Seal for a joint or juncture – Seal between relatively movable parts – Brush seal
Reexamination Certificate
1999-10-06
2001-10-30
Knight, Anthony (Department: 3626)
Seal for a joint or juncture
Seal between relatively movable parts
Brush seal
Reexamination Certificate
active
06308957
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention concerns the design of an improved brush seal. In particular, the invention relates to a brush seal in which the bristle pack is, effectively, sub-divided into upstream and downstream portions by at least one annular diaphragm.
The basic configuration of a brush seal comprising a simple bristle layer, even with a deep backing plate, does not cope well with high pressure differentials and in regions of disturbed swirling airflow on the high pressure side, for example. A number of factors seem to contribute to poor seal performance in these conditions. On one hand, conditions of high pressure difference causes the bristles to become locked together so that they behave more like a solid mass. When this occurs the bristles are slow to track relative lateral movements of the component against which the seal is produced, for example a rotating shaft, so that once pushed aside by a transient movement the bristles lock-up leaving an increased leakage gap. If the bristles become locked in their undeflected positions initial leakage is low but the tips of the bristles rapidly wear against the co-operating seal surface on the shaft, and the wear rate is increased by shaft excursions, irregularities etc. On the other hand, in disturbed conditions the bristles are caused to flutter with detrimental effect on their co-operative ability to seal effectively because the bristles become permanently splayed destroying their seal properties. Furthermore the deterioration begins with the bristles at the exposed surface and progresses deeper into the layer. Initially the seal appears to perform as expected because the sealing function falls upon bristles further into the layer until eventually all the bristles are corrupted. These two effects may be combined so that while exposed bristles are disturbed and becoming permanently splayed the burden of sealing a high pressure differential falls on bristles towards the back of the layer which are locked together and subject to rapid wear.
Previous attempts to solve these problems, but without long lasting success, have involved assemblies of multiple brush seals. However, it has been found that under dynamic conditions several stages of adjacent brush seals do not contribute equally and a major proportion of the pressure drop across the whole seal assembly is borne by a single stage. Where turbulent conditions prevail the first, upstream seal has a calming influence on the leakage flow and the second or final stage bears most of the pressure drop. The said one stage gives about equal performance to a single stage seal and overall almost nothing is gained in the long term.
SUMMARY OF THE INVENTION
The present invention seeks to provide a multi-stage brush seal having improved performance by forcing a more even distribution of pressure drop across the individual stages simultaneously creating bristle calming conditions.
Therefore, according to the present invention, there is provided a multi-stage brush seal assembly for an annular leakage gap between regions of high pressure differential comprising a plurality of close-coupled bristle pack regions positioned one adjacent another in flow sequence, with partially compliant and leaky diaphragm means located between at least two adjacent bristle pack regions to act as a barrier therebetween and which, in use, provides a controlled low-level flow between the pack regions.
Each of the close-coupled bristle pack regions may comprise, alternatively, the bristle pack of an individual brush seal or a layer of a single, sub-divided bristle pack wherein a leaky diaphragm is interposed between adjacent bristle pack layers. Furthermore the leaky diaphragm means may be realised in one of several ways. For example, the diaphragm means may comprise an annular diaphragm consisting of a multiplicity of diaphragm segments spaced circumferentially around a common support annulus. The segmented diaphragm and support annulus may be formed integrally from a single sheet of flexible material. Alternatively, the leaky diaphragm may comprise a further thin layer of bristles which may be of different size to the bristles of the pack region, the bristles of the thin layer may be disposed at a different angle or even in a different direction relative to the bristles of the pack regions.
REFERENCES:
patent: 885032 (1908-04-01), Ferranti
patent: 5318309 (1994-06-01), Tseng et al.
patent: 5688105 (1997-11-01), Hoffelner
patent: 5884918 (1999-03-01), Basu et al.
patent: 6032959 (2000-03-01), Carter
patent: 2 258 227 A (1993-03-01), None
patent: 2 281 108 A (1995-02-01), None
Knight Anthony
Oliff & Berridg,e PLC
Peavey Enoch E
Rolls Royce Plc.
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