Patent Publication Number: US-2023160479-A1

Title: Systems and methods for purging liquid from a liquid fuel supply system

Description:
BACKGROUND 
     The embodiments described herein relate generally to a liquid fuel supply system for a gas turbine engine and, more particularly, to a three-way valve used to purge liquid from a liquid fuel supply system. 
     Land-based, heavy-duty gas turbine engines are commonly used to generate electricity. At least some known gas turbine engines operate using a gaseous fuel and a liquid fuel. For example, at least some known gas turbine engines may use the liquid fuel when the gaseous fuel is unavailable or is undesirable. Moreover, when the gas turbine engine is operating on the gaseous fuel, the parallel liquid fuel supply system may store a portion of the liquid fuel in the fuel lines, for example, in standby mode. Although the liquid fuel may be drained from areas of the system near the combustors, because of the geometry and configuration of equipment within the system, some residual liquid fuel may still remain in those areas of the liquid fuel supply system that were drained. 
     With at least some known gas turbine engines, combustion of the gaseous fuel increases the operating temperatures in the combustors and in areas adjacent to the combustors, including portions of the liquid fuel supply system. The increased operating temperature of the portion of the liquid fuel supply system adjacent to the combustors may cause oxidation and/or partial decomposition of the residual liquid fuel in the liquid fuel supply system, thereby producing coke in the fuel lines and/or valves in a process known as “coking.” Over time, continued coking may create hard deposits being formed in the liquid fuel supply system. Such deposits may clog and/or foul the associated fuel lines and valves and/or may interfere with the transfer of liquid fuel through the liquid fuel supply system. Depending on the severity of the coking, the gas turbine engine may be required to shut down for maintenance. 
     To facilitate preventing fuel from becoming stagnant and thus susceptible to coking, at least some known gas turbine engines circulate purge gas through the liquid fuel supply system. For example, at least some known systems purge the liquid fuel lines with a gas, such as nitrogen, to enable the remaining liquid fuel and/or gas to be drained from the liquid fuel supply system. Despite purging the liquid fuel supply system, some residual liquid fuel may remain in the liquid fuel system because of its geometry and configuration. For example, because of the alignment of some valves and/or fittings, cavities may be formed within the liquid fuel supply system can contain residual liquid fuel and thus may be susceptible to coking. 
     BRIEF DESCRIPTION 
     In one aspect, a three-way valve for a liquid fuel supply system is provided. The three-way valve includes a housing defining a liquid fuel inlet, a purge gas inlet, and at least one drain port. The liquid fuel inlet is sized to receive liquid fuel therethrough for selectively channeling the liquid fuel to a combustor of a gas turbine engine. The purge gas inlet is sized to receive purge gas therethrough for selectively purging liquid fuel from the three-way valve. The at least one drain port is oriented to selectively channel liquid fuel from the three-way valve when purge gas is purging liquid fuel from the three-way valve. 
     In another aspect, a liquid fuel supply system is provided. The liquid fuel supply system includes a three-way valve including a housing defining a liquid fuel inlet and a purge gas inlet. The liquid fuel inlet is sized to receive liquid fuel therethrough for selectively channeling the liquid fuel to a combustor of a gas turbine engine. The purge gas inlet is sized to receive purge gas therethrough for selectively purging liquid fuel from the three-way valve. The purge gas inlet includes a purge gas port and a purge gas channel. The purge gas channel defines a purge gas channel diameter. The liquid fuel supply system also includes a fitting sized to be inserted into the purge gas inlet and oriented to channel purge gas into the purge gas inlet. The fitting defines a fitting conduit defining a fitting conduit diameter. The fitting conduit diameter is equal to the purge gas channel diameter. 
     In yet another aspect, a method of selectively purging liquid fuel from a liquid fuel supply system is provided. The liquid fuel supply system includes a three-way valve and a fitting. The method includes inserting the fitting into a purge gas inlet of the three-way valve. The three-way valve includes a housing including a liquid fuel inlet, the purge gas inlet, at least one drain port, and an outlet. The method also includes channeling liquid fuel into the liquid fuel inlet and through a housing of the three-way valve to the outlet. The method further includes stopping the flow of liquid fuel through the three-way valve. The method also includes channeling purge gas from the fitting into the purge gas inlet and through the housing to the outlet. The method further includes draining liquid fuel from the purge gas chamber through the at least one drain port. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG.  1    is a schematic diagram of an exemplary dual-fuel turbine engine; 
         FIG.  2    is a schematic diagram of a liquid fuel supply system that may be used with the turbine engine shown in  FIG.  1   ; 
         FIG.  3    is a schematic cross-sectional diagram of an exemplary three-way valve that may be used with the liquid fuel supply system shown in  FIG.  2   ; 
         FIG.  4    is a schematic cross-sectional diagram of an exemplary fitting positioned within a purge gas inlet of the three-way valve shown in  FIG.  3   ; and 
         FIG.  5    is a block diagram of an exemplary method of purging the three-way valve shown in  FIG.  3   . 
     
    
    
     Unless otherwise indicated, the drawings provided herein are meant to illustrate features of embodiments of the disclosure. These features are believed to be applicable in a wide variety of systems comprising one or more embodiments of the disclosure. As such, the drawings are not meant to include all conventional features known by those of ordinary skill in the art to be required for the practice of the embodiments disclosed herein. 
     DETAILED DESCRIPTION 
     In the following specification and the claims, reference will be made to a number of terms, which shall be defined to have the following meanings. 
     The singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. 
     “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not. 
     Unless otherwise indicated, approximating language, such as “generally,” “substantially,” and “about,” as used herein indicates that the term so modified may apply to only an approximate degree, as would be recognized by one of ordinary skill in the art, rather than to an absolute or perfect degree. Approximating language may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about,” “approximately,” and “substantially,” is not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations are identified. Such ranges may be combined and/or interchanged, and include all the sub-ranges contained therein unless context or language indicates otherwise. 
     Additionally, unless otherwise indicated, the terms “first,” “second,” etc. are used herein merely as labels, and are not intended to impose ordinal, positional, or hierarchical requirements on the items to which these terms refer. Moreover, reference to, for example, a “second” item does not require or preclude the existence of, for example, a “first” or lower-numbered item or a “third” or higher-numbered item. 
     The exemplary components and methods described herein overcome at least some of the disadvantages associated with known liquid fuel supply systems for land-based, power-generating gas turbine engines and, in particular, gaseous fuel/liquid fuel turbine engines (“dual-fuel turbine engines”). The exemplary embodiments described herein include a three-way valve and a fitting for inhibiting the formation of coke deposits in the liquid fuel supply systems for the dual-fuel turbine engines. 
     The three-way valve includes a liquid fuel inlet, a purge gas inlet, and an outlet. The fitting is coupled to the purge gas inlet to enable purge gas to be channeled into the three-way valve to remove residual liquid fuel from the three-way valve when the gas turbine engine is operating on gaseous fuel or is being maintained. As described herein, the three-way valve includes one or more drain ports that facilitate inhibiting coke formation in the liquid fuel supply system. Specifically, the drain port(s) are positioned to enable residual liquid fuel to be drained from the three-way valve. Additionally, the fitting is sized and shaped to correspond to the purge gas inlet such that any dead space between the fitting and the purge gas inlet is minimized, thus reducing an amount of residual liquid fuel that could accumulate in the dead space and facilitating reducing coking within the dead space. Accordingly, the three-way valves and fittings described herein facilitate inhibiting coke formation in the liquid fuel supply system. 
       FIG.  1    is a schematic diagram of an exemplary dual-fuel turbine engine  100 , such as a land-based turbine engine used to generate electricity. In the exemplary embodiment, turbine engine  100  uses a liquid fuel, such as heavy fuel oil, kerosene, naphtha, condensates, and/or any other suitable liquid fuel, or a gaseous fuel, such as natural gas, to operate. Turbine engine  100  includes a liquid fuel supply system  102  that supplies liquid fuel  104  to turbine engine  100  from a fuel source  106  (shown in  FIG.  2   ). In some embodiments, one or more fuel nozzles (not shown) in a combustor  116  of turbine engine  100  may receive liquid fuel  104  and one or more other fuel nozzles may receive a gaseous fuel (not shown). 
     In the exemplary embodiment, liquid fuel supply system  102  also receives a purge gas  108  from a purge gas system  110  (shown in  FIG.  2   ), for example, when turbine engine  100  is not operating on liquid fuel  104 . As used herein, “purge gas”  108  may include nitrogen, air or “instrument air,” such as supply of air that is purified or otherwise substantially excludes contaminants, and/or any other suitable gas, such as any gas (which may be pressurized) that does not pose a risk of auto-ignition and/or is otherwise inert and/or purified, as described herein. Purge gas  108  may be used and/or available from a purge gas source  112  (shown in  FIG.  2   ) available, for example, in a power plant associated with turbine engine  100 . For example, and without limitation, purge gas  108  is channeled to turbine engine  100  to facilitate inhibiting and/or reducing coking of liquid fuel  104 . In some embodiments, purge gas  108  may be heated to any suitable temperature, such as to within a range of a combustion temperature of gaseous fuel and/or liquid fuel, and/or any other suitable temperature. In alternate embodiments, purge gas  108  may be cooler than a combustion temperature, such as less than a combustion temperature by a predetermined amount. 
     In the exemplary embodiment, turbine engine  100  combusts liquid fuel  104  to produce power and purges a portion of turbine engine  100  with purge gas  108  after combustion is complete. Purging turbine engine  100  with purge gas  108  facilitates reducing coking within the fuel lines and/or valves. Residual liquid fuel  104  may remain in turbine engine  100  after combustion is complete, and purge gas  108  enables the residual liquid fuel  104  to be removed from turbine engine  100 , thus facilitating reducing coking within the fuel lines and/or valves. Specifically, a three-way valve  140  ( FIG.  2   ) within liquid fuel supply system  102  receives purge gas  108  from purge gas system  110  to enable the residual liquid fuel  104  to be purged from turbine engine  100  to facilitate reducing coking within the fuels line and/or valves. 
     In the exemplary embodiment, turbine engine  100  also includes a compressor  114 , combustor  116 , a turbine  118 , a shaft  120 , an air intake  122 , and a load  124 . Compressor  114 , turbine  118 , and load  124  are rotatably coupled to each other via shaft  120 . Air intake  122 , compressor  114 , combustor  116 , and turbine  118  are arranged in a serial configuration such that combustion air  126  is channeled from air intake  122  to turbine  118 . Additionally, liquid fuel supply system  102 , combustor  116 , and turbine  118  are also arranged in a serial configuration such that liquid fuel  104  and/or purge gas  108  are channeled from liquid fuel supply system  102  to turbine  118 . Liquid fuel supply system  102  channels liquid fuel  104  into combustor  116 , and combustor  116  combusts combustion air  126  with liquid fuel  104  to generate combustion gases  128  that are channeled to turbine  118 . 
     During operation, air intake  122  draws combustion air  126  into compressor  114 , and compressor  114  compresses combustion air  126  and channels combustion air  126  into combustor  116 . Liquid fuel supply system  102  channels liquid fuel  104  into combustor  116 , and combustor  116  combusts combustion air  126  with liquid fuel  104  to generate combustion gases  128 . Combustion gases  128  are channeled to turbine  118  to cause turbine  118  to rotate. Turbine  118  rotates shaft  120 , which rotates compressor  114  to facilitate compressing combustion air  126  and rotating load  124  to facilitate generating power. 
     Residual liquid fuel  104  may remain in turbine engine  100  after turbine engine  100  is no longer combusting or operating with liquid fuel  104  to produce power. During such operational times, residual heat within turbine engine  100  may cause coking of the residual liquid fuel  104 . Coking can negatively impact the operation of turbine engine  100 . For example, coking can reduce the flow area of liquid fuel lines. In addition, coke deposits can harden over time and cause one or more valves in liquid fuel supply system  102  to seize. Moreover, deposit fragments can flake off the fuel line surfaces, flow through open valves, and choke the fuel nozzles in combustor  116 . As such, coking can lead to uneven distribution of liquid fuel  104  in combustor  116 , which may result in tripping of turbine engine  100 . 
     Purge gas system  110  facilitates inhibiting coking within turbine engine  100  by channeling purge gas  108  through portions of turbine engine  100  to facilitate removing residual liquid fuel  104  prior to coking of the liquid fuel  104 . As such, purge gas system  110  facilitates improving the reliability and efficiency of turbine engine  100 . In addition, the operating and maintenance costs of turbine engine  100  are facilitated to be reduced. 
       FIG.  2    is a schematic diagram of liquid fuel supply system  102  for use with turbine engine  100  (shown in  FIG.  1   ). In the exemplary embodiment, liquid fuel supply system  102  includes purge gas system  110  coupled in fluid communication with liquid fuel supply system  102 . Liquid fuel supply system  102  also includes a liquid fuel forwarding skid  130 , a stop valve  132 , a liquid fuel pump  134 , a control valve  136 , a fuel flow divider  138 , and a three-way valve  140 . Liquid fuel  104  flows into liquid fuel supply system  102  from liquid fuel forwarding skid  130 . 
     During liquid fuel operation of turbine engine  100 , stop valve  132 , between forwarding skid  130  and liquid fuel pump  134 , is opened, and liquid fuel  104  is channeled to liquid fuel pump  134 . Liquid fuel pump  134  generates a positive fuel flow through control valve  136  and into fuel flow divider  138 . In the exemplary embodiment, liquid fuel pump  134  includes, for example, and without limitation, a positive displacement pump, a centrifugal pump, and/or any other fluid moving device that enables liquid fuel supply system  102  to function as described herein. 
     In the exemplary embodiment, fuel flow divider  138  divides liquid fuel  104  into a number of fuel streams equal to the number of fuel nozzles for each combustor  116  (only one of which is shown in  FIG.  2   ). When turbine engine  100  is operating on gaseous fuel, portions of liquid fuel supply system  102  may remain charged with liquid fuel  104  while portions of liquid fuel supply system  102  are purged with purge gas  108  to facilitate purging liquid fuel  104  from at least some portions of liquid fuel supply system  102 , thus reducing coking within portions of liquid fuel supply system  102 . For example, components of liquid fuel supply system  102  may remain idle while both control valve  136  and stop valve  132  remain in a closed position. 
     In at least some embodiments, instrument air actuates three-way valve  140  associated with each combustor  116  to facilitate preventing liquid fuel  104  from entering each respective combustor  116 . Purge gas  108  is then channeled into three-way valve  140 , such as continuously and/or in pulses or bursts, to facilitate purging liquid fuel  104  from three-way valve  140  to facilitate reducing coking within three-way valve  140 . In the exemplary embodiment, control valve  136  regulates (i.e., permits, prevents, and/or controls) the flow of liquid fuel  104  into three-way valve  140 . More specifically, control valve  136  facilitates controlling an amount and/or rate at which liquid fuel  104  flows into three-way valve  140 , thereby facilitating metering the flow rate into combustor  116 . Stop valve  132  and control valve  136  may include, for example, and without limitation, a proportional valve, a solenoid valve, a servo valve, and/or any other type of fluid flow control valve that enables liquid fuel supply system  102  to function as described herein. 
     During gaseous fuel operations or maintenance of turbine engine  100 , liquid fuel  104  is pressurized up to three-way valve  140 . Liquid fuel lines  142  downstream from three-way valve  140  are purged with purge gas  108  to cause purge gas  108  to displace liquid fuel  104  in liquid fuel lines  142 . In some embodiments, liquid fuel  104  in liquid fuel supply system  102  can remain stagnant for long periods, for example, and without limitation, in some instances up to approximately six months or longer. During this stagnant period, a temperature of liquid fuel  104  in liquid fuel supply system  102  may reach or exceed temperatures of at least 350° Fahrenheit (° F.) (177 degrees Celsius (° C.)) due to its proximity to combustor  116 . The combination of the increased temperature and stagnation period can lead to the formation of coke deposits, for example, in three-way valve  140  and liquid fuel lines  142  of liquid fuel supply system  102 . Moreover, liquid fuel  104  residue can exist on the inner surfaces of liquid fuel lines  142  after purge operations. Purge gas  108  can enter liquid fuel lines  142  through three-way valve  140  and prevent residual liquid fuel  104  from remaining in contact with the hot metal surfaces of the liquid fuel lines  142 , where coking may occur. 
     In the exemplary embodiment, purge gas system  110  is coupled in fluid communication with liquid fuel supply system  102 , to enable purge gas  108  (shown in  FIG.  1   ) to be channeled into liquid fuel supply system  102  to facilitate inhibiting coking in liquid fuel supply system  102 . Purge gas system  110  includes purge gas source  112  that contains purge gas  108 . Purge gas source  112  can have any size and/or shape that that enables a desired amount of purge gas  108  to be contained or produced. 
     Typically, it is more economical to operate turbine engine  100  on gaseous fuel. However, when operating on gaseous fuel, liquid fuel  104  may remain stagnant for extended periods in liquid fuel supply system  102 , as described herein. Activating purge gas system  110  enables purge gas  108  to be channeled through three-way valve  140  to facilitate inhibiting and/or reducing coking in liquid fuel supply system  102 . For example, when purge gas system  110  is activated, as described herein, purge gas  108  forces the removal of liquid fuel  104  from portions of liquid fuel supply system  102  and turbine engine  100 , such as prior to and/or during operation using gaseous fuel. For example, immediately prior to, or simultaneously with, the transition to gaseous fuel operation, control valve  136  is closed, and purge gas system  110  is activated to purge liquid fuel lines  142 . Turbine engine  100  thus transitions from liquid fuel  104  operation to gaseous fuel operation. 
       FIG.  3    is a cross-sectional schematic diagram of an exemplary three-way valve  140 . In the exemplary embodiment, three-way valve  140  includes a housing  144  that includes a purge gas inlet  146 , a liquid fuel inlet  148 , an actuator air inlet  150 , an outlet  152 , at least one drain port  154 ,  156 , a purge gas chamber  158 , an intermediate chamber  160 , a liquid fuel chamber  162 , and an actuator air chamber  164 . Three-way valve  140  also includes a spool  166  positioned within purge gas chamber  158 , intermediate chamber  160 , and liquid fuel chamber  162 , a piston  168  positioned within liquid fuel chamber  162  and actuator air chamber  164  and coupled to spool  166 , and a spring  170  that circumscribes a portion of spool  166  within purge gas chamber  158 . 
     In the exemplary embodiment, purge gas chamber  158  includes a purge gas chamber inlet  172  coupled in flow communication with purge gas inlet  146  and a purge gas chamber outlet  174  coupled in flow communication with intermediate chamber  160 . Purge gas chamber  158  is also coupled in flow communication with drain ports  154  and  156  to enable draining residual liquid fuel  104  from three-way valve  140 . In the exemplary embodiment, three-way valve  140  includes a first drain port  154  and a second drain port  156 . In alternative embodiments, three-way valve  140  may include any other number of drain ports  154  and/or  156  that enables three-way valve  140  to operate as described herein including, without limitation, less than two drain ports, or three or more drain ports. 
     Additionally, in alternative embodiments, drain ports  154  and  156  may be coupled in flow communication with intermediate chamber  160 , liquid fuel chamber  162 , and/or actuator air chamber  164 . Liquid fuel chamber  162  includes a liquid fuel chamber inlet  176  coupled in flow communication with liquid fuel inlet  148  and a liquid fuel chamber outlet  178  coupled in flow communication with intermediate chamber  160 . Actuator air chamber  164  is coupled in flow communication with actuator air inlet  150 . Intermediate chamber  160  is coupled in flow communication with purge gas inlet  146  via purge gas chamber outlet  174  and with liquid fuel chamber  162  via liquid fuel chamber outlet  178 . Additionally, intermediate chamber  160  is also coupled in flow communication with outlet  152  to facilitate discharging liquid fuel  104  and/or purge gas  108  from three-way valve  140 . 
     In the exemplary embodiment, spool  166  is sized and shaped to facilitate switching between liquid fuel  104  and purge gas  108 . Specifically, in the exemplary embodiment, spool  166  includes a purge gas section  180 , an intermediate section  182 , and a liquid fuel section  184 . As shown in  FIG.  3   , purge gas section  180  and liquid fuel section  184  are each formed with a first diameter  186 , and intermediate section  182  is formed with a second diameter  188  that is larger than first diameter  186 . Second diameter  188  is selected to enable intermediate section  182  to facilitate preventing the flow of either liquid fuel  104  through liquid fuel chamber outlet  178  or purge gas  108  through purge gas chamber outlet  174 , during operation of three-way valve  140 . 
     Specifically, in the exemplary embodiment, second diameter  188  is approximately equal to an intermediate chamber diameter  190  such that intermediate section  182  facilitates preventing the flow of either liquid fuel  104  through liquid fuel chamber outlet  178 , or purge gas  108  through purge gas chamber outlet  174 , during operation of three-way valve  140 . Purge gas section  180  is sized and shaped to enable spring  170  to circumscribe a portion of purge gas section  180  to cause spool  166  to be biased away from purge gas chamber  158 . Liquid fuel section  184  is sized and shaped to enable interfacing with piston  168  to cause spool  166  to actuate towards purge gas chamber  158 . 
     During operation, actuator air  192  is channeled into actuator air chamber  164 , actuating piston  168  and spool  166  into the position shown in  FIG.  3   . Specifically, actuator air  192  causes piston  168  and spool  166  to transition towards purge gas chamber  158  such that intermediate section  182  substantially prevents the flow of purge gas  108  through purge gas chamber outlet  174 , while enabling the flow of liquid fuel  104  through liquid fuel chamber outlet  178 . Liquid fuel  104  is channeled into and through liquid fuel inlet  148 , liquid fuel chamber  162 , intermediate chamber  160 , and outlet  152 . Liquid fuel  104  is then channeled into combustor  116  for combustion, as described above. 
     When turbine engine  100  is operating on gaseous fuel, actuator air  192  is not channeled into actuator air chamber  164  and spring  170  biases piston  168  and spool  166  away from purge gas chamber  158  and towards liquid fuel chamber  162 . Specifically, spring  170  biases piston  168  and spool  166  towards liquid fuel chamber  162  such that intermediate section  182  prevents the flow of liquid fuel  104  through liquid fuel chamber outlet  178 , while enabling the flow of purge gas  108  through purge gas chamber outlet  174 . Purge gas  108  is channeled into and through purge gas inlet  146 , purge gas chamber  158 , intermediate chamber  160 , and outlet  152  to purge residual liquid fuel  104  from three-way valve  140 . 
     Additionally, in at least some embodiments, one or more drain ports  154  and/or  156  may be opened to drain residual liquid fuel  104  from purge gas chamber  158 , such as during and/or following receipt of purge gas  108  within purge gas chamber  158 . For example, in at least some embodiments, one or more drain ports  154  and/or  156  may be initially closed when purge gas  108  is received within purge gas chamber  158 , and subsequently opened, such as in response to halting receipt of purge gas  108  and/or as purge gas  108  continues to flow into purge gas chamber  158 . Moreover, in some embodiments, purge gas  108  may be pulsed (e.g., supplied in bursts or discontinuous streams) through purge gas inlet  146  to facilitate purging residual liquid fuel  104 . 
     Without such purging, residual liquid fuel  104  may remain in three-way valve  140  after three-way valve  140  is no longer channeling liquid fuel  104 , and operational or residual heat within turbine engine  100  may cause coking of the residual liquid fuel  104  during operation or after shut-down of turbine engine  100 . As described above, coke deposits can negatively impact the operation of turbine engine  100 . For example, deposit fragments can flake off of surfaces within three-way valve  140 , flow through outlet  152 , and choke the fuel nozzles in combustor  116 . As such, coke deposits may lead to uneven distribution of liquid fuel  104  in combustor  116 , which, depending on the severity of the uneven distribution, can result in tripping, i.e., an immediate ceased operation, of turbine engine  100 . Purge gas  108  thus facilitates inhibiting the formation of coke deposits within three-way valve  140  by channeling purge gas  108  through three-way valve  140  to facilitate removing residual liquid fuel  104  from purge gas chamber  158  and, in at least some embodiments, to force residual liquid fuel  104  to drain through drain ports  154  and/or  156 . 
     For example, as described herein, in at least some embodiments, purge gas  108  may be supplied in pulses or bursts through regions of three-way valve  140 , such as through purge gas chamber  158 , while drain ports  154  and/or  156  remain closed. Pulsed bursts of purge gas  108  within three-way valve  140  may help to clear regions of three-way valve  140 , such as purge gas chamber  158 , of residual liquid fuel  104  and/or accumulated coke. Subsequently, drain ports  154  and/or  156  may be opened to release residual liquid fuel  104 , which may in some cases also contain coke and/or other debris dislodged during the pulsed purge cycle. In addition, in at least some embodiments, maintaining drain ports  154  and/or  156  in a closed position during introduction of purge gas  108  may enhance the removal of coke and other debris, for example, as a result of the fact that purge gas  108  may be introduced at high velocity, high temperature, and/or high pressure and may be contained or recirculated within portions of three-way valve  140  prior to opening drain ports  154  and/or  156 . 
     As a result of such purging, three-way valve  140  facilitates improving the reliability and efficiency of turbine engine  100 . In addition, the operating and maintenance costs of turbine engine  100  may be reduced, such as by reducing or eliminating the presence of residual liquid fuel  104  and/or accumulated coke and by, correspondingly, improving the longevity of one or more components. 
     In some embodiments, purge gas system  110  also includes a fitting  194  that enables purge gas  108  from purge gas system  110  to be directed to purge gas inlet  146  to facilitate inhibiting the formation of coke deposits within purge gas inlet  146  and three-way valve  140 .  FIG.  4    is a cross-sectional schematic diagram of an exemplary fitting  194  inserted into an exemplary purge gas inlet  146 . Purge gas inlet  146  includes a purge gas port  196  and a purge gas channel  198  coupled in flow communication with purge gas port  196  and purge gas chamber inlet  172 . In the exemplary embodiment, purge gas channel  198  may define a smooth interior surface that is substantially free of steps and/or other abrupt changes in diameter, at least within an inflow portion  250  thereof, if not over the entire length of purge gas channel  198  within fitting  194 . In some embodiments, the smooth interior surface of purge gas channel  198  further inhibits the accumulation of liquid fuel  104  residue and/or coke, for example, as a result of the smooth or step-less interior surface, which is substantially free of crevices and other regions within which liquid fuel  104  may collect. 
     Moreover, in the exemplary embodiment, purge gas port  196  includes a threaded section  200  and a beveled section  202 . Threaded section  200  includes threads  204  formed on an inner surface  206  of threaded section  200  to enable connection to fitting  194 . Purge gas channel  198  has a purge gas channel diameter  210  that is smaller than a diameter  208  of threaded section  200 . Beveled section  202  is coupled to threaded section  200  such that purge gas channel  198  is aligned at a first bevel angle  212  of between about 30° to about 40°. More specifically, first bevel angle  212  may be between about 35° to about 40° or about 37°. Beveled section  202  transitions from the diameter  208  of threaded section  200  to the diameter  210  of purge gas channel  198 . 
     As shown in  FIG.  4   , fitting  194  includes a hose or pipe connection  214  and a port connection  216 . Hose or pipe connection  214  includes threads  218  formed on an outer surface  220  to enable coupling to a hose or to pipe  222 . Port connection  216  includes a head  228 , a connection threaded section  224  extending axially from the head  228 , and a beveled tip  226  extending from the connected threaded section  224 . A fitting conduit  230  extends through hose or pipe connection  214  and through port connection  216  to enable purge gas  108  to be channeled from hose or pipe  222  into purge gas channel  198 . During operation, hose or pipe connection  214  is coupled to hose or pipe  222 , and port connection  216  is coupled to purge gas inlet  146  such that purge gas  108  is channeled from hose or pipe  222  into purge gas channel  198  and into three-way valve  140 . Specifically, hose or pipe connection  214  is coupled to hose or pipe  222  by rotating threading  218  of hose or pipe connection  214  into hose or pipe  222 , and port connection  216  is coupled to purge gas inlet  146  by rotating connection threaded section  224  into threaded section  200 . 
     The size and shape of connection threaded section  224 , beveled tip  226 , and head  228  facilitates eliminating or reducing the formation of coke deposits in purge gas inlet  146 . Specifically, connection threaded section  224 , beveled tip  226 , and head  228  are sized and shaped to complement threaded section  200  and beveled section  202  such that the formation of coke deposits is eliminated or reduced in purge gas inlet  146 . For example, as shown in  FIG.  4   , connection threaded section  224 , beveled tip  226 , head  228 , threaded section  200 , and beveled section  202  are sized and shaped to minimize dead space between fitting  194  and purge gas inlet  146 . Minimizing the dead space between fitting  194  and purge gas inlet  146  reduces the spaces where residual liquid fuel  104  can accumulate and form coke deposits. Thus, connection threaded section  224 , beveled tip  226 , and head  228  enable eliminating or reducing the formation of coke deposits in purge gas inlet  146 . 
     Further, as shown in  FIG.  4   , connection threaded section  224 , beveled tip  226 , and head  228  are sized and shaped to enable connection threaded section  224 , beveled tip  226 , and head  228  to comply with specific engineering tolerances within purge gas inlet  146 . For example, a second bevel angle  232  of beveled tip  226  substantially corresponds to first bevel angle  212  of beveled section  202 . In the exemplary embodiment, second bevel angle  232  may be between about 35° to about 40° or about 37°. Moreover, in various embodiments, second bevel angle  232  is substantially equal to first bevel angle  212 . Likewise, in at least some embodiments, second bevel angle  232  is substantially equal to first bevel angle  212  plus or minus a threshold value, such as plus or minus any desired engineering tolerance (e.g., plus or minus a tolerance in the range of 0.001° to 5.0°). Additionally, a fitting length  234  of threaded section  224  and beveled tip  226  substantially corresponds to an inlet length  236  to enable a gap  238  between beveled tip  226  and purge gas channel  198  to be maintained within a predetermined engineering tolerance. In the exemplary embodiment, fitting length  234  is defined as the length from an edge  240  of head  228  to an end  242  of beveled tip  226 , and inlet length  236  is defined as the length from an inlet  244  of purge gas port  196  to an inlet  246  of purge gas channel  198 . In the exemplary embodiment, fitting length  234  substantially corresponds to inlet length  236  to enable gap  238  to be maintained within the predetermined engineering tolerance, which may be any suitable tolerance, such as, but not limited to, less than or equal to about 2 mm. 
     Maintaining gap  238  within the predetermined engineering tolerance enables the reduction of the dead space between fitting  194  and purge gas inlet  146 , which in turn enables the reduction or inhibition of disruptions to the flow of purge gas  108  within fitting  194  and purge gas inlet  146 , the accumulation of residual liquid fuel  104  between fitting  194  and purge gas inlet  146 , and coking in the dead space between fitting  194  and purge gas inlet  146 . Specifically, fitting conduit  230  has a fitting conduit diameter  248  substantially equal to purge gas channel diameter  210 , and maintaining gap  238  within the predetermined engineering tolerance enables a smooth transition from fitting conduit  230  to purge gas channel  198  to be formed. 
     Discontinuities within a flow path may cause recirculation and/or chaotic flow patterns with the flow path. For example, if gap  238  is greater than the predetermined engineering tolerance, dead space may be formed between fitting  194  and purge gas inlet  146 , and purge gas  108  may recirculate within purge gas inlet  146 . The recirculating purge gas  108  may enable residual liquid fuel  104  to be deposited within the dead space, and such residual liquid fuel  104  may form coke deposits as described herein. 
     Conversely, maintaining gap  238  within the predetermined engineering tolerance enables formation of a smooth transition from fitting conduit  230  to purge gas channel  198 , enables reduction of recirculation of purge gas  108  within purge gas inlet  146 , enables reduction and/or elimination of residual liquid fuel  104  within purge gas inlet  146 , and enables reduction and/or elimination of coking within purge gas inlet  146 . Accordingly, connection threaded section  224 , beveled tip  226 , and head  228  are sized and shaped to enable connection threaded section  224 , beveled tip  226 , and head  228  to comply with specific engineering tolerances within purge gas inlet  146  to enable gap  238  to be maintained within the predetermined engineering tolerance, thereby reducing or eliminating the formation of coke deposits within purge gas inlet  146 . 
       FIG.  5    is a block diagram of a method  300  of purging liquid fuel  104  from a liquid fuel supply system  102 . The method  300  includes inserting  302  the fitting  194  into a purge gas inlet  146  of the three-way valve  140 . The three-way valve  140  includes a spool  166  and a housing  144  including a liquid fuel inlet  148 , the purge gas inlet  146 , at least one drain port  154  and/or  156 , an outlet  152 , a purge gas chamber  158 , an intermediate chamber  160 , and a liquid fuel chamber  162 . The spool  166  is positioned within the purge gas chamber  158 , the intermediate chamber  160 , and the liquid fuel chamber  162 . The method  300  also includes channeling  304  liquid fuel  104  into the liquid fuel inlet  148  and through the intermediate chamber  160 , the liquid fuel chamber  162 , and the outlet  152 . The method  300  further includes stopping  306  the flow of liquid fuel  104  through the three-way valve  140  by sliding the spool  166  away from the purge gas chamber  158 . The method  300  also includes channeling  308  purge gas  108  into the purge gas inlet  146  and through the intermediate chamber  160 , the purge gas chamber  158 , and the outlet  152 . The method  300  further includes draining  310  liquid fuel  104  from the purge gas chamber  158  through the at least one drain port  154  and/or  156 . 
     Exemplary embodiments of a three-way valve and a fitting for inhibiting the formation of coke deposits in a liquid fuel supply system of a dual-fuel turbine engine are thus described herein. The three-way valve includes a liquid fuel inlet, a purge gas inlet, and an outlet. The fitting is coupled to the purge gas inlet to enable purge gas to be channeled into the three-way valve to remove residual liquid fuel from the three-way valve. As described herein, the three-way valve includes one or more drain ports that, in conjunction with circulation of the purge gas, facilitate inhibiting coke formation in the liquid fuel supply system. Specifically, the drains port(s) are positioned to enable residual liquid fuel to be drained from the three-way valve. Additionally, the fitting is sized and shaped to correspond to the purge gas inlet such that any dead space between the fitting and the purge gas inlet is minimized, thus reducing an amount of residual liquid fuel that could accumulate in the dead space and facilitating reducing coking within the dead space. Accordingly, the three-way valves and fittings described herein facilitate inhibiting coke formation in the liquid fuel supply system. 
     Further aspects of the present disclosure are provided by the subject matter of the following clauses: 
     1. A three-way valve for a liquid fuel supply system, said three-way valve including a housing defining: a liquid fuel inlet sized to receive liquid fuel therethrough for selectively channeling the liquid fuel to a combustor of a gas turbine engine, a purge gas inlet sized to receive purge gas therethrough for selectively purging liquid fuel from said three-way valve, and at least one drain port oriented to selectively channel liquid fuel from said three-way valve in response to liquid fuel being purged from at least a portion of said three-way valve. 
     2. The three-way valve of any preceding clause, wherein said housing further defines a purge gas chamber coupled in flow communication with said purge gas inlet, wherein said at least one drain port is coupled in flow communication with said purge gas chamber and is oriented to selectively channel the liquid fuel from said purge gas chamber when the purge gas is received through said purge gas inlet. 
     3. The three-way valve of any preceding clause, further comprising a plurality of drain ports. 
     4. The three-way valve of any preceding clause, wherein said housing further defines: a liquid fuel chamber coupled in flow communication with said liquid fuel inlet, an intermediate chamber coupled in flow communication with said liquid fuel chamber and said purge gas chamber, an outlet coupled in flow communication with said intermediate chamber, wherein purge gas is channeled from said purge gas inlet into and through said purge gas chamber, said intermediate chamber, and said outlet. 
     5. The three-way valve of any preceding clause, wherein the liquid fuel is channeled from said liquid fuel inlet into and through said liquid fuel chamber, said intermediate chamber, and said outlet. 
     6. The three-way valve of any preceding clause, further comprising a spool positioned within said purge gas chamber, said intermediate chamber, and said liquid fuel chamber, wherein said spool is oriented to prevent liquid fuel flow into said intermediate chamber when the purge gas is purging the liquid fuel from said three-way valve. 
     7. The three-way valve of any preceding clause, wherein said spool is oriented to prevent purge gas flow into said intermediate chamber when said three-way valve is channeling the liquid fuel. 
     8. The three-way valve of any preceding clause, further comprising a spring circumscribing said spool and oriented to bias said spool away from said purge gas chamber. 
     9. A liquid fuel supply system comprising: a three-way valve comprising a housing defining: a liquid fuel inlet sized to receive liquid fuel therethrough for selectively channeling the liquid fuel to a combustor of a gas turbine engine; and a purge gas inlet sized to receive purge gas therethrough for selectively purging the liquid fuel from said three-way valve, said purge gas inlet includes a purge gas port and a purge gas channel, said purge gas channel defines a purge gas channel diameter; and a fitting sized to be inserted into said purge gas inlet and oriented to channel the purge gas into said purge gas inlet, said fitting defining a fitting conduit defining a fitting conduit diameter, wherein said fitting conduit diameter is equal to said purge gas channel diameter. 
     10. The system of any preceding clause, wherein said fitting includes a connection threaded section, a beveled tip including an end, and a head including an edge, said fitting defines a fitting length from said edge of said head to said end of said beveled tip. 
     11. The system of any preceding clause, wherein said purge gas port includes a port inlet and said purge gas channel defines a channel inlet, wherein said purge gas port defines an inlet length from said port inlet to said channel inlet. 
     12. The system of any preceding clause, wherein said end of said beveled tip and said purge gas inlet define a gap therebetween, and wherein said fitting length and said inlet length are sized such that said gap is less than or equal to a predetermined engineering tolerance. 
     13. The system of any preceding clause, wherein said purge gas port includes a threaded section and a beveled section defining a first bevel angle, and wherein said beveled tip defines a second bevel angle equal to said first bevel angle. 
     14. The system of any preceding clause, wherein said second bevel angle is equal to said first bevel angle plus or minus a threshold value. 
     15. The system of any preceding clause, wherein said first bevel angle is in a range of 30° to 40° and said second bevel angle is in a range of 30° to 40°. 
     16. The system of any preceding clause, wherein said housing further defines at least one drain port oriented to channel liquid fuel out of said three-way valve in response to the purge gas purging the liquid fuel from said three-way valve. 
     17. The system of any preceding clause, wherein said housing further defines a purge gas chamber coupled in flow communication with said purge gas inlet, wherein said at least one drain port is coupled in flow communication with said purge gas chamber and is oriented to channel the liquid fuel out of said purge gas chamber when the purge gas is received through said purge gas inlet. 
     18. The system of any preceding clause, said housing further defines a plurality of drain ports oriented to channel the liquid fuel out of said three-way valve. 
     19. The system of any preceding clause, wherein said housing defines a liquid fuel chamber coupled in flow communication with said liquid fuel inlet; an intermediate chamber coupled in flow communication with said liquid fuel chamber and a purge gas chamber; and an outlet coupled in flow communication with said intermediate chamber, wherein the purge gas is channeled from said purge gas inlet into and through said purge gas chamber, said intermediate chamber, and said outlet. 
     20. A method of selectively purging liquid fuel from a liquid fuel supply system, the liquid fuel supply system including a three-way valve and a fitting, said method comprising: inserting the fitting into a purge gas inlet of the three-way valve, the three-way valve including a housing including a liquid fuel inlet, the purge gas inlet, at least one drain port, and an outlet; channeling liquid fuel into the liquid fuel inlet and through a housing of the three-way valve to the outlet; stopping a flow of liquid fuel through the three-way valve; channeling purge gas from the fitting into the purge gas inlet and through the housing of the three-way valve to the outlet; and draining liquid fuel from the purge gas chamber through the at least one drain port. 
     While the disclosure has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the disclosure is not limited to such disclosed embodiments. Rather, the disclosure can be modified to incorporate any number of variations, alterations, substitutions, or equivalent arrangements not heretofore described, but which are commensurate with the scope of the disclosure. For example, components of each system and/or steps of each method may be used and/or practiced independently and separately from other components and/or steps described herein. Additionally, while various embodiments of the disclosure have been described, it is to be understood that aspects of the disclosure may include only some of the described embodiments, and that each component and/or step may also be used and/or practiced with other systems and methods. Accordingly, the disclosure is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims. 
     Although specific features of various embodiments of the disclosure may be shown in some drawings and not in others, this is for convenience only. Moreover, references to “one embodiment” or “an embodiment” in the above description are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. In accordance with the principles of the disclosure, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing.