Patent Publication Number: US-9845710-B2

Title: Start-up method of steam turbine plant

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-221204, filed on Oct. 24, 2013; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a start-up method of a steam turbine plant. 
     BACKGROUND 
     Conventionally, one including plural boilers for a single steam turbine is known as a steam turbine plant. Besides, one including a superheater and a reheater is known as the boiler of the steam turbine plant. As for the steam turbine plant as stated above, a steam flow rate required by a steam turbine is small from an aeration start for the steam turbine at a start-up until reaching a predetermined load. Accordingly, steam is supplied from a part of the boilers to the steam turbine from the aeration start for the steam turbine until reaching the predetermined load. Hereinafter, a part of the boilers supplying the steam to the steam turbine at the aeration start time is referred to as an aeration boiler. Besides, a remaining boiler which does not supply the steam to the steam turbine at the aeration start time is referred to as a standby boiler. 
     After reaching the predetermined load, steam of the standby boiler is merged with steam of the aeration boiler, and supplied to the steam turbine (Tie-in). Conventionally, steam pressures (reheat steam pressures) supplied from the reheaters of the aeration boiler and the standby boiler at the Tie-in time are coincident. There is a problem as described below in a conventional technology as stated above. 
     For example, a bypass pipe guiding reheat steam from the standby boiler to a steam condenser is provided at a boiler side so that the reheat steam from the standby boiler is not supplied to the steam turbine. The reheat steam pressure of the standby boiler is set to be rather high from the aeration start time so that a valve capacity of a bypass valve provided in a middle of the bypass pipe does not become large. Besides, the reheat steam pressure of the aeration boiler is set to be high in response to the standby boiler. 
     However, at a steam turbine side, a windage loss is easy to occur because a high-pressure turbine is not able to work sufficiently at the aeration start time. In particular, when a steam pressure in a vicinity of a final stage is high, a temperature of a blade of the final stage is easy to increase exceeding an allowance together with the windage loss. Accordingly, there is a possibility of an occurrence of a serious trouble such as a contact between the blade and a static part. 
     The steam from the high-pressure turbine has been bypassed to the steam condenser up to now so as to satisfy a requirement of the steam turbine side while satisfying the requirement of the boiler side. For example, a bypass pipe is provided from a middle of a low-temperature reheat steam pipe connected to an outlet of the high-pressure turbine to be connected to the steam condenser. However, it is preferable to satisfy the requirements of the boiler side and the steam turbine side without providing an additional bypass pipe as stated above. Specifically, it is preferable to suppress the windage loss and a temperature increase at the high-pressure turbine while making the valve capacity small. 
     Besides, when the reheat steam pressures of both the aeration boiler and the standby boiler are set to be high, a fuel consumption amount becomes large. Accordingly, it is required to start-up the steam turbine plant without setting the reheat steam pressures of the aeration boiler and the standby boiler high. 
     As stated above, it is required to suppress the valve capacity of the bypass valve provided at the bypass pipe connecting the standby boiler and the steam condenser as for the conventional steam turbine plant. Besides, to suppress the valve capacity, it is required not to provide the additional bypass pipe. Further, to suppress the valve capacity, it is required to suppress the fuel consumption amount. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a system diagram illustrating a steam turbine plant according to an embodiment. 
         FIG. 2  is a view illustrating a relationship between a load of a steam turbine and reheat steam pressures of an aeration boiler and a standby boiler in a start-up method of the steam turbine plant according to a first embodiment. 
         FIG. 3  is a view illustrating a relationship between a load of a steam turbine and reheat steam pressures of an aeration boiler and a standby boiler in a start-up method of the steam turbine plant according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In one embodiment, a start-up method of a steam turbine plant is a start-up method of a steam turbine plant including a steam turbine and plural boilers. The steam turbine includes a high-pressure turbine and an intermediate-pressure turbine. The plural boilers each have a superheater and a reheater. The superheater supplies high-pressure steam to the high-pressure turbine. The reheater reheats exhaust steam of the high-pressure turbine, and supplies to the intermediate-pressure turbine as reheat steam. 
     The start-up method of the steam turbine plant includes a first step and a second step. 
     The first step is performed at an aeration start time. In the first step, one of the plural boilers is set to be an aeration boiler which supplies steam to a steam turbine. Besides, in the first step, the other of the plural boilers is set to be a standby boiler which does not supply the steam to the steam turbine. A reheat steam pressure of the aeration boiler is set to be a reheat steam pressure required by the steam turbine or less. Besides, a reheat steam pressure of the standby boiler is set to be a reheat steam pressure required for the standby boiler or more. 
     The second step is performed after the aeration starts, when a load of the steam turbine becomes a predetermined value. In the second step, the reheat steam pressure of the aeration boiler is increased to a degree as same as the reheat steam pressure of the standby boiler. Then, steam from the aeration boiler and steam from the standby boiler are merged, and the merged steam is supplied to the steam turbine. 
     Hereinafter, embodiments of the present invention are described with reference to the drawings. 
       FIG. 1  is a system diagram illustrating a steam turbine plant of an embodiment. A steam turbine plant  10  of the embodiment includes a single steam turbine  11 . The steam turbine  11  includes, for example, a high-pressure turbine  111 , an intermediate-pressure turbine  112 , and a low-pressure turbine  113 . Besides, the steam turbine plant  10  of the embodiment includes, for example, a boiler  21 , a boiler  31 , and a steam condenser  41 . 
     The boiler  21  includes a superheater  211  and a reheater  212 . An outlet of the superheater  211  and an inlet of the high-pressure turbine  111  are connected by a main steam pipe  22  where a main steam separation valve  221 , a main steam stop valve  222 , and a steam control valve  223  are provided in sequence from the superheater  211  side. An outlet of the high-pressure turbine  111  and an inlet of the reheater  212  are connected by a low-temperature reheat steam pipe  23  where a low-temperature reheat steam separation valve  231  is provided. An outlet of the reheater  212  and an inlet of the intermediate-pressure turbine  112  are connected by a reheat steam pipe  24  where a reheat steam separation valve  241 , a reheat steam stop valve  242 , and an intercept valve  243  are provided in sequence from the reheater  212  side. 
     Besides, a high-pressure turbine bypass pipe  25  is provided so as to branch from an upstream side of the main steam separation valve  221  at the main steam pipe  22  to be connected to a downstream side of the low-temperature reheat steam separation valve  231  at the low-temperature reheat steam pipe  23 . A high-pressure turbine bypass valve  251  is provided in a middle of the high-pressure turbine bypass pipe  25 . Further, an intermediate-low-pressure turbine bypass pipe  26  is provided so as to branch from an upstream side of the reheat steam separation valve  241  at the reheat steam pipe  24  to be connected to the steam condenser  41 . An intermediate-low-pressure turbine bypass valve  261  is provided in a middle of the intermediate-low-pressure turbine bypass pipe  26 . 
     Similarly, the boiler  31  includes a superheater  311  and a reheater  312 . An outlet of the superheater  311  and the inlet of the high-pressure turbine  111  are connected by a main steam pipe  32  where a main steam separation valve  321 , a main steam stop valve  322 , and a steam control valve  323  are provided in sequence from the superheater  311  side. The outlet of the high-pressure turbine  111  and an inlet of the reheater  312  are connected by a low-temperature reheat steam pipe  33  where a low-temperature reheat steam separation valve  331  is provided. An outlet of the reheater  312  and the inlet of the intermediate-pressure turbine  112  are connected by a reheat steam pipe  34  where a reheat steam separation valve  341 , a reheat steam stop valve  342 , and an intercept valve  343  are provided in sequence from the reheater  312  side. 
     Besides, a high-pressure turbine bypass pipe  35  is provided so as to branch from an upstream side of the main steam separation valve  321  at the main steam pipe  32  to be connected to a downstream side of the low-temperature reheat steam separation valve  331  at the low-temperature reheat steam pipe  33 . A high-pressure turbine bypass valve  351  is provided in a middle of the high-pressure turbine bypass pipe  35 . Further, an intermediate-low-pressure turbine bypass pipe  36  is provided so as to branch from an upstream side of the reheat steam separation valve  341  at the reheat steam pipe  34  to be connected to the steam condenser  41 . An intermediate-low-pressure turbine bypass valve  361  is provided in a middle of the intermediate-low-pressure turbine bypass pipe  36 . 
     Further, an outlet of the intermediate-pressure turbine  112  and an inlet of the low-pressure turbine  113  are connected by a crossover pipe  114 . An outlet of the low-pressure turbine  113  is connected to the steam condenser  41 , and steam exhausted from the low-pressure turbine  113  is condensed to be condensed water. This condensed water is guided to a low-pressure feedwater heater  42 , and a deaerator  43  in sequence. After that, the condensed water is pressurized by a boiler feedwater pump  44 , and thereafter, is supplied to the superheater  211  and the superheater  311  via a high-pressure feedwater heater  45 . 
     Besides, control units and so on of each of valves are provided according to need though they are not illustrated. The control unit includes a processing device, an input/output processing device, a storage device, and so on. The control unit is electrically connected to a detecting device and so on detecting each valve and an operation state. As the detecting device, for example, a device detecting a temperature of components of the steam turbine  11 , a device detecting an opening degree of each valve, a device detecting a rotation speed of the steam turbine  11 , a device detecting a load, a device detecting a flow rate of steam, a device detecting a pressure of steam, a device detecting a system frequency, a voltage, and a phase when an electric power system is also turned on, and so on can be cited. 
     When the steam turbine plant  10  is started, the main steam separation valve  321  of the main steam pipe  32 , the low-temperature reheat steam separation valve  331  of the low-temperature reheat steam pipe  33 , and the reheat steam separation valve  341  of the reheat steam pipe  34  are closed from an aeration start time to a predetermined load time, and thereby, it is possible to set the boiler  31  as the standby boiler which does not supply steam to the steam turbine  11 . On the other hand, the main steam separation valve  221  of the main steam pipe  22 , the low-temperature reheat steam separation valve  231  of the low-temperature reheat steam pipe  23 , and the reheat steam separation valve  241  of the reheat steam pipe  24  are opened, and thereby, it is possible to set the boiler  21  as the aeration boiler which supplies steam to the steam turbine  11 . 
     A part of the steam generated at the boiler  21  and the boiler  31  is supplied to the steam turbine  11  according to need. Excessive steam which is not supplied to the steam turbine  11  is collected by the steam condenser  41  via the high-pressure turbine bypass pipe  25  and the high-pressure turbine bypass pipe  35 , further the intermediate-low-pressure turbine bypass pipe  26  and the intermediate-low-pressure turbine bypass pipe  36 . 
     Further, the intermediate-low-pressure turbine bypass valve  361  of the intermediate-low-pressure turbine bypass pipe  36  is adjusted, and thereby, it is possible to adjust the reheat steam pressure of the boiler  31  to be the standby boiler. Similarly, the intermediate-low-pressure turbine bypass valve  261  of the intermediate-low-pressure turbine bypass pipe  26  is adjusted, and thereby, it is possible to adjust the reheat steam pressure of the boiler  21  to be the aeration boiler. 
     As stated above, the main steam separation valve, the low-temperature reheat steam separation valve, the reheat steam separation valve, the high-pressure turbine bypass valve, the intermediate-low-pressure turbine bypass valve, and so on are provided by each boiler, and thereby, it is possible to independently adjust a steam supply and the reheat steam pressure by each boiler. 
     Next, a start-up method of a steam turbine plant according to a first embodiment is described.  FIG. 2  is a view illustrating a relationship between a load of the steam turbine  11  in the start-up method of the first embodiment and the reheat steam pressures of the boiler  21  to be the aeration boiler and the boiler  31  to be the standby boiler. 
     Hereinafter, the steam turbine plant  10  of the embodiment, namely, a case of the steam turbine plant  10  including the boiler  21  to be the aeration boiler and the boiler  31  to be the standby boiler is exemplified to be described. 
     The start-up method of the steam turbine plant of the first embodiment includes a first step  101  and a second step  102 . 
     The first step  101  is performed at an aeration start time. In the first step  101 , the boiler  21  is set to be the aeration boiler which supplies the steam to the steam turbine  11 . Besides, in the first step  101 , the boiler  31  is set to be the standby boiler which does not supply the steam to the steam turbine  11 . The reheat steam pressure of the boiler  21  being the aeration boiler is set to be a reheat steam pressure (P 1 ) required by the steam turbine  11  or less. Besides, the reheat steam pressure of the boiler  31  being the standby boiler is set to be a reheat steam pressure (P 2 ) required for the standby boiler or more as same as the conventional art. Namely, the reheat steam pressure of the boiler  31  being the standby boiler is the predetermined reheat steam pressure (P 2 ) or more in which the valve capacity of the intermediate-low-pressure turbine bypass valve  361  does not become large. Note that, the reheat steam pressure (P 1 ) is normally smaller than the reheat steam pressure (P 2 ) (P 1 &lt;P 2 ). 
     The second step  102  is performed after the aeration starts, when the load of the steam turbine  11  becomes a predetermined value. In the second step  102 , a heat steam pressure of the boiler  21  being the aeration boiler is increased to the same degree as the reheat steam pressure of the boiler  31  being the standby boiler. After that, each steam of the boilers  21 ,  31  is merged, and the merged steam is supplied to the steam turbine  11 . Here, as the steam supplied to the steam turbine  11 , main steam being high-pressure steam and reheat steam can be cited. 
     In the start-up method of the first embodiment, the reheat steam pressure of the boiler  31  being the standby boiler is set to be the predetermined reheat steam pressure (P 2 ) or more at the first step  101 . The boiler  31  is set to be the reheat steam pressure (P 2 ) or more, and thereby, it is possible to suppress the valve capacity of the intermediate-low-pressure turbine bypass valve  361 . 
     Besides, in the start-up method of the first embodiment, the reheat steam pressure of the boiler  21  being the aeration boiler is set to be low such as the reheat steam pressure (P 1 ) required by the steam turbine  11  or less at the first step  101 . The reheat steam pressure of the boiler  21  is set to be the reheat steam pressure (P 1 ) or less, and thereby, a steam pressure at a final stage of the high-pressure turbine  111  is able to be set low. The steam pressure at the final stage of the high-pressure turbine  111  is able to be set low, and therefore, it is not necessary to provide an equipment to release the steam at the final stage to the steam condenser as in the conventional art. Namely, it is not necessary to provide an equipment to release the steam from the low-temperature reheat steam pipe  23  to the steam condenser  41 . It is possible to suppress the windage loss at the high-pressure turbine  111  without providing the equipment as stated above. Namely, it is possible to suppress the windage loss at the high-pressure turbine  111  without providing a pipe connecting the low-temperature reheat steam pipe  23  and the steam condenser  41 . 
     Specifically, the reheat steam pressure of the boiler  21  being the aeration boiler is set low, and thereby, it is possible to suppress a pressure at an exhaust hood of the high-pressure turbine  111 . It is thereby possible to suppress the windage loss, and as a result, it is possible to keep a temperature increase at the final stage of the high-pressure turbine  111  within an allowance. 
     Further, the boiler  21  is set to be the reheat steam pressure (P 1 ) or less, and thereby, it is possible to suppress the fuel consumption amount compared to a conventional start-up method of a steam turbine plant. Note that in the conventional start-up method of the steam turbine plant, the reheat steam pressures of both the aeration boiler and the standby boiler are set high to be the reheat steam pressure (P 2 ) or more. 
     Here, the reheat steam pressure (P 1 ) required by the steam turbine  11  may be one capable of suppressing the windage loss at the high-pressure turbine  111 . The reheat steam pressure (P 1 ) required by the steam turbine  11  is preferably approximately 10 bar though it may be slightly different depending on a concrete configuration of the steam turbine  11 . 
     On the other hand, the reheat steam pressure (P 2 ) of the boiler  31  being the standby boiler may be the reheat steam pressure or more in which the valve capacity of the intermediate-low-pressure turbine bypass valve  361  does not become large. Here, to lower the valve capacity of the intermediate-low-pressure turbine bypass valve  361 , the reheat steam pressure (P 2 ) is preferably larger. However, when the reheat steam pressure (P 2 ) is too large, there is a possibility in which a temperature of the exhaust hood becomes too high caused by the windage loss when the reheat steam pressure of the boiler  21  being the aeration boiler is increased to the pressure in the second step. From a point of view as stated above, the reheat steam pressure (P 2 ) is preferably determined appropriately in accordance with a concrete mode of the steam turbine plant. In particular, the reheat steam pressure (P 2 ) is preferably determined appropriately by considering while comparing reduction in the valve capacity and the suppression of the temperature of the exhaust hood caused by the windage loss. 
     The first step  101  is performed as follows. Namely, as for the boiler  21  being the aeration boiler, the main steam separation valve  221 , the main steam stop valve  222 , and the steam control valve  223  of the main steam pipe  22 , the low-temperature reheat steam separation valve  231  of the low-temperature reheat steam pipe  23 , the reheat steam separation valve  241 , the reheat steam stop valve  242 , and the intercept valve  243  of the reheat steam pipe  24  are opened. Further, the high-pressure turbine bypass valve  251  of the high-pressure turbine bypass pipe  25  and the intermediate-low-pressure turbine bypass valve  261  of the intermediate-low-pressure turbine bypass pipe  26  are opened. The steam is thereby supplied to the steam turbine  11  while a steam amount from the boiler  21  is adjusted to an amount necessary for the aeration of the steam turbine  11  by a control of each valve. 
     On the other hand, as for the boiler  31  being the standby boiler, the main steam separation valve  321  of the main steam pipe  32 , the low-temperature reheat steam separation valve  331  of the low-temperature reheat steam pipe  33 , and the reheat steam separation valve  341  of the reheat steam pipe  34  are closed. It is thereby possible to set the boiler  31  to be the standby boiler which does not supply the steam to the steam turbine  11 . Note that the main steam stop valve  322  and the steam control valve  323  of the main steam pipe  32 , and the reheat steam stop valve  342  and the intercept valve  343  of the reheat steam pipe  34  may each be opened, or closed. Besides, the high-pressure turbine bypass valve  351  of the high-pressure turbine bypass pipe  35  and the intermediate-low-pressure turbine bypass valve  361  of the intermediate-low-pressure turbine bypass pipe  36  are opened. All of the steam from the boiler  31  is thereby supplied to the steam condenser  41  without being used for the aeration of the steam turbine  11  at all. 
     At this time, for example, it is possible to adjust the reheat steam pressure of the boiler  21  being the aeration boiler by adjusting the intermediate-low-pressure turbine bypass valve  261  of the intermediate-low-pressure turbine bypass pipe  26 . Besides, for example, it is possible to adjust the reheat steam pressure of the boiler  31  being the standby boiler by adjusting the intermediate-low-pressure turbine bypass valve  361  of the intermediate-low-pressure turbine bypass pipe  36 . Specifically, it is possible to make the reheat steam pressure of the boiler  21  being the aeration boiler low by adjusting a valve opening degree of the intermediate-low-pressure turbine bypass valve  261  of the intermediate-low-pressure turbine bypass pipe  26  in a direction to make it large. On the other hand, it is possible to increase the reheat steam pressure of the boiler  31  being the standby boiler by adjusting the valve opening degree of the intermediate-low-pressure turbine bypass valve  361  of the intermediate-low-pressure turbine bypass pipe  36  in a direction to make it small. 
     The second step  102  is performed as described below. Namely, the valve opening degree of the intermediate-low-pressure turbine bypass valve  261  of the intermediate-low-pressure turbine bypass pipe  26  is set to be small compared to the valve opening degree of the first step  101 . The reheat steam pressure of the boiler  21  being the aeration boiler is thereby increased to the same degree as the reheat steam pressure of the boiler  31  being the standby boiler. Besides, the main steam separation valve  321  of the main steam pipe  32 , the low-temperature reheat steam separation valve  331  of the low-temperature reheat steam pipe  33 , and the reheat steam separation valve  341  of the reheat steam pipe  34  are opened. The steam from the boiler  31  being the standby boiler and the steam from the boiler  21  are thereby made into a state in which the pressures thereof are the same to be merged, and the merged steam is supplied to the steam turbine  11 . After that, steam conditions of the aeration boiler and the standby boiler are set to be the same, and a load is increased. 
     The first step  101  is preferably performed from the aeration start time for the steam turbine  11  to an ultralow load reaching time. Here, the aeration start time is a moment when first steam is supplied to the steam turbine  11 . Besides, the ultralow load reaching time is a time when a load of the steam turbine  11  relative to a rated load is any of 10% or more and less than 30%, for example, 20%. 
     A pressurization performed at the beginning of the second step  102  is preferably performed at a constant load after the ultralow load reaching time. The load at the pressurization time is made constant, and thereby, it is possible to make controllability fine. Besides, the second step  102  is preferably performed until an intermediate load reaching time. Namely, as for the boiler  21  being the aeration boiler, the pressurization performed at the beginning of the second step  102  is preferably performed under a constant load. Besides, as for the boiler  21  being the aeration boiler, after the steam is merged with the steam of the boiler  31  being the standby boiler and supplied to the steam turbine  11 , it is preferable that the reheat steam pressure is kept until the intermediate load reaching time. Besides, as for the boiler  31  being the standby boiler, the reheat steam pressure (similar to the reheat steam pressure of the first step  101 ) at the beginning of the second step  102  is preferably kept until the intermediate load reaching time. Here, the intermediate load reaching time is a time when the load of the steam turbine  11  relative to the rated load becomes any of 30% or more and 60% or less, for example, 50%. 
     After the second step  102 , for example, the reheat steam pressure of the boiler  21  being the aeration boiler is gradually increased in accordance with an increase of the load of the steam turbine  11 . Similarly, after the second step  102 , for example, the reheat steam pressure of the boiler  31  being the standby boiler is gradually increased in accordance with an increase of the load of the steam turbine  11 . At this time, for example, the reheat steam pressure of the boiler  21  being the aeration boiler and the reheat steam pressure of the boiler  31  being the standby boiler are gradually increased such that they become the same degree with each other. 
     After the steam turbine becomes a predetermined load, for example, until the load of the steam turbine  11  reaches the rated load, the reheat steam pressure of the boiler  21  being the aeration boiler is made constant. Similarly, after the steam turbine becomes the predetermined load, for example, until the load of the steam turbine  11  reaches the rated load, the reheat steam pressure of the boiler  31  being the standby boiler is made constant. 
     Next, a start-up method of the steam turbine plant of a second embodiment is described.  FIG. 3  is a view illustrating a relationship between the load of the steam turbine  11  in the start-up method of the second embodiment and the reheat steam pressures of the boiler  21  to be the aeration boiler and the boiler  31  to be the standby boiler. 
     The start-up method of the steam turbine plant of the second embodiment includes a first step  103  and a second step  104 . 
     The first step  103  is performed at the aeration start time. In the first step  103 , the boiler  21  is set to be the aeration boiler which supplies the steam to the steam turbine  11 , and the boiler  31  is set to be the standby boiler which does not supply the steam to the steam turbine  11 . Besides, in the first step  103 , the reheat steam pressures of the boiler  21  being the aeration boiler and the boiler  31  being the standby boiler are each independently set to be the reheat steam pressure (P 1 ) or less. Namely, the reheat steam pressures of the boiler  21 , the boiler  31  may be the reheat steam pressure (P 1 ) or less, and they may be different from one another. The steam is supplied to the steam turbine  11  only from the boiler  21  being the aeration boiler. 
     The second step  104  is performed after the aeration starts, when the load of the steam turbine  11  becomes the predetermined value. In the second step  104 , the reheat steam pressures of the boiler  21  being the aeration boiler and the boiler  31  being the standby boiler are each independently increased to the reheat steam pressure (P 2 ) or more. At this time, the reheat steam pressures of the boiler  21 , the boiler  31  are preferably the reheat steam pressures at the same degree with each other. After that, the steam of both boilers are merged, and the merged steam is supplied to the steam turbine  11 . 
     In the start-up method of the second embodiment, the reheat steam pressure of the boiler  21  being the aeration boiler is set to be low at the first step  103 . The reheat steam pressure of the boiler  21  is low, and therefore, it is possible to set a steam pressure at the final stage of the high-pressure turbine  111  low. The steam pressure at the final stage of the high-pressure turbine  111  can be set low, and therefore, it is not necessary to provide the equipment to release the steam at the final stage to the steam condenser as in the conventional art. Namely, it is not necessary to provide the equipment to release the steam from the low-temperature reheat steam pipe  23  to the steam condenser  41 . It is possible to suppress the windage loss at the high-pressure turbine  111  without providing the equipment as stated above. Namely, it is possible to suppress the windage loss at the high-pressure turbine  111  without providing the pipe connecting the low-temperature reheat steam pipe  23  and the steam condenser  41 . Besides, in the start-up method of the second embodiment, the reheat steam pressure of the boiler  31  being the standby boiler is set low at the first step  103 . The reheat steam pressure of the boiler  31  is low, and therefore, the fuel consumption amount is further suppressed compared to the start-up method of the first embodiment. 
     Note that in the first step  103 , when the reheat steam pressure of the boiler  31  to be the standby boiler is lowered, it is desirable to set the pressure to a degree in which the valve capacity of the intermediate-low-pressure turbine bypass valve  361  is not affected. Here, the intermediate-low-pressure turbine bypass valve  361  is one provided at the intermediate-low-pressure turbine bypass pipe  36 . The intermediate-low-pressure turbine bypass pipe  36  is one to bypass the reheat steam from the boiler  31  to be the standby boiler. 
     The first step  103  is able to be performed as same as the first embodiment except that the reheat steam pressure of the boiler  31  being the standby boiler is adjusted to be the reheat steam pressure (P 1 ) or less. Namely, opening/closing states of each valve to set the boiler  21  to be the aeration boiler and the boiler  31  to be the standby boiler can be set to be the same as the first embodiment. 
     Besides, an adjustment of the reheat steam pressure of the boiler  21  being the aeration boiler can be performed by an adjustment of the intermediate-low-pressure turbine bypass valve  261  of the intermediate-low-pressure turbine bypass pipe  26 . Besides, an adjustment of the reheat steam pressure of the boiler  31  being the standby boiler can be performed by an adjustment of the intermediate-low-pressure turbine bypass valve  361  of the intermediate-low-pressure turbine bypass pipe  36 . Specifically, both the intermediate-low-pressure turbine bypass valve  261  of the intermediate-low-pressure turbine bypass pipe  26  and the intermediate-low-pressure turbine bypass valve  361  of the intermediate-low-pressure turbine bypass pipe  36  are adjusted in a direction in which the valve opening degrees are made large, and thereby, it is possible to lower the reheat steam pressures of the boiler  21  being the aeration boiler and the boiler  31  being the standby boiler. 
     The second step  104  is performed as, for example, described below. Namely, the valve opening degree of the intermediate-low-pressure turbine bypass valve  261  of the intermediate-low-pressure turbine bypass pipe  26  is set to be smaller compared to the case of the first step  103 . It is thereby possible to increase the reheat steam pressure of the boiler  21  being the aeration boiler. Similarly, the valve opening degree of the intermediate-low-pressure turbine bypass valve  361  of the intermediate-low-pressure turbine bypass pipe  36  is set to be smaller compared to the case of the first step. It is thereby possible to increase the reheat steam pressure of the boiler  31  being the standby boiler. Further, the main steam separation valve  321  of the main steam pipe  32 , the low-temperature reheat steam separation valve  331  of the low-temperature reheat steam pipe  33 , and the reheat steam separation valve  341  of the reheat steam pipe  34  are opened. It is thereby possible to merge the steam from the boiler  31  being the standby boiler with the steam from the boiler  21 , and to supply the merged steam to the steam turbine  11 . 
     Hereinabove, the start-up methods of the steam turbine plant according to the first and second embodiments are described. A case when two boilers are included is described as for the start-up methods of the steam turbine plant according to the first and second embodiments, but the number of boilers may be three or more. When three or more boilers are included, a boiler to be the aeration boiler or the standby boiler can be selected appropriately. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.