Abstract:
An element of an averaging orifice plate fluid flow meter for measuring the volumetric rate of fluid flow in a conduit having an interior circumference for carrying a flow of fluid comprising, a planar flow impedance plate for disposition transversely across the interior of the conduit and perpendicular to the fluid flow, a plurality of spaced apart orifices disposed in the plate where the profile of each orifice is a segment of a circle on an arc where the arc is coincident with a portion of the interior circumference of the conduit.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an orifice plate type of process fluid flow meter having a plurality of openings that average the differential fluid pressures across the plate in asymmetric profile flow. 
       BACKGROUND OF THE INVENTION 
       [0002]    In process piping the need for flow conditioners downstream of pipe elbows, valves and other flow interrupting devices is well documented in many prior art patents. Using an orifice plate flow meter with multiple apertures to self-condition disturbed and non-linear flow by averaging the differential pressure drops across a plate is the subject of U.S. Pat. No. 7,284,450. In the &#39;450 patent the background of the problem solved by the averaging orifice plate is set forth and is incorporated herein by reference. 
         [0003]    Following some period of use of the averaging orifice plate flow element disclosed in the &#39;450 patent it has been discovered that some beta ratio designs of the flow element create a blockage along the walls of the fluid carrying conduit. Such blockage diverts abrasive particulates that would otherwise travel along the pipe wall into the interior of the fluid stream, causing the particles to erode the sharp upstream edges of the orifices and thus depreciate the accuracy of the flow rate measurement. The upstream edges are especially susceptible to erosion because they are not only sharp but they are also thin, the downstream side of the edge being beveled to allow expansion of the fluid after it passes through the orifice. 
         [0004]    Accordingly, it is the primary object of the present invention to prevent fluid blockage by positioning the openings in the orifice plate proximate the pipe wall. 
         [0005]    Another object of the invention is to position the openings of an averaging orifice plate so that particulates, as well as liquids in gas applications, can pass freely through the flow meter with minimal impact on the sharp edges of the plate&#39;s orifices, whether the fluid flow in the pipe is annular or stratified. 
         [0006]    A still further object of the invention is to substantially reduce the extent of orifice edges in an averaging orifice plate flow meter that are subject to erosion. 
         [0007]    Other and still further objects, features and advantages of the present invention will be apparent upon a reading of the following detailed description of the invention, taken in conjunction with the drawings. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention relates to an orifice plate type of differential pressure fluid flow meter. The static pressure ports that are a necessary part of the flow meter are traditional in their upstream and downstream positioning and structure. 
         [0009]    The preferred form of the invention is implemented by a flat orifice plate sandwiched between pipe connecting flanges. In a secondary embodiment the orifice plate is carried by an annulus that is insertable between sections of the fluid carrying conduit where the central opening of the mounting annulus corresponds in shape and size to the interior cross section of the circular conduit carrying the fluid whose flow rate is to be measured. The planar flow impedance plate is disposed within the central opening of the annulus coincident with a plane that is perpendicular to the longitudinal axis of the central opening. Instead of having a set of circular orifices such as the ones disclosed in the &#39;450 patent, the impedance plate of both embodiments of the present invention is provided with a plurality of spaced apart openings or notches disposed in the circumference of the plate. Preferably, the circumferential openings are shaped as segments of a circle on an arc where the arc of each circle is coincident with an arc of the interior circumference of the mounting annulus. An impedance plate having four equal-spaced orifices, shaped as described above, has the resultant shape of a cruciform. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a perspective fragmentary cross sectional view of a primary flow element in the alternative form of the present invention, where the planar flow impedance plate is carried by a mounting annulus. 
           [0011]      FIG. 2  is a cross sectional side view of the orifice plate and mounting annulus shown in  FIG. 1 . 
           [0012]      FIG. 3  is a front view of the preferred form of the planar flow impedance plate of the present invention. 
           [0013]      FIG. 4  is an exploded perspective view of the preferred form of orifice plate of the present invention where the plate is mounted by being sandwiched between two adjacent pipe connecting flanges fixed to the ends of a fluid carrying conduit. 
           [0014]      FIG. 5  is a diagrammatic cross sectional view of the preferred form of mounting the impedance plate, as shown in  FIG. 4 , illustrating the asymmetric fluid profile resulting from the pipe elbow upstream of the orifice plate flow measurement device of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The successful operation of traditional orifice plate flow meters is based on Bernoulli&#39;s theorem. Bernoulli states that along any one streamline in a moving fluid, the total energy per unit mass is constant, being made up of the potential energy, the pressure energy and the kinetic energy of the fluid. Thus, when fluid passes through an orifice in a constricting pipe plate, the velocity of the fluid through the orifice increases. This increase in fluid velocity causes the dynamic pressure of the fluid immediately downstream of the orifice plate to increase, while simultaneously decreasing the static pressure of the fluid at that same point. By sensing the static pressure on the upstream and downstream sides of the orifice plate, the decrease of static pressure on the downstream side results in a measurement of the pressure differential, dP, between the upstream side of the orifice plate and the downstream side. The rate of fluid flow q is proportional to √{square root over (dP)}. 
         [0016]    Traditional prior art orifice plate flow meters work well when the velocity profile of the fluid is parabolic. That is, the fluid&#39;s velocity profile is symmetrical about the longitudinal axis of the pipe in which the fluid is flowing. In such a case, the highest velocity fluid is along the central axis of the pipe, coaxial with the centrally located orifice in the constricting pipe plate. When traveling through the orifice of a traditional orifice plate, the highest velocity fluid is the fluid that produces the pressure differential across the plate to provide the flow rate result. 
         [0017]    However, if the velocity profile is skewed a lower velocity fluid will pass through the central orifice and the downstream static pressure will be a reflection of that lower velocity fluid. The differential pressure thus produced across the constricting plate will not be a true indication of the rate of fluid flow. 
         [0018]    The present invention is adaptable to two different mounting configurations but in both of these configurations the function and operation of the peripheral notch orifices are the same. The plurality of orifices average the pressure drops across the plate at spaced apart locations in the velocity profile, but their positions next to the pipe wall prevent fluid blockage and provide the other objectives of the invention. 
         [0019]    According to one form of the present invention the flow impedance plate  2  of the primary flow element  3  is integrally incorporated into the central opening of an annular mounting ring  5 . Disposed within the ring  5  are upstream and downstream static pressure measuring ports  6  and  8 . These ports communicate with a valving manifold  23 , pressure transducers and a signal transmitter that are located exteriorly of the fluid carrying pipe through respective bores  10  and  12  located in a stem  14  projecting from the outside surface of the ring  5 . 
         [0020]    Regardless of the mounting method, the impedance plates  2  and  2   a  include a plurality of circumferentially positioned orifices  4  and  4   a.  Each of the plurality of circumferentially positioned orifices conducts a portion of the total fluid flow within the conduits  21 ,  27  and  28 . According to Bernoulli&#39;s theorem, the velocity of the fluid through each of the orifices will increase while the static fluid pressure on the downstream side of each orifice in the constricting plates  2  and  2   a  decreases. The static pressures attributable to the separate orifices will be averaged within the fluid to provide an average downstream static pressure at a downstream pressure port, such as the port  8  in the alternative form or the port  32  in flange  24  of the  FIG. 4  embodiment. The average downstream static pressure is compared with the upstream static pressure taken at an upstream pressure port, such as the port  6  in the annulus  5  or the port  34  in flange  25 , to provide an average differential pressure for whatever velocity profile may be presented to the multiple orifice plates  2  and  2   a,  resulting in an accurate measurement of the rate of fluid flow in the pipe regardless of the asymmetry of the velocity profile. 
         [0021]    As set forth above, the object of the invention is to position the orifices  4  and  4   a  in the averaging orifice plates  2  and  2   a  so that the orifices are adjacent to the inside surface of the conduit  21  (for the embodiment of  FIGS. 3 and 4 ) and adjacent to the annulus  5  in the embodiment of  FIGS. 1 and 2  so that particulates, as well as liquids in gas applications, can pass freely through the orifices in the flow meter with minimal impact on the sharp edges of the plate&#39;s orifices, whether the fluid flow in the pipe is annular or stratified. 
         [0022]    Annular wet gas flow occurs where the rate of gas flow is high and any liquid in the gas is forced into an annularly shaped ring that coats the pipe walls with liquid. Annular flow can occur in horizontally and vertically positioned pipes. On the other hand, stratified wet gas flow occurs where the rate of gas flow is relatively low, allowing the liquid in the gas to flow along the bottom portion of a horizontal pipe. Positioning of the plate orifices adjacent the pipe wall and where the lower most orifice is adjacent the bottom portion of the pipe accomplishes the objectives of the invention. 
         [0023]    When so positioned the orifices can no longer be circular but must be elongated in order to place the open areas of the flow impedance plate proximate the inside surface of either the conduit  21  or the annulus  5 , the inside of the annulus being equivalent to the inside surface of the wall of conduit  21 . To accomplish this objective the outside periphery of each orifice in the embodiment of  FIGS. 1 and 2  is formed by the inside circumferential surface  13  of the annular mounting ring  5 , which inside surface  13  is congruent with the inside surface of the fluid carrying pipe. 
         [0024]    The preferred embodiment  3   a  of the primary flow element of the present invention comprises an enlarged diameter impedance plate  2   a  that is mounted between connecting flanges  24  and  25  of two sections  27  and  28  of the fluid carrying pipe. The openings  4   a  in the plate are cut in the shape of the circle segment  17  with the outside arcs  20  all lying on a circular curve that is congruent with the inside periphery of the pipes  27  and  28 . The plate&#39;s enlarged diameter allows it to be sandwiched between the flanges  24  and  25  for mounting. The upstream and downstream pressure ports  31  and  32  are contained in the flanges  24  and  25 . 
         [0025]    The congruity of the outside of the orifices and the inside surface of the fluid carry conduit serves two important and unobvious functions. First, it allows the fluid and entrained particulates in the pipe to flow through the orifice forming peripheral notches  4  without blockage. Second, since the outer peripheral portion of each notch orifice is formed by the inside of the pipe wall or, in the alternative form, the inside perimeter  13  of the annular ring  15 , the cumulative length of the edges  14  comprising the plurality of orifices is substantially reduced. 
         [0026]    The preferred shape of the circumferential notches  4  and  4   a  in the flow impedance plates  2  and  2   a  can be described as a segment (area)  17  of a circle on an arc  20  through the circle, the arc being formed by the inside surface  13  of the annular ring  15  or the inside surface of the conduit  21 .