the embodiments described herein. Additionally, the embodiments described herein are not intended to, and should not be taken to, limit the scope of the invention. It should instead be appreciated that any suitable arrangement of a housing comprising a ceramic disc for closing the passage between the inlet and the outlet could be used as a valve for controlling the release of a substance. 
   The valves described above control the release of a fire extinguishing medium. The control of the release of the fire extinguishing medium is by the breaking of a ceramic disc. The valve therefore can only operate once, whereafter intervention is required to replace the broken ceramic disc in order to allow the valve to operate again. 
   Although a frangible disc is described, it should be appreciated that this frangible element may be any suitable shape—for example, circular or rectangular—and may not be completely flat. The shape will generally be determined by the internal shape of the housing or pipe in which the frangible element is positioned. The frangible element will generally be a flat sheet or membrane of material. 
   It will also be appreciated by one skilled in the art that the devices described herein with reference to the accompanying drawings are not only suitable for controlling the release of any pressurised fire extinguishing medium taking the form of, for example, a liquid, a gas or a powder, but are equally as suitable for controlling the release of any other suitable substance.