Abstract:
A heat recovery steam generator (“HRSG”)  40,  which is closely coupled to a gas turbine, includes a flow controls structural array  10  disposed upstream of the tubes  42  of the HRSG  40.  The structural array  10  is formed of a plurality of grate-like panels  18  secured to horizontal supports  24  mounted to the support structure of the HRSG  40.  The structural array  10  diffuses the high velocity exhaust stream  14  exiting the gas turbine and redistributes the gas flow evenly throughout the HRSG  40.  The structural array  10  reduces wear and damage of the tubes  46.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates generally to heat recovery steam generators (HRSGs), and more particularly, to a heat recovery steam generator having a structural array to control the exhaust flow exiting a gas turbine before passing through the heat recovery steam generator. 
       BACKGROUND 
       [0002]    Combined Cycle power plants employ gas turbines with Heat Recovery Steam Generators (HRSGs) that use the thermal energy in the exhaust from gas turbines to generate steam for power generation or process use. The large stationary gas turbines used in such power plants may typically have average exhaust gas velocities in the range of 200 ft/sec. The velocity of the gas turbine exhaust is not uniform however and some recent gas turbines have local exhaust gas velocities in the range of 660 ft/sec. HRSGs may have flow areas in the range of 5 to 10 times the gas turbines exit flow area and thus average entering velocities that are 5 to 10 times lower than those exiting the gas turbine. A diverging duct is therefore required to connect the gas turbine to the HRSG. A typical arrangement of the gas turbine exhaust diffuser, connecting duct and HRSG is shown in  FIG. 1 . It is desirable to locate the HRSG close to the gas turbine in a compact duct arrangement to minimize the area required for the power plant and to minimize the size and cost of the connecting duct. This can result in a high velocity jet of gas impacting the region of the front rows of heat transfer tubes in the HRSG that are in line with the gas turbine exhaust diffuser. Such high velocities can cause flow-induced vibrations that will damage the heat transfer tubes. The high aerodynamic loading on the tube banks can also cause movement of the entire front tube bank resulting in damage to components in and around the tube bank. The non-uniform velocities entering the HRSG front tube rows also reduce the heat transfer effectiveness of these rows. 
         [0003]    In some cases flow controls have been used in the diverging duct to redirect flow within the duct and improve flow distribution to the front rows of tubes in the HRSG. These flow controls would be subject to very high aerodynamic loadings in a compact duct due to close proximity to the gas turbine. In addition to the steady aerodynamic loading, the flow controls are subject to dynamic loading due to the high levels of turbulence in the duct and thermal stress due to going from ambient temperature to the high gas turbine exhaust temperature. These issues make it unlikely that flow controls located in the diverging duct  36  will survive long-term operation. 
         [0004]    As will be described in greater detail hereinafter, a structural array disposed upstream of the front tubes of an HRSG will overcome such problems, particularly when the turbine and HRSG are closely coupled. 
         [0005]    Currently there is a need for an effective and reliable means for diffusing an exhaust stream  14  from a turbine to recover heat. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Referring now to the Figures, which are exemplary embodiments, and wherein the like elements are numbered alike: 
           [0007]      FIG. 1  is a partial cut-away side elevation view of an HRSG coupled in fluid communication with a gas turbine exhaust diffuser and an HRSG in accordance with the present invention. 
           [0008]      FIG. 2  is a cross-sectional side elevation view of an HRSG having an inlet duct and a structural array disposed upstream of the tubes of the HRSG in accordance with the present invention. 
           [0009]      FIG. 3   a  is a front view of the HRSG having a structural array secured thereto in accordance to the present invention. 
           [0010]      FIG. 3   b  is a side elevation view of the structural array of  FIG. 3   a.    
           [0011]      FIG. 4   a  is a front view of a grate-like panel of the structural array of  FIG. 3   a.    
           [0012]      FIG. 4   b  is a side elevation view of the grate-like panel of  FIG. 4   a.    
       
    
    
     DETAILED DESCRIPTION 
       [0013]    A new approach to flow controls is suggested in which an array  10  of structural components is placed in front of the front row of tubes  48  to diffuse the high velocity exhaust stream  14  exiting the gas turbine (not shown) and redistribute the gas flow into the HRSG  40 . One such arrangement is shown in  FIGS. 2-4   b . Note that these figures show one possible arrangement. Other combinations could be used as long as the features discussed below are met by the design. 
         [0014]      FIG. 2  is a cross-sectional side elevation view of an HRSG having an inlet duct and a structural array disposed upstream of the tubes of the HRSG in accordance with the present invention.  FIG. 2  illustrates an HRSG  40  with a structural array  10 . 
         [0015]      FIG. 3   a  is a front view of the HRSG having a structural array secured thereto in accordance to the present invention. 
         [0016]      FIG. 3   b  is a side elevation view of the structural array of  FIG. 3   a.    
         [0017]    With reference now to  FIGS. 2 ,  3   a  and  3   b , structural array  10  is disposed upstream of the tube banks  42  of the HRSG  40 . The structural array  10  is mounted or secured to structural elements or supports  26  at the upstream end of the HRSG  40  to control the flow of the exhaust stream  14  from a turbine (not shown), e.g., a gas turbine. As shown in  FIG. 3   a , the structural array  10  extends over the upstream end of the HRSG  40  over a sufficient area to engage or control the exhaust stream  14 . 
         [0018]    In the embodiment shown, the structural array  10  comprises a plurality of grate-like panels  18 . 
         [0019]      FIG. 4   a  is a front view of a grate-like panel of the structural array of  FIG. 3   a.    
         [0020]      FIG. 4   b  is a side elevation view of the grate-like panel of  FIG. 4   a.    
         [0021]    Panels  18  are now described with reference to  FIGS. 4   a  and  4   b . Panels  18  each have a plurality of horizontal bars  20  connected to a plurality of vertical bars  22 . The bars  20 ,  22  may be solid, hollow or generally U-shaped. Furthermore, the cross section of each bar may be any geometric shape (i.e., round, oval, square, rectangular, octagonal, etc.) or U-shaped. The grid openings  12  may be uniform or irregular. Similarly, the spacing of the vertical and horizontal bars of the array may be uniform or varied. The vertical bars  22  of the panel  18  are U-shaped, wherein the orientation of the U-shaped bars are such that the openings of the bars open inwardly towards the center of the panel. While the U-shaped vertical bars  22  are shown in such an orientation, the invention contemplates that the U-shaped bars may be disposed in any orientation. 
         [0022]    Each of the panels  18  are mounted or secured (e.g., welded, bolted, or other means of attachment) to horizontal supports  24 , which are in turn attach or secured to structural supports  26  of the HRSG  40 . The mounting of the panels  18  to the structural supports  26  and not the tubes  46  of the HRSG reduce fatigue on the tubes. In the embodiment shown the horizontal supports  24  are formed of a pair of vertically disposed tubes  30  are welded together. However, the present invention contemplates that the horizontal supports  24  may be formed from any support bean. 
         [0023]    Referring now back to  FIG. 2 , in the operation of the gas turbine (not shown) and the HRSG  40  with the flow control structural array  10 , the exhaust stream  14  from the gas turbine flows through the connecting duct  34  and HRSG inlet duct  36 . The high velocity flow passes through the grate-like structural array  10 , wherein the exhaust stream  14  is diffused and further distributed across the tubes  46  of the HRSG  40 . 
         [0024]    The structural array  10  is constructed of structural components  20 ,  22 ,  24  to withstand the forces imparted by the high velocity exhaust stream  14 . Pined and/or slip connections are used where appropriate to allow for thermal expansion. The size and spacing of the components  20 ,  22 ,  24  is arranged to provide sufficient resistance to redirect part of the high velocity exhaust stream  14  to the sections of the front row tubes  48  that would have had little or no gas flow, improving the distribution of gas flow into the HRSG  40 . The structural components  20 ,  22 ,  24  are also sized and spaced such that the remaining flow passing though the array  10  is distributed through grid openings  12  into a large number of smaller jets. The smaller jets start with a diameter D the same as the grid openings  12 . These are on the order of 1/10 of the distance from the structural array  10  to the tubes  46 . This allows the small multiple jets to partially dissipate before reaching the tubes  46  and lowers the loading on the region of the tubes that would have been subjected to unacceptable velocities without the structural array  10 . 
         [0025]    The extent of the front row of tubes  46  that are protected by the structural array  10  and the diameter of the grid openings  12  will be based on physical flow modeling of the specific gas turbine and HRSG  40 . 
         [0026]    In an alternative embodiment, structural array  10  is on adjustable mounts ( 50  of  FIG. 2 ) such that the distance from the structural array and tubes  46  may be adjusted. This allows for adjustment of more or less dissipation of the exhaust jets as they impinge upon the tubes  46 . Since more diffusion of the exhaust stream  14  result in higher exhaust back pressure, the system can be interactively optimized for both backpressure and diffusion. 
         [0027]    While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.