Patent Publication Number: US-2007119227-A1

Title: Reference volume tube

Description:
TECHNICAL FIELD  
      The present invention relates to a reference volume tube which calibrates a flow meter to be tested based on a reference volume indicated through movement of a moving element by a predetermined section in a measurement tube portion having a reference volume, and more exactly, based on the volume of a fluid discharged when the moving element moves by the predetermined section and substantially equal to the reference volume.  
     BACKGROUND ART  
      It is obligatory for a meter to undergo calibration after each fixed period of time to check (test) whether or not the metering precision is within a fixed range.  
      One of the flow meter calibration (testing) methods uses a reference volume tube as a calibration device.  
      In this method, a measurement tube portion (volume tube or prover pipe) having a volume serving as a reference (hereinafter referred to as reference volume) and a flow meter constituting an object of calibration (hereinafter referred to as flow meter to be tested) are connected in series, and calibration is performed on the flowmeter to be tested based on the volume of a fluid discharged when a moving element moves by a predetermined section in the measurement tube portion and substantially equal to the reference volume. Here, assuming that an instrumental error is E, the volume (flow rate) measured by the flow meter to be tested is I, and the reference volume of the reference volume tube is Q, E is obtained as follows: E=(I−Q)/Q×100(%).  
      Reference volume tubes can be roughly classified into unidirectional provers and bidirectional provers.  
      In a method using the former, i.e., a unidirectional prover, the measurement tube portion is formed as a loop tube or a straight tube, and two detectors are provided at both ends of the measurement tube portion. Inserted into the measurement tube portion is the moving element, such as a highly elastic ball (hereinafter referred to as sphere) whose diameter is approximately 2 to 4% larger than an inner diameter of the measurement tube portion, or a piston. Through movement of the moving element in one direction between the two detectors, testing is effected on the flow meter to be tested. When the test is to be repeatedly conducted, after completion of one test, the moving element, which has reached to a terminal end of the measurement tube portion, is returned to a start end of the measurement tube portion for the next test. This can be effected manually or by connecting the terminal end and the start end of the measurement tube portion for automatic circulation (see, for example, JP 2931149 B, JP 11-304572 A, and “Chapter 4, Section 2 of Petroleum Measurement Standard Manual” (issued by American Petroleum Institute on June, 1988)).  
      In a method using the latter, i.e., a bidirectional prover, an apparatus of substantially the same construction as that of the unidirectional prover is used. After the moving element has moved in one direction between two detectors, the flow path is switched by a valve or the like, causing the moving element to move in the opposite direction (see, for example, “Chapter 4, Section 2 of Petroleum Measurement Standard Manual” (issued by the American Petroleum Institute on June, 1988)).  
      To achieve a high level of accuracy, a reference volume tube whose measurement tube portion is formed as a straight tube requires a measurement tube portion having a length dimension that is, for example, twice as large as that of a reference volume tube whose measurement tube portion is formed as a loop tube. To avoid this problem, there is also used a small volume compact prover reduced in reference volume by pulse interpolation and made small enough to be portable. Such a small volume compact prover may also be regarded as a reference volume tube in a broad sense.  
      A conventional unidirectional prover (hereinafter simply referred to as reference volume tube) will be further described with reference to  FIG. 1 .  
      A reference volume tube  1   a  is equipped with a prover pipe  2   a  having a reference volume. The reference volume is determined beforehand through precise measurement of the volume of the section between a first detector  6   a  and a second detector  7   a . A fluid having passed through a flow meter to be tested (not shown) connected to an inlet tube  3   a , flows from the inlet tube  3   a  to an outlet tube  4   a  via the prover pipe  2   a . Here, calibration is effected based on the volume of the fluid discharged when a sphere  5   a  moves within the prover pipe  2   a  through the section from the first detector  6   a  to the second detector  7   a . That is, the volume of the fluid discharged when the sphere Sa moves through the above section, is substantially equal to the reference volume which is indicated owing to the movement of the sphere  5  through the section. By comparing this reference volume with the indication value (volume) of the flow meter to be tested, calibration is effected.  
      The reference volume tube  1   a  adopts a method in which the sphere  5  is automatically circulated, and is equipped with a passage portion  8  for extracting the sphere  5   a  that has completed the section movement from the terminal end of the prover pipe  2   a  to release it at the start end of the prover pipe  2   a.    
      The passage portion  8  is provided with valves  8   a ,  8   b  and a relay portion  8   c  for keeping the sphere  5   a  on standby between the valves  8   a ,  8   b . Reference symbol  8   d  indicates a sphere lock member control device.  
      At the time of measurement, the fluid is previously caused to circulate through the prover pipe  2   a , and a steady state is maintained so that a predetermined flow velocity of approximately 3 m/secas recommended, for example, in American Petroleum Institute (API) Standard, can be achieved in a stable manner (see, for example, “Chapter 4, Section 2 of Petroleum Measurement Standard Manual” (issued by the American Petroleum Institute on June, 1988)).  
      Subsequently, the valve  8   a  is closed, and the valve  8   b  is opened to release the sphere  5   a  into the stable flow of the fluid in the prover pipe  2   a.    
      As a result, it is possible to conduct test and calibrating operation with high accuracy. According to the American Petroleum Institute Standard, inclusive of the case of a bidirectional prover described below, the reproducibility at the time of calibration should be ±0.01%. According to the Japanese Measurement Law, an accuracy of 1/3,000 to 1/5,000 is required. Thus, both standards require a high level of accuracy.  
      Next, a conventional bidirectional prover (hereinafter simply referred to as reference volume tube) will be further described with reference to  FIG. 2 .  
      Like the reference volume tube  1   a , which is a unidirectional prover, a reference volume tube  1   b  is equipped with a prover pipe  2   b  having a reference volume. The prover pipe  2   b  is equipped with two detectors  6   b ,  7   b . At both ends of the prover pipe  2   b , there are respectively provided tube sections (header portions)  9   a ,  9   b  whose diameter is larger than that of the prover pipe  2   b.    
      Normally, dimensions of the prover pipe  2   b  are determined as follows.  
      The reference volume is determined to be approximately 0.5% or more of the maximum test flow rate (per unit time). On the other hand, the flow velocity of the fluid, in other words, the moving speed of the sphere, is set to approximately 1.5 m/sec, which is lower than that of the unidirectional prover described above. Through the determination of these two values, the pipe diameter of the prover pipe is determined of necessity.  
      For example, when the maximum test flow rate is 2,000 3 /H, the reference volume is approximately 10 m 3 , and the pipe diameter (diameter) of the prover pipe is approximately 0.69 m. The distance between the two detectors corresponding to the reference volume at this time is approximately 27 m.  
      In the reference volume tube  1   b , fluid tubes  3   b ,  4   b  and the two tube sections  9   a ,  9   b  are connected to one another by a four-way valve  9   c  so as to allow switching of flow passage.  
      Prior to the measurement, a flow meter to be tested (not shown) is attached to one of the fluid tubes  3   b ,  4   b.    
      Then, the four-way valve  9   c  is operated to switch the flowing direction of the fluid. For example, communication is established between the fluid tube  3   b  to which the flow meter to be tested is attached and the tube section  9   a , and communication is established between the fluid tube  4   b  and the tube section  9   b . At this time, the fluid that has been existing since before the switching of the flowing direction stays in the prover pipe  2   b  and the tube section  9   a  in a fluid-tight fashion. Further, in the pipe section  9   a , the sphere  5   b , which has moved from the prover pipe  2   b , is arranged in advance.  
      As the valve opening of the four-way valve  9   c  increases, the flow velocity of the fluid entering the tube section  9   a  increases gradually, until it finally attains a predetermined flow velocity, with the result that the sphere  5   b , which exhibits the predetermined flow velocity along with the fluid, moves within the prover pipe  2   b  through the section between the two detectors  6   b ,  7   b , and the measurement is performed, with the sphere  5   b  further reaching the tube section  9   b.    
      In the next measurement, the four-way valve  9   c  is operated to establish communication between the fluid tube  3   b  and the tube section  9   b  and between the fluid tube  4   b  and the tube section  9   a , introducing the fluid into the prover pipe  2   b  from the tube section  9   b  with the flowing direction being changed, whereby the measurement is effected through movement of the sphere  5   b  within the prover pipe  2   b  through the section between the two detectors  7   b ,  6   b , with the sphere  5   b  reaching the tube section  9   a.    
      Of the two types of reference volume tubes  1   a  and  1   b  described above, the former, i.e., the reference volume tube  1   a , which is a unidirectional prover, involves a complicated apparatus structure due to the provision of the passage portion  8 , whereas the latter, i.e., the reference volume tube  1   b , which is a bidirectional prover, has no passage portion, which means the apparatus structure is so much the simpler.  
     DISCLOSURE OF THE INVENTION  
      However, in the latter, i.e., the reference volume tube  1   b , the operation of switching the four-way valve  9   c  requires a long time of, for example, more than 10 seconds.  
      Accurate measurement is impossible unless the reference volume tube is designed such that the sphere  5   b  causes the detector  6   b  or  7   b  to operate after the operation of switching the four-way valve  9   c  has been conducted and a state has been attained in which all the fluid having passed the flow meter to be tested flows in. In view of this, a distance of, for example, approximately 7.5 m is secured between the tube section  9   a  or  9   b  and the detector  6   b  or  7   b  (indicated by reference symbol  1  of  FIG. 2 ), respectively, thus providing a sufficient runway.  
      That is, since the start of the inflow of the fluid into the tube section  9   a  or  9   b , the sphere  5   b  moving within the tube section  9   a  or  9   b , moves, after a run-up period indicated by reference symbol  1  of  FIG. 2 , through a predetermined section in the prover pipe  2   b  together with the fluid at a predetermined flow velocity. To be more specific, the requisite time for this fluid to attain the predetermined flow velocity corresponds, for example, to the requisite time for the four-way valve  9   c  to become half-open or totally open.  
      However, when the distance between the tube section and the detector is made larger as described above, the dimension of the reference volume tube in the longitudinal direction (horizontal direction of  FIG. 2 ) becomes so much the larger, resulting, for example, in a problem of a rather large installation area for the reference volume tube.  
      A similar problem can be more or less involved not only in a reference volume tube in which a multi-way valve, such as a four-way valve, is used as the flow passage switching operation means, but also in a reference volume tube in which, for example, a plurality of single-way valves are arranged separately in the measurement tube portion of the apparatus so that they can exert a function similar to that of a four-way valve or the like.  
      The present invention has been made in view of the above problem. It is an object of the present invention to provide a reference volume tube, in which a moving element is caused to move within a prover pipe by a fluid, capable of achieving a reduction in the longitudinal dimension of the reference volume tube.  
      To achieve the object as described above, the reference volume tube according to the present invention, which is equipped with a measurement tube portion (volume tube) having a reference volume determined in a predetermined section and which calibrates a flow meter to be tested based on a volume of a fluid discharged when a moving element moves within the measurement tube portion through the predetermined section, is characterized by including a waiting means for causing the moving element to wait at a predetermined position on an upstream side of a start point of the predetermined section of the measurement tube portion.  
      Further, the reference volume tube according to the present invention is characterized in that the waiting means is a mechanical stopper for stopping the moving element.  
      In this case, the mechanical stopper may be composed of a pin to be engaged with the moving element and a hydraulic cylinder for biasing the pin.  
      Further, the reference volume tube according to the present invention is characterized in that: the reference volume tube is of a bidirectional prover type having a construction in which the fluid is movable bidirectionally within the measurement tube portion; both end portions of the measurement tube portion are respectively equipped with the waiting means; and the reference volume tube includes a multi-way valve, which is connected between the both end portions and the flow meter to be tested, for introducing the fluid into one of the both end portions through flow passage switching.  
      In this case, the measurement tube portion may be composed of a loop tube.  
      The reference volume tube according to the present invention has the waiting means for causing the moving element to wait at the predetermined position on the upstream side of the start point of a predetermined section of the measurement tube portion, so that, by canceling the stopping by the waiting means at the point in time when the flow direction has been completely switched, it is possible to cause the moving element to move at a predetermined flow velocity by the fluid. As a result, there is no need to provide a long runway as in the prior art, making it possible to reduce the longitudinal dimension of the reference volume tube, whereby it is possible, for example, to reduce an installation area for the reference volume tube. Further, it is also possible to form the reference volume tube as a portable one that can be mounted in a vehicle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic plan view of a conventional unidirectional prover-type reference volume tube.  
       FIG. 2  is a schematic plan viewof a conventional bidirectional prover-type reference volume tube.  
       FIG. 3  is a schematic plan view of a reference volume tube according to an embodiment mode of the present invention.  
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      A reference volume tube according to a preferred embodiment of the present invention will be described below with reference to  FIG. 3 .  
      A reference volume tube  10  according to this embodiment mode, shown in  FIG. 3 , is a bidirectional prover of substantially the same basic construction as the conventional example shown in  FIG. 2 .  
      The reference volume tube  10  is equipped with a loop-tube-shaped measurement tube portion (hereinafter referred to as prover pipe)  12  having a reference volume. The prover pipe  12  is equipped with two detectors  14   a ,  14   b . A volume defined by the section of the prover pipe  12  between the two detectors  14   a ,  14   b  constitutes the reference volume. The detectors  14   a ,  14   b  may be of an appropriate type, for example, mechanically operated electric switches, electronic proximity switches, or induction pick-ups may be used.  
      Tube sections  16   a ,  16   b  are provided at both ends of the prover pipe  12 , and the distance between ends of the tube sections  16   a ,  16   b  and the detectors  14   a ,  14   b  (indicated by reference symbol L of  FIG. 3 ) is substantially reduced as compared with that in the prior art. As in the conventional example, the diameter of the tube sections  16   a ,  16   b  is larger than that of the prover pipe  12 .  
      Waiting means  18   a ,  18   b  are provided at the both ends of the prover pipe  12  and, in this case, on sides of the tube sections  16   a ,  16   b  nearer to the detectors  14   a ,  14   b , that is, on upstream sides of the detectors  14   a ,  14   b  when the tube sections  16   a ,  16   b  are used as fluid inlet sides.  
      The waiting means  18   a ,  18   b  are mechanical stoppers, and are composed of pin-shaped gates  20   a ,  20   b , and hydraulic cylinders  22   a ,  22   b . The gates  20   a ,  20   b  are biased by the hydraulic cylinders  22   a ,  22   b  to radially protrude and retract within the tube sections  16   a ,  16   b.    
      When a sphere  24 , which constitutes a moving element, is situated in the tube section  16   b  as shown in  FIG. 3 , the gate  20   b  of the waiting means  18   b  provided between the sphere  24  and the detector  14   b  protrudes into the tube section  16   b , whereby the sphere  24 , which has moved from the tube section  16   b  toward the detector  14   b , as shown in  FIG. 3 , is stopped by the gate  20   b . Then, movement of the sphere  24  toward the detector  14   b  is prevented to be kept on standby at that position. On the other hand, when the gate  20   b  retracts toward the tube wall of the tube section  16   b , the standby state of the sphere  24  is canceled.  
      Instead of the hydraulic cylinders  22   a ,  22   b , the waiting means  18   a ,  18   b  can use electric cylinders, air cylinders, etc. as the driving portions. Further, they can use other appropriate driving means.  
      As long as the waiting means  18   a ,  18   b  have a function to stop the movement of the sphere  24  to keep it on standby at a predetermined position, some other mechanical stoppers may be used as the waiting means  18   a ,  18   b . For example, it is possible to adopt a construction in which advancement of the sphere  24  is hindered by a plurality of pins, narrow plates, etc. protruding toward a radial center of the tube section  16   b  from the tube wall of the tube section  16   b  and in which the sphere  24  is allowed to advance by causing these pins, plates, etc. to fall over toward the tube wall. Further, as the waiting means  18   a ,  18   b , it is also possible to use appropriate electric or electronic means.  
      Fluid tubes  26   a ,  26   b , which are used as either inlet tubes or outlet tubes through switching of the flow passages, and the two tube sections  16   a ,  16   b  are connected by a four-way valve  28  so as to allow flow passage switching.  
      In  FIG. 3 , temperature gauges for measuring fluid temperature is denoted by reference symbols  30   a ,  30   b , and indicate pressure gauges for measuring fluid pressure is denoted by reference symbols  32   a ,  32   b.    
      A flow meter to be tested (not shown) is connected to one of the fluid tubes  26   a ,  26   b , for example, the fluid tube  26   a . That is, the flow meter to be tested is connected in series with the reference volume tube  10  through the fluid tube  26   a.    
      Mounted to the flow meter to be tested is a pulse transmitter (not shown) adapted to generate and transmit pulses in a number in proportion to the flow rate.  
      The pulse signals from this pulse transmitter and detection signals (start/stop signals) from the detectors  14   a ,  14   b  are taken in by a measurement CPU (proving computer) (not shown). On the other hand, control signals are transmitted to the four-way valve  28  and the hydraulic cylinders  22   a ,  22   b  from a control CPU (flow computer) (not shown). Further, an opening signal of the four-way valve  28  is transmitted to the control CPU. The four-way valve  28  and the hydraulic cylinders  22   a ,  22   b  may be of a type which are manually operated.  
      In the state shown in  FIG. 3 , the fluid from the fluid tube  26   a  has been caused to flow from the tube section  16   a  to the tube section  16   b  to complete the measurement, with the sphere  24  having moved counterclockwise within the prover pipe  12  before reaching the tube section  16   b  to stay there.  
      When starting the next measurement, the four-way valve  9   c  is operated by the control signal of the control CPU to establish communication between the fluid tube  26   a  and the tube section  16   b  and between the fluid tube  26   b  and the tube section  16   a , thereby reversing the flowing direction of the fluid in the reference volume tube  10 . As stated above, at this time, the sphere  24  has been previously arranged in the tube section  16   b . Further, on the upstream side of the sphere  24 , the gate  20   b  has been lowered to attain the closed state by the control signal of the control CPU.  
      By starting the operation of the four-way valve  9   c , the fluid having passed through the flow meter to be tested begins to flow into the tube section  16   b  from the fluid tube  26   a . When the sphere  24  is caused to move to the position of the gate  20   b  by the fluid, which has not attained the predetermined flow velocity, the sphere  24  is engaged with the gate  20   b  and stopped at that position to be placed in a standby state. At the same time, the fluid flows into the prover pipe  12  through a gap between the sphere  24  and the tube section  16   b.    
      When the fluid attains the predetermined flow velocity, a signal indicating, for example, that the four-way valve  9   c  has attained a predetermined opening degree (normal, totally open) is received, and the gate  20   b  is opened by the control signal of the control CPU, thus canceling the standby state of the sphere  24 . The sphere  24  passes through the prover pipe  12  together with the fluid, which attained a predetermined flow velocity. Here, instead of using information on the valve opening degree of the four-way valve  9   c  as described above, it is also possible to use information on a length of the time that has elapsed since the valve operation start, the flow velocity (or the flow rate) at the flow meter to be tested, etc. as means for making a judgment as to whether the flowing direction of the fluid has been reliably changed and a predetermined flow velocity has been reached.  
      The detection signals from the detectors  14   a ,  14   b  and the pulse signals from the pulse transmitter of the flow meter to be tested when the sphere  24  passes, are taken in by the measurement CPU.  
      Then, the measurement CPU measures the number of pulses from the pulse transmitter from the moment at which the detection signal of the detector  14   b  is obtained until the moment at which the detection signal of the detector  14   a  is obtained. Through the movement of the sphere  24  through the section between the two detectors  14   a ,  14   b , a reference volume is indicated, so that, by comparing this reference volume with the indication value (volume) of the flow meter to be tested obtained from the number of pulses as measured, any error of the flow meter to be tested is measured, and further calibration is effected as needed. Instead of the pulse signals, it is also possible to use, for example, an analog signal corresponding to a voltage obtained in correspondence with the flow rate. It is also possible to use the same CPU both as the control CPU and the measurement CPU.  
      In this embodiment mode described above, the waiting means  18   a ,  18   b  are provided in the tube sections  16   a ,  16   b . Instead, it is also conceivably possible to provide the waiting means  18   a ,  18   b  at the ends of the prover pipe  12 . That is, by providing the waiting means  18   a ,  18   b  at the both ends of the prover pipe  12 , that is, on the upstream side of the detectors  14   a ,  14   b , it will be possible to omit the tube sections  16   a ,  16   b , making it possible to achieve a further reduction in the size of the reference volume tube  10 .  
      In this case, the sphere  24  being kept on standbyby the waiting means  18   a ,  18   b , serves, so to speak, as a valve provided on the upstream side of the four-way valve  9   c , and the sphere  24  cuts off the flow of the fluid until the four-way valve  9   c  attains the predetermined opening degree. At the point in time when the four-way valve attains the predetermined opening degree, the lock by the waiting means  18   a ,  18   b  is released, and the sphere  24  moves together with the fluid, which flows at the predetermined flow velocity. In this case, in order to secure the fluid flow in the opposite direction prior to the start of the next measurement with the flowing direction of the fluid changed, some contrivance is needed; for example, it is necessary to provide a discharge tube for discharging the fluid in each of the portions constituting the runways between the waiting means  18   a ,  18   b  and the detectors  14   a ,  14   b . This discharge tube is closed during the next measurement.  
      In this case, however, there is inevitably involved, though short, a change from a state in which the flow velocity of the fluid is zero to the state in which the predetermined flow velocity has been attained, during an initial period in which the sphere  24  starts to move through releasing of the waiting means  18   a ,  18   b . That is, although the requisite time for the predetermined flow velocity of the fluid to be reached is shortened as compared with the prior art, it does not mean this period of time is completely eliminated. Thus, it is to be assumed that, unlike the above-described embodiment mode, this construction does not completely eliminate the problem in the prior art but still involves a deterioration in measurement accuracy.  
      In the above-described embodiment mode, it is also possible to use, instead of a tube with a circular sectional configuration, a tube with a sectional configuration that is elliptical, rectangular, etc., using a moving element of a configuration in conformity with such sectional configuration of the tube. Further, instead of a loop tube, it is also possible to use a straight tube as the prover pipe  12 . Further, instead of being arranged on the upstream side of the prover pipe  12 , the flow meter to be tested may also be arranged on the downstream side of the prover pipe  12 .  
      Further, the present invention is not restricted to the above-described embodiment mode but is also applicable, for example, to a unidirectional prover-type reference volume tube which is provided with no flow passage switching structure and which performs test by causing fluid to flow only in one direction.