Abstract:
A safety valve for a compressed gas tank in which the plug is made of meltable material and intended to melt at a pre-determined temperature in order to release the compressed gas from the tank. The safety valve is essentially characterized in that the meltable plug is subjected to the action of a differential action sliding piston intended for reducing at least some of the effect of the gas pressure on the meltable plug.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a safety valve for a compressed-gas tank, the said valve comprising a plug made of meltable material and intended to melt at a predetermined temperature in order to release the compressed gas from the tank. 
     BACKGROUND OF THE INVENTION 
     Such safety valves are used to prevent the risk of the tank exploding when the temperature exceeds a predetermined limit. In fact, at this temperature, which may be of the order of 100° C., the plug mentioned in the introduction melts and allows the gases and excess pressure to escape. Below this critical temperature, the plug must normally remain solid and prevent any leakage of pressure and gas. 
     Unfortunately, the situation was found where the plug melts prematurely, that is to say at temperatures below its melting temperature. It was found that these instances occur when the gas is stored at high pressures and when this melting is caused not by the temperature, but by the pressure. It was discovered, in fact, that if such a plug is exposed to high pressures for a relatively long time, it may become deformed and even melt and thus release the gases below the melting temperature. There is therefore an unintentional leakage, along with the risk of an accident and risks to the environment. Moreover, the material of the plug to be replaced is extremely costly. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a new safety valve which makes it possible to avoid these disadvantages, that is to say a valve, the plug of which does not risk melting prematurely under the effect of prolonged pressure. 
     In order to achieve this object, the invention provides a safety valve of the type described in the introduction, which is essentially characterized in that the meltable plug is subjected to the action of a differential-action sliding piston intended for. educing at least some of the effect of the gas pressure on the meltable plug. 
     According to a preferred embodiment, the valve comprises a housing integral with the tank and comprising a first cylindrical part open towards the outside of the tank and a second cylindrical part with a closed bottom, having a diameter smaller than that of the first cylindrical part and communicating through its side wall with the interior of the tank, and a bush which is fastened inside the first cylindrical part and the orifice of which faces the second cylindrical part, the bush containing the meltable plug which is retained there by means of a rod integral with the said differential-action piston which is located in the said second cylindrical part. The said differential-action piston is preferably a double-action piston comprising two pistons connected by means of an axial rod, the first piston normally being located at the bottom of the second cylindrical part and the second piston normally being located between the first and the second cylindrical part in order to ensure sealing between these parts, the diameter of the latter piston being greater than the diameter of the first piston. 
     The effect of the gas pressure on the second piston consequently generates a thrust on the meltable plug, the said thrust being proportional to the pressure of the gas and to the exposed surface of the second piston. By contrast, the first piston, which is likewise exposed to the pressure of the gas, acts in the opposite direction, that is to say its effect is subtracted from the thrust exerted by the second piston on the meltable plug. Finally, the latter undergoes only a force proportional to the difference in the areas of the two pistons. The plug consequently undergoes only a slight pressure and does not risk being damaged prematurely under the effect of a high and prolonged pressure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other particular features of the present invention will be gathered from the description of an advantageous embodiment given below by way of illustration, with reference to the accompanying single figure. 
     FIG. 1 which is a view in vertical section through a safety valve according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This safety valve comprises a housing  10  with an external thread  12  allowing the valve to be screwed into the wall, not shown, of a tank containing compressed gas. The housing comprises an upper part  10   b , facing outwards when the valve is mounted on the tank, and a lower part  10   a  penetrating into the tank. 
     The lower part  10   a  of the housing  10  is a cylinder with a closed bottom, containing a differential-action piston  14  which consists of a first piston  14   a  normally located at the bottom of the lower cylindrical part  10   a  and a second piston  14   b  normally located in the neck between the two parts  10   a  and  10   b . The two pistons  14   a  and  14   b , which are connected to one another by means of an axial rod  16 , each carry a peripheral seal  18  ensuring sealing relative to the wall of the lower part of the housing  10 . The second piston  14   b  is therefore the member for closing the valve and ensures sealing between the interior of the tank and the exterior. 
     The wall of the housing  10   a  comprises a series of orifices  20 , so that the pressure in the housing  10   a  corresponds to the pressure in the tank, that is to say this pressure can be exerted in full on each of the pistons  14   a  and  14   b.    
     According to one of the particular features of the present invention, the diameter of the first piston  14   a  is slightly smaller than that of the second piston  14   b . It is therefore necessary for the inside diameter of the wall of the housing  10   a  to be greater, at the level of the piston  14   b , than the inside diameter in the bottom of the housing  10   a.    
     The upper and outer part  10   b  of the housing  10  has an inside diameter which is substantially greater than the diameter of the upper piston  14   b.    
     In the upper part  10   b  of the housing is located an upturned bush  22 , hence with an open bottom, which contains a plug  24  made of meltable material melting at a predetermined critical temperature, for example approximately 100° C. The bush  22  is made integral with the wall of the upper part  10   b  of the housing  10  by means not shown, for example by means of spokes. 
     The outer part of the piston  14   b  carries an axial rod  26  which extends in the upper part  10   b  of the housing  10  and penetrates into the bush  22  up to contact with the plug  24 . This rod  26  has a diameter slightly smaller than the inside diameter of the bush  22 . 
     During normal operation, the valve occupies the position shown in the figure. The plug  24  is solid and, under the effect of the rod  26  and by virtue of the connection, not shown, between the bush  22  and the housing  10   b , retains the piston  14  in the position shown. Consequently, the piston  14   b , by virtue of its peripheral seal  18 , prevents any leakage of gas towards the outside. Moreover, since the gas pressure acts both on the piston  14   a  and on the piston  14   b , the plug  24  undergoes only a slight thrust by the rod  26 , the said thrust being proportional to the difference in radial area between the two pistons  14   a  and  14   b.    
     When, for one reason or another, the bush  22  and the plug  24  are exposed to an abnormal temperature exceeding the critical melting temperature of the plug  24  and capable of putting the tank at risk of explosion, the plug  24  melts and its liquid material flows out of the bush  22  around the rod  26 . With the bush  22  empty, the rod  26  no longer encounters any resistance and the lift acting on the piston  14  under the effect of the differential pressure is sufficient to raise the piston  14  and cause the piston  14   b  to penetrate into the upper part  10   b  of the widened diameter housing. Here, the piston  14   b  no longer ensures any sealing relative to the outer surface and the compressed gases can from that moment escape freely from the tank. 
     In order to prevent an accumulation of gas between the bottom of the housing  10   a  and the bottom of the piston  14   a  from occurring progressively as a result of damage to the seal  18  of the piston  14   b , it is possible to ventilate this zone by providing, through the entire piston  14 , a thin axial duct which issues into the open chamber of the housing  10   b.    
     Since the meltable material of the plug  24  is usually based on welding tin, such as the material for solders, it is possible to provide a variant for the plug  24 . This variant would involve providing, instead of the upwardly closed bush  22 , a tube open at both ends and producing the head of the rod from solder or meltable metal which would be welded to this tube in order to withstand the differential pressure on the piston  14 . At the critical temperature, this weld would melt and would allow the rod  26  to slide through the tube and thus open the valve.