Patent Publication Number: US-2012024037-A1

Title: Compact provers

Description:
This invention concerns improvements in or relating to compact prover assemblies. 
     Compact provers are used to determine the accuracy of flowmeters, and particularly turbine, ultrasonic, coriolis and positive displacement flowmeters. Conventionally compact provers include a piston and cylinder, with fluid flowing into the cylinder moving the piston between two predetermined points. A flow computer measures the length of time taken for the piston to move between these points. The computer may also calculate any expansion in the cylinder circumference due to the internal pressure and temperature, and compensate for this. The resulting measured flow rate is verified by further piston runs to obtain a measured time within a small tolerance of repeatability. 
     After the piston has moved between the two predetermined points it is necessary to move the piston back to beyond the first of the predetermined points to enable a further measurement to be carried out. Whilst the piston is being moved back it is necessary for fluid to be able to move therepast. Conventionally poppet valves have been provided on the pistons which can be opened to allow fluid to pass through the pistons. The closing of the poppet valves however causes pressure fluctuations within the cylinder which is undesirable. 
     According to the present invention there is provided a compact prover assembly, the assembly comprising a piston and cylinder arrangement, the piston and cylinder arrangement including a cylinder inlet and a cylinder outlet each towards a respective end of the cylinder, a piston movable by fluid flowing through the inlet towards the outlet, means for measuring the time taken for the piston to move between predetermined points in the cylinder, the piston including a flow past arrangement, which flow past arrangement includes an opening and a cover, which cover is selectively movable substantially parallel to the piston between a closed position closing the opening, and an open position clear of the opening to permit fluid to pass through the opening and hence through the piston when the piston is being moved backwards towards the inlet. 
     The opening may be provided in a first part of the piston, with the cover provided in a second part of the piston. 
     An opening may be provided in the second part of the piston, alignable with the opening in the first part, when the cover is in the open position. 
     A plurality of openings may be provided in the first part, and a corresponding number of openings may be provided in the second part. The first and second parts may be rotatably movable relative to each other. 
     The first and second parts may be in the form of coaxially mounted discs. 
     An actuator may be provided for moving the cover between the open and closed positions. The actuator may be in the form of a piston and cylinder. The piston and cylinder may be engageable between actuation members respectively connected to the first and second parts of the piston. 
     The assembly may be arranged such that in a failure condition of the actuator the second part moves to or remains in the open position. 
     The assembly may be arranged such that the second part is automatically moved to an open condition when the piston is being moved back towards the inlet. 
    
    
     
       An embodiment of the present invention will now be described by way of example only, with reference to the accompanying drawings, in which:— 
         FIG. 1  is a diagrammatic cross sectional side view of a compact prover assembly according to the invention, sharing different conditions of use of the assembly; 
         FIGS. 2   a ,  2   b  and  2   c  are diagrammatic end views of parts of the assembly of  FIG. 1  in different conditions of use; 
         FIG. 3  is a diagrammatic cross sectional side view through part of the assembly of  FIG. 1 ; and 
         FIGS. 4   a ,  4   b  and  4   c  are diagrammatic rear end views of part of the assembly shown in  FIG. 3  in different conditions of use. 
     
    
    
     The drawings show a compact prover assembly  10  suitable for determining the accuracy of a flow meter. The assembly  10  includes a piston  20  and cylinder arrangement  12 . The arrangement  12  includes a cylinder  14  with an inlet  16  and outlet  18  towards respective ends of the cylinder  14 . The cylinder  14  locates a piston  20  mounted on a piston rod  22 . 
     Spaced sensors  24  are provided outside of the cylinder  14  adjacent to the piston rod  22 , and are interconnected so as to measure the time taken for the piston rod  22  and hence piston  20  to move a predetermined distance. The time taken for the piston  20  to move the predetermined distance is accurately measured, and any expansion in the cylinder circumference due to internal pressure and temperature is measured and used as compensation in respect of this time. This process is repeated on a number of occasions, and the data collated. 
     The piston  20  includes a flow part arrangement provided by a disc  26  which forms the front (left hand side in  FIG. 1 ) face of the piston  20 . The disc  26  has a ring of six equally spaced outer circular holes  28  and six smaller equally spaced circular holes  30 . A second disc  32  is located immediately to the right as shown in  FIG. 1  of the first disc  26  and is coaxially rotatable relative thereto. The second disc also includes an outer spaced ring of circular holes  34  and inner circular holes  36  in a similar configuration to the holes in the first disc  26 . 
     The first and second discs  26 ,  32  are mounted to the piston rod  22 . The piston rod  22  has inner and outer parts  36 ,  38  which are freely rotatable relative to each other. The inner part  36  is mounted at the left hand end to a profiled block  40  around which the second disc  32  is freely rotatable. To the left of the first disc  26  as shown, an outer sleeve  42  is provided which connects the first disc  26  to the inner part  36 . 
     At the far end of the profiled block  40  an annular end member  44  is provided and held in position by a split pin  46 . A spring washer is provided between the end member  44 , and the outer sleeve  42  to urge the first and second discs  26 ,  32  against each other. The outer part  38  of the piston rod  22  is connected to the second disc  32  by a sleeve  49  to cause rotation thereof. An annular circumferential seal  50  extends around the first disc  26 , which is sealingly engageable with the inside of the cylinder  14 . 
     An outer engagement member  52  extends radially from the outer part  38  of the piston rod  22  to permit rotation thereof. The outer engagement member  52  has a rearwardly extending finger  54 . An inner engagement member  56  is mounted to the right hand end of the inner part  36  of the piston rod  22 , which extends beyond the right hand end of the outer part  38 , to permit rotation of the inner part  36 . The inner engagement member  56  has a radially extending part and a forward facing finger  58 . The fingers  54 ,  58  are circumferentially offset so as to define a gap therebetween. A piston and cylinder actuator  60  extends between the fingers  54  and  58 . 
     A connection point  62  is provided at the right hand end of the piston rod  22  to permit connection to line means or otherwise for moving the piston  20  to the right as shown following a time recordal movement of the piston  20 . A flag member  64  is provided on the left hand end of the outer engagement member  52 , which member  64  breaks a beam from either of the sensors  24  to be detected thereby as the member  64  moves past one of the sensors  24 . The beam could be, for example, a laser, light, or magnetic beam. 
     In use, it is required to measure the time for the piston  20  to be moved by fluid flowing through the cylinder  14  to the left from the position shown in  FIG. 2   c , with the inner and outer engagement members  52 ,  56  in the position shown in the right hand most view as shown in  FIG. 1 , to the position shown in  FIG. 2   a . Initially the actuator  60  is at rest with the engagement members  52 ,  56  closer together as shown in  FIG. 4   c . In this position the respective holes  28 ,  34  and  30 ,  36  are aligned as shown in  FIG. 2   c , and fluid entering through the inlet  16  can pass through the piston  20  and back out through the outlet  18 . 
     To measure the accuracy of the flow meter the actuator  60  is operated to move through the position shown in  FIG. 4   b  to the position shown in  FIG. 4   a . In this instance the respective holes  28 ,  34  and  30 ,  36  are unaligned such that the discs  26 ,  32  provide a solid barrier to fluid entering through the inlet  16 , as shown in  FIG. 2   b . The fluid will move the piston  20  through the position  2   b  to the position  2   a,  with the sensors  24  detecting the flag member  64  and hence the time taken for the piston  20  to move the predetermined distance. 
     Once in the position  FIG. 2   a  the actuator  60  can be deactivated to return to the condition shown in  FIG. 4   c  which will cause the disc  32  to rotate relative to the disc  26  to bring the holes  28 ,  34  and  30 ,  36  back into alignment with each other. The piston  20  can then be moved back to the position shown in  FIG. 2   c , with the aligned holes permitting fluid to pass therethrough. 
     There is thus described a compact prover assembly, and particularly a piston for a compact prover, which provides significant advantages in providing a solid piston for use when proving a meter, but which readily permits fluid to pass therethrough when being moved back to a rest condition, or when in a rest condition. In contrast to poppet valves, there is no increase in pressure and thus no pressure spike produced when the piston is moving between conditions. The arrangement is though of relatively straightforward construction and can thus provide accurate and long term operation. 
     Various modifications may be made without departing from the scope of the invention. For instance the two parts of the piston may be differently formed and need not be made of two discs. A different arrangement of openings could be provided. 
     Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.