GB2068551A

GB2068551A - Vortex Shedding Flow Measuring Device

Info

Publication number: GB2068551A
Application number: GB8100391A
Authority: GB
Original assignee: Yokogawa Electric Works Ltd
Current assignee: Yokogawa Electric Corp
Priority date: 1980-01-24
Filing date: 1981-01-07
Publication date: 1981-08-12
Also published as: GB2068551B; JPS56104252A; JPS6029046B2

Abstract

A body 2 generates Karman's vortices in a fluid flowing through pipe 1 and these cause the body to oscillate laterally, varying the capacitances between the wall of recess 21 in body 2 and electrode, 51, 52. The capacitances are differentially detected to measure flow rate. To avoid discrepancies due to mechanical vibration (noise), structure 5, incorporating electrodes 51, 52, is tuned by weight 7 to vibrate in phase with body 2. <IMAGE>

Description

SPECIFICATION Vortex Shedding Flow Measuring Device The present invention relates to a vortex shedding flow measuring device making use of Karman's vortices and, more particularly, to vortex shedding flow measuring devices in which the flowing velocity or flow rate of a fluid is measured from the number of changes in the capacitance formed between the device and at least a part of the wall of a body which is vibrated by the Karma n's vortices.

The basic concept of providing an obstacle or body in a fluid flow path which causes vortices to be formed and which vortices set up vibrations or oscillations in the obstacle has been described in several patent specifications. Measurement of the vibrations has long been known to provide an indication of the velocity of flow past the obstacle.

Devices of the general type are shown in US patent specification No. 3,972,232, where a piezoelectric pickup is used for determining the vibrations of the obstacle.

Also, devices for detecting the vibration of the obstacle in terms of capacitance are disciosed in US patent specification No. 3,927,566, particularly at Figures 2 and 3. Another type of known device is shown, for example, at Figures 6 to 8 in US patent specification No. 4,186,599.

In these known devices, the measurement is made at a high sensitivity because it relies upon the detection of vibration of the body or obstacle.

On the other hand, however, these known devices involve a problem that the measurement is liable to be affected by the mechanical vibration which is transmitted through the pipe in which flows the fluid of which the rate of flow is to be measured.

According to the invention, a device for measuring the rate of flow of a fluid comprises a vortex generating body disposed in the path of the fluid to be measured, said vortex generating body having a recess formed at one end and extending in the axial direction thereof; an electrode structure having two electrodes and inserted with clearance into said recess with gaps between the respective electrodes and the wall of said recess, in such a manner that the two electrodes are set symmetrically with respect to the direction of flow of said fluid but that forces on said body due to vortex generation thereby cause variations in said gaps; and means for detecting the capacitances formed between the wall of said recess and the respective electrodes in a differential manner.

A device according to the invention can be constructed so as to have a rigid construction and exhibit a high resistance to heat and pressure, as well as a considerable sensitivity. It can also be constructed so that it is not affected by the vibration of the pipe in which the fluid flows.

In order that the invention may be clearly understood and readily carried into effect, examples thereof will now be described with reference to the accompanying drawings, in which: Figure 1 is a partly-sectioned perspective view of an embodiment of the invention; Figure 2 is a sectional view on an essential part of the embodiment shown in Figure 1; Figure 3 is a perspective view of an electrode structure incorporated in the embodiment shown in Figure 1; Figure 4 illustrates the operation of the embodiment shown in Figure 1; Figure 5 is a circuit diagram of an example of an electric circuit for the embodiment shown in Figure 1; Figure 6 is a sectional view of another embodiment of the invention; Figure 7 is a sectional view taken along the line X-X in Figure 6.

Figure 8 is a sectional view of still another embodiment of the invention; Figure 9 is a sectional view taken along the line X-X in Figure 8; and Figure 10 shows a characteristic curve representing the deflection in a vortex generating portion and an electrode structure in response to mechanical vibration in a pipe.

Referring to Figures 1 to 3, a reference numeral 1 denotes a pipe through which a fluid, of which the velocity is to be measured flows, while a reference numeral 2 designates a vortex generating body (obstacle) disposed in the pipe and shaped to generate a Karman's vortex flow in the pipe. The vortex generating body serves also as a detection means for detecting the generation of the vortices. This vortex generating body is fixed at one end to the wall of the pipe by means of a screw 3, while the other end extends to the outside of the pipe and is fixed by a screw or by welding at a flange 4. It is not essential that the said one end of the vortex generating body be fixed to the wall of the pipe. The vortex generating body 2 is provided with an axial recess or cavity 21 formed therein at one end.An electrode structure 5 is inserted into the recess 21 from the adjacent extremity of the vortex generating body with a slight clearance between the electrode structure and the wall of the recess.

As will be seen from the perspective view of Figure 3, the electrode structure is constituted by a cylindrical member having a flange 53. A pair of electrodes 51, 52 are formed on the surface of the cylinder so as to extend in the axial direction in parallel with each other. The cylindrical member may be formed of, for example, a ceramic which can sustain a high temperature, and the electrodes 51, 52 are formed on this material by spattering, evaporation, printing or the like measure. The surfaces of the electrodes 51, 52 may be coated as required. Reference numerals 61 and 62 designate lead wires which are connected to the electrodes 51, 52. These lead wires extend through the cylindrical member to the outside of the latter so as to avoid undue heating by fluid in the pipe.The electrode structure 5 is so inserted that the electrodes 51 and 52 are disposed in the recess symmetrically on opposite sides of a plane containing vector V of the fluid, and so that a capacitor is formed between the wall of the recess 21 and each of the electrodes 51,52.

The device having the described construction operates in a manner explained below. As a fluid flows through the pipe 1, Karman's vortices are formed at opposites sides of the vortex generating body 2 alternatingly and regularly, so that the vortex generating body 2 is subjected to a fluid dynamic lateral force (eg. F in Figure 7) the direction of which reverses alternatingly. The vortex generating body when subjected to the lateral force exhibits a slight displacement as shown in Figure 4. The displacement 8 of the vortex generating body 2 in response to the lateral force varies depending on various design factors such as shape of the vortex generating body 2, wall thickness of the recess, the way in which the body 2 is supported, le. whether the body is cantilevered or supported at both sides, and so forth.Practically, however, a small displacement of the order of O.02m is enough.

As the vortex generating body 2 is displaced by the lateral force, the side wall of the recess is also displaced with respect to the surfaces of the electrodes 51, 52, so that the distances between the side wall and the electrodes 51, 52 are changed to cause capacitative changes.Since the electrode structure 5 is so received in the recess 21 that the electrodes 51, 52, are arranged symmetrically on opposite sides of a plane containing the flow vector V of the fluid to be measured, ie. a plane containing the axis of the pipe 1 as shown in Figure 2, the capacitance C1 formed between the side wall of the recess and the electrode 51 and the capacitance C2 formed between the side wall of the recess and the electrode 52 are changed in a differential manner in relation to the displacement of the vortex generating body 2 due to the alternating lateral forces caused by the Karman's vortices, which displacement occurs in the direction perpendicular to the direction of flow of the fluid.

Also, the changes of these capacitances occur in the same phase as the displacement of the vortex generating body 2 due to vibration noises or the like occurring in the same direction as the flow of the fluid.

Figure 5 is a circuit diagram of an example of a circuit for detecting the changes of the capacitances between the electrodes 51,52 and the side wall Representing the area of each electrode 51, 52 by S, the gap between each electrode and the side wall by d, and the dielectric constant of air by EO, the capacitance C is given by the following equation (1).

Assuming that the vortex generating body 2 is displaced by the lateral force generated by the Karman's vortices to cause a change of gap represented by Ad, the change of capacitance is given by the following equation (2).

Ad AC=-C (2) d For instance, provided that the area S of each electrode is 800 mm2 and that the gap d is 0.1 mm, the capacitance C is calculated to be 70 pF.

If the displacement of the side wall is 0.02cm, the change AC of the capacitance is calculated as 7xlO-2pF.

The capacitances C1 and C2 are connected to opposing sides of a bridge circuit B, so that the change AC of capacitance is detected in a differential manner. On the other hand, the changes of capacitances due to the noise vibrations are negated because these changes take place at the same phase.

In the device having the described construction, the fluid dynamical lateral force acting on the vortex generating body 2 is changed into a slight displacement of the wall of the recess, and this displacement is detected electrically as a differential change in capacitance, so as to obtain pulse signals of a number corresponding to the flowing velocity of the fluid.

The device as a whole has a rigid construction and, in addition, can withstand a high temperature and can operate at a high sensitivity while avoiding the influence of noise, because it does not use any additional detection element such as a piezoelectric element strain gauge and so forth. Since the electrode structure is simply inserted into the recess, the device can be fabricated easily and the inspection and replacement of the structure can be made without substantial difficulty.

In the device shown in Figure 1, the space between the wall of the recess 21 and the electrode structure 5 may be evacuated or may be filled with an inert gas such as He or Ar. In this case, the oxidation of the electrode structure at a high temperature is avoided to ensure a higher resistance to heat, although the attaching and detaching of the electrode structure 5 is rendered more complicated.

In the embodiment of Figures 6 and 7, the recess 21 extending from one end of the vortex generating body 2 is fixed by means of screw 3 to other end of the vortex generating body so that the vortex generating body 2 is hollow along almost all its length. The electrode structure 5 is long enough to extend substantially-from one to the other end of the vortex generating body 2, and the electrodes 51, 52 are attached over a substantial length of the electrode structure 5.

This arrangement permits the area of the electrodes 51,52 to be increased, which in turn increases the detection sensitivity.

In the preceding embodiments, the electrode structure and the recess have circular crosssections. This, however, is not essential and the electrode structure and the recess can have rectangular or other cross-sections.

In Figures 8 and 9, a reference numeral 1 denotes a pipe, while a reference numeral 10 denotes a mounting portion extending from the pipe wall in the direction perpendicular to the axis of the pipe. A flange 22 is provided at one end of the vortex generating body 2, and is fixed to the mounting portion 10 by means of screws 9 through a retainer 8. The other end of the vortex generating body 2 is fixed by means of screw 3 to- the pipe wall. It is not; however, essential for'this end to be fixed.

The electrode structure 5 is provided at its outer end with a flange 53 and extends into the recess formed in the vortex generating body 2.

The electrode structure 5 is fastened at its flange 53 to the flange 22 of the vortex generating body by, for example, welding.

The electrode structure 5 is a hollow cylindrical member made of, for example, ceramics or other heat resistant material, and the electrodes 51,52 are formed on this material by, for example, spattering, evaporation, or printing. The surfaces of these electrodes 51,52 may be coated as desired.

A reference numeral 7 denotes a weight made of a metal or a ceramic located in the hollow space in the electrode structure 5. As will be explained later, this weight serves for an adjustment of the flexural rigidity of the electrode structure 5.

The operation of the device of this embodiment will be explained below with specific reference to Figure 10. The change of the fluid dynamic lateral force caused by the vortex acts on the columnar portion of the vortex generating body 2 in the pipe 1, so that only the vortex generating body is displaced while the electrode structure 5 takes a neutral position irrespective of the change in the lateral force. In consequence, the capacitance Cl and C2 formed between respective electrodes 51, 52 and the side wall of the recess 21 are changed in a differential manner, so that it is possible to know the flow velocity or flow rate by counting the number of changes in the capacitances C1, C2.On the other hand, the electrode structure 5 is displaced together with the vortex generating body in response to the mechanical vibration which acts in the direction perpendicular to the axis of the pipe.

Figure 10 shows the deflection of each element of the vortex generating body 1 (full-line curve) and the deflection of each element of four differing electrode structures 5 (broken-line curves), as observed when a vibration having an acceleration of 0.2 g is imparted to the pipe 1.

The neutral point, ie. the point at which the deflection is zero, in the vortex generating body 1 differs from that in the electrode structure 5, because of the difference between the fixing points. The deflection of the electrode structure 5 can be varied as shown by characteristic curves a, b and c, by changing the weight W of the weight 7.

In this embodiment, the weight W of the weight 7 is so adjusted as to make the deflection of the electrode structure substantially conform with the vortex generating body 2, thereby to minimise the displacement of these two members relatively to each other.

More specifically, referring to Figure 10, the parts of the electrode structure 5 are deflected following the characteristic curves a, b, c and d as the weight W of the weight 7 is selected to be Og, 3g, 6g, and 1 2g, respectively. The characteristic c is selected out of the above-mentioned characteristics a to d, because this characteristic c affords a deflection of the electrode structure 5 substantially conforming with that of the vortex generating body 2 within the region of length of the electrode structure where the electrodes 51, 52 are formed.Since the hatched areas S1 and S2 are substantially equal to each other, the relative displacement between the electrode structure 5 and the vortex generating body 2 is maintained substantially zero when the characteristic c is selected.

The deflection characteristic of each of the vortex generating body 2 and the electrode structure 5 is expressed by the following formula: M El where M, E and I represent, respectively, the mass, Young's modulus and the second moment of area. That is, the deflection characteristic can be varied by the material, cross-sectional shape and so forth. The material, cross-sectional shape and the like are preferably determined chiefly from the view points of mechanical strength, workability, coefficient of thermal expansion, cost and so forth. Apart from the above, the amount of deflection is preferably determined by the mass M.

By making the deflections of various parts of the electrode structure 5 conform with those of the vortex generating body 2, the mechanical vibration acting on the solid part of the pipe 1 causes the deflections of the vortex generating body 2 and the electrode structure 5 in conformity with each other, so that the gaps and, hence, the capacitances C1, C2 between the electrodes 51, 52 and the side wall of the recess 21 are maintained constant irrespective of the mechanical vibration. In consequence, the flow rate can be measured without being affected by the vibration of the pipe.

In the embodiment of Figures 8 and 9 the recess and the electrode structure have circular cross-section. Again, however, this is not essential and the recess and the electrode structure can have rectangular or other crosssectional shape. Also, as in the embodiment of Figure 1 the clearance between the wall of the recess and the electrode may be maintained at vacuum or may be filled with an inert gas such as He or Ar.

As has been described, it is possible to obtain a rate of flow measuring device of a rigid construction, having high heat and pressure resistances and capable of performing a measurement at a high sensitivity while avoiding the influence of various vibratory noises.

Claims

1. A device for measuring the rate of flow of a fluid comprising: a vortex generating body disposed in the path of the fluid to be measured, said vortex generating body having a recess formed at one end and extending in the axial direction thereof; an electrode structure having two electrodes and inserted with clearance into said recess with gaps between the respective electrodes and the wall of said recess, in such a manner that the two electrodes are set symmetrically with respect to the direction of flow of said fluid but that forces on said body due to vortex generation thereby cause variations in said gaps; and means for detecting the capacitances formed between the wall of said recess and the respective electrodes in a differential manner.

2. A flow measuring device as claimed in claim 1, wherein the clearance between the wall of said recess and said electrode structure is maintained under a vacuum or filled with an inert gas.

3. A flow measuring device as claimed in claim 1 or claim 2, wherein the surfaces of said electrodes are coated.

4. A flow measuring device comprising: a pipe in which flows a fluid of which the rate of flow is to be measured; a vortex generating body at least a part of which is disposed in the path of said fluid, said vortex generating body having a recess formed at one end and extending in the axial direction thereof; an electrode structure having two electrodes and received by said recess with gaps between respective electrodes and the wall of said recess, in such a manner that said electrodes are set symmetrically with respect to the direction of flow of said fluid but that forces on said body due to vortex generation thereby cause variations in said gaps, said vortex generating body and said electrode structure being arranged so that deflections of said electrode structure in response to vibrations of said pipe have a mean value substantially conforming with that of said vortex generating body along the length thereof over which said electrodes extend; and means for detecting the capacitances between the wall of said recess and said two electrodes in a differential manner.

5. A flow measuring device as claimed in claim 4, wherein a weight is attached to said electrode structure, said weight being adjustable to make the deflection of said electrode structure in response to the vibration of said pipe conform with that of said vortex generating body.

6. A flow measuring device substantially as hereinbefore described with reference to Figures 1 to 5 of the accompanying drawings.

7. A flow measuring device substantially as hereinbefore described with reference to Figures 6 and 7 of the accompanying drawings.

8. A flow measuring device substantially as herein before described with reference to Figures 8 to 10 of the accompanying drawings.