Abstract:
A method, system, and apparatus for controlling the gas temperature of gas flowing between compression stages so that the temperature of the gas always remains above the dew-point and hydrate temperature of the gas. The invention also allows for controlling inner stage compression temperatures that can be part of a new compressor assembly or retrofitted to compressors already installed and operating.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of the filing of U.S. Provisional Patent Application Ser. No. 61/798,380, entitled “Compressor Inter-Stage Temperature Control”, filed on Mar. 15, 2013, and the specification thereof is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention (Technical Field) 
     Embodiments of the present invention relate to a method, system, and apparatus for controlling the gas temperature of the gas flowing between compression stages so that the temperature of the gas always remains above the dew-point and hydrate temperature of the gas. A further embodiment of the present invention provides a method for controlling inner stage compression temperatures that can be part of a new compressor assembly or retrofitted to compressors already installed and operating. 
     2. Description of Related Art 
     To collect gases that are commonly vented by process equipment used at natural gas locations, including but not limited to well sites and processing plants, a flash gas compressor is commonly used. The flash gas compressors operate at pounds of suction pressure (usually 25 to 100 pounds per square inch gauge (“psig”) and discharge pressures generally above 100 and less than 1500 psig). The compressors generally have from one to four compression stages and the gas entering and leaving the inner compression stages is cooled by flowing through a radiator utilizing air, driven by a fan, as the heat sink. 
     During cool weather, the air circulating through the gas cooler radiator can cool the gas to a temperature below the hydrocarbon dew-point of the gas as well as cooling the gas to a temperature that causes gas hydrates to form. 
     Cooling the gas below its dew-point results in hydrocarbon liquids forming in the gas stream. The hydrocarbon liquids are separated from the gas by scrubbers installed between the compression stages. The hydrocarbon liquids that condense and are separated by the scrubbers are commonly called “recycle loops”. Depending upon the British Thermal Units of heat (“BTU”) of the gas, how low the ambient temperature is, and how high the inner stage compression pressures are, the recycle loops, when dumped back to the storage tank, can create a volume of hydrocarbon vapors that will overload the vapor recovery system or require installation of a pressurized storage tank or flaring. 
     Cooling the gas below the hydrate formation temperature causes ice crystals to form in the radiator, potentially blocking the flow of gas. There is thus a present need for a method, system, and apparatus which can consistently cool the gas between multiple stages of compression so that the gas temperature remains above the dew-point of the gases and below a temperature which would cause thermal damage to the compressor when the vapors are compressed in a subsequent stage of compression. 
     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. 
     BRIEF SUMMARY OF EMBODIMENTS OF THE PRESENT INVENTION 
     An embodiment of the present invention relates to a system for maintaining temperature of a gas between stages of compression including introducing the gas into a first stage of compression; directing the gas from the first stage of compression to a heat exchanger; directing the gas from the heat exchanger to an interstage scrubber; directing the gas from the interstage scrubber to a second stage of compression; and controlling a volume of coolant traveling through the heat exchanger by sensing a temperature of the gases that have exited a second stage of compression. 
     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 DRAWING 
       The accompanying drawing, which is incorporated into and forms a part of the specification, illustrates one or more embodiments of the present invention and, together with the description, serves to explain the principles of the invention. The drawing is only for the purpose of illustrating one or more preferred embodiments of the invention and is not to be construed as limiting the invention. 
         FIG. 1  is a flow diagram illustrating an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , a flow diagram is illustrated wherein a two stage compressor is equipped with a liquid cooling section to control the temperature of compression.  FIG. 1  also illustrates an optional gas recycle loop that includes a liquid cooling section to cool the recycle gas going to the inlet scrubber. Embodiments of the present invention can provide desirable results without this optional gas recycle loop. 
     Referring to  FIG. 1 , system  100  preferably includes gas inlet line  1 , which leads to compressor inlet scrubber  2 . Liquid level controller  56  preferably controls the liquid level in inlet scrubber  2 . A supply gas source (not shown) is preferably connected to liquid level controller  56 . Tubing line  58  preferably carries a pneumatic signal from liquid level controller  56  to motor valve  57 . Gas line  3  can connect outlet  4  of inlet scrubber  2  to suction port  5  of first stage  6  of a compressor. Gas flow line  8  can connect discharge  7  of first stage of compression  6  to inlet  9  of a tube section of heat exchanger  10 . Gas line  11  preferably connects outlet  12  of the tube section of heat exchanger  10  to inlet  13  of first stage inter stage scrubber  14 . Liquid level controller  59  preferably has a supply gas source (not shown). Motor valve  60  can be powered by a pneumatic signal traveling through line  61  from liquid level controller  59 . Gas line  16  preferably connects outlet  15  of inter stage scrubber  14  to suction port  17  of second stage of compression  19 . Gas line  20  preferably carries the compressed gas from discharge port  18  of second stage of compression  19  to point  62 . Line  63  preferably sends the gas from point  62  to be further processed or to sales. Thermostat  21  has a supply gas source (not shown). Tubing line  23  can carry the output signal from thermostat  21  to motor valve  24 . Motor valve  24  can be a throttling type motor valve. Motor valve  24  can be installed on either the inlet or outlet of heat exchanger  10 . As illustrated, line  26  preferably connects outlet  27  of motor valve  24  to inlet  28  of a shell section of heat exchanger  10 . Line  29  can connect outlet  30  of the shell section of heat exchanger  10  to point  157 . Line  72  preferably connects from point  157  to inlet  31  of air cooled radiator  32 . Line  35  connects outlet  34  of air cooled radiator  32  to inlet  36  of liquid reservoir  37 . Line  39  connects outlet  38  of liquid reservoir  37  to inlet  43  of centrifugal pump  40 . Line  44  connects the outlet  42  of centrifugal pump  40  to point  45 . Line  46  connects from point  45  to inlet  41  of throttling motor valve  24 . 
     Referring again to  FIG. 1  which illustrates an embodiment of the invention, in one embodiment the unit can operate as follows: gas to be compressed enters the compressor system through line  1  into inlet scrubber  2 . From inlet scrubber  2  the gas flows through line  3  into first stage of compression  6 . The hot compressed gas from first stage of compression  6  flows through line  8  and enters, at inlet  9 , the tube section of heat exchanger  10 . While flowing through the tube section of heat exchanger  10  the hot compressed gas is cooled by the liquid contained in the shell section of heat exchanger  10 . In one embodiment, the cooled gas exits heat exchanger  10  at the outlet  12  and flows through line  11  to enter at the inlet  13  first inter stage scrubber  14 . While flowing through first inter stage scrubber  14 , any liquids that have been condensed by cooling of the gas in heat exchanger  10  are thus preferably separated. The separated liquids can be dumped by liquid level control  59  and motor valve  60  to a storage tank or other vessels (not shown). The cooled gases exit first inter stage scrubber  14  at outlet  15  and flow through line  16  to enter the second stage of compression  19  at suction port  17 . While flowing through the second stage of compression  19  the pressure and temperature of the gas is preferably increased. The gas exits second stage of compression  19  through discharge port  18  and flows through lines  20  and  63  to repeat the cooling process previously described for second stage  19  before optionally entering a third stage of compression. An additional fourth stage of compression can also optionally be used with the same cooling process as described for the second stage of compression  19 . After discharge from the final stage of compression, the compressed gas can be directed to a sales line or other location, such as for further processing. 
     Referring again to  FIG. 1 , the operation of an embodiment of the present invention is described. An embodiment of system  100  operates by utilizing an antifreeze water solution, which can be the same as the antifreeze water solution used in car radiators. The main components of the liquid cooling process is heat exchanger  10 , air cooled radiator  32 , centrifugal pump  40 , throttling motor valve  24 , coolant reservoir  37 , and thermostat  21 . The antifreeze solution is preferably stored in reservoir  37 . From outlet  38 , the antifreeze solution preferably flows line through  39  to the inlet  43  of centrifugal pump  40 . From the outlet  42  the pressurized antifreeze solution flows through lines  44  and  46  to the inlet of throttling motor valve  24 . The amount of the opening of throttling motor valve  24  can optionally be controlled by thermostat  21 . Thermostat  21  preferably senses the temperature of the gas exiting second stage of compression  19 . Thermostat  21  is preferably set to accomplish two things. First, thermostat  21  preferably maintains a temperature in inter stage scrubber  14  that is above the hydrocarbon dew point of the compressed gases flowing through inter stage scrubber  14 . The second is to maintain the temperature of the gases exiting the second stage of compression  19  to be below the maximum temperature rating of the compressor. Usually, the maximum rated temperature of a compressor will be in a range of about 300 degrees F. From outlet  27  of throttling control valve  24 , a controlled volume of the antifreeze solution flows through line  26  to the inlet  28  of the shell section of heat exchanger  10 . While flowing through the shell section of heat exchanger  10  the antifreeze solution acts as a heat sink to cool the hot compressed gas flowing through the tube section of heat exchanger  10 . By controlling, with thermostat  21  and throttling motor valve  24 , the volume of cool liquid entering the shell section of heat exchanger  10 , the temperature of the liquid in the shell section can be maintained at the temperature required to only cool the hot gases flowing through the tube section of heat exchanger  10  enough to keep the temperature of the gas exiting the second stage of compression within an acceptable temperature operating range and so as to avoid thermal damage to the compressor. 
     The antifreeze solution preferably exits heat exchanger  10  and flows through lines  29  and  72  to the inlet of air cooled radiator  32 . Air cooled radiator  32  can optionally be a radiator similar to a radiator on a car or truck, or, as illustrated, it can be of a design where the radiator and coolant reservoir are separate units. While flowing through radiator  32 , the antifreeze solution is preferably cooled by air which is driven by a fan. The cooled antifreeze solution exits radiator  32  and flows through line  35  to inlet  36  of coolant reservoir  37 . The antifreeze solution exits coolant reservoir  37  and flows through line  39  to the inlet  43  of coolant pump  40 . Coolant pump  40  can be a centrifugal pump. When throttling motor valve  24  begins closing, the discharge pressure of centrifugal pump  40  preferably increases slightly but the pump continues running normally. 
     Although embodiments of the present invention most preferably operate via pneumatic control using pneumatically-operated components, other types of control and sources for power of operation can optionally be used in place of or in conjunction with one or more of the pneumatic controls and pneumatically-powered components. Accordingly, in one embodiment one or more electrically-operated components can be provided. Optionally, one or more manually-powered controls can be provided and an operator can operate them. 
     In one embodiment, the present invention is disposed at a well site and not at another location. In one embodiment, the present invention is skid-mounted. 
     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.