CORIOLIS MASS FLOW METER AND METHOD FOR MONITORING A CORIOLIS MASS FLOW METER

Description

[0001] The present invention relates to a Coriolis mass flowmeter and a method for monitoring the same. European patent EP 2 638 367 B1 relates to such a method, in which a measuring tube vibration in a bending vibration mode of a measuring tube outside of resonance is excited at an excitation frequency that has a fixed ratio to the natural frequency of the bending vibration mode in order to determine a value of a transfer function formed from a ratio of a sensor signal amplitude of a vibration sensor for detecting the measuring tube vibration and an excitation signal amplitude of an exciter for exciting the measuring tube vibration. WO 2021 / 047887 A1 also relates to such a method.

[0002] Changes in the value of this transfer function can be used to detect changes in the mass flowmeter at an early stage, particularly changes in the elastic properties of the measuring tube, which may be caused by abrasion or corrosion. Excitation off-resonance is advantageous because the transfer function is essentially independent of the quality factor of the vibrating measuring tube, which can vary by several orders of magnitude. On the other hand, due to the lack of resonance enhancement of the excited flexural vibration mode, the relative importance of the contribution of neighboring flexural vibration modes to the sensor signal increases, so that this contribution should not be neglected. It should also be noted that the relative importance of the contribution of neighboring flexural vibration modes varies and, in particular, is density-dependent.It is therefore the object of the present invention to provide an improved monitoring method and a Coriolis mass flowmeter with the implemented monitoring method that takes the above considerations into account. This object is achieved according to the invention by the method according to independent patent claim 1 and the Coriolis mass flowmeter according to independent patent claim 11.

[0003] The inventive method for monitoring a Coriolis mass flowmeter with at least one oscillating measuring tube for guiding a medium comprises: determining the resonant frequency of a bending vibration useful mode; determining a density measurement value of a medium guided in the measuring tube as a function of the resonant frequency; exciting a bending vibration outside of resonance with an excitation signal having an excitation signal amplitude and an excitation frequency which is µ times the resonant frequency of the bending vibration useful mode, where µ is a constant factor, where 1.05 < µ < 1.5 applies; detecting a sensor signal and determining a sensor signal amplitude of the bending vibration outside of resonance;Determining a value of an integrity function of the measuring tube, which depends on a ratio of the sensor signal amplitude to the excitation signal amplitude of the bending vibration, wherein the integrity function further depends on a density-dependent term of a transfer function, wherein the density-dependent term of the transfer function models contributions of several vibration modes to the sensor signal, wherein the integrity function is traced back to reference conditions by means of the density-dependent term of the transfer function, and / or indicates an integrity value that has no cross-sensitivities to the media density.;

[0004] In general, a transfer function T(ω) models a ratio U(ω) / I(ω) dependent on an excitation angular frequency ω = 2πf, where f is the excitation frequency and U(ω) and I(ω) denote a sensor voltage signal amplitude and an excitation current signal amplitude, respectively, at the excitation angular frequency ω. In particular, for an electrodynamic sensor, the sensor voltage signal amplitude U(ω) is proportional to the speed, i.e., proportional to the oscillation amplitude of the measuring tube at the sensor location and proportional to the oscillation frequency.

[0005] In a further development of the invention, the density-dependent term of the transfer function models a ratio of sensor signal amplitude and excitation signal amplitude as a function of the excitation frequency and the media density.

[0006] In a further development of the invention, the density-dependent term of the transfer function comprises summands that are proportional to the contribution of individual vibration modes to the sensor signal, wherein the contribution of a vibration mode depends on the ratio of the excitation frequency to the natural frequency of the vibration mode.

[0007] The transfer function can then be described, for example, as: T ω = jωe s e d ∑ A i ω , ω i ρ

[0008] Here, α and δ are the efficiency coefficients of the vibration sensor and the exciter, respectively. They describe a voltage per velocity and a force per current, respectively.

[0009] The modal contributions A i (ω, ω i (ρ)) can be described as: A i ω , ω i ρ = n i 1 + j ω ω i ρ Q i − ω ω i ρ 2

[0010] Here, ω i (ρ) is the natural frequency of a bending vibration mode, which depends on the medium density ρ, and Q i is the quality factor of the respective bending vibration mode.

[0011] The term ni denotes an effective modal excitability of the respective bending vibration mode, whereby, as shown here, the density-dependent term of the transfer function according to a further development of the invention models the contributions of individual vibration modes to the sensor signal as proportional to the effective modal excitability.

[0012] The effective modal excitability is, in particular, proportional to the quotient of the product of a normalized modal vibration amplitude at the exciter location and a normalized modal vibration amplitude at the sensor location, divided by the modal stiffness of the measuring tube. The normalization of the modal vibration amplitudes can, for example, be performed such that the integral of a modal vibration amplitude along the measuring tube always assumes the value 1.

[0013] In a further development of the invention, the density-dependent term of the transfer function relates the modal excitabilities of two vibration modes to each other by a mode-specific proportionality factor γ.

[0014] In a further development of the invention, the density-dependent term of the transfer function models the contributions of individual vibration modes to the sensor signal as damping-free.

[0015] In a further development of the invention, the density-dependent term of the transfer function models the natural frequencies of the vibration modes as proportional to each other.

[0016] In a further development of the invention, the density-dependent term of the transfer function models a mode-specific proportionality factor between the natural frequencies of two vibration modes as a function f(p) of the density measurement value.

[0017] If one further considers that the excitation angular frequency ω differs by a constant factor µ from the natural angular frequency ω 1 of a bending vibration useful mode, where for example 1.05 < µ < 1.5, the modal contributions A i can be described as: A i μ ρ = n 1 1 − μ 2 γ i 1 − μ 2 1 − μ f i ρ 2

[0018] In a further development of the invention, the function is a linear function of the density measurement value, for example: f i ρ = a i + b i ⋅ ρ − ρ ref

[0019] Here, ai and bi are mode-specific coefficients, and ρ ref is a reference density, for example 1000 kg / m 3< .

[0020] With the following definition B i μ ρ = 1 − μ 2 1 − μ f i ρ 2 the transfer function can be written as: T ω 1 μ ρ = U ω 1 μ I ω 1 μ = jω 1 μe s e d n 1 1 − μ 2 ∑ γ i B i μ ρ

[0021] An integrity function HBSI corrected for density dependence can thus be defined as: HBSI = e s e d n 1 2 π = ∑ γ i B i μ ρ − 1 U I 1 − μ 2 μf 1

[0022] This expression describes a product of sensor sensitivity es , excitation effectiveness ed , and modal compliance n 1 of the desired bending mode. Values ​​for the integrity function are easily determined by measuring values ​​of the transfer function U / I, given knowledge of the proportionality factors γ i and B i .

[0023] A simplified version of an integrity function HBSI raw is given as HBSI raw ρ = U I 1 − μ 2 μf 1

[0024] Values ​​for HBSI raw are obtained immediately after recording values ​​for the transfer function U / l.

[0025] The simplified integrity function HBSI raw exhibits a density dependence, as can be seen from comparing the definitions in equations (VII) and (VIII). Accordingly, HBSI raw ρ = HBSI ⋅ ∑ γ i B i μ ρ

[0026] In order to be able to perform reliable monitoring with the simplified integrity function HBSI raw, it is necessary to trace the values ​​of the simplified integrity function obtained at arbitrary densities ρ back to the corresponding values ​​at the reference density ρ ref . For this purpose, a correction function f dens (ρ) can be defined, for which the following applies: f dens ρ = HBSI raw ρ ref HBSI raw ρ = ∑ γ i B i μ ρ ref ∑ γ i B i μ ρ

[0027] With this correction function f dens (ρ), the simplified integrity function can be reduced to reference conditions for comparability according to: HBSI raw ρ ref = f dens ρ HBSI raw ρ

[0028] In a further development of the invention, the transfer function models the contribution of only one additional vibration mode to the sensor signal in addition to the contribution of the bending vibration mode. In this case, the transfer function can be written as: T ω 1 μ ρ = jω 1 μe s e d n 1 1 − μ 2 1 + γ 1 − μ 2 1 − μ f ρ 2

[0029] With a linear function for f(p) according to equation (IV) this becomes: T ω 1 μ ρ = jω 1 μe s e d n 1 1 − μ 2 1 + γ 1 − μ 2 1 − μ α + β ρ − ρ ref 2

[0030] A corresponding density-independent integrity function can be written as: HBSI = 1 1 + γ 1 − μ 2 1 − μ α + β ρ − ρ ref 2 U I 1 − μ 2 μf 1

[0031] With the following definition of a function C(p) C ρ = 1 − μ 2 1 − μ α + β ρ − ρ ref 2 becomes: HBSI = 1 1 + γ C ρ U I 1 − μ 2 μf 1

[0032] In this embodiment, the implementation of the method according to the invention requires only the determination of three parameters, namely the coefficients of the linear function of the density dependence α, β and the coefficient of excitability γ.

[0033] According to a further development of the invention, the coefficients of the linear function of the density dependence α, β and the coefficient of excitability γ are determined type-specifically and provided in data memories of the measuring and operating circuit of Coriolis mass flowmeters according to the invention, wherein the coefficient of excitability γ is preferably individually adapted for each example of a Coriolis mass flowmeter, preferably during an initial, in particular factory, determination of device parameters.

[0034] A corresponding correction function f dens (ρ) for the simplified integrity function HBSI raw can be written in the case of the transfer function according to equation (XIII) as: f dens ρ = 1 + γ C ρ ref 1 + γ C ρ

[0035] By multiplying a measured value of the simplified integrity function HBSI raw (ρ) with the correction function f dens (ρ), the measurement result is, according to a further development of the invention, traced back to the value of the simplified integrity function at reference conditions HBSI raw (ρ ref ).

[0036] If the value of the integrity function HBSI or the simplified integrity function HBSI raw (ρ ref ) under reference conditions changes beyond a tolerance value, this is an indication of a change in the modal stiffness of a measuring tube, a sensor sensitivity, or an excitation effectiveness. According to a further development of the invention, a corresponding error message is output in this case. It is known that the variables mentioned here exhibit cross-sensitivities to temperature. According to a further development of the invention, these cross-sensitivities are taken into account in order to specify a temperature-independent value of the integrity function HBSI or the simplified integrity function HBSI raw.

[0037] The Coriolis mass flowmeter according to the invention comprises: at least one oscillating measuring tube for conveying a medium; at least one exciter for exciting bending vibrations of the measuring tube as a function of an excitation signal; at least one vibration sensor for detecting the bending vibrations of the at least one measuring tube and for outputting a vibration-dependent sensor signal; and at least one measuring and operating circuit for supplying the exciter with an excitation signal and for detecting the sensor signal, wherein the measuring and operating circuit is configured to carry out the method according to the invention.

[0038] The invention will now be explained in more detail with reference to the exemplary embodiments illustrated in the drawings. It shows: Fig. 1a : a side view of an embodiment of a Coriolis mass flowmeter according to the invention; Fig. 1b : a spatial representation of the embodiment of the Coriolis mass flowmeter according to the invention from Fig. 1a ; and Fig. 2 : a flowchart of an embodiment of a method according to the invention; Fig. 3a : a diagram with values ​​of the density-dependent simplified integrity function HBSI raw (ρ): Fig. 3b : a diagram with density-dependent correction functions f dens (ρ), whose coefficients are calculated using the values ​​of the density-dependent simplified integrity function HBSI raw (ρ) in Fig. 3a were determined; and Fig. 3c : a diagram with density-dependent functions whose coefficients are calculated using the values ​​of the density-dependent simplified integrity function HBSI raw (ρ) in Fig. 3a were determined and which serve to reduce values ​​of the density-dependent simplified integrity function HBSI raw (ρ) to values ​​of the density-independent integrity function HBSI.

[0039] In the Figuren 1a und 1b An embodiment of a Coriolis mass flowmeter 2 according to the invention is shown, which is designed to carry out the method according to the invention. The Coriolis mass flowmeter 2 has two measuring tubes A and B mounted so as to be capable of oscillation, each of which is curved and runs parallel to one another. The Coriolis mass flowmeter 2 can be inserted into a pipeline such that the two measuring tubes A, B are flowed through by the fluid flowing in the pipeline. On the inlet and outlet sides, the measuring tubes A, B are each enclosed in flow dividers or collectors 4, 6, the latter being rigidly connected to one another by a support tube T. Thus, the inlet and outlet end sections of the measuring tubes are also coupled to the support tube T, whereby relative movements between the inlet and outlet end sections of the measuring tubes are effectively suppressed.An electrodynamic exciter 8 is arranged between the two measuring tubes A, B, by means of which the two measuring tubes A, B can be excited to bending vibrations relative to each other. The free vibration length of the measuring tubes A, B is determined by coupling elements 10, 11, with which the measuring tubes are mechanically coupled on the inlet and outlet sides. Electrodynamic vibration sensors 14, 16 are arranged between the two measuring tubes A, B, each on an inlet-side and an outlet-side section. Although... Fign. 1a und 1b While the figures show an embodiment of a Coriolis mass flowmeter with a pair of measuring tubes bent in the rest position, the invention equally applies to Coriolis mass flowmeters with a single measuring tube or with multiple pairs of measuring tubes. Likewise, instead of the illustrated measuring tubes bent in the rest position and having a mirror symmetry with respect to a transverse measuring tube plane, S-shaped measuring tubes or straight measuring tubes can also be used to implement the invention.

[0040] The Coriolis mass flowmeter 2 further comprises an operating and evaluation circuit 18 for supplying the exciter 8 with an excitation current and for detecting and evaluating measurement signals from the electrodynamic vibration sensors 14, 16. The measuring and operating circuit is particularly configured to implement the inventive method for monitoring the Coriolis mass flowmeter. This includes determining values ​​of an integrity function in order to be able to detect changes in the Coriolis mass flowmeter at an early stage.

[0041] An embodiment of the method according to the invention will now be described with reference to Fig. 2 explained.

[0042] The method 100 begins with determining 110 a resonance frequency f 1 of a bending vibration mode of the Coriolis mass flowmeter. This can be Fign. 1a und 1b In the device shown, this can be, in particular, the so-called f 1 mode, in which the two measuring tubes oscillate in phase opposition to each other without any oscillation nodes between the coupling elements. Based on this resonance frequency f 1, a density measurement value for the medium conveyed in the measuring tubes is determined in a second step 120.

[0043] To determine the integrity function, the excitation 130 of the bending vibration useful mode is carried out off-resonance at µ times the resonance frequency with an excitation current having an excitation current amplitude I, where µ in particular assumes the value 1.2.

[0044] A sensor signal is detected 140 and a sensor signal amplitude U of the bending vibration outside of resonance is determined 145.

[0045] Based on the sensor signal amplitude U and the excitation current amplitude I, a value of an integrity function is then determined. This can, for example, be a value of an integrity function HBSI or a value of a simplified integrity function HBSI raw (ρ ref ) derived from a reference density, where the reference density ρ ref is, in particular, the density of water.

[0046] As stated at the beginning, the integrity function HBSI according to equations (XV) and (XVI) is: HBSI = U I 1 − μ 2 μf 1 1 1 + γ C ρ with: C ρ = 1 − μ 2 1 − μ α + β ρ − ρ ref 2

[0047] Accordingly, the simplified integrity function HBSI raw (ρ ref ) according to equations (VIII), (XI) and XVII applies: HBSI raw ρ ref = U I 1 − μ 2 μf 1 f dens ρ with: f dens ρ = 1 + γ C ρ ref 1 + γ C ρ

[0048] The implementation of the method according to the invention for a given Coriolis mass flowmeter requires only the determination of three parameters, namely the coefficients of the linear function of the density dependence α, β and the coefficient of excitability γ. It has been shown that the coefficients of the linear function of the density dependence α, β are essentially constant for a device type, and that only the coefficient of excitability γ exhibits a certain degree of specimen scatter. Therefore, in most cases it is sufficient to determine the coefficients of the linear function of the density dependence once for a device type and only to adapt the coefficient of excitability to the specific device. The procedure for this will now be described using the Fign. 3a bis 3c explained.

[0049] When characterizing a particularly brand-new Coriolis mass flowmeter, its measuring tubes are exposed to media over a wide density range and allowed to oscillate at µ times the resonance frequency of a bending vibration mode, in particular the f 1 mode, in order to determine the respective transfer function U / I and thus the values ​​of the density-dependent simplified integrity function HBSI raw (ρ). HBSI raw ρ = U I 1 − μ 2 μf 1

[0050] Suitable media for this purpose are water as a medium with a reference density ρ ref of about 1000 kg / m 3< , air with a density ρ of about 1.2 kg / m 3< , higher densities of up to 3100 kg / m 3< can be achieved by solutions of sodium polytungstate.

[0051] Fig. 3a shows observed values ​​from the density-dependent simplified integrity function HBSI raw (ρ). The data points shown here are from different samples of the same instrument type, with the same symbols indicating data from one sample.

[0052] Fig. 3b now shows instance-specific correction functions f dens (ρ), with which values ​​of HBSI raw (ρ) can be reduced to the value of HBSI raw (ρ ref ), where f dens (ρ ref ) by definition takes the value 1 and generally applies

[0053] With: f dens ρ = 1 + γ C ρ ref 1 + γ C ρ and C ρ = 1 − μ 2 1 − μ α + β ρ − ρ ref 2 are based on the data in Fig. 3a The unknown, specimen-specific coefficients α, β, and γ are determined. It turns out that the coefficients α and β can be assumed to be constant for specimens of a device type. Only the coefficient γ exhibits specimen variance, so it must be determined specifically for each specimen.

[0054] With the previously determined coefficients, the Fig. 3c shown instance-specific functions 1 / (1+ γ* C(p)) with which a measured value of the density-dependent simplified integrity function HBSI raw (ρ) can be traced back to a value of the density-independent integrity function HBSI according to HBSI = HBSI raw ρ 1 1 + γ C ρ

Claims

1. A method (100) for monitoring a Coriolis mass flow meter having at least one vibratable measuring tube for carrying a medium, comprising: determining (110) a resonant frequency of a bending vibration useful mode of the Coriolis mass flow meter; determining (120) a density measurement value of a medium conducted in the measuring tube as a function of the resonance frequency; exciting (130) a flexural vibration out of resonance with an excitation signal which has an excitation signal amplitude and an excitation frequency which is µ times the resonance frequency of the flexural vibration utilization mode, µ being a constant factor, where 1.05 < µ < 1.5; detecting (140) a sensor signal and determining a sensor signal amplitude of the flexural vibration out of resonance; determining (145) a value of an integrity function of the measuring tube which depends on a ratio of the sensor signal amplitude to the excitation signal amplitude of the flexural vibration, characterized in that the integrity function further depends on a density-dependent term of a transfer function, wherein the density-dependent term of the transfer function models contributions of a plurality of vibration modes to the sensor signal, wherein the integrity function is traced back to reference conditions by means of the density-dependent term of the transfer function, and / or indicates an integrity value which has no cross-sensitivities to the density of the medium.

2. A method (100) according to claim 1, wherein the transfer function models a ratio of sensor signal amplitude and excitation signal amplitude as a function of the excitation frequency and the media density.

3. Method (100) according to claim 1 or 2, wherein the density-dependent term of the transfer function comprises summands that are proportional to the contribution of individual vibration modes to the sensor signal, wherein the contribution of a vibration mode is dependent on the ratio of the excitation frequency to the natural frequency of the vibration mode.

4. Method according to claim 3, wherein the density-dependent term of the transfer function models the natural frequencies of the vibration modes as proportional to one another.

5. Method according to claim 4, wherein the density-dependent term of the transfer function models a mode-specific proportionality factor between the natural frequencies of two vibration modes as a function of the density measurement.

6. A method according to claim 5, wherein the function is a linear function of the density measurement.

7. Method according to one of the preceding claims, wherein the density-dependent term of the transfer function models the contributions of individual vibrational modes to the sensor signal as damping-free in each case.

8. Method according to one of claims 3 to 7, wherein the density-dependent term of the transfer function models the contributions of individual vibrational modes to the sensor signal as being proportional to an effective modal excitatability.

9. Method according to claim 8, wherein the density-dependent term of the transfer function relates the modal excitations of two vibrational modes to one another by means of a proportionality factor.

10. Method according to one of the preceding claims, wherein the density-dependent term of the transfer function models the contribution of only one further vibrational mode to the sensor signal in addition to the contribution of the useful bending vibration mode.

11. Coriolis mass flow meter, comprising: at least one vibratable measuring tube for conducting a medium; at least one exciter for exciting bending vibrations of the measuring tube in dependence on an exciter signal; at least one vibration sensor for detecting the flexural vibrations of the at least one measuring tube and for outputting a vibration-dependent sensor signal; at least one measuring and operating circuit for supplying the exciter with an excitation signal and for detecting the sensor signal, the measuring and operating circuit being configured to carry out the method according to one of claims 1 to 10.