Patent Publication Number: US-2022221059-A1

Title: Seal device

Description:
TECHNICAL FIELD 
     The present invention relates to a seal device that seals a relative rotation portion in a rotating machine suitable for a propulsion machine of a ship or a tidal current generator, or the like. 
     BACKGROUND ART 
     Conventionally, a seal device provided in a rotating machine included in a propulsion machine of a ship or a tidal current generator, or the like seals an annular gap formed at a relative rotation portion to prevent a sealed fluid such as lubricating oil, or the like in an interior of equipment from leaking to the outside of equipment and to prevent an external fluid such as seawater from entering the interior of equipment. 
     For example, a seal device suitable for a propulsion machine of a ship disclosed in Patent Citation 1 is held by a housing with a shaft hole, through which a rotating shaft for propulsion extends, and includes three seal rings being in sliding contact with an outer peripheral surface of a liner fitted onto the rotating shaft and provided in parallel to one another in an axial direction. Air is supplied to a primary annular chamber formed between a pair of the seal rings provided on an outboard side among the seal rings, and lubricating oil is supplied to a secondary annular chamber formed between a pair of the seal rings provided on an inboard side. 
     Specifically, the air supply is adjusted so that the primary annular chamber maintains an internal pressure of a seawater pressure plus a tightening pressure on the seal ring, and the supply of lubricating oil is adjusted so that the secondary annular chamber has an internal pressure required to blow out the air supplied to the primary annular chamber through the seal rings to the outboard side, so that the internal pressures in the primary annular chamber and the secondary annular chamber are constantly adjusted according to fluctuations in seawater pressure. Accordingly, a response to fluctuations in seawater pressure is high, and entry of seawater into the ship can be prevented. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Citation 1: JP 11-304005 A (Page 4, FIG. 2) 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, the seal device in Patent Citation 1 blows the air from the primary annular chamber to the outboard side by utilizing the internal pressure in the secondary annular chamber, to which the lubricating oil is supplied, to prevent the entry of seawater into the ship, and the internal pressure in the secondary annular chamber is constantly adjusted so as to be higher than the internal pressure in the primary annular chamber. Accordingly, in case of failure of the seal rings, the high-pressure lubricating oil in the secondary annular chamber may enter the primary annular chamber through the seal rings, and the lubricating oil may be blown out from the primary annular chamber to the low-pressure outboard side together with the air to cause leakage of the lubricating oil to the outside of the ship. 
     The present invention has been made in view of such problems, and an object of the present invention is to provide a seal device capable of reliably preventing leakage of a sealed fluid to the outside of equipment. 
     Solution to Problem 
     In order to the problems described above, a seal device according to the present invention is a seal device provided at a portion where relative rotation occurs between a first member and a second member that rotates relative to the first member, comprising a first seal part facing an external fluid and a second seal part arranged in parallel to the first seal part and facing a sealed fluid in an interior of equipment to prevent entry of the external fluid and leakage of the sealed fluid, wherein the seal device further comprises an intermediate seal part arranged in parallel to the first seal part and the second seal part between the first seal part and the second seal part, a gas chamber is formed between the first seal part and the intermediate seal part, to which a gas having a higher pressure than the external fluid is supplied, and an intermediate chamber is formed between the intermediate seal part and the second seal part, to which a gas having a lower pressure than the gas supplied to the gas chamber and having a lower pressure than the sealed fluid is supplied. According to the aforesaid feature of the present invention, in case of failure in which the sealed fluid facing the second seal part enters the intermediate chamber through this second seal part, this sealed fluid can be stored in the intermediate chamber lower in pressure than the adjacent gas chamber on the equipment exterior side. Accordingly, leakage of the sealed fluid to the outside of equipment can be reliably prevented. 
     It may be preferable that the second seal part is a lip seal, and be arranged so that a lip portion thereof is pressed against the second member by the sealed fluid. According to this preferable configuration, the pressure of the sealed fluid having a higher pressure than the gas in the intermediate chamber can act as a tightening pressure on the lip seal, so that the sealed fluid is less likely to enter the intermediate chamber. 
     It may be preferable that the intermediate chamber may communicate with a collection chamber through a communication passage. According to this preferable configuration, the sealed fluid that has entered the intermediate chamber is collected in the collection chamber through the communication passage, so that the sealed fluid in the intermediate chamber can be easily discharged to the collection chamber that is a separate chamber. 
     Accordingly, the leakage of the sealed fluid to the outside of equipment can be further prevented. 
     It may be preferable that the communication passage may be provided with a check valve that prevents backflow to a side of the intermediate chamber. According to this preferable configuration, the sealed fluid collected in the collection chamber can be prevented from flowing back into the intermediate chamber through the communication passage. 
     It may be preferable that the gas supplied to the intermediate chamber is a pressure-controlled compressed gas. According to this preferable configuration, the differential pressure between the pressure-controlled compressed gas and the gas supplied to the gas chamber and the differential pressure between the pressure-controlled compressed gas and the sealed fluid can be properly maintained. 
     It may be preferable that the compressed gas is compressed air. According to this preferable configuration, it is easy to handle, and safety can be ensured. 
     It may be preferable that the sealed fluid is controlled to have a lower pressure than the external fluid. According to this preferable configuration, the sealed fluid that has entered the intermediate chamber is less likely to leak to the outside of equipment. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a stern tube seal system with a seal device according to a first embodiment of the present invention. 
         FIG. 2  is an enlarged schematic view showing the seal device according to the first embodiment. 
         FIG. 3  is a schematic diagram illustrating a stern tube seal system with a seal device according to a second embodiment of the present invention. 
         FIG. 4  is an enlarged schematic view showing the seal device according to the second embodiment. 
         FIG. 5  is a schematic diagram illustrating a modification of the stern tube seal system with the seal device according to the second embodiment. 
         FIG. 6  is an enlarged schematic view showing a seal device according to a third embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Modes for carrying out a seal device according to the present invention will be described below based on embodiments. 
     First Embodiment 
     A seal device according to a first embodiment of the present invention will be described with reference to  FIGS. 1 and 2 . In this embodiment, a seal device for a ship propulsion machine will be described as an example. Further, the left side of the drawing sheets of  FIGS. 1 and 2  will be described as a stern side (e.g., an outboard side) of the seal device, and the right side of the drawing sheets of  FIGS. 1 and 2  will be described as a bow side (e.g., an inboard side) of the seal device. In  FIGS. 2 and 4 , hatching a housing, and the like is omitted, and the compressed air and the lubricating oil that are supplied to and stored in each chamber are schematically shown by hatching. 
     As shown in  FIG. 1 , a seal device  1  according to the present invention is a shaft seal device for a ship propulsion machine, and is attached from the stern side to a stern tube  100 , through which a propeller shaft  2  with a propeller  3  for propulsion extends. The seal device  1  is used to prevent lubricating oil as a sealed fluid supplied to the interior of the stern tube  100  constituting a hull to lubricate the propeller shaft  2  and a bearing (not shown) from leaking to the outside of the ship, and to prevent seawater W as an external fluid from entering the interior of the ship. The seal device  1  is also connected with an air control unit  120 , a lubricating oil circulation unit  130 , and a collection unit  140 , which are provided inboard, by respective pipelines, to constitute a stern tube seal system. Further, a seal device  40  provided inboard is attached to the stern tube  100  from the bow side, and is a shaft seal device to prevent the lubricating oil supplied to the interior of the stern tube  100  from entering a machine room. In this embodiment, the seal device  1  on the stern side will be described, and the description on the seal device  40  on the bow side will be omitted. 
     As shown in  FIGS. 1 and 2 , the seal device  1  is provided at a portion where relative rotation occurs between a housing  10  as a first member and a liner  4  that constitutes the propeller shaft  2  as a second member rotating relative to the housing  10 . The seal device  1  mainly includes a first lip seal  21  as first seal part facing the outboard seawater W, a second lip seal  23  as second seal part arranged in parallel on the inboard side of the first lip seal  21  and facing lubricating oil that fills an oil chamber  33  in the stern tube  100 , and an intermediate lip seal  22  as intermediate seal part arranged in parallel between the first lip seal  21  and the second lip seal  23 . 
     As shown in  FIG. 2 , a first divided housing  10   a , a second divided housing  10   b , a third divided housing  10   c , and a fourth divided housing  10   d  in order from the stern side are fitted to one another in an axial direction and are integrally connected with one another by bolts or the like (not shown), so that the housing  10  is formed into a substantially cylindrical shape. The housing  10  is fixed by bolts or the like (not shown) with a flange portion of the fourth divided housing  10   d  on the bow side being in contact with the stern tube  100  on the stern side. 
     Further, in the housing  10 , an outer diameter portion of the first lip seal  21  is held in a substantially sealed manner between the first divided housing  10   a  and the second divided housing  10   b , an outer diameter portion of the intermediate lip seal  22  is held in a substantially sealed manner between the second divided housing  10   b  and the third divided housing  10   c , and an outer diameter portion of the second lip seal  23  is held in a substantially sealed manner between the third divided housing  10   c  and the fourth divided housing  10   d.    
     Further, a through hole is formed in the second divided housing  10   b , the third divided housing  10   c , and the fourth divided housing  10   d  to constitute a part of a first air supply passage  11  communicating with the air control unit  120  and a first annular chamber  31  as a gas chamber. A supply port  11   a  in communication with the first air supply passage  11  is formed on an upper side of an inner peripheral surface of the second divided housing  10   b.    
     Further, a through hole is formed in the third divided housing  10   c  and the fourth divided housing  10   d  to constitute a part of a second air supply passage  12  communicating with the air control unit  120  and a second annular chamber  32  as an intermediate chamber. A supply port  12   a  in communication with the second air supply passage  12  is formed on an upper side of an inner peripheral surface of the third divided housing  10   c . In addition, a through hole is formed in the third divided housing  10   c  and the fourth divided housing  10   d  to constitute a part of a communication passage  14  communicating with the second annular chamber  32  and the collection unit  140 . A discharge port  14   a  in communication with the communication passage  14  is formed on a lower side of an inner peripheral surface of the third divided housing  10   c.    
     As shown in  FIG. 2 , the lip seals  21 ,  22 , and  23  are made of an elastic material such as fluoro-rubber or nitrile rubber that has excellent water resistance and oil resistance. Each of the lip seals  21 ,  22 , and  23  according to this embodiment has a well-known configuration, and detailed description thereof will be omitted. 
     Further, the lip seals  21 ,  22 , and  23  are arranged in parallel in the axial direction with their outer diameter portions held in a substantially sealed manner in the housing  10 . Respective inner peripheral surfaces of lip portions  21   a ,  22   a , and  23   a  extending on the inner diameter side and extending in the axial direction toward the high pressure side are in sliding contact with an outer peripheral surface of the liner  4  fitted onto the propeller shaft  2 , so that the first annular chamber  31  is formed between the first lip seal  21  and the intermediate lip seal  22 , and the second annular chamber  32  is formed between the intermediate lip seal  22  and the second lip seal  23 . Further, the annular oil chamber  33  is formed between the second lip seal  23  and a lip seal  41  on the stern side of the seal device  40  on the bow side. 
     The first annular chamber  31  is supplied with compressed air as a compressed gas adjusted so as to have a higher pressure than the seawater W from the air control unit  120  through the first air supply passage  11 . Further, the second annular chamber  32  is supplied with compressed air as a compressed gas adjusted so as to have a lower pressure than the compressed air supplied from the air control unit  120  to the first annular chamber  31  through the second air supply passage  12 , and so as to have a lower pressure than the lubricating oil supplied from the lubricating oil circulation unit  130  to the oil chamber  33  in the stern tube  100 . 
     Further, the first lip seal  21  provided on the stern side of the first annular chamber  31  is arranged so that the lip portion  21   a  thereof faces the outboard side, and the seawater pressure P W  acts as a part of a tightening pressure on the lip portion  21   a . Further, the intermediate lip seal  22  provided on the bow side of the first annular chamber  31  and on the stern side of the second annular chamber  32  is arranged so that the lip portion  22   a  thereof faces the outboard side, that is, the side of the first annular chamber  31 , and an air pressure P 1  in the first annular chamber  31  acts as a part of a tightening pressure on the lip portion  22   a . Further, the second lip seal  23  provided on the bow side of the second annular chamber  32  is arranged so that the lip portion  23   a  thereof faces the inboard side, that is, the side of the stern tube  100 , and a hydraulic pressure P O  in the oil chamber  33  in the stern tube  100  acts as a part of a tightening pressure on the lip portion  23   a.    
     Next, the air control unit  120 , the lubricating oil circulation unit  130 , and the collection unit  140  that constitute the stern tube seal system together with the seal device  1  will be described. 
     As shown in  FIG. 1 , the air control unit  120  is a unit that supplies the compressed air supplied from a compressor (not shown) provided inboard, whose pressure is adjusted by a pressure reducing valve, a flow rate control valve, or the like (not shown), to the first annular chamber  31 , the second annular chamber  32 , and a lubricating oil tank  131  of the lubricating oil circulation unit  130  described later, through the first air supply passage  11 , the second air supply passage  12 , and the third air supply passage  13 . 
     Specifically, the air control unit  120  adjusts the air pressure P 1  in the first annular chamber  31  so that the air pressure P 1  always exceeds the seawater pressure P W  in accordance with the seawater pressure P W  that presses the lip portion  21   a  of the first lip seal  21  arranged on the stern side of the first annular chamber  31 . 
     The seawater pressure P W  fluctuates according to the draft of the ship, and the air control unit  120  adjusts the air pressure in accordance with such fluctuations in seawater pressure μW. 
     Further, the signal corresponding to the seawater pressure P W , that is, the signal corresponding to the air pressures in the first air supply passage  11  and the first annular chamber  31  is output to a pressure reducing valve  122  that is as a part of the air control unit  120  and is provided at the branch portion between the first air supply passage  11  and the second air supply passage  12 . A part of the compressed air is supplied to the second annular chamber  32  through the second air supply passage  12  as compressed air having the air pressure P 2  that has been reduced based on the output signal during the passage through the pressure reducing valve  122  so as to be lower than the air pressure P 1  of the compressed air supplied to the first annular chamber  31  and so as to be a lower than the hydraulic pressure P O  of the lubricating oil supplied to the oil chamber  33  in the stern tube  100  by a preset differential pressure. 
     The lubricating oil circulation unit  130  is a unit that supplies lubricating oil from the lubricating oil tank  131  provided inboard to the interior of the stern tube  100  through a first lubricating oil circulation passage  132  with a pump  134 , and again returns the lubricating oil from the interior of the stern tube  100  to the lubricating oil tank  131  through a second lubricating oil circulation passage  133  to circulate the lubricating oil. The signal corresponding to the seawater pressure P W  is output to a pressure reducing valve  123  that is a part of the air control unit  120  and is provided at the branch portion between the first air supply passage  11  and the third air supply passage  13 . A part of the compressed air is adjusted based on the output signal during passing through the pressure reducing valve  123  so as to have a higher pressure than the air pressure P 2  of the compressed air supplied to the second annular chamber  32 , and so as to have a lower pressure than the seawater pressure P W  by a preset differential pressure or so as to have the same pressure as the seawater pressure P W , and is then supplied to the lubricating oil tank  131  through the third air supply passage  13 . Accordingly, the lubricating oil in the lubricating oil tank  131  is brought into contact with the such pressure-reduced compressed air with the oil surface as a boundary surface, so that the hydraulic pressure P O  of the lubricating oil in the lubricating oil tank  131  is adjusted so as to be higher than the air pressure P 2  of the second annular chamber  32  and so as to be lower than or be the same as the seawater pressure P W . 
     That is, the seawater pressure P W , the air pressure P 1  in the first annular chamber  31 , the air pressure P 2  in the second annular chamber  32 , and the hydraulic pressure P O  in the oil chamber  33  are adjusted so that the air pressure P 1 &gt;the seawater pressure P W ≥the hydraulic pressure P O &gt;the air pressure P 2  is always satisfied. 
     The collection unit  140  is a unit that in case of failure in which lubricating oil enters the second annular chamber  32  from the oil chamber  33  in the stern tube  100 , collects this lubricating oil in a collection chamber  141  provided inboard through the communication passage  14 . Further, the communication passage  14  is provided with a check valve  142  to prevent the lubricating oil collected in the collection chamber  141  from flowing back into the second annular chamber  32 . 
     As described above, in the seal device  1  according to this embodiment, the compressed air, whose pressure has been reduced to the air pressure P 2  by the air control unit  120  so as to be lower than the air pressure P 1  of the compressed air supplied to the first annular chamber  31  through the first air supply passage  11 , is supplied to the second annular chamber  32  through the second air supply passage  12 . Accordingly, even in case of failure in which the lubricating oil in the oil chamber  33  facing the second lip seal  23  enters the second annular chamber  32  through the second lip seal  23 , the lubricating oil that has the air pressure P 2  lower than that in the adjacent first annular chamber  31  on the outboard side and has entered the second annular chamber  32 , can be retained, and the leakage of the lubricating oil to the outside of the ship can be reliably prevented. 
     In addition, the lip portion  22   a  of the intermediate lip seal  22  provided on the stern side of the second annular chamber  32  is arranged so as to face the side of the first annular chamber  31 , so that the lip portion  22   a  of the intermediate lip seal  22  is pressed radially inward toward the outer peripheral surface of the liner  4  due to the differential pressure between the air pressure P 1  in the first annular chamber  31  and the air pressure P 2  in the second annular chamber  32  to the seal the lubricating oil. Accordingly, the lubricating oil that has entered the second annular chamber  32  is prevented from entering the first annular chamber  31 , and the leakage of the lubricating oil to the outside of the ship can be further prevented. 
     Further, in case of failure in which the lubricating oil in the oil chamber  33  in the stern tube  100  enters the second annular chamber  32  through the second lip seal  23 , the lubricating oil that has entered the second annular chamber  32  is collected in the inboard collection chamber  141  through the communication passage  14 . Accordingly, the lubricating oil can be easily discharged from the interior of the second annular chamber  32 , so that the leakage of the lubricating oil to the outside of the ship can be further prevented. 
     Further, the pressure of the lubricating oil supplied from the lubricating oil circulation unit  130  to the oil chamber  33  in the stern tube  100  is reduced so as to be lower than the seawater pressure P W  by a preset differential pressure, so that the hydraulic pressure P O  in the oil chamber  33  is controlled to be lower than the seawater pressure μW. Accordingly, even if the lubricating oil enters the second annular chamber  32 , it is less likely to leak to the outside of the ship. 
     Further, the pressure of the compressed air supplied from the air control unit  120  to the second annular chamber  32  is reduced so as to be lower than that of the lubricating oil supplied to the oil chamber  33  in the stern tube  100  by a preset differential pressure, so that the lip portion  23   a  of the second lip seal  23  is pressed radially inward toward the outer peripheral surface of the liner  4  due to the differential pressure between the pressure in the second annular chamber  32  and the pressure in the oil chamber  33  to seal the lubricating oil. Accordingly, the lubricating oil supplied to the oil chamber  33  can be prevented from entering the second annular chamber  32 . In addition, the compressed air supplied to the second annular chamber  32  can be also prevented from entering the oil chamber  33 . 
     Further, the second annular chamber  32  is supplied with the pressure-adjusted compressed air through the air control unit  120 , and the differential pressure from the compressed air supplied to the first annular chamber  31  or from the lubricating oil supplied into the oil chamber  33  can be properly maintained. The load particularly on the intermediate lip seal  22  and the second lip seal  23  that constitute the second annular chamber  32  can be maintained substantially constant, and their long life can be extended. 
     Further, since the first annular chamber  31  and the second annular chamber  32  are supplied with the compressed air through the air control unit  120 , it is easy to handle and safety can be ensured. 
     Further, the compressed air whose pressure is adjusted so as to be air pressure P 1  equal to or higher than the seawater pressure P W  in the air control unit  120  is supplied to the first annular chamber  31  through the first air supply passage  11 , so that the seawater W is prevented from entering the annular chamber  31  from a gap between the lip portion  21   a  of the first lip seal  21  and the liner  4 . In addition, the air having the air pressure P 1  supplied to the first annular chamber  31  is blown from the gap between the lip portion  21   a  of the first lip seal  21  and the liner  4  to the outside of the ship against the seawater pressure P W , so that the fluctuations in seawater pressure P W  can be detected in the air control unit  120 . Accordingly, other additional detection tanks or pipes are not required. 
     Further, the air pressure P 2  in the second annular chamber  32  is set to be lower than the seawater pressure P W , so that the hydraulic pressure P O  in the oil chamber  33  can be less than the seawater pressure P W . Accordingly, pump equipment for increasing the pressure of lubricating oil or the lubricating oil tank  131  is not required to be installed at a high place, and the entire stern tube seal system can be simplified. 
     The second annular chamber  32  is preferably provided with a pressure control valve such as a pressure reducing valve or a relief valve so that when the high-pressure compressed air enters from the first annular chamber  31 , an excess pressure can be discharged. This can prevent air from entering the oil chamber  33 . 
     Second Embodiment 
     Next, a seal device according to a second embodiment of the present invention will be described with reference to  FIGS. 3 and 4 . The same components as those shown in the embodiment described above are designated by the same reference numerals, and overlapping description will be omitted. 
     A seal device  201  according to the second embodiment of the present invention will be described. As shown in  FIG. 3 , the seal device  201  is provided at a portion where relative rotation occurs between a housing  210  as a first member and the liner  4  that constitutes the propeller shaft  2  as a second member rotating relative to the housing  210 . The seal device  201  mainly includes the first lip seal  21 , the intermediate lip seal  22 , the second lip seal  23 , and an auxiliary lip seal  24  arranged in parallel on the bow side of the second lip seal  23 . 
     As shown in  FIG. 4 , the first divided housing  10   a , the second divided housing  10   b , the third divided housing  10   c , the fourth divided housing  210   d , and a fifth divided housing  210   e  in order from the stern side are fitted to one another in an axial direction and are integrally connected with one another by bolts or the like (not shown), so that the housing  210  is formed into a substantially cylindrical shape. 
     Further, an outer diameter portion of the auxiliary lip seal  24  is held in a substantially sealed manner between the fourth divided housing  210   d  and the fifth divided housing  210   e  in the housing  210 . In addition, an annular second oil chamber  34  is formed between the second lip seal  23  and the auxiliary lip seal  24 . 
     Further, a through hole is formed in the fourth divided housing  210   d  and the fifth divided housing  210   e  to constitute a part of a branch communication passage  234  that branches from the first lubricating oil circulation passage  132  extending from the lubricating oil circulation unit  130  and communicates with the second oil chamber  34 . A supply port  234   a  in communication with the branch communication passage  234  is formed on the lower side of an inner peripheral surface of the fourth divided housing  210   d.    
     The branch communication passage  234  is provided with an on/off valve  235 . Further, the lubricating oil tank  131  constituting the lubricating oil circulation unit  130  is configured as a pressurized tank capable of adjusting the pressure of the lubricating oil according to the pressure of the compressed air supplied from the air control unit  120  through the third air supply passage  13 . 
     Further, the second lip seal  23  provided on the stern side of the second oil chamber  34  is arranged so that the lip portion  23   a  thereof faces the inboard side, that is, the side of the second oil chamber  34 , and the hydraulic pressure P O2  in the second oil chamber  34  acts as a part of a tightening pressure on the lip portion  23   a . In addition, the auxiliary lip seal  24  provided on the bow side of the second oil chamber  34  is arranged so that the lip portion  24   a  thereof faces the inboard side, that is, the side of the stern tube  100 , and the hydraulic pressure P O1  in the oil chamber  33  in the stern tube  100  acts as a part of a tightening pressure on the lip portion  24   a.    
     As described above, in the seal device  201  according to this embodiment, the lip portion  23   a  of the second lip seal  23  is arranged so as to face the side of the second oil chamber  34 , so that the lip portion  23   a  of the second lip seal  23  is pressed radially inward toward the outer peripheral surface of the liner  4  due to the differential pressure between the second annular chamber  32  and the second oil chamber  34  to seal the lubricating oil. Accordingly, the lubricating oil supplied to the second oil chamber  34  can be prevented from entering the second annular chamber  32 . In addition, the lubricating oil supplied from the lubricating oil circulation unit  130  to the second oil chamber  34  through the branch communication passage  234  is sealed by the second lip seal  23 , so that the hydraulic pressure P O2  in the second oil chamber  34  is equal to or higher than the hydraulic pressure P O1  in the oil chamber  33  (i.e., the hydraulic pressure P O2 ≥ the hydraulic pressure P O1 ). Accordingly, the lubricating oil can flow into the oil chamber  33  from a gap between the lip portion  24   a  of the auxiliary lip seal  24  and the liner  4 , and the lubricating oil supplied to the second oil chamber  34  is less likely to enter the second annular chamber  32 . The lubricating oil that has flowed out from the second oil chamber  34  into the oil chamber  33  returns to the lubricating oil tank  131  through the second lubricating oil circulation passage  133 . 
     Further, since the branch communication passage  234  is provided with the on/off valve  235 , switching can be made so that, for example, when the second lip seal  23  is damaged, the on/off valve  235  is closed to stop the supply of the lubricating oil to the second oil chamber  34 , and the auxiliary lip seal  24  mainly seals the lubricating oil. 
     Further, the lubricating oil tank constituting the lubricating oil circulation unit is not limited to the one configured as a pressurized tank capable of adjusting the pressure of the lubricating oil according to the pressure of the compressed air supplied from the air control unit  120 . For example, as a modification of the stern tube seal system with the seal device  201  according to the second embodiment, as shown in  FIG. 5 , a lubricating oil tank  231  constituting a lubricating oil circulation unit  230  may be configured as a gravity tank that supplies the lubricating oil at a constant pressure by gravity. The lubricating oil circulation unit  230  shown in  FIG. 5  may be applied to the stern tube seal system with the seal device  1  according to the first embodiment. 
     Third Embodiment 
     Next, a seal device according to a third embodiment of the present invention will be described with reference to  FIG. 6 . The same components as those shown in the embodiments described above are designated by the same reference numerals, and overlapping description will be omitted. In this embodiment, a seal device for a center-open type tidal current generator will be described as an example. Further, the left side of the drawing sheet of  FIG. 6  will be described as an inner diameter side of the seal device, and the right side of the drawing sheet of  FIG. 6  will be described as an outer diameter side of the seal device. 
     A seal device  301  according to the third embodiment of the present invention will be described. As shown in  FIG. 6 , the seal device  301  for a center-open type tidal current generator prevents the lubricating oil from flowing to the outside of equipment, which is supplied to the interior of equipment in order to lubricate a pair of bearings  304 ,  304  provided between an annular rotor  302  including a plurality of blades  303  on the inner peripheral surface and an annular power generation unit  307  arranged on the outer diameter side of the rotor  302  and including a coil  306 , and prevents the seawater W from entering the interior of equipment. The rotor  302  is provided with a magnet  305  extending in the circumferential direction of the outer peripheral surface, and the rotor  302  that has received the tidal current from the blades  303  rotates relative to the power generation unit  307  to generate power. 
     The seal device  301  is provided at a portion where relative rotation occurs between a pair of annular housings  310 ,  310  as first members attached from the inner diameter side to a pair of annular casings  300 ,  300  arranged in front of and in back of the rotor  302  and the power generation unit  307 , and the rotor  302  as a second member rotating relative to the housings  310 ,  310 . The seal device  301  mainly includes a pair of annular first lip seals  321 ,  321  as a first seal part facing the seawater W, a pair of second lip seals  323 ,  323  as a second seal part arranged in parallel to the first lip seals  321 ,  321 , respectively, and facing the lubricating oil in the interior of equipment, and intermediate lip seals  322 ,  322  as an intermediate seal part arranged in parallel between the first lip seals  321 ,  321  and the second lip seals  323 ,  323 , respectively. 
     As shown in  FIG. 6 , a first divided housing  310   a , a second divided housing  310   b , a third divided housing  310   c , and a fourth divided housing  310   d  in order from the inner diameter side are fitted to one another and are integrally connected with one another by bolts or the like (not shown), so that the housings  310 ,  310  are each formed into a substantially cylindrical shape. The housings  310 ,  310  are fixed by bolts or the like (not shown) with respective flange portions of the first divided housings  310   a ,  310   a  on inner diameter side being fitted to the inner peripheral surfaces of the casings  300 ,  300 . 
     Further, in each of the housings  310 ,  310 , one end portion of the first lip seal  321  is held in a substantially sealed manner between the first divided housing  310   a  and the second divided housing  310   b , one end portion of the intermediate lip seal  322  is held in a substantially sealed manner between the second divided housing  310   b  and the third divided housing  310   c , and one end portion of the second lip seal  323  is held in a substantially sealed manner between the third divided housing  310   c  and the fourth divided housing  310   d . Further, a first annular chamber  331  is formed between the first lip seal  321  and the intermediate lip seal  322 , and a second annular chamber  332  is formed between the intermediate lip seal  322  and the second lip seal  323 . Further, an annular oil chamber  333  is formed between the second lip seal  323  and the bearing  304 . 
     Further, through holes are formed in the second divided housings  310   b ,  310   b  to constitute a part of the first air supply passages  311 ,  311  communicating with the air control unit  120  provided in the interior of equipment and the first annular chambers  331 ,  331  as gas chambers. 
     Further, through holes are formed in the third divided housings  310   c ,  310   c  to constitute a part of the second air supply passages  312 ,  312  communicating with the air control unit  120  and the second annular chambers  332 ,  332  as intermediate chambers. 
     As described above, in the seal device  301  according to this embodiment, the compressed air, whose pressure has been reduced to the air pressure P 2  by the air control unit  120  so as to be lower than the air pressure P 1  of the compressed air supplied to the first annular chambers  331 ,  331  through the first air supply passages  311 ,  311 , is supplied to the second annular chambers  332 ,  332  through the second air supply passages  312 ,  312 . Accordingly, in case of failure in which the lubricating oil in the oil chamber  333  in the interior of equipment, which faces the second lip seals  323 ,  323 , enters the second annular chambers  332 ,  332  through the second lip seals  323 ,  323 , the lubricating oil that has entered the second annular chambers  332 ,  332  and has the air pressure P 2  lower than that in the adjacent first annular chambers  331 ,  331  on the equipment exterior side can be stored, and the lubricating oil can be reliably prevented from leaking to the outside of the ship. 
     Although the embodiments according to the present invention have been described above with reference to the drawings, the specific configurations are not limited to these embodiments, and any changes or additions within the scope of the gist of the present invention are included in the present invention. 
     For example, it goes without saying that in the seal device according to the embodiments described above, not only the lubricating oil but also the seawater W that has entered from the outside of equipment can be collected in the second annular chamber as the intermediate chamber. Further, the external fluid is not limited to seawater, and may be, for example, freshwater. Further, the sealed fluid is not limited to lubricating oil. 
     Further, in the embodiments described above, the air control unit  120  has been described as being configured to reduce the pressure of the compressed air with the pressure reducing valves  122 ,  123 , but the present invention is not limited to this, and various pressure control valves such as a relief valve may be used to reduce the pressure of the compressed air. Further, the compressed gas is not limited to compressed air. 
     Further, in the first and second embodiments, the mode in which the lip seals are in sliding contact with the outer peripheral surface of the liner  4  fitted onto the propeller shaft  2  has been described, but the present invention is not limited to this, and the lip seals are directly in sliding contact with the outer peripheral surface of the propeller shaft  2 . Further, in the embodiments described above, each sealing part is not limited to one composed of a lip seal. 
     Further, in the third embodiment described above, the seal device  301  applied to the center-open type tidal current generator has been described, but, for example, the seal device having the configuration according to the first or second embodiment may be applied to a turbine type tidal current generator. 
     REFERENCE SIGNS LIST 
     
         
           1  Seal device 
           2  Propeller shaft (second member) 
           4  Liner 
           10  Housing (first member) 
           11  First air supply passage 
           12  Second air supply passage 
           13  Third air supply passage 
           14  Communication passage 
           21  First lip seal (first seal part) 
           22  Intermediate lip seal (intermediate seal part) 
           23  Second lip seal (second seal part) 
           24  Auxiliary lip seal 
           31  First annular chamber (gas chamber) 
           32  Second annular chamber (intermediate chamber) 
           33  Oil chamber 
           34  Second oil chamber 
           100  Stern tube 
           120  Air control unit 
           122 ,  123  Pressure reducing valve 
           130  Lubricating oil circulation unit 
           131  Lubricating oil tank 
           132  First lubricating oil circulation passage 
           133  Second lubricating oil circulation passage 
           134  Pump 
           140  Collection unit 
           141  Collection chamber 
           142  Check valve 
           201  Seal device 
           210  Housing (first member) 
           230  Lubricating oil circulation unit 
           231  Lubricating oil tank 
           234  Branch communication passage 
           300  Casing 
           301  Seal device 
           302  Rotor (second member) 
           307  Power generation unit 
           310  Housing (first member) 
           311  First air supply passage 
           312  Second air supply passage 
           321  First lip seal (first seal part) 
           322  Intermediate lip seal (intermediate seal part) 
           323  Second lip seal (second seal part) 
           331  First annular chamber (gas chamber) 
           332  Second annular chamber (intermediate chamber) 
           333  Oil chamber