Patent Publication Number: US-7213660-B2

Title: Washpipe seal

Description:
BACKGROUND OF THE INVENTION 
   This is a continuation-in-part of prior application Ser. No. 11/174,687, filed Jul. 5, 2005, which is hereby incorporated herein by reference in its entirety. 

   The present invention relates generally to so-called washpipe seals and more particularly, to a washpipe seal which is adapted to permit controlled leakage of a small portion of high pressure drilling fluid which largely passes into the so-called washpipe and into the drill string which is attached thereto. In the oil field industry particularly, a so-called swivel assembly incorporates a seal at the washpipe to confine the major portion of the drilling fluid to the washpipe. This drilling fluid is sometimes colloquially referred to as “drilling mud” or simply “mud.” 
   In the drilling system, there are high pressure abrasive fluids which are sent down to the bore hole through the washpipe and the drill string, and ultimately to the drill motor which is held at the bottom of the drill string. This abrasive fluid is supplied to the motor which drives the cutting tools under very high pressure, e.g. 5,000 PSI to 7,500 PSI or even 10,000 PSI or more. A seal is necessary between the non-rotary upper connection assembly and the rotary lower connection assembly to prevent undue leakage of this drilling fluid to the outside. As a rule, the drilling fluid leakage is taken from the vicinity of the washpipe back to a remote container where it is again picked up and supplied to the drill string. The joint in question is between the upper connection assembly, which includes a housing for the stator assembly and the lower connection assembly which includes the rotor assembly and which is affixed to and rotates with the drill string. The rotor and housing are sometimes collectively referred to as a “cartridge.” 
   According to one embodiment of the invention, the leakage of the drilling fluid, which is operated from 5,000 PSI up to perhaps 10,000 PSI or more for example, is led through a labyrinth between alternating rotor/stator discs, which preferably include lands and grooves in each rotor/stator disc. As the flow moves along between each rotor/stator it moves a radial distance about equal to the radial extent of each stator and rotor and an axial distance approximately equal to the thickness of each rotor and stator unit. 
   As a consequence of passing through this labyrinth of discs, the drilling fluid in one embodiment gradually is subject to a reduced pressure and ultimately achieves a low pressure and passes to an outlet, where it is led via a conduit back to the source of drilling fluid. Here, the drilling fluid passes back into the washpipe and undergoes a repetition of the cycle. According to the invention, only a very small portion of the drilling fluid escapes through the labyrinth seal compared to the portion of drilling fluid directed down the drill string, and the seal of the invention may thus be considered a controlled leakage seal. 
   In another embodiment, the force of the controlled leakage drilling fluid is opposed by grease within the labyrinth, whereby the grease fills most or all of the labyrinth and is present at what, in another embodiment, would be the inlet for the drilling fluid. The grease thus passes backwardly through the labyrinth of rotors and stators up to the approximate point of entry of the drilling fluid. In this embodiment, the drilling fluid is sealed at the top rather than at the bottom of the labyrinth, and the grease extends back and totally fills the labyrinth defined by the rotors and stators. 
   In either of these embodiments, it is possible and in fact probable that the cartridge, comprising the housing, or the fixed portion of the cartridges, the rotor and its included portions will need to be replaced or serviced, often fairly frequently, during the continuation of their operation. In the past the manner in which the fixed housing and the rotor have been hooked up is such that, in order to maintain it or replace parts, the system would have to be disabled, and the parts taken apart by unscrewing them on one or both sides. Thereafter, removing them would be extremely difficult because of the structure surrounding the washpipe unit. In other words, a structure that could permit the ready removal of the housing and rotor, sometimes collectively called the cartridge, and permit replacement of them with very little or no difficulty, would be highly desirable. This is done by providing a master coupling with at least one opening and preferably by attaching the cartridges with flanges which are clamped together and easily taken apart. 
   One of the features of the present invention is that the stator discs and the rotor discs are interleaved and arranged with very small clearance spaces between them. Consequently, in the stator unit there are a plurality of spacers, with a stator disc between each set of spacers. Also, there are a plurality of rotors, each one in turn separated by spacers. Consequently, there is an alternating stack of interleaved rotors and spacers which maintain the desired clearance. Each disc, in one embodiment, has a plurality of grooves of perhaps 0.005 to 0.050 inches, as well as a large plurality of lands which extend into the grooves, perhaps 0.005 to 0.040 inches by way of example. Each of the stator spacers and the rotor spacers is preferably held snugly in the desired configuration of a stack by Belleville washers or other similar units, and each of the spacers includes an O-ring or similar packing to ensure the correct alignment and spacing and to insure that there is no leakage through a secondary route. Thus, the spacers are held snugly in place by Belleville washer and the o-rings under compression. In addition, the rotor disks and the stators are held onto the inner and outer housings by keyways having locking pins associated with them, as will appear. 
   At the top of one embodiment of the novel rotor/stator assembly of the invention, is an annular space between sections of the housing and above the labyrinth. The drilling fluid pressure at this point is perhaps 5,000 to 10,000 PSI, but by reason of passing through the extended labyrinth with perhaps 16 to 20 reversals of radial direction and 16 to 20 axial movements as the drilling fluid moves along a tortuous path, the pressure confining the axial fluid gradually lowers until the fluid reaches the outlet, where it is under relatively low pressure and relatively low flow. 
   In one preferred embodiment, there is a flow meter in series relation with the drilling fluid escaping from the outlet. In this way, if there is an unexpected or sudden increase in flow, indicative of failure, steps may be taken to promptly rectify any malfunction. 
   In one embodiment of the prior art, there have been a series of seals, each seeing the highest pressure, and these seals would then fail, one at a time, until there remained only the final seal. This solution was not satisfactory, especially because of a projected seal life of only 50 to 300 hours. The present invention overcomes the difficulties with such seal designs by allowing the pressure to gradually bleed off rather than completely fail in a series of steps, each of which would see all of the pressure. 
   Because the fluid to be sealed is highly abrasive, the washpipe and the housing are preferably made from an oil field grade of steel and the rotor and stator its associated parts are made from tungsten carbide or other wear-resistant alloys, or other materials with wear-resistant coatings which are also used to provide radial and thrust bearings where the washpipe meets the upper housing. Such parts could also be made from ceramics or other like material. These bushings or bearings are greased through one or more fittings and grease passages which are provided for that purpose. 
   A known type of seal or packing is used between the lowermost portion of the washpipe and the stator and rotor housings, but this seal or packing sees only the reduced or lowermost pressure which is sensed at the outlet for the drilling fluid and accordingly, such seal is not exposed to high pressure in use. 
   It is therefore an object of the present invention to provide a new and improved seal for washpipes and similar applications, in oil fields and elsewhere. 
   A further object of the invention is to provide a washpipe seal wherein the high pressure drilling fluid moves gradually from a region of very high pressure, to a low outlet pressure from where it preferably is returned to the storage point for reuse. 
   Another object is to provide a seal which includes a housing for a plurality of stators and a housing for a plurality of rotors, with the stators and rotors being of annular disc form and being interleaved with one another. 
   A still further object of the invention is to provide a plurality of stators and rotors having interleaved portions and wherein each stator and rotor preferably includes a plurality of lands and grooves or other formations to retard the flow of drilling fluid as it works it way from the inlet chamber of the seal assembly to the outlet at the bottom portion thereof. 
   A still further object of the invention is to provide a plurality of alternating rotors and stators of annular form in which each includes a seal or packing that prevents leakage between the two sets of stators and rotors. 
   Another object of the invention is to provide a seal with a housing having a lower end member disposed in opposed relation to another housing member, the two being separated by thrust bearings and/or radial bearings each able to be greased, and maintaining such respective housings in closely spaced apart relation. 
   A further object is to provide a plurality of stators and rotors, each of which is held in place by a spacer, with the array of spacers being held in fixed position under a compressive load provided by spring means such as Belleville washers, for example. 
   Another object is to provide a construction wherein the stators are located for anti-rotation and the rotors are located for rotation by means of keyways and locking pins positioned on the outside diameter of the stator disks and the inside diameter of the rotor disks. Belleville washers are used to provide a small preload to the rotor and stator stack and the clamping of the total assembly compresses the stator and rotor spacers to fix the final position, while also providing an adequate seal between the components. 
   Another object of the invention is to provide radially inner and outer housings, each having a plurality of associated discs, with the inner housing including disks of annular form and defining a cylindrical region on the interior, able to accommodate a high volume of drilling fluid, while passing a small amount of drilling fluid to the annular headspace or chamber between the two housings. 
   A still further object of the invention is to provide a seal with controlled leakage, which includes an upper connection assembly and a housing attached thereto as well as a lower connection assembly that rotates with the washpipe in the use of the seal assembly. 
   A further object of the invention is to provide a washpipe seal which may be readily removed and replaced without altering the axial distance between the two connectors, that is, those at the top and the bottom of the assembly. 
   A still further object of the invention is to provide a system which will hold two parts of the wash pipe seal in snug, non-leaking relation but which can be readily radially removed so that the cartridge can be taken out of the system and replaced with another cartridge, with very little or no difficulty, and no changing of the ultimate distance between the upper and lower pipe flanges or connectors. 
   Another object is to provide a pair of clamps which can be separated by removing two fasteners on each end and removing the clamps, thereby enabling the cartridge to be removed radially and replaced with very little difficulty. 
   A still further object is to provide a pair of clamps which act radially and not only permit ease of removal, but also provide inherent alignment and clamping forces which prevent any possible leakage when the clamps are secured. 
   SUMMARY OF THE INVENTION 
   These and other objects and advantages of the present invention are achieved in practice by creating an outer housing with plural annular discs, an inner housing of annular form and having plural discs, with drilling fluid passing both inside and outside of the inner housing, with a small proportion of the drilling fluid passing through the inner housing being directed through a labyrinth of discs until it reaches the outlet. The invention also includes a simplified structure permitting ready removal and replacement of the cartridge without altering the position of the master coupling, which remains in a fixed position, having two conduits fastened to it by screw threads or the like on each end. 
   The manner in which these objects and advantages are achieved will become more clearly apparent when considered in conjunction with a description of the preferred embodiments of the invention and shown in the accompanying drawings in which like reference numbers indicate corresponding parts throughout. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view showing several important features of the present invention including showing the upper connection assembly, the lower connection assembly and the arrangement of the annular stator and rotor discs in interleaved relation; 
       FIG. 2  is an enlarged vertical sectional view through a portion of the apparatus of the present invention and showing one particular preferred arrangement of the stator housing, the rotor housing, plural annular rotors and stators for each housing and other features of the invention; 
       FIG. 3  is an exploded view of one annular stator and one annular rotor and showing them in spaced apart relation; 
       FIG. 4  is a further enlarged vertical sectional view of a portion of the stator/rotor assembly showing the stators and rotors with lands and grooves therein, and including the spacers between the rotor and stators and the seals therefor, and showing the tortuous path undergone by the drilling fluid; 
       FIG. 5  is a view similar to  FIG. 4 , only showing the stators and rotors without the lands and grooves; 
       FIG. 6  is a perspective view of the cartridge holder of the present invention, showing in lighter lines the master coupling and the passages through which the removable clamp and the cartridge may be removed and replaced; 
       FIG. 7  is a perspective view showing in lighter lines the master coupling and showing the cartridge in the process of being replaced and/or renewed; 
       FIG. 8  is a vertical sectional view showing the cartridge in the process of being removed and showing the two positions of the clamp holding the cartridge in position; and 
       FIG. 9  is a further enlarged view of a portion of  FIG. 8 , showing several details of construction and schematically showing the keyways and locking pins o the invention, and the threaded collar allowing release of the flanges. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
   While the invention may be embodied in a number of different forms, and may employ variations of certain parts, two preferred embodiments of the invention will be described, which include plural rotors and stators that are interleaved with each other and arranged in inner and outer annular housings, and which accommodate a pressure drop from up to 10,000 or more PSI to a much lower level adjacent the outlet. 
   Referring now to the drawings in greater detail, there is shown washpipe seal assembly generally designated  18 , and this assembly, as best shown in  FIGS. 2–4 , includes a stator assembly generally designated  20 , and a rotor assembly generally designated  22 . The stator assembly  20  includes a plurality of discs generally designated  24   a,    24   b  received within a housing generally designated  26 . This housing  26  is affixed to an upper connection assembly generally designated  30 . This assembly  30  includes a radially outer member with an O-ring  32  on its top surface  34 . The inner member is preferably in the form of a collar  36  of reduced diameter relative to the housing  26 . 
   An O-ring seal  37  is provided on the collar  36  to seal a joint between the upper connecter assembly  30  and the collar  36 . A certain amount of axial freedom of movement or play may take place between the upper connection assembly  30  and the collar  36  on the housing  26 . The stator housing generally designated  26  consists of the upper, reduced diameter collar  36 , an O-ring groove  38  and a radial flange  46  leading to the outer diameter cylindrical portion  35 . Adjacent the bottom of the stator housing  26  is a low pressure annular chamber  49  terminating in an outlet  50 . 
   An enlarged diameter portion generally designated  52  forming a lower continuation of the housing  26  is provided, and this element includes one or more grease fittings  54  connected to suitable passages  56  for grease to lubricate radial and thrust bearing assemblies generally designated  58 ,  60 . These bearings  58 ,  60  are subject to harsh environmental conditions and are preferably made from a hardened steel material, or tungsten carbide, and the bearings  58 ,  60  permit rotation between the stator assembly  20  and the rotor assembly  22 . Of course, these bearings also permit relative rotation of the rotor and stator and axial and radial guidance of these assemblies  20 ,  22 . Referring again to the spaced apart stators  24   a,    24   b,  etc., in this embodiment, there is provided an upper spacer unit  62   a  which is fixed relative to the housing  26  by a Belleville washer  66  capable of exerting a strong axial compressive force on the entire stack of spacers  62   a,    62   b,  etc. and stators  64   a,    64   b,  etc. 
   The general arrangement of stacked stators  64   a,    64   b,  etc. and the arrangement of spacers  62   a,    62   b,    62   c,  etc. is the same arrangement as the arrangement of rotors and spacers in the rotor assembly  22 . Thus, the washpipe  70  has a radial flange  72  which engages a Belleville washer  74 . This Belleville washer develops an axial load on spacers  76   a,    76   b,  etc. and these spacers  76   a,    76   b  engage the margin portions of the rotor discs  78   a,    78   b,  etc. 
   The lower connection assembly generally designated  71 , includes an axial wall  73 , a radial wall  75 , and a groove  77  accommodating an O-ring  79 . The radial bearing  58  is engaged by an extension  81  of the washpipe  70 . These portions may be affixed to a rotary member generally designated  80  and having radial and axial flanges  82 ,  84 . These components may include their own O-rings  86 ,  88  as is customary in the industry. 
   Referring again to the washpipe  70  and the housing assembly  26 , the rotation between these parts  70 ,  26  (and others fixed respectively with respect to those parts) also requires a seal  90 , and this seal or packing  90  engages opposed inner wall portions  92 ,  94 , of the washpipe  70  and the stator  35 . This form of “V-packing”  90 , however, is unlike prior art seals in that it does not see a large pressure drop and consequently it is able to endure for an extended period of time, in keeping with the life of the remainder of the seal afforded by the rotors and stators. 
   Referring now to the operation of the controlled leakage seal of the invention and in particular to  FIG. 4 , an enlarged partial sectional view of the stators, rotors and spacers is shown. Here, spacers  62   a,    62   b,  etc. hold the stators  64   a,    64   b  in a relatively fixed position and allow the rotors  78   a,    78   b  to rotate without radial or axial engagement of their counterparts  64   a,    64   b,  etc. There is a headspace region  96  provided between the radial flanges  72 ,  46  of the two housings  26 ,  70 . The spacers  62   a,    62   b,  etc. are retained in place with the aid of the gaskets or O-rings  98   a,    98   b,    98   c,  etc. and the spacers  76   a,    76   b  are held in position with the aid of O-rings or gaskets  100   a,    100   b,  etc. 
   In operation, the headspace region  96  is filled with extremely high pressure drilling fluid, and this drilling fluid is then passed through the passage  105  between the end face portion  102  of the spacer  76   a  and the end face portion  104  of the stator  64   a.  It will be noted that each of the stators  64   a,    64   b,  etc. has a plurality of lands or teeth  106   a,    106   b,  etc. disposed in offset relation to the lands  108   a,    108   b,  etc. on the rotor  78   a.  Grooves  110   a,    110   b  are disposed in an opposed relation to the lands  108   a,    108   b,  etc. 
   With the very small space  105  between the end faces  102 ,  104  being duplicated all along the axial and radial path between the stators  64   a,    64   b,  the rotors  78   a,    78   b,  etc., a tortuous path of leakage is provided for the drilling fluid as it makes its way to the annular chamber  49 . This scheme of extremely narrow passages, both axial and radial, when repeated from 16 to 20 times, has the effect of gradually lowering the pressure from the 7,000 to 10,000 PSI to a greatly reduced pressure which will appear in the lower chamber  49 . From there, the drilling fluid optionally passes through a flow meter  116 , and from there to a conduit  118  wherefrom it may be returned to the supply of drilling fluid for reuse. 
   Referring now to  FIG. 5  and another embodiment of the invention, an alternate construction of the rotor and stator disks is shown. In this embodiment, the stator consists of a reduced diameter collar  136 , a radial flange  146 , and a cylindrical axial flange  135 . Disposed inside this cylindrical shell  135  is a spacer  162   a,  sealed by an o-ring  198   a.  Several additional spacers  162   b,    162   c,  etc. are stacked inside this cylindrical shell  135 , and the additional spacers  162   b,    162   c  each include additional o-rings  198   b  and  198   c.  The head space  196  lying between the radial flange  172  of the inner cylinder or washpipe  170 , and the radial flange  146  of the cylindrical shell  135  provides a passage  173  for drilling fluid entering the head space  196 . 
   A compressive force is maintained by Belleville washer  166  acting on the spacer  162   a  and all of the components in axial alignment therewith. A Belleville washer  174  bears on the inner spacer  176   a,    176   b  and on the disks  178   a,    178   b  entrapped between them. O-rings  200   a,    200   b  seal the spaces  176   a,    176   b,  etc. The only difference in operation between embodiment of  FIG. 4  and that of  FIG. 5  is that of the configuration of the rotors/stators of the invention. 
   For example, the end face  202  of the spacer  176   a  and the end face  204  of the stator designated  164   a  lie in opposed relation with a small clearance  205 , such as 0.005 to 0.060 inches, by way of example, between them. The horizontal space  215  between the stator  164   a  and the rotor  178   a  is defined by the top and bottom faces  212 ,  214  of the stator  164   a  and the rotor  178   a.  The drilling fluid passes in the space  215  between these narrow faces  202 ,  204  and into the space  217  between surfaces between the surfaces  218 ,  220 , and so on until the drilling fluid has reached the bottom of the chamber  49  at the lower end of the washpipe  170 . 
   Consequently, the region  196  is filled with extremely highly pressurized drilling fluid, most of which travels down inside the washpipe  170 . However, a small amount of this drilling fluid is forced into to the passage  215 ,  217 , etc. between the end portions and surface portions of the rotors and stators. 
   Referring now to the stators the rotors and the means for keeping them apart, the spacers shown in the drawings are the preferred method of doing this. However it is not the only method of keeping these stators or rotors apart, since this can be done by merely forming an extension of enlarged width on the end of each rotor or stator, thereby imparting an L shape to the inboard and outboard of the stators or rotors. The O-rings such as the O-rings  198   a,    198   b,  the  200   a,    200   b,  etc. may be replaced with any suitable kind of packing, preferably a so-called block V or the like. 
   The Belleville washer is shown as being compact and having a high spring rate which is desirable in most cases. However, a wave-type spring or washer could be used or other means having a strong spring rate to hold these elements together. 
   The invention has been described showing of lands and grooves facing each other in one embodiment, and a plain ungrooved embodiment as another choice. However, it is possible to have one face grooved or the like or to have lands in the other side could be merely a planar surface. The preferred method are the tongue and groove, or lands and grooves, method illustrated or the plain, unadorned faces. Regarding the size of this space, it has been found that the most effective space depends upon the viscosity and pressure of the drilling mud, but having the lands and grooves with 0.005 to 0.050 is preferred. Some other values may be used if desired. The packing  90  has been illustrated as the most economical and effective method of creating a seal between the relatively low pressures involved at the bottom of the chamber. Needless to say, however, another type of seal could be used in this application. 
   Another entirely different method could use the apparatus of the present invention with no change or with only a slight change in the apparatus. In this case, the spaces between the rotors and the stators could be filled with grease up to approaching, or even meeting, the headspace  96 . In this instance, the grease would have to be injected into the apparatus while there were no countervailing high pressure from the drilling mud. Once this space was filled entirely with grease, the seal would otherwise, function as designed, with the drilling and pressure being counteracted with the resistance to movement of the grease. 
   Preferably, the two housings, the stators and the rotors and other parts are made from tungsten carbide or similar material, since these components see the highest pressure drilling fluid. Other components use an oil field grade steel. The o-rings and the block V or similar type seals are made from synthetic rubber or other known material. 
   Referring now to the structure for reducing or eliminating maintenance problems,  FIGS. 6–9  show a slightly modified form of the invention but one which greatly simplifies the removal of the entire cartridge shown in  FIGS. 6–9 . By cartridge is meant a removable part which includes the outer housing and the inner rotor and all their associated elements. 
   Thus, referring again to  FIGS. 6–9 , there is shown in lighter lines, the master coupling  288  which secures the upper and lower passages  290 ,  292  together in fixed axial relation and the master coupling  288  includes two enlarged openings  294 ,  296 . In  FIGS. 6–8  there is shown a large lower flange generally designated  300  and an equally large upper flange generally designated  302 . It will be seen that there is one outer housing generally designated  326  and one inner, rotary housing generally designated  322 . These elements are essentially the same as their counterparts in  FIGS. 1 and 2 . The main difference between these embodiments is that the upper collar  336  is formed separately and attached as by fasteners  338 ,  340  etc. to the housing  326 . 
   The upper fixed collar  336  is kept in snug relation to the upper flange generally designated  342 . This flange  342  has a downwardly extending, cylindrical portion  344  and a wide flange portion  346 . The portion  346  has a top surface which is planar, and mates in generally fluid-tight position with an upper flange  348  which has a cylindrical portion  350  attached in leak-proof position by threads or the like  352  to the upper inlet portion  353 . An O-ring  354  ensures a seal between the flange  348  and the flange  346  when the flanges  346 ,  348  are in facing relation and in snug engagements as will be described. 
   Referring now to the lower collar generally designated  373 , this unit is similar to its counterpart in that an upright cylindrical portion  375  is provided and a generally horizontal flange  379  is held in that position as by bearings  358 ,  360  to enable it to rotate relative to the lower portion  382 . The lower portion  382  is generally similar to its upper counterpart in  FIG. 8 , and this includes an O-ring  386  which engages the face  389  of the flange  379 . The portion  382  includes, in addition to the O-ring  386 , a horizontally extending flange  384  which is fastened by threads or the like  388  to a fixture  389  which is the uppermost portion of the drill string. The surfaces  389 ,  391  of the opposing flanges are ultimately made relatively mechanically tight and make a substantially fluid-tight connection. 
   A novel feature of the invention includes the two clamps generally designated  392  and  394 . Each of these clamps each has a pair of tapered inner surfaces  396 ,  398 ,  400 ,  402  as well as screw receiving bores  404 . The tapered marginal mating surfaces  406 ,  408  on one clamp are received within the surfaces  396 ,  398 . Accordingly, when the clamp  392  has been tightened, these surfaces  406 ,  408  remain extremely secure because their taper matches that of the surfaces  396 ,  398  on the clamp. There are counterpart surfaces for  420 ,  422  which are ultimately engaged by surfaces  400 ,  402  on the clamp  394 , thus holding the two flange  346 ,  348  together. 
   When the clamps  392 ,  394  are removed by loosening the Allen head fasteners  403 , the two halves come apart, and their construction allows the housings to slip past each other and out either one of the openings  294 ,  296  defined by the sidewalls  430 ,  432  in the master coupling  288 . 
   Referring now in particular to  FIG. 9 , it is shown that, in a preferred construction, when it is time to release the clamps and remove the cartridge, a certain amount of axial play between the upper portion fixed collar  336  and the downwardly extending cylindrical portion  344  may be gained by unscrewing the threaded collar  351 . This lowers the cylindrical portion  344  and the flange  342  downwardly and enables the lower flange  342  to be more easily separated from the upper flange  348 . 
   Thereupon, a new cartridge containing the labyrinth and the elements described in connection with  FIGS. 1 and 2 , for example, may readily be replaced, simply by slipping the entire cartridge into place, whereupon the flanges will be engaged by the clamps  392 ,  394  and tightened down. The O-rings  354 , ensure a tight seal between parts which do not undergo relative rotation. 
     FIG. 9  also schematically shows that there are keyways  355  and locking pins or blades  356  in place so that the rotor disks and the stators remain fixed with their respective housing portions  322 ,  326 . 
   It will thus be seen that the present invention provides a novel controlled leakage seal assembly having a number of advantages and characteristics, including those herein pointed out and others which are inherent in the invention.