Abstract:
Valve for pressurized or liquefied gas cylinder comprising a valve body ( 12 ) with a gas flow passage ( 16 ) running between an inlet opening ( 18 ) and an outlet opening ( 20 ); and a sealing seat ( 30 ) inside the passage ( 16 ) of the valve body with which a plugging member ( 34 ) is associated. A control head ( 14, 114 ) is mounted removably on the valve body ( 12 ) and includes an axial control rod ( 60, 118 ) cooperating with the plugging member ( 34 ) in order to move it between a closed position in which it rests on the sealing seat ( 30 ), in order to block the flow into the passage ( 16 ), and an open position in which the plugging member ( 34 ) is separated from the sealing seat ( 30 ), in order to enable the outflow. A locking means ( 82 ) is integrated with the valve body ( 12 ) and enables the plugging member ( 34 ) to be locked onto the sealing seat ( 30 ) when the control head ( 14, 114 ) is separated from the valve body ( 12 ).

Description:
FIELD 
       [0001]    The disclosure relates to a valve or valve, especially for liquefied gas or gas cylinder (or bottle). This valve is particularly well adapted for applications using ultra pure gases. 
       BACKGROUND 
       [0002]    The increasingly widespread use of special gases and the high level of purity demanded today by certain industries results in continual efforts by manufacturers of valves, valves and other gas distribution equipment to improve the quality and integrity of their products. 
         [0003]    For the semiconductor industry for example, high-quality gas distribution systems have been developed since the presence of impurities in the gases could irreparably affect the electrical properties of the components and therefore also affect production efficiency. Furthermore, these gases are expensive, highly reactive, corrosive and/or toxic and it is therefore imperative to have suitable and reliable equipment. 
         [0004]    Though the number of users of special gases may be relatively significant, there are not many sites for manufacturing such gases and they may be very distant from the users. In practice, this adds significant logistical costs to the actual costs related to the gas and to the special equipment (cylinder and valves in particular). 
         [0005]    Today, cylinders for special gases are equipped, when they leave the factory, either with manually controlled valves, or with pneumatically controlled valves, depending on the requirements of the customers/users. It often happens that one and the same factory, for example for producing semiconductors, simultaneously uses cylinders equipped with manual and pneumatic controls. A situation with which the user is frequently confronted is that the gas that he wishes to use is contained in a cylinder equipped with a manually controlled valve, while the installation in which he would have wanted to use it requires a pneumatic control, or vice versa. Furthermore, certain cylinders equipped with one type of control are reshipped partially refilled to the gas production site (or the distributer) since they cannot be used with another application which requires another type of valve control. Finally, a gas producer or distributer is confronted with the same problem, since he must extend his pool of cylinders for gas of high purity so that he has a permanent stock of cylinders equipped with manually controlled and pneumatically controlled valves, so as to be able to satisfy the requirements of his customers. 
       BRIEF DESCRIPTION 
       [0006]    In order to remedy the abovementioned problems, the present disclosure proposes a valve, especially for pressurized or liquefied gas cylinder, comprising a valve body with a gas flow passage extending between an inlet port and an outlet port. A sealing seat is arranged in the valve body passage, with which a obturating member is associated. Typically, the sealing seat is situated in a valve chamber in the gas passage, and the obturating member is generally mobile inside this chamber. A control head is mounted on the valve body and comprises an axial control rod cooperating with the obturating member so as to control its displacement between a closure position in which it rests on the sealing seat so as to block the flow in the passage and an opening position in which the obturating member is parted from the sealing seat to allow the flow of the gas in the passage. 
         [0007]    According to an important aspect of the disclosure, the control head is mounted in a removable manner on the valve body. Furthermore, a locking means comprising a stop member that is mobile outside the gas passage is integrated into the valve body, this locking means allowing the locking of the obturating member on the sealing seat when the control head is separated from the valve body, doing so when there is a pressurized gas in the cylinder on which the valve is mounted. 
         [0008]    It will be noted that the mobile stop member being mounted mobile outside the gas passage, it is not wetted by the gas, thus avoiding any contamination of the fluid passing through the valve. The mobile stop member, which may for example be arranged in a zone of the valve body intermediate between the valve chamber and the control head, is able to act directly or indirectly on the obturating member so as to lock it on the sealing seat. 
         [0009]    Advantageously, and as is typically the case for applications of high-purity gas, the valve chamber is closed in a sealed manner towards the control mechanism by an exterior sealing means. 
         [0010]    The present disclosure therefore relates to a valve whose control head is detachable from the valve body, thereby making it possible to change control head and especially to replace a manual control head by a pneumatic control head, or one based on another actuating principle. The presence of the locking means integrated into the valve body is particularly worthwhile since it makes it possible to lock the control member on the sealing seat when the control head is separated from the valve body, thereby enabling the control head to be changed while the cylinder (or the system on which the body is mounted) is pressurized. 
         [0011]    The valve therefore allows greater flexibility in managing stocks or pools of cylinders, both for gas manufacturers or distributers and also for users. Within a very short space of time a gas worker can change a control head, while the cylinder is pressurized, without any risk of gas escaping. 
         [0012]    Various embodiments are conceivable for the locking means depending on the design of the valve body. Certain preferred characteristics of such a locking means are cited below. 
         [0013]    The locking means is preferably designed to influence the operation of the valve only in a so-called “active” position, in which the obturating member is locked on the sealing seat. When locking is not desired, the locking means can be placed in the rest position, in which it does not act on the obturating member. 
         [0014]    The stop member of the locking means can take diverse forms, the objective being to be capable of activating it selectively so as to block the obturating member on its seat. According to a variant, the stop member of the locking means is axially displaceable, outside of the gas flow passage, between an active position and a rest position. This stop member can take the form of a sleeve which comprises on its exterior lateral surface a thread cooperating with a thread on a fixed part of the valve body, so that when the sleeve is rotated on the thread, it moves towards or away from the obturating member. 
         [0015]    For safety reasons, the locking means is advantageously designed so that the control head can be removed only when the locking means is in the active position, with the obturating member in the closure position. It is possible for example to use fixing screws (or other removable fixing means) to fix the control head to the valve, which screws are hidden when the locking mechanism is in the active position and are accessible for removal in the rest position. 
         [0016]    For safety reasons also, a coupling which is rigid in the axial direction, and separable, is advantageously provided between the control rod and the obturating member. A valve with a so-called “tied” valve is therefore obtained: that is to say a displacement of the control rod necessarily causes a displacement of the obturating member. Advantageously, the coupling between the control rod and the obturating member is effected outside the zone wetted by the gases, that is to say outside the gas passage, therefore typically outside the valve chamber, beyond the exterior sealing. 
         [0017]    In a preferred variant, the obturating member comprises a valve fixed at the end of a valve rod guided axially with respect to the sealing seat in a longitudinal bore of the valve body. In this case, the stop member preferably surrounds the valve rod and can be displaced axially into the active position in which it acts directly on the rod or the valve. 
         [0018]    For the tying of the valve, the valve rod advantageously comprises at its end opposite from the valve an outside thread which cooperates with a thread in a cylindrical housing of the control rod. Such coupling makes it possible, when the control rod moves axially, to transmit the axial movement to the valve rod. Furthermore, if the control rod is pivoted on itself and the valve rod is locked in rotation, this type of coupling makes it possible to generate an axial displacement of the valve rod. The valve can therefore be controlled axially by a rotary control rod (e.g. in a manual control head) or by a control rod moving axially (typically with a pneumatic control head). 
         [0019]    For its actuation, the locking mechanism preferably comprises a wheel surrounding the valve body, on the side of the valve rod and the control head. The wheel is coupled to the axial stop member in such a way that a rotation of the wheel (typically over a portion of a revolution) around the valve body generates an axial displacement of the stop member. According to a variant, the wheel is fixed by screws on a splined crown which engages in splines on the exterior periphery of the stop member-like sleeve. The wheel is secured to the splined crown by way of screws which pass through the valve body at the level of guidance slots (generally in a plane perpendicular to the axis of the control rod) which define the travel of said wheel around the valve body. 
         [0020]    The wheel can comprise a series of openings which are positioned so as to be aligned (coincide) with means for fixing the control head to the valve body, when removing the head, and so as to hide these fixing means in the rest position of the locking means. 
         [0021]    According to one execution, the valve body comprises a connector-like piece receiving the control head. The wheel surrounds the connector-like piece; the splined crown is placed in the connector-like piece and the guidance slots are provided in the connector-like piece. The control head is fixed in a removable manner to this connector-like piece, which can be made integral with the valve body or be fixed to the latter, for example by screwing. 
         [0022]    The control head can be of the manually or remotely actuated type, for example pneumatic. Specifically, any type of mechanism allowing the control rod to be displaced axially can be envisaged for the actuation head. 
         [0023]    When a control head of the pneumatic type with prestress springs is used, it advantageously comprises a locking mechanism to lock the prestress springs in a compressed position. This prevents the springs from deploying to the maximum when the actuation head is separated from the valve body. 
         [0024]    As will have been understood, the valve finds a particularly advantageous application in the guise of cylinder valve for pressurized gas of ultra-high purity. The valve body is then designed to be fixed to a cylinder and the inlet port of the flow channel typically emerges inside the cylinder. Nevertheless, the present valve can also be used as flow valve in a (fluid) gas distribution system. In this case, the valve body will preferably be adapted so that the inlet and outlet ports can be linked up to the pipes or other elements of the gas distribution system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    Other features and characteristics of the disclosure will emerge from the detailed description of a few advantageous embodiments presented below, by way of illustration, while referring to the appended drawings. The latter show: 
           [0026]      FIG. 1 : a view in longitudinal section of a preferred embodiment of a valve, with a manual control head; 
           [0027]      FIG. 2 : an exploded view of the locking means integrated into the connector part of the valve body; 
           [0028]      FIG. 3 : a view in longitudinal section of the valve body of  FIG. 1 , equipped with a pneumatic control head; 
           [0029]      FIG. 4 : a view in perspective of the valve body minus control head; 
           [0030]      FIGS. 5 and 6 : a view in perspective of the valve of  FIG. 1 , respectively of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0031]      FIG. 1  illustrates a view in longitudinal section of a preferred embodiment of the present valve  10 , which comprises a valve body and a control head designated in a general manner  12  and  14  respectively. In the variant of  FIG. 1 , the control head  14  is of the manually actuated type although in that of  FIG. 3 , the head, indicated  114 , is of the pneumatic type. As will be explained in greater detail below, the control head  14 ,  114  is mounted in a removable manner and can be separated from the valve body  12  while the latter is fixed to a pressurized cylinder. This therefore makes it possible to exchange one control head for another, whose actuating principle may be different. The replacement of the control head  14 ,  114  while the cylinder is pressurized is made possible in particular by the presence of a locking mechanism in the valve body able to act selectively on the obturating member so as to lock it in the closure position. 
         [0032]    The valve body  12  is designed to be screwed onto a gas cylinder (not shown) and comprises a gas flow passage  16  which extends between a gas inlet port  18  and a gas outlet port  20 . In the present execution, the flow passage  16  is formed by an inlet passage  22  linking the inlet port  18  to a valve chamber  24  and an outlet passage  26  linking the valve chamber  24  to the outlet port  20 . The inlet passage  22  passes through the lower part of the body  12 , which is furnished with a thread (not represented) allowing the valve to be screwed onto the cylinder. For its part, the outlet passage  26  emerges at the end of a portion forming a union  28  equipped with a removable union  29 . 
         [0033]    The end of the inlet passage  22  emerging in the chamber  24  forms a sealing seat  30  with which a obturating member cooperates so as to allow the flow towards the outlet passage  26  or to plug the passage  16  in a sealed manner. The obturating member comprises a valve  34  which is preferably secured to a valve rod  36  which passes through a longitudinal bore  38  in the upper part  68  of the valve body  12 . By axial displacement of the valve rod  36 , the valve  34  can therefore be displaced between a closure position ( FIG. 1 ) in which it rests on the sealing seat and closes the passage  16  in a sealed manner, and an opening position separated from the seat  30  in the chamber  24 , in which the gas can flow towards the outlet passage  26 . For its fixing to the valve rod  36 , the valve  34  comprises a threaded rod portion which is screwed onto a thread of the inside surface of a housing  40  at the end of the valve rod  36 . 
         [0034]    The sealing of the chamber  24  on the outside, that is to say on the side of the valve rod  36  and at the level of the longitudinal bore  38 , is achieved in a conventional manner by a flexible membrane  42  (plastic or metallic). As seen in  FIG. 1 , this membrane  42  surrounds the valve rod  36  and closes the chamber  24  on the side of the bore  38  where it is held by a mount  44 , itself held by a packing  46  solidly fixed by screwing (threads labeled  48 ) in the bore  38 . It will also be noted that the packing  46  preferably comprises a shoulder  39  limiting the axial displacement of the valve rod  34  in the opening direction. The references  50  and  52  indicate two O-ring seals. In a conventional manner also, the valve  34  can comprise, facing the seat  30 , a cavity in which are arranged a pad  54  and a stopper  56 . Finally, the valve rod  36  is locked in rotation by a pin  57  guided axially in the mount  44 . Although the exterior sealing of the valve chamber  24  is achieved here by a flexible membrane  42 , it is clear to the person skilled in the art that means of exterior sealing of diverse constructions can be employed to achieve such sealing, with or without flexible membrane, the objective being to ensure the sealing of the chamber  24  on the outside (valve rod  36  side, towards the control head  14 ). 
         [0035]    The control head  14  is mounted, removably, on the valve body  12  and comprises a control rod  60  for actuating the valve rod  36  and therefore the valve  34 . In the present variant, the control head  14  is received in an intermediate body  62 , also called a connector, of the valve body  12 . The connector  62  has a generally tubular form and defines a cylindrical housing  64  in which the lower part of the body  66  of the control head  12  is received. The connector  64  is here designed as an independent piece and is screwed by its lower part onto the upper portion  68 . In the assembled configuration of  FIG. 1 , the control head  14  is fixed to the connector  62  by way of screws (not represented) whose head is housed in the connector  62  and the threaded rod penetrates into the body  66  of the control head  14 . 
         [0036]    It will be appreciated furthermore that the control rod  60  is rigidly coupled to the valve rod  36 , so that an axial movement (along axis  72 ) of the control  60  necessarily causes a displacement of the valve  34 . For this purpose, the end of the rod  36  opposite from the valve is furnished with an outside thread  73  and will be screwed onto an inside thread  73 ′ in a cylindrical housing  75  in the control rod  60 . Such a valve  34  is termed “tied”. 
         [0037]    In the present variant, the design of the control head  14  is such that the control rod  60  is locked axially and can therefore only rotate around the axis  72  when the wheel  74  to which it is secured is actuated by its upper end. Locking in the axial direction is obtained by a radial protuberance  76  of the rod  60  at the level of a cavity  78 , in which the protuberance  76  is locked between balls  80 , therefore only allowing the rod  60  to rotate on itself. 
         [0038]    The control rod  60  being axially locked, it will be understood that the maneuvering of the wheel  74  causes a rotation of the rod  60  and of its thread  73 ′, which consequently causes the axial displacement of the valve rod  36  (locked in rotation). It will be noted here that though other types of couplings can be envisaged to tie the valve to the control rod, the present embodiment allows axial displacement of the valve, either by rotation of the control rod  60  as explained previously, or simply axial displacement of the control rod with a control head of another type. 
         [0039]    As indicated above, a means for locking the valve  34  is integrated into the valve body  12 . In the present variant, it comprises an axially displaceable sleeve  82 , which constitutes a sort of retractable stop member. More precisely, the sleeve  82  extends in the bore  38  and in the union  62 . The sleeve  82  comprises an outside thread  84  which cooperates with an inside thread  84 ′ of the packing  46 , the rotation of the sleeve  82  consequently causing an axial displacement of the latter upwards or downwards, depending on the direction of rotation. In  FIG. 1 , the valve  34  is in the closure position on its seat  30  and the sleeve  82  is in stop member against a shoulder  86  of the valve rod  36 , thus preventing any displacement of the rod  36  and therefore preventing the separation of the valve  34  from its seat  30 . In this configuration, the locking means is said to be in the active position. 
         [0040]    In this active position, the axial displacement of the valve rod  36  and of the valve  34  are prevented by the sleeve  82 , which also renders the control head inactive. It will therefore be understood that, in the active position of the locking means, the control head  14  can be removed without this having any influence on the closing of the cylinder, and therefore without any risk of gas leaks. 
         [0041]    To maneuver (axial positioning) the sleeve  82 , the locking means advantageously comprises a wheel  88  at the periphery of the connector  62  which is fixed by screws  90  to a splined crown  92 , which is engaged with exterior splines  93  provided in the upper part of the sleeve  82 . As will be better understood in  FIG. 2 , the fixing screws  90  are two in number and are diametrically opposed. Each screw  90  has its head housed in the wheel, passes through the connector  62  and is screwed into the splined crown  92 . It will be noted that the connector  62  comprises two guidance slots  94  extending in a plane essentially perpendicular to the axis  72 . By rotating the wheel  88  with respect to the valve body  12 , the splined crown  92  is thus driven and this causes the rotation of the sleeve  82  and therefore its axial displacement on the packing  46 . The amplitude of the axial displacement of the sleeve  82  depends on the pitch of the threads  84 ,  84 ′ and the angular travel of the wheel  88  (here of the order of 120°). The splines are beneficial in that they allow rotational driving of the sleeve  82  while permitting its axial displacement. 
         [0042]    Each guidance slot  94  can define at one of its ends, or at both, a clearance  95  allowing the locking of the wheel in the active and/or rest position (in which the sleeve is set back on the thread  84 ′ and does not oppose the movements of the obturating member). The reference signs  96  and  97  respectively indicate balls and springs which are positioned under the splined crown  92  and bear on the bottom of the connector  62  so as to facilitate the rotation of the crown  92  and generate an upward restoring force helping the locking in the clearances  95 . 
         [0043]    Referring again to  FIG. 2 , it will be noted that the wheel  88  comprises two series of holes  98  and  100 . The holes  98  are two in number, diametrically opposed, and receive the heads of the screws  90  for fixing to the splined crown  92 . The holes  100 , in the upper part of the wheel, are four in number and are provided so as to access the fixing screws or pins (not represented) for the control head. In the present variant, these fixing screws have their head housed in holes  102  in the connector  62  and their rod screwed into the base of the body  66  of the control head  14 . 
         [0044]    It will be appreciated that the holes  100  in the wheel and the holes  102  in the connector  62  are disposed so that they coincide only when the angular position of the wheel  88  is that which corresponds to the active position of the locking means, the holes  102  therefore being hidden by the wheel  88  in the other positions. It is therefore not possible to remove (and therefore fit) the control head  14  if the locking means is not active, a worthwhile safety criterion for users. 
         [0045]    When the present valve  10  is fitted onto a pressurized gas cylinder, the control head  14  is normally removed in the following manner. The valve  34  is placed in the closure position on its seat  30  by actuating the wheel  74 . Thereafter, the locking wheel  88  is rotated into the active position, so as to abut the sleeve  82  against the shoulder  86  of the valve rod  36 . In this position of the locking wheel  88 , the holes  100  and  102  coincide, allowing access to the screws for fixing the control head  14  as illustrated in  FIG. 5 . The fixing screws can therefore be unscrewed and withdrawn, thus detaching the body  66  from the control head  14  of the connector  62 . To withdraw the control head  14 , it then suffices to rotate it around itself, so as to uncouple the control rod  60  from the valve rod  36 .  FIG. 4  illustrates a view in perspective of the valve body  12  minus control head, with the wheel  88  in the position for activating the locking of the valve  34 . 
         [0046]    It should be noted that the exterior sealing of the valve chamber  24 , here ensured by a flexible membrane, makes it possible to prevent the gas located in the chamber  24  or more generally in the gas passage from escaping to the air when the control head  14  is removed. Additionally, as seen in the figures, the tie achieved by the separable rigid coupling between the control rod  60  and the valve rod  36  is achieved outside the chamber  24  so that this link is not located in the zone wetted by the gas. By virtue of this configuration, it is possible to remove the control head without any risk of toxic gas escaping to the outside, therefore without any risk to the operator. 
         [0047]    Once the manual control head  14  has been detached, it is of course possible to refix it to the valve body  12 , fix another manual head thereto, or indeed a remotely controlled head, for example of the pneumatic type. It will be understood that the actuating principle of the control head does not come into play per se, the criteria for mounting the control head being simply that the geometry of the base of the head fits the connector  62  so as to be fixed therein by the fixing screws through the holes  100  and  102 , and that the control rod can be coupled up to the valve rod  34 . 
         [0048]    An exemplary control head  114  with pneumatic actuation mounted on the valve body  12  is illustrated in  FIG. 3 ,  FIG. 6  showing a perspective view. The control head  114  comprises, like the manual head  14 , a body  116  which is housed in the connector  62  and an axial control rod  118  able to be coupled up to the valve rod  36 . Accordingly, the lower end of the control rod  118  comprises a cylindrical housing  120  furnished with a thread onto which the thread of the upper end of the valve rod  36  will be screwed. The valve  34  is thus tied and therefore necessarily follows the displacements of the control rod  118 . 
         [0049]    Such a control head  114  is mounted by positioning the head so as to engage the upper end of the valve rod  36  in the housing  120  and then screwing these together to couple them, by rotating the head around itself. At the same time as the valve rod  36  penetrates into the housing  120 , the body  116  of the control head  114  penetrates into the connector  62 . The rotation of the head  114  is stopped when the holes for the fixing screws in the body  116  are aligned with the holes  102  of the connector  62 , then the head  114  is immobilized by screwing the fixing screws into the body  116 . The control head  114  is then linked up to a pneumatic control hose (not represented) and the wheel  88  is rotated so as to place the sleeve  82  in the rest position, thus freeing the valve  34 . 
         [0050]    The actuating principle of the control head  114  is conventional. The control rod  118  can move axially in the body  116 , and thus controls the position of the valve  34  with respect to its seat  30 . A cover  122  is screwed onto the body  116 . Three piston assemblies are stacked in the body  116 . Each piston assembly comprises a piston  124  that can slide axially in the body  116  and a fixed partition  126 , the space between each respective piston  124  and partition  126  forming a pressure chamber  128 . The pistons  124  are spaced out along the control rod  118  and secured to the latter. Spring means  130  taking the form of a stack of Belleville washers, are disposed in a housing  132  in the cover and bear on the latter and on the first piston  124 , creating a prestress. The pistons  124  being secured to the control rod  118 , the force exerted by the Belleville washers  130  is transmitted to the control rod  118 , thereby tending to push the valve rod  36  and therefore the valve  34  downwards on its seat  30 , thus forming a so-called “normally closed” valve. Fluid intake paths are arranged in the control rod  118  to introduce a pressurized fluid, typically compressed air, into the pressure chambers  128  (the central fluid channel  134  only being shown in  FIG. 3 ). The valve  10  can be opened by injecting air compressed into the pressure chambers  128  so as to exert on the pistons  124  a force greater than the elastic force of the Belleville washers, thus causing the upward displacement of the control rod  118 . Because of the coupling in the housing  120 , the valve rod  36  follows the movement of the control rod  118  and separates the valve  34  from its seat  30 , allowing gas flow in the passage  22 . It will be noted that the coupling serves here simply to transmit an axial force since there is no rotation of the control rod  118 , unlike the case of the manual control head of  FIG. 1 . 
         [0051]    In order to avoid a downward displacement of the control rod  118  under the action of the Belleville washers  130  when the control head  114  is not mounted on the valve body  12 , the control head  114  advantageously comprises a system for locking the Belleville washers. In the variant of  FIG. 3 , this system comprises a lock-nut  136  placed on an outside thread of a pneumatic union  140  screwed onto the control rod  118 . The union  140  communicates with the central channel  134  in the control rod  118 . 
         [0052]    Typically, the Belleville washers  130  will be pre-compressed in the factory, during assembly of the control head  114 . Accordingly, the control head  114  is pressurized, thereby causing the upward displacement of the rod  118 . The operator can thereafter manually rotate the lock-nut  136  on the thread of the union  140  so as to lower the lock-nut  136  and bring it into stop member against the cover  122 . When the pressure is relaxed, the rod  118  remains locked in the up position and the Belleville washers  130  compressed. 
         [0053]    To free the Belleville washers  130  compressed by the lock-nut  136  during the first use, when the head has been mounted on the valve body  12 , pressure is raised in the head and the lock-nut  136  is unscrewed so as to return it to the position of  FIG. 3 . When subsequently removing and refitting the head, the same procedure will be adopted, that is to say the lock-nut  136  is manipulated when the head  114  is pressurized (thus limiting the friction). 
         [0054]    In the preferred variant illustrated in the figures, the seat is of the normal (or direct) type, that is to say the pressure in the cylinder tends to separate the valve from its seat. When locking the valve  34 , the sleeve  82  is displaced towards the seat  30  so as to exert a pressure force towards the sealing seat  30  by bearing on the shoulder  86 . 
         [0055]    Such a locking means is entirely transposable to the case of a valve body with inverted seat, for which the pressure in the cylinder tends to repel the valve onto the sealing seat and therefore to close the passage. In this case, the activation of the locking would correspond for example to moving the sleeve axially away from the sealing seat so as to bear on a shoulder of the valve rod and exert a force in the direction in which the gases exit, therefore preventing the possibility of the obturating member being activated in the opening direction. 
         [0056]    Finally, the valve body could comprise a membrane instead of the valve  34 . In this case, the closing of the sealing seat nevertheless involves the application of a obturating member (often not tied to the control rod) on the membrane so as to thrust it against the sealing seat. It is therefore possible to use a locking means of the type of that described above in the case of the direct seat, the locking means cooperating with the obturating member.