Patent Publication Number: US-6988418-B2

Title: Vortex flow pickup

Description:
This application is a Continuation of nonprovisional application Ser. No. 10/233,673 filed Sep. 4, 2002 now U.S. Pat. No. 6,938,496 which claims the benefit of Provisional Application No. 60/330,929, filed Nov. 2, 2001. 

   FIELD OF THE INVENTION 
   The invention relates to a vortex flow pickup for measuring the volume flow, the mass flow or the flow velocity of the fluid flowing in a direction of flow in a measuring tube, with a bluff body which serves for producing Karman vortices being arranged over a diameter of the measuring tube. 
   BACKGROUND OF THE INVENTION 
   The volume flow is defined as the volume of fluid flowing through the cross section of the measuring tube per unit of time and the mass flow is defined as the mass of fluid flowing through the cross section of the measuring tube per unit of time. 
   It is known that, during the operation of a vortex flow pickup of this type, a Karman vortex street is produced downstream of the bluff body and its pressure fluctuations are converted by a vortex sensor into an electrical signal, the frequency of which is proportional to the volume flow or the flow velocity. 
   In U.S. Pat. No. 6,003,384 there is a description of a currently customary vortex flow pickup for measuring the volume flow or the flow velocity of a fluid which is flowing in a direction of flow in a measuring tube having a tube wall, which vortex flow pickup comprises:
         a bluff body which is arranged along a diameter of the measuring tube and
           serves for producing Karman vortices and   is connected to the tube wall of the measuring tube from the inside at a first and a second fixing location, which lie opposite each other,   
           a vortex sensor, which responds to pressure fluctuations produced by the vortices, is fitted downstream of the bluff body in a bore of the tube wall of the measuring tube and seals off this bore,   the center of the bore lying together with the center of the first fixing location of the bluff body on a generatrix of the measuring tube and   the vortex sensor comprising:
           a diaphragm covering the bore, with a first surface facing the fluid and a second surface facing away from the fluid,   a wedge-shaped sensor vane, which is fastened on the first surface of the diaphragm and
               is shorter than the diameter of the measuring tube and   has principal surfaces in line with the generatrix of the measuring tube and also a front edge, and   
               a sensor element fastened on the second surface.   
               

   If the temperature of the fluid is also measured by means of a temperature sensor, the mass flow can be determined, for example calculated by means of a microprocessor, from the volume flow, the type of fluid and its properties as well as the temperature at any given time. 
   This has already been described some time ago in the case of vortex flow pickups with different types of vortex sensors. For instance, U.S. Pat. Nos. 4,048,854 and 4,404,858 each show a temperature sensor which is arranged on the tube wall of the measuring tube from the inside in such a way that it is skimmed over by the flowing fluid. 
   In JP-A 2000-2567 there is a description of a vortex flow pickup for measuring the mass flow, the volume flow or the flow velocity of a fluid which is flowing in a direction of flow in a measuring tube having a tube wall, which vortex flow pickup comprises
         a blade which is fixed on one side to the tube wall from the inside by means of a base plate and
           during operation produces Karman vortices,   is shorter than a diameter of the measuring tube and   has parallel principal surfaces aligned perpendicularly to the direction of flow and a rounded front face,
               on which a temperature sensor is arranged,   
               
           first sensor elements, fastened in the vicinity of the fixing location, for pressure fluctuations of the flowing fluid produced by the Karman vortices and   second sensor elements, fastened in the vicinity of the fixing location, for deflections of the blade produced by the flowing fluid.       

   This temperature sensor is also skimmed over by the flowing fluid and, as the inventors have found, is consequently not resistant to all fluids encountered in operation, i.e. some fluids corrode temperature sensors arranged in such a way. 
   These fluids which corrode the temperature sensor must therefore be banned from use with the vortex flow pickup by the manufacturer of the latter. However, such a ban restricts the use of these vortex flow pickups, that is the universality of their applications, and consequently also their attractiveness on the market. 
   SUMMARY OF THE INVENTION 
   One object on which the invention is based is to specify vortex flow pickups with a bluff body and with a vortex sensor fixed in the tube wall of the measuring tube and with a temperature sensor which is arranged in such a way that the respective vortex flow pickup may also be used together with those fluids which corrode the temperature sensor. 
   To achieve this object, a first variant of the invention comprises a vortex flow pickup for measuring the mass flow, the volume flow or the flow velocity of a fluid which is flowing in a direction of flow in a measuring tube having a tube wall, which vortex flow pickup comprises:
         a bluff body which is arranged along a diameter of the measuring tube and
           serves for producing Karman vortices and   is connected to the tube wall of the measuring tube from the inside at a first and a second fixing location, which lie opposite each other,   
           a vortex sensor, which responds to pressure fluctuations produced by the vortices, is fitted downstream of the bluff body in a bore of the tube wall of the measuring tube and seals off this bore,   the center of the bore lying together with the center of the first fixing location of the bluff body on a generatrix of the measuring tube and   the vortex sensor comprising:
           a diaphragm covering the bore, with a first surface facing the fluid and a second surface facing away from the fluid,   a sensor vane, which is fastened on the first surface of the diaphragm and
               is shorter than the diameter of the measuring tube,   has principal surfaces in line with the generatrix of the measuring tube and also at least one front edge, and   is provided with a blind hole, a bottom of which lies in the vicinity of the at least one front edge,   
               a temperature sensor, which is fixed on the bottom of the blind hole, and   a sensor element fastened on the second surface.   
               

   To achieve the stated object, a second variant of the invention comprises a vortex flow pickup for measuring the mass flow, the volume flow or the flow velocity of a fluid which is flowing in a direction of flow in a measuring tube having a tube wall, which vortex flow pickup comprises:
         a bluff body which is arranged along a diameter of the measuring tube and
           serves for producing Karman vortices and   is connected to the tube wall of the measuring tube from the inside at a first and a second fixing location, which lie opposite each other,   
           a vortex sensor, which responds to pressure fluctuations produced by the vortices, is fitted downstream of the bluff body in a first bore of the tube wall of the measuring tube and seals off this bore,   the center of the first bore lying together with the center of the first fixing location of the bluff body on a generatrix of the measuring tube,   the bluff body being provided with a blind hole,
           which is in line with a second bore in the tube wall and   in which a temperature sensor is fitted, and   
           the vortex sensor comprising:
           a diaphragm covering the first bore, with a first surface facing the fluid and a second surface facing away from the fluid,   a sensor vane, which is fastened on the first surface of the diaphragm and
               is shorter than the diameter of the measuring tube and   has principal surfaces in line with the generatrix of the measuring tube and also at least one front edge, and   
               a sensor element fastened on the second surface.   
               

   According to a preferred embodiment of both variants of the invention, the principal surfaces of the sensor vane form a wedge with a single front edge. 
   One advantage of the invention is that the temperature sensor has no chance of coming into contact with the flowing fluid and consequently also cannot be corroded by it. Nevertheless, the temperature sensor is arranged so close to the fluid that it senses its temperature with virtually no delay; it is in fact separated from the fluid only by the thin wall of the vortex sensor or of the bluff body and, like the remaining parts of the vortex flow pickup, these parts are produced from a metal, preferably stainless steel, and are therefore good heat conductors. 
   A further advantage of the invention is that, in a way corresponding to the book by F. P. Incropera and D. P, DeWitt “Fundamentals of Heat and Mass Transfer”, 4th edition, 1996, ISBN 0-471-30460-3, pages 114 to 119 and 407, the temperature sensor arranged in the sensor vane or in the bluff body can work together with a second temperature sensor, which is fastened on the measuring tube, preferably from the outside, that is to say likewise does not come into contact with the fluid. As known, if the second temperature sensor is provided, a more exact temperature measurement is obtained than with a single temperature sensor. 
   The invention and further advantages are now explained in more detail on the basis of exemplary embodiments, which are represented in the figures of the drawing. The same parts are designated in the different figures by the same reference numerals, which are omitted however if necessary for the sake of clarity. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a vortex flow pickup corresponding to the first variant of the invention in a perspective view, as seen in the direction of flow, and partly cut open, 
       FIG. 2  shows the vortex flow pickup from  FIG. 1  in a perspective view, as seen counter to the direction of flow, and partly cut open, 
       FIG. 3  shows the vortex sensor from  FIGS. 1 and 2  in a perspective view from below, 
       FIG. 4  shows a perspective longitudinal section of the vortex sensor of  FIG. 3 , and 
       FIG. 5  shows in a way analogous to  FIG. 2  a vortex flow pickup corresponding to the second variant of the invention in a perspective view and partly cut open. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1 to 4  are described together below, since the details cannot all be represented in each figure. The perspective views shown first in  FIGS. 1 and 2 , and serving as an overview, of an exemplary embodiment of the first variant show a partly cut open vortex flow pickup  1 , seen on the one hand in the direction of flow ( FIG. 1 ) and on the other hand seen counter to the direction of flow ( FIG. 2 ), with a vortex sensor  3  fixed to a tube wall  21  of a measuring tube  2  and protruding through a bore  22 . This is preferably a dynamically compensated vortex sensor with a capacitive sensor element, as is described in U.S. Pat. No. 6,003,384, the content of which belongs to the disclosure of this application. 
   Arranged along a diameter of the measuring tube  2 , in the interior of the latter, is a bluff body  4 , which is firmly connected to the measuring tube  2 , thereby forming a first fixing location  41 , which is represented, and a second fixing location  41 *, which is concealed. The center of the bore  22  and the center of the fixing location  41  lie on a generatrix of the measuring tube  2 . 
   The bluff body  4  has an impact surface  42 , against which, in operation, a fluid to be measured, for example a liquid, a gas or a vapor, flows. The bluff body  4  also has two lateral surfaces, of which only one (front) lateral surface  43  can be seen in  FIGS. 1 and 2 . Two separation edges are formed by the impact surface  42  and the lateral surfaces, only one (front) separation edge  44  of these being completely visible and one (rear) separation edge  45  being shown in an indicative manner in  FIG. 1 . 
   The bluff body  4  of  FIGS. 1 and 2  has substantially the shape of a straight triangular column, that is a column with a triangular cross section. However, other conventional shapes of the bluff body can also be used in the invention. 
   The flow of the fluid against the impact surface  42  leads to the formation, downstream of the bluff body  4 , of a Karman vortex street in the fluid due to the fact that vortices separate alternately at each separation edge and are carried along by the flowing fluid. These vortices generate local pressure fluctuations in the fluid, the time-related separation frequency of which, i.e. what is referred to as their vortex frequency, is a measure of the flow velocity and/or the volume flow of the fluid. 
   The pressure fluctuations are converted by means of the vortex sensor  3  into an electrical signal, which is fed to evaluation electronics, which calculate the flow velocity and/or the volume flow of the fluid in the customary way. 
   The vortex sensor  3  is fitted downstream of the bluff body  4  in the bore  22  of the tube wall  21  of the measuring tube  2  and seals off the bore  22  from the circumferential surface of the measuring tube  2 , the vortex sensor  3  being screwed to the tube wall  21 . Used for example for this purpose are four screws, of which the screws  5 ,  6 ,  7  can be seen in  FIGS. 1 and 2 , while associated bores  50 ,  60 ,  70 ,  80  are represented in  FIG. 3 . 
   Of the vortex sensor  3 , a wedge-shaped sensor vane  31 , protruding into the interior of the measuring tube  2  through the bore  22  of the tube wall  21 , and a housing cap  32  can be seen in  FIGS. 1 and 2 . The housing cap  32  runs into an extension  322 , by insertion of a thinner-walled intermediate piece  323 , cf. the cited U.S. Pat. No. 6,003,384. 
   The sensor vane  31  has principal surfaces, of which only the principal surface  311  can be seen in  FIGS. 1 and 2 . The principal surfaces are in line with the mentioned generatrix of the measuring tube  2  and form a front edge  313 . The sensor vane  31  may also have other suitable three-dimensional shapes; for example, it may have two parallel principal surfaces, which form two parallel front edges. 
   The sensor vane  31  is shorter than the diameter of the measuring tube  2 ; furthermore, it is flexurally rigid and has a blind hole  314  (can only be seen in  FIG. 4 ). In order that the blind hole  314  has an adequate diameter, wall parts protrude from the principal surfaces, of which the wall part  315  is indicated in  FIG. 2 . The blind hole  314  reaches into the vicinity of the front edge  313 , where it has a bottom. 
   Also belonging to the vortex sensor  3  is a diaphragm  33 , which covers the bore  22  and has a first surface  331 , facing the fluid, and a second surface  332 , facing away from the fluid, see  FIGS. 3 and 4 . Fixed on the surface  331  is the sensor vane  31  and fixed on the surface  332  is a sensor element  35 . Preferred are the sensor vane  31 , the diaphragm  33 , the annular rim  333  of which and the part  351  of the sensor element  35  that is fastened to the diaphragm  33  consisting of a single piece of material, for example metal, in particular stainless steel. The sensor element  35  generates the abovementioned signal, the frequency of which is proportional to the volume flow of the flowing fluid. 
   Fixed in the vicinity of the bottom of the blind hole  314  is a temperature sensor  34 . Supply leads  341 ,  342  of the temperature sensor  34  lead centrally upward through the vortex sensor  3 . 
   One of the supply leads  341 ,  342  may be omitted if the temperature sensor  34  is in electrical contact on one side with the sensor vane  31 , and is consequently at the potential of circuit zero point. The temperature sensor  34  is preferably a platinum resistor. 
   Since the sensor vane  31 , and in particular its wall part  315 , can be made adequately thin and also preferably consist of metal, the temperature sensor  34  is virtually at the temperature at any given instant of the fluid flowing past the sensor vane  31  and, because of the low thermal capacity of the arrangement, is also very capable of following temperature changes of the fluid adequately quickly and virtually without any delay. Consequently, the mass flow can be calculated in the customary way from the temperature of the fluid, measured by the temperature sensor  34 , and from the signal of the vortex sensor  3 . 
   In  FIG. 5 , a vortex flow pickup  1 ′ corresponding to the second variant of the invention is represented in a way analogous to  FIG. 2  in a perspective view and partly cut open. The parts of  FIG. 5  which are of the same type as the parts of  FIG. 2  are not explained again, but the reference numerals used for them in  FIG. 2  are provided with an apostrophe. 
   The differences between the exemplary embodiment of the second variant of the invention and the exemplary embodiment of its first variant are, on the one hand, that the bluff body  4 ′ is provided with a blind hole  46 , which is in line with a second bore  24  in the tube wall  2 ′ and in which a temperature sensor  34 ′ is fitted, and, on the other hand, that the wedge-shaped sensor vane  31 ′ has two planar principal surfaces  311 ′. The temperature sensor  34 ′ has a supply lead  341 ′. 
   The blind hole  46  may be provided to any desired depth in the bluff body  4 ′; its bottom  461  preferably lies in such a way that the temperature sensor  34 ′ is arranged in the center of the bluff body  4 ′. 
   Since the bluff body  4 ′ can be made adequately thin in the region of the blind hole  46  and, like the sensor vane  31  of  FIGS. 1 to 4 , likewise preferably consists of metal, in particular stainless steel, the temperature sensor  34 ′ is virtually at the temperature at any given instant of the fluid flowing past the bluff body  4 ′ and, because of the low thermal capacity of the arrangement, also very able to follow temperature changes of the fluid adequately quickly and virtually without any delay. Consequently, the mass flow can again be calculated in the customary way from the temperature of the fluid, measured by the temperature sensor  34 ′, and from the signal of the vortex sensor  3 ′.