Abstract:
A pressure vessel is provided having a wall with an inner surface defining a chamber about an axis and an outer surface. The wall has a thickness with at least one vent hole therein located substantially parallel with the axis. The at least one hole is located a distance from the axis, the distance providing an outer thickness of the wall between the at least one hole and the outer surface sufficient to withstand a stress generated by pressure within the pressure vessel. An inner thickness of the wall between the at least one hole and the inner surface to permit a crack to propagate from the inner surface and connect with the hole causing venting of the pressure in the chamber.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Pressure vessels used to contain high pressures, i.e., generally greater than 10,000 psi, have been used for many years in many industries. Examples are shown in U.S. Pat. Nos. 3,762,448 and 6,154,946. Most high pressure commercial applications use vessels that operate at pressures no greater than about 60,000 psi. Some of these vessels, such as those used in intensifiers pumps used in water jet cutting and in polyethylene processing extruding equipment, are subjected to high cycle fatigue loading that can lead to fatigue failure.  
           [0002]    Fatigue failure is a progressive mode of failure that occurs when stresses or strains that will not cause failure in a single application are applied by repeated loading and unloading. The failure proceeds by the initiation of a fatigue crack that occurs microscopically, followed by some crack propagation or growth until the crack obtains a sufficient size such that the structure ruptures.  
           [0003]    Fatigue cracks can propagate in fatigue by several mechanisms and under the influence of several loading modes. However, the most damaging failures generally occur when a fracture separation of one or more portions of a vessel occurs causing pieces to be launched with extremely high levels of kinetic energy capable of causing severe bodily injury or property damage.  
           [0004]    In attempting to alleviate the fatigue problems encountered in subjecting thick-walled cylinders to alternating or fluctuating, internally acting high pressures, there has been developed a process for enhancing the strength of thick-walled cylinders which are subjected to repeated internal pressures. This process is commonly referred to as “autofrettage” and involves the application of such interior pressure to the bore of the cylinder so as to plastically deform at least the inner layers of the cylinder material beyond the elastic limit or yield strength of the material and to thereby generate “negative” or residual tangential compressive stresses at the cylinder bore. These residual stresses are imparted to the inner bore surface to counteract the destructive effects of the internal cyclical or intermittent high operating pressures to which the cylinder is subjected to extend the service life of the cylinder. Exemplary patents in this regard are U.S. Pat. Nos. 4,571,969 and 4,417,459. These and other autofrettage processes suffer from drawbacks, however, which include the high equipment and manufacturing costs required to perform an autofrettage process.  
           [0005]    As an alternative to autofrettage, a process known as “compounding” for manufacturing a two part compound cylinder having improved fatigue resistance. This compounding process includes assembling together an inner sleeve and an outer sleeve by cooling the inner sleeve, heating the outer sleeve, or both, prior to assembling the inner sleeve into the outer sleeve (along with any additional intermediate layers) to achieve an interference fit. The severity and the length of the heating/cooling cycles needed are determined by the amount of interference required with higher intereferences being required for higher operating internal pressures.  
           [0006]    In order to facilitate venting of internal bore pressure should a fatigue crack initiate and propagate radially outward through the inner sleeve, preferably, a helical groove was also provided around the outer surface of the inner sleeve prior to compounding. Compound cylinders provided with this helical groove configuration are currently provided on Model SL-III Phased Intensifiers available from Ingersoll-Rand Company through its Waterjet Cutting Division located in Baxter Springs, Kans. Although, effective, compound cylinders manufactured in this manner are difficult and costly to produce given they require numerous controlled manufacturing steps both for producing an interference fit and machining a helical groove.  
           [0007]    The foregoing illustrates limitations known to exist in pressure vessels and their manufacture. Thus it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly a pressure vessel is provided including the features more fully disclosed hereinafter.  
         SUMMARY OF THE INVENTION  
         [0008]    According to the present invention, a pressure vessel is provided having a wall with an inner surface defining a chamber about an axis and an outer surface. The wall has a thickness with at least one vent hole therein located substantially parallel with the axis. The at least one hole is located a distance from the axis, the distance providing an outer thickness of the wall between the at least one hole and the outer surface sufficient to withstand a stress generated by pressure within the pressure vessel. An inner thickness of the wall between the at least one hole and the inner surface permits a crack to propagate from the inner surface and connect with the hole causing venting of the pressure in the chamber.  
           [0009]    The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with accompanying drawing figures. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    These and other advantages of the present invention will become more readily apparent upon reading the following detailed description and upon reference to the drawings in which:  
         [0011]    [0011]FIG. 1 is an isometric view of a conventional pressure vessel having end plates;  
         [0012]    [0012]FIG. 2 is a cross-sectional view of the pressure vessel shown in FIG. 1 taken along the sectional line designated “ 2 - 2 ”;  
         [0013]    [0013]FIG. 3 is an isometric view of a pressure vessel according to one embodiment of the present invention;  
         [0014]    [0014]FIG. 4 is a cross-sectional view of the pressure vessel shown in FIG. 3 taken along the sectional line designated “ 4 - 4 ”;  
         [0015]    [0015]FIG. 5A is a cross-sectional schematic view of a cylindrical pressure vessel having a longitudinal bore;  
         [0016]    [0016]FIG. 5B is a cross-sectional view taken along the sectional line designated “ 5 B- 5 B” in FIG. 5A;  
         [0017]    [0017]FIG. 6 is an isometric view of a pressure vessel according to another embodiment of the present invention;  
         [0018]    [0018]FIG. 7 is a cross-sectional view of the pressure vessel shown in FIG. 6 taken along the sectional line designated “ 7 - 7 ”;  
         [0019]    [0019]FIG. 8 is a cross-sectional view of the pressure vessel shown in FIG. 6 taken along the sectional line designated “ 8 - 8 ”;  
         [0020]    [0020]FIG. 9 is a pressure vessel according to the present invention configured for use with associated waterjet cutting and machining apparatus; and  
         [0021]    [0021]FIG. 10 is a pressure vessel according to the present invention configured for use with associated waterjet cutting and machining apparatus. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    As used herein, the term “monobloc” or “monoblock” means a vessel made out of a single continuous piece of material.  
         [0023]    The invention is best understood by reference to the accompanying drawings in which like reference numbers refer to like parts. It is emphasized that, according to common practice, the various dimensions and the associated parts as shown in the drawings are not to scale and have been enlarged for clarity.  
         [0024]    Referring now to the drawings, shown in FIGS. 1 and 2 are isometric and cross-sectional views, respectively, of a conventional pressure vessel  10  in the form of a monobloc cylinder having an inner bore  15  located axially through its length. Shown on either end of pressure vessel  10  are attachable end plates  20  with each having a bore  25  located through its thickness. End plates may be attached using threaded fasteners (not shown) as is known in the art. For these conventional pressure vessels  10 , internal cyclic loading caused by increasing and decreasing internal pressures in inner bore  15 , which are high enough, can lead to typical radial crack development. With typical crack growth, a crack initiates and propagates through the wall of pressure vessel  10  and vents to atmosphere resulting only in the loss of fluid from inner bore  15  and the capability to maintain pressure. Occasionally, however, atypical crack growth can occur with these pressure vessels in which cracks initiate radially from inner bore  15  and, prior to reaching the outer surface of the pressure vessel, propagate internally to separate a large portion of pressure vessel material prior to venting. As discussed above, this separation of material prior to venting can lead to catastrophic damage both to persons and surrounding objects.  
         [0025]    The present inventor has discovered that atypical crack growth leads to such catastrophic failure when cracks grow to separate wall material of the pressure vessel such that force exerted by the internal pressure of inner bore  15  exceeds the strength of the portion of the wall material that remains intact. According to the present invention and, as described further in detail below, venting holes are provided along the length of the pressure vessel to alleviate catastrophic failures caused by atypical crack growth.  
         [0026]    According to the present invention, shown in FIGS. 3 and 4 are isometric and cross-sectional views respectively, of a pressure vessel 40 in the form of a monobloc cylinder having end faces  46 ,  47  and an inner bore 45 located axially through its length. Located longitudinally within the wall of pressure vessel 40 is at least one vent  41  which, preferably, in one embodiment is in the form of at least one bore hole that extends from one end face  46  to the other end face  47  so that the end faces are in fluid communication. Most preferably a plurality of vents  41  are provided in a clocked fashion as shown in FIGS. 3 and 4. Although shown with four vents, it is understood that the invention is not so limited and that any number of vents may be incorporated into pressure vessel 40, provided. Preferably, for enhanced safety, at least three and more preferably four or more holes are incorporated to ensure that the crack will intercept at least one independently of the crack direction growth and relative to the safety criteria considered discussed in greater detail below. The at least one vent  41  is located in a position which does not induce unwanted stresses or otherwise weaken the vessel but which is located close enough to the inner bore 45 to intersect a propagating crack thereby permitting venting of the internal pressure held within pressure vessel 40 prior to catastrophic failure. Using the geometry of the pressure vessel and the yield stress of the wall material, the maximum distance from the longitudinal axis from which the vents  41  are to be located is determined to ensure that the area of attached material being cleaved by crack can withstand the force exerted by the fluid within the pressure vessel until venting occurs. In the case of a cylindrical pressure vessel 40 made of a ductile material (i.e., a material capable of at least 12% elongation), this may be derived as follows. Turning to the cylindrical cross-sectional schematic view in FIG. 5A, pressure vessel 40 having a longitudinal bore 45 is shown attached to closed ends having bores 25. Upon the initiation of a crack  42  in pressure vessel 40, cross-sectional view in FIG. 5B taken along the cross-section of the crack shows the various diameters defined as follows:  
                                                   D v  = Diameter of Circle Defining Vent Hole Location(s)           d = Diameter of Bore 25           D I  = Inner Diameter defined by Inner Bore 45           D o  = Outer Diameter of Pressure Vessel 40           p = Pressure of Fluid in Inner Bore 45           σ y  = Yield Stress of Material of Pressure Vessel 40                      
 
         [0027]    Thus, in order to ensure venting before a crack propagates to catastrophic failure, the resultant stress exerted by the force (F) caused by the internal pressure (p) acting on the remaining area of material (A) of pressure vessel 40 (i.e., the material bounded between D v  and D o  that has not been cleaved by crack  42 ) must be equal to or, preferably, less than the yield stress (σ y ) of the material of the pressure vessel. Thus, the diameter (D v ) of the cylinder  44  on which vents  41  are most preferably to be located is derived as follows:  
                       Equation  1:                     σ   y       =     F   A                     Equation 2:                      σ   y       =       p   ·     π   4     ·     (       D   y   2     -     d   2       )           π   4     ·     (       D                   o   2       -     D   v   2       )                         Equation 3:                     D   v       =         (           σ   y     ·   D                     o   2       +     p   ·     d   2         )       (       σ   y     +   p     )                                                       
 
         [0028]    Thus, for a cylindrical pressure vessel having the following criteria, Equation 3 yields a critical diameter (D v ) of 3.135 inches:  
                                                   d = 0.188 inches           D I  = 1.125 inches           D o  = 3.81 inches           p = 55,000 psi           σ y  = Yield Stress of Material of Pressure Vessel           D v  =0 3.135 inches                      
 
         [0029]    It is to be noted that the diameter (D v ) defines a cylinder  44  for optimally locating vents  41  as this location maximizes the amount of integral wall material surrounding inner bore 45 available to withstand pressure (p). Preferably, vents  41  are centered along the wall of cylinder  44 , although all that is minimally required is that the vents intersect this cylinder such that a crack  42  will connect with and vent internal pressure (p) through the vents before reaching a critical crack size (i.e., one that exposes the remaining attached material to exceed the yield stress of the material).  
         [0030]    It will be recognized by those skilled in the art that although discussed above with respect to locating the vent holes along a cylinder in which a crack has grown to a size which will cause the material to yield, more conservative design parameters may be desirable. For instance, depending on the specific application and use of a pressure vessel, an additional safety factor may be used to cause a leak to occur a percentage before the crack can cause yield. This is accomplished by multiplying the yield stress σ y  by a safety factor “N”. For example, when N=1 (or 100%), the yield stress is used for σ y  to give the result obtained in the calculation example set forth above.  
         [0031]    If a designer decides to use a more conservative factor, however, if N=0.9 (or 90%) times the yield stress is used in the calculation above, a smaller diameter (D v ) of 3.081 inches is obtained, thus, moving the location of the vents closer to the inner bore allowing leakage before reaching yield. Alternatively, if a less conservative design is desired, the factor “N” may be higher than 100%, however, the value used in place for the yield stress in Equation 3 above should be less than the ultimate tensile stress (σ t ) of the material to prevent catastrophic failure.  
         [0032]    Manufacture of the holes for vents  41  may be achieved using any well known machining processes, including simply drilling from either end face. In the case of longer pressure vessels, drilling from both end faces  46  and  47  can be performed to meet the holes to form a continuous vent. An alternate method of manufacture is shown in FIGS.  6 - 8  in which a pressure vessel  60  having an inner bore  65  is drilled from each of end faces  66  and  67  to form vents  61  and  62 , respectively, that overlap as shown. As shown by the cross-sectional views of FIGS. 7 and 8, the overlapping of vents  61  and  62  is achieved by locating the vents at alternate clock positions (i.e., clocking) to ensure that any propagating cracks will intersect at least one of the vents  61  and  62 .  
         [0033]    Shown in FIGS. 9 and 10 are some preferred uses of the pressure vessels of the present invention and exemplary methods of attachment of the end faces. In the preferred embodiments shown in FIGS. 9 and 10, pressure vessel 40 is a thick walled pressure vessel for containers subjected to fatigue loading. Most preferably, these containers are intensifier high pressure plunger cylinders and accumulators for use in waterjet cutting and machining apparatus having an inlet and/or outlet  52  that may be clamped into one end of inner bore 45 by an end plate  20  using threaded fasteners  51  (FIG. 9) or threadingly engaged into the end of inner bore 45 using a threaded retainer  48  and a seal  49  (FIG. 10).  
         [0034]    While embodiments and applications of this invention have been shown and described, it will be apparent to those skilled in the art that many more modifications are possible without departing from the inventive concepts herein described. For example, although described above as being particularly useful in conjunction with monobloc pressure vessels, it is envisioned that additional layers may be incorporated within the inner bore or outside of a monobloc having vents according to the present invention to provide a compound vessel having multiple layers with a layer that is vent-protected. Moreover, although described above with respect to use with waterjet cutting and machining apparatus, it is envisioned that the pressure vessels according to the present invention may be incorporated into other pressure vessels or conduits in which protection against catastrophic failure by crack growth propagation is desired. It is understood, therefore, that the invention is capable of modification and therefore is not to be limited to the precise details set forth. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims without departing from the spirit of the invention.