CA1099952A - Vortex shedding flowmeter assembly - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The present invention relates to a flow velocity measuring apparatus which provides a distinct, measurable pat-tern of vortices known as Karman Vortex Generation. The apparatus is arranged to include a plurality of vortex pro-ducing obstructions which co-operate to provide a high signal output. The flowmeter is designed to use a vibration sensor which is accessible from the exterior of the pipe carrying the fluid being measured.

Description

~ he present invention relates to vortex shedding flow measuring devices.
In the prior art several patents have illustrated 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. 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 United States Patent No. 3,972,232 wherein a piezoelectric pick-up is used for determining the vibration of the obstacle, and wherein a particular shape of obstacle is provided.
A single bar, flat plate assembly which can be inserted into a conduit is shown in Japanese Utility Model Publication No. 9015/1975. A vortex flowmeter which shows a plurality of bodies arranged in a particular orientation in relat~n to the direction of flow is shown in Japanese ~ent Disclosure No. 20553~1973. Particular attention should be given to Figure 7(h). The teaching is that a particular spacing relationship is desired between two laterally spaced "pillars"
and a downstream pillar to obtain the desired action.
Another type o~ vortex flowmeter is shown in U.S.
Patent No. 3,796,096.
The use of a bluff body as the vortex shedding obstacle or body is illus~rated in U.~. Patent No. 3,572,117, and probe rods made of a polygonal cross section are disclosed in U.~. Patent No. 2,813,42~. A flowmeter using rectangular vortex shedding probes in one embodiment, wherein the wall of the probe is intended to be the vorteæ shedding obstacle as well as the moving or sensing element is shown in U.S. Patent No. 3,927,566.
Patent No. 3,948,097 is of interest in that it provides a plurality of slots in a vortex shedding obstacle with the slots arranged to extend transverse to the flow direc-tion, and these slots are alleged to assist in the production and detection of vortices.
Another patent which illustrates three individual flowmeters in a single pipe, is United ~tates Patent No.
3,979,954. Each of the meters generates a separate signal, and the separate signals can be combined to provide an output that is substantially proportional to the flow rate o~ the fluid.
Investigations of the efects of multiple bars in flow have been made for the purpose of predicting and minimizing structural vibrations in devices such as boiler tubes.
The present invention relates to a vortex shedding flowmeter having multiple bodies or bars forming flow obstruc-tions in a single sensor assembly which provides a strong signal and satisfactory ~nearity. The sensor comprises a generally flat plate that is formed with slots that define spaced apart bars which are spaced across the diameter of the flow pipe.
The sensor can be formed from a plate which will take about the same space as an orifice plate. The width and spacing of the bars is selected to provide a stable vortex pa~tern.
Because the device provides a frequency output which is a higher frequency than conventional vortex meters using a single larger bar, a simpler sensing circuit may be used and faster response is possible. The vortex shedding bodies or bars, s~

when made in cooperation with other shsdding bodies ~rom a single plate and have sufficient deflection so the movement of the bars can be sensed as the indi~ation of vortex shedding.
The bars are aligned transversely o~ the flow direction but do not operate in connection with any other bars positioned upstream or downstream from the spaced bars.
Additionally, the construction provides ~o~ easily inserted or removable sensors without dismantling the flow-meter or the conduit carrying the fluid flow being sensed.
Because the sensor will fit in place of an ordinary orifice plate in a flow line, the sensor can be used inter-changeably with orifice plates that are now commonly used in differential pressure sensing of flow in conduits.
In the Drawings:
Figure 1 is a typical sectional view of a portion of a fluid carrying conduit having a vorteg shedding flowmeter installed therein;
Figure 2 is a plan view of the vortex shedding flowmeter of the present invention taken as on line 2--2 in Figure l;
Figure 3 is a sectional view taken as on line 3--3 in Figure 1, and illustrating the vortex generation in a ~irst stage of operation;
Figure ~ is a sectional view taken on the same line as Figure 3 with the showing of vortex generation in a second stage;
Figure 5 is a vertical sectional view of a center one of the vortex forming bars showing a signal pick-up installed therein;

Figure 6 is a vertical sectional view of a typical vortex forming bar used in a vorte~ flowmeter having a capa-citive type sensor installed therein;
Figure 7 is a sectional view taken as on line 7--7 in Figure 6;
Figure 8 is a plan view of a modified vor~ex flowmeter made according to the present invention showing a modified sensor also installed in such flowmeter; and Figure 9 is a cross sectional view of a further modified flowmeter taken generally along the same lines as Figures 3 and 4 showing a flowmeter with four vortex forming bars.
Figure 1 shows a fluid carrying conduit 10 which is carrying fluid generally in the direction indicated by arrow 11 and which has a pair of flange type couplers 12, one on each of two conduit sections, which are spaced apart to receive and sandwich a vortex flowmeter plate assembly illustrated generally at 13. The flanges are held together with suitable coupling bolts 14, and they clamp and seal onto the vortex shedding flow-meter plate 13.
The flowmeter plate 13 in this form of the inventionis made fro~ a circular plate with orifices or flow apertures cut in the plate to define the cross bars. As can perhaps best be seen in Figure 2, there is a perimeter or annular rim 15, and a center flow obstructing bar 16, a flow obstructing bar 17 adjacent a first side thereof, and a flow obstructing bar 18 adjacent a second side thereof. These bars or bodies 16, 17 and 18 are separated by suitable orifices or apertures gs~
(or slots) 21 and 22 on opposite sides of the ce~ter body 16 which are of equal size and shape, and orifices or apertures (or slots) 23 and 24 to the outside of the bars 17 and 18, which orifices 23 and 24 are also of equal shape and size. The outer edges of each of the orifices or apertures indicated at 21a, 22A, 23A, and 24A are part circular and these edges define thè
effective flow diameter D of the flowmeter assembly.
The flowmeter plate 13 is relatively thin in direction of the fluid stream 11, and the flow obstruction bodies or bars 16, 17 and 18 are formed to be substantially square (rectilinear) in cross section as shown in Figures 3 and 4. The bars 16, 17 and 18 are made to cause flow separation, causing vortices to be formed and shed from the bars along their side surfaces (the surfaces parallel to the flow direction)a The center bar 16 has a receptacle 25 defined therein, leading from the exterior edge of the rim 15 and which is open and accessible between the flanges 12. A piezoelectric sensor, which is a motion sensor, indicated generally at 26, or some other suitable motion sensor is placed into the receptacle 25 and can be held in the opening in a suitable manner. The sensor 26 is utilized for sensing vibrations of the center bar 16 caused by the formation and shedding of vortices as the fluid in ~ conduit flows past the bars.
It has been found that preerably the sum of the widths of the bars 16, 17 and 18 should be approximately .2 to O3 of the diameter of the effective flow diameter of the plate. That is, considering the width in diametral direction of each of the members W, and the diameter of the effective opening in the plate D, the sum of all of the widths of the plurality of bars or bodies 5;2 on the plate would e~ual .2D to .3D. This diameter D as shown in Figure 2 is the effective 10w diameter defined by the edges of the orifices or apertures in the meter assembly, rather than the actual diameter of the conduit.
When a plurality of spaced obstruction bodies are placed across the conduit and are commonly mounted to a rim or ring as shown, the rim can be slipped into position in place of a common orifice plate without modifying existing mounting members.
The center bar 16 is centered on a diametral line of the flow conduit, and the bars 17 and 18 are spaced an equal distance from the center bar and ~rom the sides of the conduit.
It has been found that when a plurality of the obstruction bars or bodies such as 17, 18 and 19 which are transversely aligned across the conduit are utilized, the forma-tion of vortices will tend to shift across the diameter of the conduit, so that primary or strong vortices are formed alter-nately along the sides of the bars as shown in Figures 3 and 4.
Note that no upstream or downstream bars or obstructions are used, but only the bars centered on a common plane perpendicular ; to the flow direction.
As a first step in explanation, previously formed vortices indicated at 65 and 66 will be downstream from the flow sensor, and the primary or strong vortices will be formed along the sides of the bars 16 and 17 defining orifice or opening 22. These vortices are shown at 67 and 68. Further strong vortices will be formed along the side of the body 18 adjacent the conduit side by flow through the orifice or - ~

~ .~3~
opening 23, and such a vortex is shown at 69 in Figure 3.
After the formation of these vortices 67, 68 and 69, the primary vortices will next be formed along the sides of the bars or flow obstruction bodies 16 and 18 defining orifice or openin~ 21, as shown in Figure ~, and such vortices- are shown at 70 and 71. The previous vortices 67, 68 and 69 are shown downstream slightly. Additional strong vortices will be formed along one side of the body 17 by flow through orifice or opening 24, as indicated at 72 in Figure 4. As flow continues there is alternate "switching" or oscillation of the place of formation of strong or primary vortices and such alternation will cause a lateral vibration of the in-dividual bars or bodies 16, 17 and 18. The vibration may be picked up by the transducer 26, and the frequency output will be recorded by suitable equipment well known in the prior art for recording high frequency vibrations. It should be noted that the end of the sensor shown in both Figures 2 and 5 is held snuggly at its outer end with a collar sleeve or other member which is shown only schematically in Flgures 2 and 5.
As flow continues, the vortices will alternately switch back and forth in a stable pattern. The maximum output will be achieved when the previously recited preferred relation-ship between the sum of the widths tW) of the flow obstruction bars and the overall diameter (D) of the opening is maintained.
It should be noted that there will be a Iocal re-striction at the flowmeter plate because the diameter "D", which is the effective diameter in which the bars are placed is less than the diameter of the conduit itself. The diameter D should be no less than ahout 90~ of the internal diameter of the conduit ,~

~ S 2 or pipe itself. If the open~ng diameter at the flowmeter plate is substantially less than 90% of the diameter of the pipe, turbulence will adversely affect the vortex generationO
The spacing between the individual bars is sufficiently close so that any vortex formed along the side of one bar which is adjacent another bar enhances, th ~ ugh a reinforcing action, the vortex formation along the next adjacent bar. Signal magnitudes on the order of 10 times the magnitudes obtained with a single bar can be obtained by addition of e~tra bars to obtain the desired coaction between the vortices formedO
The spacing between adjacent bars is preferably substantially twice the width (W) of the bars.
If the bars are too close, the vorte~ shedding is adversely effected and the vortex pattern described does not occur resulting in a very erratic low level signal. For example if the spacing = W the signal is not useful.
With the plurali~y of bars, it has been found that the vortices will be shed uniformly along the length of the individual bars, and each of the bars affects positively the shedding of the vortices ~rom the adjacent bars.
It has also been ~ound that while three bars are useful, two bars can also be used providing a single orifice or opening between them and openings on the outer sides of the bars (between the sides of the conduit and ~he individual bars).
Four bars can also be used if desired as will be shown. The plate from which the bars are formed is sufficiently thin so that it can be used in place of an orifice plate.
In the present device the bars are square tha~ is, L = W. While L should not be substantially greater than W

for best results, the bars can be t'~inner than they are wide. L
should not be less than .6W for strong vortex formation, how-ever.
In Figure 6, a center bar 30 of a typical vortex flow-meter, such as that shown in Figures 1 tllrough 7, is shown with a different type sensing unit. The bar 30 has an interior passagewa~T 31, and a ceralnic rod 32 is installed in this opening 31. It can be seen that the inner end 32A of the rod tightly fits into the opening 31, and the upper end 32B also tightly 10 f its in this opening. ~ center portion 33 of th e rod, which is of a reduced diameter carries capacitor plate members com-prising spaced, oppositely facing metal film layers irldicated at 34 and 35.
Suitable leads can be run from the respective capaci-tor plate members 34 and 35 up through the upper portion 32B
of the ceramic rod, and then out through the opening in the center bar 30, as with the previous sensors.
In Figure 7, a cross sectional view of the sensor is shown, and it can be seen that the side to side motion 20 of the bar 30 indicated by the double arrows 36 will cause changes in capacitance as the bar 30 vibrates back and forth.
The capacitance changes occur between the respective surfaces 34 and 35 and the surface portions on the interior of the opening which are adjacent to and aligned with these active capacitor portions. The spacing will change during vibration, and this spacing change causes capacitance changes that can be detected with suitable capacitance detection equipment. The bar, as shown is a circular cross section cylindrical member, rather than triangular or rect ilinear.

_g_ 5;~

In Figure 8, a ~urther modified form of the invention is shown. In this ~orm, an orifice plate 40 is provided with a peripheral rim 40A, and a pair of square cross section center bars 41 and 42, respectively. The bars 41 and 42 are de~ined by apertures or openings 43 and 44 which are part circular in their outer periphery, and are to the outside of the respective bars 41 and 42. A center aperture or opening 45~is defined between the two bars 41 and 42.
It can be seen that a ~irst hole 46 is drilled ~rom the edge surface of the rim 40A into the center of the bar 41, a second hole 47 is drilled from the edge surface of the rim 40A into the second bar 42. The upper portions of these holes are of larger diameter ~han the lower portions, and it can be seen that a ceramic rod 48 is inserted in the hole 46 and is securely held near its lower end 48A by reduced portion of the hole 46, and likewise a second ceramic rod 49 is insert-ed in the hole 47, and is held snuggly and securely adjacent its lower portion 49A as shown in Figure 8. The rods 47 and 48 are moved back and forth as the bars 41 and 42 flex. Vibration of the bars 41 and 42 is caused by the formation of vortices along the side edges. It should be noted t~ at the clearance of the holes 46 and 47 is selected to provide sufficient room for vibration.
The upper ends of the two bars 48 and 49 are connected by metal force bars 5~ and 51, respectively, which in turn are attached to capac:itor plates 52A and 52B. Leads 53A and 53B
as shown lead from the plates 52A and 52B respectively. The motion of the ceramic rods moves these capacitor plates alter-nately toward and away from each other as vortices are shed from bars ~1 and 42. Thus ~he capacitance as measured on leads 53A and 53~ varies at the same frequency as the ~requency of vortex shedding.
This arrangement has the advantage o~ cancelling in-phase vibration of the plates 52A and 52B induced by pipe or conduit vibration not related to vortex shedding since these vibrations will not cause relative motion of the two capacitor plates.
The mounting portions 48A and ~9A are near the inflection point of the vibrating bars in order to get maximum motion transmitted externally to the sensing uni~ at the exterior of the plate. The motion of the bars at their inflection includes a rotating component which causes large angular rotation in the rods 47 and 48. ~his angular rota~ion causes a large deflection at the end of the rods andi~fectively amplifies the motion of the bars.
In Figure 9, a cross sectional view of a typical orifice plate utilizing four bars, as opposed to the three bars and two bars shown previously, is illustrated. A vibration ~0 or motion sensor would be used in at least one of the bars and would be generally the same type as before. At least one of the bars in Figure 9 would have an opening for receiving a sen-sor.
The plate 55 as shown has bars 56, 57, 58 and 59 defined therein. The opening in the plate would be circular as previously shown, in that the outer edges of the openings between each of the bars would be rounded to define an internal diameter D as shown in Figure 2.
It has been found that in certain applications, cer-q~
tain combinations of bars in the sensor worlc better than others.For example, where water is flowing through a four inch diameter pipe at rates between one and 15 feet per second, a four bar configuration shown in Figure 9 is found to provide high out-puts. The bars shown are quarter inch square, that is, the plate itself is a quarter inch thick and the bars are a quarter inch wide. The spacing between ~e bars at the center plane of the plate (along a diametral line) is one half inch, or double the length of the bars in direction of the flow. Using a frequency to voltage analogue type output having a two second time constant, the analogue output variations are within one percent for the specific orifice plate 55 that is shown in Figure 9.
In other configurations, up to a five percent analogue output variation is found. This variation (which may be called a fluctuation or ripple in the signal) is undesirable and a low-er value gives more reliable information as to the flow rates.
Thus, in all forms of the combination the plurality of bars are utilized to enhance the output when vortices are formed along the side surfaces of the bars. It should be noted that when round bars are utilized, the facing surface portions actually are substantially half cylinders, but even with round bars, as well as with rectilinear or triangular bars, there are facing surface portions between adjacent bars. In a triangular cross section bar, one surface of the triangle could be normal to the flow direction.
In summary, the obstruction bars are formed on a plate so that they have an equal length dimens~ni (L) in direction of flow, and are spaced side by side on a common bisecting ~'r~ j2 plane transverse to the flow stream, and they are spaced suffi-ciently close laterally so that each reinforces the vortices formed at adjacent bars.
The flowmeter has bars that are centered in direction of the flow on a plane perpendicular to the flow direction and are spaced in direction transverse to the flow direction to cause the lateral alternation of primary vortex formation for strong signal output. There axe no additional posts or obstructions either upstream or downstream of -the trans-versely aligned bars that might dampen or destroy the vortexformation along alternate side surfaces of the transversely aligned bars.
Hollow or tubular square bars can be used to minimize mass and sensitivity to pipe vibrations. Triangular, round or other cross sections can also be used. Although the bars are shown as uniform in cross section along their length they could vary in cross section. The width of each bar could vary to correspond to the velocity profile along its length to enhance vortex shedding simultaneously along the entire length of each bar.
In addition, although the bars in a given flowmeter are shown to be the same width, the width of the various bars could also be individually proportioned to correspond to the velocity profile across the pipe to enhance simultaneous vortex shedding from all of the bars.
Piezoelectric and capacitive motion sensing have been described. However, other motion sensors could be employed, such as electromagnetic or optical for example.
While motion sensing of individual bars only has been described as the preferred design, where space permits, other `~

means o~ sensing vortices could be employed, such as acoustic or thermal conduction probes posi~ioned downstream (or upstream) of the vortex generating assembly.

Claims (11)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A flowmeter of the vortex shedding type for a fluid flow conduit comprising a plurality of flow obstruction bodies being contoured to cause fluid flow separation and formation of vortices, means to mount said flow obstruction bodies in said conduit in posi-tion spaced from each other in a first direction trans-verse to the flow direction, said flow conduit having a cross section dimension in the first direction of "D", each of said flow obstruction bodies having at least one surface portion generally facing a surface portion of another obstruction body in direction transverse to the flow direction, the spacing of said obstruction bodies being such that vortices formed along one of said facing surface portions affects formation of vortices formed along other of said facing surface portions and rein-forces the vortices so formed, and the total width of all flow obstruction bodies in the first direction being not substantially less than .2D, and sensor means to sense formation of vortices from at least one flow ob-struction body.

2. The flowmeter of Claim 1 wherein the obstruction bodies are generally rectilinear in cross section and are generally centered on a common plane normal to flow direction.

3. The flowmeter of Claim 2 and a common support fixedly mounting said plurality of flow ob-struction bodies.

4. The flowmeter of Claim 1 wherein said flow obstructing bodies are formed from a plate relatively thin in the direction of flow in said conduit, said plate having flow openings defined therein to form said obstruction bodies between said flow openings.

5. The flowmeter of Claim 1 wherein said flow obstruction bodies are mounted fixed in a surround-ing rim having a peripheral edge, a receptacle defined in said rim from said peripheral edge into the interior of one of said flow obstruction bodies, and said sensor comprising a piezoelectric sensor mounted in said re-ceptacle to sense vibration of said one flow obstruct-ion body.

6. The flowmeter of Claim 4 wherein there are two flow obstruction bodies comprising said flowmeter, a pair of rod means connected to each of said two flow obstruction bodies, motion sensor means connected between said rod means to detect opposed motion of said two flow obstruction bodies.

7. The flowmeter of Claim 1 wherein the flow opening being generally circular with said flow obstruction bodies being parallel to diametral lines of said flow opening and wherein the sum of the widths of said plurality of flow obstruction bodies measured transverse to the direction of flow is under .3D.

8. The flowmeter of Claim 1 wherein there are three flow obstruction bodies, and one of said flow obstruction bodies is centered on a diametral line of said flow conduit, and the other flow obstruction bodies are spaced an equal distance on opposite sides of said one centered flow obstruction body.

9. The combination of Claim 8 wherein the flow obstruction bodies each have a length "L" in direction of flow and the length "L" of each flow obstruction body is not substantially greater than its width and not sub-stantially less than .6 times its width.

10. The flowmeter of Claim 1 wherein at least one of said flow obstruction bodies is tubular for at least a portion thereof, and fixed rod means spaced from the interior of said tubular portion and fixed to the means to mount, said means to sense comprising capaci-tive means to sense changes in spacing between said rod means and the interior surfaces of said tubular portion.

11. The flowmeter of Claim 1 wherein said means to mount comprises an orifice plate having orifices therein which define four flow obstruction bodies having central longitudinal axes extending substantially parallel to each other in a first direction transverse to the flow direction and being spaced apart in a second direction perpendicular to the first direction.