Abstract:
A method and device for forming a sliding seal between a reciprocating piston ( 22 ) and the wall of a cylinder ( 8 ), wherein one side of the piston faces a first liquid in a pump chamber ( 26 ) of the cylinder ( 8 ), a first piston seal ( 24 ) and a second piston seal ( 58 ), spaced from the first piston seal ( 24 ), sealing between the piston and the cylinder ( 8 ); and wherein a second liquid is supplied under pressure between the seals ( 24, 58 ), the pressure of the second liquid between the seals ( 24, 58 ) being maintained at a level essentially equal to or slightly lower than the pressure of the pump chamber ( 26 ).

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   The present application is the U.S. national stage application of International Application PCT/NO01/00438, filed Nov. 1, 2001, which international application was published on May 8, 2003 as International Publication WO 03/038320. 
   BACKGROUND OF THE INVENTION 
   This invention relates to a method and a device for use in connection with pump pistons of reciprocating pumps. 
   Reciprocating pumps are widely used within the industry for pumping liquid at both high and low pressures. 
   In principle such pumps comprise a rotating crank eccentric with a connecting rod that has a pump piston arranged there-to, said pump piston being reciprocated, thereby providing a pumping effect. On the pressure side of the piston, suction and pressure valves are arranged. The pump piston is provided with seals which form a seal between the liquid which is be ing pumped and the surroundings. 
   When liquids containing particularly much contamination, like e.g. sand particles, are to be pumped, the seal is subjected to great wear, resulting in repairs and cost. 
   Wear occurs because the seal is subjected to excessive pressure differences which make contaminants penetrate between the seal and the piston. Thereby great wearing action occurs. 
   SUMMARY OF THE INVENTION 
   As compared to known technique the object of the invention is to provide a solution which reduces wear on pump seals and pistons that are subjected to the action of contaminated liquid. 
   This is achieved, according to the invention, in that on its side opposite the pump pressure side the (first) piston seal of the reciprocating pump is subjected to a liquid pressure which balances the liquid pressure from the pump chamber of the pump. 
   The liquid pressure applied to the back-surface of the first piston seal is formed by a clean liquid which contains a lubricator and has no contamination. 
   By subjecting the first piston seal to approximately equal liquid pressures on both sides, the possibilities of contamination entering between the seal and the pump piston are reduced. Thereby the wear on the piston seal is reduced to a substantial degree. 
   The liquid pressure acting on the back surface of the first piston seal is generated in that, relative to the pump chamber, the opposite end portion of the pump piston projects into a liquid volume of a diameter approximately equal to the diameter of the pump piston. The pressure from this chamber is directed into an annular space between the pump piston and the cylinder of the reciprocating pump, defined by the first piston seal and a second piston seal, the second piston seal sealing against leakage to the surroundings. 
   EP patent 694139 B1 discloses a sealing system which is used for sealing between a piston and a cylinder. This invention differs substantially from the present invention in that the pressure between the seals does not equal the pressure of the pump volume. 
   WO 86/03817 shows a piston and a device which may be used in pumps to supply liquid into the piston itself. 
   U.S. Pat. No. 2,615,769 shows a configuration of a pump piston with two piston rings and a pressurized liquid between them. 
   The publications EP 158457, FI 843735 and FI 65477 show different embodiments of pistons, in which liquid under pressure is used in order to seal between a piston and a cylinder wall. 
   The present invention differs from known technique in that the pressure between the first and the second piston seals approximately equals the pressure within the pump chamber at all times. According to the invention the first piston seal is thus always subjected to equal pressures on both sides. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be explained in further detail in connection with the description of an exemplary embodiment referring to accompanying drawings, in which: 
       FIG. 1  shows a section through a reciprocating pump, in which compensating liquid is supplied through the piston rod; 
       FIG. 2  shows, on a larger scale, a section of the pump of  FIG. 1 ; 
       FIG. 3  shows the same as  FIG. 2 , but with an alternative piston stopping device; 
       FIG. 4  shows a section through a second exemplary embodiment of a reciprocating pump, in which compensating liquid is supplied through the cylinder wall; and 
       FIG. 5  shows, on a larger scale, a section of the pump of  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the drawings the reference numeral  1  identifies a reciprocating pump in an embodiment essentially known, comprising a crank case  2  with a rotating crank  4 , a sliding guide housing  6 , a cylinder  8  which is fixedly connected to the sliding guide housing  6  and has an end cap  9 , and a valve housing  10  is fixedly connected in a sealing manner to the cylinder  8 . A bearing housing  12  is rotatably connected by means of a roller bearing  14  to the eccentric portion of the crank  4 . A piston rod  16  is fixedly connected at one end portion to the bearing housing  12 , and is provided, at its opposite end portion, with a spherical bearing surface  20  bearing on a guide element  18 . 
   Because of the eccentric position of the bearing  14  relative to the centre of the crank  4 , a reciprocating motion is imparted to the guide element  18 . 
   The valve housing  10  is provided with check valves, not shown, according to known technique. 
   A piston  22 , see  FIG. 2 , is disposed in a known manner in the cylinder  8  with its one end portion  22 ′ sealing by means of a first piston seal  24 , and is arranged to be reciprocated within a pump chamber  26 . 
   According to the invention, by its opposite end portion  22 ″, the piston  22  is positioned sealingly, by means of a seal  27 , in a chamber  28  in the form of a cylindrical recess  18 ′ in a guide element  18 . A helical spring  30  extends in an annular groove  32  in the piston  22  between the piston  22  and the guide element  18 . The helical spring  30  is arranged to displace the piston  22  in a direction away from the guide element  18  so that the camber  28  will be as large as possible. A stopper in the form of a bolt  34  threaded into the guide element  18 , and extending sealingly through a bore  36  through the piston  22  as is shown in  FIGS. 1 and 2 , or in the form of a groove  38  disposed externally on the piston  22  and a preferably two-part locking plate  40 , as is shown in  FIG. 3 , pre vent the piston  22  from being displaced out of the pressure chamber  28 . 
   A feed pump  42  feeds the pressure chamber  28  through a check valve  44 , a hose  46 , a bore  48  of the bearing housing  12 , a bore  50  of the piston rod  16  and a bore  52  of the guide element  18 . A bore  54  connects the through bore  36  of the piston  22  with the external cylinder surface of the piston  22  and opens into an annular space  56  between the piston  22  and the cylinder  8  defined by the first piston seal  24  and a second piston seal  58 . A seal  60  surrounds the bolt  34  and prevents liquid from flowing between the pump chamber  26  and the pressure chamber  28 . Liquid present in the pressure chamber  28  communicates with the annular space  56  through the channels/bores  36  and  54 . 
   When the feed pump  42  is started, the pressure chamber  28  is filled with clean liquid through the valve  44  and the hose/bores  46 ,  48 ,  50  and  52 . The liquid pressure on the end surface of the piston  22  within the pressure chamber  28  and the compressive force from the spring  30  displace the piston  22  in a direction from the guide element  18 , so that the piston  22  bears on the head of the bolt  34 , alternatively on the locking plate  40 . From the pressure chamber  28  liquid flows through the bores  36  and  54  to the annular space  56 . The liquid pressure from the feed pump  42  is insignificant compared to the normal working pressure of the pump  1 . 
   When the guide element  18  and thereby the piston  22  are displaced in a direction from the pump chamber  26  by means of the connecting rod  16 , bearing housing  12  and bearing  14  due to the rotation of the crank  4 , particle-containing liquid is drawn from the intake manifold  62  of the pump  1  through the valve housing  8  into the pump chamber  26 . 
   When, during their pumping stroke, the guide means  18  and the piston  22  are next displaced in a direction towards the pump chamber  26 , the particle-containing liquid present in the pump chamber  26  flows out through the valve housing  10  and further into the pressure manifold  64  of the pump  1 . During the pumping stroke a force is exerted by the liquid present in the pump chamber  26  on that end surface of the piston  22  which is inside the pump chamber  26 . This liquid pressure force displaces the piston in a direction towards the guide element  18 , where the opposite end portion  22 ″ of the piston  22  is positioned in the cylindrical recess  18 ′. The piston  22  has an equally large end area at either end portion  22 ′ and  22 ″ as the bolt  34  is cylindrical. The liquid pressure inside the chamber  28  is thus essentially identical with the pressure inside the pump chamber  26 . An insignificant pressure difference between the two chambers  26  and  28  may occur due to the moment of mass inertia of the piston, the compressive force of the helical spring  30  and the pressure from the feed pump  42 . 
   The check valve  44  prevents a flow of liquid from the chamber  28  to the feed pump  42 . The liquid pressure inside the chamber  28  propagates through the bore  36  and  54  to the annular space  56 . The first piston seal  24  is thereby subjected to essentially equal pressure from both sides. The second piston is seal  58  prevents liquid from exiting between the cylinder  8  and the piston  22 . 
   The first piston seal  24  being pressure-sealing only to an insignificant degree, it is subjected to substantially less penetration of particle-containing liquid from the pressure chamber  26 . The life of the first piston seal  24  is thus substantially extended. 
   Any leakage of clean liquid is replenished by the feed pump  42 . 
   In an alternative embodiment, see  FIGS. 4 and 5 , the pressure chamber  28  is charged with clean liquid from the feed pump  42  through the check valve  44 , a pipe  66 , a channel  68  of the cylinder  8  opening into the annular space  56 , the channel/bore  54  and the channel/bore  36 . The helical spring  30  is positioned in the pressure chamber  28 . The operation is identical to that which is described above.