GB2099515A - Shroud clearance control on a gas turbine engine - Google Patents

Abstract

A turbine stage of a gas turbine engine 10, is provided with a shroud ring 36 which is formed from segments 36a. The arrangement enables the shroud ring to be moved radially of the turbine axis by means of rams 50 and linkage 58 and thus avoid rubbing of the tips of the turbine blades 38 on the shroud ring. Stationary wedges 40 separate each segment. <IMAGE>

Description

SPECIFICATION Improvements in or relating to gas turbine engine casings This invention concerns a casing suitable for surrounding a shroudless turbine rotor in a gas turbine engine.

Shroud less turbine rotors when operating, expand more rapidly than the casing which surrounds them. This is because they are affected by both thermal and centrifugal effects, whereas the casing, being static, is only affected by thermal effects. Consequently blade tip rubbing occurs on the casing.

Numerous means have been devised, in efforts to maintain a desired gap between the turbine blades tips and the casing, during operation of an engine which includes shroudless turbine rotors.

Such means include deliberately affecting the temperature of structure surrounding the blade tips, so as to bring about appropriate changes in its dimensions, moving the structure as appropriate by pneumatic forces and effecting the movements mechanically e.g. moving segments of the casing radially by rotating a screw device which is connected to the or each segment.

The present invention seeks to provide an improved clearance maintaining mechanism.

According to the present invention there is provided a turbine casing including a shroud ring formed by segments which are adapted for movement radially of the turbine casing axis, structure surrounding said turbine casing and supporting rams, means engaging said rams and said segments so as to transmit movement of said rams to said segments, said means being arranged so as to transmit said movement at progressively changing rates to said segments, relative to the rate of movement of said rams.

The means may comprise links which engage respective rams and segments and are positioned with respect thereto for limited pivotal movement about their connection with said segments, through an arc which includes top dead centre.

Alternatively, the means may comprise cams which are effectively integral with the rams and positioned so as to engage the segments via a surface shaped such that on operation of the rams, the cams move the segments radially of the turbine casing, at a rate which changes progressively relative to the rate of movement of said rams.

Preferably resilient means are provided, by means of which the segments are urged radially outwards of the turbine casing axis.

Proximity sensing and signal generating means are provided in at least one segment, which in operation senses the proximity thereto of turbine blade tips and generates a signal on approach thereof, and actuating means connected between the proximity sensing and signal generating means and the rams for operation of the signal so as to actuate the rams and bring about movement of the moving means.

Preferably, but not restrictively, the proximity sensing and signal generating means comprises a capacitance device and the actuating means comprises a solenoid powered, ram fluid control valve, wherein the solenoid is triggered by a signal generated by said capacitance device.

The invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a diagrammatic view of a gas turbine engine, Figure 2 is an enlarged cross sectional part view of Figure 1, Figure 3 is a view in the direction of arrow 3 in Figure 2, and Figure 4 is a cross sectional part view of an alternative embodiment of the invention, and Figure 5 is a cross sectional part view of a further embodiment of the invention.

In Figure 1 a gas turbine engine 10 has a compressor 12, a combustion stage 14 and a turbine stage 1 6 in-flow series. Turbine stage 1 6 is encased in a turbine casing 18. A further casing 20 surrounds a portion of turbine casing 18 and supports a number of rams 22 of which only two are shown.

Referring now to Figure 2. Turbine casing 18 is made up from rings 24 which support static guide vanes 26, which rings are bridged by an annular, further casing 28. Further casing 28 has a pair of annular wall structures 30 joined by an inverted "U" channel 32 which includes local bosses 34.

Further casing structure 28 surrounds a shroud ring 36 which in turn surrounds a stage of turbine blades 38.

Referring now to Figure 3. Shroud ring 36 is constructed from a number of segments 36a separated by fixed wedges 40. The fit of the segments 36a between respective wedges 40, is such as to allow segments 36a to slide relative to wedges 40, in directions radially of the engine.

Each segment 36a has a cylindrical stem 42 integral therewith and now referring back to Figure 2, stems 42 slide in bores in respective bosses 34.

The outer end of each stem 42 is screw threaded to permit assembly and retention in bossses 34. Retention is by means of a nut 44 which traps a washer 46 and, in the present example, a stack of belville washers 48, between itself and a respective boss 34. Coil springs (not shown) could be substituted for belville washer 48.

Further casing 28 supports rams 22 of which only the rod 50 of one is shown in Figure 2.

The rod of each ram is connected to stems 42 via respective links 58. The arrangement of links 58 and stems 42 is such that when engine 10 is cold, shroud ring segments 36a are held in correct spaced relationship with the tips of turbine blades 36. Moreover belville washers 48 are compressed to such an extent by virtue of links 58 being trapped between them and rams 22 that they are always loaded, no matter what the magnitude of movement away from the cold position, shroud ring segments 36a may make.

A proximity sensing and signal generating device 62 in the present example, a capacitance device, is located in a shroud ring segment 36a.

When the stage of turbine blades 38 heats up during operation and of course at the same time rotates, they extend from their cold form and so their tips approach device 62. The resulting change in capacitance generates a signal which passes to ram actuating means (not shown). As an example, the actuating means (not shown) could be a solenoid connected to operate a valve (not shown) to control fluid pressure to rams 22 (Figure 1). Fluid pressure moves the rod 50 to the left as viewed in Figure 2 and allows belville washers 48 to expand which moves stems 42 along with their respective segments 36a in a direction radially outwardly, thus restoring the clearance between segments 36a and the tips of blades 38.

Contraction of the iengths of blades 38 increases the capacitance and the signal changes to enable the solenoid action to be cancelled, with the result that the rod 50 returns under spring action, to its original position. Alternatively the solenoid and fluid valve could be of the double acting kind.

The arrangement of rams 22, their respective links 58 and segments 36a, is such that when the lengths of links 36a are radially aligned with respect to the turbine axis i.e. in the top dead centre position, segments 36a are in their cold position. This means that they can only move outwards as blades 38 grow outwards from their cold position, during operation of engine 10.

Further, on moving from the top dead centre position, the links, their ends being restricted to movement along straight paths which are normal to each other, have an effect which causes segments 36a to move radially outwardly at an increasing rate relative to the movement of ram rod 50. Conversely, segments 36a move radially inwards at a reducing rate, relative to the rate of movement of ram rod 50 in a reciprocatory direction.

From the foregoing it will be seen, that if the expansion and contraction rates of blades 38 under operating conditions, can be ascertained, rams 22 can be loaded at a rate which will cause them to move at some constant speed, which will in turn move segments 36a at a speed which will at least approximate that of the expansion and contraction of the turbine blades 38.

The invention as described hereinbefore, requires that all segments 36a be moved simultaneously. Where, however, it is desired to cater for that situation in which the turbine orbits, rather than rotates about the engine axis, each segment 36a must have its own sensing and signalling device 62. Each ram 22 must be connected to a respective sensing and signalling device 62 via a respective actuating means (not shown). Any given segment 36a may then be individually moved. A stop 64 can be provided, so as to ensure prevention of excessive movement of rod 50.

Referring now to Figure 4 in which like parts have been given like reference numerals.

Rod 50 of ram 22 has a cam 66 formed at its free extremity, the working profile of cam 66 engages a pip 70 on the upper end of stem 42.

On actuation of ram 22 in the manner described hereinbefore, rod 50 and therefore cam 66 firstly moves to the left as viewed in Figure 4.

The working profile of cam 66 is such that the moment allows belville washers 48 to force stems 42 upwards, thus moving segments 36a radially outwards.

Movement of rod 50 to the right as viewed in Figure 4 results in cam 66 pushing stem 42 and associated segment 36a radially inwards. Again stops 72 may be provided, to prevent excessive movement of rod 50.

The working face of cam 66 is designed so as to move segments 36a in a manner identical with that described in connection with Figure 2 herein.

Referring now to Figure 5. Ram rods 50 are connected via a gimbal joint 54 to an annular flange 55 which in turn is connected via links 58 or cams 66, to segments 36a. Each gimbal joint 54 is so arranged as to allow relative radial movement between rods 50 and flange 55.

In operation, a ram 22 (not shown in Figure 5) on receiving a signal will tilt flange 55 and so move a respective first segment in accordance with the signal. Segments spaced symmetrically about the first segment will move progressively less distances, the further away they are, from the first segment. Orbiting of the turbine stage is thus catered for.

Signals indicating general expansion of the turbine stage will of^course, cause equal actuation of all rams and therefore, equal movement of all segments. This further embodiment permits employment of fewer rows per given number of segments.

Claims (9)

1. A turbine casing including a shroud ring formed by segments which are adapted for movement radially of the turbine casing axis, structure surrounding said turbine casing and supporting rams, means engaging said rams and said segments so as to transmit movement of said rams to said segments, said means being arranged so as to transmit said movement at progressively changing rates to said segments, relative to the rate of movement of said rams.

2. A turbine casing as claimed in claim 1 wherein said means comprises links which engage respective rams and segments and are positioned with respect thereto for limited pivoting movement about their connection with said segments, through an arc which includes top dead centre.

3. A turbine casing as claimed in claim 1 wherein alternatively said means may comprise cams effectively integral with said rams and positioned so as to engage said segments via a surface shaped such that on operation of said rams said cams move said segments radially of the turbine casing axis, at a rate which changes progressively relative to the rate of movement of said rams.

4. A turbine casing as claimed in any previous claim including resilient, corrugated washers by which said segments are urged radially outwards.

5. A turbine casing as claimed in any previous claim including proximity sensing and signal generating means in at least one segment which in operation senses the proximity thereto of turbine blade tips and generates a signal on approach thereof and, actuating means connected between said proximity sensing and signal generating means and said rams for operation by a said signal so as to actuate said rams and bring about movement of said moving means.

6. A turbine casing as claimed in claim 7 wherein said proximity sensing and signal generating means comprises a capacitance device and said actuating means comprises a solenoid powered ram fluid control valve, wherein said solenoid is triggered by a signal generated by said capacitance device.

7. A turbine casing as claimed in any previous claim and wherein the connection between rams and movement transmitting means is made via a flange which is gimballed to said rams.

8. A turbine casing substantially as described in this specification with reference to Figures 1 to 3 of the drawings.

9. A turbine casing substantially as described in this specification with reference to Figure 4 of the drawings.

1 0. A turbine casing substantially as described in this specification with reference to Figure 5 of the drawings.