Abstract:
A seal for a rotary compressor comprises a casing, at least one assembly having a rotating seal face ring which rotates as one with a sleeve intended to be mounted on a shaft of the compressor and of a stationary seal face ring mounted on the casing, the seal face rings being urged to press against one another via their rubbing face. The seal design further incorporates fluid circulation for heating leaks of compressible fluid conveyed by the compressor that occur between the seal face rings, this fluid circulation being formed in the casing and extending at least partially downstream of the seal face rings with respect to the direction of flow of the fluid stream.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a seal for a rotary compressor and to a compressor provided with such a seal. 
   DESCRIPTION OF RELATED ART 
   Rotary compressors are rotary machines intended to convey compressible fluids, and their purpose is to transfer mechanical energy to the fluid which passes through them with a view to increasing its pressure. 
   To this end they comprise a drive shaft which drives the rotation of a collection of wheels which mainly transmit to the fluid the mechanical energy supplied by the motor that drives the shaft. 
   With a view to containing the pressurized fluid within the body of the compressor, the ends of the shaft are provided with dry seals with sealing gas. 
   Such seals are conventionally provided with a casing, with a stationary seal face ring or gland washer mounted on the casing and with a rotating seal face ring which rotates as one seal sleeve mounted on the drive shaft of the compressor, these seal face rings being for example urged to press against one another by the action of a spring. 
   When the compressor is operating, the seals are subjected to high thermal loadings due, in particular, to the temperature of the fluid passing through the compressor, to the expansion of the fluid at the seal, to the shear of the film of fluid at the sealing interface between the respective friction faces of the rotating and stationary seal face rings, and to the ventilation losses brought about as the rotating seal face ring rotates through the fluid conveyed by the compressor. 
   It is known that these thermal loadings generate deformation at the rotating seal face ring and the stationary seal face ring as a result of differential expansion, and this carries the risk of leading to a loss of sealing or even to destruction of the hardware if the face rings come into contact. 
   What happens in particular is that leaks occur generally at the interface between the stationary seal face ring and the rotating seal face ring, and this causes a relatively significant local drop in the fluid temperature downstream, due to the expansion of this fluid. 
   Thus, for example, for assisted applications of recovering petroleum through the injection of natural gas, the equilibrium pressure of the gas injection compressor loop may be as high as 250 to 300 bar. The expansion brought about at the sealing interfaces of the compressor seal can locally drop the temperature to about −80° C. 
   Furthermore, since the compressible gases generally used in this type of compressor have water as a constituent, the drop in temperature is likely to give rise to the consequential formation of hydrates, for which the temperature of formation is of the order of −10° C. 
   As hydrates are solid compounds, their presence carries the risk of jamming the stationary seal face ring with respect to the rotating parts of the compressor, and this is likely to give rise to a loss of sealing when the machine is shut down, or even to prevent the latter from being started up again as long as the temperature remains lower than the hydrates formation temperature. 
   The object of the invention is to alleviate these disadvantages and to provide a seal and a centrifugal compressor which are capable of heating up the fluid downstream of the sealing interfaces of the seals while the machine is under pressurized shut down. 
   SUMMARY OF THE INVENTION 
   Thus, according to the invention, there is proposed a seal for a compressor, comprising a casing, at least one assembly consisting of a rotating seal face ring which rotates as one with a sleeve intended to be mounted on a shaft of the compressor and of a stationary seal face ring mounted on the casing, the seal face rings being urged to press against one another via their rubbing face. 
   According to one aspect of this seal, the latter further comprises means for circulating a fluid for heating the leaks of compressible fluid conveyed by the compressor and which occur between the seal face rings, the said circulating means being formed in the casing and extending at least partially downstream of the seal face rings when considering the direction of flow of the said stream of fluid. 
   The fluid flowing through the compressor seals is thus heated and this makes it possible to compensate for the cooling that is brought about during the expansion of this fluid downstream of the sealing interfaces. 
   According to another aspect of this seal, the said circulating means comprise a heating duct in communication with a supply source of heating fluid which operates independently of the means supplying the compressor with compressible fluid. 
   It is therefore possible to envisage heating the fluid within the seals even when the compressor is not running. 
   According to one particular embodiment, the casing is provided with a passage for the flow of the leaks of fluid and in which there is positioned a wall which, with the casing, delimits the said passage and which constitutes a surface for the exchange of heat energy. 
   As a preference, the face of the wall facing towards the heating duet is provided with ribs forming therein a heating coil. 
   For example, the heating fluid consists of oil. 
   According to another embodiment, the seal further comprises a heat exchanger arranged facing the rotating seal face ring. 
   According to the invention, there is also proposed a centrifugal compressor comprising a drive shaft driving the rotation of a collection of wheels able to transfer the mechanical energy supplied by the drive shaft to a compressible fluid and at least one shaft output seal. 
   According to one aspect of this compressor, the or each seal consists of a seal as defined hereinabove. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the invention will become apparent from the description which follows, given solely by way of nonlimiting example and made with reference to the appended drawings in which: 
       FIG. 1  is a schematic view in longitudinal section of a centrifugal compressor; 
       FIG. 2  is a schematic view illustrating the structure of a seal according to the prior art; and 
       FIG. 3  is a diagram illustrating the structure of the seal according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  depicts the overall structure of a centrifugal compressor, denoted by the general numerical reference  10 . 
   It is intended for handling a compressible fluid and its purpose is to transfer mechanical energy to this fluid so as to increase its pressure. 
   In the example depicted, the compressor  10  consists of a multi-cell compressor, that is to say a multi-stage compressor. The compressor  10  actually has four compression stages. 
   It essentially comprises a drive shaft  12  driven in rotation by appropriate drive means and rotating in a casing  14 . 
   This casing  14  is provided with an inlet E for the intake of compressible fluid, communicating with a supply source appropriate to the envisaged use, and with an outlet S for distributing the compressed fluid. 
   Between the inlet E and the outlet S the casing  14  is provided with the four compression stages  16 ,  18 ,  20  and  22 . 
   Each stage  16 ,  18 ,  20  and  22  comprises, from the upstream end downstream, considering the direction of flow of the fluid through the compressor  10 , an inlet guide  24 ,  26 ,  28  and  30  which guides the flow in the best direction for letting it into the compression stage, a bladed wheel  32 ,  34 ,  36 ,  38  actually transmitting the mechanical energy supplied by the drive shaft to the compressible fluid, some of the mechanical energy introduced being converted into pressure, some more being converted into speed, and a straightener or diffuser  40 ,  42 ,  44  and  46  reducing the speed of the fluid with a view to converting the dynamic pressure thereof into static pressure. 
   As can be seen in this  FIG. 1 , the last stage  22  of the compressor opens, at the downstream end, into a volute  48  of evolutive cross section constituting a final diffuser able to reduce the losses between the last compression stage and the outlet S. 
   Furthermore, the compressor is provided with seals, denoted by the general numerical references  50  and  52 , with which the casing is equipped near the shaft outlets and which contain the pressurized fluid within the casing  14 . The arrangements of these seals may be single (just one seal towards the outside), double (one seal towards the outside with sealing gas) or tandem (two seals in series towards the outside), depending on the application. 
     FIG. 2  depicts the overall structure of a seal according to the prior art. For example, this seal corresponds to the seal denoted by the general numerical reference  50 . In this  FIG. 2 , elements identical to those of  FIG. 1  bear the same reference numerals. 
   This seal comprises a rotary gland washer or rotating seal face ring  54  which rotates as one with a sleeve  56 , itself fixed to the shaft  12  of the compressor, and a stationary seal face ring  58  fixed to the casing  14  and able to move axially with respect to the latter. 
   Elastically deformable means, consisting of a spring  60 , urge the stationary seal face ring  58  to press against the rotating seal face ring  54  via their respective friction faces  62  and  64 . 
   Addition sealing means (not depicted) provide sealing between the seal face rings  54  and  58  and the elements of the compressor on which they are mounted. 
   As indicated by the arrow F, in operation, a leak of compressible fluid occurs between the sealing interfaces  62  and  64  of the seal, that is to say between the friction faces of the rotating and stationary seal face rings. 
   The casing  14  of the compressor is therefore provided with a passage  66  for flow of the leaking gas, opening, for example, into a network of flares (not depicted). 
   As indicated previously, this type of seal has one major disadvantage relating to its thermal operation. 
   What actually happens, during rotation, is that recirculation of gas from the compressor thermally conditions the seal, that is to say removes heat energy, as is known per se. When the compressor is not running, these circulations no longer exist because there is no longer any natural pressure difference, and therefore play no part either in cooling the seal or in heating the leaks. 
   On leaving the sealing interface consisting of the friction faces  62  and  64  of the seal face rings, the fluid experiences expansion leading to a consequential drop in the temperature thereof such that hydrates can be created. 
   As is well known per se, these hydrates consist of solid compounds which carry the risk of jamming the stationary seal face ring and consequently of impeding the operation of the compressor. 
   A seal that makes it possible to alleviate this drawback will now be described with reference to  FIG. 3 . 
   In the exemplary embodiment depicted in that figure, this seal consists of a seal of the tandem type, that is to say that it has two seals  68  and  70  arranged in series. 
   Each sealing proper comprises a stationary seal face ring  71  mounted on a casing  72  of the compressor and a rotating seal face ring or rotary gland washer  73  secured to a sleeve  74 , itself mounted on the drive shaft  76  of the compressor. 
   An elastic means consisting of a spring  78  urges the sealing interfaces consisting of the opposing friction faces  80  and  82  of the stationary and rotating seal face rings against one another. 
   As mentioned previously, leaks, denoted by the arrow F′, give rise to a flow of compressible fluid between the sealing interfaces  80  and  82 , which flow through a flow passage  84  produced for that purpose in the casing  72 . 
   With a view to alleviating the disadvantages associated with the expansion of the compressible fluid downstream of the sealing interfaces  80  and  82 , the casing  72  is provided with means of circulating a heating fluid, these means being arranged in the form of a heating duct  86  running at least partially downstream of the seal face rings  71  and  73 . 
   This duct  86  is connected to a supply source of heating fluid, for example an oil whose heat capacity is suitable for allowing effective transfer of heat energy to the passage  84 . 
   It will be noted that the temperature and the pressure of the fluid are chosen so as to ensure sufficient heating of the fluid in the flow passage  84  and to prevent the formation of hydrates. 
   As can be seen in  FIG. 3 , with a view to affording effective transfer of heat energy to the fluid circulating through the passage  84 , the heating duct  86  is formed in the passage  84  and is produced by placing in the latter a wall  88  which, with the casing  72 , delimits the duct  86 . 
   The wall  88  comprises, it its face facing towards the duct  86 , ribs, such as  90 , so as to form a heating coil in the duct  86 . 
   As will be appreciated, the wall  88  is also chosen to form an area for heat exchange which is large enough to, in conjunction with the temperature and pressure levels of the heating fluid circulating in the duct  86 , prevent the formation of hydrates in the fluid circulating through the leaks flow passage  84 . 
   As will be appreciated, the hydrates formation temperature depends on the composition of the fluid being handled by the compressor. As the determining of the parameters that will allow the temperature of the fluid to be raised to a level higher than this hydrate formation temperature is within the competence of the person skilled in the art, it will therefore not be described in detail hereinafter. 
   It will also be noted that the wall  88  is made of a material able to withstand relatively high pressures that there may be in these seals if the seal becomes damaged in service, so as to avoid letting flammable process gas out into the atmosphere. 
   Finally, as far as the heating fluid supply source is concerned, this preferably consists of a supply source that operates independently of the remainder of the compressor and, in particular, of the compressible fluid supply source. The independence of this supply source is also such as to allow it great availability, which is important for the reliability of this seal. 
   It is thus possible to heat up the fluid circulating through the leak flow passage  84 , even when the compressor is not running. 
   This then avoids any risk of the stationary seal face ring becoming immobilized with respect to the rotating parts of the compressor, even when the latter is not running. 
   This then gets around any need to empty or decompress the installation in which the compressor is incorporated. 
   It will finally be noted that the invention is not restricted to the embodiment envisaged. 
   This device also plays a part in cooling the dry seal when it is running in rotation. It is actually also possible, as an alternative, to supplement the compressor with an additional heat exchanger which heats up the fluid in the seal or seals, for example placed facing the rotating seal face ring.