Abstract:
A reliable and economical apparatus for relieving pressure in a large aircraft cabin oxygen supply, where multiple oxygen cylinders are used concurrently. A series-parallel array of valves actuated by changes in differential pressure between the oxygen supply and the ambient cabin atmosphere provides overpressure relief The series connection of the valves reduces the risk of open-valve failures, while the parallel connection of sets of series-connected valves reduces the risk of closed-valve failures. The small number of valves used in its design reduces the cost of the apparatus. The series-parallel structure is optionally extended to larger numbers of valves to facilitate the use of less-expensive valves and supporting components without loss of reliability.

Description:
FIELD OF INVENTION  
         [0001]    This invention relates to systems for supplying breathable oxygen, and more specifically to relief valves for systems for supplying oxygen for breathing in an aircraft cabin.  
         DEFINITIONS  
         [0002]    The term “poppet” is used here to refer to a single pressure-actuated valve mechanism  
         DISCUSSION OF PRIOR ART  
         [0003]    An oxygen storage pressure relief system monitors a bank of oxygen cylinders, perhaps as many as twenty, which supply oxygen to the passenger and crew compartments of a medium size or large aircraft. The purpose of the relief system is to prevent an overpressure condition in both the lines from the oxygen cylinders and the manifold line, by opening the line under conditions of excess pressure and venting oxygen outside the aircraft cabin just until the overpressure condition is relieved. A buildup of pressure in the lines could break a line and flood the fuselage with pure oxygen causing a fire risk.  
           [0004]    In conventional oxygen storage cylinder systems, such as that described in U.S. Pat. No. 5,159,839 (Silber et al.) there is a relief valve, made up of a single poppet valve, on each pressure reducer. See FIG. 1. Such prior art systems put a system relief valve  10  between the pressure regulator of each O 2  cylinder  15  and the relief manifold line  19 , so that the single relief manifold line  19  carries oxygen for all cylinders  15 . See FIG. 1 a  for detail. Each cylinder  15  has a DOT-required pressure relief burst disc  11  which is upstream of relief valve  10 . Relief valve  10  is located at the outlet of a pressure regulator or reducer  12  which is mounted directly to the cylinder hand valve  14 . If there are twenty cylinders  15 , there are twenty relief valves  10 .  
           [0005]    See FIG. 1 b,  which shows a single operating case of the system of FIG. 1. This prior-art approach does not provide relief of overpressure when a valve  10  on a single cylinder  15  is stuck in a shut position, thereby preventing relief of overpressure in that cylinder. The stuck-shut case is a single-point failure case, in that the behavior of the system as a whole is degraded if only one failure occurs. The probability p of failure of a single relief valve may be very small, but in a prior-art system such as that in FIG. 1 with twenty oxygen cylinders all in active service, the probability of failure of any single valve is just under twenty times p. This approximate relationship is expressed in exact form as: n×(1−p) n−1 ×p, where n is the number of valves and p is the probability of failure of a valve. For low individual-cylinder failure probabilities, the relationship holds in nearly linear fashion as the number of actively-serving oxygen cylinders increases.  
           [0006]    A second failure mode of a relief valve occurs when it leaks or remains wide open, allowing the individual cylinders to bleed to zero psig. See FIG. 1 c.  When a valve  10  on a single cylinder  15  is stuck in an open position, it vents oxygen freely. Like the stuck-shut case, the stuck-open case is a single-point failure case, in that the behavior of the system as a whole is degraded if only one failure occurs.  
           [0007]    This near-linear increase in the probability of a single-point failure makes larger prior-art systems more vulnerable to frequent valve failure and its system-wide consequences. A better-designed system would display reduced frequency of valve failure, and would restrict the consequences to the system whenever any such failure occurs.  
           [0008]    Other prior-art systems, such as that described in U.S. Pat. No. 4,148,311 (London et al.) do not even address the problem of oxygen overpressure in a system with multiple oxygen cylinders as used in large aircraft. There is a clear need for an expandable, reliable, inexpensive oxygen pressure relief system for aircraft use.  
         SUMMARY  
         [0009]    The invention is a reliable and economical apparatus for relieving pressure in a large aircraft cabin oxygen supply, where multiple oxygen cylinders are used concurrently. The invention uses a series-parallel array of valves actuated by changes in differential pressure between the oxygen supply and the ambient cabin atmosphere. The series connection of its valves reduces the risk of open-valve failures, while the parallel connection of sets of series-connected valves reduces the risk of closed-valve failures. The small number of valves used in its design reduces the cost of the invention. The invention&#39;s series-parallel structure is optionally extended to larger numbers of valves to facilitate the use of less-expensive valves and supporting components without loss of reliability.  
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0010]    [0010]FIG. 1 shows a typical prior-art oxygen supply system for the cabins of a large aircraft.  
         [0011]    [0011]FIG. 1 a shows an enlargement of the hand valve, burst disc, and pressure regulator of the prior-art system of FIG. 1.  
         [0012]    [0012]FIG. 1 b  shows the prior-art system of FIG. 1 with an individual valve poppet in a stuck shut state.  
         [0013]    [0013]FIG. 1 c  shows the prior-art system of FIG. 1 with an individual valve poppet in a stuck open state.  
         [0014]    [0014]FIG. 2 shows the invention&#39;s oxygen supply system for the cabins of a large aircraft.  
         [0015]    [0015]FIG. 3 shows the invention&#39;s relief valve in schematic form.  
         [0016]    [0016]FIG. 3 a  shows the invention&#39;s system with an individual valve poppet in a stuck shut state.  
         [0017]    [0017]FIG. 3 b  shows the invention&#39;s system with an individual valve poppet in a stuck open state.  
         [0018]    [0018]FIG. 4 a  shows the invention&#39;s system with two valve poppets in the same serial valve pair in a stuck shut state.  
         [0019]    [0019]FIG. 4 b  shows the invention&#39;s system with one valve poppet in a stuck shut state and the second valve poppet in the same serial pair in a stuck open state.  
         [0020]    [0020]FIG. 4 c  shows the invention&#39;s system with two valves in opposite serial valve pairs in a stuck open state.  
         [0021]    [0021]FIG. 4 d  shows the invention&#39;s system with two valve poppets in the same serial pair in a stuck shut state, and a valve poppet in the opposite serial pair in a stuck open state.  
         [0022]    [0022]FIG. 5 shows a cross section of the invention&#39;s series pair of valve poppets.  
         [0023]    [0023]FIG. 5 a  shows an enlarged cross section of one of the invention&#39;s serial pair of valve poppets.  
         [0024]    [0024]FIG. 6 shows the parallel connection of a pair of the serial pairs of poppets of FIG. 5.  
         [0025]    [0025]FIG. 7 shows a cross section of the invention&#39;s series of valve poppets, in an alternate embodiment.  
         [0026]    [0026]FIG. 7 a  shows the alternate embodiment&#39;s oxygen supply system in schematic form. 
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0027]    For an oxygen supply system incorporating the invention, see FIG. 2. Multiple O 2  cylinders  15  are connected to a common oxygen manifold line  19  to supply breathable oxygen via line  16  to aircraft passenger and crew compartments. Each cylinder provides oxygen through a valve mechanism, illustrated in detail in FIG. 1 a,  which includes an outlet line  18  for each safety burst disc. Outlet lines  18  connect to an aircraft overboard discharge line  17 . The inlet line  20  of inventive relief valve  5  is connected to common oxygen manifold line  19  as shown, and the outlet line  21  of inventive relief valve  5  is connected to aircraft overboard discharge line  17 . The direction of flow of O 2  in lines  17 ,  18 ,  19 ,  20  and  21  is shown by the arrows on each line.  
         [0028]    For simplicity of illustration, the hand valve, burst disc and regulator assemblies on each tank are omitted from the figures beginning with FIG. 3. As shown in FIG. 3, the invention allows the concurrent use of multiple oxygen cylinders or tanks while reducing the number of valves for the relief system to one valve with four internal poppets. In FIG. 3, multiple oxygen cylinders  25  are connected to a common oxygen manifold line  29 . The invention  5  incorporates two internal sets  22 ,  23  of serial valve poppet pairs with each set in parallel with the other, forming the inventive valve. This arrangement keep the probability of failure of exactly one valve in this system at just under four times that of a single valve poppet, whether the number of oxygen cylinders is one, ten, or twenty. Arrow  60  shows the operational path for oxygen relief in this case.  
         [0029]    [0029]FIG. 3 a  shows the failure case where a single valve poppet  42  in a serial pair  22  has failed in the closed position. In this case, its companion series valve poppet  41  can open, but oxygen cannot pass through the stuck-shut valve poppet  42 . The opposite pair  23  of valve poppets  43 ,  44  then operate to provide pressure relief as needed. The system will therefore operate normally, even with a failed valve poppet in the closed position. Arrow  60  shows the operational path for oxygen relief in this case.  
         [0030]    [0030]FIG. 3 b  shows the failure case where a single valve poppet  42  has failed in the open position. In this case, its companion series valve poppet  41  can still operate correctly, and the system will operate normally through both valve poppet pairs, even with a failed valve poppet in the open position. Arrow  60  shows the operational path for oxygen relief in this case.  
         [0031]    Given four valve poppets in all, and an overall probability p of a valve poppet failing, the probability of exactly one of the four valves failing is 4×(1−p) 3 ×p. For a system with twenty oxygen cylinders, this represents a fivefold reduction in failure probability with respect to the prior-art example, with the added advantage of continued acceptable system operation during the single-poppet failure.  
         [0032]    Dual-valve failure in the prior-art system simply exacerbates the system degradation or failure. A dual-valve failure in the invention, however, still permits normal system operation in many cases. Refer to FIGS. 4 a - 4   d.  Two stuck-shut valve poppets  41 , 42  in the same serial valve poppet pair  22  (FIG. 4 a ) do not affect the operation of the two remaining valve poppets  43 ,  44  in the second serial valve poppet pair  23 . Likewise, a stuck-open valve poppet  42  and a stuck-shut valve poppet  41  in the same serial valve poppet pair  22  (FIG. 4 b ) do not affect the operation of the two remaining valve poppets  43 ,  44  in the second serial valve poppet pair  23 . Two stuck-open valve poppets  42 ,  43  in opposite serial pairs  22 ,  23  (FIG. 4 c ) still permit the remaining valve poppets  41 ,  44  in each serial pair to operate correctly. In each figure, arrow  60  shows the operational path for oxygen pressure relief.  
         [0033]    The invention sustains proper system operation even in certain triple-failure cases. In one of these cases, both valve poppets  43 ,  44  in a serial pair  23  are stuck shut, and one valve poppet  42  in the opposite pair  22  is stuck open (FIG. 4 d ). Arrow  60  shows the operational path for oxygen relief In another case (not shown), the case of FIG. 4 b  combines with a stuck-open valve poppet in the opposite serial pair, which leaves one operational valve poppet still permitting the system to operate correctly. Finally, in a last case (not shown), the case of FIG. 4 c  combines with a third stuck-open valve poppet in either of the serial pairs, which as in the previous case leaves one operational valve poppet still permitting the system to operate correctly.  
         [0034]    The invention&#39;s serial pair of individual relief valve poppets  20  is shown in FIG. 5. Each set of individual relief valve poppets  20  includes two individual relief valve poppets  41 ,  42 , each containing a piston cylinder  410 ,  420  respectively. In each piston cylinder is a piston  421 . An oxygen cylinder manifold line is connected to the series valve poppet pair at inlet opening  401 . Inlet opening  401  connects to control passage  403 , which in turn connects freely to chamber  431  of individual relief valve poppet  41  as shown. Chamber  431  connects freely to control passage  405 , which in turn connects freely to chamber  432  of individual relief valve poppet  42  as shown. Pistons  421  separate chambers  431 ,  432  from chambers  451  in cylinders  410 ,  420  respectively as shown. Helical compression springs  441  seated in chambers  451  apply pressure against faces  461  of pistons  421 . Rods  471  extend from pistons  421  into extension cylinders  419 ,  429  to block relief valve outlet openings  483  of relief passages  481 ,  482  respectively as shown, when pistons  421  are fully displaced downward away from chambers  451 .  
         [0035]    Refer to FIG. 5 a,  showing an enlargement of part of valve poppet  41  in order to identify valve poppet seals. To prevent escape of oxygen from chamber  431  to chamber  451  and the ambient air, annular seal  421   s  is disposed around piston  421 . To prevent escape of oxygen from chamber  431  to relief passage  482 , annular seal  471   s  is disposed around rod  471 . To prevent escape of oxygen from passage  481  to passage  482  and the oxygen outlet passage via valve poppet  42 , annular seal  481   s  is disposed around the end of extension cylinder  419 . The seals of valve poppet  42  are disposed similarly. To increase the reliability of each valve poppet, double seals may be used where single seals are illustrated.  
         [0036]    For the operation of both valve poppets in the series, see FIG. 5. Via control passages  403 ,  405  to both individual relief valve poppets  41 ,  42  of the series, the oxygen from inlet  401  builds up pressure against faces  411  of pistons  421  in piston cylinders  410 ,  420  respectively. The oxygen pressure is opposed both by the force of springs  441  and the pressure of ambient air in piston cylinders  410 ,  420  on the opposite faces  461  of pistons  421 . For an oxygen pressure exceeding the opposing pressure by a predetermined amount, pistons  421  rise enough to draw rods  471  upward to open passages  481 ,  482  and let excess oxygen discharge via outlet passage  490 . In the case that either of valve poppets  41 ,  42  valve fails in an open state, the series arrangement of the valve poppets keeps the system working properly. Annular valve seals, shown in black in FIG. 5 and detailed in FIG. 5 a,  prevent oxygen and air leakage in the valve poppets. The oxygen discharged via outlet passage  490  vents to the exterior of the aircraft via an overboard discharge line.  
         [0037]    As shown in FIG. 6, the invention connects two of these valve poppet pairs  22 ,  23  in parallel with each other to oxygen manifold supply line  29 . As discussed earlier, this arrangement still protects against a valve poppet failing open, and further protects against a valve poppet failing closed. If a valve poppet fails closed, that set of pistons is useless, but with the other set of pistons in parallel, the system will still work correctly.  
         [0038]    Alternative embodiments of the invention extend its series-parallel structure to incorporate three or more valve poppets in series, and three or more sets of series valve poppets in parallel. See FIG. 7. The extension to additional individual valve poppets in series is illustrated with three individual valve poppets  61 ,  62 ,  63  with interconnecting passages  605  and  682 . The extension to additional parallel sets of such series valve poppets is exemplified in FIG. 7 a,  where three sets  22 ,  23 ,  24  each with series valve poppets  61 ,  62 ,  63  are connected to provide a complete relief valve system. Such an extension is particularly advantageous when a reliable relief valve system is constructed of lower-cost components with possibly-higher individual expected failure rates. Increasing the number of valve poppets in each series improves the system&#39;s overall reliability with respect to stuck-open valve poppet failures, and increasing the number of sets of series valve poppet sets in parallel improves the system&#39;s overall reliability with respect to stuck-shut valve poppet failures.  
         [0039]    In summary, the invention&#39;s series/parallel valve poppet arrangement allows oxygen storage cylinder systems with multiple cylinders to be designed so that there is only one system of relief valve poppets with a number of relief valve poppets well below the number of cylinders in use. The invention&#39;s design enables proper oxygen relief system operation under all conditions of single-valve-poppet failure, and under many conditions of multiple-valve-poppet failure, making the system highly reliable at low cost.  
         [0040]    Conclusion, Ramifications, and Scope of Invention  
         [0041]    From the above descriptions, figures and narratives, the invention&#39;s advantages in providing reliable, inexpensive oxygen overpressure relief in an aircraft oxygen supply system should be clear.  
         [0042]    Although the description, operation and illustrative material above contain many specificities, these specificities should not be construed as limiting the scope of the invention but as merely providing illustrations and examples of some of the preferred embodiments of this invention.  
         [0043]    Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given above.