Abstract:
A vortex flow meter for measuring fluid flow has a conduit for carrying a fluid. A region of reduced thickness is formed in a portion of the conduit. A shedding bar disposed in the conduit is coupled to the region of reduced thickness and is configured to apply a rocking motion to the region of reduced thickness about a pivot line in response to flow of the fluid. At least one reinforcing rib on the reduced thickness portion preferably extends parallel to flow of fluid.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a vortex flow meter for measuring fluid flow, and more particularly, a flow meter for use with high pressure process fluid. 
   Various differential pressure sensitive vortex flow meters have been advanced, which operate on a principle that a buff body or shedding bar placed in a fluid flow causes or generates vortices alternately on opposite sides of the shedding bar, causing variations in pressure on either side of the bar. The frequency of vortex shedding for an individual bar configuration characteristic is directly proportional to the velocity of flow in the stream. 
   Vortex flow meters are known in the prior art, and examples of vortex flow meter implementations can be found in U.S. Pat. No. 4,926,695 issued to Kleven et al. on May 2, 1990, U.S. Pat. No. 5,343,762 issued to Beulke on Sep. 6, 1994, which are incorporated herein by reference. 
   Typically, a vortex flow meter for measuring fluid flow includes a conduit having a wall surrounding a bore for carrying the fluid. The wall has a wall region of reduced thickness formed therein. The wall region of reduced thickness is sometimes referred to as a “region of reduced stiffness” or a “flexure”. A shedding bar is disposed in the bore. In a typical embodiment, the shedding bar includes an upstream extremity, a downstream extremity and an intermediate portion connecting the upstream and downstream extremities. The intermediate portion includes a region of reduced stiffness which flexes in response to disturbances or vortices within the fluid created by fluid flow around the upstream extremity to promote motion of at least a portion of the downstream extremity. Such flow meters further include sensing means coupled to the downstream extremity for sensing the motion and providing an output as a function thereof. Generally, the sensing means senses lateral motion, and is removably attached to a post extending from the wall region away from the bore, wherein the post transmits the motion to the sensing means. 
   One method of assuring that a vortex meter meets process pressure retention (strength) requirements is described in the American Society of Mechanical Engineers (ASME) Boiler Pressure Vessel Code (BPVC). To determine the factor of safety the meter has at a given pressure, a meter representative of the design is pressurized until the structure bursts. Factors are derived from testing of the first meter body produced, and calculations are performed based on various factors of the material utilized to make the device. For example, material characteristics, including composition and manufacturing processes, are factored into a calculation to determine the maximum pressure to which a device may be rated. 
   For higher pressures, the flexure often presents a weak point in the structure, which can tear or burst open when exposed to higher pressures. Since the flexure must be designed to be thin enough to permit movement of the post that is coupled to the buff body or shedding bar, conventional flexures have difficulting providing the required safety factor at high pressures. 
   SUMMARY 
   A vortex flow meter for measuring fluid flow has a conduit for carrying a fluid. A region of reduced thickness is formed in a portion of the conduit. A shedder bar disposed in the conduit is coupled to the region of reduced thickness and is configured to apply a rocking motion to the region of reduced thickness about a pivot line in response to flow of the fluid. At least one reinforcing rib on the reduced thickness portion extends parallel to flow of the fluid. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial cut away view of a flow meter in situ according to an embodiment of the present invention. 
       FIG. 2  is an enlarged view of the flexure portion of the flow meter of  FIG. 1 . 
       FIG. 3  is a cross-sectional view of a flow meter taken along line  3 — 3  in  FIG. 1 . 
       FIG. 4  is an expanded view of the region of reduced thickness taken in cross-section along line  3 — 3  in  FIG. 1 . 
       FIG. 5  is a top view of the region of reduced thickness of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
   In order to qualify for use in a particular pressure rating, users are required to withstand a burst pressure test that gives effective safety factor according to the ASME BPVC. 
       Burst   &gt;       (     safety   ⁢           ⁢   factor     )     ×     PR     (     F   ×   R     )             
 
where PR is the maximum operating pressure for a pressure rating, F is a casting quality factor, and R is the ratio of actual to minimum tensile strength of the material utilized in the particular implementation. The safety factor may depend on the specific usage and the implementation.
 
   The present invention includes a modification to conventional flexures (sometimes referred to as regions of reduced wall thickness) to include a rib to extend across the flexure in the direction of flow on either side of the center line of the flexure. Additionally, the size of a center rib disposed on the center line of the flexure is increased. With these enhancements, the flexure is able to retain (withstand) a pressure that is approximately 150% that of a conventional meter. Additionally, the side ribs increase the sensitivity of the meter to alternating pressure created by the shedding bar, by reducing the countervailing force on the sensor due to pressure on the rigid center flexure of the meter. 
     FIG. 1  illustrates an embodiment of a vortex flow meter of the present invention. Flowmeter  10  includes a conduit  12  having a wall  14  surrounding a bore  16 . Bore  16  carries a fluid, which may be a liquid or a gas, generally along a bore axis  18 . Shedding bar  20  is a vortex-generating obstruction. Pivoting member  22  extends from a hole  24  formed in wall  14  into bore  16 . Fluctuating fluid pressures act on the shedding bar and on pivoting member  22 , such that pivoting member  22  moves in response to the fluctuating pressures. 
   A center rib  26  is disposed in hole  24  and coupled to flexure  28 . The flexure  28  is generally coupled to wall  14 . Post  30  extends from the flexure  28 . A sensing device  32  (shown in phantom) is coupled to the flexure  28 , preferably by attachment to the post  30 , and senses the motion of pivoting member  22 . The sensing device  32  generates an output indicative of the sensed motion and communicates the output via lead  34  to circuitry  36 . Typically, circuitry  36  is adapted to communicate the sensed motion to a control center  38  via a communications link  40  (which may be a two-wire, three-wire or four-wire loop, or which may be a wireless communications link  40 ). 
   In a preferred embodiment, reinforcing ribs  42  are disposed on the flexure on either side of centerline  27  (or pivot line) of the flexure  28  (reference numeral  27  is shown in  FIGS. 2 and 5 ). In general, the center rib  26  and the reinforcing ribs  42  extend parallel to the direction to the bore axis  18  (parallel to the direction of flow). 
   In one embodiment, the reinforcing ribs  42  are positioned on the flexure between a center of the center rib  26  and the outside edge of the flexure  28 . The position may be quantified by considering a flexure  28  of diameter D, where the reinforcing ribs  42  are positioned at approximately 0.25D on either side of the center rib  26 . In an alternative embodiment, the reinforcing ribs  42  are centered between the edge of the center rib  26  and the outside edge of the flexure  28  (at approximately 0.22D, for example, relative to the edge of the flexure  28 ). In this embodiment, the center rib  26  is approximately the same width as the post  30  and extends the entire diameter of the flexure  28 . In an alternative embodiment, the center rib  26  is a fraction of an inch wider than the post  30  and extends the full diameter of the flexure. The reinforcing ribs are approximately the same relative height and width. In one embodiment, the reinforcing ribs have a height and a width that are approximately the same as the thickness of the flexure. It will be understood by a worker skilled in the art that the flexure  28  is curved so as to match the wall of the pipe. Thus, the height of the reinforcing ribs  42  is relative to the surface of the flexure  28 , but varies relative to a fixed point according to a radius of curvature of the pipe. 
   Depending on the implementation, the rib size may be adjusted to account for different pressures and different flexure sizes. Additionally, the location of the reinforcing ribs  42  may be adjusted toward or away from the center line. In a preferred embodiment, the reinforcing ribs  42  are substantially centered between the center line of the flexure  28  and the outer edge of the flexure  28 . 
   It is appreciated that these reinforcing ribs  42  add to the burst pressure strength of the meter. Adding the reinforcing ribs  42  to a meter typically increases the burst pressure of the meter by 50%. Additionally, the reinforcing ribs  42  increase the sensitivity of the flow meter to the alternating pressure from the fluidic vortices, by reducing the countervailing force on the sensor due to pressure on the flexure  28  of the meter  10 . 
     FIG. 2  shows an expanded view of the flexure  28  of  FIG. 1 . As shown, the conduit  12  has an opening  24  in which a flexure  28  is disposed. Center rib  26  extends a full diameter of the flexure, and is positioned on a center line  27  of the flexure  28 . In this embodiment, reinforcing ribs  42  are disposed on either side of the center rib  26  and extend to the outer edge of the flexure  28 . The reinforcing ribs  42  are positioned at approximately a midpoint between the center line  27  and the outer edge of the flexure  28 . Post  30  extends from the center rib  26  and away from the conduit  12 . As previously discussed, in a preferred embodiment, the sensor is coupled to the shedding bar via the post  30 . 
     FIG. 3  shows a cross sectional view of the vortex flow meter  10  of  FIG. 1  taken along line  3 — 3 . Flowmeter  10  includes a conduit  12  having a wall  14  surrounding a bore  16 . Bore  16  carries a fluid, which may be a liquid or a gas, generally along a bore axis  18 . Shedding bar  20  is a vortex-generating obstruction. Pivoting member  22  (shown in  FIG. 1 ) extends from the wetted side of the flexure  28 . Fluctuating fluid pressures act on the shedding bar and on pivoting member  22 , such that pivoting member  22  moves in response to the fluctuating pressures. 
   In this embodiment, center rib  26  is disposed in hole  24  and coupled to flexure  28 . The flexure  28  is generally coupled to wall  14 . Post  30  extends from the flexure  28 . Reinforcing ribs  42  are disposed on the flexure on either side of the center rib  26  on either side of a centerline of the flexure  28 . In general, the center rib  26  and the reinforcing ribs  42  extend parallel to the direction to the bore axis  18  (parallel to the direction of flow). Flange element  46  couples a transmitter housing  48  (shown in phantom) to the meter body. The transmitter housing  48  may also contain circuitry for processing sensed data into a signal for transmission to a control center. 
     FIG. 4  illustrates an expanded cross-sectional view of the flexure  28 . Flexure  28  is disposed in opening  24  within conduit  12 . Center rib  26  is centered on the flexure  28 , and reinforcing ribs  42  are approximately centered between the center rib  42  and the outer circumferential edge of the flexure  28 . The center rib  26  and the reinforcing ribs  42  extend in parallel to each other and to the direction of fluid flow. Pivoting member  22  is disposed on shedding bar  20  and is coupled to the post  30  to cause the post  30  to move responsive to fluidic pressures experienced by the pivoting member  22  and the shedding bar  20 . 
   In general, the flexure  28  (region of reduced thickness) has a thickness T. The reinforcing ribs  42  have a width W and a height H. It should be understood that the flexure  28  is curved to match the curvature of the pipe wall, such that the height H of the reinforcing ribs  42  is relative to the curved surface of the flexure  28 . In a preferred embodiment, the width W and the height H are approximately equal to the thickness T. 
     FIG. 5  shows the flexure  28  from a top view looking into opening  24 . The flexure  28  has a diameter D, which can be measured from edge to edge along the outer (non-wetted) surface of the flexure  28 . In one preferred embodiment, the position of the reinforcing ribs  42  may be determined relative to the centerline (pivot line)  27  of the flexure  28 , such that the reinforcing ribs  42  are centered at approximately 0.25D. 
   As previously discussed, the primary method of determining the pressure rating for a meter is a burst pressure test. In these tests, the flow meter is connected to a testing device and then pressurized until the meter fails because it can no longer retain the pressure. In all test cases, failures occurred in the flexure area, which is the thinnest wall section. 
   For the purposes of the ASME code, the safety factor for a meter is determined by the following equation: 
       SF   =         F   ×   BP       P   r       ×       T   s       T   a             
 
where SF is the safety factor, F is a casting quality factor, BP is burst pressure, P r  is rated pressure, T s  is the material&#39;s tensile strength specification, and T a  is actual tensile strength of the material tested as determined by a test specimen from the heat the casting is taken from.
 
   Adding a reinforcing rib  42  to the flexure  28  between the centerline and the edge of the flexure  28  significantly increases the ultimate burst pressure. In one example test, the reinforcing ribs were approximately 0.035 inches wide and 0.070 inches tall and extended parallel to the center rib  26 . The reinforcing ribs  42  were positioned at the area where the maximum total displacement occurred in the prior art design. Calculations predicted a greater than 30 percent increase in burst pressure. The tested device showed improvement in burst pressure of 50 percent. 
   Further analysis of the reinforced flexure design showed improved sensitivity over conventional flexure designs. Calculations predicted an increase in sensitivity of approximately 6 percent. In testing, the reinforcing ribs  42  increased sensitivity of the meter up to 20 percent in some embodiments. 
   It is also important to note that the center rib contributes to a restorative force tending to restore pivoting member  22  to its equilibrium position. The restoring force tends to increase the natural frequency of vibration of the pivoting member, which ensures that the natural frequency of the pivoting member  22  is greater than the highest vortex frequency encountered during operation. The reinforcing ribs may actually improve this restorative force. 
   Additionally, it should be noted that in the embodiments shown, the reinforcing ribs are offset from a pivot line of the region of reduced thickness. In a preferred embodiment, the reinforcing ribs are approximately centered between a pivot line and an edge of the region of reduced thickness, making the offset distance approximately the same. However, in some embodiments, it may be desirable to vary the position of the reinforcing ribs. For example, the position of the reinforcing ribs may be offset from the midpoint between the pivot line and the edge, by approximately the same distance. Alternatively, the reinforcing ribs may be spaced from the pivot line by slightly different distances so as to locate the reinforcing rib in a particular location on the region of reduced thickness. 
   Finally, while the above-discussion has largely described the reinforcing ribs as extending substantially parallel to the pivot line (or center rib), in some embodiments, it may be desirable to change the angle of reinforcing ribs relative to the center line. For example, in one embodiment, it may be desirable to position the reinforcing rib on the flexure adjacent to the pivot line and extending at an angle from the pivot line. In such an embodiment, the position of the reinforcing rib relative to the pivot line or the circumferential edge would vary depending on where the measurement was taken. By changing the angle, it may be possible to adjust the sensitivity of the meter while maintaining the improved pressure retention advantages. 
   Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.