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FIELD OF THE INVENTION 
       [0001]    The present invention relates to methods and apparatus for protecting against the influx of air into piping carrying a combustible gas under negative pressure, and particularly to such methods and apparatus for protecting against such influx of air at flanged and threaded piping connections. 
       BACKGROUND OF THE INVENTION 
       [0002]    Natural gas is commonly found in subsurface geological formations such as deposits of granular material (e.g., sand or gravel) or porous rock. Production of natural gas from these types of formations typically involves drilling a well a desired depth into the formation, installing a casing in the wellbore (to keep the well bore from sloughing and collapsing), perforating the casing in the production zone (i.e., the portion of the well that penetrates the gas-bearing formation) so that gas can flow into the casing, and installing a string of tubing inside the casing down to the production zone. Gas can then be made to flow up to the surface through a production chamber, which may be either the tubing or the annulus between the tubing and the casing. The gas flowing up the production chamber is conveyed through an intake pipeline running from the wellhead to the suction inlet of a wellhead compressor. The compressed gas discharged from the compressor is then conveyed through another pipeline to a gas processing facility and sales facility as appropriate. 
         [0003]    When natural gas is flowing up a well, formation liquids will tend to be entrained in the gas stream, in the form of small droplets. As long as the gas is flowing upward at or above a critical velocity (the value of which depends on various well-specific factors), the droplets will be lifted along with the gas to the wellhead. In this situation, the gas velocity provides the means for lifting the liquids, and the well is said to be producing by “velocity-induced flow”. Because liquids in the gas stream can cause internal damage to most gas compressors, a gas-liquid separator is provided in the intake pipeline to remove liquids from the gas stream before entering the compressor. The liquids may be pumped from the separator and reintroduced into the gas flow at a point downstream of the compressor, for eventual separation at the gas processing facility. Much more commonly, however, the liquids are collected in a tank on the well site. 
         [0004]    In order to optimize total volumes and rates of gas recovery from a gas reservoir, the bottomhole flowing pressure should be kept as low as possible. The theoretically ideal case would be to have a negative bottomhole flowing pressure so as to facilitate 100% gas recovery from the reservoir, resulting in a final reservoir pressure of zero. In order to reduce the bottomhole pressure to a negative value, or to a very low positive value, it would be necessary to have a negative flowing pressure (i.e., lower than atmospheric pressure) in the intake pipeline. This can be readily accomplished using well-known technology, such as by providing a wellhead compressor of sufficient power. 
         [0005]    However, negative pressure in a natural gas pipeline would present an inherent problem, because any leak in the line (such as at pipeline joints) would allow the entry of air into the pipeline, because air would naturally flow to the area of lower pressure. This would create a risk of explosion should the air/gas mixture be exposed to a source of ignition. In addition to the explosion risk, entry of air into the pipeline also creates or increases the risk of corrosion inside the pipeline. For these reasons, the pressure in the intake pipeline is typically maintained at a positive level (i.e., higher than atmospheric). Therefore, in the event of a leak in the intake pipeline, gas in the pipeline will escape into the atmosphere, rather than air entering the pipeline. The explosion and corrosion risks are thus minimized or eliminated, but in a way that effectively limits ultimate recovery of as reserves from the well. 
         [0006]    One way of minimizing or eliminating explosion and corrosion risks, while facilitating the use of negative pressures in the intake pipeline, would be to provide an oxygen sensor in association with the pipeline. The oxygen sensor would be adapted to detect the presence of oxygen inside the pipeline, and to shut down the compressor immediately upon detection of oxygen. This system thus would more safely facilitate the use of compressor suction to induce negative pressures in the intake pipeline and, therefore, to induce negative or low positive bottomhole flowing pressures. However, this system has an inherent drawback in that its effectiveness would rely on the proper functioning of the oxygen sensor. If the sensor malfunctions, and if the malfunction is not detected and remedied in timely fashion, the risk of explosion and/or corrosion will become manifest once again. This fact highlights an even more significant drawback in that this system would not prevent the influx of air into the pipeline in the first place, but is merely directed to mitigation in the event of that undesirable event. 
         [0007]    Canadian Patent No. 2,536,496 (Wilde) and corresponding U.S. Pat. No. 7,275,599 teach methods and apparatus for minimizing and protecting against the risk of explosion arising from the influx of air into a pipeline carrying a combustible gas under negative pressure, without relying on oxygen sensors or other devices that are prone to malfunction. In accordance with the teachings of CA 2,536,496 and U.S. Pat. No. 7,275,599, the intake pipeline running between the production chamber of a natural gas well and the suction inlet of an associated wellhead compressor is completely enclosed within a vapour-tight jacket containing natural gas under positive pressure (i.e., higher than atmospheric). The intake pipeline is thus “blanketed” by natural gas under positive pressure and thus not exposed to the atmosphere. This arrangement allows gas to be drawn into the compressor through the intake pipeline under a negative pressure, without risk of air entering the intake pipeline should a leak occur in the pipeline. Should such a leak occur, there would merely be a harmless transfer of gas from the positive pressure jacket into the intake pipeline. Should a leak develop in the positive pressure jacket, any gas leaking therefrom would escape into the atmosphere, and entry of air into the positive pressure jacket would be impossible. System components other than piping, such as compressors and separators, may be similarly enclosed within a positive pressure gas jacket in accordance with CA 2,536,496 and U.S. Pat. No. 7,275,599. 
         [0008]    Although the methods and apparatus taught by CA 2,536,496 and U.S. Pat. No. 7,275,599 have proved highly effective in actual use, it may be desirable in certain situations to provide protection against air influx into piping and equipment components containing gas under negative pressure without complete enclosure in a positive pressure gas jacket. For example, in absence of material defects, the risk of air influx through the walls of pressure-rated piping and vessels will typically be far less than the potential risk of air influx at bolted flanged connections between piping sections, or where piping sections connect to pressure vessels. If effective protection against air influx can be provided at flanged connections, it may be unnecessary to provide complete or even partial positive pressure gas jacketing. 
         [0009]    Bolted flanged connections typically use gaskets to prevent leakage through the connection. However, there are no perfect or foolproof gaskets, and fugitive emissions of gas through gasketed flanged connections are a common reality. Such fugitive emissions are typically small in terms of volume or rate of gas leakage, and therefore do not pose a safety hazard in situations where the piping involved is carrying gas at a pressure higher than atmospheric, because any gas leakage through the gaskets will be to atmosphere. This might not be desirable from an environmental standpoint, but it does not create a fire or explosion hazard. 
         [0010]    The situation is different in the case of a flammable gas under partial vacuum. In this situation, deficiencies or defects in the gaskets can result in the higher-pressure air leaking into the stream of flowing gas (or into non-flowing gas in a storage vessel), thereby causing a serious hazard even when only small volumes of air are involved. For this reason, gasketing technology per se cannot be relied on to provide an acceptable solution to the problem of air leakage through bolted flanged connections into conduits or vessels containing flammable gas under negative pressure. 
         [0011]    For these reasons, there is a need for apparatus and methods for protecting against influx of air through flanged and other types of piping connections into piping and vessels carrying gas under negative pressure. As well, there is a need for apparatus and methods for providing improved or enhanced protection against the escape of harmful or hazardous gases (such as but not limited to “sour” gas) from flanged and other types of piping connections The present invention is directed to these needs. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0012]    In general terms, the present invention provides methods and apparatus for preventing migration of either gaseous or liquid fluids through a piping connection, from a region of higher pressure to a region of lower pressure. In many if not most practical applications, the invention will be used to prevent migration of a gas through a piping connection, and the invention is described and illustrated in that context in this patent document. It is to be understood, however, that the methods and apparatus of the invention can also be adapted for applications intended to prevent migration of liquids through a piping connection. 
         [0013]    In accordance with one embodiment of the present invention, the inward migration of air through a bolted flanged connection, and into a vessel or piping containing a flammable gas under negative pressure, may be prevented by providing double seal means between the mating faces of the two flanges being connected, with the double seal means being configured to form an annular chamber into which a blanketing gas is introduced, at a pressure higher than atmospheric. 
         [0014]    In a preferred embodiment, the double seal means comprises a pair of generally concentric, spaced-apart inner and outer ring-shaped gaskets (the term “ring-shaped” in this context not to be construed as restricted to circular rings, but inclusive of rings of other configurations). The annular chamber is thus defined by the flange faces, the outer edge of the inner gasket, and the inner edge of the outer gasket. However, the double seal means could take other forms without departing from the principles and scope of this embodiment of the invention. When gaskets are used, they do not necessarily have to be made of resilient materials commonly used for many types of gaskets; for example, solid metal ring gaskets could be used in appropriate applications. In other variants, the double seal means could be in the form of a unitary double-sealing gasket that has an annular recess formed into one face, such that the recess defines the required annular chamber when the unitary double-sealing gasket is clamped between the two flange faces. What is important is for the double seal means to provide an inner seal and an outer seal against the flange faces, with the inner and outer seals being spaced so as to form an annular chamber. 
         [0015]    The blanketing gas is introduced into the annular chamber through a gas inlet channel drilled or otherwise formed in one of the flanges. The other flange may be provided with a similar gas outlet channel, from which blanketing gas can flow to another blanketed flanged connection (and so on), to facilitate positive-pressure gas blanketing of multiple flanged connections using a single source of blanketing gas. 
         [0016]    Any air that might for any reason tend to migrate inward through the outer gasket will be at a lower pressure than the blanketing gas, which will thus block the air from migrating further inward toward the vessel or piping. The blanketing gas pressure is maintained at a level sufficient to ensure that it remains higher than atmospheric notwithstanding any fugitive emissions of blanketing gas inward through the inner gasket or outward through the outer gasket. Suitable pressure gauges and gas valves will preferably be provided in association with each blanketed flange (or each group of blanketed flanges served by a common blanketing gas source), to facilitate monitoring and regulation of the blanketing gas pressure. 
         [0017]    The blanketing gas may be the same type of gas as the gas under negative pressure, as will commonly be convenient when using blanketed flange assemblies in association with natural gas production facilities. As an alternative, the blanketing gas may be an inert gas, such as nitrogen (by way of non-limiting example). 
         [0018]    In alternative embodiments, the present invention provides methods and apparatus for preventing migration of gas (or liquid) through non-flanged piping connections (threaded or unthreaded) from a region of higher pressure to a region of lower pressure. For example, in an NPT piping connection (i.e., a connection using tapered threads in accordance with the U.S. National Pipe Thread standard), the engagement between the internal (female) threads of a first pipe and the external (male) threads of a second pipe can provide a primary circumferential seal. The first pipe end may be provided with an unthreaded and at least substantially cylindrical extension section extending beyond the internally-threaded section, such that in the assembled connection, the extension section of the first pipe end extends over an unthreaded region of the outer surface of the second pipe end (also referred to herein as a cylindrical interface region). A secondary circumferential seal is provided in the cylindrical interface region, and a circumferential annular chamber is formed either in the inner cylindrical wall of the extension section of the first pipe or in the interface region of the second pipe, with the circumferential annular chamber being disposed between the secondary circumferential seal and the primary seal formed by the engagement of the tapered male and female threads. 
         [0019]    The circumferential annular chamber is in fluid communication with a source of positive-pressure blanketing gas (i.e., at a pressure higher than that of a process gas flowing through the first and second pipes). Accordingly, any tendency of the process gas to migrate outward through the primary circumferential seal (e.g., the threaded connection) will be prevented by the higher-pressure blanketing gas. The blanketing gas in this application will preferably be an inert gas such as nitrogen, such that any leakage of blanketing gas through the secondary circumferential seal will be environmentally, benign. This embodiment is particularly advantageous and beneficial for applications where the process gas is sour gas. 
         [0020]    For threaded piping connections having untapered threads (e.g., machine threads) by providing primary and secondary circumferential seals in the form of O-rings or other suitable known seal means, with a circumferential annular chamber being provided or formed between the primary and secondary seals. The position of the seals relative to the engaged threads is not critical; for example, there could be one seal on each side of the threads, or both seals could be provided on one side of the engaged threads. 
         [0021]    The same general principle may also be applied in the context of non-threaded piping connections. 
         [0022]    The principles of the present invention may be readily applied for purposes other than preventing migration of air into a vessel or piping carrying gas under negative pressure. For example, in the production of “sour gas” (i.e., natural gas containing significant amounts of hydrogen sulphide), a primary concern is to prevent migration of sour gas from production piping and equipment into the atmosphere. In conventional gasketed flanged connections, there is a risk of fugitive sour gas emissions to atmosphere when the sour gas in the vessel or piping is at or higher than surrounding atmospheric pressure. Gas-blanketed piping and equipment flanges, in accordance with the present invention, may be used to prevent such fugitive emissions. In this application, an inert blanketing gas such as nitrogen is introduced into the annular chamber of each flanged connection, at a pressure higher than the pressure of the sour gas in the vessel or piping. The inert blanketing gas thus blocks any outward migration of sour gas past the inner gasket of the blanketed flange assembly. The blanketing gas pressure is maintained at a level sufficient to ensure that it remains higher than atmospheric notwithstanding any fugitive emissions of blanketing gas outward through the outer gasket. 
         [0023]    Accordingly, in a first aspect the present invention provides a piping connection assembly comprising a first pipe having a first end; a second pipe having a first end; a first annular flange mounted to the first end of the first pipe, said first flange having an annular connection face; a second annular flange mounted to the first end of the second pipe, said second flange having an annular connection face; connection means for connecting the first and second flanges with their connection faces in juxtaposition; and double seal means disposed between the two flange connection faces and configured to form an annular chamber. A gas inlet channel extends through a selected one of the flanges so as to be in fluid communication with the annular chamber, such that a gas flowing into the gas inlet channel will flow into the annular chamber. 
         [0024]    In a second aspect, the present invention provides a piping connection assembly comprising a first pipe having a female end, and a second pipe having a male end; connection means for connecting said female end of the first pipe and said male end of the second pipe; primary seal means extending around the circumference of the male end of the second pipe, said primary seal means providing a seal between the first and second pipes; secondary seal means extending around the circumference of the male end of the second pipe, said secondary seal being axially spaced from the primary seal and providing a seal between the first and second pipes; an annular chamber extending around the circumference of the male end of the second pipe, said annular chamber being disposed between the primary and secondary seals; and a gas inlet channel extending through the wall of a selected one of the first and second pipes so as to be in fluid communication with the annular chamber, such that a gas flowing into the gas inlet channel will flow into the annular chamber. 
         [0025]    In a third aspect, the present invention teaches a method of providing enhanced protection against migration of gas through a connection between two fluid-carrying pipes, said method comprising the steps of providing primary and secondary seals extending around the connection, said primary and secondary seals being spaced apart, and each of said primary and secondary seal means providing a seal between the first and second pipes; providing an annular chamber extending around the circumference of the male end of the second pipe, said annular chamber being disposed between the primary and secondary seals; and providing a gas inlet channel in fluid communication with the annular chamber and with a source of a blanketing gas, such that blanketing gas can flow through the gas inlet channel into the annular chamber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    Embodiments of the invention will now be described with reference to the accompanying figures, in which numerical references denote like parts, and in which: 
           [0027]      FIG. 1  is a schematic diagram of a natural gas wellhead and associated piping and equipment components, enclosed within a positive pressure gas jacket in accordance with prior art technology. 
           [0028]      FIG. 2  is cross-sectional detail through a bolted flanged piping connection with positive pressure gas blanketing in accordance with a first embodiment of the present invention. 
           [0029]      FIG. 3  is a cross-section through a wellhead assembly with gas-blanketed flanges in accordance with a second embodiment of the present invention, with the wellhead assembly incorporating a gas-blanketed shut-off valve. 
           [0030]      FIG. 4  is a cross-section through a generic non-flanged connection between two piping sections, with gas blanketing in accordance with a third embodiment of the present invention. 
           [0031]      FIG. 5  is a cross-section through a taper-threaded connection between two piping sections, with gas blanketing in accordance with a fourth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0032]    The present invention will be best understood after first reviewing methods and apparatus taught by CA 2,536,496 and U.S. Pat. No. 7,275,599 for protecting against air influx into piping and equipment components conveying or containing gas under negative pressure.  FIG. 1  (which is not to scale) schematically illustrates a typical natural gas well W penetrating a subsurface formation F containing natural gas. Well W is lined with a casing  20  which has a number of perforations conceptually illustrated by short lines  22  within a production zone (generally corresponding to the portion of the well penetrating the formation F). As conceptually indicated by arrows  24 , formation fluids including gas, oil, and water may flow into the well through the perforations  22 . A string of tubing  30  extends inside the casing  20 , terminating at a point within the production zone. The bottom end of the tubing  30  is open such that fluids in the wellbore may freely enter the tubing  30 . An annulus  32  is formed between the tubing  30  and the casing  20 . The upper end of the tubing  30  runs into a surface termination apparatus or “wellhead.” (not illustrated), of which various types are known in the field of gas wells. 
         [0033]    Tubing  30  serves as a production chamber to carry gas from well W to a production pipeline  40  having an upstream section  40 U which carries the gas through a gas-liquid separator  70  to the suction manifold  42 S of a gas compressor  42 . Separator  70  divides the upstream pipeline into section  40 U′ on the wellhead side of separator  70 , and section  40 U″ on the compressor side of separator  70 . Production pipeline  40  also has a downstream section  40 D which connects at one end to the discharge manifold  42 D of compressor  42  and continues therefrom to a gas processing facility (not shown). As schematically indicated, liquids  72  separated from the gas flowing through intake pipeline  40 U′ will accumulate in a lower section of separator  70 . In the usual case, liquids  72  flow from separator  70  to a storage tank  80  on the well site. 
         [0034]    The apparatus shown in  FIG. 1  provides for production of gas under negative pressure, in which case liquids  72  removed from the gas stream by separator  70  will also be under negative pressure, and for this reason a vacuum pump  74  is provided as shown. Liquids  72  flow under negative pressure through a pump inlet line  78  to pump  74 , which pumps liquids  72 , now under positive pressure, through a liquid return line  76  into downstream section  40 D of production pipeline  40  at a point Z downstream of compressor  42 . Alternatively, liquids  72  may be pumped to an on-site storage tank  80 . 
         [0035]    As illustrated in  FIG. 1 , upstream pipeline sections  40 U′ and  40 U″, separator  70 , and pump inlet line  78  are fully enclosed by a vapour-tight positive pressure jacket  50  that defines a continuous internal chamber  52 . A gas recirculation pipeline  60  extends between, and is in fluid communication with, downstream section  40 D of production pipeline  40  (at point X located between compressor  42  and point Z) and a selected pressure connection point Y on positive pressure jacket  50 . As shown in  FIG. 1 , pressure connection point Y may be located in upstream pipeline section  40 U″ between compressor  42  and separator  70 . By means of recirculation pipeline  60 , a portion of the gas discharged from discharge manifold  42 D of compressor  42  may be diverted into positive pressure jacket  50 , such that upstream pipeline sections  40 U′ and  40 U″, separator  70 , and pump inlet line  78  are entirely enclosed by a “blanket” of gas under positive pressure. Positive pressure jacket  50  thus enshrouds all components of the apparatus containing combustible fluids under negative pressure between the wellhead and suction manifold  42 S of compressor  42  with a blanket of gas under positive pressure, thereby preventing the entry of air into the combustible fluids present in any of those components. 
         [0036]    Turning now to the present invention,  FIG. 2  illustrates a gas-blanketed flanged piping connection in accordance with one embodiment of the invention. A first end  110 A of a first pipe  110  is fitted with an annular flange  112 , which has a planar annular end face  112 A and bolt holes  113 . A first end  120 A a second pipe  120  is fitted with an annular flange  122 , which has a planar annular end face  122 A and bolt holes  123  configured to match bolt holes  113  in annular flange  112 . An annular outer gasket  131  is positioned on (and preferably bonded to) either end face  112 A of flange  112  or end face  122 A of flange  122 , with outer gasket  131  being sized such that outer gasket  131  is entirely disposed radially inward of bolt holes  113 . An annular inner gasket  132  is positioned on (and preferably bonded to) either end face  112 A of flange  112  or end face  122 A of flange  122 , with inner gasket  132  being sized such that outer gasket  131  is entirely disposed radially inward of outer gasket  13 . 1 , such that when flanges  112  and  122  are bolted together using bolts  115  as shown in  FIG. 2 , a continuous annular space  140  is formed between outer gasket  131  and inner gasket  132 . 
         [0037]    A gas outlet channel  116  is drilled or otherwise formed in flange  112  on first pipe  110 , with gas outlet channel  116  extending between a first end  116 A and a second end  116 B. First end  116 A of gas outlet channel  116  is located at a selected point on flange  112  other than end face  112 A thereof, and is adapted for connection with a gas outlet conduit  150 . Second end  116 B of gas outlet channel  116  is in fluid communication with annular space  140 . In preferred embodiments, and as shown in  FIG. 2 , first end  116 A of gas outlet channel  116  is located on the outer perimeter face  112 B of flange  112 . 
         [0038]    A gas inlet channel  126  is drilled or otherwise formed in flange  122  on second pipe  120 , with gas inlet channel  126  extending between a first end  126 A and a second end  126 B. First end  126 A of gas inlet channel  126  is located at a selected point on flange  122  other than end face  122 A thereof, and is adapted for connection with a gas inlet conduit  160 . Second end  126 B of gas inlet channel  126  is in fluid communication with annular space  140 , preferably but not necessarily at a point diametrically opposite from second end  116 B of gas outlet channel  116 . In preferred embodiments, and as shown in  FIG. 2 , first end  126 A of gas inlet channel  126  is located on the outer perimeter face  12213  of flange  122 , but this is by way of example only; gas inlet channel  126  can be located and routed in a variety of ways without departing from the concept of the present invention. A pressure gauge  162  is installed in conjunction with gas inlet conduit  160 , and a valve  164  is installed in gas inlet conduit  160  at a point between first end  126 A of gas inlet channel  126  (at flange  122 ) and pressure gauge  162 . 
         [0039]    To put the embodiment of  FIG. 2  into practice, flanges  112  and  122  are bolted together as shown, with gaskets  131  and  132  being sufficiently compressed to form substantially vapour-tight seals against both end face  112 A of flange  112  and end face  122 A of flange  122 . A flow of a “blanketing” gas is introduced into gas inlet conduit  160 , whereupon opening valve  164  will cause the blanketing gas to flow into annular chamber  140  between gaskets  131  and  132 . The blanketing gas exits annular chamber  140  via gas outlet channel  116  and gas outlet conduit  150 , which may be connected to another gas-blanketed flange assembly (preferably with its own valve and pressure gauge). 
         [0040]    The blanketing gas pressure is maintained at a level higher than atmospheric pressure, thus protecting the connection against influx of air into pipes  110  and  120  when carrying a flammable gas under negative pressure. The blanketing gas pressure may be monitored by means of pressure gauge  162 . For installations having multiple gas-blanketed flange assemblies, a leak in the gas-blanketing system will be detectable from discrepancies between readings of the pressure gauges  162  associated with the various flange assemblies. In such event, one or more of valves  164  associated with the flange assemblies can be closed as required to isolate each flange assembly in turn, in order to pinpoint the source of the leak. 
         [0041]    In an alternative embodiment, a pressure switch (not shown) can be used in association with an assembly of multiple gas-blanketed flange assemblies served by a common source of blanketing gas. The pressure switch is programmed to automatically shut off the flow of gas within the piping if the pressure of the blanketing gas drops below a preset value. 
         [0042]    In the embodiment shown in  FIG. 2 , gas outlet channel  116  is formed in one flange (flange  112 ), and gas inlet channel  126  is formed in the other flange (flange  122 ). However, this is by way of example only, and persons skilled in the art will appreciate that gas outlet channel  116  and gas inlet channel  126  may be formed in either flange without departing from the principles and scope of the present invention. Moreover, it is not necessary for gas outlet channel  116  to be formed in one flange and for gas inlet channel  126  to be formed in the other flange; in alternative embodiments, both gas outlet channel  116  and gas inlet channel  126  may be formed in a selected one of the flanges. 
         [0043]      FIG. 3  provides just one example of how the principles of the present invention can be adapted to a variety of practical situations.  FIG. 3  conceptually illustrates an assembly associated with the wellhead of a well producing natural gas under negative pressure generally as shown in  FIG. 1 . Natural gas G NEG  under negative pressure flows upward through production tubing  30  disposed within well casing  20 . The upper ends of casing  20  and tubing  30  terminate at a wellhead flange  25 , with the open upper end of tubing  30  being supported by a conventional tubing hanger (not shown) and sealingly disposed, in conjunction with annular packing means  23 , in an opening  27  in wellhead flange  25 . In the embodiment shown in  FIG. 3 , wellhead flange  25  has a downwardly extending collar  25 A which receives casing  20 . 
         [0044]    A valve housing  200  (formed in the illustrated embodiment from two pieces of pipe of different diameters with a swedge transition) has a lower end welded to a lower valve housing flange  202 , which is bolted to wellhead flange  25 . The upper end of valve housing  200  is welded to an annular upper valve housing flange  204 . A first extension tube  30 A has a lower end threaded into an opening in lower valve housing flange  202 , and an upper end connected to a shut-off valve  210  disposed within valve housing  200  (with valve stem  212  extending through the wall of valve housing  200 ). A second extension tube  30 B has a lower end connected to shut-off valve  210 . A housing annulus  215  is thus formed between extension tubes  30 A and  30 B and shut-off valve  210 , and the inner wall surface of valve housing  200 . 
         [0045]    A pipe stub  220  has a lower end welded to a flange  222 , which is bolted to upper valve housing flange  204 . Flange  222  has an opening  223  through which second extension tube  30 B upwardly extends and forms an upper annulus  225  between second extension tube  30 B and the inner wall surface of pipe stub  220 . The upper end of second extension tube  30 B is connected to an upper stub flange  224 . Upper annulus  225  is in fluid communication with valve housing annulus  215  through opening  223 , which is of larger diameter than second extension tube  30 B. A production pipeline  40  has an upstream end  40 U connected to an annular flange  240 , which is bolted to upper stub flange  224 . 
         [0046]    The connection between wellhead flange  25  and lower valve housing flange  202  is a gas-blanketed assembly generally as shown in  FIG. 2 . A first inlet gas conduit  160 - 1  connects, via a first fitting  166 - 1  in the perimeter of wellhead flange  25 , to a first gas inlet channel  126 - 1  which leads to a first annular space  140 - 1  formed between flanges  25  and  202  and spaced concentric gaskets  131 - 1  and  132 - 1 . A first outlet gas channel  150 - 1  extends through lower valve housing flange  202  so as to be in fluid communication with first annular space  140 - 1  and valve housing annulus  215 . Positive-pressure gas G POS  flows through gas inlet channel  126 - 1  into first annular space  140 - 1  and thence through first outlet gas channel  150 - 1  into valve housing annulus  215  and thence into upper annulus  225  through opening  223  in flange  222 , thus providing positive pressure gas blanketing to extension tubes  30 A and  30 B and shut-off valve  210 , through which flows negative-pressure gas G NEG . 
         [0047]    The connection between flanges  224  and  240  is a gas-blanketed assembly generally as in  FIG. 2 . Positive-pressure blanketing gas G POS  is supplied to this assembly through a second gas inlet conduit  160 - 2  leading from upper annulus  225  (via a second fitting  166 - 2  through the wall of pipe stub  220 ) to a second annular space  140 - 2  formed between flanges  224  and  240  and spaced concentric gaskets  131 - 2  and  132 - 2 . A second outlet gas channel  150 - 2  extends from second annular space  140 - 2  through flange  202  for connection to another gas-blanketed connection served by the same source of blanketing gas. 
         [0048]    The connection between flanges  204  and  222  does not require positive pressure gas blanketing, as it is not exposed to negative-pressure gas G NEG . 
         [0049]    The use and operation of gas-blanketed flanges in accordance with the present invention may be readily understood with reference to the Figures and the preceding description. In installations where multiple flanged connections are to be blanketed, each such connection would be generally as shown in  FIG. 2 . Blanketing gas from a suitable source flows through gas inlet conduit  160  and gas inlet channel  126  into annular chamber  140 , from which the blanketing gas exits through gas outlet channel  116  and gas outlet conduit  150 , with gas outlet conduit  150  serving as the gas inlet conduit for purposes of another blanketed flange, and so on. 
         [0050]    In preferred usage, the pressure of the blanketing gas will be monitored and regulated by means of pressure gauge  162  used in conjunction with valve  164 , thereby facilitating detection of any pressure drops necessitating an increase in the blanketing gas inlet pressure. A single pressure gauge  162  in conjunction with a single valve  164  can be used in association with a system of multiple blanketed flanges served by a common source of blanketing gas. However, it is preferable to provide a pressure gauge  162  and a valve  164  in association with each blanketed flange assembly to facilitate temporary isolation of individual flange assemblies, which will be beneficial for purposes of locating any leaks in the blanketing gas system. 
         [0051]    Typically, there will be little or no flow of blanketing gas through the gas inlet and outlet conduits once blanketing gas has been initially delivered to the annular chambers of all gas-blanketed flanges in the system. In alternative embodiments, however, blanketing gas could be circulated through the system of gas-blanketed flanges. 
         [0052]    In simple situations where it is necessary or desirable to provide gas blanketing to a single flanged connection only, the assembly would be generally as shown in  FIG. 2 , except that there would be no need for gas outlet channel  116  and gas outlet conduit  150 . 
         [0053]      FIGS. 4 and 5  illustrate embodiments of the present invention for use with non-flanged piping connections. In the general case shown in  FIG. 4 , a first pipe  310  has a female end  310 A adapted for connection with a male end  320 A of a second pipe  320  by suitable connection means, in conjunction with longitudinally-spaced primary and secondary circumferential seals  331  and  332 . In the assembled connection, first circumferential seal  331  is proximal to the end of second pipe  320  and secondary circumferential seal  332  is proximal to the end of first pipe  310 . A circumferential annular chamber  240  is formed in a region between primary and secondary circumferential seals  331  and  332 , in either first pipe  310  or second pipe  320  (or, alternatively, formed partially in each of first and second pipes  310  and  320 ). 
         [0054]    Circumferential annular chamber  240  is in fluid communication with a source of positive-pressure blanketing gas by means of a gas inlet conduit  160  and a gas inlet channel  126  extending through the wall of first pipe  310 . A gas outlet channel  116  preferably extends through the wall of first pipe  310  at a location diametrically opposite from gas inlet channel  126 , for connection to a gas outlet conduit  150  which carries blanketing gas to another piping connection in a multiple blanketed-flange system. As in the embodiments shown in  FIGS. 2 and 3 , a pressure gauge  162  and a gas valve  164  are preferably provided in association with either gas inlet conduit  160  or gas outlet conduit  150 . 
         [0055]      FIG. 5  illustrates a particular embodiment of the general case of  FIG. 4 , in which first second pipes  310  and  320  have tapered NPT threads. In this embodiment, primary circumferential seal  331  takes the form of the engagement between tapered female threads  315  of first pipe  310  and tapered male threads  325  of second pipe  320 , with tapered threads  315  and  316  also serving, as the means for connecting first second pipes  310  and  320 . Secondary circumferential seal  332  is provided in the form of an O-ring disposed within a circumferential groove in second pipe  320 . However, persons skilled in the art will readily appreciate that secondary circumferential seal  332  can take a variety of other forms in accordance with known sealing technologies. 
         [0056]    In piping connections configured as in  FIGS. 4 and 5 , any tendency of a gas flowing within first and second pipes  310  and  320  to migrate outward through primary circumferential seal  331  (e.g., the threaded connection of  FIG. 5 ) will be counteracted by the higher-pressure blanketing gas G POS  introduced into circumferential annular chamber  240 . Blanketing gas G POS  in such practical applications will preferably be an inert gas such as nitrogen, so that any leakage of blanketing gas G POS  through secondary circumferential seal  332  will be environmentally benign. 
         [0057]    Persons skilled in the art will readily appreciate that the concept and principles of the present invention will be operative in any assembly in which there is a mechanical connection of some type between two gas-containing sections of pipe, with associated primary and secondary seals configured to create a annular chamber disposed between the primary and secondary seals, plus means for introducing a blanketing gas into the annular chamber. The particular embodiments described and illustrated herein (i.e., in conjunction with flanged and threaded piping connections) are specific examples of the general case, and the present invention is not restricted or limited to such exemplary embodiments. 
         [0058]    It will be readily appreciated by those skilled in the art that various modifications of the present invention may be devised without departing from the essential concept of the invention, and all such modifications are intended to come within the scope of the present invention and the claims appended hereto. It is to be especially understood that the invention is not intended to be limited to illustrated embodiments, and that the substitution of a variant of a claimed element or feature, without any substantial resultant change in the working of the invention, will not constitute a departure from the scope of the invention. 
         [0059]    In this patent document, any form of the word “comprise” is to be understood in its non-limiting sense to mean that any item following such word is included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one such element. Any use of any form of the terms “connect”, “fasten”, “engage”, “couple”, “attach”, or any other term describing an interaction between elements is not meant to limit the interaction to direct interaction between the subject elements, and may also include indirect interaction between the elements such as through secondary or intermediary structure. Relational terms such as “parallel”, “perpendicular”, “planar”, “coaxial”, “concentric”, “coincident”, “intersecting”, “equal”, and “equidistant” are not intended to denote or require absolute mathematical or geometrical precision. Accordingly, such terms are to be understood as denoting or requiring substantial precision only (e.g., “substantially parallel”) unless the context clearly requires otherwise.

Summary:
Migration of air through a flanged connection into piping containing a gas under negative pressure is prevented by disposing concentric inner and outer gaskets between the flange faces, so as to form an annular chamber into which a blanketing gas is introduced at a pressure higher than atmospheric. Air tending to migrate through the outer gasket is blocked by the higher-pressure blanketing gas in the annular chamber. The blanketing gas pressure is maintained at a level higher than atmospheric notwithstanding any fugitive emissions through the outer gasket. The annular chambers of multiple flanged connections may be interconnected to blanket multiple flanged connections using a single source of blanketing gas. The blanketing gas may be the same type as the gas under negative pressure. In sour service applications, an inert blanketing gas may be used to prevent leakage of sour gas to atmosphere through flanged connections. In alternative embodiments, the principles of the invention may be adapted for use with other types of connections including threaded piping connections, and for use with piping carrying liquids.