METHOD FOR DETERMINING A MASS FACTOR OF THE GAS PHASE AND / OR THE MASS FLOW RATE OF THE GAS PHASE, OF A MULTIPLASING MEDIUM FLOWING IN A MEASURING TUBE WITH A LIQUID PHASE AND A GAS PHASE AND MEASURING SENSOR THEREBY

Description

[0001] The present invention relates to a method for determining a mass fraction of the gas phase of a multiphase medium flowing in a measuring tube, comprising a liquid phase and a gas phase, and measuring instruments for this purpose.

[0002] It is known to characterize a mass flow of a multiphase medium by means of differential pressure measurements, as disclosed, for example, in the American patent application US 2011 / 0259119 A1, which teaches a determination of the Lockhart-Martinelli parameter by measuring two pressure drops in a V-cone arrangement, wherein the first measurement includes the V-cone tip and the second measurement is carried out several pipe diameters apart in the trailing section of the V-cone.

[0003] On the one hand, the teaching of the above prior art sacrifices measurement accuracy, and on the other hand, it requires large measuring arrangements. The object of the present invention is to remedy this. This object is achieved according to the invention by the method according to independent claim 1 and the device according to independent claim 8.

[0004] The method according to the invention serves to determine a mass fraction of the gas phase and / or the mass flow rate of the gas phase of a multiphase medium with a liquid phase and a gas phase flowing in a measuring tube, wherein the medium comprises, for example, a vapor, in particular saturated vapor, wherein the measuring tube has a separation edge exposed to the flow of the medium and at least three pressure tap points, wherein the influence of the separation edge on the flow of the medium is different at the at least three pressure tap points, wherein the method comprises: determining a first pressure drop between a first and a second of the at least three pressure tap points, each exposed to the flowing medium; determining a second pressure drop between two pressure tap points, each exposed to the flowing medium.wherein one of the pressure tap points for determining the second pressure drop is a third of the at least three pressure tap points; and determining a value of the mass fraction of the gas phase and / or a value of the mass flow rate of the gas phase as a function of the first pressure drop and the second pressure drop, wherein the first pressure tap point is arranged in the flow direction of the medium upstream of the separation edge, wherein both the second pressure tap point and the third pressure tap point are positioned downstream of the separation edge with respect to the flow direction, wherein the second pressure tap point is arranged in a surface section of a solid whose normal vector forms an angle of not more than 30° with a longitudinal axis of the measuring tube, for example not more than 15° and in particular not more than 5°, and wherein the third pressure tap point is arranged in a surface section of a solid,whose normal vector with a cross-section through the measuring tube at the location of the third pressure tap point encloses an angle of no more than 20°, for example no more than 10° and in particular no more than 5°, wherein the second pressure tap point is positioned at or near the position of a pressure minimum, such that the pressure at the second pressure tap point is no more than 10%, in particular no more than 5%, of the pressure drop between the first pressure tap point and the second pressure tap point above the pressure minimum.

[0005] The position of the pressure minimum can be, for example, in the center of the flow shadow of a V-cone or in the flow shadow of a radial step of a pipe expansion.

[0006] According to the invention, the third pressure tap point is positioned in the flow direction after the second pressure tap point at or near the position of a pressure maximum, in particular the next pressure maximum, so that the pressure at the second pressure tap point is no more than 6%, in particular no more than 3%, of the pressure drop between the first pressure tap point and the second pressure tap point below the pressure maximum.

[0007] The position of the pressure maximum can occur, particularly on the pipe wall, with no or minimal axial offset compared to the position of the pressure minimum.

[0008] In a further development of the invention, the value of the mass fraction of the gas phase and / or the mass flow rate is determined as a function of a strictly monotonic function of the ratio of the first pressure drop to the second pressure drop. This strictly monotonic function of the ratio of the first pressure drop to the second pressure drop can, for example, be the ratio itself, its logarithm, or the difference of the two pressure drops divided by the sum of the two pressure drops. The exact form of the function is irrelevant, as long as it defines a one-to-one relationship to the quotient of the first pressure drop divided by the second pressure drop.

[0009] In connection with the present invention, first and second pressure drops were determined for various mass fractions of a gas phase in a gas-liquid mixture at different mass flow rates for different sensor types. The resulting quotients depend on the mass fraction of the gas phase and are essentially independent of the flow rate. Thus, the mass fraction of the gas phase can be determined using the quotient or a strictly monotonically dependent function thereof.

[0010] In a further development of the invention, the method comprises determining the mass flow rate of the gas phase, which is determined as a function of at least one of the pressure drops, a density value of the gas phase, and either the mass fraction of the gas phase or a strictly monotonic function of a ratio of the first pressure drop to the second pressure drop.

[0011] The mass flow rate can be expressed, for example, as the product of a base term, which is proportional to the kinetic energy of the gas phase, and a loss factor, which describes losses due to friction and vortex separation. The base term depends, in a manner known per se, on the pressure drop and the density of the gas phase. Specifically, the base term is determined to be proportional to the square root of the product of one of the pressure drops and a density value of the gas phase. Furthermore, the geometric characteristics of the sensor are incorporated into the base term.

[0012] The loss factor depends on the mass fraction of the gas phase or on a strictly monotonic function of the ratio of the first pressure drop to the second pressure drop and must be determined for different types of sensors.

[0013] In a further development of the invention, the mass flow rate of the gas phase is determined as a function of the Froude number of the gas phase. The Froude number also influences the loss factor and can be considered as a further parameter in the type-specific determination of the loss factors.

[0014] In a further development of the invention, the density value of the gas phase is determined on the basis of an absolute pressure measurement and, if it is not saturated steam, a temperature measurement, in particular at the first pressure tap point, at this position the pressure measurement is not yet affected by the effect of the separation edge and should yield a representative pressure measurement value for the gas phase in the measuring tube.

[0015] The above positioning achieves the highest possible sensitivity for determining the mass fraction of the gas phase. Positioning the third pressure tap point several diameters away from the separation edge, as in US 2011 / 0259119 A1, is far below the maximum and thus sacrifices sensitivity and measurement accuracy.

[0016] The displacer body can be designed in a substantially rotationally symmetrical manner with respect to the longitudinal axis of the measuring tube and in particular as a V-cone.

[0017] The measuring device according to the invention for determining a mass fraction of the gas phase and / or the mass flow rate of the gas phase of a multiphase medium with a liquid phase and a gas phase flowing in a measuring tube, in particular for carrying out the method according to the invention, comprises: a measuring tube which has a measuring tube body with an inner surface, which inner surface defines a lumen for guiding the flowing medium, wherein the measuring tube has a longitudinal direction Z in which the medium is to be guided; a cut-off edge which is arranged in the lumen; at least three pressure tap points, the positions of which differ from each other with respect to the cut-off edge; several pressure sensors for detecting pressure measurements at each of the pressure tap points and / or for detecting pressure differences between each of the pressure tap points; a measuring and operating circuit,which is set up to determine a first pressure drop between a first and a second of the at least three pressure tap points, each exposed to the flowing medium; to determine a second pressure drop between two pressure tap points, each exposed to the flowing medium, wherein one of the pressure tap points for determining the second pressure drop is a third of the at least three pressure tap points; and to determine a value of the mass fraction of the gas phase and / or the mass flow rate of the gas phase of the medium as a function of the first pressure drop and the second pressure drop, assuming a multiphase medium containing a liquid phase and a gas phase, wherein the first pressure tap point is arranged upstream of the separation edge with respect to a longitudinal direction of the measuring tube in the flow direction.wherein both the second pressure tap point and the third pressure tap point are positioned downstream of the separation edge in the flow direction, wherein the second pressure tap point is located in a surface section of a solid whose normal vector forms an angle of no more than 30° with a longitudinal axis of the measuring tube, for example no more than 15° and in particular no more than 5°, and wherein the third pressure tap point is located in a surface section of a solid whose normal vector forms an angle of no more than 20° with a cross-section through the measuring tube at the location of the third pressure tap point, for example no more than 10° and in particular no more than 5°, wherein the second pressure tap point is positioned at or near the position of a pressure minimum, such that the pressure at the second pressure tap point is no more than 10%,in particular, not more than 5% of the pressure drop between the first pressure tap point and the second pressure tap point above the pressure minimum, wherein the third pressure tap point (116) is located in the longitudinal direction of the measuring tube at approximately the same position as the second pressure tap point (114), wherein the second pressure tap point is positioned at or near the position of a pressure minimum for a flow Re = 4000 with the tube diameter as the characteristic length, such that the pressure at the second pressure tap point is not more than 10%, in particular not more than 5%, of the pressure drop between the first pressure tap point and the second pressure tap point above the pressure minimum.

[0018] According to the invention, the third pressure tap point is positioned in the flow direction after the second pressure tap point at or near the position of a local pressure maximum for a flow of Re = 4000 with the pipe diameter as the characteristic length, so that the pressure at the second pressure tap point is no more than 6%, in particular no more than 3%, of the pressure drop between the first pressure tap point and the second pressure tap point below the pressure maximum.

[0019] The invention will now be explained in more detail with reference to the exemplary embodiments shown in the drawings, which illustrate: Fig. 1 : a schematic representation of a first embodiment of a measuring sensor according to the invention; Fig. 2 : an exemplary diagram showing the mass fraction of the gas phase as a function of the quotient of the pressure drops; Fig. 3 : an exemplary diagram showing a damping factor as a function of the mass fraction of the gas phase and the Froude number; Fig. 4 : a flowchart for an embodiment of the method for determining the mass fraction of the gas phase; Fig. 5a : a flowchart for an embodiment of the method for determining the mass flow rate of the gas phase; and Fig. 5b : a modification of the exemplary embodiment of the method for determining the mass flow rate of the gas phase.

[0020] The in Fig. 1 The illustrated first embodiment of a measuring sensor 100 according to the invention comprises a cylindrical measuring tube 110 with a displacement body 120 mounted therein, which here is designed in the form of a so-called V-cone, i.e., the effective part of the displacement body 120 comprises a first conical section 122 with a diameter increasing in the flow direction or in the direction of the longitudinal axis Z of the measuring tube and a second conical section 124 adjoining it with a diameter decreasing in the flow direction Z. The common circumference of the two conical sections 122, 124 forms a separation edge 126 at which vortices of a medium flowing through the measuring tube 110 detach.The sensor 100 has a first pressure tap point 112 upstream of the displacer body 120 in the flow direction Z. An absolute pressure gauge p1 is connected to this first tap point. The gauge is located approximately one diameter of the measuring tube 110 from the separation edge 126 and is largely unaffected by the effects of the displacer body. A second pressure tap point 114 is located at the apex of the second conical section 124, i.e., in the flow shadow of the displacer body 120 on the axis of the measuring tube. A pressure minimum is to be expected at the location of the second pressure tap point 114. A third pressure tap point 116 is located downstream of the separation edge 126 on the wall of the measuring tube 110. The third pressure tap point 116 is located approximately at the same position along the longitudinal direction of the measuring tube as the second pressure tap point 114.

[0021] A first differential pressure sensor Δp ​​1 and a second differential pressure sensor Δp ​​2 are connected to the second and third pressure tap points, respectively, each detecting the pressure difference relative to the first pressure tap point. The sensor 100 further comprises a measuring and operating circuit 130 for operating the pressure sensors and for evaluating their measured values ​​by means of a microprocessor 132, in particular according to the method according to the invention.

[0022] The diagram in Fig. 2 The dashed line shows the mass fraction of the gas phase to the total mass of the flowing medium as a function γ(Δp₁, Δp₂), which is defined as γ(Δp₁, Δp₂) = (Δp₁ / Δp₂) ≤ 0.5. To create the curve, defined, non-condensable mass flows of a gas, in particular air, were mixed with defined mass flows of a liquid at room temperature at different flow rates to produce a variety of mass fractions X of the gas phase and passed through the sensor to be characterized. The resulting pressure drops Δp₁, Δp₂ were recorded. The specified mass fractions X were then modeled as a function of γ(Δp₁, Δp₂). Curve 4 shows a resulting fit for X(γ(Δp₁, Δp₂)). The corresponding model is stored in the sensor, thus enabling the determination of the mass fraction of the gas phase during measurement operation based on the two pressure drops Δp1 and Δp2.This is of particular interest for the characterization of (saturated) steam streams, especially in power plants.

[0023] To determine the mass flow rate of a flowing medium with a gas phase and a liquid phase based on a differential pressure measurement, a basic mass flow rate, determined assuming a lossless Bernoulli effect, must be multiplied by a correction factor that takes into account pressure losses depending on the composition of the medium and the Froude number of the medium.

[0024] The diagram in Fig. 3 The graph shows correction factors that were experimentally determined for a sensor being characterized, based on defined mass flows with defined mass fractions of the gas phase. The dashed curve corresponds to a Froude number of 140, the dotted curve to a Froude number of 100, and the solid curve to a Froude number of 60.

[0025] The Froude number Fr was determined for the various mass flows according to: Fr = c sv , g g ⋅ h

[0026] The Froude number is calculated using an empty tube velocity of the gas phase. c sv,g The gas phase is formed in front of the flow obstruction and a gap height h, which is defined as a characteristic length. The empty pipe velocity of the gas phase is determined from the mass flow rate of the gas phase and its density, assuming that the entire pipe cross-section is available for the flowing gas phase. For example, in the first embodiment, the gap height is derived from the pipe diameter and the diameter of the V-cone at the separation edge. h = D Rohr − D VCone 2

[0027] The basic mass flow rate dm / dt g,b of the gas phase according to Bernoulli is, for the sensor of the first embodiment: m ˙ g , b = A 1 − β 4 ⋅ 2 ⋅ ρ p 1 ⋅ Δp 1

[0028] A denotes the narrowest cross-sectional area, β the diameter ratio of measuring tube and V-cone, and ρ(p 1 ) the density at the first pressure tap.

[0029] The actual mass flow rate of the gas phase should be obtained from the basic mass flow rate by multiplying it by the correction factor K 1. m ˙ g = K 1 x Fr ⋅ m ˙ g , b

[0030] Accordingly, when characterizing a sensor, the correction factors K are to be determined based on given actual mass flow rates and on the basis of measured baseline mass flow rates according to: K 1 x Fr = m ˙ g m ˙ g , b

[0031] The determined correction factors are stored in a data memory of the measuring and operating circuit of a sensor.

[0032] The methods according to the invention can be carried out using the previously described sensors and the data obtained for their characterization, as described below.

[0033] The in Fig. 4 illustrated embodiment of a method 300 for determining the mass fraction of the gas phase of a two-phase medium which contains a liquid phase and a gas phase, and which, for example, includes a measuring sensor according to Fig.1 The process begins with the detection of a first pressure drop Δp₁ between a first pressure tap point and a second pressure tap point, as well as a second pressure drop Δp₂ between the first pressure tap point and a third pressure tap point. This is followed by the determination of a strictly monotonic function γ(Δp₁, Δp₂) of the first and second pressure drops, which is given here as the square root of the quotient Δp₁ / Δp₂. In a final step, the mass fraction X(γ(Δp₁, Δp₂)) is determined based on a stored model, such as that found, for example, in Fig. 2 is shown.

[0034] The in Fig. 5 illustrated embodiment of a method 400 for determining the mass flow rate of the gas phase of a saturated vapor which contains a liquid phase (condensate) and a gas phase, and for example a measuring sensor according to Fig.1 The process begins with the measurement of an absolute pressure p1 at a first pressure tap point unaffected by any flow obstruction, a first pressure drop Δp1 between the first and a second pressure tap point, and a second pressure drop Δp2 between the first and a third pressure tap point. This is followed by the determination of the density of the gas phase ρ(p1). In a saturated steam application, density determination based on pressure is sufficiently accurate; for other media, an additional temperature measurement at the first pressure tap point may be necessary.

[0035] With the density ρ(p 1 ) and the pressure drop Δp 1 available, the conditions are met to determine a basic mass flow rate for the gas phase according to Bernoulli: m ˙ g , b = A 1 − β 4 ⋅ 2 ⋅ ρ p 1 ⋅ Δp 1

[0036] However, the basic mass flow rate is not representative, as the measured pressure drop Δp1 is influenced by the second phase, in this case condensate. Determining a correction factor K1 requires first determining the Froude number, which is calculated based on the basic mass flow rate.

[0037] The determination of the correction factor K 1 as a function of the mass fraction X of the gas phase X and the Froude number follows, whereby the determination of the mass fraction X of the gas phase is carried out as in the procedure according to Fig. 4 The correction factor K1 is derived from the determined input variables based on the modeled dependency, as exemplified in Fig. 3 is shown.

[0038] The actual mass flow rate dm / dt of the gas phase is obtained by multiplying 460 of the basic mass flow rate by the correction factor K 1 .

[0039] Based on the actual mass flow rate dm / dt of the gas phase, the Froude number is recalculated.

[0040] If a subsequent test 480 reveals that the Froude number deviates from the previously calculated Froude number by more than a limit value G, the procedure steps 450 ff are repeated iteratively until a newly determined Froude number deviates from the previous Froude number by no more than the limit value G. Then an actual value of the mass flow rate is output, and the procedure begins again by acquiring new input variables.

[0041] One in Fig. 5b presented modification of the procedure according to Fig. 5aThis concerns the determination of the correction factor K 1. This can be calculated in one step of 450' as a function of γ(Δp 1 , Δp 2 ) and the Froude number without explicitly determining the mass fraction X of the gas phase.

Claims

1. A method (300, 400) for determining a mass fraction of the gas phase and / or the mass flow rate of the gas phase of a multi-phase medium flowing in a measuring tube with a liquid phase and a gas phase, wherein the medium has a steam, for example, in particular saturated steam, wherein the measuring tube has a tear-off edge which the medium flows toward, and at least three pressure tapping points, wherein the effect of the tear-off edge on the flow of the medium differs at the at least three pressure tapping points, wherein the method comprises: Determining a first pressure drop (310; 410) between a first and a second of the at least three pressure tapping points, each of which is exposed to the flowing medium; Determining a second pressure drop (310; 410) between two pressure tapping points, each of which is exposed to the flowing medium, wherein one of the pressure tapping points for determining the second pressure drop is a third of the at least three pressure tapping points; and Determining a value of a mass fraction (330) of the gas phase and / or the mass flow rate (460) of the gas phase based on the first pressure drop and the second pressure drop, wherein the first pressure tapping point is arranged in front of the tear-off edge in the flow direction of the medium, and wherein both the second pressure tapping point and the third pressure tapping point are positioned behind the tear-off edge relative to the flow direction, wherein the second pressure tapping point is arranged in a surface section of a solid body whose normal vector creates, together with a longitudinal axis of the measuring tube, an angle not exceeding 20°, for example not exceeding 10°, and in particular not exceeding 5°, and wherein the third pressure tapping point is arranged in a surface section of a solid body whose normal vector creates, together with a cross-section through the measuring tube at the location of the third pressure tapping point, an angle not exceeding 20°, for example not exceeding 10°, and in particular not exceeding 5°, wherein the second pressure tapping point is located at or near the position of a pressure minimum, so that the pressure at the second pressure tapping point is equal to no more than 10%, in particular no more than 5%, of the pressure drop between the first pressure tapping point and the second pressure tapping point above the pressure minimum, characterized in that the third pressure tapping point is approximately located at the same position as the second pressure tapping point in the longitudinal direction of the measuring tube, and in that the third pressure tapping point downstream of the second pressure tapping point in the flow direction is located at or near the position of a pressure maximum, in particular the closest one, so that the pressure at the second pressure tapping point is equal to no more than 6%, in particular no more than 3%, of the pressure drop between the first pressure tapping point and the second pressure tapping point below the pressure maximum.

2. The method (300) as claimed in claim 1, wherein the value of the mass fraction of the gas phase and / or the mass flow rate of the gas phase is / are determined based on a strictly monotonic function of a ratio of the first pressure drop to the second pressure drop.

3. The method as claimed in claim 1 or 2, further comprising determining the mass flow rate of the gas phase, which is determined based on at least one of the pressure drops, a density value of the gas phase, and either the mass fraction of the gas phase or a strictly monotonic function of a ratio of the first pressure drop to the second pressure drop.

4. The method (400) as claimed in claim 3, wherein the mass flow rate of the gas phase is also determined based on the Froude number of the medium.

5. The method as claimed in claim 3 or 4, wherein the mass flow rate is determined proportionally to the root of the product from one of the pressure drops and a density value of the gas phase of the medium.

6. The method as claimed in claim 5, wherein the density value of the gas phase is determined on the basis of an absolute pressure measurement and, if necessary, a temperature measurement, in particular at the first pressure tapping point.

7. The method as claimed in one of the preceding claims, wherein a displacement body is arranged in the measuring tube, wherein the tear-off edge runs in a cross-sectional plane of the measuring tube with a maximum cross-sectional circumference of the displacement body.

8. A measuring transducer (100) for determining a mass fraction of the gas phase and / or the mass flow rate of the gas phase of a multi-phase medium flowing in a measuring tube with a liquid phase and a gas phase, in particular for carrying out a method as claimed in one of the preceding claims, comprising: A measuring tube (110), which has a measuring tube body with an inner lateral surface, said inner lateral surface delimiting a lumen for conducting the flowing medium, wherein the measuring tube has a longitudinal direction Z in which the medium is to be conducted; A tear-off edge (126), which is arranged in the lumen; At least three pressure tapping points (112, 114, 116), whose positions relative to the tear-off edge (126) change depending on each other; Multiple pressure sensors for detecting pressure measured values (p1) at each of the pressure tapping points (112, 114, 116) and / or for detecting pressure differences (Δp1, Δp2) between two pressure tapping points (112, 114, 116) in each case; A measuring and operating circuit (130), which is configured to determine a first pressure drop (Δp1) between a first and a second of the at least three pressure tapping points, each of which is exposed to the flowing medium; to determine a second pressure drop (Δp2) between two pressure tapping points, each of which is exposed to the flowing medium, wherein one of the pressure tapping points for determining the second pressure drop is a third of the at least three pressure tapping points; and to determine a value of the mass fraction (X) and / or the mass flow rate of the gas phase of the medium based on the first pressure drop and the second pressure drop, assuming that the medium has multiple phases, including a liquid phase and a gas phase, wherein the first pressure tapping point is arranged in front of the tear-off edge based on the longitudinal direction of the measuring tube in the flow direction, and wherein both the second pressure tapping point and the third pressure tapping point are positioned behind the tear-off edge (126) in the flow direction, wherein the second pressure tapping point is arranged in a surface section of a solid body whose normal vector creates, together with a longitudinal axis of the measuring tube, an angle not exceeding 20°, for example not exceeding 10°, and in particular not exceeding 5°, and wherein the third pressure tapping point is arranged in a surface section of a solid body whose normal vector creates, together with a cross-section through the measuring tube at the location of the third pressure tapping point, an angle not exceeding 20°, for example not exceeding 10°, and in particular not exceeding 5°, wherein the second pressure tapping point (114) is located at or near the position of a pressure minimum for a flow where Re = 4000 with the tube diameter as the characteristic length, so that the pressure at the second pressure tapping point is equal to no more than 10%, in particular no more than 5%, of the pressure drop between the first pressure tapping point and the second pressure tapping point above the pressure minimum, characterized in that the third pressure tapping point (116) is approximately located at the same position as the second pressure tapping point (114) in the longitudinal direction of the measuring tube, and in that the third pressure tapping point downstream of the second pressure tapping point in the flow direction is located at or near the position of a local pressure maximum for a flow of Re = 4000 with the tube diameter as the characteristic length, so that the pressure at the second pressure tapping point is equal to no more than 6%, in particular no more than 3%, of the pressure drop between the first pressure tapping point and the second pressure tapping point below the pressure maximum.