BACKGROUND 
       [0001]    Embodiments of the present invention relate to a method for starting a gas turbine. Such method is potentially applicable to every kind of gas turbine, such as the ones used for mechanical power (compressors or pumps drive) and for power generation (electrical generators). 
         [0002]    A gas turbine is known from the state of the art, comprising a combustion chamber provided with one or more nozzles. Such nozzles are used to inject fuel, which is then burned inside the combustion chamber. The hot exhaust gases that exit the combustion chamber are then used to move an impeller attached to a shaft, thus providing mechanical work. 
         [0003]    An apparatus for regulating the flow of fuel in such gas turbine is also known. The apparatus comprises a fuel line, which in operation is connected with a fuel inlet and with the nozzles of the gas turbine. Indeed, the fuel line has the function of transferring fuel from the inlet to the nozzles. A regulating valve is placed along the fuel line, so that the flow of fuel can be controlled. Thus, in order to start the turbine, the regulating valve is opened to a defined stroke and a small flow of fuel is allowed to enter inside the combustion chamber. A spark then ignites the fuel and, afterwards, the flow of fuel can be increased further, until a desired operating condition is achieved. 
         [0004]    An aspect of the prior art is that, in case of a malfunction of the regulating valve, too much fuel can be allowed inside the combustion chamber before the ignition. This is potentially extremely dangerous, as it can lead to an explosion of the gas turbine or its exhaust duct with high risk of injuries or fatalities of personnel. For this reason, the known turbines are provided with a safety device designed to interrupt the flow of fuel in case of malfunction. Indeed, such safety device may comprise a flow meter placed on the fuel line downstream of the regulating valve 
       BRIEF DESCRIPTION 
       [0005]    A first embodiment of the present invention therefore relates to a method of starting a gas turbine. Such method comprises the step of providing an apparatus for regulating the flow of fuel in a gas turbine. The apparatus comprises a main line fluidly connectable with a fuel source and with a nozzle array for transferring fuel from the fuel source to the nozzle array. The apparatus also comprises an auxiliary line fluidly connectable with the fuel source and with the nozzle array for transferring fuel from the fuel source to the nozzle array. 
         [0006]    The method comprises the step of keeping the main line sealed while increasing the auxiliary line fuel flow rate. The method also comprises the step of firing the gas turbine while keeping the main line sealed. After the combustion has started in the gas turbine, and when auxiliary line reaches about its maximum capacity, the main line is opened to increase the main line fuel flow rate. The auxiliary line maximum flow rate is less than the main line maximum flow rate. 
         [0007]    In an embodiment, the gas turbine can be started in an intrinsically safe manner, since the auxiliary line alone does not provide enough fuel to enable a catastrophic failure of the gas turbine. Only after the fuel combustion has taken place the main line is opened. 
         [0008]    Another embodiment of the invention relates to an apparatus for regulating the flow of fuel in a gas turbine. Such apparatus comprises a main line that can be connected in fluid communication with a fuel source and with a nozzle array for injecting fuel into a combustion chamber of a gas turbine. The apparatus also comprises a main flow regulator placed on the main line and configured to vary the flow of fuel on the main line up to a main line maximum flow rate. 
         [0009]    An auxiliary line is placed in fluid communication with the fuel source and the nozzle array for transferring fuel from the fuel source to the nozzle array. An auxiliary flow regulator is placed on the auxiliary line and is configured to vary the flow of fuel on the auxiliary line up to an auxiliary line maximum flow rate. The auxiliary line maximum flow rate is less than the main line maximum flow rate. 
         [0010]    A third embodiment of the invention relates to a method of upgrading a previous apparatus for regulating the flow of fuel in a gas turbine. The previous apparatus is connected to a nozzle array and to a fuel source for injecting fuel into a combustion chamber of a gas turbine. The previous apparatus comprises a main line in fluid communication with the fuel source and the nozzle array, so as to transfer fuel from the fuel source to the nozzle array. The previous apparatus also comprises a main flow regulator placed on the main line and configured to vary the flow of fuel on the main line up to a main line maximum flow rate. 
         [0011]    The method of upgrading the previous apparatus itself comprises the steps of providing an auxiliary line and placing it in fluid communication with the fuel source and with the nozzle array, so as to transfer fuel from the fuel source to the nozzle array. The auxiliary is sized so that its maximum flow rate is less than the main line maximum flow rate. An auxiliary flow regulator is then placed on the auxiliary line to vary the flow of fuel on the auxiliary line up to an auxiliary line maximum flow rate. 
         [0012]    In an embodiment, this allows to apply the above described method for starting a gas turbine to an apparatus not specifically designed for it. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Further details and specific embodiments will refer to the attached drawings, in which: 
           [0014]      FIG. 1  is a schematic representation of an apparatus for regulating the flow of fuel in a gas turbine according to a first embodiment of the present invention; 
           [0015]      FIG. 2  is a schematic representation of an apparatus for regulating the flow of fuel in a gas turbine according to a second embodiment of the present invention; 
           [0016]      FIG. 3  is a schematic representation of an apparatus for regulating the flow of fuel in a gas turbine according to a third embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    The following description of exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the embodiments of the present invention. Instead, the scope is defined by the appended claims. 
         [0018]    Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. 
         [0019]    Therefore, an apparatus for regulating the flow of fuel in a gas turbine will be described by referring to the attached figures, in which will be indicated with the number  1 . 
         [0020]    Such apparatus  1  is designed to transfer the fuel to a combustion chamber “C”, which is provided with a plurality of nozzles  3 . Indeed, the combustion chamber may comprise one or more array “N” of nozzles  3 . As shown for example in  FIG. 3 , the nozzle array “N” (itself not part of the invention) comprises a first  3   a  and a second set  3   b  of nozzles  3 . In the embodiment shown in  FIG. 3  the first set  3   a  of nozzles  3  comprises the primary nozzles  3 . The second set  3   b  comprises the secondary nozzles  3 . 
         [0021]    In further embodiments, not shown, there may be as many set of nozzles  3  as it is deemed necessary by the project specifications. 
         [0022]    The combustion chamber “C” is not considered as part of the present invention, and will not be described in further detail. 
         [0023]    The apparatus  1  comprises a fuel source  2 . Indeed, the fuel source  2  comprises one fuel inlet  2   a , associated with a respective inlet strainer  2   b.    
         [0024]    A main line  4  for the transfer of fuel is placed in fluid communication with the fuel source  2 . Also, the main line  4  is placed in fluid communication with one or more nozzles  3 . Indeed, in the context of the present disclosure the main line  4  is considered to be the path of the fuel between the fuel source  2  and the nozzles  3 . It is to be noted that such path may comprise more than one parallel physical paths between the fuel source  2  and the nozzles  3 . 
         [0025]    Along the main line  4  is placed a sealing device  5 , also referred to as “block and bleed” in the technical field. The sealing device  5  comprises a set of valves  5   a ,  5   b ,  5   c . A first  5   a  and a second valve  5   b  are arranged serially along the main line  4 . A third valve  5   c  is connected in fluid communication with the main line  4  between the first  5   a  and the second valve  5   b  so that, when opened, can vent the gas entrapped between the first  5   a  and the second valve  5   b . This arrangement is such that during the functioning of the gas turbine, the third valve  5   c  is closed while both the first  5   a  and the second valve  5   b  are kept open. In this way, fuel can flow along the main line  4 . When the gas turbine is not working both the first  5   a  and the second valve  5   b  are kept closed, and the third valve  5   c  is kept open. In this way, any fuel leaks from the first  5   a  and the second valve  5   b  will be allowed to vent away from the main line  4  by the third valve  5   c , thus preventing any potential accumulation of fuel inside the combustion chamber “C”. 
         [0026]    In an embodiment, a vent valve  6  is present on the main line  4  downstream of the sealing device  5 . 
         [0027]    In the embodiment shown in  FIG. 3 , the main line  4  branches to reach the first  3   a  and the second set  3   b  of nozzles  3 . Indeed, the main line comprises a main branch  4   a  which branches into a primary  4   b  and into a secondary branch  4   c . The primary branch  4   b  is placed in fluid connection with the main branch  4   a  and with the first set  3   a  of nozzles  3 . Similarly, the secondary branch  4   c  is placed in fluid connection with the main branch  4   a  and with the second set  3   b  of nozzles  3 . In this arrangement, the above described sealing device  5  and vent valve  6  are placed on the main branch  4   a  of the main line  4 . 
         [0028]    The main line  4  can comprise an expansion zone  12 . As shown in  FIG. 3 , the expansion zone  12  is placed between the main branch  4   a  and the primary  4   b  and secondary branches  4   c . A divergent portion  13  connects the main branch  4   a  to the expansion zone  12 . A convergent portion  14  connects each of the primary  4   b  and secondary  4   c  branches to the expansion zone  12 . In an embodiment, this helps to dampen any pressure fluctuation on the main line  4  before it reaches the nozzles  3 . 
         [0029]    In an embodiment, a main flow regulator  7  is placed on the main fuel line  4  downstream of the sealing device  5 . Also, the main flow regulator  7  is placed downstream of the vent valve  6 . Such main flow regulator  7  is configured to vary the flow of fuel on the main fuel line  4  up to a main line maximum flow rate. In the embodiment from  FIG. 3 , each of the primary  4   b  and secondary  4   c  branch is provided with its own flow regulator. With more detail, the apparatus  1  may comprise a primary flow regulator  7   a  on the primary branch  4   b  of the main line  4 . Similarly, the apparatus  1  may comprise a secondary flow regulator  7   b  on the secondary branch  4   c  of the main line  4 . The primary  7   a  and the secondary flow regulator  7   b  are substantially similar to the main flow regulator  7  described above. If the specifications require it, the primary  7   a  and the secondary  7   b  flow regulators may be sized differently. It is to be noted that in this arrangement no main flow regulator  7  is actually present. 
         [0030]    An auxiliary line  8  is placed in fluid communication with the fuel source  2 . The auxiliary line is also placed in fluid communication with at least some of the nozzles  3 , so that it can transfer fuel from the inlet  2   a  to the nozzles  3 . The purpose of the auxiliary line  8  is to provide an intrinsically safe way to start the gas turbine, as its maximum flow can be sized so as to be enough to start the gas turbine but not too much as to create the danger of an explosion. 
         [0031]    With more detail, in one embodiment of the invention the pipes of the auxiliary line  8  has a lesser internal diameter than the pipes of the main line  4 . Additionally, in an embodiment, the auxiliary line  8  comprises a convergent portion  10  directly attached to the main line  4  upstream of the main flow regulator  7 . In particular, the auxiliary line  8  branches from the main line  4  upstream of the sealing device  5 . 
         [0032]    In the embodiment shown in  FIG. 1 , the auxiliary line  8  connects back to the main line  4  upstream of the main flow regulator  7 . 
         [0033]    In the embodiment shown in  FIG. 2 , the auxiliary line  8  connects back to the main line  4  downstream of the main flow regulator  7 . 
         [0034]    In the embodiment shown in  FIG. 3 , the auxiliary line  8  connects back to the primary branch  4   b  of the main line  4 , in particular downstream of the expansion zone  12 . Similarly to the embodiment shown in  FIG. 2 , the auxiliary line  8  connects to the primary branch  4   b  downstream of the primary flow regulator  7   a . Another variant is possible (not shown in the drawings) in which the auxiliary line  8  connects to the primary branch  4   b  upstream of the primary flow regulator  7   a.    
         [0035]    The auxiliary line  8  comprises a divergent portion  11  directly attached to the main line  4  or, in case of the embodiment from  FIG. 3 , to the primary branch  4   b  of the main line  4 . 
         [0036]    The auxiliary line  8  is provided with its own auxiliary sealing device  15 . Such auxiliary sealing device  15  is substantially similar to the sealing device  5  described above. 
         [0037]    The auxiliary line  8  is provided with an auxiliary flow regulator  9 , which is configured to vary the flow of fuel on the auxiliary fuel line  9  itself. Indeed, the auxiliary flow regulator  9  ensures that the flow along the auxiliary line  8  can never exceed the auxiliary line  8  maximum flow rate. This maximum flow rate is less than the maximum flow rate of the main line  4 . The auxiliary flow regulator  9  is in particular placed directly downstream with respect to the auxiliary sealing device  15 . 
         [0038]    According to a first embodiment, the auxiliary flow regulator  9  is an orifice. This simplistic embodiment can be employed if the fuel composition is relatively constant (a variation of about 5% of the calorific content of the fuel is admissible). According to another embodiment, the auxiliary flow regulator  9  is a valve. This allows to vary the composition of the fuel without constraints, at the cost of greater complexity. 
         [0039]    In detail, the auxiliary line  8  maximum flow rate is less than the Lower Explosive Limit. The Lower Explosive Limit is defined as the minimum concentration of fuel in a fuel/air mixture such as a spark can ignite the mixture and cause it to explode. In other words, no explosion is possible if the fuel concentration is below the Lower Explosive Limit. Such Lower Explosive Limit generally corresponds to a fuel flow rate of about 15% of the maximum total flow rate. The exact value of the LEL depends on the phisical/chemical properties of the fuel, on the temperature of the air, and on other general phisical properties so that its precise value can easily be computed by the person skilled in the art if all such properties are known or can be reasonably assumed to be inside a specific range (such as is generally the case during the design of a gas turbine). 
         [0040]    The apparatus  1  may also comprise a flame detector (not shown in the drawings) active on the combustion chamber of the gas turbine. In an embodiment, the flame detector is optical, and ensures that a control system of the apparatus  1  can react to any changes in the combustion chamber. 
         [0041]    In order to start the gas turbine, the main shaft of the gas turbine is initially moved by a starting engine (both are not shown in the drawings). Thus an initial air flow is established in the gas turbine and, in particular, in the combustion chamber “C”. In this way, the combustion chamber “C” is purged from eventual residue of fuel which may be still inside. 
         [0042]    After the purging phase, a spark is started inside the combustion chamber “C”. The fuel flow is then slowly increased inside the combustion chamber “C”. With more detail, the main line  4  is kept sealed, while the fuel flow rate in the auxiliary line is increased. In other words, the gas turbine is fired while keeping the main line  4  sealed. Indeed, the gas turbine is fired using only fuel from the auxiliary line. 
         [0043]    With additional detail, in the embodiment shown in  FIG. 2  the main flow regulator  7  is opened at a preset stroke. The auxiliary flow regulator  9  is controlled in order to achieve a predetermined pressure value upstream of the main flow regulator  7 . With more detail, the preset pressure value is not constant, but is a function of the rotational speed of the turbomachine. Since the main flow regulator  7  is fixed at this stage, the fuel flow inside the combustion chamber “C” is only a function of the pressure upstream of the main flow regulator  7 . 
         [0044]    The combustion can therefore start in the gas turbine. With additional detail, the method comprises a step of detecting a flame inside the combustion chamber. In an embodiment, such step is performed before the main line  4  opening step. In particular, such step is performed by the above mentioned flame detector. 
         [0045]    Afterwards, the main line  4  is opened in order to increase the main line  4  fuel flow rate. With more detail, after the auxiliary flow regulator  9  is completely opened the gas turbine is warmed up for several minutes. Afterwards, the sealing device  5  is gradually opened, in particular with reference to the first  5   a  and second valve  5   b . The main flow regulator  7  can then be controlled in order to further increase the flow of fuel inside the combustion chamber “C”. 
         [0046]    In the embodiments from  FIGS. 1 and 3  the startup sequence is similar to the one described above. However, the auxiliary flow regulator  9  is not controlled as a function of a preset pressure, but is placed in sonic condition and targets a predetermined auxiliary line  8  flow rate directly. Additionally, after the auxiliary line  8  maximum flow rate is achieved, the main flow regulator  7  is opened at a preset stroke. Afterwards, the sealing device  5  is gradually opened, in particular with reference to the first  5   a  and second valve  5   b . During this phase the auxiliary flow regulator  9  is gradually closed, so that the total flow rate can remain constant. After the auxiliary flow regulator  9  is completely closed, the main flow regulator  7  can be controlled in order to further increase the flow of fuel inside the combustion chamber “C”. 
         [0047]    It is to be understood that even though numerous characteristics and advantages of various embodiments have been set forth in the foregoing description, together with details of the structure and functions of various embodiments, this disclosure is illustrative only, and changes may be made in detail, especially in matters of structure and arrangement of parts within the principles of the embodiments to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. It will be appreciated by those skilled in the art that the teachings disclosed herein can be applied to other systems without departing from the scope and spirit of the application.
A method of starting a gas turbine is provided, comprising the step of providing an apparatus for regulating the flow of fuel in a gas turbine, the apparatus comprising a main line connecting a fuel source to a nozzle array; and an auxiliary line connecting the fuel source to the nozzle array. The method further comprises the steps of keeping the main line sealed while increasing the auxiliary line fuel flow rate; firing the gas turbine while keeping the main line sealed; and after the combustion has started in the gas turbine, opening the main line for increasing the main line fuel flow rate; wherein the auxiliary line maximum flow rate is less than the main line maximum flow rate.