Transition piece frame support

Abstract: A transition piece for connection between a gas turbine combustor and a stage of the gas turbine, the transition piece (10) being generally tubular and having an upstream end for connection to the gas turbine combustor and a downstream or aft end for connection to the turbine stage, the aft end defined by radially inner (64) and outer (56) sides, and wherein the aft end is formed with a peripheral rib (24,72,74) extending about the end and wherein at least one of the radially inner and outer sides has a frame support secured thereto and extending substantially completely between the opposite sides.

Description: This invention relates generally to gas turbine structural support systems with high thermal gradients combined with high mechanical loads which produce potentially unacceptably high stress levels. In particular, the invention relates to a redesign of the aft end of the transition piece of a gas turbine. The transition piece in a gas turbine is a tubular member of compound shape which typically connects a combustor of the combustion system to the first stage of the turbine. In conventional systems, the aft mount of the transition piece, by which the transition piece is connected to the turbine stage, is welded to and protrudes from the transition piece body upstream of the aft end frame. A well known problem with the gas turbine transition piece is the tendency for the aft end opening to deflect closed due to creep at high metal temperatures. This unwanted deflection is caused by higher pressure on the exterior than on the interior of the tubular transition piece. As may be recalled, the aft end of the transition piece must transition to an annular sector in order to pass hot combustion gas from the combustor to the turbine. This annular geometry is inherently weak against external pressure loading. The creep phenomenon is one of the design limits which determines the minimum number of combustors and maximum gas temperature for the gas turbine. An additional design limit is thermal stress fatigue cracking of the transition piece. In US-A-5,414,999 an integral strengthening frame is formed at the aft end of the transition piece. This thickened frame incorporates the mounting hardware for attaching the transition piece to the turbine stage. It was found, however, that simply making the aft end frame wall thicker increases thermal stresses and does not increase the operating life of the part. With reference now to Figures 1-3, a conventional transition piece 10 is illustrated including an integral aft frame 12. The integral frame include one to three or more ribs, and as shown, includes a pair of peripheral upstanding ribs 14, 16 (Figure 3) extending about the aft end opening of the transition piece. Mounting hardware 18 is located upstream of the frame, but may be integrated with the frame in accordance with US-A-5,414,999. The ribs 14, 16 serve three functions: 1) structural stiffening of the aft end which, due to the annular geometric shape, is weak at resisting the external pressure on the transition piece; 2) attachment for labyrinth seals; and 3) increased cooling surface area. As a result of the incorporation of such ribs, however, large thermal gradients exist in the ribs, causing large thermal stresses. Moreover, any increase in bending strength of the ribs (i.e., the rib section modulus), to better resist the pressure loading, causes an increase in thermal stress. Accordingly, the maximum allowable thermal stress limits the rib section modulus which, in turn, limits the circumferential span of the transition piece (i.e., the number of combustors for a given metal temperature). Current designs use the deepest rib that will not crack due to thermal fatigue while the rib width is limited by heat transfer and sealing concerns. According to the present invention, there is provided a transition piece for a gas turbine as specified in claim 1 of the accompanying claims. The invention herein, in general terms, involves attaching a structural frame to the aft end of the gas turbine transition piece. This has the advantage of being able to support the pressure load which otherwise causes the transition piece aft opening to deflect closed due to creep deformation, while not producing the undesirable high thermal stresses caused by rib stiffeners or increased wall thickness. More specifically, in a first exemplary embodiment, the invention provides an external frame for surrounding the aft end of the transition piece, with attachments to the transition piece integral frame at the radially inner and outer mid-spans, thereby resisting the pressure tending to force the aft opening closed. This external frame is isolated from the hot combustion gas and thus operates at a much lower temperature than the transition piece itself. In a second exemplary embodiment, a pair of support bars are attached along the radially outer and radially inner sides of an integral aft end frame, respectively. In each case, the support bar is secured to the aft end frame at a mid-span location by a clamp, while at remote ends, the bar is merely supported in saddles in a prestressed condition such that an outward force (away from the transition piece interior) is applied to the respective radially inner and outer walls to counteract the inwardly directed gas pressure during operation. In addition, by simply supporting (as opposed to clamping) the bars in saddles at their respective remote ends, the transition piece is free to expand thermally during operation. In a third exemplary embodiment, a support bar is employed across the radially inner side of the aft end in the manner described immediately above, but the radially outer side of the aft end is provided with axially extending pins located mid-span and at the remote ends. These pins are designed to be. received in a center hole and two end slots, respectively, formed in a nozzle retaining ring of the turbine stage. More specifically, the center pin of the transition piece is received within a complementary hole in the retaining ring while the outer pins are received within elongated slots in the retaining ring, again allowing the transition piece to expand thermally during use. In a fourth exemplary embodiment, the radially inner side of the transition piece aft end is reinforced by a support bar (rectangular cross section stock) clamped mid-span to the transition piece frame, and grooved at its opposite ends to receive saddles projecting from the transition piece. Thus, in accordance with its broader aspects, the invention here relates to a generally tubular transition piece for connection between a gas turbine combustor and a is stage of the gas turbine, the transition piece having an upstream end for connection to the gas turbine combustor and a downstream or aft end for connection to the turbine stage, the aft end defined by upper and lower sides, and wherein the aft end is formed with a peripheral rib extending about the end; and characterised by at least one of said upper and lower sides having a frame secured to the peripheral rib. Additional objects and advantages will become apparent from the following detailed description with reference to the accompanying drawings, in which: FIGURE 1 is a perspective view of a conventional gas turbine tran5ition piece incorporating an aft end frame and mounting hardware located 30 upstream of the aft end frame; FIGURE 2 is a front elevation of the aft end frame portion of the transition piece illustrated in Figure 1; FIGURE 3 is a cross section taken along the line 3-3 of Figure 2; FIGURE 4 is a front elevation of the aft end frame of a transition piece in accordance with this invention; FIGURE 5 is a section taken along the line 5-5 of Figure 4; FIGURE 6 is a partial section taken along the line 6-6 of Figure 4; FIGURE 7 is a partial section taken along the line 7-7 of Figure 4; FIGURE 8 is a partial perspective view of a gas turbine transition piece in accordance with a second exemplary embodiment of the invention; FIGURE 9 is a front elevation of the aft end frame of the transition piece illustrated in Figure 8; FIGURE 10 is a partial perspective of the aft end of the transition piece in accordance with a third exemplary embodiment of the invention; FIGURE 11 is a side elevation of a gas turbine transition piece and associated turbine stage in accordance with the embodiment of Figure 10; FIGURE 12 is a partial section taken along the line 12-12 of Figure 11; FIGURE 13 is a partial front elevation of a gas turbine transition piece in accordance with a fourth exemplary embodiment of the invention; and FIGURE 14 is a perspective view of the aft end frame of the transition piece illustrated in FIGURE 13. Turning to Figures 4 through 7, a new transition piece aft end design is shown in accordance with a first exemplary embodiment of the invention. The generally tubular transition piece 20 is formed with an aft end frame 22 which includes an upstanding peripheral rib 24, adjacent the downstream edge 26 of the aft end frame. The aft end frame 20 and the upstanding rib 24 extend completely around the aft end opening 28. An external frame 30 also surrounds the aft end frame opening 28, and is secured to the upstanding rib 24 of the transition piece as described below. For convenience, the lower side of the aft end of the transition piece as viewed in the Figures is regarded as the radially inner side while the upper side is regarded as the radially outer side, relative to a horizontal, longitudinal axis of the turbine rotor about which the combustors and associated transition pieces are arranged. The rib 24 is formed with a mounting flange 32 extending in upstream and downstream directions from the rib 24, but only at a mid-span location of the radially outer side 24a of the rib 24. Here, the frame 30 is fixed to the rib 24 and flange 32 via a clamp 34 and a pair of associated bolts (not shown) extending through pairs of aligned bolt holes 36, 38 (one pair shown in Figure 5). Flange 32 is received within mating grooves 40, 42 provided in the frame 30 and clamp 34, respectively. At the same time, the radially inner side 24b of the rib 24 is formed with a forwardly projecting hook 44 which is received within a mating groove 46 formed in the frame 30 in the mid-span region of the inner side 24b of the rib 24. The remaining peripheral area of the external frame 30 has a cross section as shown in Figure 7 and thus permits room for thermal expansion. Conventional face style labyrinth seals 48 may be used between the transition piece and the turbine first stage nozzle, but other seal arrangements are contemplated as well. In any event, some flow of air similar to the amount that currently leaks through the seals is required in the gap between the transition piece rib 24 and the external frame 30. The above described embodiment increases the bending strength of the transition piece aft end without necessarily also increasing the thermal stresses associated with a rib stiffener or increased wall thickness. The clamping arrangement only at the mid-span of the radially outer side 24a constrains all degrees of freedom between the transition piece 20 and the external frame 30. The radially inner connection along side 24b provides constraint only between radial degrees of freedom of the transition piece 20 and external frame 30. At the same time, the frame 30 is nevertheless isolated from the hot combustion gases. As a result, the frame 30 operates at much lower temperature than the transition piece 20, and thus is not subject to creep deformation. Moreover, by being attached to the transition piece 20 with minimal constraints, the hot transition piece 20 can thermally expand inside the frame 30 without creating high thermal stresses. Turning now to Figure 8, another exemplary embodiment is illustrated. In this case, the transition piece 50 is fitted with saddle supports 52 and 54 at opposite ends of the radially outer side or edge 56 of the integral aft end frame 58, and similar supports 60 and 62 at opposite ends of the radially inner side or edge 64. Each saddle support is formed with a rod receiving groove 66 extending transverse to the longitudinal axis of the combustor. In addition, clamps 68 and 70 are welded to the sides 56, 64, respectively, each clamp having upper and lower elements 68a, b and 70b, a, respectively, which include "half" grooves permitting support rods 72, 74 to be clamped therebetween as described further below. The support bar or rod 72 is prestressed and clamped between elements 68a and b such that an outward force is exerted on the mid-section of the transition piece, as indicated by arrow A in Figure 9. This outward force ' counteracts the outside gas pressure during operation. Similarly, a prestressed support bar 74 is clamped between elements 70a, b to provide a similar effect on the radially inner side of the transition piece, causing a force to be exerted on the mid-section of the radially inner side, indicated by arrow B. By allowing the rods 72, 74 to slide in the saddles 52, 54 and 60, 62, respectively, the transition piece 50 is free to expand thermally during operation. Figures 10-12 illustrate yet another embodiment of the invention which is similar in some respects to the embodiment shown in Figures 7-9. In fact, the radially inner side or edge 64' of the transition piece 50' is provided with a support rod 74' and associated saddles 60', 62' and clamp 70' which are essentially identical to the arrangement shown in Figures 7-9. The radially outer side 56' of the transition piece 50', however, is formed with projecting bosses 76, 78 and 80, each having an axially projecting pin 82, 84 and 86, respectively. These pins are adapted to seat in openings formed in a nozzle retaining ring 88 fixed to the first turbine stage. As best appreciated from Figures 11 and 12, the retaining ring 88 is formed with a round hole 90 for receiving the pin 84, and slots 92 and 94, adapted to receive pins 82 and 86. Slots 92 and 94, like the saddles 60', 62', allow the transition piece 50' to expand thermally during operation. Figures 13 and 14 illustrate a final embodiment of the invention, wherein an external support rod is applied only to the radially inner side of the transition piece aft end. Specifically, the transition piece 96 has an aft end frame 98 to which is welded a pair of end projections 100 and 102 and a center boss or mounting flange 104. An arcuate support rod 106 (of preferably rectangular cross section) is formed with grooves 108 at opposite ends thereof (only one shown), adapted to receive projections 100 and 102. At the same time, mounting flange 104 is received in a center recess 110 in the support rod 106, allowing the rod to be securely bolted in place, in radially spaced relationship to the radially inner side or edge 98b of the frame. Here again, the opposite ends of the rod are free to slide relative to the projections 100 and 102, allowing for thermal expansion of the transition piece 96.

Claims: A generally tubular transition piece (20) for connection between a combustor and a stage of a gas turbine, said transition piece having an upstream end adapted for connection to the combustor and a downstream end (22) adapted for connection to the stage of the gas turbine, said transition piece downstream end having an opening (28) defined, with respect to the centerline of the gas turbine rotor axis, by radially inner and outer walls connected by opposing side walls; and, characterized by support means (30) rigidly clamped and/or bolted to said downstream end at least at one of said radially inner and outer walls. The transition piece (20) recited in claim 1 wherein the downstream end is formed with a peripheral rib (24); and, said support means is a frame (30) clamped and/or bolted to said peripheral rib. The transition piece (20) recited in claim 2 wherein the frame (30) extends completely around the peripheral rib (24) of the downstream end. The transition piece (20) recited in claim 3 wherein the frame (30) is rigidly fixed to said peripheral rib only at a mid-span location of said radially outer wall, thereby permitting the transition piece to expand thermally with respect to said frame. The transition piece (20) recited in claim 1 wherein the support means (30) includes a first support rod (74) attached to the downstream end and extending along said radially inner wall and a second support rod (72) attached thereto and extending along said radially outer wall. The transition piece (20) recited in claim 5 wherein each of said first and second support rods (74, 72) is rigidly clamped to the respective radially inner and outer walls. The transition piece (20) recited in claim 5 wherein the first and second support rods (74, 72) are prestressed to exert outwardly directed forces on said transition piece. The transition piece (20) recited in claim 5 wherein each support rod (74,72) is slidably supported at opposite ends thereof in saddles (60,62,52,54) affixed to the respective radially inner and outer walls thereby permitting the transition piece to expand thermally. The transition piece (20) according to claim 1 wherein said support means (30) includes a prestressed support rod (106) affixed to substantially a mid-point (70) of said radially inner wall and further wherein said support rod is slidably supported at remote ends (100,102) thereof to permit said transition piece to expand thermally. The transition piece (20) according to claim 9 wherein said radially outer wall is adapted to be supported by a retaining ring (88) on the stage of the gas turbine. The transition piece (20) according to claim 10 wherein said support means comprises in combination the radially outer wall including a first axially oriented pin (84) adapted to be engaged by a complementary hole (90) in said retaining ring (88) at a mid-span location, and said radially outer wall also including second and third axially oriented pins (82) at remote ends thereof which are adapted to be engaged in elongated slots (94,92) in said retaining ring.

Inventors: ANDERSON RODGER ORVAL, BARNES JOHN EUGENE, STEBER CHARLES EVAN, ANDERSON, RODGER ORVAL, BARNES, JOHN EUGENE, STEBER, CHARLES EVAN
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1995
Main classification: F01D9
IPC codes: F01D9, F02C7, F23R3