Patent Publication Number: US-6334294-B1

Title: Combustion crossfire tube with integral soft chamber

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to crossfire tubes extending between adjacent combustors in a land-based gas turbine. 
     The annular arrangement of combustors in a stationary, or land-based gas turbine with interconnecting crossfire tubes is generally well known as disclosed in, for example, commonly owned U.S. Pat. No. 4,249,372. As disclosed in the &#39;372 patent, a typical cross ignition assembly comprises tubular members extending between aligned openings in adjacent combustors that are held in place by means that position the opposite ends of the tubular members or crossfire tubes in fluid communication with the adjacent combustion chambers. The purpose of the crossfire tubes is to provide for the ignition of fuel in one combustion chamber from ignited fuel in an adjacent combustion chamber, thereby eliminating the need for a separate igniter in each combustor. Specifically, when chamber to chamber crossfire is desired, it is accomplished by a pressure pulse of hot gases transferring from a firing chamber to an unfired chamber through the crossfire tube. The crossfire tubes also serve the purpose of equalizing to some extent the pressures between combustion chambers. 
     Current crossfire tube design includes a plurality of purge air holes (usually six) arranged about the circumference of the crossfire tube, approximately midway along its axial length. An arrangement of this type is disclosed in commonly owned U.S. Pat. No. 5,896,742. Purge air is fed to the crossfire tube purge air holes at approximately the compressor discharge pressure so as to prevent unwanted migration of oil (unburned fuel) between adjacent combustors. This purge air, however, opposes the cross-firing pressure pulse, and can actually prevent firing of the adjacent combustor. Specifically, the purge air feed pressure and flow rate both inherently resist the crossfire pressure pulse. Inhibiting oil intrusion, however, is controlled by the purge air flow rate combined with the convergence of the tube shape. Thus, a minimum discharge flow rate out of the crossfire tubes is necessary to inhibit oil intrusion into the crossfire tube. Oil intrusion is a source of auto ignition, which, in turn, is a cause of failure of the gas turbine to correctly operate. Accordingly, the purge air flow rate must be sufficient to inhibit oil intrusion, but the combined pressure and flow rate must not be too great to keep a pressure pulse of hot gases from transferring from one combustion chamber to the other through the crossfire tube. Compressor discharge pressure and the crossfire tube purge air hole size set the flow rate inside the crossfire tubes. Attempts to balance the crossfire performance and resistance to oil intrusion by varying the hole size, however, have not been successful. It has been found that the feed pressure and flow rate are either too high to consistently achieve good crossfire or are too low to inhibit oil intrusion. 
     Thus, the problem to be solved is that the flow rate cannot be decreased to improve crossfire performance without risking an increase of oil intrusion, and possibly other harmful effects elsewhere in the combustion turbine. 
     Some model gas turbines have reduced the purge air feed pressure to the crossfire tubes by mechanical blockage for reasons divorced from crossfire tube performance. These gas turbines have good crossfire performance but, since they do not operate on oil, they do not have the same design constraints with respect to oil intrusion into the crossfire tubes. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with this invention, both purge air feed pressure to the crossfire tube and the purge air flow rate within the crossfire tube are separately affected by creating a pressure drop mechanism upstream of the purge air feed holes into the crossfire tube. This is accomplished by creating an integral chamber as part of the crossfire tube assembly that will operate such that during crossfire, the purge air is temporarily “stalled” in the integral chamber. This is made possible by reducing the purge feed pressure such that the re-light pulse from the firing chamber to the unfired chamber can overcome the pressure drop across the purge feed holes. 
     Thus, in accordance with the broader aspects of the invention, there is provided a crossfire tube for connecting adjacent combustors in a gas turbine, the crossfire tube comprising a hollow tubular body having opposite end portions adapted to be secured to the adjacent combustors; an annular chamber surrounding a mid-section of the hollow tubular member; a first plurality of purge air holes in an outer wall of the chamber, and a second plurality of purge air holes in an inner wall of the chamber opening into the crossfire tube. 
     In another aspect, the invention provides a crossfire tube assembly for connecting adjacent combustors in a gas turbine, the crossfire tube assembly comprising a hollow tubular body having opposite end portions and a plurality of purge air feed holes arranged in a circumferential array about a mid-section of the hollow tubular body and adapted to feed purge air into the hollow tubular body; the hollow tubular body having means for reducing compressor discharge air pressure prior to entry into the hollow tubular body. 
     In still another aspect, the invention relates to a method of supplying purge air to a crossfire tube located between a pair of adjacent combustors in a gas turbine, the crossfire tube including a hollow tubular body adapted for connection between the adjacent combustors, the method comprising establishing a chamber about the tubular body; utilizing compressor discharge air as crossfire tube purge air; feeding the purge air at a first pressure approximately equal to the compressor discharge air pressure into the chamber to thereby reduce the pressure to a second, lower pressure; and subsequently feeding the purge air from the chamber into the tubular body of the crossfire tube at the lower pressure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross section of a prior art crossfire tube construction; and 
     FIG. 2 is a cross section of a crossfire tube in accordance with this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A known crossfire tube configuration is shown in FIG.  1 . The crossfire tube  10  extends between a first combustor  12  and a second combustor  14 , and is surrounded by a vessel  16  that is open to the flow of compressor discharge air. The crossfire tube itself comprises a pair of tubular sections joined in a telescoping relationship. Specifically, the crossfire tube includes a first section  18  that is slidably received within a second section  20 , and secured therein mechanically and/or by any other suitable means. A plurality of purge air feed holes  22  are drilled in the section  18  adjacent the telescoping joint with section  20 . The tubular sections  18  and  20  each taper from a larger diameter in the mid-section of the crossfire tube, to smaller diameters at the ends  24 ,  26 , respectively, where the ends are joined to the combustors  12  and  14 . The taper at the ends of the crossfire tube causes the purge air flow to accelerate and be forced against the tube walls so that the purge air fills the entire cross section of the opposite ends of the tube. This arrangement, however, has not been completely successful in inhibiting oil intrusion into the crossfire tube at some flow conditions, and may also inhibit good crossfire at other pressure and flow conditions, due to the acceleration of purge air in a direction opposite the crossfire pressure pulse. 
     Turning to FIG. 2, a crossfire tube in accordance with an exemplary embodiment of the invention is arranged between adjacent combustors  30  and  32 , and surrounded by pressure vessel  34  similar to vessel  16 . The crossfire tube is essentially a hollow, tubular body  28  made up of two mating tubular sections  36  and  38 . The smaller diameter or inner section  38  is adapted to slide into a larger diameter portion of an outer section  36  in telescoping relationship. The inner section  38  is configured, however, to include axially spaced, raised annular lands  40 ,  42  that engage and support the interior surface of the outer section  36 . This arrangement also creates an annular chamber  44  radially between the inner and outer sections  38 ,  36 , extending axially between the lands  40 ,  42  generally in the central area of the crossfire tube. A first plurality of purge air feed holes  46  (also referred to as “sleeve holes” to facilitate differentiation vis-a-vis the purge air feed holes  48 ) are formed in the outer section  36 . A second plurality of purge air feed holes  48  are formed in the inner section  38 , axially between the lands  40 ,  42  and opening into the crossfire tube. Note that outer section  36  also forms an “outer wall” of the chamber  44  while inner section  38  also forms an “inner wall” of the chamber. The purge air feed holes  48  are also axially spaced from the sleeve holes  46 . The end portions  50 ,  52  of the crossfire tube may taper to approximately equal smaller diameters where the end portions are secured to the adjacent combustors  30 ,  32  in otherwise conventional fashion. 
     With this arrangement, compressor discharge air will flow into the vessel  34  (as indicated by the flow arrows), through the sleeve holes  46  in the outer section  36  and into the chamber  44 . The purge air then flows through purge air feed holes  48  into the crossfire tube  28 , flowing in opposite directions towards combustors  30 ,  32 . By flowing through holes  46 , a pressure drop is created such that a lower pressure exits in chamber  44  so that the purge air entering the air feed holes  48  is at a pressure less than the compressor discharge air pressure. In other words, the pressure pulse necessary to establish good crossfire performance is not resisted by the full compressor discharge pressure. The purge air pressure within chamber  44  and the purge air flow rate into the crossfire tube can be independently varied by selecting the appropriate sizes for holes  46  and  48 , as well as the number of such holes about the inner and outer sections  38 ,  36 . 
     The volume of the chamber  44  must be adequate to accumulate suspended purge air flow for a short period without raising the pressure in the chamber significantly, or else the flow into the crossfire tube during crossfire will increase and the temperature of the crossfire relight pulse will be diluted. 
     In one specific example, successful results were achieved using six sleeve holes  46  and six purge air feed holes  48 . The sleeve hole diameter was 0.297 inch while the purge air feed hole diameter was 0.344 inch. With a purge air mass flow rate of 0.192 pps, the pressure upstream of the sleeve holes  46  was 121 psi and the reduced pressure in the chamber  44  upstream of the purge air feed holes  48  was 118.9 psi. 
     The success of the above design is predicated on two separate factors: (1) an ability to closely control the purge air pressure and flow rate via the unique mechanical arrangement; and (2) the ignition pressure pulses&#39; response to a reduction in purge air pressure and change in purge air flow rate. These functions are addressed in connection with specific application of the invention to different model turbines to determine the size and number of holes  46  and  48  in sections  36  and  38  of the crossfire tube. The success is also dependent on flow rates that are sufficient to inhibit oil intrusion. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.