Method for determining a mixture density measurement value, method for determining a flow measurement value on the basis of a differential pressure measurement value and the mixture density measurement value, and measuring point for same

Abstract

The method according to the invention is used to determine a mixture density measurement value ρm of a gas-laden liquid by means of a Coriolis mass flowmeter having a measuring tube for guiding the gas-laden liquid. The method comprises: ascertaining (210) an initial mixture density value on the basis of at least one natural frequency of a bending vibration mode of the measuring tube; ascertaining (220) a mass flow measurement value; ascertaining (230) an initial gas volume fraction value αs on the basis of the initial mixture density value ρs and a liquid density value ρi; ascertaining (240) a final gas volume fraction value αf on the basis of the initial gas volume fraction value αs and the mass flow measurement value; and (250) ascertaining the mixture density measurement value on the basis of the liquid density value and the final gas volume fraction value αf.

Description

[0001] Method for determining a mixture density measurement value, method for determining a flow rate measurement value based on a differential pressure measurement value and the mixture density measurement value, and a measuring point for this purpose.

[0002] The invention relates to a method for determining a mixture density measurement value, a method for determining a flow measurement value based on a differential pressure measurement value and the mixture density measurement value, and a measuring point for this purpose.

[0003] Flow measurements based on a differential pressure reading require a density measurement as an auxiliary variable to determine a volumetric or mass flow rate. A Coriolis mass flow meter, connected in series with the differential pressure meter, is frequently used for density measurement. This is straightforward for homogeneous liquids; however, if the liquid contains free gas bubbles, the determination of a mixture density cannot be directly derived from the natural frequency of a bending mode of the Coriolis flow meter. Instead, an initial mixture density value is first determined based on the natural frequency, and a gas volume fraction is then calculated based on this initial value. The mixture density is subsequently determined based on the gas volume fraction and a value for the density of the liquid.However, with free gas bubbles, decoupling can occur between the gas bubbles and the liquid, rendering the underlying density model, which essentially assumes a homogeneous medium, unsuitable for determining the mixture density. Such a mixture density measurement is therefore not readily suitable as a basis for characterizing the medium. It is thus known to determine a gas volume fraction based on such a mixture density measurement, which depends on a relative deviation of the mixture density from the liquid density. The effect of the decoupling between free gas bubbles and the medium must also be compensated for. A mixture density measurement can then be calculated based on the corrected gas volume fraction. Approaches for determining a mixture density measurement are described, for example, in WO 2023 / 196960 A1 and WO 2021 / 169721 A1.Although satisfactory correction algorithms exist for the effect of decoupling at high flow rates, these leave much to be desired, especially at low flow rates; it is therefore the object of the present invention to remedy this.

[0004] The problem is solved according to the invention by the method for determining a mixture density measurement value according to claim 1, the method for determining a flow measurement value according to claim 10 and the measuring point according to claim 12.

[0005] The inventive method for determining a mixture density measurement value of a gas-laden liquid using a Coriolis mass flow meter with a measuring tube for guiding the gas-laden liquid comprises: Determining a mixture density starting value p sbased on at least one natural frequency of a bending vibration mode of the measuring tube; determining a mass flow rate measurement; determining a gas volume fraction starting value oc s based on the initial mixture density value p s and a liquid density value pi; determining a final gas volume fraction value oct based on the initial gas volume fraction value a s , and the mass flow rate measurement; and determining the mixture density measurement based on the liquid density value and the gas volume fraction final value.

[0006] In a further development of the invention, the initial gas volume fraction value oc s proportional to the relative deviation of the initial mixture density value p s from the liquid density value pi, i.e.:

[0007] Pl Ps er«. = -

[0008] pi

[0009] In a further development of the invention, the final gas volume fraction value oct is proportional to the initial gas volume fraction value oc. sand inversely proportional to a flow-dependent correction term, which is a monotonically increasing function of the mass flow rate measurement, and in particular assumes a value less than 1.1 when the flow rate is zero. In a further development of the invention, the correction term has a sigmoidal function or a logistic function whose argument is a linear function of the mass flow rate measurement dm / dt.

[0010] In a further development of the invention, the correction term has a sigmoid function or a logistic function whose argument is a linear function of the reciprocal of the liquid density value pi.

[0011] In a further development of the invention, the flow-dependent correction term k(dm / dt) has the following form: where kmax is a monotonically decreasing function of the initial gas volume fraction value, where kmax > 1 , where kmax < 2, and where k(dm / dt) tends towards kmax for maximum mass flow measurements.

[0012] In a further development of the invention, the flow-dependent correction term kmax is a linear function of a power of the initial gas volume fraction oc. s is, which in particular has the following form: where e is a negative exponent with a magnitude less than 1, and where a and c are constants.

[0013] In a further development of the invention, the mixture density measurement value is determined as the product of the gas volume fraction final value and the liquid density value.

[0014] In a further development of the invention, the liquid density value is determined when the liquid is free of free gas bubbles. A liquid is considered to be free of free gas bubbles within the meaning of the invention, in particular, if fluctuations in the initial mixture density value remain below a threshold value, above which an inhomogeneous medium is assumed.

[0015] In a further development of the invention, the liquid, when it is free of free gas bubbles, has a viscosity of no more than 40 cP, in particular no more than 25 cP.

[0016] The inventive method for determining a flow rate measurement value by means of a differential pressure measuring point comprising a differential pressure sensor and a Venturi tube as an operative pressure transmitter for the differential pressure sensor, wherein the Venturi tube is through which a gas-laden liquid flows, wherein the Venturi tube is connected in series with a Coriolis mass flow meter, wherein the method comprises: determining a mixture density measurement value with the Coriolis mass flow meter using a method according to one of the preceding claims; acquiring a differential pressure measurement value by means of the differential pressure sensor, wherein; and determining a flow rate measurement value for the flow through the Venturi tube based on the differential pressure measurement value and the mixture density measurement value.

[0017] The measuring point according to the invention comprises: a Coriolis mass flow meter; a differential pressure measuring device with a Venturi measuring tube as an differential pressure sensor which is connected in series with the Coriolis mass flow meter; and a calculation arrangement for carrying out the method according to the invention for determining a flow measurement value.

[0018] In a further development of the invention, the Coriolis mass flow meter has an inlet opening and an outlet opening for the gas-laden liquid, wherein a flow axis is defined that passes through the center of a cross-section of the inlet opening and the center of a cross-section of the outlet opening, the flow axis deviating from the vertical by no more than 30°, for example no more than 15°, and in particular no more than 5°. The invention will now be explained in more detail with reference to the embodiment shown in the drawings. These show:

[0019] Fig. 1 : a schematic representation of an embodiment of a measuring point according to the invention;

[0020] Fig. 2: a flowchart of an exemplary embodiment of the present

[0021] Invention;

[0022] Fig. 3a: a diagram showing flow-dependent density measurement errors on

[0023] Basis of a mixture density starting value without further corrections;

[0024] Fig. 3b: a diagram showing flow-dependent density measurement errors on

[0025] Basis of a mixture density starting value after determining a mixture density measurement value according to Hemp; and

[0026] Fig 3c: a diagram showing flow-dependent density measurement errors on

[0027] Basis of a mixture density starting value with a determination of the mixture density measurement value according to the invention.

[0028] The embodiment of a measuring point 100 according to the invention, shown in Fig. 1, comprises a pipeline 110 in which a Coriolis mass flow meter 120 and a differential pressure flow meter 130 are installed in a vertical orientation in a hydraulic series connection. The Coriolis mass flow meter 120 has two parallel measuring tubes 122, 124 for guiding a medium flowing in the pipeline. Coriolis mass flow meters with only one measuring tube or four measuring tubes are equally suitable for carrying out the invention. The Coriolis mass flow meter 120 further comprises a measuring and operating circuit 142 for exciting and evaluating measuring tube oscillations and for determining at least one initial mixture density value and a mass flow measurement value.The differential pressure flow meter 130 has a Venturi tube 132 as a differential pressure sensor, which is connected in series with the measuring tubes 122, 124 of the Coriolis mass flow meter, and a measuring and operating circuit 143, which is configured to determine at least one flow-dependent differential pressure measurement value. The measuring point 100 further comprises a calculation arrangement 140, which in the simplest case includes the measuring and operating circuits 142, 143 of the two flow meters connected via a communication path, in particular a data bus. Optionally, the calculation arrangement 140 further comprises an additional flow computer 144 which is connected to the two measuring and operating circuits 142, 143 via a communication path, in particular a data bus.If no flow computer is available, it is advantageous for the calculation of the final mixture density value to be performed by the measuring and operating circuit 142 of the Coriolis mass flow meter 120. This final mixture density value is then transmitted to the measuring and operating circuit 143 of the differential pressure flow meter 130, which then calculates a flow rate based on a differential pressure measurement and the final mixture density value. If a flow computer 144 is available, process steps which perform calculations based on primary measured variables, such as the initial mixture density value, the mass flow rate measurement of the Coriolis mass flow meter, and the differential pressure measurement, can be carried out by the flow computer 144.

[0029] An embodiment of the method according to the invention will now be explained with reference to Fig. 2.

[0030] The method 200 according to the invention comprises determining 210 a mixture density start value based on at least one natural frequency of a bending vibration mode of the at least one measuring tube. For this purpose, a bending vibration mode of the at least one measuring tube is excited, and the associated natural frequency is determined, for example, by amplitude maximization or phase control. Details of this are known to those skilled in the art. The method 200 further comprises determining 220 a mass flow rate measurement value dm / dt, which is carried out in a known manner based on a phase or time difference between the vibration signals of an inlet-side and an outlet-side vibration sensor. To determine a correct mixture density measurement value, a gas volume fraction start value is first determined 230. s based on the initial mixture density value p sand a liquid density value pi, where any deviation of the initial mixture density value from the liquid density value pi is assumed to be caused exclusively by a gas volume fraction of the mixture with a density of zero. For pressure ranges up to a few MPa, this assumption leads to satisfactory results. The initial gas volume fraction value s In this case, this corresponds to a relative density deviation of the initial mixture density value p. s from the liquid density value pi, i.e.:

[0031] Pl Ps a s = -

[0032] pi

[0033] The inventive method 200 further comprises determining 240 a gas volume fraction final value at based on the gas volume fraction initial value s and the mass flow rate measurement value dm / dt. The final gas volume fraction value at is initially proportional to the initial gas volume fraction value. sapplied, and then divided by a correction factor k, which takes the following form: where kmax (as) is a linear function of a power of the initial gas volume fraction value s is, which in particular has the following form: where kmax > 1 , where k max < 2.

[0034] The flow rate-dependent factor S(dm / dt) is in particular a sigmoidal or logistic function whose argument is a linear function of the mass flow rate measurement. Furthermore, the argument of the sigmoidal or logistic function is preferably a linear function of the reciprocal of the fluid density value π, whereby the argument is, to a first approximation, proportional to the volumetric flow rate or the flow velocity. In principle, any type of sigmoidal function is suitable, for example:

[0035] SW = 77^ + c -

[0036] S(x) = tanh(x) + C, or where C is a constant and where x is a linear function of a quotient of mass flow rate measurement dm / dt and liquid density value pi, i.e.

[0037] . dm / dt x = k - 1- z ,

[0038] Pl where k and z are constants.

[0039] One embodiment of the sigmoid function has the following form: where 3 < k < 6 and -6 < z < -3.

[0040] In a final step, the mixture density measurement value p is determined. m by multiplying the gas volume fraction final value with the liquid density value pm.

[0041] Optionally, the method 200 for flow measurement can be further developed based on a flow-dependent differential pressure measurement value, in particular on a Venturi tube as a differential pressure transmitter, which is arranged in series with the Coriolis mass flow meter, with which the mixture density measurement value p mis determined. In this case, the acquisition of a differential pressure measurement Ap and calculation follows.

[0042] 270 of a flow rate measurement as a function of the differential pressure measurement Ap and the

[0043] Mixture density measurement p m .

[0044] Figures 3a, 3b, and 3c show relative density measurement errors (pmix - pt) / pt as a function of mass flow rate for different gas loadings, where the gas loadings cover the same value ranges in all three diagrams. Here, pt denotes the actual or true mixture density, and pmix the mixture density measurement determined according to the respective method.

[0045] In Fig. 3a, pmix = ps. This shows the relative density measurement error of the initial mixture density value. It is obvious that the initial mixture density value p s is unsuitable as a measurement value for mixture density over the entire flow range, since the mixture density ps is systematically underestimated with increasing mass flow.

[0046] In Fig. 3b, pmix = ps = i represents the relative density measurement error of a mixture density measurement where the correction term k has no flow dependence, i.e., the sigmoid function S is permanently set to 1. In this case, the correction term has the maximum value k = max, independent of the flow rate. This corresponds to Hemp's approach. However, this approach is also unsuitable for use as a mixture density measurement over the entire flow rate range.

[0047] In Fig. 3c, pmix = p m Accordingly, the relative density measurement error of the mixture density measurement value determined according to the invention is p m shown. Accordingly, the latter is suitable to serve as a mixture density measurement value over the entire flow range.

Claims

Patent claims 1. Method for determining a mixture density measurement value p m a gas-laden liquid by means of a Coriolis mass flow meter with a measuring tube for guiding the gas-laden liquid, comprising: Determine (210) a mixture density start value based on at least one natural frequency of a bending vibration mode of the measuring tube; Determining (220) a mass flow rate measurement; Determining (230) a gas volume fraction starting value oc s based on the initial mixture density value p s and a liquid density value pi Determining (240) a final gas volume fraction value at based on the initial gas volume fraction value a s , and the mass flow rate; and (250) determining the mixture density value based on the liquid density value and the gas volume fraction final value ar.

2. The method according to claim 1, wherein the initial gas volume fraction value ocs is proportional to the relative deviation of the initial mixture density value p s from the liquid density value pi, i.e.: Pl Ps er«. = - pi 3. The method of claim 1 or 2, wherein the final gas volume fraction value ar is proportional to the initial gas volume fraction value oc. s and inversely proportional to a flow-dependent correction term, which is a monotonically increasing function of the mass flow rate measurement, and in particular assumes a value less than 1.1 when the flow rate is zero.

4. Method according to claim 3, wherein the correction term comprises a sigmoid function or a logistic function whose argument is a linear function of the mass flow measurement.

5. Method according to claim 3 or 4, wherein the correction term comprises a sigmoid function or a logistic function whose argument is a linear function of the reciprocal of the liquid density value.

6. Method according to any one of claims 3 to 5, wherein the flow-dependent correction term k(dm / dt) has the following form: ( where kmax is a monotonically decreasing function of the initial gas volume fraction oc s is, Where kmax ■ > 1 , where kmax < 2, and where k(dm / dt) tends towards kmax for maximum mass flow measurements.

7. The method of claim 6, wherein the flow-dependent correction term kmax is a linear function of a power of the initial gas volume fraction value, which in particular has the following form: where e is a negative exponent with a magnitude less than 1, and where a and c are constants.

8. Method according to one of the preceding claims, wherein the mixture density measurement value is determined as the product of the gas volume fraction final value and the liquid density value.

9. Method according to any of the preceding claims, wherein the liquid density value is determined in a state of the liquid in which the liquid is free of free gas bubbles.

10. Method according to any of the preceding claims, wherein the liquid, when free of free gas bubbles, has a viscosity of not more than 40 cP, in particular not more than 25 cP.

11. Method for determining a flow rate measurement using a differential pressure measuring point comprising a differential pressure sensor and a Venturi tube as an intermediate pressure transmitter for the differential pressure sensor, wherein the Venturi tube is through which a gas-laden liquid flows, and wherein the Venturi tube is equipped with a A Coriolis mass flow meter is connected in series, the method comprising: Determining a mixture density measurement value using the Coriolis mass flow meter by a method according to one of the preceding claims; Acquiring (260) a differential pressure measurement Ap using the differential pressure sensor; and Determine (270) a flow rate measurement for the flow through the Venturi tube based on the differential pressure measurement Ap and the mixture density measurement.

12. Measuring point comprising a Coriolis mass flow meter; a differential pressure measuring device with a Venturi measuring tube, as a differential pressure sensor which is connected in series with the Coriolis mass flow meter; and a calculation arrangement for carrying out the method according to claim 10.

13. Measuring point according to claim 11, wherein the Coriolis mass flow meter has an inlet opening and an outlet opening for the gas-laden liquid, wherein a flow axis is defined which passes through a center of a cross-section of the inlet opening and a center of a cross-section of the outlet opening, wherein the flow axis deviates from the vertical by no more than 30°, for example no more than 15° and in particular no more than 5°.

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