Abstract:
A vent for use in a gaseous fuel supply circuit of a gas turbine is provided. The vent includes an inlet in flow communication with the gaseous fuel supply circuit, a first outlet in flow communication with the gaseous fuel supply circuit and configured to release gaseous fuel at atmospheric pressure, a first valve coupled between the inlet and the first outlet, wherein the first valve includes a second outlet configured to channel the gaseous fuel towards a combustion device. The system also includes a second valve coupled between the inlet and the second outlet, and a control device configured to selectively open and close the first and second valves based on a pressure of the gaseous fuel.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Phase patent application of International Application Serial No. PCT/EP2012/075916, filed on Dec. 18, 2012, the disclosure of which is incorporated by reference in its entirety, and which claims priority to French Patent Application No. 1162219, filed on Dec. 21, 2011, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     The field of the disclosure relates generally to gas turbines and, more particularly, to evacuation devices, or vents for gaseous fuel powering the gas turbines. 
     At least some known gas turbines are powered by a gaseous fuel routed through a supply circuit. The gaseous fuel supply circuit generally used to regulate a flow and a pressure of gaseous fuel that powers the gas turbine and used to ensure rapid security of the circuit. To achieve such rapid security, the supply circuit comprises several valves fitted in series on fuel routing pipes of the gaseous fuel supply circuit. 
     In cases where emergency shutdown of the system is required, such as a case of detection of gas leakage or excess speed of the gas turbine, the gaseous fuel supply circuit must be drained within a very short period, preferably within 30 seconds, to obtain a residual pressure in the closed circuit substantially equal to the atmospheric pressure. The residual gaseous fuel in the supply circuit is then released to the atmosphere. 
     Furthermore, during turbine startup, it is also necessary to supply the turbine with a gaseous fuel having a temperature greater than a determined value to obtain the conditions required in the combustion chamber to be able to start the turbine. For this purpose, the supply circuit comprises one or more heating devices capable of gradually increasing the gaseous fuel temperature while being routed towards the combustion chamber. However, if the gaseous fuel has not reached the desired temperature, it cannot power the combustion chamber, and is generally released to the atmosphere. 
     However, the discharge of gaseous fuel to the atmosphere has several disadvantages such as from an environmental point of view (i.e., toxicity of gaseous fuel), and from a security point of view. The volume of gaseous fuel released to the atmosphere may be difficult to disperse due to the large volume released, a density of gaseous fuel, or topography of the site where the gaseous fuel is released. 
     JP document 08200650 discloses a gaseous fuel routing device comprising one vent that releases the gaseous fuel. 
     BRIEF DESCRIPTION 
     In one aspect, a vent system for use in a gaseous fuel supply circuit of a gas turbine is provided. The system includes an inlet in flow communication with the gaseous fuel supply circuit, a first outlet in flow communication with the gaseous fuel supply circuit and configured to release gaseous fuel at atmospheric pressure, a first valve coupled between the inlet and the first outlet, wherein the first valve includes a second outlet configured to channel the gaseous fuel towards a combustion device. The vent also includes a second valve coupled between the inlet and the second outlet, and a control device configured to selectively open and close the first and second valves based on a pressure of the gaseous fuel. 
     In another aspect, a gas turbine is provided. The gas turbine includes a combustor and a gaseous fuel supply circuit coupled upstream from the combustor that includes a vent. The vent includes an inlet in flow communication with the gaseous fuel supply circuit, a first outlet in flow communication with the gaseous fuel supply circuit and configured to release gaseous fuel at atmospheric pressure, a first valve coupled between the inlet and the first outlet, wherein the first valve includes a second outlet configured to channel the gaseous fuel towards a combustion device. The vent also includes a second valve coupled between the inlet and the second outlet, and a control device configured to selectively open and close the first and second valves based on a pressure of the gaseous fuel. 
     In yet another aspect, a method of operating a gas turbine is provided. The method includes selectively channeling a flow of gaseous fuel towards a first valve and a first outlet, the first valve including a second outlet towards a combustion device the first outlet towards an atmospheric environment, coupling a second valve upstream from the first valve, and selectively opening and closing the first and second valves based on a pressure of the gaseous fuel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an exemplary gas turbine; 
         FIG. 2  is a schematic diagram of an exemplary gaseous fuel supply circuit; and 
         FIG. 3  is a graph of an exemplary vent control model in an emergency shutdown phase. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure resolve the problems described above. In particular, embodiments of the present disclosure include a vent, or gaseous fuel evacuation device, which facilitates limiting the discharge of gaseous fuel to the atmosphere, particularly at the time of emergency shutdown or gas turbine startup. Embodiments of the present disclosure also preserve the security conditions required, such as a depressurization period of the supply circuit, a maximum residual vacuum pressure in the supply circuit, and/or reliability. 
     The embodiments described herein include a vent for the gaseous fuel supply circuit of a gas turbine. The vent includes an inlet to be connected to the gaseous fuel supply circuit, a first outlet capable of releasing the gaseous fuel at atmospheric pressure, and a first valve coupled between the inlet and the first outlet. The vent also comprises a second outlet capable of providing gaseous fuel through combustion at atmospheric pressure, such as a flare, a second valve coupled between the inlet and the second outlet, and a control device capable of controlling the opening and closing of the first and second valves based on the pressure of gaseous fuel. 
     In the exemplary embodiment, the vent provides an independent combustion device of the gas turbine, and facilitates burning gaseous fuel before being released to the atmosphere. The combustion device, for example a flare, also facilitates releasing the residual gaseous fuel present in the gaseous fuel supply circuit, and obtaining a residual pressure substantially equal to the atmospheric pressure as the combustion takes place at a pressure that is almost identical or substantially equal to the atmospheric pressure. Finally, the combustion device can be easily sized to obtain the combustion of a given flow of gaseous fuel, thus facilitating the limited period of purge of the supply circuit. 
     Furthermore, the vent also facilitates limiting the risks of combustion of gaseous fuel in the supply circuit by providing an outlet towards the atmosphere when the gaseous fuel pressure is less than a determined value. In particular, according to one aspect of the present disclosure, the vent facilitates limiting transfer between the outlet towards the combustion device and the outlet towards the atmosphere. Thus, it is possible to maintain the circuit in a stable and safe state, by maintaining pressure in the supply circuit at a level substantially equal to that of the atmospheric pressure. Furthermore, the vent facilitates limiting the discharge of gaseous fuel to the atmosphere by opening the outlet towards the atmosphere only at the end of each drain stage of the supply circuit when the gaseous fuel pressure is almost identical to the atmospheric pressure. 
     In one embodiment, the vent also comprises a third valve coupled between the inlet and the second outlet and the control device is capable of controlling the opening and closing of the third valve based on the temperature of the gaseous fuel in the supply circuit. The third valve may be used specifically for the start phases when it is then controlled by the temperature of gaseous fuel, and it closes when the temperature of gaseous fuel reaches a desired value. In this case, no gaseous fuel is released to the atmosphere in the start phase of the gas turbine. 
     Also, according to another aspect, embodiments of the present disclosure include a supply circuit for a gas turbine comprising a vent such as described before. 
     The supply circuit may also have a gaseous fuel pressure measurement device, and the control device may receive measurements from the pressure measurement device. 
     The supply circuit may also comprise an insulation valve and a regulation valve coupled downstream from the insulation valve, and the vent inlet may be coupled between the insulation valve and the regulation valve. During depressurization of a supply circuit, a quantity of gas is present between the insulation valve and the regulation valve. As such, the vent is coupled between these two valves to facilitate burning a quantity of gaseous fuel in the combustion device. 
     The supply circuit may also comprise a gas control valve coupled downstream of the regulation valve and a vent to the atmosphere coupled between the regulation valve and the gas control valve. In the portion of the supply circuit located between the regulation valve and the gas control valve, the quantity of gaseous fuel present therein is relatively limited, such that a vent direct to the atmosphere is possible. 
     Embodiments of the present disclosure also relate to a gas turbine comprising a supply circuit as described before. 
     In another aspect, embodiments of the present disclosure relate to a gaseous fuel evacuation process present in a gaseous fuel supply circuit of a gas turbine, in which the gaseous fuel to be evacuated is routed towards a combustion device at atmospheric pressure, for example a flare, when the pressure of the gaseous fuel is greater than a determined pressure, for example, the flare usage pressure, and the gaseous fuel to be evacuated is routed towards the atmosphere until a pressure substantially equal to the atmospheric pressure is obtained in the gaseous fuel supply circuit. 
     In one embodiment, the gaseous fuel present in the supply circuit is evacuated towards the flare when the gas turbine is shutdown urgently or when the gas turbine is in startup mode and the gaseous fuel temperature is less than a fixed temperature. 
     In one embodiment, the gas turbine comprises an insulation valve and a regulation valve coupled downstream of the insulation valve, and the gaseous fuel evacuated when the gas turbine is shutdown urgently is the gaseous fuel contained between the insulation valve and the regulation valve. 
     Other advantages and features of the present disclosure will appear on examination of the detailed description of a mode of execution of the present disclosure which is not limited. 
       FIG. 1  is a schematic diagram of an exemplary gas turbine  1  powered with gaseous fuel from, for example, a tank  2 . Gas turbine  1  is generally used in electric power plants to drive the generators and to produce electric energy. Gas turbine  1  comprises an axial compressor  3  with one rotor shaft  4 . Air is channeled towards inlet  5  of axial compressor  3 , is compressed by axial compressor  3 , and is then routed towards a combustion chamber  6 . Combustion chamber  6  is also powered by a gaseous fuel, for example natural gas which, during combustion, produces hot gases with high energy capable of driving a turbine  7 . The gaseous fuel may be routed from tank  2  to combustion chamber  6  through a gaseous fuel supply circuit  8  which comprises an inlet (not shown) connected to tank  2  and an outlet (not shown) connected to combustion chamber  6 . 
     In turbine  7 , the energy from the hot gases is converted during operation and at least a portion of the hot gas is used to drive compressor  3 , via rotor shaft  4 , and at least a portion of the hot gas is used to drive an electricity production generator  9  through a shaft  10 . Exhaust gases are discharged from turbine  7  through an outlet  11 , and may be used for other applications. 
       FIG. 2  is a schematic diagram of an exemplary gaseous fuel supply circuit  8  of combustion chamber  6 . Supply circuit  8  comprises an inlet  12  to receive the gaseous fuel, and outlets  13  to provide combustion chamber  6  with gaseous fuel, and a routing line  14  connecting inlet  12  with outlets  13 . 
     Gaseous fuel routing line  14  successively comprises, in the direction of circulation of gaseous fuel, an insulation valve  15  (i.e., a Safety Shut-Off valve) connected to inlet  12 , a regulation valve  16  (i.e., a Stop Ratio Valve) coupled downstream of insulation valve  15 , and supply lines (not shown), for example three, coupled in parallel downstream of regulation valve  16  and each one comprising a gas control valve  17  (i.e., a Gas Control Valve) coupled upstream of an outlet  13  towards combustion chamber  6 . 
     Insulation valve  15  is a safety valve that insulates gaseous fuel supply circuit  8  from the supply circuit of combustion chamber  6 . As such, valve  15  facilitates interrupting the gaseous fuel supply if a gas turbine operation problem occurs, or in case it is shutdown. 
     Regulation valve  16  also facilitates interrupting the gaseous fuel supply of combustion chamber  6 , but particularly facilitates controlling the gaseous fuel pressure in routing line  14 , between regulation valve  16  and control valves  17 , which varies based on a current speed of gas turbine  1 . 
     Control valves  17  determine the quantity of gaseous fuel delivered by supply line  13  in combustion chamber  6 . In particular, valves  17  may be a sonic-type valve. 
     Routing line  14  may also comprise, upstream of insulation valve  15 , a heating device  18  capable of increasing the temperature of gaseous fuel before it powers combustion chamber  6 . 
     Routing line  14  also includes a temperature sensor  19  and a pressure sensor  20 . Temperature sensor  19  is coupled downstream of heating device  18  between the heating device  18  and insulation valve  15 , and facilitates monitoring the temperature of gaseous fuel powering combustion chamber  6 . Pressure sensor  20  is coupled between insulation valve  15  and regulation valve  16  and facilitates monitoring the pressure of gaseous fuel in supply circuit  8  during operation of gas turbine  1  or during emergency shutdown. 
     In the exemplary embodiment, a vent  21  is coupled on the routing line  14 . More specifically, vent  21  has a main pipe  22  tapped on routing line  14  between insulation valve  15  and regulation valve  16 . A first vent valve  23  is coupled to main pipe  22 , which facilitates releasing the gaseous fuel present in routing line  14  between the insulation valve  15  and the regulation valve  16  to the atmosphere. 
     Vent  21  also has a secondary pipe  24 , tapped on main line  22 , between the tap on routing line  14  and first vent valve  23 . Secondary pipe  24  has a second vent valve  25  towards a combustion device  26 , for example a flare, an insulation valve  27  of the vent towards the flare, a parallel pipe  28 , and a third vent valve  29  towards combustion device  26 . 
     Second vent valve  25 , insulation valve  27 , and combustion device  26  are coupled in series on secondary pipe  24  between the tap on main pipe  22  and the discharge to the atmosphere. Third vent valve  29  is coupled on parallel pipe  28 , and parallel pipe  28  is fitted on secondary pipe  24  in parallel to second vent valve  25 . An inlet (not shown) of parallel pipe  28  is tapped on secondary pipe  24  upstream of second vent valve  25  and an outlet (not shown) of parallel pipe  28  is tapped on secondary pipe  24  downstream of second vent valve  25 , but upstream of insulation valve  27 . 
     Combustion device  26  facilitates burning the gaseous fuel before discharging it to the atmosphere. Insulation valve  27  facilitates closing secondary pipe  24  and thus insulating combustion device  26  of routing line  14 , either through secondary pipe  24  or through parallel pipe  28 . Second vent valve  25  facilitates controlling the gaseous fuel flow circulating from routing line  14  towards combustion device  26 . 
     In one embodiment, first vent valve  23  is a valve open by default with an opening diameter of about 3 inches, second vent valve  25  is a valve closed by default with an opening diameter of about 3 inches, and third vent valve  29  is a valve closed by default with an opening diameter of about 1 inch. In the exemplary embodiment, third vent valve  29  is used in the start phases, while second vent valve  25 , which has a wider opening and thus a greater flow, is used in the emergency shutdown phases when the gaseous fuel present in routing line  14  must be rapidly vented. 
     Furthermore, supply circuit  8  may also have a secondary vent  30  to the atmosphere. More particularly, secondary vent  30  has a main pipe  31  tapped on routing line  14  between regulation valve  16  and control valves  17 . A fourth vent valve  32  to the atmosphere is coupled on main pipe  21  and is between regulation valve  16  and control valves  17 . Fourth vent valve  32  facilitates releasing the gaseous fuel present in routing line  14 . In particular, when the quantity of residual gaseous fuel between regulation valve  16  and control valves  17  is less than the quantity of residual gaseous fuel between insulation valve  15  and regulation valve  16 , supply circuit  8  has only a single vent  21  to combustion device  26 , between insulation valve  15  and regulation valve  16 . 
     Vents  21  and  30  facilitate evacuating the gaseous fuel found in routing line  14  between insulation valve  15  and control valves  17 . 
     In the exemplary embodiment, supply circuit  8  has a control device  33 . Control device  33  that receives the information provided by the pressure and temperature sensors  19  and  20 , and is capable of controlling the different valves of supply circuit  8  based on the mode of operation of gas turbine  1 . 
     Thus, when gas turbine  1  is in a normal shutdown phase, valves  16 ,  17 ,  23 ,  25 ,  27  and  29  are closed, and valves  15  and  32  are opened. Valves  23 ,  25 ,  27  and  29  remain closed to facilitate limiting depressurization of gaseous fuel routing line  14 . 
     In the start phases, the gaseous fuel is heated before entering combustion chamber  6 . Insulation valve  15  is opened and heating device  18  is used. Temperature sensor  19  indicates to control device  33  if the determined temperature is reached. When the temperature of the gaseous fuel is less than the required temperature, valves  16  and  17  are closed and the gaseous fuel is vented. Valves  29  and  27  are opened to vent the gaseous fuel between valves  15  and  16  towards combustion device  26 , and valve  32  is opened to vent the gaseous fuel between valve  16  and valves  17  to the atmosphere. Valves  23  and  25  remain closed. 
     When the gaseous fuel reaches the desired temperature, gas turbine  1  starts operating and valves  15 ,  16  and  17  are opened, while valves  23 ,  25 ,  27 ,  29  and  32  are closed. As such, there is no venting and the gaseous fuel is channeled towards combustion chamber  6 . 
     In case of emergency shutdown of gas turbine  1 , valves  15 ,  16  and  17  of routing line  14  are all closed, and the residual gaseous fuel in routing line  14  is vented. As such, valve  32  is opened to vent the gaseous fuel remaining between valve  16  and valves  17  towards the atmosphere. Furthermore, the gaseous fuel between valve  15  and valve  16  is firstly vented towards combustion device  26 . More specifically, valves  25  and  27  are opened to facilitate rapid evacuation of the gaseous fuel towards combustion device  26 . Valves  23  and  29  remain closed during this period. 
     When the pressure of the residual gaseous fuel remaining between valves  15  and  16 , and measured by sensor  20 , is less than a determined value, for example the supply pressure of combustion device  26 , valves  25  and  27  are closed and valve  23  is opened to finish venting the remaining residual gaseous fuel towards the atmosphere. In particular, valve  23  is opened only after closure of valves  25  and/or  27 . 
       FIG. 3  is a graph of an exemplary vent control model in an emergency shutdown phase, such as for valves  25  and  23  (i.e., curves (B) and (C), respectively).  FIG. 3  also shows a variation in pressure of the gaseous fuel measured by device  20  in supply circuit  8  (curve (A)). Thus, the model has a first part during which safety valve  15  is closed, and a second part in which the remaining gaseous fuel is vented in supply circuit  8 . For this, second valve  25  is gradually opened until it is completely open such that the gaseous fuel remaining in the supply circuit  8  is fed to combustion device  26 . As such, a rapid drop is observed in the pressure of the gaseous fuel which is burnt in combustion device  26 . 
     When the pressure of the gaseous fuel in supply circuit  8  falls below a determined value, second valve  25  is gradually closed to facilitate reducing transfer between combustion device  26  and supply circuit  8 . When second valve  25  is completely closed, first valve  23  may then be gradually opened to ensure routing line  14  is at atmospheric pressure, and to discharge the residual gaseous fuel to the atmosphere. Thus, a pressure equal to the atmospheric pressure is obtained in supply circuit  8 . 
     Thus, the embodiments described herein vent the gaseous fuel contained in the supply circuit by increasing the quantity of gaseous fuel released towards a combustion device while reducing the vent period. Furthermore, control of the vent valves based on the pressure in the supply circuit and according to the opening status of other vent valves facilitates safely operating the vent phase. 
     This written description uses examples to disclose the embodiments of the present disclosure, including the best mode, and also to enable any person skilled in the art to practice embodiments of the present disclosure, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the embodiments described herein is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.