This application claims priority to British application number 0324076.9, filed 14 Oct. 2003, the entirety of which is incorporated by reference herein.
1. Field of the Invention
This invention relates to sealing arrangements in general, in particular a sealing arrangement between first and second relatively rotatable bodies of a machine, more particularly a turbo machine.
2. Brief Description of the Related Art
The development of flexible seal, e.g. brush seal, or leaf or foil seal, technology over the last two decades has principally been in the field of gas turbines. In such applications, the pressure drop across the seal assembly will not usually be more than 25 bar.
However, in steam turbine applications, sealing arrangements will often have to be designed for pressure drops significantly greater than 25 bar, sometimes more than 50 bar. By way of example, in a high pressure cylinder the steam inlet pressure may be of the order of 200 bar and the exhaust pressure may be of the order of 80 bar. In an intermediate pressure cylinder the inlet pressure may be about 80 bar and the exhaust pressure about 5 bar. Thus, a pressure drop across a balance piston and a pressure drop between the exhaust pressure and the external atmospheric pressure will often be considerably higher than typical applications of flexible seals.
Various seal arrangements using flexible seals such as brush seals have been proposed with the aim of achieving good efficiency and long life. EP-A-0 836 040 discloses a sealing arrangement comprising a series of sealing units 1 (one of which is shown in
GB-B-2 301 635 discloses a somewhat similar sealing arrangement (as shown in
With flexible (e.g. brush or leaf) seals, non-linear movement of the flexible elements, due to friction between the flexible elements and their supporting structure, can result in situations where one of the seals in a multiple seal configuration is exposed to a significantly larger proportion of the total pressure drop than the design intent and lead to failure of the flexible element. Once a flexible seal has failed, further failures will occur in a cascade fashion due to the increased loading on the remaining seals.
Resistors in series are a simple electrical analogue of a fluid sealing arrangement in which a pressure drop is carried across multiple seals of any type. For incompressible conditions, if all resistances are equal each seal will be exposed to the same fraction of the total pressure drop. For compressible conditions, if the effective leakage area of each seal is the same, the pressure drop (Δp) across individual seals will increase in the downstream direction because a larger pressure drop is required for the same leakage flow as density decreases. This is true regardless of seal type (brush seal, labyrinth seal, other). The pressure distribution is dependent on the relative magnitude of the resistances of the various seals, not their absolute values.
Now consider multiple flexible seals (e.g. brush seals or leaf/foil seals) in series. By way of an example, consider three identical spring-backed sealing glands, each consisting of a brush seal (or leaf/foil seal) mounted in the middle of six conventional labyrinth seal restrictions (fin seals), as shown in
Under normal operation, the design of the brush seals in the example is such that their performance (resistance to fluid flow) is equivalent to that of a conventional single labyrinth restriction with a clearance to the rotor of 0.1 mm. The brush seals are designed to operate satisfactorily under pressure loads of up to 25 bar (predetermined maximum pressure drop (Δpmax) or rated pressure drop). The sealing arrangement can be modelled as 18 resistors of magnitude 1 in series with three resistors of magnitude 10 (based on leakage areas). From the electrical analogue, the pressure drop (Δp) across each brush seal will be 100*10/(3*10+18*1)=21 bar.
The worst recoverable leakage (loss of performance or decrease in resistance) of each brush seal will occur if, following some large radial vibration of the rotor, say, the brush seal bristles become temporarily stuck up behind the backing ring, owing to friction. Under these circumstances the brush seal will perform as a single labyrinth restriction with a clearance equal to that between the brush seal backing ring and the rotor, 1.0 mm in this case, say. Suppose now that, as a result of a large rotor vibration event, the bristles of two of the three brush seals become temporarily stuck up behind the backing ring, but those of the third seal are able to quickly recover to the normal position. The system is now operating as 20 labyrinth restrictions of 1.0 mm in series with the equivalent of a single restriction of 0.1 mm. Under these circumstances, the pressure drop across the one ‘unstuck’ brush seal will be 100*10/(10+20*1)=33 bar. So, this seal would suffer permanent damage from over-loading. Once damage has occurred to one seal, the loading on the other brush seals will be increased when their bristles become free again and consequently the likelihood of further bristle pack over-loading is increased. Once damage has started to occur, any such system has the potential to cascade rapidly towards total failure in this manner.
It would therefore be desirable to be able to provide a sealing arrangement including flexible seals which functioned within acceptable design limits during both normal and abnormal operating conditions such as described above, without suffering permanent damage.
One aspect of the present invention includes providing a sealing arrangement between first and second relatively rotatable bodies of a machine in which, in operation, a relatively high pressure (P1) exists on one side of the sealing arrangement and a relatively low pressure (P2) exists on the other side, the pressure difference (P1-P2) being up to a predetermined maximum pressure difference (ΔPmax), the sealing arrangement comprising a series of seals, the pressure dropping across each successive seal from the high pressure side of the sealing arrangement to the low pressure side, at least two of the seals being flexible seals, each flexible seal being designed to function under a pressure drop (Δp) up to a predetermined maximum pressure drop (Δpmax) across the flexible seal and to provide a given resistance to fluid flow, each flexible seal being capable of recovering from an abnormal condition, due to normal operation of the machine, in which the resistance of the flexible seal to fluid flow decreases from a normal level to a substantially lower level, the series of seals being arranged in such a manner that the pressure drop across each flexible seal is less than the predetermined maximum pressure drop (Δpmax) when the pressure difference (P1-P2) between the high pressure side of the sealing arrangement and the low pressure side is equal to the said predetermined maximum pressure difference (ΔPmax) and another one of the flexible seals has suffered a recoverable decrease in its resistance to fluid flow.
The said predetermined maximum pressure difference (ΔPmax) may be more than 20 bar, for example.
The said predetermined maximum pressure drop (Δpmax) may be in the range from 10 to 30 bar, for example.
In particular, in the above example, if the performance of each individual flexible seal (e.g. brush seal) is degraded (e.g. by shortening the bristles in some circumferential sections of the seal, or by drilling by-pass holes through the seal, allowing leakage around the back of the seal, or by installing the seals with increased radial clearance, or by careful control of the leakage around the back of the spring-backed gland, or if the seal is split into segments leaving gaps between them), limiting the pressure drop caused by the seal, so that the performance of the flexible seal is reduced to being equivalent to a labyrinth (fin seal) restriction with a clearance of 0.15 mm, then under the same scenario as above (bristles of two brush seals stuck behind the backing ring), the maximum pressure drop (Δpmax) that the third brush seal would see is now reduced to 25 bar. Thus, in this case, this seal would not be over-loaded and performance of the system would be unchanged when the bristles of the remaining brush seals returned to their normal operating positions.
There will be a penalty in increased leakage flow through the sealing arrangement if the brush seals are protected in this way. Replacing three of 21 labyrinth restrictions with a clearance of 1.0 mm by three brush seals with an equivalent labyrinth clearance of 0.1 mm reduces leakage flow to 44% ((21*1)/(18*1+3*10)) of the leakage through the sealing arrangement with 21 labyrinth restrictions. If the performance of each brush seal is degraded to a labyrinth clearance equivalents of 0.15 mm, the leakage increases to 55% of the pure labyrinth system.
In summary, limiting the pressure drop caused by the flexible seal enables the seal to be protected against over-pressurisation whilst maintaining a leakage performance that is still significantly better than a corresponding all-labyrinth version of the sealing arrangement. Clearly, the trade-off between leakage performance and flexible seal protection depends upon the number of seals and their operating clearances. The values quoted above are specific to the example used by way of explanation. In arrangements with a greater number of seals (balance pistons and end-glands usually have more than three gland rings) the impact on leakage performance of load protection for the flexible seals would typically be less than that seen in the example.
The invention will be described further, by way of example only, with reference to the accompanying drawings, in which:
The brush seal 14 shown in
The brush seal 14 is one of a series of seals in a sealing arrangement between first and second relatively rotatable bodies of a turbine. In a steam turbine, for example, the sealing arrangement may be between a stationary casing and a balance piston, which is part of the rotor. Alternatively or additionally, the sealing arrangement may be provided between a stationary casing and a trunnion on the rotor or at any other location between the stationary casing and the rotor where sealing against a large pressure drop is required. The sealing arrangement comprises a plurality of the brush seals as well as fin seals (labyrinth elements); there may, for example, be at least one fin seal upstream of each brush seal.
As can be seen in
In
In
In
Various modifications may be made within the scope of the invention. For example the brush 11 may be replaced by a flexible leaf or foil. The flexible sealing element (e.g. brush, leaf, or foil) may be carried by any convenient supporting element which provides fluid resistance in parallel to the flexible sealing element; the supporting element may be rigid or deformable. The number of brush seals (or leaf or foil seals) may be varied. A balance piston may, for example, be designed with six sealing units (substantially as shown in
While the invention has been described in detail with reference to exemplary embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. Each of the aforementioned documents is incorporated by reference herein in its entirety.
Number | Date | Country | Kind |
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0324076.9 | Oct 2003 | GB | national |