Abstract:
A gas seal for providing a seal between a pair of relatively rotatable components is provided with a stand-still seal which will seal between the components when the components are stationary, the stand-still seal having a sealing element mounted in sealing relationship with respect to one of said components and being displaceable into sealing engagement with the other component when the components are stationary with respect to one another.

Description:
FIELD OF THE INVENTION 
   The present invention relates to seals and in particular to improvements in and relating to gas seals. 
   BACKGROUND OF THE INVENTION 
   With gas seals of, for example, the type disclosed in EP 0,499,370 and EP 0,578,377, the disclosures of which are incorporated herein by reference thereto, groove areas are provided in one of a pair of opposing sealing faces whereby, upon rotation of one of the sealing faces relative to the other, a cushion of product gas is built up between the sealing faces to lubricate and create a seal between the sealing faces. When such seals are not rotating, they rely on engagement between the sealing faces to provide a static seal. Even though the grooved sealing face will normally have a continuous circumferential dam formation which will engage the other sealing face, gas seals of this type are subject to leakage under static conditions. When used in high pressure applications, for example on gas compressors, the expansion of the gas as it leaks across the gas seal will cause cooling of the gas which may even lead to liquefaction of the gas. The presence of liquefied gas in the gas seal is detrimental to the efficient operation of the gas seal under dynamic conditions. In order to address this problem auxiliary compressors have hitherto been used to pass warm gas through the gas seal. A further problem with leakage under static conditions is that the leaking gas must be vented or flared to atmosphere, with the consequent adverse affect on the environment. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention, a gas seal for providing a seal between a pair of relatively rotatable components comprises a first seal face member mounted in rotationally fixed relationship and sealed with respect to one of said components and a second seal face member mounted in rotationally fixed relationship and sealed with respect to the other component, the first and second seal face members being urged towards one another by spring means, grooves being provided in one of said first and second seal face members to create a hydrodynamic force opposing the load applied by the spring means, when one seal face member rotates relative to the other, and a stand-still seal acting between the relatively rotatable components, the stand-still seal comprising a sealing element mounted in sealing relationship with respect to one of said components and being displaceable into sealing engagement with the other component when the components are stationary with respect to one another. 
   In accordance with the present invention, when the components are rotating relative to one another the stand-still seal will be retracted and the seal will be provided between the components by means of the gas seal, in conventional manner. However, when the components are stationary relative to one another, the stand-still seal is moved into sealing engagement with the other component, thereby providing a static seal between the components. The pressure across the gas seal may then be equalised thereby preventing leakage and cooling of the process gas. 
   According to a preferred embodiment of the invention the stand-still seal comprises an annular piston which is slidably mounted in a housing coaxially of a rotating component, the piston being displaceable axially into sealing engagement with a radial face of a component mounted for rotation with the other component. The piston may be moved between a retracted and engaged position by hydraulic, pneumatic or electrical means. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention is now described, by way of example only, with reference to the accompanying drawings in which: 
       FIG. 1  illustrates a seal in accordance with the present invention; 
       FIG. 2  illustrates the seal shown in  FIG. 1 , with a stand-still seal in a deployed position; 
       FIG. 3  illustrates a gas compressor with stand still seals on either side of the compressor chamber; and 
       FIG. 4  shows a partial view of another embodiment of the seal shown in  FIGS. 1 and 2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates the seal assembly  10  for the shaft  12  of a gas compressor, the seal assembly  10  providing the seal between the compressor chamber  14  and a shaft bearing  16 . 
   The seal assembly  10  is mounted in a bore defined by a housing  18  coaxial of the shaft  12 . A sleeve member  20  is secured to the shaft  12  for rotation therewith and sealed with respect thereto by means of sealing elements  22  and  24 . A flange formation  26  extends radially outwardly of the sleeve member  20  at the end thereof adjacent the compressor chamber  14 . 
   A first seal face member  30  is mounted on the face of flange formation  26  remote from the compressor chamber  14 , for rotation therewith. The first seal face member is sealed to the flange formation  26  by means of sealing element  32 . An annular member  34  is mounted on the external diameter of the sleeve member  20 , the annular member  34  abutting the seal face member  30  to locate it axially. 
   A second seal face member  36  is mounted on a carrier ring  38  which is slidably located between annular member  34  and an annular member  40  secured to the housing  18 . The carrier ring  38  is sealed with respect to annular member  40  by means of sealing element  42  and to the second seal face member  36  by sealing element  44 . A plurality of annular spaced compression springs  46  act between annular member  40  and the carrier ring  38 , to urge the second seal face member  36  towards the first seal face member  30 . 
   A grooved area  48  is provided in the outer portion of a sealing face  50  of the first seal face member  30 , so that upon rotation of the shaft  12 , processed gas will be pumped between the faces  50  and  52  of the first and second seal face members  30  and  36 , to generate a load which opposes that applied by the springs  46  and creates a gas cushion which will lubricate the sealing faces and provide a seal. 
   A key formation  60  acts between the annular member  34  and sleeve member  20  to transmit torque therebetween. A third seal face member  62  is mounted in similar manner to the first seal member  30  to a flange formation  64 , formed at the end of annular member  34  remote from the compressor chamber  14 . A fourth seal face member  66  is mounted in similar manner to the second seal face member  36  and is sealed with respect to an annular member  68  secured to the housing  18 . The fourth seal face member  66  is urged towards the third seal face member  62  by means of a plurality of angularly spaced compression springs  70 . The first and second and third and fourth seal face members thereby constitute a pair of gas seal assemblies spaced axially of one another and defining a sealed chamber  72  therebetween. A passageway  74  from the chamber  72  allows product gas leaking passed the first gas seal assembly  30 / 36  to be removed from the chamber  72 , the leaking gas being vented or flared to atmosphere; recycled or mixed with fuel gas powering the compressor. 
   A labyrinth seal assembly  76  is provided outboard of the gas seal assembly  62 / 66  to prevent lubricant from the bearing  16  from reaching the gas seal assembly  62 / 66 . Alternatively a carbon ring contacting seal can be used in this location. 
   A labyrinth seal  80  is mounted between the first seal face member  30  and the compressor chamber  14  on an annular member  82  which is secured to the housing  18  and sealed with respect thereto by means of sealing elements  84  and  86 . The annular member  82  defines a closed annular cylinder  90  which extends coaxially of the shaft  12  and opens to the radial face of annular member  82  remote from the compressor chamber  14 . An annular piston  92  is located in the cylinder  90 , the piston  92  being sealed with respect to the inner and outer walls of the cylinder  90  by means of sealing elements  94  and  96 . An annular sealing element  98  is also provided in the outer end of the piston  92 . 
   The cylinder  90  is connected to a source of gas under pressure, for example an accumulator  110  by means of passageway  100 , whereby pressure a may be applied to the piston  92  forcing it axially out of cylinder  90 , so that the sealing element  98  is forced into sealing engagement with the surface of flange formation  26  adjacent the compressor chamber  14 , thereby isolating the gas seal assembly  30 / 36  from the compressor chamber  14 . 
   As illustrated in  FIG. 3 , stand-still seals as described above are provided on both sides of the compressor chamber  14 . Gas under pressure is selectively delivered to the cylinder  90  from an accumulator  110  via line  112 , or vented from the cylinder  90  via link  114 , by means of an electronically controlled change-over valve  116 . The electronically controlled change-over valve  116  has an electronic interlock preventing connection of the cylinder  90  to the accumulator  110  when the shaft  12  is rotating. The accumulator  110  is charged with process gas from the discharge side of the compressor, via line  118  and non-return valve  120 . 
   With the seal assembly described above, under normal operation when shaft  12  is rotating, no pressure is applied to cylinder  90  and consequently the pressure of the product gas acting on the free end of piston  92  will force it away from flange formation  26 , so that the sealing element  98  is clear of the rotating face of flange  26 . The gas seal assemblies  30 / 36  and  62 / 66  operate in conventional manner to provide a seal, any gas leaking across the gas seal assembly  30 / 36  being removed from chamber  72  via passageway  74 . 
   When shaft  12  is stationary, fluid under pressure is applied to the piston  92  forcing it into engagement with the adjacent face of flange formation  26  thereby isolating the gas seal assembly  30 / 36  from the product gas in compression chamber  14 . The gas between the piston  92  and gas seal assembly  30 / 36  may then be vented, via a passageway  102 , to remove the pressure differential across the gas seal  30 / 36  and thereby prevent any leakage across the gas seal  30 / 36  and cooling of the product gas upon expansion. 
   To restart the compressor, the chamber between the piston  92  and gas seal assembly  30 / 36  is first re-pressurised via passageway  102 . The hydraulic pressure in cylinder  90  is then vented so that the piston retracts under the pressure of process gas in the compressor chamber  14  and the compressor can then be started in the normal way, thereby preventing wear on the sealing element  98 . 
   The various sealing elements  32 , 24 , 32 , 42 , 44 , 62 , 84 , 86 , 94 , 96  and  98  have been illustrated as elastomeric O-rings. Other forms of sealing element, for example spring energised polymer seals, may however be used, particularly for the seals  94 , 96  and  98  on the piston  92 . 
   While in the above embodiment process gas is used to control the stand-still seal, an alternative supply of gas may be used, either from a pressurised source of by means of a compressor. Alternatively, a source of hydraulic fluid under pressure may be used. 
   In an alternative embodiment, movement of piston  92  between its retracted and deployed positions may be controlled by electromagnetic means, for example an electrical solenoid  120 . Preferably the solenoid when energised will hold the piston  92  in the retracted position, the piston  92  being biased to the deployed position so that when the compressor stops the solenoid is de-energized so that the piston  92  moves into engagement with flange  26  to form a seal. 
   While the invention has been described by way of example, with reference to a double gas seal, it may equally well be used in a single gas seal. Moreover, the standstill seal may be provided on the outboard side of the seal rather than inboard, the chamber formed between the gas seal and the stand-still seal being pressurised to balance the pressure across the gas seal and prevent leakage thereacross.