SEALS FOR GAS ISOLATORS

Abstract: A seal for a gas isolator, comprising a cantilever leaf spring (1) to be attached to a fixed frame or a movable member of an isolator and a bias spring (2) acting on the leaf spring, wherein the seal is given an initial curvature corresponding to an initial deflection of 10 % to 90 % of the final free deflection, preferably 25 % to 75 % thereof, more preferably 40 % to 60 % thereof.

Description: SEALS FOR GAS ISOLATORS This invention relates to seals for gas isolators, and in particular provides a low stress bias spring seal with improved damping. GB-1308801 describes a seal for a gas isolator which comprises a cantilever leaf spring to be attached to a fixed frame or a movable member of the isolator and a bias spring acting on the leaf spring to tension the leaf spring so as to curve the latter when the seal is not in the sealing position, the bias spring being itself tensioned to press the leaf spring flat against a seating when the seal is in the sealing position. An alternative form of seal is described in GB-2060824 which provides a seal for a gas isolator which comprises a cantilever leaf spring to be attached to a fixed frame or a movable member of the isolator and a bias spring acting on the leaf spring, wherein the leaf spring is bent or curved when unstressed and wherein in the non-sealing position of the seal the bias spring makes only touch contact with the leaf spring, so that the leaf spring is unstressed in the said non-sealing position, whereby in the sealing position of the seal the stress induced in the bias spring will be the same as that induced in the leaf spring where the leaf spring and the bias spring have the same section modulus per unit length of seal. The increasing size of gas turbines is creating a need for larger isolating equipment. One of the main problems which confronts the designer of such equipment is that of the differential expansion which occurs between the fixed frame and the moving blade member. The amount of differential expansion which can be accommodated is limited by the width of the support bar under the seal. This bar has the dual function of clamping the seal to the blade or frame member and to prevent flutter of the seal when it is in the non-sealing position. Merely to shorten the support bar under the seal is not acceptable in the case of the low stress bias spring seal according to GB-2060824. because only touch contact exists between the bias spring tip and the seal itself providing a minimal damping effect. The bias spring seal depicted in GB-1308801, in which the seal is flat in the unloaded condition, provides a high contact force in the free position thus providing a high damping effect. The present invention aims to combine these features in such a way as to attain an optimum combination of properties. The seal is given an initial curvature corresponding to an initial deflection of, for example, half the final free deflection. It can be demonstrated that the maximum stress in the bias spring amounts to 1.5 times that corresponding to that induced by the free deflection of the seal. The present invention provides a seal for a gas isolator, comprising a cantilever leaf spring to be attached to a fixed frame or a movable member of the isolator and a bias spring acting on the leaf spring, wherein the seal is given an initial curvature corresponding to an initial deflection of 10% to 90% of the final free deflection, preferably 25% to 75% thereof, more preferably 40 to 60% thereof. For the seal in GB-1308801 the maximum stress would be 2 times that induced by the free deflection of the seal, whilst for the seal in GB-2060824 the stress would be equal to that induced by the free deflection of the seal. By varying the initial curvature the maximum stress and the damping effect can be varied at will. In a preferred embodiment the edge of the member to which the seal is fixed can be brought forward. In the free position, should the seal start to flutter, this extension causes a shortening in length in the upward part of the vibration cycle, thus providing additional damping effect. The increased damping effect obtained by the seal according to the present invention may be considered as follows, in the general case. In the free position of the seal, where the bias spring acts on the leaf spring, the deflection of the leaf spring from its flat position may be denoted x, said deflection being measured in the direction perpendicular to the plane of the leaf spring in its flat position, and at the point where the tip of the bias spring contacts the leaf spring. In the unloaded position of leaf spring alone, said deflection of the leaf spring is equal to x-fx, or x(l-f). where f is in effect a damping factor. In the unloaded position of the bias spring alone, the perpendicular distance from the tip of the bias spring to the plane of the leaf spring in its flat position is equal to x+fx, or x(l+f). If the damping factor f is set equal to 1. then the seal according to GB-1308801 is obtained. If f is set equal to 0, then the seal according to GB-2060824 is obtained. According to the present invention, f is set equal to 0.1 to 0.9. and preferably equal to 0.25 to 0.75, more preferably from 0.4 to 0.6, and still more preferably about 0.5. The invention will be further described, by way of example, only with reference to the accompanying drawings, wherein: Figure la, lb and lc illustrate the calculation of the above-mentioned damping factor, wherein figure la illustrates the free position of the seal, figure lb the unloaded position of the leaf spring alone, and figure lc the unloaded position of the bias spring alone; and Figure 2 shows a seal according to the invention, wherein the edge of the member to which the seal is fixed is brought forward. In the free position of the seal, as shown in figure la, where a bias spring 2 acts on a leaf spring 1, the deflection of the leaf spring from its flat position is denoted x, in the direction perpendicular to a theoretical flat plane 3 of the leaf spring, and at the point 4 where the tip of the bias spring contacts the leaf spring. In the unloaded position of the leaf spring alone, as shown in figure lb, the deflection of the leaf spring is equal to x-fx, or x(l-f), where f is in effect a damping factor. In the unloaded position of the bias spring alone, as shown in figure lc, the perpendicular distance from the tip of the bias spring to the theoretical plane 3 of the leaf spring is equal to x+fx. or x(l+f). As mentioned above, the damping factor f is set equal to 1, then the seal according to GB-1308801 is obtained. If f is set equal to 0, then the seal according to GB-2060824 is obtained. According to the present invention, f is set equal to 0.1 to 0.9, preferably 0.25 to 0.75. more preferably 0.4 to 0.6. As shown in figure 2, a seal according to the invention comprising a leaf spring 1 and a bias spring 2 acting thereon is fixed to a member 5. Numerals 6 and 7 indicate support bars which have the dual function of clamping the seal to the member 5 and of preventing fluttering of the seal in its non-sealing position. The member 5 may be a movable member (blade) of the isolator or a fixed frame of the isolator. According to a preferred embodiment of the present invention as shown in figure 2, the edge of the member 5 is extended or brought forward by a distance d from the position which it would conventionally occupy. As a result, in the free position, should the seal start to flutter, this extension causes a shortening in length in the upward part of the vibration cycle, thus providing an additional damping effect.

Claims: CLAIMS : 1. A seal for a gas isolator, comprising a cantilever leaf spring (1) to be attached to a fixed frame or a movable member of an isolator and a bias spring (2) acting on the leaf spring, characterized in that the seal is given an initial curvature corresponding to an initial deflection of 10% to 90% of the final free deflection. 2. A seal as claimed in claim 1, characterized in that the seal is given an initial curvature corresponding to an initial deflection of 25% to 75% of the final free deflection. 3. A seal as claimed in claim 2, characterized in that the seal is given an initial curvature corresponding to an initial deflection of 40% to 60% of the final free deflection. 4. A seal as claimed in any of claims 1 to 3, characterized in that the edge of the member (5) to which the seal (1, 2) is fixed is extended or brought forward by a distance (d) relative to said seal.

Inventors: SQUIRRELL ANTON FREDERICK, SQUIRRELL, ANTON, FREDERICK
Applicants: GROVAG GROSSVENTILTECH, GROVAG GROSSVENTILTECHNIK AG
Application year: 1989
Main classification: F16K1
IPC codes: F01D11, F16K1