Patent Publication Number: US-2023160474-A1

Title: High pressure seals

Description:
FIELD 
     The invention relates to pressure systems and to components therefor. 
     BACKGROUND 
       FIG.  1    schematically illustrates a pumping system  1  for pumping fluid from an inlet  3  to an outlet  5 . The system comprises a linear actuator  7  with a pump head  9  at each end. 
       FIG.  2    schematically illustrates an example of a pump head  9  comprising a cylinder  11  defining a pump chamber  13 , a plunger  15  and one-way flow paths  17 . A sealing system  19  seals the annular space between the plunger  15  and the cylinder  11  so that as the plunger  15  is stroked, by the linear actuator  7 , fluid is positively displaced via the flow paths  17 . 
     In some applications, such as water jet cutting, it is desirable to deliver ultra-high pressures such as 90 ksi. Delivering such pressures places extraordinary loads on all of the components and in particular on the sealing system  19 . 
     U.S. Pat. No. 7,247,006 discloses various sealing systems. A portion of  FIG.  3    of that patent is reproduced as  FIG.  3    herein. 
     The sealing system of that figure comprises a seal support  21  and a bearing  23  inside the seal support. A seal  25  sits in front of and is axially supported by components  21 ,  23  to sealingly engage the plunger  15 ′. The seal support is compressed against a complementary formation at the rear of the cylinder  11 ′ to define a tangential sealing area  27  and collapse the seal support  21  against the bearing  23 . 
     The sealing arrangement of  FIG.  3    corresponds to a commercial product with which the present inventors are familiar. Whilst that product is popular, the present inventors have recognised room for improvement. The tangential sealing interface  27  seems to reduce the life of the cylinder  11 ′ by adding to the hoop stress therein. When the sealing assembly wears out essentially the whole sealing system, including the seal support  21  and bearing  23 , must be replaced. This replacement operation is not straightforward. The force applied to the seal support  21  to compress it against the rear of the cylinder  11 ′ must be accurately controlled to avoid damaging the bearing  23 . 
     With the foregoing in mind, the present invention aims to provide improvements in and for sealing systems. 
     It is not admitted that any of the information in this patent specification is common general knowledge, or that the person skilled in the art could be reasonably expected to ascertain or understand it, regard it as relevant or combine it in any way before the priority date. 
     SUMMARY 
     One aspect of the invention provides a sealing system to seal an annulus, between an interior and an exterior axially movable within the interior, to hold fluid pressure; 
     the sealing system comprising
         an inner seal to surround, and sealingly engage, the exterior;   a seal support to surround, and axially support, the inner seal; and   an outer seal to surround the seal support and sealingly engage the seal support and the interior.       

     Preferred embodiments advantageously seal within the interior whereby, relative to the sealing arrangement of  FIG.  3   , the high pressure fluid is presented with a much smaller effective area over which axial forces are generated, thereby reducing the cyclical axial loading placed upon the surrounding components. 
     The seal support is preferably shaped to be deformed, by the fluid pressure, to tighten a fit between the seal support and the exterior to at least slow extrusion via the fit. Preferably a rear of the inner seal is set forward of a rear of the outer seal. 
     A peripheral support ring to sit behind and axially support a periphery of the outer seal may be provided. Preferably there is an interference fit between the peripheral support ring and the interior. The peripheral support ring may have a substantially rectangular profile but preferably has a rearwardly converging interior. 
     The sealing system optionally comprises an outer resilient seal ring to squeeze the outer seal. Preferably the outer seal comprises a body portion having a radial thickness. The body portion may be dimensioned to axially separate the outer resilient seal ring from the rearward-facing portion by at least the radial thickness. Preferably it is dimensioned to axially separate the outer resilient seal ring from the peripheral support ring by at least the radial thickness. 
     The outer resilient seal ring may be a polyurethane seal ring. There may be a back stop for the seal support, or the seal support and the support ring, to axially bear against. The seal support and back stop are configured for the seal support to have radial freedom with respect to the back stop. The seal support may have an exterior portion forwardly converging from the rearward-facing face for bearing against a back stop, although the exterior of the seal support is preferably substantially cylindrical. Preferably the back stop is configured to be clamped against a rearward-facing portion of a body defining the space. The back stop may be predominantly steel. 
     Optionally an inside support ring sits behind and axially supports an inside of the outer seal. The inside support ring may have a rearwardly diverging exterior. 
     The sealing system may comprise an inner resilient seal ring to squeeze the inner seal. The inner seal is preferably at least predominantly thermoplastic. Preferably the seal support predominantly consists of copper-based material and/or of an integral body of material. The outer seal is preferably at least predominantly thermoplastic. 
     Another aspect of the invention provides a pressure system comprising 
     the sealing system; 
     an outer body defining the space; and 
     an inner body defining the exterior. 
     The pressure system may be a pump. 
     The pressure system may be capable of delivering at least 50 ksi, or more preferably at least 87 ksi. 
     Also disclosed is a seal support for a sealing system; 
     the sealing system being configured to sealingly engage a cylindrical exterior and comprising an inner seal to surround the cylindrical exterior; 
     the cylindrical exterior being axially movable relative to a space; 
     the sealingly engaging being to hold fluid pressure within the space; 
     the seal support comprising
         a backing ring portion to axially support the inner seal; and   a seal-surrounding portion to surround the inner seal;       

     the seal support being shaped to be deformed, by the fluid pressure, to tighten a fit between the seal support and the cylindrical exterior to at least slow extrusion via the fit. 
     The seal support preferably comprises a rearward-facing face for bearing against a back stop. A front of an innermost portion for engaging the cylindrical exterior may be in the range of 1.25%±1.25%, e.g. in the range of 1.25%±0.5%, of the diameter of the cylindrical exterior in front of the rearwardly directed annular face. 
     Preferred variants substantially comprise, rearward of the rearward-facing face, an outer diameter in the range of the 135%±10% of the diameter of the cylindrical exterior. 
     A seal support face may be at the front of the innermost portion to bear against the inner seal. Preferably there is a curved transition from the seal support face to the seal-surrounding concavity. Most preferably the seal-surrounding concavity substantially has a diameter in the range of 114%±5% of the diameter of the cylindrical exterior. 
     The seal support may comprise, in front of the seal-surrounding concavity, an inwardly-open annular groove to receive a resilient ring for squeezing the inner seal. Preferably an interior of the seal-surrounding portion comprises, forward of the inwardly-open annular groove, one or more outwardly-set portions radially outward with respect to the seal-surrounding concavity to define a passage, external to an inner seal exterior portion extending from conformal contact within the seal-surrounding concavity, for energising the resilient ring. 
     Preferably an exterior portion forwardly converges from the rearward-facing face. 
     The seal support may predominantly consist of copper-based material, e.g. bronze or more preferably an alloy thereof such as an aluminium bronze alloy or a bronze-nickel alloy. Preferably the seal support is an integral body of material. 
     Also disclosed is a sealing system comprising the seal support and the inner seal. 
     Preferably the inner seal has 
     an interior; 
     an exterior separated from the interior by a wall-thick thickness; and 
     a length at least three times the wall thickness. 
     The inner seal may be at least predominantly thermoplastic; e.g. at least predominantly UHMWPE. 
     The sealing system may comprise an inner resilient seal ring to sealingly engage the inner seal and the seal support and squeeze the inner seal. Preferably, the inner resilient seal ring is a polyurethane seal ring. 
     The sealing system preferably comprises an outer seal to surround the seal support and sealingly engage the seal support and a cylindrical interior of the space. The outer seal may be at least predominantly thermoplastic; e.g. at least predominantly UHMWPE. Preferably there is an interference fit between the outer seal and the cylindrical interior. The sealing system preferably comprises an outer resilient seal ring to squeeze the outer seal. The outer seal may comprise an outwardly-open annular groove to carry the outer resilient seal ring. Preferably, the outer resilient seal ring is a polyurethane seal ring. 
     The outer seal preferably comprises a periphery and, about the periphery, a rearward-facing portion by which the outer seal is axially supportable. Optionally the outer seal comprises a body portion having a radial thickness and being dimensioned to axially separate the outer resilient seal ring from the rearward-facing portion by at least the radial thickness. 
     The sealing system may comprise a support ring to sit behind and axially support an or the periphery of the outer seal. Preferably the support ring has a substantially rectangular profile. Preferably there is an interference fit between the support ring and the cylindrical interior. Preferably the support ring is axially supported by an or the back stop. The support ring may be formed of metal such as one of stainless steel, aluminium and aluminium bronze. 
     The support ring and seal support are configured to together define a rearwardly-narrowing annular space, in which case the outer seal is preferably shaped to fill the rearwardly-narrowing annular space. 
     The sealing system may comprise an or the back stop to axially support the seal support. The seal support and back stop may be configured for the seal support to have radial freedom, with respect to the back stop, to self-align with respect to the cylindrical exterior. The back stop may carry a bearing for aligning the cylindrical exterior. Preferably the back stop is configured to be clamped against a rearward-facing portion of a body defining the space. The back stop may be predominantly (e.g. entirely) steel. 
     Also disclosed is a sealing system for sealing an annulus; 
     the annulus being defined by a cylindrical interior and a body within the cylindrical interior; 
     the sealing system comprising
         an outer seal to surround the body and sealingly engage the body and the cylindrical interior;   a support ring to sit behind and axially support a periphery of the outer seal.       

     Preferably, there is an inner resilient seal ring to sealingly engage the inner seal and the seal support and inwardly urge the inner seal. The seal support may comprise an inwardly-open annular groove to carry the inner resilient seal ring. 
     The first ring and seal support may be configured to together define a rearwardly-narrowing annular space. The outer seal may be shaped to fill the rearwardly-narrowing annular space. The seal support may comprise a rearwardly-divergent portion to define an interior of the rearwardly-narrowing annular space. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    schematically illustrates a pumping system; 
         FIG.  2    schematically illustrates a pumping head; 
         FIG.  3    is a half-section view of a sealing system; 
         FIG.  4    is a half-section view of a sealing system; 
         FIG.  5    is an enlargement of detail A in  FIG.  4   ; 
         FIG.  6    is a half-section of another sealing system; and 
         FIG.  7    is a half-section view of a sealing system. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Sealing system  29  serves to seal an annulus AN defined between a cylinder  11 ″ and a plunger  15 ″. In this case the plunger is cylindrical although other shapes are possible. The sealing system comprises an inner seal  31 , a seal support  33 , an outer seal  35 , a peripheral support ring  37 , a back stop  39 , an inner resilient seal ring  41  and an outer resilient seal ring  43 . 
     The inner seal  31  takes the form of a short sleeve about six times longer than its radial thickness. The outer corners are rounded. The seal is a single integral body formed of suitably compliant material. Ultra-high molecular weight polyethylene (UHMWPE) is preferred, although other polymers may be suitable. 
     As the wording is used herein, “integral” refers to a single continuous body of material; two bodies may be integrated by welding but not by conventional mechanical fastening. 
     The seal support  33  surrounds and supports the inner seal  31 . Preferably, the seal support  33  is an integral body. The seal support  33  may be formed of a bronze-based material or other suitable bearing material to ride along the plunger  15 ″. The plunger is preferably formed of zirconia. Most preferably the plunger is highly polished or otherwise has a similar degree of smoothness. 
     An interior of the seal support  33  presents a forward-facing surface  33   a  to the rear of the inner seal  31  to axially support the inner seal  31 . Preferably there is a corner (as opposed to a radius) at the juncture of features  33   a ,  33   g . In this example, the forward-facing support face  33   a  is substantially radial to define a substantially perpendicular circular corner  33   f ,  33   g . The cylindrical interior  33   g  is the innermost portion and is dimensioned to ride along the plunger  15 ′. 
     A curved transition, in this case, a radiused transition  33   h  connects the support face  33   a  to a seal-surrounding concavity  33   i . Preferably, the curved transition substantially has a radius in the range of 3.8%±2%, or more preferably 3.8%±1% of the plunger diameter. In this example the concavity  33   i  is substantially cylindrical. Preferably the concavity is concentric to the cylindrical interior  33   g.    
     An inwardly-open annular groove  33   j  sits in front of the concavity  33   i  to accommodate resilient ring  41 . A further cylindrical seal-surrounding concavity  33   k  sits in front of the groove  33   j  and is set radially outwards with respect to the concavity  33   i . In this way, whilst the cylindrical exterior portion of the seal  31  sits in conformal contact with the concavity  33   i , an annular clearance is defined between that exterior and the seal-surrounding concavity  33   k , via which clearance the ring  41  is exposed to the high pressure fluid. This exposure tends to axially compress the ring  41  and thereby urge the ring  41  to increase in radial thickness. In this way, the ring  41  is energised by the high pressure fluid to radially act between the seal support  33  and the inner seal  31  to squeeze the seal  33 . 
     Of course there are other ways by which the resilient ring carrying space might be vented to the high pressure side of the sealing system. By way of example, a circular array of axially extending laser drilled holes might be formed. Preferably plural, e.g. two, inwardly-open cut-outs  331  are equispaced about the seal support  33 . 
     The seal support  33  further comprises an inwardly-directed lip  33   m  to sit in front and axially retain the inner seal  31  during assembly. The inner seal receiving space, in this case axially limited by portions  33   a ,  33   m , is longer than the inner seal  31  to give the inner seal  31  room to axially grow under radial pressure. For similar reasons, the groove  33   j  is preferably longer than the ring  41  is wide. 
     The seal support  33  presents a cylindrical exterior portion  33   b  to the outer seal  35 . Behind this cylindrical portion is a rearwardly-diverging conical portion  33   c  that sits behind the outer seal  35 , to support an inside of the outer seal  35 , and in register with the peripheral support ring  37 . 
     The back stop  39  is formed of stainless steel and comprises an annular flange  39   a  axially projecting into the cylinder  11 ″ and surrounding a rear  33   d  of the seal support  33 . The back stop  39 , and in this example the annular flange  39   a , presents a forward-facing support surface  39   b . In this case, the support surface  39   b  is substantially radial. The support surface  39   b  abuts a rear of the peripheral support ring  37  and a rearward annular face  33   e  of the seal support immediately behind the conical portion  33   c.    
     In this example, the rearward annular face  33   e  is a single radial face. Some variants of the sealing system are configured for the face  33   e  and a rearmost face of the seal support  33  to both draw axial support from the back stop  30 . This may be achieved by carefully controlling the axial tolerances on both parts so that upon assembly the face  33   e  abuts the back stop  39   a  and there is potentially a gap of upto a few microns at the rearmost interface, which gap is quickly closed when the sealing assembly  29  is exposed to high pressure. In this example, the rear of the backing ring portion presents a surface area to the back stop  39  much greater than the corresponding surface area presented by the portion  33   e  whereby, after the axial gap has been closed, the seal carrier  33  is supported mostly by this rearward face. The firm axial loading at the face  33   e , and the acute corner at its outer periphery, limit the extrusion of the outer seal  35  through the interface of portions  33   e ,  39   a.    
     The outer seal  35  is another integral body of material. Preferably, the outer seal  35  is formed of UHMWPE, although the other materials contemplated in respect of the inner seal  31  may also be suitable. Conveniently, the inner seal  31  and the outer seal  35  may be formed of the same material. 
     A rear of the outer seal  35  defines a rearwardly-tapering annular flange shaped to occupy the rearwardly-narrowing space internally bounded by the surface  33   c  and externally bounded by the support ring  37 . In this example, the outer seal  35  also bears against and is axially supported by the back stop  39 . 
     The peripheral support ring  37  is an integral body pressed into the cylinder  11 ″. An outward rear corner of the outer seal  35  is shaped to conformally fit the front and interior of the support ring  37 . In this case, the support ring  37  has a rectangular profile and in turn the corresponding corner of the outer seal has a stepped profile. Preferably, the profile of the peripheral support ring  37  is about twice as axially long as it is radially thick. 
     The outer seal  35  is preferably pressed into the cylinder  11 ′ to form an interference fit therewith. Most preferably, the outer seal  35  is dimensioned for a radial interference in the range of 0.3%±0.2% of the plunger diameter. Optionally, the outer seal is dimensioned for a radial interference of 0.3%±0.2% of the plunger diameter with the seal support  33 . These interferences help to maintain sealing contact with the cylindrical interior of the cylinder  11 ″ as that cylinder expands under pressure. The resilient seal ring  43  encircles the outer seal  35  and inwardly urges the outer seal  35  to sealingly engage the exterior of the seal support  33 . In this example, the outer seal ring  43  is a square-profiled polyurethane ring. 
     This variant of the outer seal  35  comprises an outwardly-open annular groove  35   b  to accommodate the ring  43 . The full radial thickness of the ring  43  is supported from behind by the outer seal  35  whilst a short radial lip  35   c  defines a forward extent of the groove  35   b  and serves to retain the seal  43  during assembly, etc. The ring  43  is also exposed to and energised by the high pressure fluid. 
     Preferably, the support ring  37  is pressed into the cylinder  11 ″ so as to achieve an interference fit that compresses the support ring  37 . Preferably, the degree of compression is selected to be commensurate with the expansion of the cylinder  11 ″ when the system is pressurised. In this way, the hoop stress in the support ring  37  is limited and in operation the ring  37  does not materially contribute to hoop stress within the cylinder  11 ″. 
     The pressing operation can be relatively straightforward. The ring  37  and outer seal  35  may be pressed in by a required axial amount without careful attention to the pressing force. Of course, there are other ways of achieving an interference fit, e.g. the cylinder  11 ″ might be heated and/or the support ring  37  might be cooled. One or both of these thermal operations might be sufficient to enable the support ring  37  to be simply placed. Alternatively, one or both of the thermal operations might be employed in combination with pressing. 
     The back stop  39  is preferably another integral body of material. In this case, it is formed of stainless steel. The back stop  39  encircles the plunger  15 ″ and has an inner bore dimensioned to clear the plunger. The inner bore carries an inwardly-open annular groove  39   b  in which a resilient seal (e.g. O-ring) is mountable to bear against the plunger  15 ″. This seal principally serves to stop oil moving forward but also provides a small degree of back up if some fluid leaks rearwardly from the sealing system  29 . 
     The flange  39   a  embraces the rear  33   d  of the seal support. The portions  39   a ,  33   d  are dimensioned for a radial clearance, in this case a nominal radial clearance of 0.1 mm, whereby the seal support  33  has radial freedom with respect to the back stop  39 . In this way, the seal support  33  is radially constrained by the relatively compliant outer seal  35  so that whilst the water pressure causes the seal support  33  to bear down on the plunger, that downward bearing is evenly distributed. In this case, the seal support  33  serves as a bearing by which the plunger  15 ″ is radially restrained. In other implementations of the concept, the plunger  15 ″ might be radially restrained by other means, in which case the radial freedom of the seal support  33  would help to accommodate a few microns of misalignment. 
     According to preferred variants of the sealing system, the task of sealing across the annulus AN is essentially divided by the seal support  33  into two sub-tasks. Components  31 ,  41  seal between components  15 ″,  33  whilst components  35 ,  43  seal between components  33 ,  11 ″. 
     The present inventors have recognised that in the context of ultra-high pressure (50,000 psi or more) polymer seals  31 ,  35  distribute pressure in a manner analogous to a highly viscous liquid and deform over time. In particular, the polymer seals have a tendency to extrude through any discontinuity in the axial supports backing them up. 
     The present inventors have christened the nominal radial gap between the exterior of the plunger  15 ″ and the cylindrical interior  33   g  the ‘extrusion gap’. In the context of a 16 mm diameter plunger, a nominal radial clearance of 15 μm (about 0.1%) is preferred. 15 μm is about ¼ of the thickness of a typical human hair and no surface is perfectly cylindrical. As such, when the seal support  33  is placed over the plunger  15 ″, there is no readily observable gap. Rather, there is a sliding fit by which the low friction material of the seal support can slide over the polished exterior of the plunger  15 ″. 
     This fit corresponds to an extrusion gap through which the seal  31  might be easily forced if not for the support  33  being shaped to be deformed by the fluid pressure to tighten the fit. 
     In this particular example, the (in profile) square solid body of material at the rear of the seal support constitutes a backing ring portion whilst portions forward of the face  33   a  constitute a seal-surrounding portion. 
     Part of the seal-surrounding portion is directly exposed to the high pressure fluid whilst other parts are exposed to that pressure via the fluid-like behavior of the seals  31 ,  35 . Fluid pressure outwardly drives the seal support  33  along its interior forward of the seal support surface  33   a . This outward driving is counteracted by inward driving along the exterior of the seal support forward of the face  33   a.    
     The axial spacing of the surfaces  33   a ,  33   e , or more to the point the spacing of the corresponding sealing interfaces, impacts on the magnitude of the inward and outward forces. Relatively moving the surface  33   e  rearwards increases the net inwards force. It is also to be born in mind that since the illustrated profile is a revolved profile rather than a parallel piped profile, per unit length the relevant circumferential area over which the radical forces act are larger on the exterior than they are on the corresponding interior of the seal support. This difference also contributes to a net inward force. Applying these principles, the shape of the seal support  33  can be selected so that the seal support is deformed, by fluid pressure, to tighten a fit between the seal support and the plunger exterior to at least slow extrusion via the fit. Preferably the inward deformation is driven predominantly, or more preferably substantially only, by fluid pressure. 
     Preferred variants substantially eliminate the nominal 15 μm gap to maintain a substantially conformal fit and thereby limit the extrusion of the seal  31  via the interface of components  15 ″,  33   g . Essentially, the seal support  33  is configured to be pressed against the plunger  15 ″ by the fluid pressure whereby, over time, as the components  15 ″,  33  wear, the fit can relax for a period of time whilst maintaining extrusion limiting conformal contact. By carefully controlling the geometry, the extent of pressing can be controlled so as to balance seal-retention and seal-service-life on the one hand vs generating heat (that would increase the tendency of the seal  31  to flow) and wear on the plunger  15 ″ on the other hand. 
     In this particular implementation, the seal support face  33   a  is about 0.2 mm in front of the rearward face  33   e . That 0.2 mm corresponds to about 1.25% of the 16 mm plunger diameter. The diameter of concavity of  33   i  is preferably in the range of 114%±5%, more preferably 114%±2.5%, of the plunger diameter. The outer diameter of the seal support  33  (i.e. the external diameter of the cylindrical portion forward of the ramp  33   c ) is not as critical. As this diameter grows, the wall thickness of the seal support and the surface area differential between the inside and the outside of the seal support both grow and to some extent these factors cancel each other out. Preferably this OD is in the range of 136%±10%, or more preferably 136%±5%, of the plunger diameter. 
     The backing ring portion may be upwards of 10% of the plunger diameter long. Preferably it is in the range of 20%±10%, or more preferably in the range of 20%±5%, long. The seal-surrounding portion is preferably at least 30% of the plunger diameter long, more preferably it is in the range of 55%±10% of the plunger diameter long. 
     In this particular implementation, the seal support  33  has an elastic modulus in the vicinity of 144 kN/mm 2 . Whilst other materials might be adopted, preferably the seal support has an elastic modulus in the vicinity of 144 kN/mm 2 ±25 kN/mm 2 . Other materials might be used, e.g. a stiffer material might be used by increasing the 0.2 mm dimension. 
     Whilst the innermost portion  33   g  of the seal support  33  is preferably cylindrical, other profiles might be adopted. By way of example, tests with rearwardly-diverging, slightly conical interiors showed potential for sealing against very high pressures, but wore faster than cylindrical interiors. 
     The controlled inward deformation in conjunction with the press fitted support ring  37  combine to form an advantageous sealing system with the advantages noted throughout, although each of these two things might be separately employed without the other. By way of example, the controlled deformation of the inner components of the system  29  might be employed in combination with an external sealing system akin to the tangential sealing interface  27 . Likewise, variants of the outer seal and support ring might be usefully employed about an inner body that is not shaped for controlled inward deformation. 
     The sealing system  29  and variants thereof may be employed in a variety of contexts. Advantageously, they may be employed in pumping systems, such as pumping systems akin to the system  1 . Other variants might be usefully employed in other contexts, such as in the context of a hydraulic intensifier or a hydraulic ram—e.g., instead of the plunger  15 ″ being driven by a linear actuator to create pressure in the cylinder  11 ″, the cylinder  11 ″ might be pressurised to drive the plunger  15 ″. 
     Preferred variants of the sealing arrangement  29  are capable of sealing against 90 ksi, have a reasonable service life, and are relatively inexpensive to service. In particular, typically the back stop  39  can be left in place and reused, whilst the relatively inexpensive components  31 ,  33 ,  35 ,  37 ,  41 ,  43  are periodically replaced. Advantageously, one variant of the sealing system takes the form of a service kit comprising consumable components  31 ,  33 ,  35 ,  37 ,  41 ,  43 . 
     A preferred service operation entails removing the cylinder body  11 ″ whilst the back stop  39  and plunger  15 ″ are left in place. The sealing system  31 ,  33 ,  35 ,  37 ,  41 ,  43  can then be assembled by hand and fitted over the end of the plunger and slid backwards by hand until sitting loosely in abutment with the back stop  39 . The cylinder  11  can then be replaced. For this purpose, the rear opening of the cylinder  11 ″ preferably has a lead-in  11   a ″, e.g. a 1 mm×20° lead-in. As the cylinder  11 ″ is reversed by hand, the lead-in  11   a ″ serves to guide it over the sealing system  29  as the forward portions of the system  29  are guided relatively into the interior of the cylinder. The cylinder  11  can then simply be pushed on by hand, or may require a gentle tap with a soft mallet. In this way, the annular flange  39   a  presses the ring  37  into the cylinder  11 ″. This is a much simpler operation than utilising hydraulic rams to create an accurate axial pre-load across the sealing interface  27 . 
     A preferred form of the sealing system  29  comprises an UHMWPE inner seal  31 , a bronze-nickel seal support  33  and a steel back stop  39 . UHMWPE has a very low coefficient of friction, low moisture absorption, and is resistant to tearing. Other suitably compliant materials might be used, although preferably the inner seal is formed of lower friction material than the material of the seal support  31 . 
     Bronze-nickel is also wear resistant and lower friction than many other materials. The back stop may be formed of material that is higher friction than the material of the seal support since it preferably does not contact the plunger. Resilient rings  41 ,  43  have a hardness in the vicinity of 90 Shore A Duro. Preferably, the ring  41  is formed of suitable resilient material having a lower Young&#39;s modulus than the material of the inner seal  31 . The plunger  15 ″ is formed of zirconia, partly because it is very hard and stiff. Whilst other materials might be used, preferably the plunger  15 ″ is formed of material harder and/or stiffer than the material of the seal support  33 . Corrosion-resistant materials are preferred throughout. 
       FIG.  6    illustrates a sealing system  129  comprising an inner seal  131 , a rear  133   d , a face  133   e , an inner resilient ring  141 , and portions in proximity to those components as described in respect of the inner seal  31 , rear  33   d , face  33   e  and resilient ring  41  and features in proximity to those components. In place of  FIG.  5   &#39;s conical surface  33   c , profiled outer seal  35  and anti-extrusion ring  37 , the sealing system of  FIG.  6    comprises an outer seal  135  energised by a resilient ring  143 . The system  135 ,  143  is akin to an inversion of the system  131 ,  141 . The ring  143  is carried within an outwardly-open annular groove of the seal support  133  and radially compresses the outer seal  135  against the interior of the cylinder  111 ″. This annular groove is vented to the high pressure side to energise the ring  143 . 
     The outer seal  135  is essentially a tubular sleeve. In this example, a forward outer corner of the sleeve is equipped with a lead-in  135   b , which in this example is a chamfer. The lead-in assists with assembly. 
     The outer seal  135  is backed up by the forward annular face  139   b  of the backstop  139 . The cylinder  111 ″ and the back stop  139  have complementary conical faces  111   b ″,  139   c ″ to form a seal to impede extrusion of the outer seal  135 . 
     In this case, the rearwardly-divergent surfaces  111   b ″,  139   c  are conical. Other shapes are possible. The surface  111   b  rearwardly diverges at a slightly larger angle of divergence and has a slightly larger inner dimension (inner diameter in this case) than the surface  139   c . This leads to a conical contact patch between the surfaces  111   b ″,  139   c , the interior of which is defined by the interior of the cylinder  111   b  and the outer bounds of which depend on the axial compression and the resilience of the materials. 
     Eliminating the ring  37  simplifies the construction. Moving the resilient ring  143  to the inside of the outer seal extends the life of the ring. The inventors have recognised that, as the pumping chamber is periodically pressurised, the cylinder  111 ″ periodically expands, which expansion wears the components in contact therewith. The seal  135  is more durable and has a lower coefficient of friction than the ring  143 . 
       FIG.  7    illustrates a sealing system  229 . Variants of the sealing system  229  may incorporate the features and the potential variations described in connection with sealing systems  29 ,  129 . 
     The sealing system  229  serves to seal between the interior of the cylinder  211  and the exterior of the plunger  215  and comprises inner seal  231 , seal support  233 , outer seal  235 , peripheral support ring  237   a  and inside support ring  237   b . Relative to the sealing system  29 , the exterior of the seal support  233  is reprofiled. The conical portion  33   c  is removed and essentially replaced by the inside support ring  237   b . This change avoids stress within the seal carrier associated with axial loading on the face  33   e.    
     The exterior of the seal support  233  is substantially cylindrical and comprises a shallow lead-in at its forward end. The inside support ring  237   b  has a cylindrical interior to complement the exterior of the seal support  233 . The components  233 ,  237   b  are carefully toleranced to provide a fit that allows for the ring  237   b  to have an internal diameter ranging from 70 microns smaller up to 10 microns larger than the external diameter of the seal support  223  (in the context of a 16 mm diameter plunger, similar figures may be scaled up or down to suit other sealing systems). 
     The inside support ring  237   b  is backed up by the back stop  239 , or more specifically by the annular flange  239   a  of the back stop  239 . In this case, the inside support ring  237   b  has a triangular profile and presents a radial rear face to a radial front of the flange  239   b . The peripheral support ring  237   a  also has a triangular profile. The support rings  237   a ,  237   b  radially bracket and conformally contact a tapered-in-profile rear of the outer support ring  235 . 
     A rear of the inner seal  231  is defined by the face  233   a  positioned 1 mm in front of the face  239   b . In this way the system  229  is configured for the seal carrier  233  to be radially compressed by the fluid pressure to increase (e.g. create) pressure at the interface between components  215 ,  233 . Generally speaking, increasing the 1 mm dimension increases the corresponding pressure on the interface. Preferably this dimension is in the range of 0.2 mm to 2 mm. The 0.2, 1 and 2 mm dimensions are advantageous in the context of a 16 mm diameter plunger but may be scaled up or down to suit sealing about other cylindrical exteriors. The ideal value for this key dimension will depend on a range of factors including the preferred interface pressure, the pressure against which the sealing system is sealing, and the radial thickness of the seal-surrounding portion. Generally speaking, for a given preferred interface pressure, the ideal dimension will vary in negative relation to the operating pressure. 
     The cylinder  211  has a planar radial rear  211   a  presented rearwardly towards the back stop  239 . This face is an abutment for providing force in reaction to a preload by which the components  211 ,  239  are pressed towards each other. 
     The flange  239   a  is dimensioned to fit inside the cylinder  211  with not more than a small degree of interference, e.g. without contact. Preferably the flange  239 A and the cylinder  211  present complementary cylindrical faces to each other. 
     The back stop  239  comprises a radial annular abutment face  239   c  outwards of annular setback  239   d . In this way, the components  211 ,  239  are co-operatively configured to abut each other at locations at least twice, preferably at least three times, a radius of the interior of the cylinder  211  from a centre line of the sealing system. Moving these abutments portions outwards helps to take stress away from the stressed inboard areas of the cylinder  211 . 
     The annular setback  239 D defines an annular region of clearance at the back of the cylinder  211 , which annular region of clearance is internally bounded by the cylindrical interior of the cylinder  211 . 
     The present inventors have recognised that under cyclical high pressure loading the various components expand and contract at differing rates. By leaving the complementary cylindrical faces as the only points of potential contact between the components  211 ,  239  radially inwards from the abutment face  239 C, the potential for the faces of these two components to slide over each other as a result of the cyclical expansion and contraction is reduced so as to avoid the potential surface damage associated therewith and thereby extend the life of the components. There are other ways in which this annular region of clearance might be created, e.g. whilst the abutment face  239 C takes the form of a proud feature on the front of the back stop  239 , in another variant the cylinder  211  might have a proud feature about its outer periphery. 
     In this example, the portions  211   a ,  239   c  are planar radial faces to transmit the axial preload without generating radial forces unlike the force at the interface  27  that comprises a radial component outwardly urging the cylinder  11 ′. 
     In this example, the face  239   c  is a circular face save for a single radial break. The radial break is but one potential example of a drain provided to give fluid somewhere to go (and thereby potentially minimise damage) should the outer seal  235  fail. 
     The described geometry advantageously minimises sliding interactions between the components  211 ,  239  associated with cyclical expansion and contraction of the components in operation. 
     In this example, the components  211 ,  239  present planar radial faces sitting in direct contact with each other. Such faces are relatively easy to produce accurately although there are other options. By way of example, in principle, net zero radial force could be produced with various complementary stepped profiles etc. Potentially components  211 ,  239  might be separated by one or more spacers. 
     The invention is not limited to the various examples disclosed herein. Rather the invention is defined by the claims. 
     The term “comprises” and its grammatical variants has a meaning that is determined by the context in which it appears. Accordingly, the term should not be interpreted exhaustively unless the context dictates so.