GB2393223A

GB2393223A - Brush seal with angled bristles

Info

Publication number: GB2393223A
Application number: GB0221676A
Authority: GB
Inventors: Simon Hogg
Original assignee: Alstom Technology AG; Alstom SA
Current assignee: General Electric Technology GmbH
Priority date: 2002-09-18
Filing date: 2002-09-18
Publication date: 2004-03-24
Also published as: GB0221676D0

Abstract

A brush seal 5 for sealing relatively rotating components eg the rotor shaft 2 of a turbine has bristles 6 which are arranged in a plane perpendicular to the axis of rotation 3 and make an angle r with the radial direction 21 which is less than 45 degrees over the entire length of the bristles. The tips 10 of the bristles are spaced from the shaft to leave a clearance 9 of up to 0.3 mm. The angle of inclination is preferably 30 degrees. This reduces aerodynamic stiffness and overall radial stiffness of the seal, which in turn reduces frictional heating of the surface of the shaft. The brush seal consists often rows of bristles having a free bristle length of 15 mm and a diameter of 0.1 mm, made of a cobalt or nickel based superalloy.

Description

< Brush Seal for Turbine Technical Field

The invention pertains to brush seals providing a seal at the rotor shaft of a turbine. Background Art

Brush seals are known for use as seals between relatively rotating components as described for example in GB 2 066 382. They can be applied to fluid flow Us machines such as steam or gas turbines. They consist of an annular array of bristles attached to one of the components and extending in a radial or near radial direction across a gap between the components. The bristle can either touch the surface of the other component or extend only so far as to leave a clearance between the bristle tips and the surface. In both cases frictional contact between so the components is to be expected during rotation of the components. For this reason the bristles are fabricated of a high temperature resistant material able to withstand the heat due to friction.

The bristles are characterized by the so-called radial stiffness found when a force z5 is exerted on the bristle tip in a radially outward direction. This radial stiffness has two major components. The first component is the mechanical stiffness which is due to the material used and the manner in which the bristles are mounted. The second component is the aerodynamic stiffness that is due to a force exerted on the bristles by a leakage flow along the length of the bristles towrad the bristle tips, 30 which exists in addition to the leakage flow between the bristle tips and the surface of the component that the seal is directed to. The leakage flow along the length of the bristles causes frictional forces that contribute to the overall radial stiffness of the brush seal.

35 When applied to turbine rotor shafts the bristles of the brush seal are typically arranged at a 45 angle or greater angle to the radial direction and in a plane perpendicular to the main flow through the turbine. Such brush seals are disclosed

for example in US 6,105,966 This orientation at 45c or greater angle serves to avoid shaft pick-up and reduce heating of the bristles and shaft due to frictional contact. Problems however, can arise in the case that the turbine rotor shaft is not 5 completely regular in shape about the 360 . Bends and bumps on the rotor shaft can give rise to increased frictional forces on the bristles of the seal. These forces cause bristles to heat up and in some cases to melt together. This can lead to a loss in flexibility and effectiveness of the seal. Furthermore, as a rotor shaft can have machining high-spots or minor bends, the frictional heating from the brush to seal can be irregular around the circumference of the shaft. Such circumstances can cause an increase of the initial bending and deformation of the rotor, possibly to such an extent as to cause rotor failure. Until today, this problem of frictional heat has been circumvented by allowing a greater clearance between the bristle tips and rotor surface. This solution however, has the drawback of a greater as leakage flow through the clearance and a reduced seal effectiveness.

US 5944,320 discloses a brush seal for rotatable machine components, such as a turbine rotor shaft. The bristles of the seal arranged in a bristle pack are oriented with respect to the radial direction at a first angle ranging between 30 and 60.

20 The end portion of each bristle is oriented toward the circumferential surface of the rotor shaft and angled with respect to the radial direction at a second angle that is smaller than the first angle. A brush seal with this angled construction of the bristles is siad to have the fundamental functional feature in that it maintains the elastic Reflectability of the brush seal relative to radial as well as axial movements 2s of the machine components. The brush seal of this design furthermore retains this functional feature in particular independently of the direction of rotation of the machine component.

Summary of Invention

In view of the described background art it is an object of the invention to provide a

brush seal for sealing against rotatable machine components, such as a turbine 35 rotor shaft, that can accommodate irregularities of the rotor shape and avoid frictional heating and the damages resulting from it. Additionally, leakage flow through the brush seal shall be minimised.

A brush seal for sealing between first and second relatively rotating machine components, such as a turbine stator and rotor shaft respectively, is attached to the first component and comprises a plurality of bristles that extend toward the surface of the second machine component. A small clearance extends from the 5 tips of the bristles to the surface of the second component.

According to the invention the bristles of the brush seal are oriented over their entire length at an angle with respect to the radial direction that is smaller than 45 . to Brush seals with bristles oriented at such an angle are characterized by a reduced radial stiffness. This results in a reduced torque or frictional force exerted by the one machine component on the brush seal. By the reduction of frictional force in the case of contact of the bristle ends with a rotor surface a reduction of heating of the rotor shaft and of the risk of rotor damage is achieved.

As the brush seal according to the invention develops less frictional heat upon contact with a rotor surface the brush seal can be designed with a smaller clearance between bristle tips and rotor surface than is typical for brush seals of the state of the art. Hence the leakage flow for a brush seal according to the 20 invention can be kept small while the risk of frictional heating and possible damages is low. As a result, the lifetime of the brush seal as well as the rotor shaft is increased. Furthermore, a brush seal according to the invention having reduced risk of frictional heating allows a greater range of application in different types of machines. The choice of angle of orientation of the brush seal according of the invention is based on the newly found fact that brush seals with bristles oriented at angles smaller than 45 to the radial have a reduced aerodynamic stiffness compared to brush seals oriented at greater angles. Furthermore, it has been found that the 30 aerodynamic stiffness is the dominant component of the two components contributing to the radial stiffness of the brush seal. These facts result in that short bristles exert less frictional forces on the surface of a machine component than long bristles.

35 The angle of orientation according to the invention results in a bristle length that is slightly shorter in comparison to bristles that are oriented at greater angles. A shorter length results in a greater mechanical stiffness that contributes to the radial stiffness. However, as the aerodynamic stiffness is the dominant factor over

the mechanical bristle forces and as the bristles oriented at a small angle according to the invention have a reduced aerodynamic stiffness, the brush seal has as a result a reduced overall radial stiffness. As such, the brush seal exerts less frictional forces on the machine component. In the case of a brush seal 5 applied to rotor shaft of a turbine this invention results in a reduced shaft heating during operation.

A design of a bristle pack with the aim of reducing frictional forces on the machine component and with only the consideration of the mechanical stiffness of the 0 bristis would result in a bristle pack with long bristles that are oriented at a large angle. However, such a design would lead to a contrary result.

This invention is based on the newly found facts concerning aerodynamic stiffness as a function of angle of orientation and the unexpected fact that short bristles exert smaller rubbing forces than long bristles do.

Brief Description of the Drawings

20 Figure 1 shows a brush seal arrangement according to the invention as applied to the rotor shaft of a turbine Figure 2 shows the annular brush seal arrangement according to the view along the lines loll and arranged at an angle according to the invention Figure 3 shows a graph demonstrating the development of the frictional torque as 26 a function of the pressure drop across the brush seal and for different inclination angles of the bristles with respect to the radial direction.

30 Best Modes for Carrying out the Invention Figure 1 shows a brush seal 5 applied to a rotor shaft 2 of a turbine having a rotor axis 3. The brush seal 5 is attached to a stator component 1 of the turbine. Its bristles 6 are arranged between a backing ring 7 and a front plate 8 which are also as attached to the stator component 1. The bristles 6 extend from the stator component 1 toward the surface 11 of the rotor shaft 2 in a direction perpendicular to the rotor axis 3. They extend as far as to leave a small gap or clearance 9

between the bristle ends 10 and the rotor shaft surface 11. A small leakage flow 4 of the working fluid of the turbine passes through the gap 9.

Figure 2 shows the brush seal in its annular arrangement about the rotor shaft 2 5 rotating in the direction 20. The bristles 6 are arranged between the stator component 1 toward the rotor shaft surface 11 and aligned along a direction toward the direction of rotation 20 of the shaft and with respect to the radial direction 21 by an angle 0. This angle, according to the invention, is less than 45 , and preferably about 30 .

no A typical brush seal consists of ten rows of bristles having a length of 15 mm, ( this length being the free-bristle length), and a diameter of 0.1 mm. The wire bristle material is typically a Cobalt or Nickel based superalloy. The clearance between bristle ends and rotor can be up to 0.3 mm. The pressure drop across such a brush seal is for example 5 to 10 bars as is typical for many 15 turbomachinery applications. It can be used with working fluids being a gas or a gas/fluid mixture at an operating temperature of of up to 675 C.

The inclination angle of the brush seal of less than 45 with respect to the radial results in a reduced aerodynamic stiffness of the bristles. Their mechanical go stiffness is slightly greater compared to that of bristles inclined at greater angles due to their decreased length. However, the overall radial stiffness or torque is reduced as shown in Figure 3.

Figure 3 shows the frictional torque experienced by the bristles as a function of the pressure drop Ap across the brush seal in bars. The several curves demonstrate the torque experienced by the bristles oriented at inclination angles to 60 , 40 , and 30 with respect to the radial direction. Clearly the torque decreases with decreasing inclination angle. For a bristle oriented at an angle of 30 the torque is only about 25% of the torque for a bristle oriented at 45 and that is operated at 30 the same conditions, that is same pressure loading.

The absolute value of the torque T/l in Nm per unit length of seal exerted by the brush seal on the rotating shaft depends upon a large number of variables assopciated with the seal geometry and operating conditions. Typical values for the torques are in the range of 1 to 10 Nm per meter length of seal if the brush as seal is designed for operating conditions up to 10 bar of pressure drop.

Claims

1. A brush seal (5) for sealing between a first machine component (1) and a 5 second machine component (2) that are rotating relatively to one another, where one of the machine components (1,2) is rotating about a rotation axis (3), and the brush seal (5) is attached to the first machine component (1) and comprises a plurality of bristles (6) that extend from the first machine component (1) toward the surface (11) of the second machine component (2), and where a clearance (9) Jo extends from the tips (10) of the bristles (6) to the surface (11) of the second component (2), and the bristles (6) of the brush seal (5) are oriented at an inclination angle (a) to the radial direction (21) and in the plane perpendicular to the rotation axis (3), characterized in that the inclination angle (a) of the bristles (6) is less than 45 15 over the entire length of the bristles (6).

2. A brush seal according to claim 1 characterized in that the angle of inclination (a) is 30 .

3. A brush seal according to claim 1 or 2 characterized in that the clearance (11) is in the range of up to 0.3 mm.

4. A brush seal according to one of claims 1 to 3 characterized in that the first machine component (1) is a stator component of a turbine and the second machine component (2) is a rotor shaft rotating about a rotor axis.