AU641213B2

AU641213B2 - Ultrasonic gas/liquid flow meter

Info

Publication number: AU641213B2
Application number: AU70592/91A
Authority: AU
Inventors: Winfried Russwurm; Alexander Von Jena
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1989-12-15
Filing date: 1990-12-13
Publication date: 1993-09-16
Anticipated expiration: 2010-12-13
Also published as: NO922336L; DE3941546A1; EP0505483B1; FI100273B; NO305624B1; WO1991009282A1; NO922336D0; ES2057857T3; DE59006793D1; ATE109886T1; FI922731A0; AU7059291A; EP0505483A1

Abstract

An ultrasonic gas/liquid flow meter with a measurement tube (1) of rectangular cross-section is particularly suitable for use as a domestic gas meter.

Description

Ultrasound aas/liauid flowmeter The present invention relates to an ultrasound flowmeter as specified in the precharacterizing clause of Claim 1.

Ultrasound flowmeters of tubular shape are known, into whose tubular interior ultrasound is allowed to penetrate as a pencil beam through a crosi-section of the medium, which in contrast is significantly larger obliquely with respect to the flow axis or to the tube axis through the gas or liquid flowing there, in order to measure the change in propagation time of the ultrasound resulting from the flow speed. Using the value for the flow speed determined thus, the quantity of the gas or the liquid flowing through per unit of time cani-be determined from the tube cross-section and correction parameters which are known per se. In order to have an active ultrasound path La in the flowing medium which is as long as possible, but which results in a technically sensible length of the flowmeter, multiply reflected S* 20 ultrasound paths, running obliquely with respect to the !tube axis, especially in a "W"-shape, have been provided in the tube interior. The ultrasound transducer [sic] is emitted obliquely with respect to the tube axis from a I* transmitting transducer arranged in the tube wall, is 25 aligned onto the opposite tube wall where it is once again reflected further to the opposite wall, is again reflected to the wall which is then in turn opposite and is finally reflected back to the receiving transducer.

*a The use of a quadrilateral measuring tube having 30 a rectangular cross-section H B where H:B 2:1, in order to be able to detect the entire flow virtually .o *uniformly using the ultrasound, for a heat-quantity metgr through which water passes, is disclosed in Siemens Forschungs- und Entwicklungsberichte (Siemens Research 35 and Development Reports), Volume 15 (1986) pages 126 to 134, Figure 11.

1A It is normal to allow such an acoustic sound path in the tube interior to be passed through alternately in the one direction and in the other direction by the ultrasound, namely once with the flow direction and once against the flow direction of the medium. The corresponding variable for evaluating the flow through the tube in terms of volume is obtained from the resulting difference signal. Efforts are directed at having as a long a propagation time difference as possible for the ultrasound with respect to the length of the flowmeter, i.e. with respect to the distance between the transmitting transducer and the receiving transducer. The functions of the two transducers as transmitting transducer and as receiving transducer are interchanged with one another for the path in the flow direction, on the one hand, and for the path against the flow direction, on the other hand.

A limit condition which is generally to be met is that the pressure drop resulting, for example, from refinements of the tube in the region of the flowmeter does not exceed a specific value and is preferably also easily adjustable.

The transducers can be fitted in a simple manner to the flat wall surfaces of such a quadrilateral tube, and transitions to the normal, round cross-section of the remaining pipeline are available.

The object of the present invention is to optimize a flowmeter having a rectangular measuring tube, known per se, for a special *application, namely for domestic fuel gas and for a flow quantity measurement range of 5 1/h to 30 cbm/h with respect to its measurement accuracy of (sic) its functional reliability.

In accordance with the invention there is disclosed ultrasound flowmeter having a measuring tube through which fluid flows and having ultrasound transmitting/receiving transducers which are fitted on one and the same side wall of the measuring tube, placed at a suitable distance 30 L from one another, for a "W"-shaped ultrasound path having multiple reflections in the measuring tube, wherein the measuring tube has a .AD/18610 -2rectangular cross-section with respect to the flow direction with dimensions H .B such that the breadth dimension B is essentially at right angles to the ultrasound path in the measuring tube, the height dimension H is approximately 30mm, the dimension ratio between the height H and breadth B is from 4:1 to 10:1 and in that the fitment, alignment and size dimensioning of the transducers and of their emitting/receiving surface are selected such that the ultrasound passes through this cross-section (H B) homogeneously on the ultrasound path.

The invention is based on the idea of dimensioning the cross-section of the ultrasound radiation, which is as homogeneous as possible, through the measurement space.such as that as large a received measurement signal as possible, with an optimum propagation time range of the ultrasound signal within the range of flow speeds for the predetermined quantity measurement range, is produced for the domestic fuel gas, which is defined as a fluid.

For the multiplicity of the above parameters which are to be taken into account, the range, which is tight in accordance with the claims, for the dimension ratio H:B of the rectangular measuring tube having the 0* *o oo 861o 3 dimension H approx. 30 mm has been found to be the range of the optimum for separation of the useful signal of the W-shaped path with respect to the parasitic signal of the V-shaped path. The lower limit of the optimum region is first reached at an H:B ratio of 4:1 rather than the ratio of H:B 2.1 which is suitable for water in a heat quantity meter. The upper' limit is at 10:1, preferably even 6:1. The dimension ratio preferably lies between 5:1 and 6:1. Such cross-sections have ultrasound propagation times which can be detected favorably for the entire range of flow values resulting from the predetermined quantity measurement range, with good uniformity of the gas flow and homogeneity.

Furthermore, it is also an object of the present invention to keep as small as possible the signal of the V-shaped path of the ultrasound that occurs in a flowmeter according to the invention for which the W-shaped ultrasound path between the input transducer an output transducer is specified as the measurement path.

20 The following configurations are also used primarily for further explanation of the invention.

Considered together, Figs. 1 and 2 show the view in the axial direction of the flow (Fig. 1) and the side vie of the tube (Fig. 2) cut away in the longitudinal direction. H and B specify the above-defined dimensions for the height and breadth of the rectangular tube 1. 2 and 3 designate the side walls and 4 and 5 the lower side C wall and the upper side wall. Fig. 2 shows the same reference symbols. The transducers 11 and 12 are inserted into the upper side wall 5, namely with their transmitting and receiving surface 111, 112 respectivey directed obliquely with respect to the side wall 5 (as shown in Fig. The angle alpha of the W-shaped ultrasound path 21 is designated a in Fig. 2, namely related to the side walls 4 and 5. The surfaces 111 and 112 are aligned such that the beam path 21 strikes these surfaces Sat right angles.

4 However, in principle it is not possible to avoid a component of acoustic energy also being emitted at such an angle from the surface 111 or 112 respectively or being received by these surfaces respectively which leads to a V-shaped path 22 in the tube interior of the tube 1.

The V-shaped path having the angle beta b) has a different propagation time from the desired W-shaped path, which advantageously has a larger acoustic path length in the tube 1 than is the case for the V-shaped path.

In Fig. 2, the entire length of the flowmeter is specified by L. The remaining drawings in Fig. 2 relate to the physical explanation of the invention given in the following text.

For a flowmeter, according to the invention 'and in accordance with Figs. 1 and 2, having triple sound reflection on the lower and upper side wall, the effective flow speed veff is o• Vf v* cosa (1) where alpha is the angle between the axial flow direction 20 and the emission or receiving direction of the transducers 11 and 12, and where v* is the flow speed reduced over the sound path. v* is related to the time differential volume flow dV/dt: eC *i dV/dT [sic] v* F (2) 25 where F is equal to the cross-sectional area H x B. The length L of the sound path in the moving medium results from L, (N+11 H 4H for N 3 rangement) (3) S* sin a sin a where N number of reflections. Using the length of the measurement path L between the penetration points of the 5 ultrasound through the (imaginary) upper side wall (see also Fig. 2) to the flowing medium L, H cot a 4 H cot a for N= 3 ("W"-arrangement) (4) the achievable propagation time difference is given by Delta t 2 L vf 2L v* 2 H v cot a c c c Although the flowing medium is located between the tube inlet and the penetration point (Fig. this is not detected by the ultrasound beam. This preliminary length L depends on the incidence angle alpha and on-the radius r of the transducer, without taking into account S the transducer housing wall thickness: 15 L, r (6) sin a S: The sound path Lw within the transducer pockets is ly r cot a (7) S*oo This sound path causes additional signal attenuation and is therefore kept as small as possible.

The non-usable component 2L/L calculated over the entire length L La 2L becomes SL L r and _L r and La 4 H La L-2L 4 H cos a 2L, r L 2 H cosa r (8) This component depends on the incidence angle alpha on the transducer r:adius r and on the height H of the tube cross-section. The higher the tube and the smaller the transducer radius r, the larger is the usable component. The usable component becomes larger as the t. angle alpha becomes smaller (until, when alpha 0, the 6 limiting case of the sound passing through the measuring tube in a straight line without reflections (no longer a arrangement) is produced). The volume of the transducer pocket, which is likewise unusable for measurement, Vt r 3 cot a (9) 2 becomes smaller for a predetermined r as alpha increases.

The influence of the cross-sectional expansion and narrowing on turbulence and pressure loss hence becomes smaller.

In the arrangement, the contribution of the parasitic sound path 22, which is emitted at the ahgle delta a with respect to the normals of the trans- 15 ducer surface 111, 112 sound path) and is reflected S: once at the angle beta (N is observed as a superimposed disturbance signal.

Local separation of the sound paths can be achieved.

.O 20 The relationship delta a a-b applies. The "distance between the transducer centerpoints L m is constant for both sound paths. For the sound path, this is made up of L. La 2L (Fig. Using the relationship 25 L_ r cot a cos a and from the angle delta a becomes Delta a arctan([sic](1- H tan a) r cos a 2H (11) This means that, when a o 0° and 90*, a larger delta a is achieved by means of a smaller r and a larger 7 a. When r is not equal to 0 and a is fixed, a larger delta a can still be achieved over the height H. If r is very much smaller than H or r 0, (11) can be expressed more simply by delta a a-arctan(tan a) (12) 2 Delta a is thus independent of the height H. In this case, the series development for arctan and tan (x) provides the approximation formula delta a a a 3 (measured on the arc) 2 8 (13) A,large delta a means good suppression of the signal amplitude. The incidence angle should therefore also be *15 made as large as possible and the condition r very much I smaller than H should be met. The distance between the impact points $*se Ld La/4 H cot a (14) is intended to be at least Ld greater than 2L, i.e. La greater than 8Ly, in order to keep the coincidence of the two beam paths as small as possible and facilitate reduction of the amplitude by reflection-attenuating elements. Since L. is generally fixedly specified, attention must be paid to suitable dimensioning of Ly.

Time separation of the sound paths can also be achieved.

The length of the sound path for r 0 can be described by the sound path for the path being described by in accordance with and The path length difference dL can be described for the simple case where r 0 or r is very much less than H, by 8 dL H 4-2(3 cos 2 a 1) 1/2 sin a Using the relationships tup for "upstream" and town for "downstream" for the flow-dependent propagation times, the time difference td between the arrival of the signal and the signal can be represented as td(up) dL (C-Vff) (16) td(down) dL (c+vff) 15 a.

a a egos

OS

S.C

:20 0e e as

S

25 bee

S..

The larger td is, the better the signal can.be separated in time from the signal, i.e. evaluation errors are reduced by superimposition effects (time multiplexing). It must be noted that, in the case of upstream flow, the time td increases and decreases downstream, that is to say the time separability also decreases. dL should therefore be made large enough that both signal components can still be sufficiently separated in time at maximum flow. A larger a, i.e. a steeper incidence, and a larger height H increase dL.

A limit condition mentioned above is the permissible pressure drop delta p, which depends on the measuring tube geometry as follows: delta p prop V 2 L U prop

F

dv. z dt .L U

F(

(17) 0 C. O

C

where L length of the measuring tube and U the circumference of the cross-sectional area described by F.

This means that a minimum pressure drop occurs in the case of a square measuring tube shape.

When designing an ultrasound measuring tube according to the arrangement, in general the maximum measuring tube length La, the maximum volume flow dV/dt, the maximum pressure drop delta p when using a specific 9 medium and the maximum installed height H are specified.

A delta t which is as large as possible is to be achieved with a disturbance influence which is as small as possible. The incidence angle can be determined from this maximum effective transducer separation La (distance between the two penetration points of the transducer beam on the inner top surface) and the maximum installed height H,: tan a 4Hmax (18) La The cross-sectional area or the breadth B of the measuring tube is determined by the pressure loss. It is changed in a simple experimental procedure until *the maximum pressure loss is reached at maximum flow. The 15 cross-sectional area resulting from this determines the achievable propagation time difference delta t: t= 2L dV/dt (19) B.c From (18) and the ratio of the height to 20 the breadth of the measuring tube is given by: H F. tan 2 a c delta t 2

L.

2 tan 2 a B 64 (dV/dt) 16 F In an elongated tube (La greater than H, La 25 greater than H/B will be set to be greater than/equal to 2. This means a measuring tube with a cross-section which is distinctly non-square, up to an extreme ratio of the edge lengths.

SIn summary, for r constant and the angle alpha S 30 a) differing from 0° and 90°, i.e. the beam path with reflections, and N 3, the following criteria result for the selection of the incidence angle a: 'A good, relative utilization of the measuring 10 tube length Equation 8 can be achieved by an a which is as small as possible. A small volume of the transducer pockets Equation 9, short sound path in the transducer pockets Equation 7, large angle for the signal-to-noise ratio Equations 11, 12 and 13, a good absolute utilization of the measuring tube length Equation 6 and a good time separation of the sound paths Equations 15 and 16 can be achieved over an angle a which is as large as possible. Since the signal-to-noise ratio, i.e. the separation of the sound paths, the avoidance of turbulence and an optimum absolute utilization of the measuring tube length, are decisive for the quality of the measurement, in practice an a which is as large as possible a greater than 350) should be selected. In principle, in the case of a specified La according to Equation 18, a is limited only by the height which is S restricted in accordance with the installation dimensions of the tube. However, since important variables (see above) can be optimized via H, H should be made as large 20 as possible. Nevertheless, a large H means additional signal attenuation as a result of the long sound paths L, which can, hcwever, be compensated for by an adequately high sound level.

Practical numerical example: Measuring tube length La 150 mm, pressure drop delta p 2 mbar, maximum volume flow dV/dt 0.00167 m 3 medium air (speed of sound c 340 m/s) at room temperature, r 5 mm. Experimentally, the pressure drop permits the cross-sectional area F 1.2 cm

Z

The maximum installed height is H 31.5 mm. In consequence, Sthe incidence angle a 400 and the breadth B 3.8 mm.

This produces the value 8.25 for the ratio H/B. The usable propagation time difference will be delta t 36 ps. Further variables: angle for the signal-tonoise ratio (using Equation 11) delta 16.10, Lv 7.8 mm, L 165.5 mm [sic], 2,1/L 0.094, i.e. 90.6% of the measuring tube length is used. The volume of the 11 measuring tube is V 19.9 cm 3 the volume of the transducer pockets V, 2 x 0.47 cm 3 Since 8L, 62 mm is less than L a 150 mm, there is good local selectability of the sound paths. The variable dL is determined to be 33.2 mm. With a non-moving medium, the signal hence occurs approximately 98 ps before the signal.

As can be seen from the numerical example, a ratio H:B results which is more extreme than in the known case. However, a further advantage can be seen: in the case of a square cross-section having the same area, H B 11 mm and hence the side facing the transducers is broader than the actual transducers. This would lead to non-homogeneous passage of sound in the measuring space with consequent, serious measurement errors. For an H/B ratio of more than 4, the side facing the transducers is considerably narrower than the transducers. A measuring tube cross-section having an extreme edge ratio S: of 4:1 to 10:1 may also be able to minimize turbulence 0 20 occurring on the flow inlets or on the transducer pockets, or occurring upstream and dragged into the I. measuring tube, and hence contribute further to the measurement accuracy, reproduceability and signal stability of the ultrasound measurement process.

Using a measuring tube 1 dimensioned according to the invention and having a rectangular cross-sectio?,, and using the transducer arrangement and dimensioni-'3 for homogeneous through-radiation, reliable flow measurements *ew can be carried out for the range 5 Itr/h to 30 cbm/h. For example, a dimensioning H:B having (specified) 30 mm to or 6 mm is appropriate. In the case of a maximum flow quantity, the pressure drop may rise to 25 mbar. Using electronic recorrection, such a flowmeter can be used as an accurate test set from 30 cbm [sic] down to 1 Itr/h.

Claims

1. Ultrasound flowmeter having a r Jring tube through which fluid flows and having ultrasound transmitting/receiving transducers which are fitted on one and the same side wall of the measuring tube, placed at a suitable distance Lm from one another, for a "N"-shaped ultrasound path having multiple reflections in the measuring tube, wherein the measuring tube has a rectangular cross-section with respect to the flow direction with dimensions H B such that the breadth dimension B is essentially at right angles to the ultrasound path in the measuring tube, the height dimension H is approximately 30mm, the dimension ratio between the height H and breadth B is from 4:1 to 10:1 and in that the fitment, alignment and size dimensioning of the transducers and of their emitting/receiving surface are selected such that the ultrasound passes through this cross-section (H B) homogeneously on the ultrasound path.

2. An ultrasound flowmeter substantially as described herein with reference to the drawing. DATED this EIGHTH day of JULY 1993 Siemens Aktiengesellschaft Patent Attorneys for the Applicant SPRUSON FERGUSON S* o *ooo /1861o