Abstract:
In sealing arrangement  41  between first and second relatively rotatable bodies of a machine such as a steam turbine, a relatively high pressure p 0  exists on one side of the sealing arrangement and a relatively low pressure p f  exists on the other side. The sealing arrangement comprises a series of sealing elements or units S 1 , S 2 , S 3 , at least one of which comprises a brush seal. A seal bypassing device, preferably comprising a pressure relief valve  42 - 44  limits the pressure drop across the brush seal.

Description:
This application is a continuation of International Application No. PCT/EP2003/50684 filed Oct. 3, 2003, which claims priority pursuant to Article 4of the Paris Convention to British Application No. 022294.6 filed Oct. 3, 2002. 

   BACKGROUND 
   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. 
   The development of brush 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. 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, for example. In an intermediate pressure cylinder the inlet pressure may be about 80 bar and the exhaust pressure about 5 bar, for example. Thus, in each case, the pressure drop across the balance piston and the pressure drop between the exhaust pressure and the external atmospheric pressure will be considerable. 
   Various seal arrangements using 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  FIG. 1 ) between a stationary casing  2  and the rotor  3  of a turbo machine. Each sealing unit  1  includes several brush seals  4  fixed on a carrier  6  mounted on the casing  2  so as to be movable to a limited extent in the radial direction. Fin seals  7  are fixed to the rotor  3 . 
   GB-B-2 301 635 discloses as somewhat similar sealing arrangement (as shown in  FIG. 2 ), in which the carrier  6  has a only a single brush seal  4 , which is mounted so as to be capable of limited radial movement relative to the carrier  6 . The fin seals  7  are provided on the carrier  6 . 
   It has been found that, in sealing arrangements comprising a series of brush seals, the individual brush seals are prone to failure. It appears that, once one of the brush seals has failed, further failures occur in a cascade fashion because of the increased loading on the remaining seals. It would therefore be desirable to be able to provide a sealing arrangement utilising a brush seal and having a sufficiently long life. 
   SUMMARY 
   The present invention provides a sealing arrangement between first and second relatively rotatable bodies of a machine in which, in operation, a relatively high pressure exists on one side of the sealing arrangement and a relatively low pressure exists on the other side, the sealing arrangement comprising a series of sealing elements, at least one of which is a brush seal, the pressure dropping across each successive sealing element from the high pressure side of the sealing assembly to the low pressure side, and a seal bypassing device which limits the pressure drop across the brush seal. 
   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. 
   To regulate the pressure drop across the brush seal, a conduit may communicate between a first region in the machine on the high pressure side of the sealing arrangement and a second region, within the sealing arrangement, on the lower pressure side of the brush seal. Alternatively, a first conduit communicating between the high pressure side and the lower pressure side of the sealing arrangement may include a valve providing a pressure which is intermediate the high pressure and the low pressure, a second conduit communicating between the valve and a region, within the sealing arrangement, on the lower pressure side of the brush seal. 
   Preferably, the seal bypassing device comprises a pressure relief valve. In general terms, a pressure relief valve may comprise means for sensing a pressure difference (which may be an absolute value or a pressure ratio) and acting to reduce the pressure difference when the sensed pressure difference exceeds a given threshold. 
   A suitable pressure relief valve may comprise a valve member which opens communication from a first valve chamber (communicating with the higher pressure side of the brush seal) to a second valve chamber (communicating with the lower pressure side of the brush seal) when the pressure difference between the chambers exceeds a given threshold. The pressure relief valve may include a spring biasing the valve member toward a position in which it blocks communication between the valve chambers. The valve member may have a relatively small area exposed to the pressure in the first chamber and a relatively large area exposed to the pressure in the second chamber, the pressure difference threshold being a function of the exposed area (and the stiffness of the spring). 
   The brush seal may form part of the sealing unit comprising at least one sealing element, the seal bypassing device limiting the pressure drop across the sealing unit, the sealing arrangement comprising a series of such sealing units. In the simplest case, there may be two such sealing units, each comprising a single brush seal. In other embodiments, the sealing unit may comprise at least one brush seal and at least one fin seal. The sealing unit may comprise two, three, or more brush seals. The sealing unit may be mounted on either of the relatively rotatable bodies so as to be movable to a limited extent in a direction towards and away from the other body. 
   The invention will be described further, by way of example only, with reference to the accompanying drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an axial cross-section through a known sealing arrangement; 
       FIG. 2  is an axial cross-section through another known sealing arrangement; 
       FIG. 3  is an axial cross-section through part of a steam turbine; 
       FIG. 4  is an axial cross-section through a first embodiment of a sealing arrangement according to the present invention; 
       FIG. 5  is an axial section through a second embodiment of a sealing arrangement according to the present invention; 
       FIG. 6  is a diagram illustrating a third embodiment of a sealing arrangement according to the present invention; and 
       FIG. 7  is an axial section through part of a fourth embodiment of the sealing arrangement according to the present invention. 
   

   DETAILED DESCRIPTION 
   Referring first to  FIG. 3  of the accompanying drawings, this shows one cylinder of a steam turbine with a casing  2  and a rotor  3  which includes a balance piston  11  and two trunnions  12 . High pressure steam (for example at 200 bar) is introduced through the inlet of the cylinder and passes through the blades  14  before entering an exhaust chamber (in which the pressure may, for example, be 80 bar) between inner and outer parts of the casing  2 . The diameter of the balance piston  11  is selected so that the thrust produced on the rotor  3  due to the pressure drop across the balance piston substantially balances the thrust due to the total effect of the pressure drop across each of the rows of moving blades on the rotor  3 . A sealing arrangement  17 , between the casing  2  and the balance piston  11 , is subjected to substantially the full pressure drop from the inlet pressure to the exhaust pressure (for example, a pressure drop of 120 bar). A sealing arrangement  17   a , between each trunnion  12  and the casing  2 , is subjected to the pressure drop between the exhaust pressure and atmospheric pressure. 
   The embodiments of sealing arrangements described below are primarily designed to be applied to the sealing arrangement  17  between the casing  2  and the balance piston  11 , but they are also applicable to the sealing arrangement  17   a  between the casing  2  and a trunnion  12 . The embodiments are, of course, applicable to other types of steam turbine designs. 
   A first embodiment of a sealing arrangement  21  between the casing  2  and the rotor  3  is shown in  FIG. 4  and comprises a series of three sealing units S 1 , S 2 , S 3 , each of which is similar to the sealing unit  1  shown in  FIG. 2  and more fully described in GB-B-2 301 635. Each sealing unit includes a single brush seal  4  and several fin seals  7 . (The number of fin seals  7  could be reduced, or the fin seals could be omitted, the number of brush seals could be greater than one and the sealing elements could be fixed to the casing  2 .) Although the sealing arrangement has been shown as having three sealing units, this number of sealing units is arbitrary. 
   If the sealing arrangement  21  is between the casing  2  and the balance piston  11 , the pressure p 0  at the high pressure side of the sealing arrangement  21  (the left-hand side in the drawing) is substantially equal to the steam inlet pressure, and the pressure p f  at the low pressure side is substantially equal to the steam exhaust pressure. Intermediate pressures p 1  and p 2  prevail between the sealing units S 1  and S 2  and between the sealing units S 2  and S 3  respectively. For maximum life of the sealing arrangement, p 0 −p 1 =p 1 −p 2 =p 2 −p f  so that, if p 0  is 170 bar and p f  is 80 bar, then p 1  should be 140 bar and p 2  should be 110 bar. In order to ensure this, the casing  2  is provided with conduits  22  and  23  communicating respectively between, on the one hand, regions  24  and  25  between the sealing units and, on the other hand, regions in the machine which are on the high pressure side of the sealing arrangement and are at respective pressures higher than p f , these pressures being such that the required values of p 1  and p 2  are maintained. It will be seen that the conduits  22  and  23  provide bypasses with respect to the seal units S 1  and S 2 , respectively. In the case in which the sealing arrangement is on the balance piston  11 , the conduits  22  and  23  tap off steam from appropriate locations in the cylinder expansion (or from elsewhere in the steam cycle). 
   Accordingly, if a temporary abnormality of the sealing arrangement occurs during operation, tending to disrupt the pressure distribution along the sealing arrangement, the bypass conduits  22  and  23  maintain the correct values of the intermediate pressures p 1  and p 2  proportionally between the high pressure p 0  and the low pressure p f . In this way the pressure drop across the brush seals  4  is limited, thereby reducing the risk of failure of the brush seals. 
   The sealing arrangement  31  according to a second embodiment, as shown in  FIG. 5 , is similar to the sealing arrangement  21 , except for the conduits communicating with the regions  24  and  25  between the sealing units. First conduits  32  and  33  each communicate between the high pressure side and the low pressure side of the sealing arrangement  31 . Each conduit  32  ( 33 ) includes a control valve  34  ( 35 ) having a port  34   a  ( 35   a ) connected to a respective second conduit  36  ( 37 ) which communicates with the respective region  24  ( 25 ) between the sealing units S 1  and S 2  (S 2  and S 3 ). Each control valve  34  ( 35 ) is acted on by the pressures p 0  and p f  and is configured to provide at the respective port  34   a  ( 35   a ) a pressure which is intermediate the pressures p 0  and p f  and substantially equal to the desired value of the respective pressure p 1  (p 2 ). 
   The sealing arrangement  41  according to the third embodiment shown diagrammatically in  FIG. 6  differs from the previous embodiments in that each sealing unit S 1 , S 2 , S 3  is capable of being bypassed by a pressure relief valve  42 ,  43 ,  44 , respectively. Sealing units each comprise a brush seal and may be as described above. If any one of the brush seals starts to become overloaded during operation (for example, owing to brush seal hysteresis caused by friction), the corresponding pressure relief valve will open in order to maintain a safe operating pressure drop across the brush seal. In this way, damage to the brush seal can be avoided and the brush seal therefore remains available to resume normal operation when the pressure drop along the sealing arrangement  41  has readjusted to a more uniformed distribution. 
     FIG. 7  shows a ball and spring design for a pressure relief valve. This Figure shows, by way of example, the pressure relief valve  42  associated with the first sealing unit S 1 , which in this case consists of a single brush seal  4  fixed to the casing  2 . The pressure relief valve includes a valve member or ball  46  which is urged by a spring  47  to block communication between a narrow first valve chamber  48  and a wide second valve chamber  49 . The first valve chamber  48  communicates directly with the high pressure side of the sealing arrangement  41  and is thus subject to the high pressure p 0 , and the second valve chamber  49  communicates directly with the space between the sealing units S 1  and the next sealing unit S 2  (not shown) and is thus subject to the intermediate pressure p 1 . The ball  46  opens communication from the first valve chamber  48  to the second valve chamber when the pressure difference p 0 −p 1  exceeds a given threshold value, which is a function of the biasing force of the spring  47  and the exposed areas of the ball. The same ball and spring design pressure relief valve can also be used for the values  43  and  44  in  FIG. 6 . 
   In each of the sealing arrangements described above, the number of brush seals in each sealing unit may be varied, each sealing unit may be provided with one or more sealing elements other than brush seals, and the sealing arrangement may include additional sealing units which do not include brush seals. 
   Furthermore, as mentioned above, the number of sealing units may be varied. For example, a balance piston may be designed with six sealing units for sealing against a total pressure drop of, for example, 120 bar. Each sealing unit might contain a single brush seal capable of accommodating a pressure drop of 30 bar. Any of the embodiments described above could be used to regulate the pressure drop across each sealing unit to limit the pressure drop to 20 bar, thus eliminating any risk of over-pressurisation of any individual brush seal.