Abstract:
The present application describes a heat recovery steam generator. The heat recovery steam generator may include a superheater, a first turbine section, a first main steam line in communication with the superheater and the first turbine section, and a first prewarming line positioned downstream of the first main steam line such that a flow of steam from the superheater preheats the first main steam line without entry into the first turbine section.

Description:
TECHNICAL FIELD 
       [0001]    The present application relates generally to a combined cycle power plant and more particularly relates to systems and methods for prewarming heat recovery steam generator piping so as to provide stress mitigation as well as improvements in overall power plant start up time. 
       BACKGROUND OF THE INVENTION 
       [0002]    Generally described, a combined cycle power plant uses a combination of a gas turbine and a steam turbine to produce electrical power. Specifically, a gas turbine cycle may be operatively combined with a steam turbine cycle by way of a heat recovery steam generator (“HRSG”) and the like. 
         [0003]    Because the steam turbine cycle is driven from the exhaust of the gas turbine, the HRSG may not become fully operational until the gas turbine cycle has increased the steam turbine cycle to a suitable temperature. For example, the temperature of the exhaust gas gradually increases at startup as the firing temperature of the gas turbine increases. Although the hot exhaust gases from the gas turbine flow through the HRSG, a considerable period of time may elapse before an initially cold HRSG is capable of generating steam at a sufficient pressure and temperature. In conventional systems, the gas turbine thus may be kept at relatively low loads until the temperature of the HRSG increases to a level where the HRSG can generate steam at a desired pressure and temperature. 
         [0004]    The superheated steam generated in the HRSG may be located several hundred feet away from the steam turbine or more. The piping connecting the HRSG and the steam turbine thus may be several hundred degrees cooler than the superheated steam being generated. Flowing superheated steam through pipes with cooler metal temperatures with respect to the steam, however, may cause a temperature reduction of the steam upon admission into the steam turbine. This drop in temperature may lead to a loss of equipment life or delays in allowing steam admission to the turbine due to inadequate time for the pipes to warm up. Similarly, if steam admission is delayed to allow the pipes to warm up gradually, increased operational costs may be incurred as the generated steam is bypassed to the condenser or otherwise as opposed to being used to generate work. There is thus a desire for an improved heat recovery steam generator system and associated piping startup and warming procedures. Preferably, such improved systems and procedures should mitigate the stress within the piping while also providing improved start up times. 
       SUMMARY OF THE INVENTION 
       [0005]    The present application thus describes a heat recovery steam generator. The heat recovery steam generator may include a superheater, a first turbine section, a first main steam line in communication with the superheater and the first turbine section, and a first prewarming line positioned downstream of the first main steam line such that a flow of steam from the superheater preheats the first main steam line without entry into the first turbine section. 
         [0006]    The present application further provides for a method of starting up a heat recovery steam generator. The method may include the steps of generating a flow of steam in a superheater, directing the flow of steam through a first main steam line, diverting the flow of steam through a first prewarming line until the flow of steam reaches a predetermined temperature, and directing the flow of steam to a first turbine section once the predetermined temperature is reached. 
         [0007]    The present application further provides for a heat recovery steam generator. The heat recovery steam generator may include a superheater, a first turbine section, a first main steam line in communication with the superheater and the first turbine section, a first prewarming line positioned downstream of the first main steam line such that a flow of steam from the superheater preheats the first main steam line without entry into the first turbine section, a second turbine section, a reheater downstream of the first turbine section, a second main steam line in communication with the reheater and the second turbine section, and a second prewarming line positioned downstream of the second main steam line such that the flow of steam from the reheater preheats the second main steam line without entry into the second turbine section. 
         [0008]    These and other features and improvements of the present application will become apparent to one of ordinary skill in the art upon review of the following detailed description when taken in conjunction with the several drawings and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic view of a known heat recovery steam generator system. 
           [0010]      FIG. 2  is a schematic view of a heat recovery steam generator system as may be described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    Referring now to the drawings, in which like numerals refer to like elements throughout the several views,  FIG. 1  shows a conventional heat recovery steam generator (“HRSG”) system  100 . A flow of steam  105  may be cycled through a series of turbine stages, including a high pressure stage  110 , an intermediate pressure stage  120 , and a low pressure stage  130 . A condenser  140  may be connected to the low pressure stage  130  and indirectly to the high pressure stage  120  via a bypass line  145  or otherwise. The condenser  140  collects steam, water, or mixtures thereof used in the various stages  110 ,  120 ,  130 . 
         [0012]    A flow of the superheated steam  105  may be provided by a high pressure superheater  160 . The high pressure superheater  160  may be heated by exhaust from the gas turbine or otherwise. The high pressure superheater  160  directs the superheated steam flow  105  to the high pressure stage  120  via a high pressure stage main steam line  170 . A high pressure stage main steam line isolation valve  180  may be positioned between the high pressure superheater  160  and the high pressure stage main steam line  170  so as to control the steam flow therethrough. Likewise, a high pressure stage main steam line control valve  190  may be positioned between the high pressure stage main steam line  170  and the high pressure stage  110  so as to control the pressure and steam flow therethrough. 
         [0013]    After the flow of steam  105  drives the high pressure stage  110 , the flow of steam  105  may be directed towards a reheater  200  via a cold reheat line  210 . A cold reheat line valve  220  may be positioned thereon. A high pressure stage cascade bypass line  230  also may be positioned downstream of the high pressure superheater  160  and in communication with the cold reheat line  210 . A high pressure stage cascade bypass line valve  240  may be positioned thereon. 
         [0014]    The output of the cold reheat line  210  and/or the high pressure stage cascade bypass line  230  then may be heated in the reheater  200 . The reheater  200  also may be heated by exhaust from the gas turbine or otherwise. The flow of steam  105  from the reheater  200  may be directed towards the intermediate pressure stage  120  via an intermediate pressure stage main steam line  250 . The flow between the reheater  200  and the intermediate pressure stage main steam line  250  may be controlled by a hot reheat isolation valve  260 . Likewise, the flow and pressure between the intermediate pressure stage main steam line  250  and the intermediate pressure stage  120  may be controlled by an intermediate pressure stage main steam line control valve  270 . A hot reheat bypass line  280  also may be positioned downstream of the reheater  200  and in communication with the condenser  140 . A hot reheat bypass line valve  290  may be positioned thereon. Although only one of each of the above-referenced components is shown, any number of similar components may be used in the overall HRSG system  100 . 
         [0015]    Because the steam turbine cycle derives mechanical energy from the flow of steam  105 , the steam turbine cycle components and the associated steam lines may operate at extremely high temperatures. These components and the steam lines, however, may fall outside of a desired operating range. For example, the HRSG system  100  may be in a “cold” thermal state condition after an extended period of time of non-operation. If the HRSG system  100  is simply turned on when cold, the sudden thermal expansion may cause physical stress on the components and steam lines which may lead to a reduced useful lifetime and/or damage. 
         [0016]      FIG. 2  shows a heat recovery steam generator (“HRSG”) system  300  as may be described herein. The HRSG system  300  may be largely identical to the HRSG system  100 . The HRSG system  300  also may includes a high pressure stage prewarming line  310 . The high pressure stage prewarming line  310  may be positioned downstream of the high pressure stage main steam line  170  and just upstream of the high pressure stage main steam line control valve  190 . The high pressure stage prewarming line  310  may extend to the high pressure stage cascade bypass line  230 . A high pressure stage prewarming valve  320  also may be positioned thereon. The high pressure stage prewarming line  310  may be positioned as close as possible to the high pressure stage main steam line control valve  190  so as to ensure that as much of the high pressure stage main steam line  170  may be warmed as possible. 
         [0017]    Likewise, an intermediate pressure stage prewarming line  330  may extend from just upstream of the intermediate pressure stage main steam line control valve  270  to the hot reheat bypass line  280  about the condenser  140 . An intermediate pressure stage prewarming line valve  340  may be positioned thereon. 
         [0018]    In use, the flow of steam  105  from the high pressure superheater  160  and the reheater  200  may be used to warm the high pressure stage main steam line  170  and the intermediate pressure stage main steam line  250  as opposed to being dumped directly or indirectly to the condenser  140  via the bypass lines  145 ,  280  or otherwise without performing useful work. Specifically, the high pressure stage main steam line isolation valve  180  and the high pressure stage prewarming line valve  320  may be opened while the high pressure stage main steam line control valve  190  and the high pressure stage cascade bypass valve  240  may be shut so as to warm the high pressure stage main steam line  170 . 
         [0019]    Likewise, the hot reheat isolation valve  260  and the intermediate pressure stage prewarming line valve  340  may be opened while the intermediate pressure stage main steam line control valve  270  and the hot reheat bypass valve  290  may be shut so as to warm the intermediate pressure stage main steam line  250 . This preheating thus warms the main steam lines  170 ,  250  and, once the preheating sequence is complete, ensures that the flow of steam  105  entering the stages  110 ,  120  is at approximately at the proper temperature. Once the flow of steam  105  maintains such a temperature for an appropriate amount of time, the preheating sequence may cease and steam  105  may flow through the stages and otherwise in a normal manner. 
         [0020]    In the case of the use of multiple superheaters or outlets, the high pressure stage prewarming line valve  320  and the high pressure stage main steam line isolation valve  180  may be allowed to open once the high pressure stage cascade bypass valve  240  reaches a predetermined stroke so as to ensure that steam is available for prewarming purposes. The remaining prewarming line valves  340  and the isolation valves  260  may open once the outlet pressure of the high pressure superheater  160  is higher than the pressure within the high pressure stage main steam line  170  so as to ensure a positive flow. This provides operational flexibility for the prewarming so as to account for the gas turbine operating at different loads. 
         [0021]    The HRSG system  300  as is described herein thus heats the main steam lines  170 ,  250  with no additional energy costs. This flow  105  also may be used with respect to the reheater  200  and otherwise. The prewarming concept also may be applied to the low pressure section  130  and otherwise. 
         [0022]    It should be apparent that the foregoing relates only to certain embodiments of the present application and that numerous changes and modifications&#39; may be made herein by one of ordinary skill in the art without departing from the general spirit and scope of the invention as defined by the following claims and the equivalents thereof.