GB2050546A - Duct joint - Google Patents

Abstract

A flexible/sliding duct joint for coupling a gas turbine flame tube to the turbine inlet duct. The joint can accommodate thermal differential movement of the two ducts in all directions, axial displacement, angular misalignment, axis displacement, and all while maintaining a controlled radial force between the two ducts. The ducts overlap one another and in the annular gap so formed house one or several split rings 18 which have alternate inner and outer 'lands' or prominences, which engage the duct surfaces and act as leaf springs between the surfaces. <IMAGE>

Description

SPECIFICATION Duct joints This invention relates to duct joints for ducts which are subject to thermal or other stresses such as may cause differential movement of the adjoining ends of two ducts. The invention is particularly, but not exclusively, concerned with combustors for gas turbine engines and the joint between the downstream end of a flame tube and a turbine inlet duct which may be a transition duct or a turbine nozzle ring.

It is known for a gas turbine flame tube within a combustion chamber casing to be attached at the upstream or burner end to a relatively cool casing and at the other, downstream end to be attached to a very hot turbine nozzle or entry ring. There is sometimes a transition duct between flame tube and turbine nozzle ring, and in the 'can-annular' type of combustion system in which a plurality of flame tubes are disposed in a ring around the axis of the turbine, the transition duct geometry changes from tubular (round) at its flame tube end (upstream end) to a segment of an annulus at its turbine nozzle end (downstream end).

Movement of these parts relative to their cooler attachment points and the turbine casing, due to thermal distortion, may be summarized as follows.

The flame tube is subject to axial growth, i.e.

lengthening, at its downstream end; radial expansion (relative to its own centre) at the same end; angular movement in an outer radial direction of its downstream end due to heat from adjacent flame tubes in the annular array, the movement being relative to the turbine axis. Movement of a flame tube in this latter manner is shown by the solid and chain dotted lines in the diagrammatic view shown in Figure 1. The transition duct is subject to radial and axial expansion relative to the cooler casing. The movement may be complicated by the transition duct geometry, and may not be uniform: thus, one sector of its upstream end may move to a greater or lesser degree than the remainder.

One object of the present invention is therefore to provide a duct joint which will accommodate differential duct movements such as outlined above.

According to one aspect of the present invention, a duct joint comprises an inner and an outer duct portion, one surrounding the other concentrically, the two portions being separated by a metallic joint member which has a plurality of lands bearing upon the outer surface of the inner duct portion and a plurality of lands bearing upon the inner surface of the outer duct portion, the inner and outer lands being out of register so as to permit resilient displacement of one duct portion with respect to the other. The lands on the joint member are preferably regularly disposed, the inner lands alternating with the outer lands.

The radial thickness of the joint member at any point may be comparable with the radial dimension of the gap between the inner and outer duct portions, the joint member having a gap between its ends to accommodate changes of length. Preferably, the spacing of successive lands is large compared to the radial dimension of the gap between the inner and outer duct portions.

Alternatively, the two portions may be separated by a plurality of the joint members bearing against each other in an axial direction, the joint members being arranged out of register to each other so that the total lands are distributed uniformly around the inner and outer duct portions. The inner duct portion or the outer portion may be formed with end stops limiting axial movement of the joint members thereon. The end stops may be provided by an annular recess in the surface of the respective duct portion.

The inner and outer duct portions may surround a further duct portion which is a clearance fit within the inner duct portion and is fixed to the outer duct portion.

According to another aspect of the invention, a gas-turbine combustion chamber may include a flame tube coupled to a turbine inlet duct by a duct joint as aforesaid. The inner duct portion may then be fixedly connected to the turbine inlet duct, the joint member being out of the stream of the gas flow which is diverted by the further duct portion. The outer duct portion may then have holes for the passage of cooling air from outside the flame-tube past the duct joint to cool the interior of the inner duct portion.

One embodiment of a duct joint in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 is a diagrammatic perspective view of a flame tube for a gas turbine combusion chamber; Figure 2 is a cross sectional view on the axis of a flame tube incorporating a duct joint according to the invention; and Figure 3 is a cross sectional view on the line Ill-Ill of Figure 2.

The gas-turbine combustion chamber incorporating the duct joint of the present embodiment comprises a cylindrical case (not shown) containing an annular array of flame tubes such as shown in Figure 1. These flame tubes 11 are mounted at one end of the casing so as to pivot on a bracket 13 in a plane containing the axes of the combustion chamber and of the flame tube. A fuel nozzle fixed at the bracket 13 injects fuel into the flame tube in known manner, the combustion gases passing down the flame tube to a transition duct 12 which forms an inlet duct to the turbine (not shown). At the turbine end of the transition duct the inlet to the turbine is of annular form so that each transition duct has to couple to a segment of an annul us at the downstream end and to the circular flame tube at the upstream end.

The beginnings of its change of cross section are shown to the right of Figure 2, it getting narrower in the radial direction (of the combustion chamber) and wider in the circumferential direction transverse to the plane of Figure 2.

The downstream end of the flame tube 11 and the upstream end of the transition duct 12 are then coupled together by a kinematic joint 1 7 in accordance with the invention and illustrated in Figures 2 and 3.

Referring to these Figures, the duct joint comprises an outer duct portion 19 which is a bell-mouthed extension of the flame tube 1 1, an inner duct portion 14 which is an extension of the transition duct 12, and two joint members 18 housed in the annular space between the outer and inner duct portions. The joint members 18 are shown in end view in Figure 3 and can be seen to be in the general form of piston rings, being split at one point 16 around the circumference. Each ring 18 is of metal, suitably stainless steel in the present application, and is formed with three lands 25 on the inner surface, bearing upon the duct portion 14, and three lands 26 on the outer surface, bearing upon the duct portion 19. The lands are regularly spaced around the ring, the outer lands alternating with the inner lands.In the absence of the inner duct portion 14 the free position of the ring would be such that the inner lands 25 would be on a slightly smaller circle than the outer surface of the inner duct portion 14. The ring is thus stressed slightly in the position shown in Figure 3, each section of the ring between two outer lands acting as a leaf spring bearing at a mid-point on the inner duct portion, and vice versa. A controlled force is thus exerted outwardly on the flame tube and inwardly on the transition duct.

The surfaces of the duct portions on which the lands 25 and 26 bear, are preferably coated with a wear resistant material such as tungsten carbide.

Clearly, the flexibility of the joint will depend upon the ring dimensions and the closeness of the lands in relation to the radial dimension of the annular gap between the duct portions. For a useful degree of flexibility in the application in question, the land spacing (on one side of the ring) must be large compared to the radial dimension of the gap, say more than five times as much. In the example illustrated, this ratio is about 10:1.

To obtain a useful degree of stiffness in the same application with such spacing ratios, it is necessary that the radial dimension of the ring at any point is substantial, dependent on the temperature differential between the inner and outer duct portions in operation. it may in some cases be comparable with the radial dimension of the gap.

The axial dimension of the ring must, of course, be limited to permit angular misalignment of the two ducts, but further stiffness can be incorporated, as shown in Figure 2, by employing two or more rings in series axially, and which may be in contact.

The ring or rings are mounted in an annular groove in the inner duct portion 14, the groove being formed by radial flanges 20 and 21. A seal ring 22, which is a clearance fit in the radial gap, may be included in this groove to prevent the passing of hot gases through the 'relief' portions of the rings 18. In the case of a plurality of rings 1 8.

they are preferably arranged out of register with each other so that the total lands are distributed uniformly around the circumference to spread the load on the inner and outer duct portions. Pins may be inserted through the outer duct portion 19 to locate in the 'split' of each ring and prevent rotation of the rings.

In order to keep the joint components out of the path of the combustion gases the flame tube 11 is continued inside the joint, to provide a further duct portion 15 at least partially overlapping the joint, but leaving an annular clearance 23 with the inner duct portion 14. Holes 24 in outer duct portion 19 then allow cool air to be drawn in past the joint, through the clearance gap 23 and into the hot gas strearh to cool the interior of transition duct 12.

The ring 18 may be formed from a plain ring with lands built up by spray coating for example.

In a modification, the outer duct portion 19 could be made an extension of the transition duct 12 and the inner duct portion a continuation of the flame tube 11.

Claims (12)

1. A ductjoint comprising an inner and an outer duct portion, one surrounding the other concentrically, the two portions being separated by a metallic joint member which has a plurality of lands bearing upon the outer surface of the inner duct portion and a plurality of lands bearing upon the inner surface of the outer duct portion, the inner and outer lands being out of register so as to permit resilient displacement of one duct portion with respect to the other.

2. A ductjoint according to Claim 1 , wherein the lands on said joint member are regularly disposed, the inner lands alternating with the outer lands.

3. A ductjoint according to Claim 1 or Claim 2, wherein the radial thickness of the joint member at any point is comparable with the radial dimension of the gap between the inner and outer duct portions, the joint member having a gap between its ends to accommodate changes of length.

4. A duct-joint according to any preceding claim, wherein the spacing of successive lands is large compared to the radial dimension of the gap between the inner and outer duct portions.

5. A duct-joint according to any preceding claim, wherein said two portions are separated by a plurality of said joint members bearing against each other in an axial direction, the joint members being arranged out of register to each other so that the total lands are distributed uniformly around the inner and outer duct portions.

6. A duct-joint according to Claim 5, wherein said inner duct portion or said outer portion is formed with end stops limiting axial movement'of the joint members thereon.

7. A ductjoint according to Claim 6, wherein said end stops are provided by an annular recess in the surface of the respective duct portion.

8. A duct-joint according to any preceding claim wherein said inner and outer duct portions surround a further duct portion which is a clearance fit within said inner duct portion and is fixed to said outer duct portion.

9. A gas-turbine combustion chamber including a flame tube coupled to a turbine inlet duct by a duct-joint according to any preceding claim.

10. A gas-turbine combustion chamber including a flame tube coupled to a turbine inlet duct by a duct-joint according to Claim 8, wherein said inner duct portion is fixedly connected to said turbine inlet duct, the joint member being out of the stream of the gas flow which is diverted by said further duct portion.

11. A gas-turbine combustion chamber according to Claim 10, wherein said outer duct portion has holes therein for the passage of cooling air from outside the flame-tube to cool the duct-joint in passing to the interior of the flame tube.

12. A duct-joint substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.