Abstract:
An apparatus and method for controlling a combustor on a vapor recovery process system which prevents excessive oxygen from entering a natural transmission line and which prevents the need to vent natural gas to the atmosphere.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This Application is a continuation-in-part of “Liquid Hydrocarbon Slug Containing Vapor Recovery System”, Ser. No. 12/043,685, filed on Mar. 6, 2008, and the specification and claims thereof are incorporated herein by reference. This application also claims priority to and the benefit of the filing of U.S. Provisional Patent Application Ser. No. 61/089,383, entitled “Combustor Control”, filed on Aug. 15, 2008, and the specification thereof is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention (Technical Field) 
         [0003]    Embodiments of the present invention relate to a method and apparatus for controlling a hydrocarbon combustor as part of a vapor recovery process system. 
         [0004]    2. Description of Related Art 
         [0005]    On natural wells or well pads where a vapor recovery process system, as described in U.S. patent application Ser. No. 12/043,685, has been applied, there is a need to automatically control the operation of a combustor (sometimes called a flare). The purpose of a combustor is to burn hydrocarbon emissions generated by the dumping of hydrocarbon liquids from the higher pressure of the flowing natural gas line to the lower pressure of the storage tank. The physics of the generation of hydrocarbon emissions during the reduction of applied pressure on hydrocarbon liquids has been previously described. 
       BRIEF SUMMARY OF EMBODIMENTS OF THE PRESENT INVENTION 
       [0006]    In one embodiment, the present invention relates to a method for controlling a combustor on a vapor recovery process system which includes allowing hydrocarbon vapors to flow from a tank to a control valve, sensing an oxygen level within the hydrocarbon vapors via an oxygen sensor, opening the control valve when the oxygen level reaches a predetermined level, causing the hydrocarbon vapors to flow to a combustor, and igniting the hydrocarbon vapors in the combustor. The hydrocarbon vapors can be caused to flow to the combustor for a predetermined duration of time, which duration can optionally be user-selectable. The hydrocarbon vapors can flow to a combustor until a predetermined oxygen level is achieved. The oxygen sensor can be disposed upstream from a natural gas compressor, which can optionally be in a tank and/or can optionally be disposed such that it measures combined vapors from a low-pressure separator and a tank. In one embodiment, the method can include monitoring a pressure of the tank. 
         [0007]    The method can also include attempting to maintain a pressure within the tank of at least 3 ounces per square inch. The natural gas compressor can be activated when a pressure of the tank is greater than a predetermined level and the oxygen level is below a predetermined level. 
         [0008]    An embodiment of the present invention also relates to a method for preventing excessive oxygen from entering a natural gas transmission line which includes operating a vapor recovery process system, wherein vapors from hydrocarbons are compressed with a compressor in a natural gas compressor for transmission in a natural gas transmission line; providing a combustor communicably coupled to a control valve; activating the control valve when the compressor stops such that hydrocarbons are caused to flow to the combustor; and deactivating the control valve. In one embodiment of the method, the control valve can be deactivated based at least partially on an output of an oxygen sensor. The control valve can be deactivated based at least partially on a tank pressure and/or based at least partially upon an output of an oxygen sensor. The compressor can be deactivated based at least partially on a tank pressure and/or an output of an oxygen sensor. 
         [0009]    In one embodiment of the method, hydrocarbons can be selectively caused to flow to either the combustor or the compressor. Optionally, the compressor or the combustor can be operated so as to maintain a tank pressure of at least 3 ounces per square inch or of at least 5 ounces per square inch. In one embodiment the compressor and the combustor are operated so as to maintain a tank pressure of at least 3 ounces per square inch. The method can also include igniting the hydrocarbons in the combustor. 
         [0010]    An embodiment of the present invention also relates to a vapor recovery process apparatus which includes a hydrocarbon containment tank, a compressor, an oxygen sensor positioned upstream of the compressor, and a hydrocarbon combustion chamber, the chamber burning hydrocarbons based at least in part on a reading from the oxygen sensor. In one embodiment, the combustion chamber burns hydrocarbons based at least in part on a pressure of the tank. The compressor can operate at least in part based on a reading from oxygen sensor. Optionally, a valve is provided which can be operated at least in part based on a reading from the oxygen sensor. Hydrocarbon vapors can flow to the compressor when the valve is in a first position and the hydrocarbon vapors can flow to the combustion chamber when the valve is in a second position. 
         [0011]    Optionally, the compressor can be operated to maintain a pressure of at least 3 ounces per square inch in the tank. The combustion chamber can be operated to maintain a pressure of at least 3 ounces per square inch in the tank. 
         [0012]    Objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0013]    The accompanying drawings, which are incorporated into and form a part of the specification, illustrate one or more embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating one or more preferred embodiments of the invention and are not to be construed as limiting the invention. In the drawings: 
           [0014]      FIG. 1  is a schematic diagram illustrating a stand-alone, slug containing vapor recovery process system of an embodiment of the present invention, wherein a combustor is provided for burning off hydrocarbon vapors; 
           [0015]      FIG. 2  is a schematic diagram which illustrates a configuration of the intermediate pressure slug containing vessel of an embodiment of the present invention; and 
           [0016]      FIG. 3  is a schematic diagram illustrating the operation of pipe coils of an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    When a vapor recovery process system is applied to a natural gas well or well pad, the generated hydrocarbon emissions are recovered and compressed back into the sales line, and, except where an oxygen sensor is being used, the combustor is shut off while the vapor recovery process system is operating. Accordingly, no generated hydrocarbon emissions are burned. To prevent generated hydrocarbon emissions from being vented to the atmosphere when the vapor recovery process system is not operating, a combustor is preferably automatically turned on when the engine on the vapor recovery process system stops. 
         [0018]    Referring now to  FIGS. 1-3 , flow line  77  preferably allows hydrocarbon vapors to flow from tank  22  to control valve  78 . Electrical line  79  is preferably connected from controller  28  to solenoid  81 , which is most preferably a three-way solenoid. Although controller  28  is most preferably a programmable logic controller, a microcontroller, microprocessor, or other type of controller can be used and will provide desirable results. Control valve  78  is preferably open or closed depending upon whether the vapor recovery process system engine is running or stopped or whether oxygen sensor  84  (see  FIG. 3 ) has opened control valve  78  to vent oxygen from tank  22 . Tubing line  80  preferably carries a pressure signal from solenoid  81  to control valve  78 . Flow line  83  preferably allows the hydrocarbon vapors in tank  22  to flow to combustor  82  when control valve  78  is open. 
         [0019]    In operation of an embodiment of the present invention, solenoid  81  preferably receives through line  79  an electrical signal from controller  28 . When the engine on the vapor recovery process system is operating normally, solenoid  81  is closed and control valve  78  is closed. When the engine on the vapor recovery process system stops, or when oxygen sensor  84  senses oxygen in tank  22 , controller  28  preferably sends an electrical signal to solenoid  81 . Solenoid  81  opens sending a pressure signal through tubing line  80  opening control valve  78 . The opening of control valve  78  allows the hydrocarbon vapors from tank  22  to flow through flow lines  77  and  83  to combustor  82 . Combustor  82  preferably has a pilot flame that ignites the hydrocarbon vapors, thereby preventing the vapors from polluting the atmosphere. When the engine on the vapor recovery process system is restarted or the oxygen sensor is not requiring the tank to be vented, the system is preferably reversed and the hydrocarbon vapors from tank  22  are again routed to the vapor recovery process system for compression into flow line  26 . 
         [0020]    When measurable oxygen is present in the vapors being compressed, by the vapor recovery process system, into flow line  26 , the combustor is preferably automatically turned on to vent and purge the oxygen from tank  22 . While operating, the vapor recovery process system maintains a positive pressure of about  8  ounces per square inch on the storage tank; so, the only time oxygen can be in the collected vapors going to the vapor recovery process system from tank  22  is after a thief hatch or other opening in the tank has been opened to the atmosphere thereby allowing the pressurized hydrocarbon vapors to be released to the atmosphere as well as allowing air to enter the tank. There are two routine tank operations that typically require the thief hatch on the tank to be opened to the atmosphere. One of the routine operations, where the thief hatch is opened to the atmosphere, is when the tank fluid level is gauged. Tank gauging can occur several times a week. The other routine operation where the tank thief hatch is commonly opened to the atmosphere is when the fluids in the tank are pumped into a tanker truck. 
         [0021]    Oxygen sensor  84  is preferably installed on the vapor recovery process system to prevent oxygen from entering flow line  26  while the vapor recovery process system is running and air has been introduced into storage tank  22 . A preferred oxygen sensor is supplied by Teledyne Instruments, although any suitable oxygen sensor may be used in accordance with the present invention.  FIG. 3  is a schematic illustration of an embodiment of the present invention wherein electrical line  85  connects oxygen sensor  84  to controller  28 . 
         [0022]    As previously noted, oxygen sensor  84  preferably senses the collected vapors while the vapor recovery process system is running. The sampling connection for oxygen sensor  84  is preferably installed in flow line  53  (see  FIG. 3 ). When the vapor recovery process system is operating, flow line  53  preferably contains collected vapors from both low-pressure separator  14  and tank  22 . When oxygen sensor  84  senses an oxygen content of about 25 parts per million (“ppm”) to about  50  ppm in the collected vapors being compressed by the vapor recovery process system, oxygen sensor  84  sends an electrical signal through electric line  85  to controller  28 . Controller  28  then preferably sends an electric signal to solenoid  81  and, as previously described, the sequence of opening tank  22  to the combustor begins. During the time that tank  22  is opened to the combustor, the vapor recovery process system continues collecting and compressing the vapors from low-pressure separator  14 . Since tank  22  is the oxygen source and since tank  22  is opened to the combustor, oxygen sensor  84  will return to its operating range (from about 25 to about 50 ppm) in a short period of time, at which point the electric signal to controller  28  preferably stops. Once the electric signal from oxygen sensor  84  is stopped, controller  28  causes tank  22  to stop venting to the combustor. 
         [0023]    When the pressure in tank  22  returns to a pressure of from about  3  to about  20  ounces per square inch and more preferably from about  5  to about  10  ounces per square inch and most preferably from about  7  to about  8  ounces per square inch, the vapor recovery process system again begins to collect the vapors from tank  22 . If the venting and purging of tank  22  has not been long enough to rid tank  22  of a sufficient quantity of oxygen, oxygen sensor  84  restarts the venting sequence. To prevent an on and off venting cycle from occurring, controller  28  is preferably field programmed to have a venting cycle duration of virtually any user-selected length. The length is most preferably programmed such that it is sufficient to lower the oxygen level of the vapors from tank  22  such that the level of the oxygen content of the combined vapors (from low-pressure separator  14  and tank  22 ) is reduced to a permissive level. Most preferably, that oxygen content is lowered until it is within the operating range of oxygen sensor  84  for any particular application. In some applications, several days of operations on each location may be required in order to determine the length of time required for the venting cycle to adequately complete the oxygen purging of tank  22 . In one embodiment, oxygen sensor  84  can optionally be disposed within tank  22 . In this embodiment, oxygen sensor  84  or interfacing circuitry is most preferably calibrated such that the percentage of permissive oxygen is increased over that which is permissive at a point past the combined vapors since the percentage of oxygen which occurs in tank  22  will be significantly reduced after the combination of the vapors. 
         [0024]    Although the invention has been described in detail with particular reference to these preferred embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents. The entire disclosures of all references, applications, patents, and publications cited above are hereby incorporated by reference.