Abstract:
A sealing system for a high pressure fuel cylinder has an inner seal and an outer seal sealing an insert into a boss of the cylinder. Permeation rates as a result of the differential pressures across the seals are balanced across both the inner and outer seals to maintain a constant pressure in an intermediate space between the seals. Permeation is balanced by selecting suitable seal materials or by seal geometry or by providing a pressure relief device to the intermediate space to release excess pressure built up in the intermediate space beyond a desired intermediate pressure. Maintaining the intermediate pressure lower than the cylinder pressure and higher than atmospheric pressure results in lower pressure differentials across the seals, extending the seal life. A pressure switch or gauge is provided to monitor the pressure in the intermediate space. Changes in the pressure are indicative of a leak in one or both of the seals.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a regular application claiming priority of U.S. Provisional Patent application Ser. No. 60/564,605 filed on Apr. 23, 2004, the entirety of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     Embodiments of the invention relate to sealing systems for pressure vessels and more particularly, to sealing systems for high pressure fuel cylinders and additionally, incorporating means for maintaining and monitoring seal integrity.  
       BACKGROUND OF THE INVENTION  
       [0003]     High pressure cylinders are known typically for carrying and supplying alternative fuels to vehicles such as compressed natural gas (CNG) vehicles and for hydrogen fuel cells. Due to the rigid safety requirements for such vessels, it is desirable to provide systems for sealing the boss or neck of the vessel that are robust and capable of maintaining the high pressure in the cylinders without fear of leaking.  
         [0004]     Typically, conventional cylinders are sealed at an outer aspect of the boss using a face seal to provide sealing between an insert, such as a plug or a valve assembly, and the cylinder. The seal is rated to be able to contain at least the maximum pressure in the cylinder. Should the seal fail however, the contents of the vessel would be vented to atmosphere causing a loss of fuel from the cylinder and creating a combustion hazard, particularly if the vehicle is in an enclosed structure such as a garage. Failure of the seal is typically without warning and may result in a situation where a vehicle is stranded without sufficient fuel. Most often the operator is unaware of such a leak or failure of the seal until after a significant portion of the fuel within the cylinder has already escaped.  
         [0005]     In an incident reported in the media, Toyota recalled a number of fuel cell vehicles as a result of leaking. Applicant believes that it was determined to be a leak at the valve-cylinder interface and when examined resulted due to failure of an O-ring.  
         [0006]     Conventional seals used in high pressure fuel cylinders are subjected to large pressure differentials across the seal between the high pressure interior of the cylinder, as high as 700 bar (10,000 psi) for hydrogen storage cylinders, and atmosphere. The high pressure differential acts to reduce the life of the seal, despite using highly effective sealing materials.  
         [0007]     The automotive industry has attempted to provide more reliable sealing by utilizing different seal materials, seal types and seal gland configurations. Further, tapered threads combined with sealing paste or tape has been used to prevent leaking however, Applicant believes this induces additional circumferential stress in the cylinder neck.  
         [0008]     Ideally, a sealing system for high pressure fuel cylinders is capable of withstanding large pressure differentials for extended lifetimes without need for frequent replacement and more preferably is equipped with a means for detecting seal failure before the contents of the cylinder are vented.  
       SUMMARY OF THE INVENTION  
       [0009]     In one embodiment a unique apparatus, method and system for sealing a high pressure from a lower pressure, such as a high pressure fuel cylinder from atmospheric pressure, utilizes an inner seal spaced from an outer seal by an intermediate space. An intermediate pressure is maintained in the intermediate space to reduce the pressure differential across both the inner seal and the outer seal and thus extend seal life.  
         [0010]     Typically, the intermediate space is formed about an insert fit within a boss of the high pressure fuel cylinders and the inner and outer seals seal between the insert and the cylinder boss.  
         [0011]     In another embodiment a means for monitoring the intermediate pressure is fluidly connected to the intermediate space to permit detection of changes in the intermediate pressure, which are indicative of a leak in either or both the inner and outer seal.  
         [0012]     Thus, in one broad aspect of embodiments of the invention a method for sealing a fluid at a first high pressure from a second lower pressure comprises: providing an inner seal capable of sealing the fluid at the first high pressure; providing an outer seal capable of sealing the fluid at the first high pressure, the outer seal being spaced from the inner seal for forming an intermediate space therebetween, the intermediate space having an intermediate pressure being lower than the first high pressure and higher than the second lower pressure; and providing means for maintaining the intermediate pressure in the intermediate space for reducing a pressure differential at the inner seal.  
         [0013]     In another broad aspect of embodiments of the invention, apparatus for sealing a fluid at a first high pressure from a second lower pressure comprises: an inner seal capable of sealing the fluid at the first high pressure; an outer seal capable of sealing the fluid at the first high pressure, the outer seal being spaced from the inner seal for forming an intermediate space therebetween, the intermediate space having an intermediate pressure being lower than the first high pressure and higher than the second lower pressure; and means for maintaining the intermediate pressure in the intermediate space for reducing a pressure differential at the inner seal.  
         [0014]     Further, in another broad aspect of embodiments of the invention a system adapted for sealing a boss in a high pressure cylinder and for indicating the integrity of said sealing comprises: an insert adapted to fit within the boss; an inner seal, adapted to be positioned between the insert and the boss, and capable of sealing the fluid at the first high pressure; an outer seal, adapted to be positioned between the insert and the boss, and capable of sealing the fluid at the first high pressure, the outer seal being spaced from the inner seal for forming an intermediate space therebetween, the intermediate space having an intermediate pressure being lower than the first high pressure and higher than the second lower pressure; means for maintaining the intermediate pressure in the intermediate space for reducing a pressure differential at the inner seal; and means for monitoring the intermediate pressure fluidly connected to the intermediate space for detecting a change in the intermediate pressure being indicative of a lack of integrity of the inner seal, the outer seal or both.  
         [0015]     Preferably the intermediate pressure is maintained by balancing an inflow and outflow from the intermediate space, typically as a result of permeation across and around the inner and outer seals. Permeation is balanced by selecting a seal material for the inner and outer seals and the differential pressure, by selecting a seal geometry for the inner and outer seals or by providing a pressure relief device fluidly connected to the intermediate space for releasing pressure therefrom.  
         [0016]     The means for monitoring the pressure in the intermediate space may be any suitable pressure monitoring means such as a pressure switch or a mechanical pressure gauge fluidly connected to the intermediate space. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a partial sectional view of an embodiment of the present invention illustrating an inner seal and an outer seal between an insert and a cylinder boss and having an optional monitoring port for determining integrity of the seals;  
         [0018]      FIGS. 2   a - 2   c  are dimensional sectional views according to  FIG. 1  illustrating machining of the cylinder boss to accommodate an insert using the sealing system of an embodiment of the present invention and more particularly,  
         [0019]      FIG. 2   a  is a section view of the cylinder boss;  
         [0020]      FIG. 2   b  is a detailed section view of an outer valve seat; and  
         [0021]      FIG. 2   c  is a detailed view of a taper or chamfer adjacent an inner sealing surface to permit insertion of the insert into sealing engagement with the cylinder boss; and  
         [0022]      FIGS. 3   a - 3   c  illustrate a plug insert adapted for insertion into the cylinder boss according to  FIGS. 2   a - 2   c  and more particularly,  
         [0023]      FIG. 3   a  is a side view of the plug insert;  
         [0024]      FIG. 3   b  is a detailed sectional view of an annular groove adjacent a bottom end of the plug insert for accommodating the inner seal and a backup-ring;  
         [0025]      FIG. 3   c  is a plan view of a top of the plug insert;  
         [0026]      FIG. 3   d  is a partial sectional view of an insert illustrating parameters for calculation of a barrier thickness;  
         [0027]      FIG. 4  is a schematic illustrating the relationship between permeation and differential pressure between inner and outer seals; and  
         [0028]      FIG. 5  is a schematic illustrating a sectional view of an insert according to an embodiment of the invention and pressures at and between seals between the insert and a structure containing a fluid at a high pressure. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0029]     Embodiments of the invention are described herein in the context of high pressure cylinders used to fuel vehicles. One of skill in the art would understand that the sealing arrangement described herein is applicable to any situation wherein gases are stored in vessels at high pressure.  
         [0030]     Having reference to  FIGS. 1-5 , a system  1  for reliably sealing a high pressure cylinder  2  is shown. The system  1  comprises an inner seal  10 , exposed to a first high pressure P op  and an outer seal  11 , exposed to a second lower pressure, such as atmospheric pressure P atm , each seal  10 , 11  being capable of containing the desired operating pressure or first high pressure P op  of the cylinder  2  for a gas of interest, including, but not limited to, compressed natural gas and compressed hydrogen. While discussed herein in the context of separating a high pressure from atmospheric pressure, embodiments of the invention are applicable to any arrangement separating a high pressure from a lower pressure.  
         [0031]     Having reference to  FIGS. 1, 4  and  5 , the inner and outer seals  10 , 11  are selected to maintain an intermediate pressure P m , which is lower than the first high pressure P op  of the cylinder  2 , in an intermediate space  12 , between the inner and the outer seals  10 , 11 . One effect of the intermediate pressure P m  is to reduce the pressure differential ΔP (P op −P m ) at the inner seal  10 , prolonging the life of the inner seal  10 .  
         [0032]     Seal permeability is a factor to be considered when attempting to maintain the pressure P m  between the inner and outer seals  10 , 11 . Seal permeability is dependant primarily on two factors, a material from which the seal  10 , 11  is made and a geometry of the seal  10 , 11  itself. As shown in Tables A and B, reproduced in part, respectively, from Peacock, R. N. “Practical selection of elastomer materials for vacuum seals.” Journal of Vacuum Science Technology Vol. 17 No. 1 (January/February 1980):330-336 and Parker Seals, Parker O-Ring Handbook, Table 3-19, Gas Permeability Rates, Pages 3-27-3-35, Parker Hannifin Corporation, 2360 Palumbo Drive, Lexington Ky. 40509 USA, the entirety of which are incorporated herein by reference, different elastomers have different gas permeability rates for different fuel types.  
                                                             TABLE A                           Helium   Nitrogen   Oxygen   Carbon Dioxide   Water       Polymer   (K × 10 8 )   (K × 10 8 )   (K × 10 8 )   (K × 10 8 )   (K × 10 8 )                                Fluoroelastomer    9-16   0.05-0.3    1.0-1.1   5.8-6.0   40       Buna-N   5.2-6     0.2-2.0   0.7-6.0   5.7-48    760       Buna-S   18   4.8-5     13   94   1800       Neoprene   10-11   0.8-1.2   3-4   19-20   1400       Butyl   5.2-8     0.24-0.35   1.0-1.3     4-5.2   30-150       Polyurethane   —   0.4-1.1   1.1-3.6   10-30   260-9500       Propyl   —   7   20   90   —       Silicone   —   —    76-460    460-2300   8000       TEFLON ™   —   0.14   0.04   0.12   27       KEL-T ™   —   0.004-0.3    0.02-0.7    0.04-1     —       Polyimide   1.9   0.03   0.1   0.2   —                  
 
         [0033]     K=permeation constant in sccms −1 cm −2  cmatm −1   
                                                     TABLE B                               Tem-                       pera-               Gas or       ture   Temper-   Permeability       Liquid   Elastomer   ° C.   ature ° F.   (1)                                Acetylene   Butyl   25   77   1.26       Acetylene   Butyl   50   122   5.74       Acetylene   Natural   25   77   74.5       Acetylene   Natural   50   122   192       Acetylene   Nitrile   25   77   18.7       Acetylene   Nitrile   50   122   67.4       Hydrogen   Butadiene   25   77   31.6       Hydrogen   Butadiene   50   122   76       Hydrogen   Butyl (B0318-70)   35   95   16.1       Hydrogen   Butyl (B0318-70)   82   180   68.2       Hydrogen   Butyl (B0318-70)   124   255   273       Hydrogen   Ethylene Propylene   38   100   28.9-111        Hydrogen   Ethylene Propylene   40   104   111       Hydrogen   Ethylene Propylene   38   100   45.3           (E0529-65)       Hydrogen   Ethylene Propylene   93   200   187-544           (E0529-75)       Hydrogen   Ethylene Propylene   94   202   544       Hydrogen   Ethylene Propylene   94   201   252           (E0529-65)       Hydrogen   Ethylene Propylene   152   306    599-1730           (E0529-75)       Hydrogen   Ethylene Propylene   155   311   1730       Hydrogen   Ethylene Propylene   151   304   591           (E0529-65)       Hydrogen   Ethylene Propylene   93   200   160           (E0529-75)       Hydrogen   Fluorocarbon-Viton ™   38   100   180       Hydrogen   Neoprene   38   100   10.3-32.1       Hydrogen   Nitrile   39   103   11.9       Hydrogen   Niltrile (N0741-75)   79   175   47.0-125        Hydrogen   Nitrile   80   176   88.2       Hydrogen   Niltrile (N0741-75)   121   250   98.8-330        Hydrogen   Polyacrylate (A0607-70)   38   100   49.6       Hydrogen   Polyacrylate (A0607-70)   91   195   174       Hydrogen   Polyacrylate (A0607-70)   153   307   927       Hydrogen   Polysulfide   25   77   102       Hydrogen   Polyurethane (P0642-   39   103   19.3           70)       Hydrogen   Polyurethane (P0642-   39   102   4.89           90)       Hydrogen   Polyurethane (P0642-   66   151   70.4           70)       Hydrogen   Polyurethane (P0642-   67   152   21.3           90)       Hydrogen   Polyurethane (P0642-   94   202   155           70)       Hydrogen   SBR   25   77   30.1       Hydrogen   SBR (G0244-70)   38   101   46.2       Hydrogen   SBR (G0244-70)   84   183   245       Hydrogen   SBR (G0244-70)   122   251   539       Hydrogen   Silicone   Room       1880-488        Hydrogen   Silicone   25   77   495       Hydrogen   Silicone (S0684-70)   39   103   1010       Hydrogen   Silicone   93   200   1570-2070       Hydrogen   Silicone (S0684-70)   91   195   2070       Hydrogen   Silicone   149   300   3300-8760       Hydrogen   Silicone (S0684-70)   156   313   4300       Hydrogen   FEP PTFE   −74   −101   .0113       Hydrogen   FEP PTFE   −46   −51   .180       Hydrogen   FEP PTFE   −18   0   1.05       Hydrogen   FEP PTFE   10   50   3.90       Hydrogen   FEP PTFE   25   77   9.89       Hydrogen   FEP PTFE   38   100   10.1       Hydrogen   FEP PTFE   50   122   24.7       Hydrogen   FEP PTFE   66   151   22.5       Hydrogen   FEP PTFE   75   167   49.5       Hydrogen   FEP PTFE   100   212   89.9       Hydrogen   FEP PTFE   25   77   17.8       Hydrogen   FEP PTFE   30   86   42.0       Hydrogen   FEP PTFE   50   122   63.8       Methane   Butadiene   25   77   9.77       Methane   Butyl   25   77   .56       Methane   Fluorocarbon   30   86   .12       Methane   Natural   25   77   22.7       Methane   Neoprene   25   77   2.6       Methane   Nitrile   25   77   2.4       Methane   Silicone   25   77   705       Methane   Silicone   30   86   443       Methane   FEP PTFE   25   77   .702-.83        Methane   FEP PTFE   30   86   1.05       Methane   FEP PTFE   50   122   2.02       Methane   FEP PTFE   75   167   4.50       Methane   FEP PTFE   100   212   8.99       Methane   FEP PTFE   30   86   1.13       Methane   FEP PTFE   50   122   3.0       Propane   Butadiene   25   77     22-40.5       Propane   Butyl   25   77   1.28       Propane   Natural   25   77   126       Propane   Neoprene   25   77   5.4       Propane   Polysulfide   25   77   1.09       Propane   Silicone   25   77   3080                  
 
         [0034]     Std cc cm/cm 2 sec.bar  
         [0035]     Further, as demonstrated in the following formulas, permeation can be calculated dependant upon the material selected or upon the geometry or size of the barrier presented by the seal  10 ,  11 , for a particular material. Permeation is defined as the passage of a gas under pressure into, through and out a solid material by diffusion and solution to the low pressure side. Assuming an equilibrium state, the rate of gas permeation can be calculated using the following formula:  
       Q   =     KA   ⁢       (       P   1     -     P   2       )     d           
 
 where, 
        K=permeation constant  
       [         cm   3     ·   cm       s   ·     cm   2     ·   atm       ]       
    A=area of the barrier [cm 3 ]    P 1 =high side pressure [atm]    P 2 =low side pressure [atm]    d=barrier thickness [cm]       
 
         [0041]     For example, if the gas to be contained is helium, the permeation rate from a 2.000″ port, on a 350 bar cylinder using a nitrile (Buna-N) seal, can be estimated using the following values in the above equation:  
         [0042]     The average permeation constant for helium gas through nitrile (Buna-N):  
       K   =     5.6   ×       10     -   8       ⁡     [         cm   3     ·   cm       s   ·     cm   2     ·   atm       ]             
 
         [0043]     The area of the barrier: 
 
 A=π·D·t=π· 5.44·0.27=4.6 cm 2  
 
         [0044]     The high and low side pressures: 
        P 1 =345 atm     P 2=1  atm        
 
         [0047]     The barrier thickness, as shown in  FIG. 3   d:  
        d=0.36 cm        
 
         [0049]     Substituting the above values into the permeation rate equation yields:  
       Q   =       5.6   ×         10     -   8       ⁡     [         cm   3     ·   cm       s   ·     cm   2     ·   atm       ]       ·       4.6   ⁢           ⁢     cm   2     ⁢     (     345   -   1     )     ⁢           ⁢   atm       0.36   ⁢           ⁢   cm           ⁢     
     ⁢           =       2.46   ×     10     -   4       ⁢       cm   3     s       =     0.88   ⁢           ⁢       cm   3     hr               
 
         [0050]     In a preferred embodiment of the invention, for use in a 700 bar (10,000 psi) cylinder  2 , the inner and outer seals  10 , 11  are selected or configured to maintain a maximum intermediate pressure P m  of 350 bar (5000 psi) in the intermediate space  12  therebetween, thus significantly reducing the pressure differential ΔP at the inner seal  10 . Each of the seals  10 , 11  is selected to be capable of containing the full operating pressure or first high pressure P op  of 700 bar (10,000 psi) so that in the event of a failure of the inner seal  10 , the contents of the cylinder  2  are not vented to atmosphere. The inner and outer seals  10 , 11  however, are selected so that the permeation across the inner seal  10  is compensated for or balanced by the permeation at the outer seal  11 , effectively maintaining the lower intermediate pressure P m  therebetween.  
         [0051]     Optionally, as shown in  FIG. 5 , a small orifice  200  may be provided from the intermediate space  12  to atmosphere and possibly fit with a pressure relief device  201  to permit a controlled release of pressure between the seals  10 , 11  to maintain the desired intermediate pressure P m  therebetween.  
         [0052]     As shown in  FIGS. 1, 2   a - 2   c  and  3   a - 3   c , typically, the inner seal is a circumferential seal  10 , the circumferential seal  10  positioned for sealing between an insert  100 , threaded into a boss  101  of the cylinder  2 , and the boss  101 . Preferably, the circumferential seal  10  is fit within an annular groove  102  in the insert  100  for sealing against a finished sealing surface  103  of the cylinder boss  101 . The cylinder boss  101  is machined to provide the suitable sealing surface  103  to prevent any leaking due to poor sealing therebetween. The outer seal  111  is spaced from the inner seal  10  and is preferably a compression seal  11  in sealing arrangement between an end  104  of the boss  101  and a top  105  of the insert,  100 . Configuration of the seals  10 , 11  is not critical to embodiments of the invention disclosed herein and therefore both inner and outer seals  10 , 11  may be circumferential seals, compressions seals or the like.  
         [0053]     Preferably, a backup ring  106  is positioned adjacent the inner seal  10  and in the annular groove  102 . The backup ring  106  is typically manufactured from a material, such as nitrile, having a greater durometer rating than the inner seal  10  so as to provide a surface against which the seal  10  may be compressed and to prevent extrusion of the seal  10  from the annular groove  102 . The backup ring  106  may be a split ring or a deformable ring.  
         [0054]     In the preferred embodiment, as shown in  FIG. 1 , a monitoring port  110  is provided having access and being fluidly connected to the intermediate space  12  between the seals  10 , 11 . The monitoring port  110  is used to house instrumentation for monitoring the integrity of the inner and outer seals  10 , 11 . Advantageously, due to the intermediate pressure P m  between the inner and outer seals  10 , 11  being maintained at a pressure lower than the first high pressure P op  in the cylinder  2 , should the inner seal  10  leak or fail completely, the intermediate pressure P m  between the inner and the outer seals  10 , 11  will exhibit a measurable change in pressure and rise to be in equilibrium with the first high pressure POP inside the cylinder  2 , the rise being detectable at the monitoring port  110 . Preferably, means for monitoring the pressure (not shown), such as a pressure switch, a mechanical pressure gauge or other pressure indicator is fluidly connected to the monitoring port  110  for continually monitoring the intermediate pressure P m  between the inner and outer seals  10 , 11 .  
         [0055]     Further, if for some reason the outer seal  11  should fail, rather than the inner seal  10 , the change or drop in the intermediate pressure P m  would be observed and signal a need for service. In both cases, the redundancy in the sealing arrangement would allow the cylinder  2 , and ultimately, a vehicle, (not shown) to which the cylinder  2  is supplying fuel, to remain in use until it could be removed from service and the inner and outer seals  10 , 11  replaced.  
         [0056]     Optionally, additional apparatus (not shown) such as a burst disc, an on-off valve, gas sensors, flow restrictors or regulators, pressure regulators, check valves or gas filters, may be fit within the monitoring port  110 .  
         [0057]     In one embodiment of the invention, wherein the insert  100  and the cylinder boss  101  are manufactured using aluminum, threads  120  used for threading the insert  100  into the boss  101  are not self-centering to avoid sharp edges which may result in the insert  100  galling to the boss  101  during installation or removal. Further, the sealing surface  103  is polished to remove any spiral or radial tool marks, scratches or gouges which would impair sealing thereto.  
         [0058]     An additional advantage of positioning the inner seal  10  into the boss  101  of the cylinder  2  is realized during manufacturing high pressure cylinders  2  which undergo autofrettage as part of the manufacturing process. Autofrettage pressures have the potential to cause deformation of the boss  101  of the cylinder  2 , thus, positioning a seal  10  at an inner surface adjacent the containment portion of the cylinder  2  acts to protect the boss  101  from the high pressure P op , preventing costly rework of the boss  101  or rendering the cylinder  2  defective.