Abstract:
An apparatus and method for separating the output fluids of a well by way of efficiently applying heat to the fluids. Separation takes place in a sealed vessel to which heat is applied to the fluid by contact with a multi-pass firetube assembly through which hot exhaust gas is expelled from a forced draft burner. The firetubes forming the assembly are of smaller diameter than used in conventional separators and not subject to high leakage experienced in conventional separating vessels, and the heat transfer is highly efficient resulting in less fuel consumption and less emission of greenhouse gases.

Description:
CROSSED REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of convention priority on Canadian Patent Application No. 2,570,719, filed Dec. 8, 2006 entitled Heated Separation System For Well Fluids. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates to the gravity treating of fluid such as produced oil and water from a well, and more particularly, to a heating system for applying heat to the fluids to enhance separation. 
       BACKGROUND OF THE INVENTION 
       [0003]    Various types of separation systems are presently used in oil field facilities at the well head where oil, water and natural gas occur together naturally. 
         [0004]    A common type of such a system uses a treating vessel wherein the produced fluids of a well slowly travel through the vessel in order to basically allow the water to sink and the oil to float. The fluid properties determine the amount of resident time required for separation. Baffles and the like along the path of flow are commonly used to enhance this process, and again while properties of the fluids determines the amount of time required for separation, it is not unknown to utilize the addition of heat to the fluids as well to accelerate the separation. 
         [0005]    As indicated, such systems are used in field facilities where oil, water and natural gas occur together, and thus, in known systems natural gas, which was otherwise considered as being a waste, has been utilized to fire relatively small natural draft burners for heating the fluids. The efficiency of such natural draft burners was considered irrelevant because the fluid gas was in the past a waste product. Over the past decade or so, however, the separating vessels have become much larger causing the heating requirement to significantly increase, and as a result, it is difficult in many operations for the field facilities to provide enough fuel gas to fire larger burners of the type used in the past. Moreover, because of more recent escalating prices of natural gas amounts of gases produced and previously considered to be a waste product is now considered a saleable commodity. Furthermore, government regulations have become more strict to the point it is difficult for the typical natural draft burner systems to meet such new standards. 
         [0006]    Also, due to the nature of the known natural draft systems, which must utilize very large firetube diameters, such systems are known to encounter severe leakage characteristics and overall produce high amounts of greenhouse gases. 
       SUMMARY OF THE INVENTION 
       [0007]    It is an object of the present invention to provide in a separation vessel a heating system which provides more rapid and efficient heating of the fluids being separated without emitting significant greenhouse gases. 
         [0008]    According to one aspect of the invention, there is provided a heating system in a separating apparatus for applying heat to fluids introduced from a well, the apparatus including a vessel defining an internal, sealed chamber provided with an inlet for the well fluids and outlets for separated fluids. The heating system includes a forced draft burner having a hot exhaust outlet, and a multi-pass firetube assembly disposed within the vessel and having an inlet connected to the exhaust outlet of the forced draft burner and an outlet end external of the vessel. The firetube assembly thereby forms a repeated exhaust gas flow path therethrough from the forced draft burner to the atmosphere via the outlet, thus providing external firetube surfaces within the vessel and exposed to the fluids for transfer of heat thereto. 
         [0009]    Another aspect of the invention resides in a method of heating well fluids for separation at a well head site by introducing the fluids into a sealed inner chamber of a vessel and exposing the fluids to heat. The method includes the step of providing a multi-pass firetube assembly formed of continuously connected firetubes of relatively small diameter within the vessel to be exposed externally to the well fluids and having an inlet end and an outlet end external of the vessel, and wherein a forced draft burner having an exhaust gas outlet thereof is connected to the inlet end of the firetube assembly. The method includes the step of operating the forced draft burner to drive the exhaust gases thereof through the firetube assembly to thereby cause heat transfer from the exhaust gases of the forced burner to the well fluids. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]      FIG. 1  is a side view of the separation vessel incorporating the heating system of the present invention; 
           [0011]      FIG. 2  is a partial side view of a known type of separating vessel incorporating a system commonly presently used in the field; 
           [0012]      FIG. 3  is a side view of a portion of the vessel of  FIG. 1  and showing in dashed lines the multi-pass firetube assembly of the present invention, and, 
           [0013]      FIG. 4  is a cross section view taken through the separating vessel as indicated at  4 - 4  of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    Referring first briefly to  FIG. 1 , it can be seen that there is present a separating unit  10  formed by an elongated, sealed vessel  11  of circular cross section to which is transferred from an adjacent well (not shown) production material which normally consists of oil, water, natural gas, and possibly other impurities through vessel pipe  12 . After entering the vessel, the production materials flow towards the opposite end of the vessel during which separation of the constituents of well production occurs. Eventually, the fluids leave the vessel through separate outlets which includes oil and gas outlets shown as  13  and  15 , respectively. 
         [0015]    Returning now, however, to the features of the conventional natural draft burner system, reference is made to  FIG. 2  of the drawings, wherein such a burner system is shown for adding heat to the separation process generally designated  17 . The heating system thereof is generally designated  18  contained in an elongated separation vessel  20 . A natural draft burner is designated  21  and is located externally of the vessel at one end thereof. Exhaust gas produced by burner  21  flows into one end of a firetube  22  at a first end projecting from the end of the vessel  20 . The firetube is shown as being of a large diameter in order to permit the natural convention of the exhaust gases to pass through the single pass large diameter firetube consisting of a first flight  22   a  extending lengthwise within the vessel  17 , and a U-turn connection  22   b  joined to a return flight  22   c , which exits the end of the vessel  20  and is delivered to a stack  23 . 
         [0016]    As previously indicated, such natural gas burners used in the arrangement shown in  FIG. 2 , are inefficient, and for the amount of fuel consumed, the transfer of heat to the fluids within the vessel  17  is low. Subsequent to the original introduction of this known design the separation vessels have become much larger, with heating requirements becoming considerably greater to the extent that the facilities are unable to provide enough on site natural gas to fire these larger natural draft burners of conventional designs. Moreover, with the increase in price of natural gas the cost of operation is becoming prohibitive. Also, because of the lower efficiency of the large heat transfer firetube necessary in the use of the natural draft burner, high emissions are produced, typically 16,000 tonnes per year in CO 2  equivalent green house gases. In addition with the use of the large firetube diameters required with the natural draft burner, acceptable seals are very difficult, if not impossible to achieve in many cases. While it has been found that most users of the conventional system are prepared to accept some leakage, as a result of these other above described features of the now commonly used natural draft systems, it is becoming difficult for the users to meet the newer and stricter government standards. 
         [0017]    Returning now to the present invention as illustrated in  FIGS. 1 and 3 , it can be seen that the elongated cylindrically shaped vessel  11  is mounted on supports  25 , 25  for installation near a well head site (not shown). The product flow from the well enters the vessel through a line or inlet pipe  12 , ( FIG. 1 ) and flows towards the outlet end of the vessel during which time the oil and water content of the well output, which are in contact with the firetubes within the vessel, are separated and discharged separately from the vessel. Increasing the temperatures of the production flow towards the output end enhances separation, and thus the efficient transfer of heat from the exhaust gases produced by a forced draft burner  26 , is essential to a more rapid rate of separation. 
         [0018]    The hot exhaust issuing from forced draft burner  26  is injected into an inlet end  29  of a multi-pass firetube assembly  27  ( FIG. 3 ) as indicated by arrow  28 . After passing through the firetube assembly  27 , the then relatively cooled exhaust gases pass to atmosphere through an exhaust stack  30  which is connected to an outlet end of the multi-pass firetube assembly  27 . An important feature of the present invention is that in use with the forced draft burner  26 , in the exhaust gases are introduced into the inlet of the multi-pass firetube assembly  27  under significant pressure, thus making possible the use of a firetube diameter of considerably smaller diameter than now commonly used in the field. Also because of the pressure produced at the outlet of the forced draft burner  26  at its connection to an inlet end of the firetube assembly  27 , a much longer total length of firetube is possible. Accordingly, as illustrated, the firetube assembly is formed by multi-passes the separate flights of which are laterally and/or vertically spaced from each other for effect exposure to the outer circumferences of the different flights. 
         [0019]    With reference to  FIG. 3 , it may be seen that the output of the forced draft burner  26  is directed into an inner end of a first flight or length  32  of the multi-pass firetube assembly  27 , as indicated by arrow  28 . At the extreme outer end the first flight  32 , it is connected by a U-tube connecting firetube  33  to the outer end of a return flight  34 . The inner end of the return firetube flight  34  is in turn connected to the inner end of firetube flight  36  by a U-tube connecting firetube  35 . The outer end of the firetube flight  36  is connected by a U-tube connecting firetube  37  to a second return firetube flight  38 , the inner end of which firetube  38  is connected to the exhaust stack  30  through outlet  31 . 
         [0020]    An initial inlet and outlet portion of  29  and  31  of the multi-pass firetube assembly pass through an end plate  40  which is bolted to and properly sealed to a flanged open ended portion  41  of the vessel  11 . Thus, by providing an outwardly turned flange about the open-ended portion  41 , a fastening system, such as a bolting arrangement may be provided for quick fastening and removal of the end plate  40  from the portion  41  whereby the forced draft burner  26 , exhaust stack  30  as well as the inner end of the multi-pass firetube assembly  27 , at one end, is supported by the vessel. Along the length of the multi-pass firetube assembly  27  individual supporting structures (not shown) are provided within the vessel  11 . The arrangement of the firetube assembly is thus such that by removing the end plate  40 , the entire heating unit, including the forced draft burner  26 , multi-pass firetube assembly  27  and exhaust stack  30  can be removed as a unit from the vessel for any required maintenance to or overall replacement of the assembly can be carried out. 
         [0021]    Operation of the heating system of the present invention has shown to be highly efficient in relation to the conventional system described herein, and avoids serious problems of leakage as described in relation to units now in use. For example, as compared with green house gas emissions indicated above in relation to the conventional unit, test indications are that the emissions of the heating system of the present invention will be in the order of 6,700 tonnes per year. Additionally the more efficient combustion and heat transfer to the separation fluids results in faster separation and significant savings in gas consumption. Maintenance costs are also reduced, and more efficient separation of the products of the well are achieved with the higher heat transfer rate to the products. 
         [0022]    While a single embodiment of the invention has been shown and described, variations within the spirit of the present invention as defined in the accompanying claims will be obvious to those skilled in the art.