Abstract:
A burner assembly ( 100 ) for flaring low calorific gases, such as methane with high carbon dioxide content, may be configured to provide a gradual decrease in flow velocity. The burner assembly ( 100 ) may include a conical deflector ( 140 ) that creates a relatively large recirculation zone ( 154 ) downstream of the deflector ( 140 ), thereby to stabilize fluid flow. A swirl inducing structure positioned in a final stage of the burner assembly ( 100 ) further stabilizes the fluid flow and flame at different gas flow rates.

Description:
BACKGROUND OF THE DISCLOSURE 
       [0001]    Hydrocarbons are widely used as a primary source of energy, and have a significant impact on the world economy. Consequently, the discovery and efficient production of hydrocarbon resources is increasingly important. As relatively accessible hydrocarbon deposits are depleted, hydrocarbon prospecting and production has expanded to new regions that may be more difficult to reach and/or may pose new technological challenges. During typical operations, a borehole is drilled into the earth, whether on land or below the sea, to reach a reservoir containing hydrocarbons. Such hydrocarbons are typically in the form of oil, gas, or mixtures thereof which may then be brought to the surface through the borehole. 
         [0002]    Well testing is often performed to help evaluate the possible production value of a reservoir. During well testing, a test well is drilled to produce a test flow of fluid from the reservoir. During the test flow, key parameters such as fluid pressure and fluid flow rate are monitored over a time period. The response of those parameters may be determined during various types of well tests, such as pressure drawdown, interference, reservoir limit tests, and other tests generally known by those skilled in the art. The data collected during well testing may be used to assess the economic viability of the reservoir. The costs associated with performing the testing operations are significant, however, and may exceed the cost of drilling the test well. Accordingly, testing operations should be performed as efficiently and economically as possible. 
         [0003]    One common procedure during well testing operations is flaring a gas flow associated with the well effluent. Many types of burners and flares are known that can efficiently combust gas flows having relatively high colorific content (i.e., a relatively high percentage of methane) without producing significant smoke or fallout. That is because, with a high calorific content, a high velocity gas jet may thoroughly mix with minimal risk of blowing out the flame. 
         [0004]    It is more difficult, however, to cleanly burn gas flows having low calorific content, also known as “lean gases.” Lean gas flows may have a relatively high proportion of inert gases, such as nitrogen, which dilute the flammable content of the gas and therefore increase the risk of quenching the flame. Other inert gases, such as carbon dioxide, do not simply dilute the gas but may also actively inhibit flame when present in certain concentrations, such as greater than 35% of the gas flow content. Even at concentrations less than 35%, the flame inhibiting inert gases such as carbon dioxide may significantly increase the risk of flame blow-off. 
         [0005]    Various burner designs have been proposed for combusting gas having a low calorific content. In general, the proposed burners require complex gas flow paths that are susceptible to clogging, have complex designs that complicate construction and maintenance, and/or are otherwise unsuitable for flaring waste fuel during well testing operations. 
       SUMMARY OF THE DESCRIPTION 
       [0006]    In accordance with certain aspects of the disclosure, a burner assembly is provided for flaring a low calorific gas. The burner assembly may include a burner pipe disposed along a burner pipe axis and having an inlet pipe having an inlet pipe cross-sectional area extending substantially perpendicular to the burner pipe axis, an intermediate pipe coupled to the inlet pipe and having an intermediate pipe cross-sectional area extending substantially perpendicular to the burner pipe axis that is greater than the inlet pipe cross-sectional area, and an expander pipe coupled to the intermediate pipe and having an expander pipe cross-sectional area extending substantially perpendicular to the burner pipe axis that is greater than the intermediate pipe cross-sectional area. A hub may be disposed within a downstream portion of the expander pipe and have a hub upstream end facing the intermediate pipe and a hub downstream end. A plurality of guide vanes may interconnecting the expander pipe and the hub, and a deflector may be coupled to the hub and have a deflector exterior surface with a substantially frustoconical shape extending radially outwardly from the burner pipe axis and axially downstream of the hub downstream end, wherein the deflector exterior surface is oriented at a deflector surface angle relative to the burner pipe axis. 
         [0007]    In accordance with additional aspects of the disclosure, a method of flaring a low calorific gas may include flowing the low calorific gas through a burner pipe disposed along a burner pipe axis, the burner pipe including an inlet pipe having a relatively small cross-sectional area, an intermediate pipe having an intermediate cross-sectional area, and an expander pipe having a relatively large cross-sectional area, wherein the low calorific gas flows successively through the inlet pipe, intermediate pipe, and expander pipe. A central portion of the relatively large cross-sectional area of the expander pipe may be obstructed with a hub disposed at a downstream portion of the expander pipe to create a perimeter gas flow along the expander pipe. The perimeter gas flow may be rotated about the burner pipe axis to create a swirling gas flow exiting the expander pipe. A recirculation flow may be generated downstream of the expander pipe by directing the swirling gas flow radially outwardly along an exterior surface of a deflector, the deflector exterior surface having a substantially frustoconical shape. 
         [0008]    The summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Embodiments of burner assemblies and flaring methods suitable for combusting gas flows having low calorific content are described with reference to the following figures. The same numbers are used throughout the figures to reference like features and components. 
           [0010]      FIG. 1  is a perspective view of a burner assembly for a low calorific content gas flow constructed according to the present disclosure. 
           [0011]      FIG. 2  is a side elevation view, in cross-section, of the burner assembly of  FIG. 1  operating with a low superficial velocity gas flow. 
           [0012]      FIG. 3  is a side elevation view, in cross-section, of the burner assembly of  FIG. 1  operating with an intermediate superficial velocity gas flow. 
           [0013]      FIG. 4  is a side elevation view, in cross-section, of the burner assembly of  FIG. 1  operating with a high superficial velocity gas flow. 
       
    
    
       [0014]    It should be understood that the drawings are not necessarily to scale and that the disclosed embodiments are sometimes illustrated diagrammatically and in partial views. In certain instances, details which are not necessary for an understanding of the disclosed methods and apparatuses or which render other details difficult to perceive may have been omitted. It should be understood, of course, that this disclosure is not limited to the particular embodiments illustrated herein. 
       DETAILED DESCRIPTION 
       [0015]    So that the above features and advantages of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to the embodiments thereof that are illustrated in the accompanying drawings. It is to be noted, however, that the drawings illustrate only typical embodiments of this disclosure and therefore are not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments. 
         [0016]    Burner assemblies and methods are disclosed herein for use with a gas flow having a low calorific content, such as waste effluent from a supply line formed during well testing operations. The generic term used to describe such waste effluent is often roughly termed a gas flow to be combusted. In general, the assemblies and methods are adapted to decelerate the superficial velocity of the gas flow provided by the supply line to prevent flame blow-off, and to create a large recirculation zone downstream of the burner to ensure flame stability. 
         [0017]      FIG. 1  illustrates a burner assembly  100  adapted to combust a low calorific content gas flow across a wide range of superficial gas velocities. The gas flow may be communicated to the burner from any source, such as a supply line of a test well (not shown). The gas flow includes a flammable component, such as methane, as well as one or more inert gases, such as nitrogen, water vapor, and/or carbon dioxide. 
         [0018]    The burner assembly  100  includes a burner pipe  102  disposed along a burner pipe axis  104  and having a plurality of stages. In the illustrated embodiment the burner pipe  102  has three stages; however other embodiments of the burner pipe may have a different number of stages. More specifically, the burner pipe  102  may include an inlet pipe  105 , an intermediate pipe  106  having an intermediate pipe upstream end  108  coupled to the inlet pipe  105  and an intermediate pipe downstream end  110 , and an expander pipe  112  coupled to the intermediate pipe downstream end  110 . The stages of the burner pipe  102  are sized so that the gas flow successively encounters a larger cross-sectional area within the burner pipe  102 . Accordingly, the inlet pipe  105  may have an inlet pipe cross-sectional area that is relatively small, the intermediate pipe  106  may have an intermediate pipe cross-sectional area that is larger than the inlet pipe cross-sectional area, and the expander pipe  112  may have an expander pipe cross-sectional area that is larger than the intermediate pipe cross-sectional area. 
         [0019]    In the illustrated embodiment, the inlet pipe  105 , intermediate pipe  106 , and expander pipe  112  are shown as having generally cylindrical shapes. Accordingly, the relative sizes of the cross-sectional areas of the pipes may be determined based on their respective diameters. For example, the inlet pipe  105  may have an inlet pipe diameter D 1 , the intermediate pipe  106  may have an intermediate pipe diameter D 2 , and the expander pipe  112  may have an expander pipe diameter D 3 . Furthermore, as shown in  FIG. 2 , the intermediate pipe diameter D 2  is larger than the inlet pipe diameter D 1 , and the expander pipe diameter D 3  is larger than the intermediate pipe diameter D 2 . It will be appreciated, however, that the inlet, intermediate, and expander pipes  105 ,  106 ,  112  may be provided in non-cylindrical shapes. 
         [0020]    The expander pipe  112  may include an expander pipe upstream end  114  coupled to and fluidly communicating with the intermediate pipe  106 , and an expander pipe downstream end  116  open to atmosphere and therefore defining a burner pipe outlet  118 . A hub  120  may be disposed in a downstream portion of the burner pipe  102  adjacent the expander pipe downstream end  116 . In the illustrated embodiment, the hub  120  is concentric with, and has an overall profile shape that is substantially symmetrical relative to, the burner pipe axis  104 . The hub  120  may include a hub upstream end  122  generally facing the intermediate pipe  106 , a hub downstream end  124  opposite the hub upstream end  122 , and a hub side wall  126  connecting the hub upstream and downstream ends  122 ,  124 . The hub upstream end  122  may have a conical shape defining an apex  128  disposed substantially along the burner pipe axis  104 . The hub side wall  126  may be cylindrical and have a diameter D 4  defining a maximum hub cross-sectional area extending substantially perpendicular to the burner pipe axis  104 . To create a perimeter gas flow along the inside surface of the expander pipe  112 , as described in greater detail below, the hub  120  may be sized to obstruct a central portion of an expander chamber  119  defined by the expander pipe  112 . In some applications, the maximum hub cross-sectional area may be approximately 30 to 50% of the expander pipe cross-sectional area to create the desired perimeter gas flow. The hub downstream end  124  may be substantially planar as shown in  FIG. 2 . 
         [0021]    A plurality of guide vanes  130  may extend between the expander pipe  112  and the hub  120  to hold the hub  120  in position within the expander pipe  112  and to impart a rotation to the gas flow, as described in greater detail below. The number of guide vanes  130  may be selected so that there are a sufficient number to produce the desired rotational flow but not so many as to restrict flow or create a significant risk of catching debris entrained in the gas flow. Accordingly, approximately 3 to 8 guide vanes  130  may be provided in the burner assembly  100 . Each guide vane  130  may include a guide vane upstream surface  132  facing upstream toward the intermediate pipe  106  and oriented at a guide vane angle a relative to the burner pipe axis  104 . In some embodiments, the guide vane angle a may be approximately 20 to 45 degrees. Additionally, the guide vanes may be configured to have profiles that increase the efficiency with which rotation is imparted to the gas flow. 
         [0022]    A deflector  140  may be positioned downstream of the burner pipe  102  to stabilize the flame during operation. As shown in  FIGS. 1 and 2 , the deflector  140  may have a deflector upstream end  142  coupled to the downstream end  124  of the hub  120 , and a deflector downstream end  144 . The deflector  140  may include a deflector exterior surface  146  having a substantially frustoconical shape. More specifically, the deflector exterior surface  146  may extend radially outwardly from the burner pipe axis  104  and axially downstream from the deflector upstream end  142  to the deflector downstream end  144 . Accordingly, the deflector upstream end  142  may define a deflector upstream end diameter D 5  that is smaller than a deflector downstream end diameter D 6  defined by the deflector downstream end  144 . The deflector downstream end diameter D 6  may be sized relative to the expander pipe diameter D 3  to induce the desired gas flow pattern downstream of the burner pipe  102 . For example, the deflector downstream end diameter D 6  may be approximately 60 to 80% of the expander pipe diameter D 3 . Additionally, the deflector exterior surface  146  influences the flow pattern produced by the deflector  140 . In the illustrated embodiment, the deflector exterior surface  146  is oriented along a deflector surface angle β relative to the burner pipe axis  104 . In some applications, the deflector surface angle β may be approximately 20 to 45 degrees to produce the desired gas flow pattern. 
         [0023]    In operation, the gas flow is communicated to the burner assembly  100 . As the gas flow travels through the burner pipe  102 , the successively larger cross-sectional areas of the inlet pipe  105 , intermediate pipe  106 , and expander pipe  112  will reduce the superficial velocity of the gas flow. As the gas flow enters the expander pipe  112  from the intermediate pipe  106 , the relatively large and abrupt change in cross-sectional area may produce an internal recirculation zone  150  in the upstream portion of the expander pipe  112 . 
         [0024]    The hub  120  may obstruct a central portion of the gas flow through the downstream portion of the expander pipe  112 , thereby to create a perimeter gas flow  152 . The guide vanes  130  may impart a rotation of the perimeter gas flow generally centered about the burner pipe axis  104 , thereby to create a swirling gas flow, which may be substantially helical, as the gas flow exits the expander pipe  112 . Downstream of the burner pipe  102 , the deflector  140  directs the swirling gas flow radially outwardly, which creates a relatively large exterior recirculation zone  154  downstream of the deflector  140 . This exterior recirculation zone  154  further reduces gas flow velocity, thereby promoting stable and efficient combustion of the gas flow. 
         [0025]    Additionally, the burner assembly  100  is equipped with a set of pilot burners  155  needed for ignition of flame and stabilization of gas burning. The set of burners  155  may be positioned at the outer edge of the expander pipe  112 .  FIGS. 1 and 2  depict two pilot burners installed at the opposite sides of the expander pipe  112  in the zone of low flow velocity. However, the number and positions of pilot burners  155  may vary in size, type and location, deepening on the parameters of the operation, cost, safety requirements and/or convenience for an operator. 
         [0026]    The burner assembly  100  may create stable combustion of low calorific content gas flow under a variety of gas flow pressures and related superficial velocities.  FIG. 2 , for example, illustrates a sub-sonic gas flow through the burner. The superficial velocity of the gas flow may be determined by dividing the gas flow rate Q by the cross-sectional area A of the body through which it flows. With a known gas flow rate Q, the cross-sectional area A of the intermediate pipe  106  may be sized so that the superficial gas velocity Q/A is less than a sonic speed of the gas. When the superficial gas velocity is sub-sonic, the burner assembly  100  will decelerate the gas flow through the successive stages of the burner pipe  102 , and the swirling gas flow pattern exiting the burner pipe  102  will be directed over the deflector  140  to create the exterior recirculation zone  154 . 
         [0027]      FIG. 3  illustrates a gas flow rate that is substantially equal to the sonic flow rate in the intermediate pipe  106 . The burner assembly  100  operates in substantially the same fashion as noted above, with the exception that the incoming gas flow pressure and/or intermediate pipe cross-sectional area are selected so that the superficial gas velocity in the intermediate pipe  106  is substantially equal to the sonic velocity of the gas. As the superficial gas velocity achieves the sonic velocity in the inlet pipe  105 , a pattern of oblique shock waves  160  is generated within the intermediate pipe  106 . The shock wave pattern  160  is formed due to the increase in cross-sectional area of the intermediate pipe  106  as compared with inlet pipe  105 . The shock wave pattern  160  is illustrated in  FIG. 3  as a series of substantially conical structures. Traveling further downstream the burner pipe  102 , the shock wave cells  160  dissipate and the gas flow expands in the expander pipe  112  to flow at a sub-sonic velocity. The remainder of the gas pattern around the hub  120 , through the guide vanes  130 , and over the deflector  140  is substantially the same as that described above in connection with  FIG. 2 . 
         [0028]      FIG. 4  illustrates a gas flow having a superficial gas velocity that is at a supersonic velocity in the intermediate pipe  106 . In  FIG. 4 , the gas flow does not near the sonic or sub-sonic velocity until it flows through the expander pipe  112 . As shown in  FIG. 4 , the supersonic velocity of the gas will generate shock wave cells  162  within the expander pipe  112  that partly dissipate the energy of the gas flow. As the gas flow approaches the hub  120 , a direct shock wave  164  may be formed at the upstream apex  128  of the hub  120 . The gas flow may continue around the hub  120 , through the guide vanes  130 , and over the deflector  140  substantially as described above in connection with  FIGS. 2 and 3 . 
         [0029]    In view of the foregoing, burner assemblies and methods are provided that may efficiently combust low calorific content gas under a variety of pressures. As noted above, a gas flow pattern conducive to a stable flame is produced under subsonic, sonic, and supersonic gas velocities through the burner pipe  102 . The low amount of swirling induced by the guide vanes  130  stabilizes the gas flow and shortens the flame length. The conical deflector  140  further keeps the flame near the burner pipe outlet, thereby reducing the possibility of flame blow-off. In addition to creating the perimeter flow pattern, the hub  120  also helps prevent flashback by obstructing flow through the central portion of the expander pipe  112 . 
         [0030]    Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from the burner assembly and methods for flaring low calorific content gases disclosed and claimed herein. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures.