Abstract:
An isolating valve includes a body having a canal for the passage of fluid, which is intended to be shut off by upstream and downstream spherical plugs, that can be actuated independently of one another between positions allowing the passage of fluid through the canal and positions of shutting off the canal, and a sealing checker, opening into the canal and between the two spherical plugs. The upstream plug is equipped with an upstream seat of the “simple piston effect” type and has no downstream seat. The downstream plug is equipped with an upstream seat of the “simple piston effect” type and with a downstream seat of the “double piston effect” type.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Phase Entry of International Application No. PCT/FR2012/052425, filed on Oct. 23, 2012, which claims priority to French Patent Application Serial No. 1159649, filed on Oct. 25, 2011, both of which are incorporated by reference herein. 
     BACKGROUND 
     The present invention relates to an isolation valve, designed in particular for equipping a liquefied gas storage tank. 
     Conventionally, a gas storage tank comprises isolation means allowing any flow of gas out of the tank to be prevented. To this end, it is known to equip the tank with a movable safety valve mounted in a gas extraction pipe and blocked by a fusible pin. In normal operation, the safety valve is held by the pin in a position wherein it does not impede the flow of the liquefied gas. In the event of fire, the pin yields and frees the safety valve, which blocks the flow of gas. In practice, such a safety valve only makes it possible to limit the flow of gas, but does not afford a total seal. A valve is therefore added downstream of the safety valve, said valve providing the sealing function even if it is subjected to high temperatures in the event of a fire. This valve has the following disadvantages. 
     First of all, a valve is designed to control the flow of fluid through a duct and not to provide an isolation function. Consequently, such a valve is generally not perfectly sealed. Moreover, actuation of such a valve, even if it is motorized, is relatively slow. Indeed, the valve is actuated by rotation of a shaft, and transition from the fully open position to the fully closed position requires several turns. 
     It is known, moreover, to use for different applications, valves offering a good seal, such as in particular valves of the “double block and bleed” or DBB type, described in particular in documents EP 1 618 325 and WO 2010/131039. A valve of this type conventionally comprises a body including a fluid flow channel, designed to be plugged by upstream and downstream spherical plugs, operable independently of one another between positions for fluid flow through said channel and positions plugging the channel. The body is also equipped with bleeding means leading into the aforementioned channel, between the upstream and downstream plugs. 
     The use of two means of isolation in series, consisting of the upstream and downstream plugs, makes it possible to guarantee that no fluid escapes, even in the event of failure of one of the isolation means. The bleeding means make it possible to check the proper sealing of the plugs. To this end, it is sufficient to close the plugs, to bleed the part of the channel located between the two plugs a first time, then checking with a second bleed that at the end of a predetermined period no fluid has escaped downstream of the upstream plug. 
     The “double block and bleed” type valve also comprises, for each upstream and downstream plug, an upstream seat and a downstream seat, associated with elastic return means so as to come to bear sealingly against the spherical plugs around the fluid flow channel. Such a valve comprises several dead volumes, that is to say confined volumes capable of capturing fluid. 
     In the case where both plugs are in the plugging position, a first dead volume is formed in the fluid flow channel, between the upstream and downstream plugs. A second dead volume is formed by a volume external to the aforementioned channel, located between the outer wall of the upstream plug and the body. The second dead volume also comprises the internal volume of the upstream plug which, in the plugging position of the upstream plug, communicates with the aforementioned external volume. 
     Finally, a third dead volume is formed by a volume external to the aforementioned channel, located between the outer wall of the downstream plug and the body. The third dead volume also comprises the internal volume of the downstream plug which, in the plugging position of the downstream plug, communicates with the aforementioned external volume. 
     In the event of fire, the gas caught inside these dead spaces expands and can cause the cracking, even the explosion of the valve body, causing the complete destruction of the valve. The gas can then escape through the cracks that are formed, thus supplying fuel to the fire. In order to allow the escape of the gas contained in the second and third dead volumes, the seats of the upstream and downstream plugs can be of the “simple piston effect” or SPE type. In this case, the seats are designed to detach from the corresponding plugs in the event of overpressure in the dead volumes. 
     Such a design has the following disadvantages. First of all, gas can escape to the outside, downstream, due to detachment of the downstream seat from the downstream plug. Even if only a limited volume of gas is involved (corresponding to the third dead volume), this gas nevertheless supplies fuel to the fire. Furthermore, the seats do not allow to evacuate the gas contained in the first dead volume, between the upstream and downstream plugs, the expansion of this gas which may cause, as indicated above, cracking or explosion of the valve body. 
     The invention has in particular the aim of providing a simple, effective and economical solution to this problem. 
     SUMMARY 
     To this end, it proposes an isolation valve comprising a body including a fluid flow channel, designed to be closed by upstream and downstream spherical plugs, capable of being actuated independently of one another between positions allowing fluid flow through said channel and positions plugging the channel, and sealing checking means leading into said channel between the two spherical plugs, the spherical plugs being equipped with seats associated with elastic return means so as to come to bear sealingly against the spherical plugs around said channel, a dead volume being defined between the outside wall of each spherical plug and the body, outside the fluid flow channel, each seat being designed to be subjected, on the one hand, to the pressure of the fluid situated in said channel and, on the other hand, to the pressure of the fluid contained in the dead space, characterized in that the upstream spherical plug is equipped with an upstream seat and has no downstream seat, the downstream spherical plug being equipped with an upstream seat and a downstream seat, the upstream seats being capable of separating from the spherical plugs in the event of an overpressure inside the corresponding dead volumes, the downstream seat being designed to remain in sealing abutment on the downstream spherical plug in the event of overpressure inside the corresponding dead volume. 
     In this manner, in the event of overpressure inside the dead volumes, for example if the valve is subjected to a fire, the gas contained in each of the dead volumes can escape upstream without risking cracking of the body. Moreover, the gas can normally not escape downstream, so that it cannot fuel the fire. Advantageously, the upstream seat of the upstream plug comprises an annular spherical segment or frusto-conical surface, designed to come into sealing contact with an upstream plug and covered, at least in part, with a layer of tungsten carbide. 
     The fluid intended to pass through the valve can have contaminants or abrasive particles which threaten to damage the seat and affect the sealing between the seat and the spherical plug, in particular when the plug is actuated. The tungsten carbide layer exhibits high hardness, which protects the upstream seat. The tungsten carbide deposit, however, has a high cost. It is therefore reserved by preference for the seat that is most subject to this type of damage, that is to say the upstream seat of the upstream plug. 
     According to one feature of the invention, the upstream seat and/or the downstream seat of the downstream plug comprise annular inserts design to come to bear sealingly against the upstream plug. Such inserts are less resistant to contaminants and to abrasive particles but they provide the sealing function at a lower cost. These inserts can be made of polymer. 
     Preferably, the upstream seat and/or the downstream seat of the downstream plug also comprise an annular spherical segment or frusto-conical surface, capable of coming into sealing contact against the downstream plug in the event of deterioration of the inserts. In this manner, even in the event of deterioration of the inserts, sealingly is provided between the corresponding seats and the downstream plug. Such a deterioration can arise either from wear, or from a temperature rise due to a fire. 
     According to another feature of the invention, the upstream seats of the upstream and downstream plugs are movable in rotation relative to the body, along a longitudinal axis, each upstream seat comprising a first and a second surfaces, oriented respectively in the direction opposite the plug and in the direction facing the plug, designed to be subjected to the pressure of the fluid in the dead volume so that this pressure being applied to the first surface tends to apply the seat onto the plug this pressure being applied to the second surface tends to detach the seat from the plug, the projection of the second surface onto a plane perpendicular to the longitudinal axis being greater than the projection of the first surface onto said perpendicular plane, so that the resulting force applied by the pressure of the fluid in the dead volume to the seat tends to detach it from the corresponding plug. Both upstream seats are therefore of the “simple piston effect” or SPE type, and allow the gas contained in the dead volumes and subjected to an overpressure to escape upstream. 
     Advantageously, the downstream seat of the downstream plug is movable in translation, relative to the body, along a longitudinal axis, and comprises a first and a second surfaces, oriented respectively in the direction opposite the downstream plug and in the direction facing the downstream plug, designed to be subjected to the pressure of the fluid in the dead volume so that that pressure, applied to the first surface, tends to apply the seat onto the downstream plug and that pressure, applied to the second surface, tends to detach the seat from the downstream plug, the projection of the first surface onto a plane perpendicular to the longitudinal axis being greater than that of the second surface so that the resulting force applied by the pressure of the fluid in the dead volume onto the downstream seat tends to press it against the downstream plug. 
     The downstream seat equipping the downstream plug is of the “double piston effect” or DPE type, and normally prevents gas from escaping downstream. The valve can comprise at least one annular seal providing the sealing between each upstream seat and the body, the seal being accommodated in an annular groove of the body or of the upstream seat. 
     In this manner, the pressure of the fluid contained in the corresponding dead volume and which enters into the groove, is applied on the one hand to the surface of the seal and, on the other hand, on the surface of the groove opposite the seal. The projections of these surfaces being identical, the resulting forces cancel. In other words, such a structure makes it possible to reduce the active portion of the first surface of the upstream seat so as to reduce the force tending to press the seat against the corresponding plug. Thus, ultimately, the force resulting from the pressure of the fluid on the first and second surfaces tends to separate the upstream seat from the corresponding plug. 
     Moreover, the valve can comprise at least one annular seal providing the sealing between the downstream seat of the downstream plug and the body, the seal being mounted in an annular space defined between an outer cylindrical wall of the seat and an inner cylindrical wall of the body, having a greater diameter than the outer wall of the seat. Such a structure has the effect of increasing the second surface of the downstream seat compared with the first surface, so that the resulting force tends to press the downstream seat against the downstream plug in the event of overpressure inside the corresponding dead volume. Preferably, the sealing checking means comprise bleeding means leading into the section of the fluid flow channel located between the upstream and downstream spherical plugs. 
     The invention also relates to a storage tank for gas, particularly liquefied gas, comprising a pipe for extracting the gas contained in the tank, characterized in that the extraction pipe comprises a valve of the aforementioned type. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood and other details, features and advantages of the invention will appear upon reading the following description given by way of a non-limiting example with reference to the appended drawings wherein: 
         FIG. 1  is a sectional view of a prior art “double block and bleed” valve; 
         FIG. 2  is a partial longitudinal section illustrating schematically the operation of seats of the “simple piston effect” type equipping a spherical plug according to the prior art; 
         FIG. 3  is a partial longitudinal section view illustrating schematically the operation of seats of the “double piston effect” type equipping a spherical plug according to the prior art; 
         FIG. 4  is a front view of a valve according to the invention; 
         FIG. 5  is a side view of the valve of  FIG. 4 ; 
         FIG. 6  is a longitudinal section view of the valve of  FIG. 4 ; 
         FIG. 7  is a detail view illustrating the structure of the upstream seat of the upstream plug; 
         FIG. 8  is a view corresponding substantially to  FIG. 7 , illustrating schematically the operation of the upstream seat of the upstream plug; 
         FIG. 9  is a detail view illustrating the structure of the upstream seat of the downstream plug; 
         FIG. 10  is a view corresponding substantially to  FIG. 9 , illustrating schematically the operation of the upstream seat of the downstream plug; 
         FIG. 11  is a detail view illustrating the structure of the downstream seat of the downstream plug; 
         FIG. 12  is a view corresponding substantially to  FIG. 11 , illustrating schematically the operation of the downstream seat of the downstream plug; and 
         FIG. 13  is a detail view illustrating the structure of the bleeding means. 
     
    
    
     DETAILED DESCRIPTION 
     A “double block and bleed” or DBB type valve, according to the prior art, is shown in  FIG. 1 . It corresponds to the one already described in document WO 2010/131039 and comprises a body  1 ′ including an upstream end on which is fixed a first flanged connector  2 ′, designed to be connected for example to a gas extraction line of a liquefied gas storage tank, and a downstream end on which is fixed a second flanged connector  3 ′ which can be un-connected with any pipe and therefore left in the open. The body  1 ′ and the flanges  2 ′,  3 ′ comprise a fluid flow channel  4 ′ extending along the longitudinal axis A′ of the valve. 
     Upstream and downstream spherical plugs  5 ′,  6 ′ are mounted inside the body  1 ′, each plug  5 ′,  6 ′ comprising a central cylindrical through hole  7 ′ and an outer spherical surface  8 ′. Each plug  5 ′,  6 ′ is mounted about an axis B′ formed by an upper shaft  9 ′ and a lower shaft  10 ′, mounted in sealed bearings  11 ′ of the body  1 ′. Actuating means such as handles  12 ′ or wheels are fastened to the upper shafts  9 ′. Each plug  5 ′,  6 ′ can thus be actuated independently, by pivoting the handle  12 ′ or operating wheel through a quarter turn, between a position allowing flow of the fluid ( FIG. 1 ) wherein the cylindrical hole  7 ′ of the plug  5 ′,  6 ′ extends in alignment with the channel  4 ′, and a closed position (not shown) wherein the cylindrical hole  7 ′ of the plug  5 ′,  6 ′ extends perpendicular to the channel  4 ′. 
     The valve additionally comprises bleeding means (not visible in  FIG. 1 ) leading, on the one hand, into the section  13 ′ of the fluid flow channel  4 ′ located between the upstream and downstream spherical plugs  5 ′,  6 ′ and, on the other hand, to the outer surface of the body  1 ′. Each plug  5 ′,  6 ′ is moreover equipped with an upstream seat  14 ′,  16 ′ and with a downstream set  15 ′,  17 ′ taking the form of rings accommodated inside the body  1 ′ and the inner walls whereof extend substantially in alignment with the channel  4 ′. The seats  14 ′,  15 ′,  16 ′,  17 ′ are movable in translation along the longitudinal axis A′ relative to the body  1 ′. Each seat  14 ′,  15 ′,  16 ′,  17 ′ comprises one end subjected to the force of return springs  18 ′, and another end coming to bear sealingly against the outer surface  8 ′ of the plug  5 ′,  6 ′. Sealing gaskets are also provided between the body  1 ′ and the seats  14 ′,  15 ′,  16 ′,  17 ′. 
     When the plugs  5 ′,  6 ′ are in the open position, that is in the position allowing flow of fluid through the valve, more precisely through the channel  4 ′, the fluid can flow freely from upstream to downstream, the fluid being prevented from flowing through different junction planes by sealing gaskets  19 ′. In this position, dead volumes  20 ′, wherein gas can be confined, are formed outside the channel  4 ′ between the body  1 ′ and the outer walls  8 ′ of the upstream and downstream plugs  5 ′,  6 ′. When the two plugs  5 ′,  6 ′ are in the closed position, that is in the plugging position, the valve comprises a first dead volume  13 ′ formed in the fluid flow channel  4 ′, between the upstream and downstream plugs  5 ′,  6 ′. 
     A second dead volume is formed by the volume  20 ′ external to the channel  4 ′, located between the outer wall  8 ′ of the upstream plug  5 ′ and the body  1 ′. The second dead volume also comprises the internal volume  7 ′ of the upstream plug  5 ′ which, in the closed position of the upstream plug  5 ′, communicates with the aforementioned external volume  20 ′. 
     Finally, a third dead volume is formed by the volume  20 ′ external to the channel  4 ′, located between the outer wall  8 ′ of the downstream plug  6 ′ and the body  1 ′. The third dead volume also comprises the internal volume  7 ′ of the downstream plug  6 ′ which, in the closed position of the downstream plug  6 ′, communicates with the aforementioned external volume  20 ′. As indicated previously, both in the open position and in the closed position of the plugs  5 ′,  6 ′, the gas imprisoned in the dead volumes expands in the event of fire and can cause cracking, or even explosion of the body  1 ′ of the valve. The gas can then escape through the cracks created, thus supplying fuel to the fire. 
     In the prior art, it is known to equip the plugs  5 ′,  6 ′ either with seats of the “simple piston effect” type or with seats of the “double piston effect” type.  FIG. 2  shows a spherical plug  5 ′ equipped with upstream and downstream seats  14 ′,  15 ′ of the “simple piston effect” or SPE type. In this example, the upstream seat  14 ′ comprises, from upstream to downstream, a first cylindrical portion  21 ′ and a second cylindrical portion  22 ′ with a larger diameter than the first portion  21 ′. The downstream end face of the seat  14 ′ comprises a frusto-conical portion  25 ′ wherein is provided an annular groove  26 ′ for mounting an annular seal  27 ′, designed to be pressed against the outer surface  8 ′ of the spherical plug  14 ′. An O-ring  28 ′ is moreover mounted in a groove  29 ′ of the first portion  21 ′ of the seat  14 ′, and provides a seal between the seat  14 ′ and the body  1 ′. 
     The upstream face  30 ′ of the second portion  22 ′ forms a shoulder which supports springs  18 ′ mounted in recesses in the body  1 ′. The springs  18 ′ exert longitudinally oriented forces so as to press the upstream seat  14 ′ against the plug  5 ′. The plug  5 ′ is also equipped with a downstream seat  15 ′, the structure whereof is symmetrical with that of the upstream seat  14 ′ with respect to the pivot axis B′ of the plug  5 ′. 
     The operation of the upstream and downstream seats  14 ′,  15 ′ of the “simple piston effect” type is the following. It is assumed that the annular seal  27 ′ of the upstream seat  14 ′ is deteriorated so that it no longer completely provides its sealing function, a leakage of fluid occurring from the channel  4 ′ toward the dead volume  20 ′. The pressure P 1 ′ in the channel  4 ′ upstream of the upstream seat  14 ′ is greater than P 2 ′, that in the dead volume  20 ′, which in turn is greater than P 3 ′, that in the channel  4 ′ downstream of the downstream seat  15 ′. 
     Considering the position of the different seals  27 ′,  28 ′ and the shape of the seat  14 ′, the fluid enters into the interior of the groove  29 ′ and into the annular space  31 ′ located between the frusto-conical portion  25 ′ of the upstream seat  14 ′ and the outer surface  8 ′ of the plug  5 ′. Thus, upstream of the plug  5 ′, the fluid exerts a pressure P 1 ′ on all the faces of the upstream seat  14 ′ with which it is in contact, directly or indirectly (for example by way of the seal  28 ′). In the remainder of the description, head losses will be neglected, and also the forces exerted by the springs  18 ′, in order to facilitate understanding. 
     The surface or surfaces subjected to the fluid pressure P 1 ′, directly or indirectly (for example by way of the seal  28 ′), and oriented in the direction opposite to the spherical plug  5 ′, are called the first surface, the surface or surfaces subjected to pressure P 1 ′, directly or indirectly, and oriented toward the spherical plug  5 ′ being called the second surface. The pressure P 1 ′ exerted on the first surface of the upstream seat  14 ′ generates a force Fc′ oriented along the longitudinal axis A′ and tending to press the upstream seat  14 ′ against the spherical plug  5 ′ while, on the contrary, the pressure P 1 ′ exerted on the second surface of the upstream seat  14 ′ generates a force Fd′ oriented along the axis A′ and tending to detach the upstream seat  14 ′ from the spherical plug  5 ′. The force Fc′ depends on the projection of the first surface onto the plane perpendicular to the axis A′ and the force Fd′ depends on the projection of the second surface onto the aforementioned perpendicular plane. 
     In the case of  FIG. 2 , the projection of the first surface onto the plane perpendicular to the axis A′ is greater than the projection of the second surface onto the plane perpendicular to the axis A′ (as can be seen in  FIG. 2 , the difference between these two projections being denoted Δ 1 ), so that the value (norm of the vector) of the force Fc′ is greater than that of the force Fd′, the resulting force R′ being directed from upstream to downstream and tending to press the upstream seat  14 ′ against the spherical plug  5 ′. In other words, due to the particular geometry of the upstream seat  14 ′, the pressure P 1 ′ exerted on the first and second surfaces tends to press the upstream seat against the spherical plug  5 ′. 
     In addition, considering the position of the different seals  27 ′,  28 ′ and the shape of the downstream seat  15 ′, the fluid leaving the dead volume  20 ′ at pressure P 2 ′ enters into the interstice  32 ′ located between the body  1 ′ and the downstream seat  15 ′, into the recesses of the springs  18 ′ of the downstream seat, and into the groove  29 ′ accommodating the seal  28 ′ of the downstream seat  15 ′. As before, the surface or surfaces of the downstream seat  15 ′ subjected to the fluid pressure P 2 ′, directly or indirectly, and oriented in the direction opposite to the spherical plug  5 ′, are called the first surface, and the surface or surfaces of the downstream seat  15 ′ subjected to pressure P 2 ′, directly or indirectly (for example by way of the seal  28 ′), and oriented toward the spherical plug  5 ′ being called the second surface. The pressure P 2 ′ exerted on the first surface of the downstream seat  15 ′ generates a force Fc′ oriented along the longitudinal axis A′ and tending to press the downstream seat  15 ′ against the spherical plug  5 ′ while, on the contrary, the pressure P 2 ′ exerted on the second surface generates a force Fd′ oriented along the axis A′ and tending to detach the downstream seat  15 ′ from the spherical plug  5 ′. 
     In the case of  FIG. 2 , for the downstream seat  15 ′, the projection of the first surface onto the plane perpendicular to the axis A′ is less than the projection of the second surface onto the plane perpendicular to the axis A′ (as can be seen in  FIG. 2 , the difference between the two projections being denoted Δ 2 ), so that the value (norm of the vector) of the force Fc′ is less than that of the force Fd′, the resulting force being directed from upstream to downstream. In other words, due to the particular geometry of the downstream seat  15 ′, the pressure P 2 ′ exerted on the first and second surfaces tends to detach the downstream seat  15 ′ from the spherical plug  5 ′. Recall that the pressure P 3 ′ is lower than the pressure P 2 ′, which in turn is lower than the pressure P 1 ′, so that the effect of pressure P 3 ′ on the downstream seat  15 ′ is negligible and the effect of pressure P 2 ′ on the upstream seat  14 ′ is also negligible. It is noted that the seats  14 ′,  15 ′, of the “simple piston effect” or SPE type, are, in normal operation, pressed against the corresponding plug  5 ′ but open in the event of overpressure into the dead volume  20 ′ so as to allow the fluid from the dead volume  20 ′ to escape toward the channel  4 ′. 
       FIG. 3  represents a spherical plug  5 ′ equipped with upstream and downstream seats  14 ′,  15 ′ of the “double piston effect” or DPE type. In this example, the upstream seat  14  comprises, from upstream to downstream, a first cylindrical portion  21 ′, a second cylindrical portion  22 ′ with a diameter greater than that of the first portion  21 ′, and a third cylindrical portion  23 ′ with a diameter greater than that of the second portion  22 ′. The downstream end face of the upstream seat  14 ′ comprises a frusto-conical portion  25 ′ wherein is provided an annular groove  26 ′ for mounting an annular seal  27 ′, designed to be sealingly pressed against the outer surface  8 ′ of the spherical plug  5 ′. An O-ring  28 ′ is also mounted around the first portion  21 ′ of the upstream seat  14 ′, in an annular space  33 ′ defined between the outer surface of the first portion  21 ′ of the seat  14 ′ and an internal cylindrical surface of the body  1 ′, with a diameter substantially identical to that of the second portion  22 ″ of the seat  14 ′. This O-ring  28 ′ provides the sealing between the upstream seat  14 ′ and the body  1 ′. 
     The upstream face of the third portion  23 ′ forms a shoulder  30 ′ supporting the springs  18 ′ mounted in recesses in the body  1 ′. The springs  18 ′ exert forces oriented longitudinally, so as to press the upstream seat  14 ′ against the plug  5 ′. The plug  5 ′ is also equipped with a downstream seat  15 ′, the structure whereof is symmetrical with that of the upstream seat  14 ′ with respect to the pivot axis B′ of the plug  14 ′. 
     The operation of the “double piston effect” type seats is as follows. It is assumed that the annular seal  27 ′ of the upstream seat  14 ′ is deteriorated, so that it no longer fully accomplishes its sealing function, a fluid leak occurring from the channel  4 ′ toward the dead volume  20 ′. The pressure P 1 ′ in the channel  4 ′ upstream of the upstream seat  14 ′ is greater than P 2 ′, that in the dead volume  20 ′, which in turn is greater than P 3 ′, that in the channel  4 ′ downstream of the downstream seat  15 ′. 
     Considering the position of the different seals  27 ′,  28 ′ and the shape of the upstream seat  14 ′, the fluid enters inside the annular space  33 ′ surrounding the first portion  21 ′ of the upstream seat  14 ′ and into the annular space  31 ′ located between the frusto-conical portion  25 ′ of the upstream seat  14 ′ and the outer surface  8 ′ of the plug  5 ′. Thus, upstream of the plug  5 ′, the fluid exerts a pressure P 1 ′ on all the faces of the upstream seat  14 ′ with which it is in contact, directly or indirectly (for example by way of the seal  28 ′). Hereafter in the description, head losses and forces exerted by the springs  18 ′ will be neglected in order to facilitate understanding. 
     The surface or surfaces of the upstream seat  14 ′ subjected to the fluid pressure P 1 ′, directly or indirectly (for example by way of the seal  28 ′), and oriented in the direction opposite to the spherical plug  5 ′, are called the first surface, and the surface or surfaces subjected to pressure P 1 ′, directly or indirectly, and oriented toward the spherical plug  5 ′, being called the second surface. The pressure P 1 ′ exerted on the first surface generates a force Fc′ oriented along the longitudinal axis A′ and tending to press the upstream seat  14 ′ against the spherical plug  5 ′ while, on the contrary, the pressure P 1 ′ exerted on the second surface generates a force Fd′ oriented along the axis A′ and tending to detach the upstream seat  14 ′ from the spherical plug  5 ′. The force Fc′ depends on the projection of the first surface onto the plane perpendicular to the axis A′ and the force Fd′ depends on the projection of the second surface onto the aforementioned perpendicular plane. 
     In the case of  FIG. 3 , for the upstream seat  14 ′, the projection of the first surface onto the plane perpendicular to the axis A′ is greater than that of the second surface (as can be seen in  FIG. 3 , the difference between these two projections being denoted Δ 1 ), so that the value (norm of the vector) of the force Fc′ is greater than that of the force Fd′, the resulting force R′ being directed from upstream to downstream and tending the press the upstream seat  14 ′ against the spherical plug  5 ′. In other words, due to the particular geometry of the upstream seat  14 ′, the pressure P 1 ′ exerted on the first and second surfaces tends to press the upstream seat  14 ′ against the spherical plug  5 ′. 
     Moreover, considering the position of the different seals  27 ′,  28 ′ and the shape of the downstream seat  15 ′, the fluid leaving the dead volume  20 ′ at pressure P 2 ′ enters into the interstice  32 ′ located between the body  1 ′ and the downstream seat  15 ′, into the housing springs  18 ′, and into the annular space  33 ′ located between the first portion  21 ″ of the downstream seat  15 ′ and the body  1 ′. For the downstream seat  15 ′, the surface or surfaces subjected to the fluid pressure P 2 ′, directly or indirectly, and oriented in the direction opposite to the spherical plug  5 ′, are called the first surface, and the surface or surfaces subjected to pressure P 2 ′, directly or indirectly, and oriented toward the spherical plug  5 ′ being called the second surface. For the downstream seat  15 ′, the pressure P 2 ′ exerted on the first surface generates a force Fc′ oriented along the longitudinal axis A′ and tending to press the downstream seat  15 ′ against the spherical plug  5 ′ while, on the contrary, the pressure P 2 ′ exerted on the second surface generates a force Fd′ oriented along the axis A′ and tending to detach the downstream seat  15 ′ from the spherical plug  5 ′. 
     In the case of  FIG. 3 , for the downstream seat  15 ′, the projection of the first surface onto the plane perpendicular to the axis A′ is greater than the second surface, so that the value (norm of the vector) of the force Fc′ is greater than that of the force Fd′, the resulting force R′ being directed from downstream to upstream. In other words, due to the particular geometry of the downstream seat  15 ′, the pressure P 2 ′ exerted on the first and second surfaces tends to press the downstream seat  15 ′ against the spherical plug  5 ′. Recall that the pressure P 3 ′ is lower than the pressure P 2 ′, which in turn is lower than the pressure P 1 ′, so that the effect of pressure P 3 ′ on the downstream seat  15 ′ is negligible and the effect of pressure P 2 ′ on the upstream seat  14 ′ is also negligible. 
     It is noted that the “double piston effect” or DPE type seats  14 ′,  15 ′ are, in normal operation, pressed against the corresponding plug  5 ′, and do not open in the event of overpressure in the dead volume  20 ′, the fluid thus remaining confined inside the dead volume  20 ′ with no possibility of escape. As previously indicated, the use of a valve of the “double block and bleed” type according to the prior art, comprising four seats of the “simply piston effect” type or four seats of the “double piston effect” type, cannot be considered when this valve must withstand fire. Indeed, in the event of fire, the construction of the valve does not allow the gas under pressure to escape from all the dead volumes, so that there exists a risk of cracking or explosion of the valve body. 
       FIGS. 4 to 6  illustrate a valve according to the invention, of the “double block and bleed” or DBB type. This comprises a body  1  including an upstream end on which is fixed a first flanged connector  2 , designed to be connected for example to a pipe for extracting gas from a liquefied gas storage tank, and a downstream end on which is fixed a second flanged connector  3  which can be un-connected to any pipe and thus left in the open. The body  1  and the flanges  2 ,  3  comprise a channel  4  for fluid flow extending along the longitudinal axis A of the valve. 
     Upstream and downstream spherical plug  5 ,  6  are mounted in the body  1 , each plug  5 ,  6  comprising a central cylindrical through hole  7  and an outer spherical surface  8 . Each plug  5 ,  6  is mounted around an axis B formed by an upper shaft  9  and a lower shaft  10 , mounted in sealed bearings  11  of the body  1 . Actuation means such as handles or wheels are mounted on the upper shafts  9 . Each plug  5 ,  6  can thus be actuated independently, by pivoting the handle or operating wheel through a quarter turn, between a fluid flow position ( FIG. 6 ) wherein the cylindrical hole  7  of the plug  5 ,  6  extends in alignment with the channel  4 , and a plugging position (not shown) wherein the cylindrical hole  7  of the plug  5 ,  6  extends perpendicular to the channel  4 . 
     The valve also comprises bleeding means  34  ( FIG. 13 ) leading, on the one hand, into the section  13  of the fluid flow channel  4  located between the upstream and downstream spherical plugs  5 ,  6  and, on the other hand, to the outer surface of the body  1 . These bleeding means  34  will be better described hereafter, with reference to  FIG. 13 . 
     As previously indicated, each plug  5 ,  6  is also equipped with at least one seat  14 ,  16 ,  17  taking the form of a ring accommodated in the body  1  and the internal wall whereof extends substantially in alignment with the channel  4 . The seats  14 ,  16 ,  17  are mounted movable in translation along the longitudinal axis relative to the body  1 . Each seat  14 ,  16 ,  17  comprises an end subjected to the force of the return springs  18 , and another end coming to bear sealingly against the outer surface  8  of the corresponding plug  5 ,  6 . 
     More particularly, the upstream plug  5  is equipped with an upstream seat  14  of the “simple piston effect” type and has no downstream seat, the downstream plug  6  being equipped with an upstream seat  16  of the “simple piston effect” type and a downstream seat  17  of the “double piston effect” type. When the plugs  5 ,  6  are in the open position, that is in the position allowing flow of the fluid through the valve, more precisely through the channel  4 , the fluid can flow freely from upstream to downstream, the fluid being prevented from flowing through different junction planes by sealing gaskets  19 . In this position, a dead volume  20 , wherein gas can be confined, is located outside the channel  4 , between the body  1  and the outer surface  8  of the downstream plug  6 . 
     When the two plugs  5 ,  6  are in the closed position, that is to say in the plugging position, the valve comprises a first dead volume  13  formed in the fluid flow channel  4  between the outer surfaces  8  of the upstream and downstream plugs  5 ,  6 . This is also connected to the dead volume formed by the volume  20  external to the channel  4 , situated between the outer surface  8  of the upstream seat  14  and the body  1 , and to the internal volume  7  of the upstream seat  14  which, in the closed position of the upstream seat  14 , communicates with the aforementioned external volume  20 . 
     A second dead volume is formed by the volume  20  external to the aforementioned channel  4 , located between the outer surface  8  of the downstream plug  6  and the body  1 . The second dead volume also comprises the internal volume  7  of the downstream plug  6  which, in the closed position of the downstream plug  6 , communicates with the aforementioned external volume  20 . In the event of overpressure inside dead volumes, in particular in the event of fire, the upstream seats  14 ,  16  detach from the upstream and downstream plugs  5 ,  6 , the downstream seat  17  remaining pressed against the downstream plug  6 . In this manner, the fluid under pressure contained in the dead volumes  7 ,  13 ,  20  can escape upstream, for example toward a tank, no fluid normally being able to flow downstream of the downstream plug  6 . This ensures that fluid, such as for example gas, does not fuel the fire. 
       FIG. 7  is a detail view showing the structure of the upstream seat  14  equipping the upstream plug  5 . This seat  14  comprises, from upstream to downstream, a first cylindrical portion  21 ″, a second cylindrical portion  22 ″ having a larger diameter than that of the first portion  21 ″, and a third cylindrical portion  23 ″ having a diameter greater than that of the second cylindrical portion  22 ″. The downstream end face of the upstream seat  14  comprises a frusto-conical portion  25  (or having the shape of a segment of a sphere), bounded by upstream and downstream annular gaps  35 ,  36 . The frusto-conical portion  25  is designed to come into sealing contact against the upstream plug  5  and is covered, at least in part, with a layer of tungsten carbide  60 . 
     Springs  18  mounted in recesses in the body  1  bear against the upstream end of the upstream seat  14 . The springs  18  exert forces oriented longitudinally, so as to press the upstream seat  14  against the upstream plug  5 . An annular recess  37  is provided at the upstream end of the upstream seat  14 , said recess leading upstream and radially outward. A seal  38 , made of polytetrafluoroethylene for example, is mounted in this recess  37 . An annular groove  39  is provided in the body, facing the first portion  21 ″ of the seat  14 , and is used to accommodate an O-ring  40  providing a seal between the seat  14  and the body  1 . The seal  40  is for example made of a synthetic elastomeric material of the type known under the brand name Viton® AED. 
     Another seal  41 , made of graphite for example, is mounted around the first portion  21 ″ of the upstream seat  14  and downstream of the aforementioned groove  39 , in an annular space  42  defined between the outer surface of the first portion  21 ″ of the seat  14  and an inner cylindrical surface of the body  1 , with a diameter substantially identical to that of the second portion  22 ″ of the seat  14 . This seal  41  is designed to limit the upstream flow of fluid but does not provide a complete seal between the seat  14  and the body  1 , said seal being provided by the O-ring  40  mounted in the groove  39 . 
     The operation of this “simple piston effect” type seat  14  will now be described with reference to  FIG. 8 . In this figure, certain non-functional elements have been removed, partially or totally, in order to facilitate understanding. Thus the seal  41  in particular, which does not provide complete sealing, has been removed. The operation of the upstream seat  14  of the upstream plug  5  is similar to that of the seats in  FIG. 2 . 
     It is assumed that the fluid contained in the first dead volume ( 7 ,  13 ,  20  when the two plugs are in the closed position) is subjected to an overpressure P 2 , for example in the event of a fire. The pressure P 2  in a dead volume  7 ,  13 ,  20  is therefore greater than P 1 , that inside the fluid flow channel  4 . Considering the position of the O-ring  40 , that of the frusto-conical portion  25  and the non sealing character of the seal  41 , the fluid leaving the dead volume  7 ,  13 ,  20  at pressure P 2  enters the annular spaces provided between the upstream seat  14  and the upstream plug  5  and between the upstream seat  14  and the body  1 . In particular, the fluid penetrates into the annular space  42  and into the groove  39 . 
     As before, the surface or surfaces of the upstream seat  14  subjected to the fluid pressure P 2 , directly or indirectly (for example by way of the seal  41 ), and oriented in the direction opposite to the spherical plug  5 , are called the first surface, the surface or surfaces of the upstream seat  14  subjected to pressure P 2 , directly or indirectly, and oriented toward the spherical plug  5  being called the second surface. The pressure P 2  exerted on the first surface generates a force Fc oriented along the longitudinal axis A and tending to press the downstream seat against the spherical plug while, on the contrary, the pressure P 2  exerted on the second surface generates a force Fd oriented along the axis A and tending to detach the upstream seat  14  from the spherical plug  5 . For the upstream seat  14  of  FIGS. 7 and 8 , however, the projection S 1  of the first surface onto the plane perpendicular to the axis A is less than S 2 , that of the second surface, so that the value (norm of the vector) of the force Fc is less than that of the force Fd, the resulting force R being directed from downstream to upstream. In other words, due to the particular geometry of the upstream seat  14 , the pressure P 2  exerted on the first and second surfaces tends to separate the upstream seat  14  from the upstream spherical plug  5 . 
     Recall that the pressure P 2  is considerably greater than the pressure P 1 , so that the effect of the pressure P 1  on the upstream seat  14  is negligible. Thus, as indicated previously, when the plug  5  is closed, this upstream seat  14 , of the “simple piston effect” type, opens in the event of an overpressure in the dead volume  7 ,  13 ,  20  relative to the pressure inside the fluid flow channel  4 , so as to allow the fluid in the dead space  7 ,  13 ,  20  to escape toward said channel  4 . 
       FIGS. 9 and 10  are detail views corresponding respectively to  FIGS. 7 and 8  and illustrating respectively the structure of the upstream seat  16  equipping the downstream plug  6  and the operation of this upstream seat  16 . The structure of the upstream seat  16  equipping the downstream plug  6  is similar to that of the upstream seat  14  equipping the upstream plug  5 , only the differences between these two seats  14 ,  16  being detailed hereafter. The front face of the upstream seat  16  equipping the downstream plug  6  includes a frusto-conical portion  25  (or a spherical segment) wherein is provided an annular groove  42  used to accommodate an insert  43  made for example of a polymer such as that known under the trade name DEVLON. 
     This insert  43  also protrudes slightly from the frusto-conical portion  25 , the sealing abutment of the upstream seat  16  on the downstream plug  6  being accomplished by this insert  43 . The frusto-conical portion  25 , however, is capable of coming into sealing contact against the downstream plug  6 , in the event of deterioration of the insert  43 , particularly in the event of a fire. 
       FIG. 10  illustrates the behavior of this seat  16  in the event of overpressure inside the second dead volume  20 . It is assumed for example that the insert  43  has disappeared under the influence of the elevated temperature due to a fire. It is noted that the seal  41 , if it is made of graphite, withstands very high temperatures. The seal  40  is also not deteriorated, considering the fact that it is protected within a groove  39  and separated from the high-temperature regions by considerable thicknesses of material. 
     The behavior of this seat  16  is identical to that of the upstream seat  14  equipping the upstream plug  5 : the pressure P 2  exerted on the first surface generates a force Fc oriented along the longitudinal axis A and tending to press the upstream seat  16  against the spherical plug  6  while, on the contrary, the pressure P 2  exerted on the second surface generates a force Fd oriented along the axis A and tending to detach the upstream seat from the spherical plug  6 . For this upstream seat of  FIGS. 9 and 10 , the projection S 1  of the first surface onto the plane perpendicular to the axis A is less than S 2 , that of the second surface, so that the value (norm of the vector) of the force Fc is less than that of the force Fd, the resulting force R being directed from downstream to upstream. In other words, due to the particular geometry of the upstream seat  16 , the pressure P 2  exerted on the first and second surfaces tends to separate the upstream seat  16  from the downstream plug  6 . 
       FIGS. 11 and 12  are detail views illustrating respectively the structure of the downstream seat  17  equipping the downstream plug  6  and the operation of this downstream seat  17 . This downstream seat  17  comprises, from downstream to upstream, a first cylindrical portion  21 ″, a second cylindrical portion  22 ″ with a greater diameter than that of the first portion  21 ″, a third cylindrical portion  23 ″ with a diameter greater than that of the second portion  22 ″, and a fourth cylindrical portion  24  with a diameter greater than that of the third portion  23 ″. The upstream end face of the downstream seat comprises a frusto-conical portion  25  (or shaped like a segment of a sphere), wherein is provided a groove  42  accommodating an insert  43  similar to that of  FIGS. 9 and 10 , made for example of a polymer such as that known under the trade name DEVLON. 
     As before, this insert  43  protrudes slightly from the frusto-conical portion  25 , the sealing abutment of the downstream seat  17  on the downstream plug  6  being accomplished by this insert  43 . The frusto-conical portion  25  is however capable of coming into sealing contact against the downstream plug  6 , in the event of deterioration of the insert  43 , particularly in the event of a fire. 
     An O-ring  40 , made for example of a synthetic elastomeric material of the type of that known under the trade name Viton® AED, is mounted around the first portion  21 ″ of the seat  17 , in an annular space  39 ″ defined between the outer surface of the first portion  21 ″ of the seat  17  and an inner cylindrical surface of the body  1 , with a diameter substantially identical to that of the second portion  22 ″ of the seat  17 . This O-ring  40  provides the sealing between the downstream seat  17  and the body  1 . Another seal  41 , made of graphite for example, is mounted about the second portion  22 ″ of the seat  17 , in an annular space  42  defined between the outer surface of the second portion  22 ″ of the seat  17  and an inner cylindrical surface of the body  1 , with a diameter substantially identical to that of the third portion  23  of the seat  17 . This seal  41  is designed to limit downstream flow of fluid but does not provide a complete seal between the seat  17  and the body  1 , said seal being provided by the aforementioned O-ring  40 . 
     The downstream face of the fourth portion  24  forms a shoulder  30  supporting the springs  18  mounted in recesses in the body  1 . The springs  18  exert forces oriented longitudinally so as to press the downstream seat  17  against the downstream plug  6 . 
     The operation of this “double piston effect” type seat  17  will now be described with reference to  FIG. 12 . In this figure, certain non-functional elements have been removed, partially or totally, to facilitate comprehension. The operation of the downstream seat of the downstream plug  6  is similar to that of the seats of  FIG. 3 . 
     It is assumed that the fluid contained in the second dead volume  20  is subjected to an overpressure P 2 , for example due to exposure to a fire. The pressure P 2  in this dead volume is therefore greater than P 1 , that inside the fluid flow channel  4 . It is also assumed that the insert  43  has vanished under the influence of the elevated temperature due to the fire. It will be noted that the seal  41 , if it is made of graphite, withstands very high temperatures. The seal  40  is also not deteriorated, considering the fact that it is protected inside the annular space  39 ″ and is separated from high-temperature regions by considerable thicknesses of material. 
     Considering the position of the O-ring  40 , that of the frusto-conical portion  25  and the non sealing nature of the seal  41 , the fluid leaving the dead volume at pressure P 2  enters into the annular spaces provided between the downstream seat and the downstream plug and between the downstream seat and the body. In particular, the fluid enters into the annular spaces  39 ″ and  42 . As before, the surface or surfaces of the downstream seat  17  subjected to the fluid pressure P 2 , directly or indirectly (for example by way of the graphite seal  41 ), and oriented in the direction opposite to the spherical plug  6 , are called the first surface, and the surface or surfaces of the downstream seat  17  subjected to pressure P 2 , directly or indirectly, and oriented toward the spherical plug  6  being called the second surface. 
     The pressure P 2  exerted on the first surface generates a force Fc oriented along the longitudinal axis A and tending to press the downstream seat  17  against the spherical plug  6  while, on the contrary, the pressure P 2  exerted on the second surface generates a force Fd oriented along the axis A and tending to detach the downstream seat  17  from the spherical plug  6 . For the downstream seat  17  of  FIGS. 11 and 12 , however, the projection of the first surface onto the plane perpendicular to the axis A is greater than that of the second surface, so that the value (norm of the vector) of the force Fc is greater than that of the force Fd, the resulting force R being directed from downstream to upstream. In other words, due to the particular geometry of the downstream seat  17 , the pressure P 2  exerted on the first and second surfaces tends to press the downstream seat  17  onto the spherical plug  6 . 
     Recall that the pressure P 2  is considerably greater than the pressure P 1 , so that the effect of the pressure P 1  on the downstream seat  17  is negligible. Thus, as indicated earlier, this “double piston effect” type downstream seat  17  does not open, even in the event of overpressure in the dead space, so as to avoid any downstream flow of fluid. 
       FIG. 13  illustrates the bleeding means  34  mounted between the two plugs  5 ,  6  of the valve according to the invention. These means  34  comprise a first bleed channel  44   a , provided in the body and leading radially into the fluid flow channel  4 , in the region  13  located between the upstream and downstream plugs  5 ,  6 . The first bleed channel  44   a  is in fluid connection with a first end of a second bleed channel  44   b , provided in the body  1 . The second end of the second channel  44   b  leads to the outer surface of the body  1 , by way of ordinary connection means  45 . 
     A plug  46  is mounted at the junction between the first and second bleed channels  44   a ,  44   b . It comprises a radial rod  47 , one end whereof bears a conical head  48  which is inserted into the corresponding end of the first channel  44   a  so as to plug it, and the second end whereof comprises actuating means  49 . The rod  47  comprises a thread cooperating with a tapped thread of a nut  50  fixed on the body  1 . The rod, and hence the conical head, is movable between a plugging position shown in  FIG. 13 , wherein the head  48  is in abutment with the corresponding end of the first channel  44   a , and a bleed position (not shown), wherein the head  48  is withdrawn from said end of the first channel  44   a . Such plugs  34  are known from the prior art. The invention thus proposes a valve of the “double block and bleed” type making it possible to guarantee downstream sealing simply and reliably, even in the event of fire.