k ^ n 21 418 15 7 - 1 DEVICE FOR KEEPING THE A M LAR OUTLET MOUTH OF THE
GAS VOLUTE ALVAYS CENTERED ABOUT THE NOZZLE ASSEMBLY IN A GAS-TURBINE This invention relates to a device which enables the annular outlet mouth of the gas volute to be kept always centered about the nozzle assembly even in gas turbines of high compression ratio and high fire temperature in which high loads act on the volute. In all gas turbines provided with a single combustion chamber there is a volute the function of which is to distribute the hot gases from the combustion chamber annularly to the array of nozzles. As the volute, together with the combustion chamber walls, represents the hottest part of the turbine it must necessarily be of the lightest possible sheet metal construction to limit thermal stresses to a minimum, with the result that it is unable to withstand high localized loads. Again, the connection between the annular outlet mouth of the volute and the nozzle support cannot be rigid because as their masses are different and they operate under different working conditions, these two members assume very different temperatures with consequent considerable differential expansion. There is therefore the problem of keeping the annular outlet mouth 3 of the volute centered about the array of nozzles, and ensuring that it is so, because any misalignment results in loss of turbine efficiency due to the sharp steps created in the gas passage duct. The volute is subjected to mechanical loads deriving from its weight (generally negligible) and in particular from the gas pressure and overall motion differences between the inlet and outlet. These mechanical loads, which become greater the higher the turbine working pressures and thus the higher its compression ratio, create an upward force which moves the annular outlet mouth of the volute upwards. In the current state of the art the classical system usually used to guide said movement and to keep any two parts subjected to considerable differential expansion concentric with each other is to connect said parts together by a set of radial keys which make it possible for the two parts to undergo relative sliding while remaining concentric. Such a method cannot however be applied to gas turbines of high compression ratio and high fire temperature because the consequenit considerable loads acting on the volute would have to be transmitted through said radial keys and specifically through only those- keys in a horizontal plane, and this load concentration at the keys would induce intolerable stress states in the volute which as stated has to be of very light construction because of the high gas temperature.
The object of the present invention is to obviate said drawback by providing a device for connecting the annular outlet mouth of the volute to the nozzle support which although dispensing with the 1 7 use of keys enables effective centering to be maintained, even in gas turbines of high compression ratio. This is attained substantially in that said connection is made by two retaining half-rings which are fixed to the nozzle support to embrace the annular outlet mouth of the volute, the upper halfring forming with the outer flange of said mouth a predetermined gap which compensates the differential expansion between the volute and nozzle support, to thus keep said volute centere d about the nozzles during normal working.
In this respect, during normal working the annular outlet mouth of the volute rests against the upper half-ring by the effect of said upward force, and therefore if the temperatures attained by the volute and the nozzle support ring are known, an exact calculation can be made of the gap to be left between the outer flange of the volute outlet mouth and the upper half-ring so that said mouth is centered about the nozzles during normal working. According to a further characteristic of the present invention said lower retaining half-ring has an inner radius greater than that of the upper half-ring. In this manner, if differential expansion occurs between the annular outlet mouth of the volute and the retaining half-ring which is greater than that predicted, the risk of serious coaction is avoided because the annular mouth can further expand into the greater empty space available. Thus, the device for keeping the annular outlet mouth of the gas volute always centered about the nozzle assembly in a gas turbine is characterised according to the present invention by comprising two retaining half-rings which are fixed to the nozzle support ring to embrace said annular mouth,, the lower half-rinS having an inner radius greater than that of the upper half-ring, which forms with the outer flange of said annular outlet mouth of the volute a predetermined gap arranged to compensate the differential expansion between said annular mouth and said nozzle support ring. The invention is described in detail hereinafter with reference to the accompanying drawings which illustrate a preferred embodiment thereof given by way of non-limiting example in that technical or constructional modifications can be made thereto but without leaving the scope of the present invention. In said drawings: Figure I is a partial lateral;=,ection through a gas turbine using the device of the invention; Figure 2 is a partial front section on the line A-A of Figure 1.
In the figures, the reference numeral 1 indicates a gas turbine casing which on bearings 2 rotatably supports the shaft 3 carrying the discs 4 of the turbine blades 5. The combustion chamber 6 is connected to a gas volute 7 the annular outlet mouth 8 of which faces the zone comprising the nozzles 9, which are supported radially by a nozzle support ring 10 supported by the turbine casing 1. In order to keep the annular outlet mouth 8 always centered about the nozzle assembly 9, two retaining half-rings 12 and 13 are fixed on the nozzle support ring 10 by bolts 11 to embrace said annular mouth 8 in such a manner as to leave a predetermined gap 14 between the upper half-ring 12 and the outer flange 7' of said annular outlet mouth 8. In addition, the lower half-ring 13 has a il 191 4 1 j larger inner radius than the upper half-ring 12 (see Figure specifically), so that a greater enpty space 15 is available.
2 CLAIMS
1. A device for keeping an annular outlet mouth of a gas volute centered about a nozzle assembly in a gas turbine, comprising two retaining halfrings which are adapted to be fixed to a nozzle support ring to embrace said annular mouth, the lower half-ring having an inner radius greater than that of the upper halfring, which forms with an outer flange of said annular outlet mouth of the volute a predetermined gap arranged to compensate for the differential expansion between said annular mouth and said nozzle support ring.
2. A device according to claim 1, substantially as hereinbefore described with reference to, and as shown in, the drawings.
f b,ed 1989 atThe Patent Office, State House, 66 71 High HoIbc.-n. London WClR 4TP. Further copies maybe obtained from The Patent Office. Sales Branch, St Mary Cray. Orpington. Kent BR5 3RD. Printed by Multiplex techniques ltd. St Mary Cray. Kent. Con. 1187 11