Abstract:
A pressure vessel for transporting compressed gas at very high pressure without failing is disclosed. The pressure vessel includes an inverted neck at the head so that pressure from compressed gas is applied to the neck in a converging manner. A plug valve is inserted into the opening defined by the neck to seal the pressure vessel. As pressure from the compressed gas enclosed in the pressure vessel increases, engagement between the neck and the plug also increases. Therefore, the pressure vessel can withstand higher levels of pressure from compressed gas before failing during hydro and burst testing.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an apparatus for transporting compressed gas. Specifically, embodiments of the invention relate to pressure vessels for transporting compressed gas at very high pressure without failing. 
         [0003]    2. Description of the Related Art 
         [0004]    Gases or fluids under pressure and similar materials require specialized pressurized containers for transportation. Natural gas and similar materials are often procured at locations that are remote from refineries and storage facilities, as well as the end users of the materials. Large volumes of pressurized gas are transported from field to market using various forms of transportation. 
         [0005]    One way to transport gas between locations is through the use of pressure vessels. In general, a pressure vessel is a heavy steel module containing gas so that the gas can be moved from one location to another. A neck provides an opening to the pressure vessel and a plug or valve inserted into the neck prevents any gas from entering or exiting the pressure vessel. 
         [0006]    Pressure vessels can withstand the pressure of enclosed gas up to a certain level. At high pressure (generally around 7500 psi for heavy duty composite reinforced pressure vessels), pressure vessels fail because they are unable to contain the gas. In particular, the high pressure exerts force outwardly on the internal walls of the head, causing the head to expand outwardly and the plug or valve to disengage from contact with the neck. When the plug is not fully engaged with the interior walls of the neck, the enclosed pressurized gas escapes from the pressure vessel through the space between the plug and the neck. This scenario presents a problem, as various gases are contained and transported at a pressure of 3000 psi this requires a test pressure of 5000 psi and a burst rating of 7500 psi. Generally, safety guidelines require the burst rating be at least 2.4 times the operating pressure. As explained below, compliance with this standard is difficult with existing technology. Typical composite reinforced pipe uses a ½″ metal shell with ½″ of composite reinforcement and ⅞″ heads welded to the ends of the metal shell. Because composite reinforcement doubles the hoop strength of the shell, the head becomes the probable point of failure. 
         [0007]      FIG. 1  is a diagram of a cross-sectional view of a pressure vessel head in the prior art.  FIG. 1  depicts an area of weakness that may precipitate failure in a pressure vessel head  110  when the pressure vessel contains gas at high pressure. This illustration includes a pressure vessel head  110  that forms one end of a pressure vessel (not shown) and a neck  120  that provides an opening in the pressure vessel head  110 . Note that the neck  120  protrudes distally from the pressure vessel head  110 . The head  110  and neck  120  combination is typically extruded from a unitary blank to avoid weaknesses that result when the neck is welded to the head. Welding the neck to the head creates a shift spot at the weld. This less flexible spot can cause premature cyclic failure. Typically, one seeks to get at least 10,000 to 20,000 cycles out of a pressure vessel. The weld also has the potential to introduce flaws during the welding process. These factors taken together result in a welded neck being a significant failure point. 
         [0008]    An o-ring  130  and plug  140  are engaged in the opening of the neck  120  to prevent gas contained in the pressure vessel from escaping. The gas contained in the pressure vessel creates pressure on an interior portion of the pressure vessel head  110  in the direction of the arrows  150  depicted in  FIG. 1 . The pressure  150  is directed in an outward direction from inside the pressure vessel head  110 , pushing against the internal walls of the pressure vessel head  110  and neck  120 . 
         [0009]    When pressure  150  from the gas contained in the pressure vessel is high enough, the force from the pressure  150  causes the neck  120  and the opening created by the neck  120  to expand. When the neck  120  expands away from the plug  140 , the plug  140  and o-ring  130  no longer provide an effective seal of the opening to prevent gas from escaping from the pressure vessel (even if the plug  140  may still loosely rest in the opening defined by the neck  120 ). More often the failure is catastrophic with the plug  140  being ejected at high speed. The failure to contain gas at high pressure presents a problem of a pressure vessel in the prior art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The invention is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one. 
           [0011]      FIG. 1  is a diagram of a cross-sectional view of a pressure vessel head in the prior art. 
           [0012]      FIG. 2  is a diagram of an exterior side view of one embodiment of a pressure vessel. 
           [0013]      FIG. 3  is a diagram of a cross-sectional view of one embodiment of a pressure vessel. 
           [0014]      FIG. 4  is a diagram of a cross-sectional view of one embodiment of a pressure vessel head. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 2  is a diagram of an exterior side view of one embodiment of a pressure vessel  200 . The pressure vessel  200  is used to transport gas from one location to another. 
         [0016]    The composite reinforced pressure vessel  200  includes a cylindrical shell  210  and arcuate head  230  welded together  240  to form a container for holding gas. The cylindrical shell  210  and arcuate head  230  may be manufactured from metal, metal alloy, or elastic metal composite. Examples are steel, stainless steel, high strength low alloy steel, carbon steel, monel, inconel, hastelloy, and titanium. The weld  240  between the cylindrical shell  210  and arcuate head  230  may be overmatched in strength and/or volume. To overmatch the weld  240  in terms of strength, the weld  240  may be composed of higher strength metal than the metal of the cylindrical shell  210  and arcuate head  230 . For example, the weld material can have a tensile strength of 10%-12% greater than the material of the cylindrical shell  210  and arcuate head  230 . To overmatch the weld  240  in terms of volume, a larger volume of material per unit area may be used for the weld  240  than for the cylindrical shell  210  and arcuate head  230 . For instance, the volume per unit may be 15%-18% greater for the weld  240  than for the cylindrical shell  210  and arcuate head  230 . 
         [0017]    The cylindrical shell  210  is wrapped circumferentially with a composite reinforcement  220  to strengthen the cylindrical shell  210  to improve the hoop strength of the walls of the vessel  200 . This yields a two time increase in hoop strength of the shell. Composite reinforced pressure vessels are described in U.S. Pat. No. 6,779,565. Otherwise, the increased ability of the pressure vessel  200  to withstand higher pressure because of the improved arcuate head  230  would be limited by the lower pressure tolerance of the cylindrical shell  210 . 
         [0018]    The material used to make the composite reinforcement  220  varies depending on the reinforcement characteristics desired for the pressure vessel  200 . For example, the composite reinforcement  220  can be made with isopolyester resin matrix, polyester, aramid, or other glass fiber materials. Characteristics such as strength, heat distortion temperature, and elongation are taken into consideration when selecting the material used to make the composite reinforcement  220 . For example, in high temperature environments, an isopolyester with lower elongation would be desired for the composite reinforcement  220 , while in low temperature environments, an isopolyester with higher elongation would be desired for the composite reinforcement  220 . In addition, the thickness of the composite reinforcement  220  may be selected based on factors such as operating pressure and the strength of the cylindrical shell  210 , arcuate head  230 , and the weld  240 . While any thickness of composite reinforcement  220  would provide further reinforcement to the cylindrical shell, a composite reinforcement  220  of greater thickness provides more reinforcement than a composite reinforcement  220  of lesser thickness. In general, a composite reinforcement  220  would add about 20% to the weight of the steel pressure vessel  200  and increase the pressure capability of the pressure vessel  200  to contain gas by about 100%. 
         [0019]    The opening  250  of the pressure vessel  200  is defined by a neck (not shown) in the arcuate head  230 . The opening  250  would receive an o-ring (not shown) and plug (not shown) to seal the pressure vessel  200 . When the pressure vessel  200  is sealed, gas enclosed in the pressure vessel  200  cannot escape the pressure vessel  200  and gas outside the pressure vessel  200  cannot enter the pressure vessel  200 . The opening  250  can be fitted with a pump and/or valve to load and unload gas to and from the pressure vessel. Since the neck is inverted, it protrudes internally (rather than distally) from the surface of the arcuate head  230  at the opening  250 , in contrast to the neck  120  depicted in prior art  FIG. 1 . 
         [0020]      FIG. 3  is a diagram of a cross-sectional view of one embodiment of a pressure vessel. The pressure vessel  200  may be used to contain and transport gas. 
         [0021]    As described in  FIG. 2 , the pressure vessel  200  includes a cylindrical shell  210  and arcuate head  230  welded together  240 . The cylindrical shell  210  is wrapped circumferentially with a composite reinforcement  220 . 
         [0022]    Gas may be contained for transport in an interior section  310  of the pressure vessel  200 . Gas may be loaded to and unloaded from the interior section  310  of the pressure vessel  200  through an opening  250  defined by the neck  320 . 
         [0023]    The neck  320  is inverted such that it protrudes towards the concave interior of the arcuate head  230 , rather than distally from the external surface of the arcuate head  230 . The arcuate head  230 , including the neck  320 , may be extruded from a single unit of material. One feature of the extrusion process is that the neck  320  is thickest towards the external surface of the arcuate head  230  and decreases in thickness towards an interior portion of the arcuate head  230 . The neck  320  includes a threaded region to receive a reciprocally threaded plug (not shown) through the opening  250  to seal the pressure vessel  200 . The neck  320  also includes an o-ring seat  330  proximate to the external surface of the arcuate head  230 . The o-ring seat  330  engages an o-ring (not shown) coupled with the plug to enhance the seal of the pressure vessel  200 . A second head  231  may be welded to the opposite end of the pressure vessel. In some embodiments, head  231  is a blank head (as shown), i.e., it has no neck or opening. In other embodiments, head  231  may be identical to head  230 . 
         [0024]      FIG. 4  is a diagram of a cross-sectional view of one embodiment of a pressure vessel head that would be welded to a cylindrical shell (not shown) to create a container for transporting gas. This figure presents the view of an arcuate head/shell  230  sealed with a plug  420  and o-ring  410  so that gas enclosed at an interior section  310  of the pressure vessel cannot escape the pressure vessel and gas outside the pressure vessel cannot enter the pressure vessel. In one embodiment, tapered threads may be used instead of using an o-ring. In another embodiment, tapered threads are used in conjunction with an o-ring. 
         [0025]    The pressure vessel head includes an arcuate shell  230  and integrally formed inverted neck  320 . The arcuate shell  230  can define a portion of an ellipse, such as a two-to-one ellipse. In addition, the arcuate shell  230  can be made of steel having a nominal thickness of ⅞″. The neck  320  includes a threaded region and o-ring seat  330  to engage a reciprocally threaded plug  420  coupled with an o-ring  410  to seal the pressure vessel. Since the neck  320  is inverted, the o-ring seat  330  resides in a thicker portion of metal than with distally oriented necks. The thicker metal is better suited to accommodate the o-ring seat  330  without creating an inherent weakness. 
         [0026]    When the plug  420  and o-ring  410  are in place, the gas enclosed in the interior section  310  of the pressure vessel creates a force on the interior walls of the arcuate shell  230 . Specifically, the pressure converges on the neck  320  in the direction of the dotted arrows  430 . At high pressure, rather than expanding the neck and the opening defined by the neck so that the plug no longer provides an effective seal (as occurs in the prior art), the pressure  430  inside the arcuate shell  230  is situated to increase engagement between the neck  320  and the plug  420  at the protruding end of the neck  320 . This configuration permits pressure vessels having an inverted neck  320  to withstand high pressure from compressed gas of up to around 8550 psi before the plug  420  is forcefully blown out of the opening defined by the neck  320 . Tests on the distally extended necks found failure pressures between 6500 and 7500. 
         [0027]    Testing has shown that high pressure from compressed gas in the pressure vessel may cause the o-ring  410  to disengage from the o-ring seat  330  prior to a failure of the pressure vessel. This is because the o-ring  410  may lose contact with the o-ring seat  330  while the plug  420  is still at least partially engaged with the threaded portion of the neck  320 , particularly towards the end of the plug  420  interior to the pressure vessel head. However, the pressure from the gas on the neck forces stronger engagement between the metal constituents of the neck  320  and plug  420  and maintains the seal of the pressure vessel, so that enclosed gas can not escape the pressure vessel and external gas can not enter the pressure vessel. The internally protruding neck also reduces an overall pressure vessel length/unit of gas carried. This can be desirable for transport and storage. 
         [0028]    In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes can be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.