Abstract:
The present invention provides a tank vent device for venting gas from a tank  10  comprising a gas inlet  107  for receiving gas from a vent outlet of the tank, a gas outlet  109  for discharging the gas received, and a pressure valve  70  connecting the gas inlet and gas outlet, wherein the pressure valve is connected to a control pressure input  108  and is controllable by control pressure supplied at the control pressure input, whereby, in use, the pressure valve is controlled by the control pressure to open and close so as to regulate the flow of gas between the gas inlet and the gas outlet. The present invention also provides a vent tank  10 , methods of venting gas from a tank including a method including the steps of connecting a tank vent device to a tank and supplying gas at a lower pressure than the pressure of the tank to the tank vent device and a method of refuelling an aircraft.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention concerns the venting of gas from fuel tanks. In particular, it concerns venting fuel tanks of aircraft. Also particularly, but not exclusively, this invention concerns a tank vent device for venting gas from a tank. The invention also concerns a vent tank, a method of venting gas from a tank and a method of refuelling an aircraft including performing the method of venting gas. 
         [0002]    It is important to vent a tank for two reasons. Firstly, a tank must be vented to enable the tank to be efficiently filled with fuel. If there is inadequate venting, the pressure build up in the tank is significant and can make it very difficult to put further fuel in the tank. Secondly, a pressure build up can result in a large positive pressure differential between the tank and the atmosphere. A negative pressure differential could also arise (depending on the atmospheric pressure). Any pressure differential puts undue strain on the tanks and can cause damage to them and their surrounding structure. This is especially important in relation to aircraft fuel tanks, as the fuel tanks are often located in the wings or tail and it is important to minimise and, preferably, entirely prevent, wing/tail damage. 
         [0003]    Venting of aircraft tanks is normally performed by allowing gas to vent from the fuel tanks via a passive vent. The gas can be vented directly from the fuel tank itself or via a dedicated vent tank. 
         [0004]    GB 2 321 639 A discloses a fuel vapour recovery system for an automotive vehicle. The system includes first and second vapour recovery canisters and a bypass flow element between the canisters. When the bypass flow element is open, vapour can pass directly through the bypass element to the second canister without first passing through the first canister. When the bypass flow element is closed, vapour must first pass through the first canister before reaching the second canister. The bypass flow element is opened when the pressure differential across the element is large enough to push a check valve open. 
         [0005]    The vapour recovery system of GB 2 321 639 A can effectively contain the fuel vapour, that would otherwise be vented into the atmosphere as VOCs (volatile organic compounds) and cause air pollution. The fuel vapour can also be reused/recycled. 
         [0006]    However, use of the vapour fuel recovery system of GB 2 321 639 A results in a vapour flow restriction (even when the bypass flow element is open). Therefore, the system causes pressure to build up in the fuel tank during refuel and therefore slows down the refuelling rate that can be achieved and puts undue strain on the tank. The system is also not suitable for use with jet fuel or for use on an aircraft. 
         [0007]    U.S. Pat. No. 5,575,441 discloses a device for preventing fuel spillage and the venting of fuel vapour to the atmosphere. The device is attached to the outlet of a dump mast on a wing of a military aircraft during refuel. An adaptor assembly of the system is attached on one side to the outlet by a groove corresponding to the shape of the outlet walls. A standard fuel hose is attached to the opposite side of the adaptor assembly and feeds into a fuel container of a fuel truck. Suction is applied to the fuel hose in order to urge fuel and vapour out of the dump mast into the fuel container. However, the system disclosed contains no safety features that could prevent over or under pressure of the fuel tank or prevent the possibility of flame propagation via the fuel hose into the fuel tank. The system, however, does contain an alarm which can be used to show fuel flow problems or hose kinking. The alarm is controlled by sensors. Hence, electronics are used to control and provide warnings in relation to the vent system. This means that electrical power would be in close proximity to highly volatile fuel vapour, directly linked to the fuel tank. Failure of the safety system by an electrical short would give rise to a risk of fuel vapour ignition. 
         [0008]    WO 2007/138366 discloses the use of a standard coupling (known as the API  1004  coupling) connected to an aircraft vent during refuel of the aircraft tanks. The coupling is connected back to a fuel tanker via a discharge line. The tanker collects the vapour and later transports it to a gas storage terminal. The gas can then be used for electricity production and supply. The coupling includes an “overpressure vent valve” which prevents the aircraft tank becoming overpressured by opening when the pressure in the aircraft tank increases too much. This coupling system is controlled solely based on the pressure in the aircraft tank. In other words, gas is extracted into the discharge line when the valve is open but not when it is closed. There is no independent control of the system. Furthermore, the system disclosed contains no safety features that could prevent the possibility of flame propagation via the discharge line into the aircraft tank. 
         [0009]    The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved tank vent device, vent tank and method of venting gas from a tank. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention provides, according to a first aspect, a tank vent device for venting gas from a tank, the device comprising a gas inlet for receiving gas from a vent outlet of the tank, a gas outlet for discharging the gas received from the vent outlet of the tank, and a pressure valve connecting the gas inlet and gas outlet, wherein the pressure valve is connected to a control pressure input and is controllable by control pressure supplied at the control pressure input, whereby, in use, the pressure valve is controlled by the control pressure to open and close so as to regulate the flow of gas between the gas inlet and the gas outlet. 
         [0011]    Providing a pressure valve that can be controlled by a separate control pressure means that the pressure valve can be opened and closed depending on a separate control. The opening and closing of the pressure valve is not simply dependent on the pressure differential of the gas inlet and gas outlet. This means the pressure valve can be opened and closed in dependence on different or additional conditions. 
         [0012]    In general, the tank will be at a pressure greater than atmospheric pressure due to the increase in pressure from refuelling the tank. Preferably, the gas outlet is arranged to be connected to a low pressure gas supply. Providing a low pressure gas supply means that the tank pressure can be lowered so as to reduce a positive pressure differential between the tank and the atmosphere. In addition, providing a low pressure gas supply means that the tank pressure can be lowered to a pressure below that of ambient pressure. The tank pressure should not be lowered to a pressure to give an undesirably large negative pressure differential between the tank and the atmosphere. 
         [0013]    Preferably, the gas pressure at the control pressure input of the pressure valve is provided by the same source that provides the gas pressure in the gas outlet. This makes the tank vent device simpler. 
         [0014]    Preferably, the gas inlet and gas outlet both act on a first side of a moveable diaphragm of the pressure valve and wherein the control pressure input acts on a second, opposite side of the diaphragm. The moveable diaphragm preferably acts to seal the gas inlet and gas outlet from each other. More preferably, the diaphragm of the first pressure valve is arranged to be moved away from the gas inlet and gas outlet by low pressure supplied at the control pressure inlet. 
         [0015]    More preferably, the cross-sectional area of the gas inlet and the cross-sectional area of the gas outlet acting on the diaphragm of the first pressure valve are dimensioned such that, in use, when the gas inlet experiences a higher than atmospheric pressure from the vent tank and the gas outlet experiences a lower than atmospheric pressure from a low pressure supply, there is little overall combined pressure force acting on the diaphragm from the combination of the gas inlet and gas outlet. This means that the pressures experienced in the gas inlet and gas outlet have a negligible effect on the pressure valve. This allows the first pressure valve to be controlled essentially solely based on the pressure force supplied by the control pressure. 
         [0016]    Preferably, movement of the diaphragm away from the gas inlet and gas outlet causes the first pressure valve to open and thereby allow the gas inlet to be connected with the gas outlet such that gas in the gas inlet can flow to the gas outlet. This allows gas from the vent tank to be vented through the first pressure valve and through the vent tank device. The gas can then be collected for later recycling or re-use or can simply be collected to prevent it escaping into the atmosphere and causing air pollution. 
         [0017]    Preferably, the device further comprises a second pressure valve for controlling the pressure force supplied to the first pressure valve by the control pressure. This allows the first pressure valve to be controlled by the second pressure valve. More preferably, the second pressure valve controls the pressure force supplied to the first pressure valve by controlling the area over which the control pressure is supplied to the first pressure valve. This means that the control pressure supplied to the tank vent device can be at a constant pressure, whilst still allowing it to supply a range of forces on the first pressure valve. It also allows the first pressure valve to be controlled without the use of electronics. 
         [0018]    Preferably, the second pressure valve is controllable based on a pressure differential between two pressure inputs including a first pressure input corresponding to the pressure in the tank being vented. This allows the second pressure valve to be controlled based on the tank pressure. This, in turn, allows the tank pressure to control opening and closing of the first pressure valve. This means that gas flow from the tank (the gas inlet) can be controlled (and therefore the venting of the tank to be controlled) based on the pressure in the tank. More preferably, the gas at the first pressure input of the second pressure valve comes from the same source as the gas in the gas inlet. This means a separate inlet from the tank is not needed. 
         [0019]    Preferably, a second pressure input to the second pressure valve corresponds to a base pressure. More preferably, the base pressure corresponds to a “target” tank pressure. Preferably, the base pressure corresponds to atmospheric pressure. This allows the first pressure valve to be controlled (and therefore the venting of the tank to be controlled) based on the pressure differential between the tank and the atmosphere (or another base pressure). It also allows the tank to be vented such that the pressure in the tank approaches a “target” pressure, which could be atmospheric pressure. This means that the overall force and strain exerted on the tank can be reduced. 
         [0020]    Preferably, the second pressure valve comprises a moveable diaphragm, with the first pressure input on one side of the diaphragm and a second pressure input on the second, opposite side of the diaphragm. 
         [0021]    More preferably, the diaphragm of the second pressure valve is connected to a piston, such that when the diaphragm moves in a first direction towards the second pressure input side, the piston is caused to move in the same direction from a closed position against a first seat to an open position. When in the closed position, the piston may seal off the control pressure input from the first pressure valve. More preferably, the first seat is on the second, opposite side of the first pressure valve, such that lifting of the piston from the first seat opens the control pressure input of the first pressure valve, enabling a pressure force to be supplied to the first pressure valve by the control pressure. Even more preferably, when the piston is in the fully open position, it rests against a second seat, in between the first seat and the second pressure valve. 
         [0022]    Preferably, the diaphragm of the second pressure valve is also connected to a second piston, such that when the diaphragm moves in a second direction towards the first pressure input side, the second piston is caused to move in the second direction towards a restricting position with respect to the control pressure input of the first pressure valve. More preferably, once the second piston is in its restricting position, further movement of the diaphragm in the second direction causes the first piston to move in the second direction towards its closed position against the first seat to seal off the control pressure input of the first pressure valve. 
         [0023]    Preferably, in use, the control pressure is supplied in between the first seat and second seat such that the control pressure can be supplied to the diaphragm of the first pressure valve through an opening associated with the first seat. 
         [0024]    Preferably, the control pressure is supplied between the second seat and the second pressure valve such that the control pressure can be used to hold the first piston against the second seat. 
         [0025]    Preferably, the device is arranged to receive a coaxial pipeline such that the gas outlet can be connected to an inner section of the coaxial pipeline and that the control pressure can be supplied in an outer section of the coaxial pipeline, such that, in use, when the outer section of the coaxial pipeline is cut or severed, atmospheric pressure is supplied as the control pressure, causing the first pressure valve to close. This means that if there is a fire or if the pipeline gets damaged in another way, the outer section is ruptured first. This means that atmospheric pressure is supplied as the control pressure, thereby increasing the pressure force supplied to the first pressure valve, thereby closing the first pressure valve. Hence, the first pressure valve is closed and prevents flow of gas between the gas inlet and gas outlet as soon as the pipeline is damaged. Therefore, upon further damage of the pipeline (i.e. rupture of the inner section), the gas from the atmosphere cannot flow past the first pressure valve and cannot reach the gas inlet or vent tank through the vent tank device. This is an important feature because it means that in the event of a fire (or other damage to the pipeline), the vent tank device automatically shuts down and does not allow the device to be used as a bypass to a flame arrestor associated with the vent tank. Hence, any fire occurring outside the vent tank is prevented from reaching the vent tank through the vent tank device. Providing this safety feature with a single pipeline means that handling and storage of the ground refuelling equipment is easier and less awkward. 
         [0026]    More preferably, the device is arranged to receive a low pressure gas supply in the inner section of the coaxial pipeline and thereby supply a low pressure gas to the gas outlet. Even more preferably, the device is arranged such that the low pressure gas supply can also be supplied to the outer section of the coaxial pipeline, such that the low pressure gas supply to the gas outlet is the same gas supply as the control pressure supply to the control pressure input. 
         [0027]    Preferably, the device further comprises a probe for connecting to a vent tank, the probe comprising a nozzle connected to the gas inlet such that gas can be vented from the tank, through the nozzle to the gas inlet. This allows the device to be directly inserted into a valve in the vent tank. Preferably, the probe further comprises a locking mechanism for securing the probe to the tank during use. 
         [0028]    Preferably, a shroud is mounted on the probe, such that, in use, the shroud provides a seal around the probe. This prevents any gas, for example, gas from a flame arrestor passage, that is not sealed between the vent tank and the tank vent device from escaping. More preferably, the shroud is provided with a bleed port so that gas, and in the case of a system failure; fuel, contained by the shroud can be captured by the bleed port and transferred into the gas inlet. 
         [0029]    Preferably, the shroud can be moved between a deployed position, wherein, in use, the shroud provides a seal around the probe, and a collapsed position, such that the device can be stored. Even more preferably, the shroud comprises a lever mechanism, such that, when in the deployed position, the levers are over-centred so as to help maintain the shroud in the deployed position. 
         [0030]    According to a second aspect of the invention there is also provided a tank vent assembly comprising the tank vent device as described above, and a coaxial pipeline connected to the gas outlet of the tank vent device for connection to a low pressure and control pressure supply. The inner section of the coaxial pipeline may be connected to the gas outlet and the outer section of the coaxial pipeline may be connected to the control pressure input. 
         [0031]    According to a third aspect of the invention there is also provided a vent tank comprising a valve assembly for connecting to the vent tank device as described above, the valve assembly comprising a valve such that when the probe is inserted in the valve assembly, the valve can be opened to allow gas in the vent tank to vent into the probe, and a securing mechanism for releasably securing the probe to the vent tank. Preferably, the valve assembly further comprises a rotatable joint to allow the probe to move rotationally in relation to the tank. Providing a rotatable joint means that no loading is imported into the wing structure from the probe. This allows the structural weight of the vent tank to be minimized whilst still being able to use a relatively long probe. A long probe aids installation in the valve assembly. 
         [0032]    According to a fourth aspect of the invention there is also provided an aircraft wing comprising the vent tank as described above. 
         [0033]    According to a fifth aspect of the invention there is also provided an aircraft comprising the vent tank as described above or the wing as described above. 
         [0034]    According to a sixth aspect of the invention there is also provided a method of venting gas from a tank, wherein the method includes the steps of providing a tank vent device as described above or the tank vent assembly as described above, connecting the device or assembly to a vent tank as described above, and venting gas from the vent tank through the gas inlet and gas outlet of the device/assembly. 
         [0035]    According to a seventh aspect of the invention there is also provided a method of venting gas from a tank, the method including the steps of providing a pressure valve to control the venting of gas from the tank, and providing an independent control pressure to control opening and closing of the pressure valve. Preferably, the control pressure is lower than the pressure of the tank. Preferably, the method includes the step of changing the area over which the control pressure is supplied to the pressure valve so as to vary the pressure force on the pressure valve. More preferably, the area is changed based on a pressure differential between the pressure of the tank and a base pressure. Preferably, the method includes the step of providing a low pressure supply to an output of the pressure valve. 
         [0036]    According to an eighth aspect of the invention there is also provided a method of refuelling an aircraft, including performing the method of venting gas from a tank on the aircraft as described above and simultaneously supplying liquid fuel to a fuel tank on the aircraft. 
         [0037]    It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0038]    Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: 
           [0039]      FIG. 1  shows a partial side view of a vent tank, according to an embodiment of an aspect of the invention; 
           [0040]      FIG. 2  shows a plan view of the flame arrestor and valve assembly from inside the vent tank of  FIG. 1 ; 
           [0041]      FIG. 3   a  shows a rear view of a tank vent assembly according to an embodiment of an aspect of the invention; 
           [0042]      FIG. 3   b  shows a side view of the tank vent assembly of  FIG. 3   a;    
           [0043]      FIG. 3   c  shows a front view of the tank vent assembly of  FIGS. 3   a  and  3   b;    
           [0044]      FIG. 3   d  shows a rear view of the tank vent assembly of  FIGS. 3   a  to  3   c;    
           [0045]      FIG. 3   e  shows a rear section view of the tank vent assembly of  FIGS. 3   a  to  3   d;    
           [0046]      FIG. 4  shows two sections through at least part of the coaxial pipeline of the tank vent assembly of  FIGS. 3   a  to  3   e;    
           [0047]      FIG. 5  shows a side section view of the tank vent assembly of  FIGS. 3   a  to  3   e;    
           [0048]      FIG. 6   a  shows a side view of part of the tank vent assembly of  FIGS. 3   a  to  3   e  and  5 ; 
           [0049]      FIG. 6   b  shows a side view of part of the tank vent assembly of  FIGS. 3   a  to  3   e  and  5 ; 
           [0050]      FIG. 7   a  shows a side section view of a meter valve assembly of the tank vent assembly of  FIGS. 3   a  to  3   e  and  5 ; 
           [0051]      FIG. 7   b  shows a side section view of a meter valve assembly of the tank vent assembly of  FIGS. 3   a  to  3   e  and  5 ; 
           [0052]      FIG. 7   c  shows a side section view of a meter valve assembly of the tank vent assembly of  FIGS. 3   a  to  3   e  and  5 ; 
           [0053]      FIG. 7   d  shows a side section view of a meter valve assembly of the tank vent assembly of  FIGS. 3   a  to  3   e  and  5 ; 
           [0054]      FIG. 7   e  shows a side section view of a meter valve assembly of the tank vent assembly of  FIGS. 3   a  to  3   e  and  5 ; 
           [0055]      FIG. 7   f  shows a side section view of a meter valve assembly of the tank vent assembly of  FIGS. 3   a  to  3   e  and  5 ; 
           [0056]      FIG. 8  shows a cross section through Section A-A shown in  FIG. 7   a;    
           [0057]      FIG. 9   a  shows a side section view of the tank vent assembly of  FIGS. 3   a  to  3   e  and  5 , shown in position in the valve assembly of the vent tank of  FIGS. 1 and 2 ; 
           [0058]      FIG. 9   b  shows another side section view of the tank vent assembly of  FIGS. 3   a  to  3   e  and  5 , shown in position in the valve assembly of the vent tank of  FIGS. 1 and 2 ; 
           [0059]      FIG. 10  shows an enlarged view of the tank vent assembly in the valve assembly; and 
           [0060]      FIG. 11  shows a side section view of a meter valve assembly of a tank vent assembly according to a second embodiment of an aspect of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0061]      FIGS. 1 and 2  show partial views of a vent tank  10  in the outboard wing of an aircraft. The vent tank  10  conventionally includes a flame arrestor  11  and a NACA (National Advisory Committee for Aeronautics) duct  12 . The vent tank  10  also includes a valve assembly situated within the NACA duct  12 . The valve assembly includes a socket  13  which is bolted into place within the NACA duct. 
         [0062]    A ball portion  14  of the valve assembly is contained within the socket  13  to provide a rotatable gimble joint of the valve assembly to the vent tank  10 . The ball portion  14  has a bore including an internal circular groove  14   b . At the top of the ball portion is a slanted valve seat  14   a . A valve casing  15  is mounted above the ball portion  14  and includes an upper opening  15   a  to the inside of the vent tank  10 . 
         [0063]    Within the valve casing  15  is a moveable cap  16 . The cap  16  includes a slanted valve seat  16   a  at the bottom. This valve seat  16   a  corresponds to the slanted valve seat  14   a  of the ball portion  14  such that the moveable cap  16  can rest closed on the ball portion  14 . The moveable cap  16  is mounted on the outside of an inner valve frame  18 . The inner valve frame  18  contains gas inlet slots  19 . 
         [0064]    A spring  17  in the valve casing between a top inner surface of the casing and an upper portion of the moveable cap valve seat  16   a . Hence, the spring  17  acts to keep the moveable cap closed against the ball portion  14 . 
         [0065]      FIGS. 3   a  to  3   e  show different views of a tank vent assembly according to an embodiment of the present invention. The tank vent assembly is designed to fit in the valve assembly in the vent tank  10 , as described above. 
         [0066]    The tank vent assembly includes a probe  100  with a lower (gas outlet) section  101 , a middle section  102  and an upper (gas inlet) section  103 . 
         [0067]    At the upper section  103 , there are gas inlet slits  104  around the probe circumference. The gas inlet slits  104  extend into the inner hollow of the probe  100  which forms a gas inlet channel  107 . There is also a gas inlet bleed opening  105  at the top of the probe  100 . The gas inlet bleed opening  105  leads to a gas inlet bleed channel  106  extending down the probe. 
         [0068]    Below the gas inlet slits  104  is an upper part of a ball rack lock mechanism  50 . A ball race outer collar  54  has six rotatably mounted ball bearings  55  in its circumference. (There should, ideally, be at least three ball bearings.) The collar  54  and ball bearings  55  can be rotated with respect to an inner shaped collar  56 . In a locked position (as shown in  FIG. 6   a ), the ball bearings  55  rest in grooves of the inner collar  56 . In a locked position (shown in  FIG. 6   b ), the ball bearings rest in mini grooves on the top of spokes of the inner collar  56 . Hence, in the unlocked position, the ball bearings  55  are nearer the centre and in the locked position, the ball bearings  55  are further outwards. 
         [0069]    The outer collar  54  is connected to and can be rotated by a lever collar  52  at the lower section  101  of the probe  100 . The lever collar  52  is connected to a lever  51 . The lever  51  can be pivoted from a position flush with the probe and held in a lever slot  53  to an extended position where it can be rotated with respect to the probe  100  to rotate the lever collar  52  and outer collar  54 . 
         [0070]    When in the locked position, the ball bearings  55  are forced outwards to co-operate with the circular groove  14   b  in the ball portion  14  of the valve assembly. Hence, the probe  100  is locked in the valve assembly by rotation of the lever  51  and lever collar  52 . The probe  100  can be released by rotation of the lever  51  in the opposite direction. 
         [0071]    The tank vent assembly also has an umbrella shroud  30  having a shroud covering  31  mounted on the middle section  102  of the probe  100 . The shroud covering is shown in a collapsed position in  FIGS. 3   a  to  3   c  and in a deployed position in  FIG. 3   d . A narrow part of the shroud covering  31  is attached to a shroud connector  33  fixedly mounted on the probe  100 . The shroud connector  33  contains a bleed port  32  connected to the gas inlet channel  107 . Eight shroud arms  34   a ,  34   b  and six others (not shown) are pivotally mounted around the outer circumference of the shroud connector  33 . The other ends of the shroud arms are connected around the circumference of the widest part of the shroud covering  31 . 
         [0072]    Connecting levers  35   a ,  35   b  and six others (not shown) are pivotally connected to the shroud arms  34   a ,  34   b  etc. approximately one third of the length of the shroud arms from the connector  33 . The other ends of the connecting levers  35   a ,  35   b  etc. are connected to the outer circumference of a handle lever slider  37 , slidably mounted on the probe  100  below the shroud connector  33 . Also pivotally connected at two points on the handle lever slider  37  are the upper ends of two handle levers  36   a ,  36   b . These handle levers  36   a ,  36   b  are pivotally connected at their lower ends to two handles  38   a ,  38   b  and the two handles  38   a ,  38   b  are pivotally connected to the probe  100 . 
         [0073]    A shroud spring  39  is positioned around the probe underneath the handle lever slider  37  so as to bias the slider  37  upwards (as in  FIG. 3   d ). 
         [0074]    A cap  40  is shown in  FIG. 3   a  placed on the top of the probe  100 . The cap  40  is connected by a lanyard  41  to the outside of a valve meter assembly  60  (described later). 
         [0075]    A coaxial pipeline  20  is connected to the bottom of the probe  100 . The pipeline  20  has an inner tube  21  and an outer tube  22 . As shown in the lower figure of  FIG. 4 , the pipeline  20  also has re-inforcement indents  24 , allowing the pipeline to flex without causing damage to the pipeline  20 . 
         [0076]    As shown in  FIG. 5 , in the lower section  101  of the probe  100  is a gas outlet channel  109  and a control pressure input channel  108 . The coaxial pipeline  20  is attached to the lower section of the probe  100  so that the inner tube  21  is connected to the gas outlet channel  109  and the outer tube  22  is connected to the control pressure input channel  108 . 
         [0077]    As also shown in  FIG. 5 , a meter valve assembly  60  is mounted on the lower section  101  of the probe  100 . The meter valve assembly  60  has a shaft section  62  connected to the lower section  101  of the probe  100  and a pressure chamber section  63  connected to the shaft section  62 . A plate  64  is mounted to the other side of the pressure chamber section  63 . Two gas ports  65  are located on the exterior side of the plate  64 . The gas ports  65  allow air at atmospheric pressure into the pressure chamber section  63 . A cover  61  is mounted over the plate  64  and the gas ports  65 . The cover  61  acts to protect the gas ports  65  and the meter valve assembly  60  but also allows air to reach the gas ports  65 . 
         [0078]    The meter valve assembly  60  is shown in more detail in  FIGS. 7   a  to  7   f.    
         [0079]    A first pressure valve  70  is located in the shaft section of the meter valve assembly  60 . The first pressure valve  70  has a non-flexible diaphragm  71  slidably moveable in a chamber  72  formed in the shaft section  62 . A spring  73  is also contained in the chamber  72  between the rear of the diaphragm  71  and a disc  74  at the rear of the chamber  72 . Hence, the diaphragm  71  is biased away from the disc  74 . In its farthest position from the disc  74 , the diaphragm  71  is in its closed position abutting against a dividing wall between the gas inlet channel  107  and gas outlet channel  109 . 
         [0080]    The disc  74  has an opening  75  in the centre and a raised area around the reverse of the opening. The raised area forms a valve seat  74   a.    
         [0081]    The control pressure input channel  108  leads to a control pressure inlet  76  on the reverse side of the disc  74 . 
         [0082]    A second disc  78  is located behind the first disc  74 . The control pressure input channel  108  also leads to a second control pressure inlet  77  in between the first disc  74  and the second disc  78 . The second disc  78  has a slanted annular ring opening  79 . The slant of the opening provides a valve seat on the front side of the second disc  78 . 
         [0083]    A second pressure valve  80  is located in the pressure chamber section  63  of the meter valve assembly  60 . The second pressure valve  80  has a non-flexible diaphragm  82  slidably moveable in a chamber  81  formed in the pressure chamber section  63 . Two springs  83   a  and  83   b  are also contained in the chamber  81 ; one  83   a  between the front of the chamber  81  and the front of the diaphragm  82  and one  83   b  between the rear of the diaphragm  82  and the rear of the chamber  81 . The diaphragm  82  has a box structure  84  at its centre. The box structure  84  is connected to the spring  83  so as to bias the diaphragm  82  to a central, default position along the chamber  81 . 
         [0084]    The gas ports  65  on the rear wall  64  of the pressure chamber  81  provide atmospheric pressure to the rear side of the diaphragm  82 . The gas inlet bleed channel  106  leads to the front side of the diaphragm  82 . 
         [0085]    A piston assembly  90  is associated with the box structure  84 . A first piston  92  of the piston assembly  90  is slidably mounted with respect to the box structure  84 . The first piston  92  has a protruding stop portion  92   a  at its rear end, which is contained within the box structure  84  such that the first piston  92  cannot completely slide out of the box  84 . The first piston  92  extends out of the box  84  through an opening at the front of the box. The first piston  92  has a first piston head  93  at its front end. The first piston  92  also has a spring holding protrusion  92   b  in between the piston head  93  and protruding stop portion  92   a . A spring  94  is held between the spring holding protrusion  92   b  and the front wall of the pressure chamber  81  such as to bias the first piston  92  away from the chamber  81  into an extended position. 
         [0086]    A second piston  95  is slidably mounted within a hole through the centre of the first piston  92 . The second piston  95  has a needle head at its front end. The second piston  95  has a stop  95   a  on a section of the second piston  95  that extends out of the rear of first piston  92 . The stop  95   a  is contained within the box structure  84  such that the second piston  95  cannot completely slide out of the box  84 . A spring  96  is contained within the box  84  between the stop  92   a  of the first piston  92  and the stop  95   a  of the second piston  95 . A rear end of the second piston  95  extends out the back of the box  84  and through the plate  64  at the rear of the pressure chamber  81 . In line with the second piston  95 , behind the plate  64 , is a damper  91  that damps movement of the second piston  95 . 
         [0087]    The first piston  92  is moveable between two positions. A first position is an extended position where the front of the first piston head  93  abuts the valve seat  74   a  of the first disc  74  of the first pressure valve. In this position, the opening  75  in disc  74  is closed off and air from the first control pressure inlet  76  cannot reach the pressure chamber  72  of the first pressure valve  70  (shown in  FIGS. 7   a  and  7   e ). This causes the first pressure valve  70  to close. A second position is a retracted position where the back of the first piston head  93  abuts against the slanted opening  79  of the second disc  78 . In this position, the opening  75  in disc  74  is open and air from the first control pressure inlet  76  can reach the pressure chamber  72  of the first pressure valve  70  (shown in  FIGS. 7   b ,  7   c ,  7   d  and  7   f ). This allows the first pressure valve  70  to open. 
         [0088]    The second piston  95  is also moveable between two positions. A first position is an extended position where the needle piston head extends forwards past the piston head  93  of the first piston  92  and is located in the opening  75  of the first disc  74  of the first pressure valve. In this position air from the first control pressure inlet  76  is restricted in reaching the pressure chamber  72  of the first pressure valve  70 . Gas flow is restricted through opening  75  by the second piston  95  in this first extended position even when the first piston head  93  is not abutting the valve seat  74   a  (shown in  FIGS. 7   d  and  7   e ). A second position is a retracted position where the needle piston head of the second piston  95  is contained within the first piston  92 . In this position, second piston  95  does not affect the control of air through the various openings (shown in  FIGS. 7   a ,  7   b ,  7   c  and  7   f ). 
         [0089]      FIG. 8  shows a cross section A-A of part of  FIG. 7   a . The probe  100  has various channels for the outlet of the gas inlet bleed channel  106 , gas inlet channel  107 , control pressure input channel  108  and gas outlet channel  109  into the meter valve assembly  60 . The shaft section  62  of the meter valve assembly  60  is attached to the probe by screws  110 . 
         [0090]    In use, with reference to  FIGS. 9   a  and  10 , the cap  40  is lifted from the upper section  103  of the probe  100 . The upper end of the probe  100  is inserted into the valve assembly of the vent tank  10 . This causes the moveable cap  16  to lift upwards against the spring  17  and slide up the inner valve frame  18 . 
         [0091]    The ball race lever  51  is lifted from the lever retaining slot  53  and rotated by 15 to 30 degrees clockwise so as to rotate the lever collar  52 . This rotates the ball race outer collar  54  so the ball bearings  55  rotate. The ball bearings  55  are then retained in the mini grooves on the top of the spokes of the inner collar  56  and therefore protrude further outwards into the circular groove  14   b  in the ball portion  14  of the gimble joint. This locks the probe  100  in place in the valve assembly and mechanically suspends it from the vent tank  10 . The lever  51  can then be lowered and locked into place to prevent the probe from detaching.  FIG. 9   a  shows the probe  100  locked in place in the valve assembly. 
         [0092]      FIG. 10  shows that the probe  100  can be rotated by up to about 15 degrees with respect to the vent tank  10  due to the gimble joint formed by the ball portion  14  and socket  13  of the valve assembly on the vent tank  10 . This reduces loads being applied to the tank or wing by the probe  100 . The gimble joint  13 ,  14  is located as close to the underside skin structure as practical. 
         [0093]    The umbrella shroud  30  can then be deployed into position (shown in  FIG. 9   b ) by rotating handles  38   a ,  38   b  upwards (not shown in  FIGS. 9   a  and  9   b  for clarity). This cause handle levers  36   a ,  36   b  to be pushed upwards. This, in turn pushes up handle lever slider  37  and connecting levers  35   a ,  35   b . The connecting levers  35   a ,  35   b  push upwards on shroud levers  34   a ,  34   b  at the point of connection with the connecting levers. This causes the shroud levers  34   a ,  34   b  to rotate about the shroud connector  33  to deploy the shroud covering  31  against the underside of the vent tank  10  and covering the NACA duct  12  and flame arrestor  11  opening. The periphery of the shroud covering  31  forms an air tight seal against the tank  10  underside. The shroud  30  is designed to maintain small differential pressures but not to act as a restrictive barrier where high-pressure differentials are encountered. The umbrella shroud covering  31  captures any fuel vapour expelled from the flame arrestor  12  and these fuel vapours are drawn into the gas inlet channel  107  via bleed port  32  at the base of the umbrella shroud  30 . The umbrella shroud  30  also acts to damp out movement of the probe by reacting small loads into the wing structure. 
         [0094]    Once the probe  100  is inserted into the valve assembly, gas in the vent tank  10  can flow through the opening  15   a  in the valve outer casing  15  and into the gas inlet bleed channel  106  through the gas inlet bleed opening  105  and into the gas inlet channel  107  through the gas inlet slits  104 . This is because the moveable cap  16  has been lifted by insertion of the probe  100  such that seat  16   a  lifts off seat  14   a , allowing air to reach the gas inlet slits  19  on valve inner frame  18  and allow air up into the moveable cap  16 . 
         [0095]    Hence, in use, gas at the vent tank pressure is supplied to gas inlet channel  107  and gas inlet bleed channel  106 . 
         [0096]    In addition, a low pressure gas supply is supplied to the coaxial pipeline  20  in the inner  21  and outer  22  tubes. This low pressure is supplied from a vacuum device which is mounted to a refuel bowser or adjacent to a hydrant system. Typically, the low pressure supply is at 5 psi. 
         [0097]    Referring again to  FIGS. 7   a  to  7   f , the different configurations of the meter valve assembly  60  in different operating conditions will be described. 
         [0098]      FIG. 7   a  shows the meter valve assembly  60  in its default position before refuelling has started. The pressure chamber  81  is exposed to atmospheric pressure (approximately 15 psi) on both sides of the diaphragm  82 . On the front side of the diaphragm  82 , it is exposed to air at atmospheric pressure from the vent tank  10  through gas inlet bleed channel  106 . The vent tank  10  experiences atmospheric pressure as it is exposed to atmospheric pressure via the NACA duct  12  and flame arrestor  11  prior to insertion of the probe  100 . On the rear side of the diaphragm  82 , it is exposed to air at atmospheric pressure from the outside through gas ports  65 . Hence, the diaphragm  82  is in its central, default position. The first piston  92  is in its extended position with the piston head  93  of the first piston  92  abutting the seat  74   a  of disc  74  to close off opening  75 . The first pressure valve  70  is in its closed position with the diaphragm  71  of the first pressure valve  70  abutting the dividing wall between the gas inlet channel  107  and gas outlet channel  109 . The low pressure force on the diaphragm  71  of the first pressure valve  70  from the gas outlet channel  109  is not substantial. 
         [0099]    As the first pressure valve  70  is closed, the low pressure supply (5 psi) in the gas outlet channel  109  does not reach the vent tank  10  through the gas inlet channel  107 . 
         [0100]    In  FIG. 7   b , refuelling has started and the pressure in the vent tank  10  increases (to, say, 20 psi). This causes the pressure in gas inlet channel  107  and gas inlet bleed channel  106  to also increase (to 20 psi). The increase in pressure force on the diaphragm  71  of the first pressure valve  70  from the gas inlet channel  107  is not substantial because the area of the diaphragm  71  exposed to the gas inlet channel  107  is small compared to the area of the diaphragm  71  exposed to the gas outlet channel  109 . However, the increased pressure in the gas inlet bleed channel  106  exposes the front side of the diaphragm  82  to air at a higher pressure (20 psi) than atmospheric pressure. The rear side of the diaphragm  82  is still exposed to air at atmospheric pressure (15 psi) from the outside through gas ports  65 . Hence, the diaphragm  82  and box  84  are pushed backwards against spring  83 . The box  84  pulls the stop  92   a  of the first piston  92  backwards, causing the first piston  92  to move backwards to its retracted position with the piston head  93  of the first piston  92  abutting the slanted opening  79  in second disc  78 . This opens opening  75  in the first disc  74 . Low pressure (5 psi) coming from control pressure input channel  108  and second control pressure inlet  77  holds the first piston head  93  against the second disc  78 . However, the first pressure valve  70  remains closed so no fuel vapour can yet be drawn through the gas inlet channel  107  and gas outlet channel  109 . 
         [0101]    In  FIG. 7   c , as the opening  75  in disc  74  is now open, suction pressure (5 psi) coming from control pressure input channel  108  is also applied through the first control pressure inlet  76  and opening  75  to the rear side of the diaphragm  71  of the first pressure valve  70 . The entire rear side of the diaphragm  71  is exposed to the suction pressure (at 5 psi) from the control pressure input channel  108 , whereas only part of the front side of the diaphragm  71  is exposed to the suction pressure (at 5 psi) from gas outlet channel  109 . The gas outlet channel  109  is dimensioned such that the ratio of the area of the rear side of the diaphragm  71  to the area of the front side of the diaphragm exposed to the gas outlet channel  109  is greater than 1.75. This ensures that the suction force from the first control pressure inlet  76  on the rear of the diaphragm  71  is more than the suction force from the gas outlet channel  109  on the front of the diaphragm  71 . Hence, this ensures that the diaphragm  71  moves away from the gas outlet channel  109 , towards the first control pressure inlet  76 . This causes the diaphragm  71  to move backwards against spring  73  away from the dividing wall between the gas inlet channel  107  and gas outlet channel  109  and open the first pressure valve  70 . 
         [0102]    As the first pressure valve  70  is open, the low pressure supply (at 5 psi) in the gas outlet channel  109  does reach the vent tank  10  through the gas inlet channel  107 . Fuel vapour moves from the gas inlet channel  107  to the gas outlet channel  109 , aided by the suction applied at the gas outlet channel  109 . 
         [0103]    In  FIG. 7   d , the pressure in the vent tank  10  has been reduced (to, say, 13 psi) by the application of low pressure (at 5 psi) to the gas inlet channel  107  through the now open first pressure valve  70 . This causes the pressure in gas inlet channel  107  and gas inlet bleed channel  106  to also decrease (to approximately 13 psi). The decrease in pressure force on the diaphragm  71  of the first pressure valve  70  from the gas inlet channel  107  is not substantial. However, the decreased pressure in the gas inlet bleed channel  106  exposes the front side of the diaphragm  82  of the second pressure valve to air at a lower pressure (approximately 13 psi) than atmospheric pressure. The rear side of the diaphragm  82  is still exposed to air at atmospheric pressure from the outside through gas ports  65 . Hence, the diaphragm  82  moves forwards against spring  83 . The box  84  pushes the stop  95   a  of the second piston  95  forwards against spring  96 , causing the second piston  95  to move forwards towards its extended position, extending outside of the first piston  92 . This pushes the needle piston head of the second piston  95  into opening  75  in the first disc  74 . Hence, this reduces the area that the suction pressure can flow through and consequently reduces the suction pressure force applied to the rear of the diaphragm  71  of the first pressure valve  70 . This causes the diaphragm  71  to move back towards its closed position abutting the dividing wall of the gas inlet channel  107  and gas outlet channel  109 . This restricts the low pressure supply from the gas outlet channel  109  to the gas inlet channel  107  and therefore restricts the low pressure supply to the vent tank  10 . 
         [0104]    In  FIG. 7   e , the pressure in the vent tank  10  has been reduced further by the continued application of low pressure to the gas inlet channel  107  through the open (albeit restricted) first pressure valve  70  (to, say, 12 psi). (In particular, the pressure in the vent tank  10  would reduce rapidly if refuel was no longer taking place or where refuel is reaching its end and the fuel tank is almost full so the corresponding ullage volume is very small.) This causes the pressure in gas inlet channel  107  and gas inlet bleed channel  106  to also further decrease. The further decrease in pressure force on the diaphragm  71  of the first pressure valve  70  from the gas inlet channel  107  is not substantial. However, the further decreased pressure in the gas inlet bleed channel  106  exposes the front side of the diaphragm  82  to air at an even lower pressure. The rear side of the diaphragm  82  is still exposed to air at atmospheric pressure (at 15 psi) from the outside through gas ports  65 . Hence, the diaphragm  82  is moved further forwards against spring  83 . The box  84  pushes the stop  95   a  of the second piston  95  further forwards against spring  96 , causing the second piston  95  to move further forwards towards its extended position, extending outside of the first piston  92 . This pushes the needle piston head of the second piston  95  to be inserted further into opening  75  in the first disc  74 . Hence, this further reduces the suction pressure coming from the first control pressure inlet  76  and hence, reduces the low pressure applied to the rear of the diaphragm  71  of the first pressure valve  70 . If the pressure at the front of the diaphragm  82  of the second pressure valve  80  is low enough, the box  84  pushes the stop  95   a  of the second piston  95  further forwards against spring  96 , so as to completely compress spring  96  or so as to impart a sufficient force on the spring to unseat the first piston head  93 . At this point, further movement of the box  84  forwards, causes the spring  96  to push on the stop  92   a  of the first piston  92 , pushing the first piston  92  forwards so the first piston head  93  is unseated from the second disc  78 . (This “unseat” force needed is equal to the sum of the suction pressure force applied by the ports of the second control pressure inlet  77 . The suction pressure force for each port is the area of the port multiplied by the control pressure applied at the control pressure input channel  108 .) The first piston head  93  is pushed forwards so as to abut the seat  74   a  of the first disc  74 . This completely closes the opening  75  in the first disc  74  and therefore, shuts off suction pressure (at 5 psi) to the rear side of the diaphragm  71  of the first pressure valve  70  coming through first control pressure inlet  76  from control pressure input channel  108 . 
         [0105]    This causes the diaphragm  71  to move fully back towards its closed position abutting the dividing wall of the gas inlet channel  107  and gas outlet channel  109 . This closes the low pressure supply from the gas outlet channel  109  to the gas inlet channel  107  and therefore stops the low pressure supply to the vent tank  10 . 
         [0106]    The opening and closing of the first pressure valve  70  and second pressure valve  80  continues to control supply of low pressure to the vent tank  10 . The first  70  and second  80  pressure valves open and close based on the pressure in the vent tank  10 , received from gas inlet bleed channel  106 . 
         [0107]    Hence, it can be seen that the embodiment of the invention described has an automatic mechanical arrangement that modulates low pressure supplied to the vent tank  10  and therefore protects the wing of the aircraft and the tanks of the aircraft from over or under pressure between the tank pressure and atmospheric pressure. This reduces and/or prevents damage to and strain on the tanks and wings and means that the weight of the wings and tanks does not need to be increased to cope with the venting of the tanks by the vent tank assembly. In addition, the aircraft can be refuelled quicker as the additional back pressure in the tank resulting from refuelling is reduced. The target level of pressure reduction in the vent tank is the same as the pressure increase due to refuel. In other words, the system acts to modulate the tank pressure to be the same or similar to atmospheric pressure and therefore to overcome the rise in pressure from the internal restrictions in the refuel and vent systems. The vent tank system is not intended to “pull” the fuel through the tank but to react to the increase in pressure. 
         [0108]    In addition, the fuel vapour can be collected in a tank. The fuel vapour can simply be contained or it can be returned to liquid. The liquid fuel can either be used in power ground equipment, for example, at the airport, or re-processed for vehicle use. The containment tank used to contain the fuel vapour is fitted with a non-return valve (and could be filled with multiple non-return valves) to prevent a flame that may start in the containment tank from propagating into the vent tank  10  via the tank vent assembly. 
         [0109]    Once refuel is completed, and the pressure in the tank  10  is stable, the low pressure supply can be shut-off, the umbrella shroud  30  can be collapsed and the ball race lock mechanism  50  can be unlocked by rotating the ball race lever  51  anti-clockwise. The probe  100  can then simply be removed from the valve assembly. 
         [0110]    In use, whilst the first pressure valve  70  is open and low pressure is applied to the gas inlet channel  107 , the flame arrestor  12  is bypassed.  FIG. 7   f  represents a situation where the outer tube  22  of the coaxial pipeline is damaged (for example, being severed or by a ground fire) at rupture point  120 . 
         [0111]    The rupture point  120  exposes the outer tube  22  of the coaxial pipeline  20  to atmospheric pressure. This means that atmospheric pressure is supplied to the control pressure input channel  108 . Hence, atmospheric pressure is also supplied to the rear side of the diaphragm  71  of the first pressure valve  70  through first control pressure inlet  76  and opening  75  of the first disc  74 . This causes the diaphragm  71  to move to its closed position abutting the dividing wall between the gas inlet channel  107  and gas outlet channel  109 . This closes the first pressure valve  70  and shuts off the connection between the gas outlet channel  109  and gas inlet channel  107 . This happens regardless of the position of the first piston  92 , second piston  95  or diaphragm  82  of the second pressure valve  80 . In other words, the first pressure valve  70  is closed off regardless of the vent tank  10  pressure. 
         [0112]    Hence, it can be seen that the embodiment of the invention described has an automatic mechanical arrangement that means that the flame arrestor  11  is not bypassed in the event of damage to the pipeline  20 . This means that in the event of a fire, a flame cannot bypass the flame arrestor  12  to propagate into the vent tank  10  though the vent tank assembly. It is advantageous that this automatic cut-off function is achieved by mechanical means without the need for electronics that would otherwise have to be incorporated in close proximity to the fuel vapour. This would further increase the risk of fire or explosion. In addition, electronic components may be more prone to failure. The operation of the embodiment described is totally dependent upon the flow of the air. 
         [0113]    It is also impossible to breech the inner tube  21  of the hose without first cutting through the outer tube  22 . Furthermore, the second pressure chamber  81  surrounds the first pressure chamber  72 . Additionally, each time the device is used, this function, which is shared with the working function of the valve, is tested. If the function does not work, the device does not work in the first place. 
         [0114]    In the above example a low pressure supply of 5 psi is used. However, the low pressure supplied at the gas outlet channel  109  and/or control pressure input channel  108  simply has to be at a lower pressure than atmospheric pressure. It does not have to produce a substantial suction pressure. 
         [0115]      FIG. 11  shows a meter valve assembly of a tank vent device according to a second embodiment. In this figure and in the description below, reference numerals corresponding to similar features as in the first embodiment have been numbered to add  500  to the original numbering. I.e. feature  75  in the first embodiment is numbered as  575  in the second embodiment. 
         [0116]    A pressure valve  570  is located in a shaft section of the meter valve assembly  560 . The pressure valve  570  has a diaphragm  571  slidably moveable in a chamber  572  formed in the shaft section. A spring  573  is also contained in the chamber  572  between the rear of the diaphragm  571  and a disc  574  at the rear of the chamber  572 . Hence, the diaphragm  571  is biased away from the disc  574 . In its farthest position from the disc  574 , the diaphragm  571  is in its closed position abutting against a dividing wall between a gas inlet channel  607  and a gas outlet channel  609 . 
         [0117]    The disc  574  has an opening  575  in the centre. A control pressure input channel  608  leads to a control pressure chamber  581  on the reverse side of the disc  574 . 
         [0118]    In use, gas at the vent tank pressure is supplied to gas inlet channel  607 . In addition, a low pressure gas supply is supplied to the control pressure input channel  608 . 
         [0119]    In use, the diaphragm  571  is caused to move between an open position, allowing gas to flow from the gas inlet channel  607  to the gas outlet channel  609 , and a closed position, abutting the dividing wall of the gas inlet channel  107  and gas outlet channel  109 . This closes the low pressure supply from the gas outlet channel  609  to the gas inlet channel  607  and therefore stops the low pressure supply to the vent tank  510  (not shown). 
         [0120]    The opening and closing of the pressure valve  570  controls supply of low pressure to the vent tank  510 . The pressure valve  570  opens and closes based on the force provided by the control pressure in the control pressure chamber  581 , coming from the control pressure input channel  608 . 
         [0121]    Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described. 
         [0122]    For example, the tank that the fuel vapour is vented from may not be a vent tank. Instead, fuel vapour may be vented from the aircraft (or other vehicle) straight from the fuel tank itself. Hence, in the context of the present invention, the term “vent tank” is used to describe any tank that gas can be vented from, including a specific tank that gas first vents to from a separate fuel tank. 
         [0123]    As an alternative, the supplies at the gas outlet channel  109  and control pressure input channel  108  could be different and independent. This would likely require further safety features. 
         [0124]    As another alternative, instead of, or in addition to, the second piston  95  moving forwards when the diaphragm  82  moves forwards to reduce the area that suction pressure can flow through, the needle piston head could open an air port that connects the chamber to ambient air thus reducing the effect of the control pressure. 
         [0125]    As another alternative, the valve assembly may be manufactured and designed as part of the NACA duct. As another alternative, the valve assembly may be retro-fitted to an aircraft as part of the existing NACA duct. 
         [0126]    As an alternative use, the vent tank assembly may be used to vent gas from a automotive vehicle or any other type of vehicle or equipment with a fuel tank that requires venting. 
         [0127]    Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.