WO2005073676A1

WO2005073676A1 - Flow unit for a coriolis type mass flow meter

Info

Publication number: WO2005073676A1
Application number: PCT/HU2005/000009
Authority: WO
Inventors: András KISS; Béla NAGY; Tamás OLÁH
Original assignee: MMG Flow Méréstechnikai Kft.
Priority date: 2004-02-02
Filing date: 2005-01-31
Publication date: 2005-08-11
Also published as: HUP0400330A2; HU0400330D0

Abstract

The invention is a flow unit for a Coriolis type mass flow meter, the unit comprising at least one flow tube (1) with an inlet pipe section (2) and an outlet pipe section (3), as well as a rigid body (4) to which an inlet end (2v) of the inlet pipe section (2) and an outlet end (3v) of the outlet pipe section (3) are rigidly secured. The inlet pipe section (2) and the outlet pipe section (3) are parallel with each other, and the flow tube (1) comprises : a first ‘S’ shaped pipe section (5) having a first arc (5a) connected to an end of the inlet pipe section (2) opposite its inlet end (2v) and being arranged in a way that it curves away from the outlet pipe section (3), and a second arc (5c) curving in a direction opposite to that of the first arc (5a) ; a second ‘S’ shaped pipe section (6) having a first arc (6a) connected to an end of the outlet pipe section (3) opposite its outlet end (3v) and being arranged in a way that it curves away from the inlet pipe section (2), and a second arc (6c) curving in a direction opposite to that of the first arc (6a) ; and a middle pipe section (7) arranged perpendicularly to the inlet pipe section (2) and the outlet pipe section (3), and connected with the second arc (5c) of the first ‘S’ shaped pipe section (5) and with the second arc (6c) of the second ‘S’ shaped pipe section (6).

Description

FLOW UNIT FOR A CORIOLIS TYPE MASS FLOW METER

TECHNICAL FIELD

The invention relates to a flow unit for a Coriolis type mass flow meter.

BACKGROUND ART

Known Coriolis type mass flow meters comprise a flow unit which includes a rigid body and at least one flexible flow tube secured thereto. The flow tube has an inlet pipe section and an outlet pipe section, the ends of which are secured to a body having a rigidity much higher than that of the flexible flow tube. The flow tube, in a way known per se, is forced into vibration by a vibrating apparatus linked to the tube. The medium to be measured flows through the flow tube and Coriolis forces are generated as a result of the interaction between the mass flow of the medium and the vibration of the tube. These Coriolis forces result in a further vibration of the flexible flow tube, which vibration is added to the oscillation generated by the vibrating apparatus mentioned above. Furthermore, a vibration detector apparatus known per se is also connected to the flow tube, consequently on the basis of the description above, the vibration movement measured by this vibration detector apparatus also carries the vibration component generated by the mass flow of the medium. A signal processing unit associated with the vibration detector apparatus selects the effect of vibration components caused by the mass flow, calculating therefrom the mass flow of the medium.

Numerous shapes of the flow tubes applied in prior art Coriolis type mass flow meters are known, for example mass flow meters with flow units containing U, Δ (delta), Ω, B and I shaped flow tubes. Such mass flow meters are described for example in HU 200 234 and US 5 627 326. It is a disadvantage of prior art approaches that as a result of the relatively simple shape of the flow tube, it is not possible to dimension the natural frequencies (resonance frequencies) of the flexible tube appropriately. This has an importance because the vibration frequency and the natural frequencies close to it may not have just any interrelated value. It is to be avoided for example that the natural frequency associated with the distorted natural shape of the Coriolis forces generated (the distorted shape developing during the vibration at the natural frequency) and the frequency used for the measurement and generated by the vibrating apparatus linked to the flexible tube are too close. The proximity of the two frequencies results in detrimental interferences, which mainly deteriorate stability and linearity. At the same time, the two frequencies mentioned above may not be too far from each other, because this would result in a very high rigidity of the flexible tube against the distortion shape generated by the Coriolis forces, which reduces the ratio of the vibration component generated as a result of the mass flow as against the vibration component produced by vibration, i.e. the sensitivity is reduced.

It is a further disadvantage of prior art solutions that the pipe shapes do not fit into a relatively compact space, and therefore a relatively large casing is required for designing mass flow meters. A large casing may only be designed in a less rigid way, and when its natural vibrations interfere with the measured vibrations, this deteriorates the result of the measurement.

It is also a disadvantage of prior art solutions that the inlet pipe section and the outlet pipe section are fixed to the rigid body relatively far from each other, and as a result it is difficult to avoid the vibration of the inlet pipe end and the outlet pipe end in relation to each other. Indeed, the oscillation of pipe ends in relation to each other makes a disadvantageous influence on the accuracy of measurement.

DISCLOSURE OF INVENTION

During our experiments we have come to the conclusion that if the inlet pipe section and the outlet pipe section are arranged in parallel with their ends secured to the rigid body and these pipe sections are routed to a central pipe section via 'S' shaped pipe sections, the arc angles of the 'S' shaped pipe sections, the bending radii associated with the arcs and the lengths of the straight pipe sections associated with the arcs can be selected so as to allow the appropriate dimensioning of the natural frequencies (resonance frequencies) of the flexible flow tube.

Accordingly, the invention is a flow unit for a Coriolis type mass flow meter, the unit comprising at least one flow tube with an inlet pipe section and an outlet pipe section, as well as a rigid body to which an inlet end of the inlet pipe section and an outlet end of the outlet pipe section are rigidly secured. The inlet pipe section and the outlet pipe section are parallel with each other, and the flow tube comprises a first 'S' shaped pipe section having a first arc connected to an end of the inlet pipe section opposite its inlet end and being arranged in a way that it curves away from the outlet pipe section, and a second arc curving in a direction opposite to that of the first arc, a second 'S' shaped pipe section having a first arc connected to an end of the outlet pipe section opposite its outlet end and being arranged in a way that it curves away from the inlet pipe section, and a second arc curving in a direction opposite to that of the first arc, and a middle pipe section arranged perpendicularly to the inlet pipe section and the outlet pipe section, and connected with the second arc of the first 'S' shaped pipe section and with the second arc of the second 'S' shaped pipe section.

In addition to the appropriate dimensioning, a further advantage of the invention is that the relatively long flexible tube can be fitted into a smaller space, which enables the implementation of a smaller volume stiff casing. In this way the natural frequencies of the casing may not interfere with the measuring vibration frequency of the flexible flow tube. Designing a stiffer, i.e. higher natural frequency casing is possible more cost efficiently in the case of pipe shapes according to the invention (with a smaller volume casing). In the flow unit according to the invention, the shape of the flow tube allows the advantageous increasing of the ratio of displacements caused by the Coriolis forces, as against the total displacement.

Preferred embodiments of the invention are described in the dependent claims.

The straight pipe sections applied in the embodiments according to claims 2 and 3 facilitate the design and dimensioning, make it easier to place the vibrating and detector apparatuses, and simplify the bending technology of the curves.

The embodiments according to claims 4 and 5 make it possible to design the flow tube in a compact way.

The embodiment according to claim 6 has been proven to be especially advantageous from the aspects of appropriate dimensioning and compact design.

Claim 7 describes a preferred vibration method of the flow unit.

The embodiment according to claim 8 minimises the vibrations coming out from the measuring system.

BRIEF DESCRIPTION OF DRAWINGS

The invention will hereinafter be described on the basis of preferred embodiments depicted by the drawings, where Figs. 1-7 show front views of preferred tube shapes of a flow unit according to the invention, Fig. 8 shows a schematic 3-dimensional view of a flow unit comprising two flow tubes according to the invention, Fig. 9 shows a schematic view in cross section of a prior art vibrating apparatus, Fig. 10 shows a schematic view in cross section of a prior art detector apparatus, Fig. 11A shows the front view of a flow unit with casing according to the invention, Fig. 11 B shows the side view of an encased flow unit as per Fig. 11A, Fig. 12 shows a preferred embodiment of fixing the flow tube to the rigid body and Fig. 13 shows another preferred embodiment of fixing the flow tube to the rigid body.

MODES FOR CARRYING OUT THE INVENTION

The flow unit shown in Fig. 1 comprises a rigid body 4 and a flow tube 1. The flow tube 1 is arranged along a plane. An inlet end 2v of an inlet pipe section 2 and an outlet end 3v of an outlet pipe section 3 are secured rigidly to the rigid body 4. According to the invention, the inlet pipe section 2 and the outlet pipe section 3 are arranged in parallel, side by side. It is advantageous to arrange these pipe sections close to each other, because in this way it can be ensured properly that they are clamped without vibration in a rigid way in relation to each other.

According to the invention, the flow tube 1 has a first 'S' shaped pipe section 5 having a first arc 5a connected to an end of the inlet pipe section 2 opposite its inlet end 2v and being arranged in a way that it curves away from the outlet pipe section 3, and a second arc 5c curving in a direction opposite to that of the first arc 5a. Preferably, a straight pipe section 5b is arranged between the two arcs.

Furthermore, the flow tube 1 has a second 'S' shaped pipe section 6 having a first arc 6a connected to an end of the outlet pipe section 3 opposite its outlet end 3v and being arranged in a way that it curves away from the inlet pipe section 2, and a second arc 6c curving in a direction opposite to that of the first arc 6a. Again, between these two arcs, preferably a straight pipe section 6b is arranged. A middle pipe section 7 is arranged perpendicularly to the inlet pipe section 2 and the outlet pipe section 3, and is connected with the second arc 5c of the first 'S' shaped pipe section 5 and with the second arc 6c of the second 'S' shaped pipe section 6, preferably via straight pipe sections 8 and 9. The middle pipe section 7 is straight, and at its ends there are arcs joined to the straight pipe sections 8 and 9.

It is a common feature of the preferred embodiments shown in Figs. 1 to 7 that the flow tube 1 is arranged symmetrically along a plane defined by the inlet pipe section 2 and the outlet pipe section 3, and the middle pipe section 7 is longer than the distance between the inlet pipe section 2 and the outlet pipe section 3.

A further common characteristic is that the arc angle of the first arcs 5a, 6a is at least 90°. By enabling a lateral expansion of the pipe shape, the flow tube length that can be fitted into a given space of the casing may be extended.

The embodiments shown in the figures differ from each other in the arc angles of the arcs of the 'S' shaped pipe sections 5 and 6. In the embodiment shown in Fig. 1 , the arcs of the 'S' shaped 5 and 6 pipe sections have an arc angle of 180°. In the embodiment shown in Fig. 2, the arcs of 'S' shaped pipe sections 5 and 6 have an equal arc angle of between 90° and 180°. In the embodiment shown in Fig. 3, the arcs of the 'S' shaped pipe sections 5 and 6 have an arc angle of 90°.

From the aspect of practical implementation, it is especially advantageous and it is a common characteristic of the embodiments depicted in Figs. 4 and 5 that the arc angle of the first arcs 5a, 6a and that of the second arcs 5c, 6c are different, but both are larger than 90°. In the embodiment shown in Fig. 4, the arc angle of the first 5a, 6a arcs is smaller than that of the second arcs 5c, 6c, and in the embodiment shown in Fig. 5, the arc angle of the first arcs 5a, 6a is larger than that of the second arcs 5c, 6c. In these embodiments, the relatively long flow tube is located in a relatively compact space, consequently the flow unit can be housed in a small and rigid casing. In addition, these tube shapes have been proven to be advantageous also from the aspect of the ability to induce vibration and the ability to detect the vibrations. In the embodiment shown in Fig. 6, the first arcs 5a, 6a of the 'S' shaped pipe sections 5 and 6 have an arc angle of 90°, and the second arcs 5c, 6c have an arc angle of less than 90°. In the embodiment shown in Fig. 7, the first arcs 5a, 6a of the 'S' shaped pipe sections 5 and 6 have a 90° angle and the second arcs 5c, 6c have an arc angle of between 90° and 180°.

Fig. 8 shows a schematic 3-dimensiona! view of a flow unit comprising two flow tubes 1 and 1'. The possible excitation points 11 and 11' of the vibrating apparatus of the mass flow meter are the intersection points of the centreline and symmetry axis of the flow tubes 1 and 1'. The vibrating apparatus generates periodically changing forces in a direction perpendicular to the plane of the above mentioned centrelines, thereby forcing the flexible flow tubes 1 and V into a vibrating movement. In order to compensate the vibrations (to keep the centre of gravity of the vibrating system in a constant position), the flow tube V has the same geometry and material as the flow tube 1 , and the two tubes are arranged in parallel with each other.

In the two flow tubes 1 , 1', the flowing of the medium to be measured is ensured by flow distributing passages shown by a dotted line in Fig. 8 and featured in the rigid body 4. Between the excitation points 11 and 11', the vibrating apparatus generates a periodical force, by which the two parallel pipes are forced into a counter-phase vibration.

The vibrating apparatus is able to generate symmetrical distortion shapes in relation to the symmetry line of the flow tube 1 , 1 ' (natural shapes in the case of vibration at a natural frequency), which distortion shapes have 0 or even number of nodes (a minimum point of vibration amplitude). It is advantageous if the nodes of the distortion shape having two nodes are located at the ends or in the vicinity of the middle pipe section 7, preferably at the points 14, 14' and 15, 15'.

When two vibrating apparatus are applied in a symmetrical location and operated in a counter-phase, it becomes possible to excite distortion shapes with an odd number of nodes. In the case of pipe shapes according to the invention, a single node distortion shape can be advantageously formed by vibrating apparatuses located in or in the vicinity of points 14, 14', and points 15, 15'.

The detection of vibration can be implemented advantageously by a detector apparatus fitted at the detection points 12, 12' and 13, 13'. These detection poi nts are situated in the case of the tube shapes according to the invention preferably at or in the vicinity of the opposite ends of the inlet pipe section 2 and the outlet ipe section 3 of the 'S' shaped pipe sections 5, 6.

A schematic cross-sectional view of a vibrating apparatus is depicted by way of example in Fig. 9. The apparatus comprises an electric coil 16 and a permanent magnet 17 located in the centreline of the coil. The permanent magnet 17 is secured at the excitation point 11 to the flow tube 1 , and the coil 16 is fastened at the excitation point 11' to the flow tube 1'. The arrangement may of course be reversed. The coil 16 and the permanent magnet 17 are not in contact, and an interaction between them is only established via a magnetic field, as a result of an alternating current fed in a way not shown into the coil 16. Fig. 9 also shows the holder 19 of the magnet 17 and the holder 18 of the coil 16.

A schematic cross-sectional view of a detector apparatus is depicted by way of example in Fig. 10. The apparatus comprises an electric coil 20, a coil holder 23, magnets 21 and 22, a magnet mounting 24 and a magnet holder 25. Detection is made by the coil 20, in which the magnetic field of the permanent magnets 21 , 22 induces a voltage as a result of the counter-phase vibration. The coil 20 and the magnets 21 , 22 are not in contact, and they are only in interaction via the magnetic field.

A signal processing unit known perse is connected to the flow unit accord ing to the invention. This signal processing unit processes the signal of the detector apparatus, and in the course of this it performs two functions. On the one hand , in an appropriate phase it feeds back the signal of the detector apparatus to the vibrating apparatus, thereby creating an electromechanical oscillator that keeps the flexible flow tubes in vibration. On the other hand, by analysing the signal from the two detector apparatus, it calculates the oscillation component proportional to the mass flow and generated by the Coriolis forces, then from this component it performs the calculation of mass flow.

A front view and a side view of an encased mass flow meter comprising a flow unit as per Fig. 8 are shown in Figs. 11A and 11 B. The casing of the unit is represented by a base plate 33 and a cap 30 secured thereto. The passages of the rigid body 4 can be connected by adjoining tubes 31 and 32 into the tube that carries the mass flow to be measured.

For implementing the rigid body, examples are shown in Figs 12 and 13. The rigid body 4', 4" is advantageously a robust metal block, which is preferably made of the same material as that of the flexible flow tube 1 , 1'. In a case shown in Fig. 12, the ends 2v and 3v of the flow tube are fixed by butt weldings to the rigid body 4'. The welded cross sections at the ends 2v and 3v, which cross sections are liable to break anyway, would be subjected to a heavy fatigue load as a result of the vibration of the flow tube. To reduce this effect, tension releasing plates 10a, 10b are preferably located on the flow tube at a small distance from its ends 2v and 3v. The tension releasing plates 10a, 10b are preferably fixed to the flexible tube by soldering or mechanical clamping and not by welding. The tension releasing plates 10a, 10b connect the parallel flow tubes 1 , 1' shown in Fig. 8.

Fig. 13 shows a different embodiment of the rigid body 4". In this embodiment, it is not necessary for the flow tube and the fixing rigid body 4" to be made of an identical material. In this case the design of the fastening is such that the medium to be measured is not in contact with the rigid body 4", because the flow tube goes through it. If the method of fixing is not welding, the tension releasing plates 10a, 10b shown in Fig. 12 may be omitted. In this case the flow distribution between the parallel vibration tubes may not be provided by passages designed inside the rigid body 4". This is solved by a further flow distribution unit (not shown) attached to the ends of the vibration tubes. The material of the flow tube 1 , 1' according to the invention must meet two requirements. On the one hand, it must have a stable elastic modulus, because this is the most important material characteristic that determines the mass flow conversion factor. On the other hand, the material must have the best possible chemical resistance to the medium to be measured in the given application. These requirements are met by most stainless steels. In most cases materials of the standards signs 1.4404 or SS 316L can be used, but in the case of a special corrosion load, the material called Hastelloy C22 or glass may also be applied.

The wall thickness of the vibrating flow tube must be selected according to the pressure of the medium. It is advisable to apply pipe dimensions standardised for different pressure stages. In dimensioning, the load resulting from a periodical distortion caused by vibration must also be taken into consideration. This means that the safety factor of calculations applying to a static (no vibration) status must be selected higher. The diameter of the vibrating tube must be determined subject to the highest rate of the operating flow range. The medium velocity associated with the maximum flow is preferably selected to be 10 to 12 m/s in the case of fluids and in the range of 30 to 50 m/s in the case of gases.

Claims

1. A flow unit for a Coriolis type mass flow meter, the unit comprising at least one flow tube with an inlet pipe section and an outlet pipe section, as well as a rigid body to which an inlet end of the inlet pipe section and an outlet end of the outlet pipe section are rigidly secured, c h a r a c t e r i s e d in that the inlet pipe section (2) and the outlet pipe section (3) are parallel with each other, and the flow tube (1) comprises a first 'S' shaped pipe section (5) having a first arc (5a) connected to an end of the inlet pipe section (2) opposite its inlet end (2v) and being arranged in a way that it curves away from the outlet pipe section (3), and a second arc (5c) curving in a direction opposite to that of the first arc (5a), a second 'S' shaped pipe section (6) having a first arc (6a) connected to an end of the outlet pipe section (3) opposite its outlet end (3v) and being arranged in a way that it curves away from the inlet pipe section (2), and a second arc (6c) curving in a direction opposite to that of the first arc (6a), and a middle pipe section (7) arranged perpendicularly to the inlet pipe section (2) and the outlet pipe section (3), and connected with the second arc (5c) of the first 'S' shaped pipe section (5) and with the second arc (6c) of the second 'S' shaped pipe section (6).

2. The flow unit according to claim 1 , characterised in that the second ares (5c, 6c) of the 'S' shaped pipe sections (5, 6) are connected to the middle pipe section (7) via respective straight pipe sections (8, 9).

3. The flow unit according to claim 1 or claim 2, characterised in that the first arcs (5a, 6a) and the second arcs (5c, 6c) of the 'S' shaped pipe sections (5, 6) are connected to each other via respective straight pipe sections (5b, 6b).

4. The flow unit according to any of claims 1 to 3, characterised in that the flow tube (1) is arranged along a plane determined by the inlet pipe section (2) and the outlet pipe section (3) symmetrically, and the middle pipe section (7) is longer than the distance between the inlet pipe section (2) and the outlet pipe section (3).

5. The flow unit according to any of claims 1 to 4, characterised in that the arc angle of the first arcs (5a, 6a) is at least 90°.

6. The flow unit according to claim 5, characterised in that the arc angle of the first arcs (5a, 6a) and the arc angle of the second arcs (5c, 6c) are different, but both are larger than 90°.

7. The flow unit according to claim 4, characterised by comprising a flow tube (1) vibrated in a direction perpendicular to the plane defined by the inlet pipe section (2) and the outlet pipe section (3).

8. The flow unit according to any of claims 1 to 7, characterised by comprising two flow tubes (1 , 1') of identical shape, arranged side by side in parallel with each other.