Abstract
The invention relates to a Coriolis mass flow meter, comprising a housing with an inlet and an outlet for a fluid medium, which are arranged along a flow axis (d), at least one measuring tube configured to allow the fluid medium to flow through it in a flow direction (x) and arranged between the inlet and the outlet, wherein the measuring tube includes at least one section with an oval cross-section, so that the measuring tube in this section comprises, perpendicular to the flow direction (x), a longer axis (a) and a shorter axis (b), a vibration exciter (D) configured to cause the measuring tube to vibrate in a vibration direction (f), and two vibration sensors for detection of the movements of the measuring tube, wherein the longer axis (a) of the oval cross-section of the measuring tube is oriented essentially in the vibration direction (f). Moreover, the invention relates to a method for manufacturing a Coriolis mass flow meter with little pressure dependence.
Claims
1. A Coriolis mass flow meter, comprising: a housing with an inlet and an outlet for a fluid medium, which are arranged along a flow axis (d); at least one measuring tube configured to allow the fluid medium to flow through it in a flow direction (x) and arranged between the inlet and the outlet, wherein the measuring tube comprises at least one section with an oval cross-section such that the oval cross-section of the measuring tube comprises, perpendicular to the flow direction (x), a longer axis (a) and a shorter axis (b); a vibration exciter (D) configured to cause the measuring tube to vibrate in a vibration direction (f); and two vibration sensors for detection of the movements of the measuring tube, wherein the longer axis (a) of the oval cross-section of the measuring tube is oriented along the vibration direction (f).
2. The Coriolis mass flow meter according to claim 1, wherein: the at least one measuring tube arranged between the inlet and the outlet comprises two measuring tubes, arranged between the inlet and the outlet, the two measuring tubes each being U-shaped, wherein the two measuring tubes are connected to a fixing element in a region of the inlet and/or the outlet such that a position of the two measuring tubes relative to each other is fixed, and wherein the two measuring tubes each comprise at least one section with an oval cross-section.
3. The Coriolis mass flow meter according to claim 1, wherein: the oval cross-section of the measuring tube in is elliptical.
4. The Coriolis mass flow meter according to claim 1, wherein: The oval cross-section of the measuring tube comprises two curved wall sections and two straight wall sections, respectively lying across from one another.
5. The Coriolis mass flow meter according to claim 1, wherein: the measurement tube has a ratio of the longer axis (a) to the shorter axis (b) of less than 1.17 and greater than 1.01.
6. The Coriolis mass flow meter according to claim 1, wherein: an angle () between the vibration direction (f) and the longer axis (a) of the oval cross-section of the measuring tube is at most five degrees.
7. The Coriolis mass flow meter according to claim 1, wherein: the at least one section of the measuring tube with the oval cross-section is arranged in at least one area of the measuring tube in which an angle () between the flow direction (x) and the flow axis (d) exists.
8. The Coriolis mass flow meter according to claim 1, wherein: the measuring tube has an oval shape between a fixing element arranged in an area of the inlet and a fixing element arranged in an area of the outlet.
9. The Coriolis mass flow meter according to claim 1, wherein: the measuring tube has an oval shape over an entire length.
10. A method for manufacturing a Coriolis mass flow meter having at least one measuring tube configured to allow a fluid medium to flow through the measuring tube in a flow direction (x) and is caused to vibrate by a vibration exciter (D), comprising: providing a measuring tube with at least one oval section in which the measuring tube comprises, perpendicular to the flow direction (x), a longer axis (a) and a shorter axis (b); and arranging the measuring tube in the Coriolis mass flow meter such that the longer axis (a) of the oval section of the measuring tube is oriented along a vibration direction (f) and is configured such manner that the oval section of the measuring tube is rounded by internal pressure prevailing during operation and stiffness in the vibration direction (f) decreases.
11. The method according to claim 10, wherein: a length of the oval section of the measuring tube and a ratio of the longer axis (a) to the shorter axis (b) of the measuring tuber are coordinated such that a dependence of a mass flow measurement and/or a dependence of a density measurement on a pressure of the fluid medium are reduced.
12. The method according to claim 10, wherein: a length of the oval section of the measuring tube and a ratio of the longer axis (a) to the shorter axis (b) of the measuring tube are coordinated in such a manner that an optimal reduction of the dependence of both a mass flow measurement and a density measurement on a pressure of the fluid medium is achieved.
13. The method according to claim 10, wherein: the measuring tube has a ratio of the longer axis (a) to the shorter axis (b) of less than 1.17 and greater than 1.01.
14. The method according to claim 10, wherein: using finite element analysis to determine at least one of a length of the oval section of the measuring tube, a ratio of the longer axis (a) to the shorter axis (b) of the measuring tube a cross-sectional shape of the oval section of the measuring tube.
15. The Coriolis mass flow meter according to claim 5, wherein: the ratio of the longer axis (a) to the shorter axis (b) is less than 1.15 and greater than 1.02.
16. The Coriolis mass flow meter according to claim 15, wherein: the ratio of the longer axis (a) to the shorter axis (b) is less than 1.1 and greater than 1.04.
17. The Coriolis mass flow meter according to claim 16, wherein: the ratio of the longer axis (a) to the shorter axis (b) is less than 1.08 and greater than 1.05.
18. The Coriolis mass flow meter according to claim 6, wherein: the angle () between the vibration direction (f) and the longer axis (a) of the oval cross-section of the measuring tube is at most four degrees.
19. The Coriolis mass flow meter according to claim 18, wherein: the angle () between the vibration direction (f) and the longer axis (a) of the oval cross-section of the measuring tube is at most three degrees.
20. The Coriolis mass flow meter according to claim 19, wherein: the angle () between the vibration direction (f) and the longer axis (a) of the oval cross-section of the measuring tube is at most two degrees.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The invention is explained in greater detail below with the help of the examples shown in the figures, which show schematically:
[0021] FIG. 1 is a side view of a Coriolis mass flow meter;
[0022] FIG. 2 is the measuring tube inside the housing of the Coriolis mass flow meter according to FIG. 1;
[0023] FIG. 3 is the arrangement of two measuring tubes inside the housing of a Coriolis mass flow meter;
[0024] FIG. 4 is a side view of the extension of a measuring tube;
[0025] FIG. 5 is a circular cross-section of a measuring tube;
[0026] FIG. 6 is an oval cross-section of a measuring tube;
[0027] FIG. 7 is a cross-section of a measuring tube with two round sections and two flat sections respectively lying across from one another;
[0028] FIG. 8 is a flow chart of the method and
[0029] FIG. 9 is the correlation between pressure dependence of the density measurement and the ovality of the measuring tube in the region of a branch.
DETAILED DESCRIPTION
[0030] Identical parts or parts acting in an identical manner are designated by identical reference numbers. Recurring parts are not designated separately in each figure.
[0031] FIG. 1 shows a Coriolis mass flow meter 1 with a transmitter 2 and a housing 3. The transmitter 2 of the Coriolis mass flow meter 1 accommodates the electronics for, among other things, the vibration exciter and the vibration sensors, as well as a control unit 5. It is connected to the housing 3 via a collar 34. During operation, the Coriolis mass flow meter 1 with its housing 3 is fitted into a pipeline transporting the fluid to be measured. The Coriolis mass flow meter 1 especially comprises a connecting piece 30, which in turn comprises an inlet 31 for connection to a supply line 40 and an outlet 32 for connection to a discharge line 41 of the pipeline. The pipeline into which the Coriolis mass flow meter 1 is fitted defines the flow axis d. The flow axis d designates the direction in which the fluid would flow in the pipeline if it was not led through the Coriolis mass flow meter 1.
[0032] Moreover, the Coriolis mass flow meter 1 comprises a tube housing 33, in which the at least one measuring tube 4 is accommodated, as depicted in FIG. 2. FIG. 2 also shows the extension of the measuring tube 4 through the housing 3 from the inlet 31 via the tube housing 33 to the outlet 32. The extension of the measuring tube 4, which is U-shaped in the example shown, also defines the flow direction x of the fluid inside the measuring tube 4 and thus inside the Coriolis mass flow meter 1. The measuring tube 4 is respectively fixed in both the area of the inlet 31 and the area of the outlet 32 by a fixing element 35, which is configured as a gusset plate in the present example. These fixing elements 35 are all the more important in cases in which more than one measuring tube 4, e.g. two measuring tubes 4 (see, e.g., FIG. 3), are used. As is evident from FIG. 2, arranged on the measuring tube 4 is a vibration exciter D, which is implemented to cause the vibration, in particular resonant vibration, of the measuring tube 4 during the operation of the Coriolis mass flow meter 1. In FIG. 2, the vibrations excited by the vibration exciter D are respectively directed into and out of the paper plane. A first vibration sensor S1 and a second vibration sensor S2 are arranged on the measuring tube 4 in the flow direction x upstream and downstream of the vibration exciter D. The vibration sensors S1, S2 detect the movements of the measuring tube 4 and in particular the vibration induced by the vibration exciter D. Moreover, arranged on the measuring tube 4 is a temperature sensor RTD, which is, e.g., configured as a resistance thermometer.
[0033] FIG. 3 illustrates the spatial arrangement of two parallel measuring tubes 4. FIG. 4 shows the geometry of such a measuring tube 4. Both measuring tubes 4 are respectively connected to the inlet 31 and the outlet 32. In these regions, they are respectively attached to each other via a fixing element 35, configured here as a gusset plate, so that their position relative to each other is fixed. In the example shown, the measuring tubes 4 have an essentially U-shaped extension, which also corresponds to the flow direction x of the fluid flowing through the measuring tubes 4. Each of the measuring tubes 4 comprises in particular two curves 44, two branches 43 and a curve segment 42 connecting the branches 43. The curves 44 here designate those sections of the measuring tubes 4 in which the fluid is respectively led into and out of the U-shaped protrusion. In the curves 44 and the curve segment 42, the flow direction x deviates from the flow axis d to an extent that is particularly small and especially minimal. The branches 43 designate those sections of the measuring tubes 4 in which the flow direction x deviates from the flow axis d to an extent that is particularly large and especially maximal. The curve segment 42 in turn describes the arcuate connection of the U-shaped protrusion between the two branches 43. The vibration direction f is also indicated in FIG. 3. The vibration direction f results from the vector of the angular velocity of the vibration induced by the vibration exciter D being parallel to the flow axis d and lying on the same. In the branches 43 of the measuring tubes 4, the flow direction x deviates from the flow axis d to the greatest extent. This is shown by the angle 3 between the flow direction x and the flow axis d (i.e. a line running parallel to the flow axis d) in FIG. 4. In the example shown, the angle 3 is particularly large in particular in the branches 43. In the curves 44 and the curve segment 42, by contrast, the angle 3 is particularly small. For this reason, the Coriolis forces acting on the fluid led through the measuring tube 4 are weakest in the area of the curves 44 and the curve segment 44. The sections of the measuring tubes 4 with an oval cross-section in accordance with the invention are thus preferably located in the region of the curves 44 and/or the curve segment 42. For example, a section with an oval cross-section can be provided only in a curve 44 or in the curve segment 42 of each measuring tube 4. However, it is preferred that at least one section with an oval cross-section is provided in each curve 44 and, particularly preferably, also in the curve segment 42. In the example shown, the measuring tubes 4 have an oval cross-sectional shape along the entire length of their curve 44, in particular in both curves 44, and in the curve segment 42. Over the length of the branches 43, by contrast, the measuring tubes 4 of the example shown have a circular cross-sectional shape.
[0034] FIGS. 5, 6 and 7 respectively show different cross-sectional shapes of the measuring tubes 4. FIG. 5 shows a circular cross-section as present, for example, in the branches 43 of the measuring tube 4 according to FIGS. 3 and 4. Specifically, FIG. 5 shows the cross-section along line A-A of FIG. 4. The diameter of the tube in the sections with a circular cross-section is thus equal in all directions and is designated by a.sub.0 in FIG. 5. By contrast, FIGS. 6 and 7 show oval cross-sectional shapes of the measuring tubes 4. For example, FIG. 6 shows a measuring tube 4 with an elliptical cross-section. The cross-section thus has unequal diameters, wherein the maximum diameter corresponds to the longer major axis a of the ellipse, while the minimum diameter corresponds to the shorter minor axis b of the ellipse.
[0035] FIG. 7 shows an alternative oval cross-section of a measuring tube 4. The cross-section according to FIG. 7 includes two round sections 45 and two flat sections 46, respectively lying across from one another. Round and flat here refers to the respective configuration of the tube wall. In the region of the round sections 45, the tube wall is rounded, in particular with a curvature corresponding to a circular tube as shown, e.g., in FIG. 5. In the region of the flat sections 46, by contrast, the tube wall is flat, i.e. planar. The ovality of the measuring tube 4 according to FIG. 7 is created by the combination of flat sections 46 and round sections 45. Accordingly, the cross-section through the measuring tube 4 according to FIG. 7 also comprises a longer axis a and a shorter axis b corresponding to the respective maximum and minimum tube diameters in the cross-section.
[0036] As indicated by line B-B in FIG. 4, sections with an oval cross-section according to FIG. 6 or FIG. 7 are arranged, for example, in the curves 44 and the curve segment 42 of the measuring tube 4. In this regard, they may be arranged either in only one curve 44 or the curve segment 42, or in both curves 44, or in both curves 44 and the curve segment 42 of the measuring tube 4. The length of the respective sections with an oval cross-section can also be adapted to the specific application. As also indicated in FIGS. 6 and 7, the measuring tubes 4 are arranged in such a manner that the longer axis a is oriented in the vibration direction f, i.e. parallel to the vibration direction f. The vibration direction f, which is indicated by the dashed line in FIG. 7, merely serves to illustrate the angle between the vibration direction f and the longer axis a. The angle , the size of which is exaggerated in FIG. 7 for the purpose of illustration, should be as small as possible. This way, the reduction of the stiffness of the measuring tube 4 in the vibration direction f due to a rounding of the tube has the greatest impact. The sections of the measuring tubes 4 with an oval cross-section can be introduced into tubes originally comprising a circular cross-section, e.g., through mechanical methods. For example, the sections with an oval cross-section can be introduced by pressing and/or by hydroforming. Additionally or alternatively, roll forming or other mechanical processes are also possible.
[0037] FIG. 8 shows a flow chart of the method 6. The method 6 starts at step 60 with the determination of the required length of the oval section of the measuring tube 4, its ovality (i.e. the ratio of the longer axis a to the shorter axis b) and/or its cross-sectional shape by means of the finite element method (FEM). With this numerical method, the design of the measuring tube 4 can be adapted to the working conditions of the specific application at hand so that the required tube geometry is known in advance. Step 61 typically takes place simultaneously with the determination of the tube geometry in step 60 or is included therein. In step 61, the parameters of the length of the oval section of the measuring tube, its ovality and its cross-sectional shapes are coordinated in such a manner that the dependence of the measurement of the Coriolis mass flow meter 1 on the pressure of the fluid medium is reduced. The measurement can relate to the measurement of the mass flow and/or the measurement of the density of the fluid medium. Should either the mass flow measurement or the density measurement be particularly important for the current application, the measuring tube 4 can be configured in such a manner that the dependence of the respective measurement on the pressure essentially disappears. If both measurements are important, it is at least possible to reach a compromise in the design of the measuring tube 4, according to which an optimal reduction of the dependence of both the mass flow measurement and the density measurement on the pressure of the fluid medium is achieved. Although the pressure dependence of the respective measurements will not disappear completely in this case, it can be reduced simultaneously for both measurements. The measuring tube 4 determined in this manner is then provided in step 62 of the method 6. The measuring tube 4 thus comprises at least one oval section in which the measuring tube 4 comprises, perpendicular to the flow direction x, a longer axis a and a shorter axis b. In step 63 of the method 6, the measuring tube 4 is then arranged inside the Coriolis mass flow meter 1 in such a manner that the longer axis a of the oval section of the measuring tube 4 is oriented essentially in the vibration direction f. As a result of the tube geometry of the measuring tube 4 described above, the measuring tube 4 is configured so that its oval section is rounded by the internal pressure prevailing during the operation of the Coriolis mass flow meter 1 and the stiffness in the vibration direction f decreases. This effect counteracts the stiffening of the measuring tube 4 caused by the increased internal pressure so that these two influences at least partially cancel each other out. In this manner, the pressure dependence of the measurement overall can be reduced with the Coriolis mass flow meter 1 manufactured with the method 6 according to the invention.
[0038] FIG. 9 shows illustratively the results of a calculation using FEM. In the diagram shown in FIG. 9, the ovality of the section of the measuring tube 4 with an oval cross-section is plotted on the abscissa as a quotient of the longer axis a and the shorter axis b. The ordinate shows the pressure dependence of the resonance frequency of the measuring tube 4. Specifically, the FPC value plotted on the ordinate is calculated from the derivative of the resonance frequency in accordance with the pressure, divided by the resonance frequency at 20 C. Since, as explained above, the density of the fluid medium is calculated from the resonance frequency, FIG. 9 shows the overall dependence of the density measurement on the pressure of the fluid medium for different ovalities of the measuring tube 4. The object of the invention is thus to minimize this pressure dependence, i.e. the value of FPC, as far as possible. The diamonds in the diagram respectively show values of FPC, which are calculated using FEM, at the corresponding ovality. As is evident from this figure, the calculated values result in a more or less straight line, which has been extrapolated to an FPC value of zero by means of the thin line. Accordingly, the measurement of the density of the fluid medium would become completely independent of the pressure approximately at an ovality of the measuring tube 4 of 15%, i.e. the rounding of the measuring tube 4 would lead to a complete compensation of the increase in stiffness of the measuring tube 4 caused by the rise in pressure. An analogous calculation can also be performed for the pressure dependence of the measurement of the mass flow. The results of the calculation using FEM described above were based on a model in which the section of the measuring tube 4 with an oval cross-section was located in one of the branches 43. This was for the purely practical reason that the calculation using FEM is easier to perform in this case. As already described above, however, the effects of the section with an oval cross-section are greater if the latter is arranged, for example, in the curves 44 and/or in the curve segment 43 of the measuring tube 4. Therefore, it must be expected in these cases that the respective influence of the ovality on the pressure dependence of the density measurement and the mass flow measurement is even greater.
