GB2222456A

GB2222456A - Flowmeters working on the coriolis principle

Info

Publication number: GB2222456A
Application number: GB8919407A
Authority: GB
Inventors: Andreas Back-Pedersen; Ole Buris Nielsen; Ller Hans Christian M
Original assignee: Danfoss AS
Current assignee: Danfoss AS
Priority date: 1988-08-26
Filing date: 1989-08-25
Publication date: 1990-03-07
Also published as: IT8967719A0; DK416989D0; SE8902496L; FR2635866A1; IT1233178B; SE8902496D0; DK416989A; ES2014875A6; JPH02107925A; NL8902140A; GB8919407D0; DE3829062A1; DE3829062C2

Description

FLOWMETERS WORKING ON THE CORIOLIS PRINCIPLE 2,,,22456 This invention

relates to flowmeters working on the Coriolis principle.

A known flowmeter working on the Coriolis principle comprises an attaching means connected, on the one hand, to an inlet-connection tube and an outlet-connection tube, and on the other hand to two adjacent measuring-tube bight portions-which are arranged to be oscillated in opposite senses by an exciter of oscillations with, where n is an integer, the nth natural frequency fn corresponding to the nth natural mode of oscillation, and are provided with sensors for monitoring a parameter depending on the relative movement of the bight portions.

In such a flowmeter known from European patent specification EP-A-0 239 679, the attaching means consists of a substantial block member carrying three connecting orifices at each of its ends. one pair of orifices is connected to two straight connection tubes. A respective measuring-tube bight portion is connected to each of the two other pairs. Appropriate connecting passages extend inside the block member. Each measuring-tube bight portion consists of a straight tube section, two adjoining 180 degree bends and two end sections of substantially equal length connected to the bends. The ratio of the length of the measuring-tube bight portions to their height is about 2:1.

Numerous other forms of bight portion are also known, for example, circular or tennis racket shape bight portions are disclosed in German patent specification DE-AS 28 22 087.

In all cases, the measuring sensitivity or quality factor of the flowmeter is significantly limited.

This invention is based on the problem of providing a flowmeter of the afore-mentioned kind which has a substantially higher measuring sensitivity.

The present invention provides a flowmeter working on the Coriolis principle, the flowmeter comprising an attaching means connected, on the one hand, to an inlet-connection tube and an outlet-connection tube, and on the other hand to two adjacent measuring-tube bight portions which are arranged to be oscillated in opposite senses by an exciter of oscillations with, where n is an integer, the nth natural frequency fn corresponding to the nth natural mode of oscillation, and are provided with sensors for monitoring a parameter depending on the relative movement of the bight portions, characterized in that the arrangement of the measuring-tube bight portions is such that the (n + I)th natural frequency fn + 1 corresponding to the (n + 1)th natural mode of oscillation lies in the range 0.7 fn to 1-5 fn inclusive but outside a range in which fn produces resonance.

The above-mentioned problem is solved by the feature that the arrangement of the measuring-tube bight portions is such that the (n + 1)th natural frequency fn + 1 corresponding to the (n + 1)th natural mode of oscillation lies in the range 0.7 fn to 1.5 fn inclusive but outside a range in which fn produces resonance.

In operation, the measuring-tube bight p6rtions are excited by the exciter of oscillations at one of their-natural frequencies, generally the first natural frequency. They therefore move towards and away from each other in the associated natural mode of oscillation. By reason of the Coriolis forces, on this movement is superimposed a movement which to a large extent corresponds to the mode of movement when exciting oscillations using the (n + 1)th frequency, that is the (n + I)th natural mode of oscillation. According to the invention, constructional measure are taken so that the (n + I)th natural frequency lies relatively closely adjacent to the nth natural frequency. That characteristic leads to a marked deflection in dependence on the Coriolis forces. The sensitivity of the flowmeter is correspondingly high. The (n + 1)th natural frequency may be as close as possible to the exciting nth natural frequency; one has only to take care that this excitation will not lead to resonance of the (n + 1)th natural frequency, that is, that the response bandwidth curves do not intersect or intersect only in regions where the damping is high (for example, 8OdB) and that therefore no influence on the phase shift in the sensor region results.

If fn + 1 is greater than fni the deflection caused by the Coriolis force is for the most part determined by the stiffness of the measuring-tube bight portions and not by the mass. Consequently, changes in the density of the fluid to be measured have a comparatively small influence on the deflection dependent on the Coriolis force, which feature likewise contributes to an improvement in measuring sensitivity.

It is therefore particularly favourable if n is equal to 1, that is excitation takes place with the first natural oscillation and the deformation of the measuring-tube bight portion by the Coriolis forces corresponds to the second natural mode of oscillation. That results in comparatively large deflections from the start so that the improvements aimed at are particularly substantial.

It is particularly advantageous if f2 lies in the range 1.2 f, to 1.3 f, inclusive, especially if f2 lies in the range 0.75 f, to 0.85 f, inclusive.

A further improvement is obtained if the quality factor Q of the oscillatory system constituted by the measuring-tube bight portions is at least equal to 3000 at the nth as well as at the (n + 1)th natural frequency (fn, fn + 1)- Preferably, the quality factor Q is greater than 4000. Since that corresponds to a remarkably narrow bandwidth, the natural frequencies can be even closer to each other, which leads to correspondingly large deflections dependent on the Coriolis forces.

In a flowmeter in which the measuring-tube bight portions each comprise a straight tubing seciion, two substantially 180 degree bends adjoining the straight tubing section, and two end sections of substantially equal length connected to the two bends and extending from opposite faces of a block member, it is advisable for the length of the measuring-tube bight portions to be at least 6 times their height. It is of particular advantage for the length of the measuring-tube bight portions to be in the range 8 to 12 times inclusive their height. Preferably, the length of the measuring-tube bight portions is substantially 10 times their height. During oscillation, the tubing of the measuring-tube bight portions is stressed in bending and torsion. By reason of the long length, even small torsional stresses will result in a large deflection. Consequently, not only does the second natural frequency lie comparatively low but the tube is also stressed mechanically to a comparatively small extent, which leads to a correspondingly long life. Another advantage resides in the fact that the entire meter has comparatively small dimensions transversely to the straight tubing sections. The flowmeter can therefore be put readily in a protective tube, which arrangement is desirable for reasons of safety.

Preferably, the length of the block member is no more than 15% of the length of the measuring-tube bight portions. Nore preferably, the length of the block member is less than 5% of the length of the measuring-tube bight portions. The end sections are therefore secured practically in the middle of the measuring-tube bight portions so that on the one hand comparatively long end sections are available for the torsional stress and on the other hand deformations of the block member by reason of external influences such as temperature effects will have only little effect on the end result.- In a development, the block member carrying the measuring-tube bight portions is connected by way of at least one pair of connecting tubes providing a resilient connection to a further block member to which the inletand outlet-connection tubes lead. The connecting tubes providing the resilient connection ensure that external influences, especially vibrations, are kept away from the measuring-tube bight portions and the block member carrying them. In particular, that arrangement can also avoid energization from the outside at the (n + 1)th natural frequency.

It is also favourable for at least the measuring-tube bight portions and the adjoining tube sections to consist of a single continuous tube bent several times, and the tube section between the bight portions, and the said tube sections to be fixed at their ends in three inter-connected tube holder means. In that way, one ensures that there will be no soldered join over the entire length of the measuring-tube bight portions and the tube sections fixed in the tube holder means. The natural frequency relationships can - therefore be very accurately determined even in the case of mass production. There is no danger of different natural frequencies occurring by reason of unequal soldered joints. In addition, the omission of soldered joints gives a higher strength. Nor is there a danger of the medium to be measured undergoing undesirable reactions through contact with solder.

Desirably, the three tube holder means are arranged substantially parallel to one another in one plane. That simplifies production and assembly. Although the continuous single tube is deformed spirally, that deformation is insignificant so far as the measurement is concerned because the co-operating sections of the measuring-tube bight portions continue to be juxtaposed in parallel.

The present invention also provides a flowmeter comprising an attaching means connected. on the one hand, to an inlet-connection tube and an outlet-connection tube, and on the other hand to two adjacent measuring-tube bight portions which are arranged to be oscillated in opposite senses by an exciter of oscillations with, where n is an integer, the nth natural frequency fn corresponding to the nth natural mode of oscillation, and are provided with sensors for monitoring a parameter depending on the relative movement of the bight portions, characterized in that the measuring-tube bight portions are provided at the antinodes of the nth natural oscillation and of the (n + 1)th natural oscillation with mass elements, including components of the exciter of oscillations and the sensors, and their masses are so tuned to each other that the deflection caused by the Coriolis forces is substantially independent of the density of the fluid to be measured.

It is unavoidable that the measuring-tube bight portions must have components of the exciter of oscillations and sensors attached to them. That, however, influences the oscillatory behaviour insofar that the deflections caused by the Coriolis forces change according to the density, that is, the specific gravity of the fluid to be measured. If, however, the said antinodes of the oscillations are provided with mass elements and these masses are tuned to each other, the influence of the fluid density on the deflection can be eliminated. The size of the masses required can be determined by experiment or by calculation.

In the simplest case, if the measuring-tube bight portions are arranged to be excited at the first natural frequency fl, a respective mass element is arranged at approximately the middle of the bight portions and at approximately the middle of half portions of the bight portions, as is already known for exciters of oscillations and sensors without the afore-mentioned adaptation of the masses.

The solutions in accordance with the second aspect of the invention are also applicable in conjunction with solutions in accordance with the first aspect of the invention.

Flowmeters working on the Coriolis principle constructed in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic representation of a first flowmeter embodying the invention; Figure 2 is a perspective view of a second flowmeter embodying the invention; Figure 3 is a diagram showing the excitation force F against deflection u and the associated damping D against frequency f; Figure 4 is a diagram of the phase shift at the sensor against frequency; Figure 5 illustrates the oscillatory behaviour when mass elements are applied; and Figure 6 illustrates the second natural mode of oscillation and the associated Coriolis force.

Referring to the drawings, a flowmeter 1 is provided in a protective tube 2. Two inlet and outletconnection tubes, 3 and 4 respectively, each provided with a respective flange 5 and 6 lead from positions of attachment to an external system to opposite ends bf a block member 7. The connection tubes 3 and 4 are connected to two connecting tubes 8 and 9 providing resiliency which lead to a second block member 10. The second block member 10 carries two measuring-tube bight portions 11 and 12 in series with each other and with the connecting tubes. The measuring-tube bight portions have a length L which is a multiple, in this case 10-fold, of the bight portion height H. The block member 7 has a very short length 1; the length 1 amounts to less than 5% of the length L.

In the middle of the bight portions is an exciter of oscillations 13 with which the two bight portions are relatively moved towards and away from each other at the first natural frequency f, corresponding to the first natural mode of oscillation. Near the middle of each half portion of a bight portion are sensors 14 and 15 which produce measuring signals dependent on the relative movement of the measuring-tube bight portions 11 and 12. The flow through the apparatus can be calculated in known manner from the two measuring signals. The exciter of oscillations 13- comprises two elements 13a and - 11 13b, each connected to a respective bight portion. Similarly, the sensors 14 and 15 have two elements 14a, 14b and 15a, 15b, respectively, one of each pair being connected to a respective measuring-tube bight portion.

In Figure 2, integers corresponding to those in Figure 1 are given reference numerals increased by'100. The exciter of oscillations and sensors are not illustrated in order to simplify the figure. The main difference from the first embodiment is that the block member 107 and the adjoining sections of the connection tubes 105 and 106 are located completely within the space bounded by the measuring-tube bight portions 111 and 112. The radial dimension of the flowmeter is therefore still less than in the case of the first embodiment. A protective tube of smaller diameter can be employed.

The measuring-tube bight portion 11 consists of a straight end section 116, a 180 degree bend 117, a straight tube section 118, a 180 degree bend 119 and a further straight tube section 120. The second measuring- tube bight portion 112 consists of a straight tube section 121, a 180 degree bend 122, a longer straight section 123, a 180 degree bend 124, and a straight section 125. The entire tube formation is bent out of a single continuous tube R. Because of the many straight tubing sections, only six bends have to be provided.

The block member 107 consists of an upper portion 126 and a lower portion 127 which, between them, form two tube holder means 128 and 129 for receiving those tubing sections which extend between the connection tubes 105 and 106 and the associated connecting tubes. The block member 110 consists of an upper portion 130 and a lower portion 131 which, betweeen them, form three tube holder means of which only the tube holder means 132 and 133 are visible in the drawings. The tube holder 132 serves to receive the tubing section between the two measuring-tube sections 120 and 121. The two other two holder means serve to receive the tubing sections between the measuring-tube end sections 116 and 125 and the adjacent connecting tube. The bent tube is inserted at the appropriate positions in the lower portions of the block members 107 and 109. The tubing is then fixed by laying the upper portion over the top and connecting the two parts of the block members. That connection of the two parts of the block members can be achieved by, according to the material in question, soldering, welding, adhesive bonding, screws or even a frictional connection.

Figure 3 shows the'state of excitation of the oscillatory system against frequency f. The state of excitation is, on the one hand, given as a deflection V per unit force F and, on the other hand, damping D in decibels (dB). Two resonances are present, at the first natural frequency f, and the second natural frequency f2, At both points of resonance, the oscillatory system has a high quality factor Q of over 4000 resulting in very narrow response bandwidths. The quality factor Q is, as usual, defined by the equation 1r Q -- U (t) ln I-J(t_+_T_)l in which u is the amplitude and T the period. That expression corresponds to the ratio of the amplitude to the reduction in amplitude per oscillation.

The first natural frequency leads to the first natural mode of oscillation at which the measuring-tube bight portions do not form a node between their fixed ends. Excitation at the second natural frequency leads to the second natural mode of oscillation at which a node is produced at the middle of a bight portion and the first half portion of a bight portion oscillates in the opposite sense to the second half portion. That type of deflection corresponds to the deformation of the measuring- tube bight portions as brought about by the Coriolis forces.

Because the two natural frequencies are closely juxtaposed, a comparatively large deflection uc is brought about by the Coriolis forces Fc. That is because they are proportionally related by the formula:

uc a 1 FC 1 - fl] 1 f2 2 The small spacing of f, and f2 is achieved by the long length L of the measuring-tube bight portions in relation to the height H. Upon deflection of the tube loops, the straight tubing sections 116, 120, 121 and 125 are deformed not only in torsion but also in bending. For that reason, the desired deformations can be achieved by selected combinations of torsion and bending. The torsional deformation over a long end section has the additional advantage that the stresses occurring are less substantial and therefore the life of the apparatus is longer.

Figure 4 shows the phase shift T (Fc,, Uc) which depends on the Coriolis force Fc and the Coriolis deflection Uc against excitation frequency f. Influence of the excitation frequency on the phase shift and thus on the measuring result can be feared only between the limits z, and z2, outside the limits, the conditions are substantially constant. For the exciting first natural frequency fl, however, a lower value than for f2 applies because in the region below z, the deflection as a result of the Coriolis force depends on the stiffness of the oscillatory system but above Z2 on the mass of that system. Accordingly, one there obtains different deflections according to the density of the fluid to be measured. If, however, it is a question only of the measuring-tube bight portion itself, one can keep the deflection substantially independent of the fluid density.

The measuring-tube bight portion is, however, provided with mass elements, for example, the elements 13a,.4a and 15a. These again lead to the deflection being dependent on the fluid density and this will be described in conjunction with Figure 5. Consideration will be given to the first natural mode of oscillation., the bight portion being shown in elongate form. The representation a shows that by applying masses m, and m2 substantially in the region of the middle of each bight half portion, the mode of oscillation changes from that depicted in full lines at a lower fluid density to the one shown in broken lines at a higher fluid density. In the representation b, a mass m3 is shown in the middle of the bight portion. That mass leads to the first natural mode of oscillation changing from the one shown in full lines at a lower fluid density to the one shown in broken lines at a higher fluid density. The respective change depends on the magnitude of each of the individual masses. By adapting the masses m,, % and 1B3 in relation to each other, the departures shown in broken lines can be made to cancel each other out as shown in representation c. The measuring sensitivity becomes - 16 therefore substantially independent of the fluid density. The measuring accuracy is correspondingly high. In the previously described examples, the excitation was with the first natural oscillation, the deflection as a result of the Coriolis forces corresponding to the second natural mode of oscillation. In Figure 6, representation d shows the excitation taking place with the second natural frequency, the bight portion, shown in elongated form, oscillating in the second natural mode of oscillation. That results in Coriolis forces as shown in representation e, which leads to deflections according to the third natural mode of oscillation. An analogous result applies to excitation at higher natural frequencies.

As one practical example, the flowmeter of Figure 2 can have the following parameters:

Length L in the range 35 to 45 cms inclusive, Height H in the range 4 to 5 cms inclusive, External tube diameter in the range 8 to 10 mm inclusive, Tube thickness 1 mm, Tube material stainless steel, Quality factor > 4000, First natural frequency 135 Hz, and Second natural frequency 175 Hz.

z R It is favourable if fl lies between 100 and 150 Hz. That range is at the upper limit of the first natural frequency suitable for the oscillation and therefore makes it easier to provide the second natural frequency near to the first as a result of constructional measures.

Many changes can be made from the illustrated constructions without departing from the invention as defined in the appended claims. Thus, the measuring-tube bight portions may be arranged one above the other instead of side by side. The block members 7 and 10 may have a different shape and arrangement.

Attention is drawn to the description and claims of our two related copending patent applications filed on the same day as the present application.

Claims

CLAIMS:

1. A flowmeter working on the Coriolis principle, the flowmeter comprising an attaching means connected, on the one hand, to an inlet- connection tube and an outlet-connection tube, and on the other hand to two adjacent measuring-tube bight portions which aie arranged to be oscillated in opposite senses by an exciter of oscillations with, where n is an integer, the nth natural frequency fn corresponding to the nth natural mode of oscillation, and are provided with sensors for monitoring a parameter depending on the relative movement of the bight portions, characterized in that the arrangement of the measuring-tube bight portions is such that the (n + 1)th natural frequency fn + 1 corresponding to the (n + I)th natural mode of oscillation lies in the range 0.7 fn to 1.5 fn inclusive but outside a range in which fn produces resonance.

2. A flowmeter as claimed in claim 1, wherein fn + 1 is greater than fn3. A flowmeter as claimed in claim or claim 2, wherein n is equal to 1.

4. A flowmeter as-claimed in claim 2, wherein f2 lies in the range 1.2 f, to 1.3 f, inclusive.

5. A flowmeter as claimed in claim 2., wherein f2 lies in the range 0.75 f, to 0.85 f, inclusive.

6. A flowmeter as claimed in any preceding claim, wherein the quality factor Q of the oscillatory k R system constituted by the measuirng-tube bight portions is at least equal to 3000 at the nth as well as at the (n + 1)th natural frequency (fn,, fn + l) - 7. A flowmeter as claimed in claim 6, wherein the quality factor Q is greater than 4000.

8. A flowmeter as claimed in any preceding claim, in which the measuringtube bight portions each comprise a straight tubing section, two substantially 180 degree bends adjoining the straight tubing section, and two end sections of substantially equal length connected to the two bends and extending from opposite faces of a block member, the length of the measuring-tube bight portions being at least 6 times their height.

9. A flowmeter as claimed in claim 8, wherein the length of the measuringtube bight portions is in the range 8 to 12 times inclusive their height.

10. A flowmeter as claimed in claim 9, wherein the length of the measuring-tube bight portions is substantially 10 times their height.

11. A flowmeter as claimed in any one of claims 8 to 10, wherein the length of the block member is no more than 15% of the length of the measuring-tube bight portions.

12. A flowmeter as claimed in claim 11, wherein the length of the block member is less than 5% of the length of the measuring-tube bight portions.

13. A flowmeter as claimed in any one of claims 8 to 12, wherein the block member carrying the measuring-tube bight portions is connected by way of at least one pair of connecting tubes providing a resilient connection to a further block member to which the inletand outlet-connection tubes lead.

14. A flowmeter as claimed in any preceding claim, wherein at least the measuring-tube bight portions and the adjoining tube sections consist of a single continuous tube bent several times, and the tube section between the bight portions, and the said tube sections are fixed at their ends in three inter-connected tube holder means.

15. A flowmeter as claimed in any preceding claim, wherein the three tube holder means are arranged substantially parallel to one another in one plane.

16. A flowmeter working on the Coriolis principle, the flowmeter comprising an attaching means connected, on the one hand, to an inletconnection tube and an outlet-connection tube, and on the other hand to two adjacent measuring-tube bight portions which are arranged to be oscillated in opposite senses by an exciter of oscillations with, where n is an integer, the nth natural frequency fn corresponding to the nth natural mode of oscillation, and are provided with sensors for monitoring a parameter depending on the relative movement of the bight portions, characterized in that the measuring-tube bight portions are provided at the Z I antinodes of the nth natural oscillation and of the (n + 1)th natural oscillation with mass elements, including components of the exciter of oscillations and the sensors, and their masses are so tuned to each other that the deflection caused by the Coriolis forces is substantially independent of the density of the fluid to be measured.

- 17. A flowmeter as claimed in claim 16, wherein the measuring-tube bight portions are arranged to be excited at the first natural frequency fl, a respective mass element is arranged at approximately the middle of the bight portions and at approximately the middle of half portions of the bight portions.

18. A flowmeter as claimed in claim 16 or 17, wherein the flowmeter is claimed in any one of claims 1 to 15.

19. A flowmeter substantially as herein described with reference to, and as illustrated by, Figures 1, 3, 4, 5, and 6 of the accompanying drawings.

20. A flowmeter substantially as herein described with reference to, and as illustrated by, Figures 2, 3, 4, 5, and 6 of the accompanying drawings.

Published 1990 at The Patent Office. State House, 6#3.7 t High Holburn, London WC 1R 4TP. Further copies May be obtained I!rOM The Patent Sales Branch, St Mary Cray, Orpuigton, Kent BRS 3RD. Printed by Multiplex techniques ltd. St Mary Cray, Kent, Com 1157