GB2355049A

GB2355049A - Differential lay brush seal

Info

Publication number: GB2355049A
Application number: GB9923499A
Authority: GB
Inventors: David W Richards
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1999-10-05
Filing date: 1999-10-05
Publication date: 2001-04-11
Also published as: GB9923499D0

Abstract

A brush seal having a bristle pack composed of two or more layers of bristles which are laid at different angles of inclination and may have different wire thicknesses. In one form the bristles in the upstream layer are laid at substantially zero angle of inclination ie in a radial direction. In the next layer the bristles are inclined at a conventional angle. There may be a further downstream layer in which the bristles are substantially radial.

Description

2355049 DIFFERENTIAL LAY BRUSH SEAL The invention concerns a brush seal in

which the bristle pack is composed of several layers of bristles laid at different angles of inclination.

Conventionally, the design of brush seals as applied to gas turbine engines consists of a number of rows of inclined bristles arranged between two side plates, with usually some sort of shield in front to contact turbulence. Figure 1 shows a schematic illustration of a section through the axis of a conventional annular brush seal. There is a gap 2 between the front shield 4 and the bristle pack 6 to prevent pinching which would inhibit movement. The complex dynamic relationship between these items and the associated rotor 8 all determine the efficiency of the seal but it is known that the gap between the rotor and the seal backing ring 10 and shield 4 inside diameter plays a large part in determining seal performance. As this gap increases the seal performance deteriorates.

The make up of this gap is shown in Figure 1, which has to be arranged so that under the worst condition there will not be contact. This safety margin is only required very infrequently and so compromises the seal performance for most of the time the engine is running.

The inclined bristles 6 which fill this gap 2 blow down onto the rotor 8 during engine running with the result that the extra bristle tip load causes wear. When the pressure is removed the bristles return to their natural position and progressively the gap between the bristle tips and the rotor surface is widened, in some cases as far as the backing ring 10 diameter. The bristles will again blow down during the next cycle but leakage has increased in the interim and the increasing flow under the bristle will to some extent resist the blow down. There is a resulting general deterioration in sealing performance over time, following a fairly rapid reduction from a very good initial condition.

According to the present invention there is provided a brush seal having a bristle pack composed of two or more layers of bristles laid at different angles of inclination.

The invention will now be described in greater detail with reference to the accompanying drawings, in which:

Figure 1, as mentioned above, shows a schematic illustration of a section through the axis of a conventional annular brush seal, Figure 2 shows a similar section through one form of brush seal constructed according to the invention, and Figure 3 shows a detail illustration of the differential lay angle between two bristles layers in the seal of Figure 2.

Referring now to Figures 2 and 3, in accordance with the present invention, a seal comprises at least two layers of bristles, in the illustrated embodiment three layers 12,14,16 in which the front layer 12, and in the case of the three layers, the rear layer 13, are at or near zero degrees, with respect to a radial direction, and the middle section 14 at the conventional inclination in the range 30-60 degrees. The layers consist of the most advantageous number of rows of the most suitable wire type and diameter, not necessarily the same as each other. In this way the largest gap is set by the zero degree bristles and not the backing ring/shield. The bore of the seal inevitably is a compromise between contact by the zero degree bristles and sealing by the inclined bristles during blown down. With appropriate matching of materials, this can be reduced to just the component tolerances and the thermal growth. The other factors being accommodated by the seal wearing to size.

When fluid pressure is applied, the radial or zero degree bristle pack does not change diameter but the inclined bristles blow down as usual. The result is a seal which, when compared to an existing design of seal, does not seal so well as that seal when new (with bristles contacting the rotor) but which does not deteriorate as much or as fast as that existing seal in use, and which does not continue to deteriorate beyond a given condition.

The radial bristles do not blow down and so only wear to the diameter required by the engine, and this is less than the conventional backing ring clearance. Because of this, the sealing performance does not deteriorate beyond this condition. Although stiff, the contact with the rotor still has a deflecting component due to rotation and is comparable to sideplate contact. The diameter that results is the minimum required by the engine, eliminating the extra safety gap required by the solid rings. The gap between the shield and the bristles is now not needed, so reducing the flow of air up into the seal thereby blow down of the inclined bristle is better controlled. The wear on the inclined bristles which blow down to close the gap under pressure still reduces these in length, sometimes as far as the backing ring diameter at ambient pressure, but this no longer reduces the efficiency of the seal as at present because the zero degree bristles sets the maximum gap that can occur.

When applied to a seal allowed to "float" in its housing, the zero degree bristle centralises the seal about the rotor further reducing the required clearance and resulting wear.

In summary, therefore, the permanent gap usually formed by the backing ring is reduced. Seal performance closely follows backing ring gap which must be as small as possible, but must allow for all the possible positional errors of the rotor, the worst possible case. Some of these occur only rarely if ever, but the safety is designed in and the loss of sealing is permanent in all installations, needed or not. If the permanent gap is formed by rows of 90 degree or near 90 degree bristles, several of these allowances can be omitted and the gap reduced. Safety concerns are mitigated by the fact that should a rub occur, the bristles will still deflect, and even if they wear will only increase in size to the minimum necessary. The alternative bristles must be at or near 90 degrees to form a row that will now blow down like the angled bristles, and will not normally wear. These rows will set the minimum seal performance, and the angled rows will still blow down and form an improved seal as normal.

The seal may be produced from the same size wire for all the bristles because "blow down" would be eliminated for the secondary bristles. Also the bore size would be such that would just allow the 90 degree bristles to wear to size for the minimum clearance, and that the inclined rows would still blow down onto the rotor.

Claims

1 A brush seal having a bristle pack composed of two or more layers of bristles laid at different angles of inclination. 2 A brush seal as claimed in claim 1 wherein at least a bristle layer on the upstream side of the bristle pack comprises bristles laid at substantially zero angle of inclination. 3 A brush seal as claimed in claim 1 or claim 2 comprising a bristle layer on the upstream side and a bristle layer on the downstream side wherein the bristles are laid at substantially zero angle of inclination and the bristles of an intermediate layer are laid at an inclined angle. 4 A brush seal as claimed in any preceding claim having a backing plate and a front shield. 5 A brush seal substantially as hereinbefore described with reference to the accompanying drawings.