GB2101318A

GB2101318A - Ultrasonic flowmeter

Info

Publication number: GB2101318A
Application number: GB08213889A
Authority: GB
Inventors: Christopher Alan Watson
Original assignee: Deutsche ITT Industries GmbH; ITT Industries Inc
Current assignee: TDK Micronas GmbH; ITT Inc
Priority date: 1981-05-29
Filing date: 1982-05-13
Publication date: 1983-01-12

Abstract

In a flowmeter in which ultrasonic pulses are transmitted alternately from two oppositely disposed transducers (38, 38'), the transducers (38,38') are mounted on the outside of bodies (36,36') "transparent" to the ultrasonic vibrations. The bodies each have a face exposed inside a pipeline to a fluid flowing therein. The faces are shaped to prevent debris from becoming trapped therein or therearound. The transducers are mounted in a way that, with the shaped bodies, there is an angle of refraction phi such that phi = O (Fig. 2) or phi -> O (Fig. 7) (not shown) such that the pulse transmission is substantially direct from one transducer to the other, and vice versa. In an alternative arrangement (Fig. 3) (not shown), two transducers (41) (42) are mounted on a single body (43) and receive reflected vibrations from an opposing surface (44). <IMAGE>

Description

SPECIFICATION Ultrasonic flowmeter This invention relates to flowmeters, and more particularly to an ultrasonic flowmeter.

An ultrasonic flowmeter is described in our US Patent No. 41 85498. U.K. Patent Application No.

2,037,986 also describes such a flowmeter.

The prior art, however, does not disclose an efficient way of transmitting ultrasonic vibrations and of keeping the transducer mountings clear of debris.

According to one aspect of the present invention there is provided an ultrasonic flowmeter comprising: a pipe section for a fluid to flow there-through, said pipe section having a wall with at least one opening extending completely therethrough, said pipe section having an internal surface about an axis: a first assembly mounted in a position fixed relative to said pipe secton and sealing said opening shut, said first assembly including a first solid body having a fluid free side and a side opposite thereto, said opposite side lying in contact with said fluid; a first transducer fixed relative to said first solid body fluid free side to transmit vibrational energy through said first solid body and subsequently to and through said fluid, and to receive vibrational energy; and a second assembly having a second body and a second transducer to transmit and to receive vibrational energy, said first and second assemblies being constructed and mounted in a manner to cause said vibrational energy to be transmitted alternately from and to be received alternately by said first and second transducers at central locations thereon, the line of travel of said energy centrally from one transducer to a central location on the other taking into account whatever refraction is caused by said first and second solid bodies.

According to another aspect of the present invention there is provided an ultrasonic flowmeter comprising: a pipe section for a fluid to flow there-through, said pipe section having an axis, a wall with at least one opening extending completely therethrough, said pipe section having a cylindrical internal surface of revolution, an assembly mounted in a position fixed relative to said pipe section and sealing said opening shut, said assembly including a solid body having a fluid free side and a side opposite thereto, said opposite side lying in contact with said fluid; a first transducer fixed relative to said solid body fluid free side to transmit vibrational energy through said solid body and subsequently to and through said fluid and to receive vibrational energy; and a second transducer to transmit and to receive vibrational energy, said first and second transducers being constructed and mounted in a manner to cause said vibrational energy to be transmitted alternately from and to be received alternately by said first and second transducers at central locations thereon, said first body having two end surfaces over which said body has uniform thicknesses, said end surfaces intersecting in a line at a radius larger than the inside radius of said pipe section, said line of intersection lying in a plane normal to said axis, said transducers being fixed to respective portions of said fluid free side over respective ones of said end surfaces.

In accordance with the flowmeter of the present invention, the above-described and other disadvantages of the prior art are overcome by providing transducer locations such that, when refraction is considered, vibration is transmitted with maximum efficiency to each transducer. This is true even though transducer mounting bodies are shaped to keep the area clear of debris.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a diagrammatic view of one embodiment of an ultrasonic flowmeter constructed in accordance with the present invention; Fig. 2 is a longitudinal sectional view of a second embodiment of the present invention; Fig. 3 is a longitudinal sectional view of a third embodiment of the present invention; Fig. 4 is a transverse sectional view of the embodiment taken on the line 44 shown in Fig. 3; Fig. 5 is a top plan view of a portion of the embodiment taken on the line 5-5 shown in Fig. 4; Fig. 6 is a longitudinal sectional view which may be the same as or similar to that of Fig. 3; Fig. 7 is a longitudinal sectional view of the embodiment of the present invention shown in Fig. 1;; Figs. 8 and 9 are perspective views of a transducer mounting body shown in Fig. 7; Fig. 10 is an inverted side elevational view of the body shown in Figs. 7, 8 and 9.

Figs. 11 and 1 2 are longitudinal sectional views of respective fourth and fifth alternative embodiments of the present invention; Fig. 1 3 is a diagrammatic view of downstream vibration transmission across a pipeline; Fig. 14 is a vector diagram of acoustic and flow velocities: Fig. 15 is a diagrammatic view of upstream vibration transmission; and Fig. 1 6 is a vector diagram of acoustic and flow velocities.

In the drawings, in Fig. 1, an ultrasonic flowmeter 20 is shown including a pipe section 21 and transducer assemblies 22 and 23 connected to and from a flow computer 24 via leads 25 and 26, respectively.

An indicator 27 is connected from flow computer 24.

Flow computer 24 and indicator 27 may be entirely conventional and may be, if desired, identical to those disclosed in our U.S. Patent No. 4,185,498, which is equivalent to GB Patent Specification No.

1 6021 85. See also this patent for the theory of operation of flow computer 24 and indicator 27. Flow computer 24 alternatively transmits pulses from transducers (not shown in Fig. 1) in assemblies 22 and 23. From these, flow computer 24 produces pulses in a counter, not shown, the total count of which is indicated by indicator 27. The said total count is directly proportional to the total flow through pipe section 21. The flow computer 24 may be calibrated to cause indicator 27 to indicate total flow in units of volume.

In Fig. 2, assembly 22' includes a shell 28 fixed to a body 36 in a flanged cylinder 29 via a pin 30 and/or otherwise.

Assembly 23' similarly has a shell 31, and a pin 33. Angle A is 15 degrees. Angles A" and A"' are 90 degrees. Pipe section 21' has two portions 34 and 35 fixed to and sealed within cylinder 29. The body 36 is sealed within cylinder 29 by an O-ring 37. A transducer 38 is fixed against body 36 by epoxy 39. A transducer 38' is similarly mounted by epoxy 39' against a face of body 36'. Assembly 23' also has an O-ring 37'. Line 40 shows the two directions in which the wave front travels. Transducers 38 and 38' are fixed to portions of bodies 36 and 36', respectively. The said portions of bodies 36 and 36' have a uniform thickness. There is no refraction of a wave from bodies 36 and 36' into the fluid or liquid inside cylinder 29. Thus , the angle of refraction is 00.The surfaces of bodies 36 and 36' exposed to the fluid or liquid are shaped to keep the respective areas free of debris.

The velocity of fluid, flowing in pipe section 21 ' is determined by flow computer 24 and indicator 27 (Fig. 1) as follows: Transducers 38 and 38' are alternately pulsed. Each received pulse at transducer 38 is compared with the output of a voltage controlled oscillator (VCO) state, and the frequency and period of the VCO is adjusted so that the transit time is an integral number of periods. The same or a similar electronic servo also corrects another VCO responsive to pulses received by transducer 38'.

The difference between the VCO frequencies is then directly proportional to the volume flow rate through pipe section 21'.

In Fig. 3, transducers 41 and 42 may be bonded to a body 43 equivalent to body 36 (Fig. 2). The waves transmitted from transducers 41 and 42 reflect from surface 44. An agent (not shown) such as epoxy 39 (Fig. 2) may be employed for bonding transducers 41 and 42. In Fig. 3, angle B is 1 5 degrees.

In Fig. 4, note that surface 44 is rectangular and flat. For other views of transducers 41 and 42, see both of Figs. 4 and 5.

In Fig. 6, B = 15 degrees G' = 15 degrees H' = 1 5 degrees K' = 1 5 degrees M' = 15 degrees N' = 1 5 degrees Fig. 7 includes 20, 21, 22 and 23 from Fig. 1. See the refraction at D + C (D = 1 35 degrees and C = 1 5 degrees) (47700) since the faces of the bodies are not parallel. Also E = 45 degrees and F = 5 degrees. Bodies 45 and 46 in Fig. 7 serve a purpose similar to those of bodies 36 and 36' in Fig. 2. Line 47 in Fig. 7 is equivalent to line 40 in Fig. 2.

In Fig. 7, shells 48 and 49, and rings 50 and 51 surround bodies 45 and 46. Electrical connectors 52 and 53 are also provided.

Surfaces 54 and 55 are cylindrical surfaces of rings 50 and 51, respectively, having vertical parallel axes.

Body 45 is shown again in Figs. 8, 9 and 10, where typically: G = 2.00 inches H = 1.86 inches K = 5.00 degrees In Fig. 11, M = 5.0 degrees N = 46.2 degrees P = 41.2 degrees Q = 15.0 degrees R = 15.0 degrees S = 15.0 degrees In Fig. 12, A' = 29.3 degrees B' = 19.3 degrees C' = 10.0 degrees D' = 29.3 degrees E' = 29.3 degrees F' = 29.3 degrees (No angles or other dimensions herein are critical.) In a pipe section where flow is only in the direction of the pipe, a pulse propogated into a moving fluid at a sonic velocity Vs an angle a to the flow, will have a velocity component Vs sin a in the direction normal to flow. It has to travel a distance D across the pipe and will therefore take a transmit time D Vssin α This is independent of flow velocity VF.How then do ultrasonic flowmeters of this type work? The sound propagation direction is normal to the wave front. Strictly speaking this is not the case, but for the situation where the transducer faces are parallel to each other and normal to the mean sonic path direction, the error in making this assumption is negligible. This analysis was dealt with by McCartney (McCartney et al. J. Acoust Soc. Am. 65(1) Jan 1979, pg. 50-55 "Ray Theory for Acoustic Velocimetry").

For the case where the transducer faces are parallel but not normal to the mean sonic path direction, a modified set of equations result.

An analysis follows which still assumes propagation in a direction normal to wave front (straight paths) but considers the effect of "beam movement", that is the effective shift of the ray path due to flow velocity.

A transducer and pipe configuration is shown in Figs. 13-16.

For the downstream case in Figs. 13 and 14, VFsin(α-ss) = Vssin α (1) Vssin α tan (α-ss) = Vscos α + VF (2) Angle ss is created by a flow velocity of a finite magnitude.

x-y D D sin 0 tan(α-ss) tan a (3) Thus, x-y VF+Vscos α 1 VF = - = Dsin # Vssin α tan α Vssin α (4) Thus, for the downstream case y = YD DV,sin 0 Vssin α (5) For the upstream case (Figs. 15 and 16):: VFsin (α+#) = Vssin # (6) Vssin α tan (α+#) = Vscos α-VF (7) y-x D D sin 0 tan a tan (α+#) (8) Therefore, y-x 1 Vscos α-VF VF = - = Dsin # tan α Vssin α Vssin α Therefore, for the upstream case y = Yu V,Dsin 0 =x+ (10) Vssin α The upstream and downstream transit times, T, and T2, respectively, as seen by the electronics will be the sum of the times traversing both transducers and the fluid, and also an additional electronic delay T.Hence:

where VE is the sonic velocity in the material on which the transducers are mounted (e.g., 36 and 36' in Fig.2).

AF is the difference between the upstream and downstream frequencies. If the difference frequency is divided by N in the computer circuit, substituting (11) and (12) into #F 1 1 N T2 T1 (13)

ButVF2 < < VsVE' so ignoring the 2nd term, the sensitivity AF/V, is:

If a is close to zero, the transducers must be spaced lengthwise a great distance. If a = 90 degrees, AF/VF is very small, Vs does not have a component to add to or to substract from V,. Thus, a = 75 degrees might be typical.

The angle 0 preferably is small, but not so small as to make a = 90 degrees. If desired, 0 45 degrees.

Typically,

Some other typical values: 2x - + T = 2.0 microseconds VE Vs#1500 meters/second The sensitivity AF/V, preferably should be as large as possible and constant. Preferably Vs"'VE is held constant by holding the temperature constant or by other compensation means. The ratio Vs/VE is temperature sensitive. VE is dependent upon the material of bodies 36 and 36', for example. Vs is dependent upon the material of the liquid. The constants N, a and 0 are selected for a high and constant ratio of AF/V,, efficiency and linearity.

Bodies 36 and 36' in Fig. 2 may be made of any one of many materials including but not limited to glass or machinable glass, titanium, an epoxy or aluminium.

The angle 0 is preferably in the range 0-45 in degrees.

To review, Vs is the sonic velocity of the flowing fluids; VE is the sonic velocity in the bodies 36 and 36' or the like; D is the inside diameter of the pipe; V, is the velocity of flow; N is a dividing factor in flow computer 24; and xis the distance travelled by the wave through 36 and 36' or the like.

The phrase "pipe section" as used herein and in the claims is hereby defined to mean a pipe having an internal surface which has a rectangular, square, round or other cross section. Compare Fig. 2 with Fig. 4.

Fig. 2 deals with a particular configuration relevant to a particular pipe diameter only. It is not the general case.

If a (Fig. 13, for example) is defined as 75 degrees, then the separation between crystal centres can be defined as D tan 150.

If α = 750, then using standard transducer bodies the pipe diameter is 2.0".

If a = 750 in Fig. 3, then the pipe diameter is 1.0".

The general case refers to Fig. 1 or to Fig. 7 with the transducers mounted offset from each other.

Fig. 7 is not restricted to the convex faced transducer, and indeed the 15 complement (C on Fig. 7) is not the only choice. Using concave or parallel interface transducers as in Fig. 2, the sonic path angle could be as required. It could be 270.

Note: 750 path gives 50% sensitivity 630 path gives 80% sensitivity 450 path gives 100% sensitivity AF Output Frequency N sin 2a as in ratio: V, Flow Velocity D Note: sin 2a = sin 2(90 - a) sin 2x75 = sin 30 = 0.50 sin 2x63 = sin 54 = 0.81 sin 2 x45 = sin 90 = 1.00

Claims

1. An ultrasonic flowmeter comprising: a pipe section for a fluid to flow therethrough, said pipe section having a wall with at least one opening extending completely therethrough, said pipe section having an internal surface about an axis; a first assembly mounted in a position fixed relative to said pipe section and sealing said opening shut, said first assembly including a first solid body having a fluid free side and a side opposite thereto, said opposite side lying in contact with said fluid; a first transducer fixed relative to said first solid body fluid free side to transmit vibrational energy through said first solid body and subsequently to and through said fluid, and to receive vibrational energy; and a second assembly having a second solid body and a second transducer to transmit and to receive vibrational energy, said first and second assemblies being constructed and mounted in a manner to cause said vibrational energy to be transmitted alternately from and to be received alternately by said first and second transducers at central locations thereon, the line of travel of said energy centraily from one transducer to a central location on the other taking into account whatever refraction is caused by said first and second solid bodies.

2. An ultrasonic flowmeter as claimed in claim 1, wherein said first and second bodies produce angles of refraction the sum of which is substantially equal to zero.

3. An ultrasonic flowmeter as claimed in claim 1, where said first body has an angle of refraction equal to + 0, and said second body has an angle of refraction equal to - 4. An ultrasonic flowmeter as claimed in claim 3, wherein < , = 0.

5. An ultrasonic flowmeter as claimed in claim 3, wherein the angle of incidence is about 50 degrees and the angle of refraction is about 1 5 degrees.

6. An ultrasonic flowmeter as claimed in claim 1, wherein said first body has two flat end surfaces on said opposite side intersecting in a first line in one plane normal to said axis, said second body also having two flat end surfaces intersecting in a second line parallel to said first line and lying in another plane normal to said axis.

7. An ultrasonic flowmeter as claimed in claim 6, wherein said first body end surfaces and said second body end surfaces are of the same size and shape, and are symmetrical about respective planes through said first and second lines normal to said axis, said first and second bodies being of a substantially uniform thickness over one of said end surfaces of each, said first transducer being fixed to said first solid body fluid free side at the location of said first body uniform thickness, said second body having a fluid free side, an opposite side and a uniform thickness over at least one of said end surfaces thereof, said second transducer being fixed to said second body fluid free side over the location of said second body uniform thickness.

8. An ultrasonic flowmeter as claimed in claim 7, wherein one of said first body end surfaces is parallel to one of said second body end surfaces, said first and second transducers lying on a line normal to said one end surfaces centrally therethrough.

9. An ultrasonic flowmeter as claimed in claim 8, wherein said first and second lines are each positioned at a radius from said axis larger than the inside radius of said pipe section.

10. An ultrasonic flowmeter as claimed in claim 8, wherein said first and second lines are each positioned at a radius from said axis smaller than the inside radius of said pipe section.

1 2. An ultrasonic flowmeter comprising: a pipe section for a fluid to flow therethrough, said pipe section having an axis, a wall with at least one opening extending completely therethrough, said pipe section having a cylindrical internal surface of revolution, an assembly mounted in a position fixed relative to said pipe section and sealing said opening shut, said assembly including a solid body having a fluid free side and a side opposite thereto, said opposite side lying in contact with said fluid, a first transducer fixed relative to said solid body fluid free side to transmit vibrational energy through said solid body and subsequently to and through said fluid and to receive vibrational energy; and a second transducer to transmit and to receive vibrational energy, said first and second transducers being constructed and mounted in a manner to cause said vibrational energy to be transmitted alternately from and to be received alternately by said first and second transducers at central locations thereon, said first body having two end surfaces over which said body has uniform thicknesses, said end surfaces intersecting in a line at a radius larger than the inside radius of said pipe section,said line of intersection lying in a plane normal to said axis, said transducers being fixed to respective portions of said. fluid free side over respective ones of said end surfaces.

13. An ultrasonic flowmeter as claimed in claim 12, wherein a line through the centre of said first transducer perpendicular to a corresponding said one end surface passes through a point on the opposite inside of said pipe section, another line through said second transducer perpendicular to a corresponding other end surface passing through the same said point.

14. An ultrasonic flowmeter as claimed in claim 13, wherein said pipe section has a flat interior surface through the said point and parallel to said line of intersection.

1 5. An ultrasonic flowmeter substantially as herein described with reference to and as illustrated in Fig. 2, Figs. 3,4,5 and 6, Figs. 7,8,9 and 10, Fig. 11 or Fig. 12, with or without reference to Fig. 1 and with or without reference to Figs. 13 to 16, of the accompanying drawings.