Patent Publication Number: US-2012042773-A1

Title: Pump Piston Device

Description:
This invention relates to a pump piston. More particularly, it relates to a pump piston for reciprocating movement in a pump cylinder in which, between the pump piston and the pump cylinder, an annular space is formed which is defined by a first seal and a second seal, the annular space being pressurized. 
     In piston pumps working with particle-containing or non-lubricating fluids, seal wear may be a considerable problem, in particular in the case of high pressures. 
     According to the prior art, it is sought to relieve the problem by arranging scrapers or several successive seals, for example, around the piston. 
     WO document 2005/080836 discloses a pump in which the piston is provided with two seals with an intermediate annular space. 
     U.S. Pat. No. 5,701,797 and EP patent application 1975409 deal with pumps in which there are two seals arranged in the cylinder pipe. 
     NO patent 321183 discloses a piston in which, on a portion of the external jacket surface of the piston facing the pressure side of the piston, an axially movable sleeve is formed, sealing against the piston. Externally, the piston and the sleeve are each provided with a respective seal sealing against the cylinder wall. The area between the external seals of the piston and the sleeve is pressurized by means of a fluid. During the pump stroke, the piston and the sleeve are moved towards the fluid which is to be pumped. The sleeve is thereby moved somewhat along the piston, so that pressure builds up between the two seals. The sleeve is of such a design that the pressure is approximately the same on both sides of the seal surrounding the sleeve. The movement of the sleeve on the piston and the relatively large cross-sectional area of the sleeve reduce the volumetric efficiency of the pump. 
     The invention has for its object to remedy or reduce at least one of the drawbacks of the prior art. 
     The object is achieved according to the invention through the features which are specified in the description below and in the claims that follow. 
     A pump piston is provided for reciprocating movement in a pump cylinder in which, between the pump piston and the pump cylinder, an annular space is formed, defined by a first seal and a second seal, the annular space being pressurized and the first seal and the second seal being in a common piece of material which constitutes the pump piston. The pump piston is characterized by a compensation piston being movable in a compensation cylinder in the pump piston, the compensation piston communicating, by its first end portion, with the pressure side of the pump piston, and, at its second end portion, with the annular space via a piston channel. 
     Thus, it is possible to form the piston in a simple way, as the first seal and the second seal can be arranged in a common piece of material. 
     The pump piston may be crank-operated or moved in other known ways, for example hydraulically or pneumatically. 
     A shoulder may prevent the compensation piston from being moved out of the compensation cylinder. 
     The annular space may be supplied with pressure fluid via a check valve and a cylinder channel. The pressure fluid may thereby be supplied to the annular space at a considerably lower pressure than the operating pressure of the piston pump, as the check valve will close to the return flow of pressure fluid during the working stroke of the piston pump. 
     The operation of the pump piston is described in the characterizing part of the application. 
     A pump piston in accordance with the invention exhibits the advantageous features from NO 321183 with respect to seal life, with just an insignificant reduction in the volumetric efficiency. 
    
    
     
       In what follows is described an example of a preferred embodiment which is visualized in the accompanying drawings, in which: 
         FIG. 1  shows schematically a piston pump which is provided with a pump piston in accordance with the invention, the piston being moved in its suction stroke; and 
         FIG. 2  shows schematically the same as  FIG. 1  but during the working stroke of the piston pump. 
     
    
    
     In the drawings, the reference numeral  1  indicates a piston pump comprising a pump casing  2  with a pump cylinder  4 , a crankcase  6  and a valve block  8  for the medium which is to be pumped. 
     A driven crank  10  is supported in the crankcase  6  and connected, in a manner known per se, to a pump piston  12  by means of a crank rod  14 . The pump piston  12  is reciprocatable in the pump cylinder  4 . 
     The valve block  8  is formed with an inlet valve  16  and an outlet valve  18 . 
     The pump piston  12  is provided with a first surrounding seal  20  which is relatively close to its pressure side  22 , and a second surrounding seal  24  on the crank side of the first seal  20 . 
     An annular space  26  is defined by the pump cylinder  4 , the pump piston  12  and the seals  20 ,  24 . 
     A compensation cylinder  28  has been bored into the pump piston  12  from the pressure side  22  of the pump piston  12 . A compensation piston  30  is sealingly and movably arranged in the compensation cylinder  28 , a compensation piston chamber  32  being defined by the compensation cylinder  28  and the compensation piston  30 . The compensation piston chamber  32  communicates with the annular space  26  via a piston channel  34 . 
     A shoulder  36  prevents the compensation piston  30  from being moved out of the compensation cylinder  28 . A supporting spring  38  may be arranged in the compensation piston chamber  32  and be arranged to move the compensation piston  30  in the direction towards the shoulder  36 . 
     A feed pump  40  supplies the annular space  26  with pressure fluid via a pipe  42 , a check valve  44  and a cylinder channel  46 . The pressure from the feed pump  40  is restricted by an overpressure valve  48 . 
     During operation, the annular space  26  is filled with a preferably lubricating pressure fluid by the feed pump  40  via the pipe  42 , the check valve  44  and the cylinder channel  46 . An appropriate maximum pressure, which may be relatively low in relation to the maximum pressure of the piston pump  1 , has been set on the overpressure valve  48 . 
     Pressure fluid flows from the annular space  26  via the piston channel  34  into the compensation piston chamber  32 , whereby the compensation piston  30  is moved until it hits the shoulder  36 . 
     During the suction stroke of the piston pump  1 , see  FIG. 1 , the pump piston  12  is moved in the direction towards the crank  10 , the inlet valve  16  opens and pumped liquid enters the piston pump  1 . The compensation piston  30  stays in its position, bearing against the shoulder  36 , because of the pressure in the compensation piston chamber  32  and a possible underpressure in the piston pump  1 . 
     When the piston pump  1  starts its pump stroke, see  FIG. 2 , the inlet valve  16  closes, whereby the pressure in the piston pump  1  increases until the outlet valve  18  opens. The increased pressure in the piston pump  1  works against the compensation piston  30  which is moved inwards in the compensation cylinder  28 . The pressure in the compensation piston chamber  32  increases by the check valve  44  closing. The annular space  26  thereby gets the same pressure as the piston pump  1 . 
     Thus, during the pressure stroke of the piston pump  1 , there is approximately the same pressure on both sides of the first seal  20 , which substantially reduces seal wear, even when pumping particle-laden liquid. 
     The second seal  24 , sealing against a clean lubricating fluid, receives full pumping pressure.