Method for determining a density measurement and Coriolis mass flow transmitter for carrying out the method

Description

[0001] The invention relates to a method for determining a measured density value of a medium using a Coriolis mass flowmeter with two oscillators mounted in a pipeline, as well as to such a Coriolis mass flowmeter for installation in a pipeline and for implementing the method according to the invention. Flowmeters of this type are disclosed, for example, in DE 10 2015 104 931 A1.

[0002] Furthermore, DE 10 2018 133 318 A1 discloses a vibration-type flow transducer with two curved, parallel measuring tube pairs, wherein the partial flow rates through the two measuring tube pairs are recorded separately and added to a total flow rate.

[0003] DE 10 2016 007 905 A1 discloses a method for operating a vibration-type measuring sensor with at least two oscillators, each formed by a pair of measuring tubes, wherein the pairs of measuring tubes are arranged in parallel flow terms, wherein the two oscillators have independent oscillator oscillations with different natural frequencies for corresponding oscillation modes, wherein the method evaluates the relationship between measurement results of the two oscillators for a measured variable and outputs a signal in the event of deviations from an expected relationship.

[0004] DE 10 2016 125 616 A1 discloses a vibration-type flow transducer with two curved, parallel measuring tube pairs. The measuring tubes of one measuring tube pair are each located in the same plane as one measuring tube of the other measuring tube pair and are coupled to the measuring tube of the other measuring tube pair in the other plane at the apex of the measuring tube center. The resulting diagonal coupling causes the measuring tubes of the two measuring tube pairs to oscillate at the same frequency and in antiphase.

[0005] WO 2020 / 088 837 A1 discloses a vibration-type flow transducer with two curved, parallel measuring tube pairs, with independent flow measurement in the two measuring tube pairs, wherein the credibility of the respective measurement results is checked and, in case of doubt, the doubtful measured values ​​of one measuring tube pair are replaced by substitute values ​​based on the measured values ​​of the other measuring tube pair.

[0006] DE 10 2018 112 002 A1 discloses a vibration-type measuring sensor for determining the density, mass flow and / or viscosity of a flowable medium.It comprises: an oscillator which has at least one oscillatable measuring tube for guiding the medium and which has at least one oscillation mode whose natural frequency depends on the density of the medium, an exciter for exciting the oscillation mode; at least one oscillation sensor for detecting oscillations of the oscillator; and an operating and evaluation circuit which is configured to apply an excitation signal to the exciter, to detect signals from the oscillation sensor, to determine current values ​​of the natural frequency of the oscillator and fluctuations in the natural frequency based on the signals from the oscillation sensor, and to determine a value characterising density fluctuations of the medium, wherein the value depends on a function which is proportional to the fluctuation of the natural frequency and has a natural frequency-dependent normalisation.

[0007] These transmitters are factory calibrated and installed in a calibrated state at a measuring point. This installation can expose the transmitter to mechanical stresses, resulting in minimal detuning of the oscillators, which particularly affects frequency-dependent density measurements.

[0008] It is the object of the present invention to remedy this situation.

[0009] The method according to the invention is used to determine a density measurement value of a medium by means of a Coriolis mass flow measuring transducer mounted in a pipeline, which has at least two oscillators, each comprising at least two counter-oscillating measuring tubes, wherein the measuring tubes of a first of the oscillators run above the measuring tubes of a second of the oscillators, wherein the measuring tubes are arranged in a fluidically parallel manner and open into collectors on the inlet and outlet sides, wherein the mounting of the measuring transducer in the pipeline causes mechanical stresses which influence the oscillators via the collectors, wherein the medium is guided in the measuring tubes, wherein the method comprises the following steps: Exciting at least one oscillation mode of the first oscillator and the second oscillator; determining the respective natural frequency of the excited oscillation modes; determining a preliminary density measurement value based on the respective natural frequencies; determining a deviation between the preliminary density measurements; determining a corrected density measurement value using a model which determines and corrects the influence of the mechanical stresses on the density measurement based on the deviation.

[0010] In more detailed investigations into the effects of mechanical stresses caused by the installation of the measuring transducer in a measuring point, the inventors of the present invention discovered that the mechanical stresses affect the two oscillators differently, i.e., they are detuned to different degrees, causing them to produce different preliminary density measurements for a medium flowing through them. Since the deviations between the two preliminary density measurements correlate with the absolute value of the detuning, the extent of the stress-dependent detuning can be determined and corrected according to the invention.

[0011] In a further development of the invention, the model models the influence of mechanical stresses in the form of bending moments.

[0012] In a further development of the invention, the model models the influence of the bending moments on the oscillators under the assumption that the bending moments have axes that are perpendicular to a longitudinal axis of the measuring transducer and that run in the direction of the oscillations of the oscillators.

[0013] In most measuring points, the transducers are arranged so that the measuring tubes oscillate in a horizontal plane. This means that, according to the above definition, the axis of the bending moments also runs horizontally. This is particularly true when the effects of gravity, due to tolerances in the support of the pipeline, lead to sagging or overly supported pipe sections to which the transducer is connected. This is likely the most common cause of installation-related misalignments, which can be corrected using the method according to the invention.

[0014] In a further development of the invention, an effective density measurement value is determined as a mean value or a mean value of the preliminary density measurements weighted by the respective flow rate through the measuring tubes of the oscillators, wherein the corrected density measurement value is determined on the basis of the effective density measurement value and a correction term which is determined on the basis of the deviation.

[0015] In a further development of the invention, the corrected density measurement value is determined on the basis of both preliminary density values ​​and the correction terms assigned to them, which are determined on the basis of the deviation.

[0016] In a further development of the invention, at least one correction term comprises a polynomial of the deviation, in particular a polynomial of the first or second degree.

[0017] In a further development of the invention, at least one correction term is added to the effective density measurement value.

[0018] In a further development of the invention, it is checked whether the medium is single-phase, whereby the density measurement value corrected by means of the model is determined only for single-phase media.

[0019] In a further development of the invention, the first bending vibration mode of the oscillators is excited, whereby the preliminary density measurement values ​​are determined on the basis of the natural frequencies of these bending vibration modes.

[0020] In a further development of the invention, the corrected density measurement value deviates from the actual density of the medium by no more than 0.5 kg / m 3< , in particular no more than 0.3 kg / m 3< , in particular no more than 0.2 kg / m 3<.

[0021] The Coriolis mass flow measuring transducer according to the invention for installation in a pipeline comprises: at least two oscillators, each comprising at least two measuring tubes which can oscillate against one another, wherein in the installed state the measuring tubes of a first of the oscillators run above the measuring tubes of a second of the oscillators, wherein the measuring tubes are arranged in a fluidically parallel manner and open into collectors on the inlet side and outlet side, wherein the installation of the measuring transducer in the pipeline can cause mechanical stresses which influence the oscillators via the collectors, wherein measuring tubes serve to guide a medium whose density is to be determined with the Coriolis mass flow measuring transducer, wherein the Coriolis mass flow measuring transducer further comprises a measuring and operating circuit which is set up to carry out the method according to one of the preceding claims with the Coriolis mass flow measuring transmitter.

[0022] The invention will now be explained in more detail with reference to the exemplary embodiments illustrated in the drawings. It shows: Fig 1 : an exemplary voltage distribution in a measuring transducer; Fig. 2 : a diagram of preliminary and corrected density measurements at varying mechanical stresses; Fig. 3 : Flowchart of an embodiment of the method according to the invention; and Fig. 4 : an embodiment of a measuring transducer according to the invention.

[0023] Fig. 1 shows exemplary simulation results for mechanical stresses in a generic measuring transducer for an installation situation in a pipeline, when bending moments are introduced into the measuring transducer 10 due to tolerances in the support of the pipeline. Fig.1 Only a quarter of the measuring transducer 10 is shown, whereby it can be assumed that the mechanical stresses in the non-illustrated parts are essentially mirror-symmetrical to the stress distribution in the illustrated quarter with respect to a transverse plane and a longitudinal plane of the measuring transducer. Quantitative details are not important in the representation of the stresses. It should only be noted that greater mechanical stresses occur in the light-colored zones of a component than in the dark-colored zones of the component.

[0024] The measuring transducer 10 comprises an outer oscillator 12 with two parallel, curved measuring tubes, and an inner oscillator 14 with two parallel, curved measuring tubes. The measuring tubes each open into a collector 18 on the inlet and outlet sides, which carries a flange 20 for mounting the measuring transducer 10 in a pipeline. The two collectors 18 are connected to each other by a rigid support tube 16.

[0025] In this context, it should only be noted that the mechanical stresses introduced into the measuring tubes of the outer oscillator 12 lead to a more extensive area of ​​increased mechanical stress than in the measuring tubes of the inner oscillator 14, as is immediately apparent from a comparison of the light-colored areas. Accordingly, stronger voltage-dependent detuning is to be expected for the outer oscillator 12 than for the inner oscillator 14.

[0026] This expectation is confirmed by the measurement results in Fig. 2 This study confirms the density measurements taken during the installation of a mass flow transmitter in a pipeline, with the introduced stresses increasing over time. The transmitter was filled with air under standard conditions, so a density of approximately 1.2 kg / m 3 must have been measured.

[0027] Curve (a) shows preliminary density measurements from the outer oscillator, while curve (b) shows preliminary density measurements from the inner oscillator. As expected, the density measurements from the outer oscillator deviate more from the target value than those from the inner oscillator.

[0028] Finally, curve (c) shows a density measurement value corrected by the method according to the invention, which satisfactorily agrees with the target value.

[0029] The method according to the invention will now be described using a Fig. 3 The flow diagram of an exemplary embodiment is explained in more detail below. The method 100 begins by exciting at least one oscillation mode of the first oscillator and the second oscillator (110), wherein the fundamental bending oscillation mode is generally excited, i.e. the mode in which the excited mode has no oscillation nodes and which has the lowest natural frequency of all oscillation modes of the respective oscillator. In a second step, the respective natural frequency of the excited oscillation modes (120) is determined. This is followed by the determination of a preliminary density measurement value (130) based on the respective natural frequencies. For a measuring transducer installed without voltage, the two preliminary density measurements should essentially agree.However, if deviations exist and it can be ruled out that these deviations have other causes, such as gas loading of a liquid medium, then a deviation is indicative of an influence of mechanical stress on the density measurement. The next step is to determine a deviation between the preliminary density measurements (140). On this basis, a corrected density measurement value (150) is finally determined using a model which, based on the deviation, determines and corrects the influence of mechanical stress on the density measurement. In general, the correction function can be a polynomial in the difference between the density measurements, whereby sufficient measurement accuracy can be achieved with a linear function of the deviation or a second-degree polynomial. The procedure is explained below using linear terms, but it can also be used with polynomials.

[0030] First, a corrected density partial measurement value can be determined for each oscillator, according to ρ corr , i = ρ vorl , i + a i ⋅ ρ diff + b i , where i = 1, 2 is an index for the first or second oscillator, respectively, and ρ vorl,i denotes the preliminary density measurement value of the respective oscillator, where ai and bi are oscillator-specific coefficients, and where ρ diff is the deviation between the preliminary density measurements. The resulting corrected partial density measurements ρ vorl,i for the oscillators should essentially agree. An average of the two partial density measurements ρ corr,i, possibly weighted by the respective mass flow rates through the measuring tubes of the oscillators, can then be output as the corrected density measurement ρ corr.

[0031] Density measurement values ​​corrected by the method according to the invention are achieved which deviate from the actual density of the medium by no more than 0.5 kg / m 3< , in particular no more than 0.3 kg / m 3< , in particular no more than 0.2 kg / m 3<.

[0032] The Fig. 4 The illustrated embodiment of a measuring transducer 10 according to the invention comprises an outer oscillator 12 with two parallel, curved measuring tubes 12a, 12b, and an inner oscillator 14 with two parallel, curved measuring tubes 14a, 14b. The measuring tubes each open into a collector 18 on the inlet and outlet sides, which carries a flange 20 for mounting the measuring transducer 10 in a pipeline. The two collectors 18 are connected to one another by a rigid support tube 16.

[0033] To excite flexural vibrations of the measuring tubes of the oscillators 12, 14, the measuring transducer has an electrodynamic exciter (not shown here) for each oscillator, which acts between the measuring tubes of the respective oscillator. In addition, the measuring sensor has an inlet-side and an outlet-side electrodynamic vibration sensor (not shown here) for each oscillator to detect the vibrations of the measuring tubes of the oscillator. Details of this are generally known to those skilled in the art and are explained in more detail, for example, in DE 10 2015 104 931 A1.

[0034] The measuring transducer according to the invention further comprises a measuring and operating circuit 32 with a computing unit to drive the exciters, detect signals from the vibration sensors, and carry out the method according to the invention.

Claims

1. Method (100) for determining a measured density value of a medium by means of a Coriolis mass flow transducer (10) which is mounted in a pipeline and has at least two oscillators (12, 14) which each comprise at least two measuring tubes oscillating with respect to one another, the measuring tubes (12a, 12b) of a first of the oscillators running above the measuring tubes of a second of the oscillators (14a, 14b), wherein the measuring tubes (12a, 12b, 14a, 14b) are arranged in parallel in terms of flow and open into collectors (18) on the inlet side and outlet side, wherein the mounting of the transducer in the pipeline causes mechanical stresses which influence the oscillators via the collectors, wherein the medium is guided in the measuring tubes, wherein the method (100) comprises the following steps: (110) Excitation of at least one oscillation mode each of the first oscillator and the second oscillator; (120) Determine the respective natural frequency of the excited oscillation modes; (130) Determine a preliminary density measurement value based on the respective natural frequencies; (140) Determine a deviation between the preliminary density readings; (150) Determination of a corrected density measurement value by means of a model which determines and corrects the influence of the mechanical stresses on the density measurement on the basis of the deviation.

2. The method according to claim 1, wherein the model models the influence of the mechanical stresses in the form of bending moments.

3. The method according to claim 2, wherein the model models the influence of the bending moments on the oscillators under the assumption that the bending moments have axes which are perpendicular to a longitudinal axis of the transducer and which extend in the direction of the oscillations of the oscillators.

4. Method according to claim 2 or 3, wherein an effective density measurement value is determined as an average value or an average value, weighted with the respective flow rate through the measuring tubes of the oscillators, of the preliminary density measurement values, wherein the corrected density measurement value is determined on the basis of the effective density measurement value and a correction term which is determined on the basis of the deviation.

5. Method according to claim 2 or 3, wherein the corrected density measurement value is determined on the basis of both preliminary density values and correction terms assigned to them in each case, which are determined on the basis of the deviation.

6. Method according to claim 4 or 5, wherein at least one correction term comprises a polynomial of the deviation, in particular a polynomial of the first or second degree.

7. The method according to claim 4, 5 or 6, wherein at least one correction term is added to the effective density measurement value.

8. Method according to one of the preceding n claims, wherein it is checked whether the medium is single-phase, and wherein the density measurement value corrected by means of the model is determined only for single-phase media.

9. Method according to one of the preceding claims, wherein the first bending vibration mode of the oscillators is excited in each case, and preliminary density measurement values are determined on the basis of the natural frequencies of these bending vibration modes.

10. Method according to one of the preceding claims, wherein the corrected density measurement value does not deviate from the actual density of the medium by more than 0.5 kg / m3, in particular not more than 0.3 kg / m3, in particular not more than 0.2 kg / m3.

11. Coriolis mass flow transmitter (10) for mounting in a pipeline, comprising: at least two oscillators (12, 14), which each comprise at least two measuring tubes (12a, 12b, 14a, 14b) capable of oscillating relative to one another, wherein in the installed state the measuring tubes (12a, 12b) of a first of the oscillators (12a) extend above the measuring tubes (14a, 14b) of a second of the oscillators (14), the measuring tubes (12a, 12b, 14a, 14b) being arranged in parallel in terms of flow and opening into collectors (18) on the inlet side and outlet side, the mounting of the transmitter in the pipeline being capable of causing mechanical stresses, which influence the oscillators (12, 14) via the collectors (18), wherein the measuring tubes (12a, 12b, 14a, 14b) serve to guide a medium whose density is to be determined with the Coriolis mass flow rate transmitter (10), wherein the Coriolis mass flow rate transmitter further comprises a measuring and operating circuit with computing unit (32) which is set up to carry out the method according to one of the preceding claims with the Coriolis mass flow rate transmitter.