Pressure transducer for measuring the flow of gaseous and liquid fluids and corresponding method

Description

[0001] The invention relates to a differential pressure sensor for measuring flows of gaseous and liquid fluids and a corresponding method for determining the measuring accuracy of a differential pressure sensor. background

[0002] The invention relates to a differential pressure sensor for measuring the flow of gaseous and liquid fluids, comprising a tubular body arranged between a first and a second connection element. A fluid flows into the tubular body through the first connection element and out through the second connection element. The tubular body has a first section with a first diameter and, downstream of this, a second section with a second diameter that is smaller than the first diameter. A first pressure measuring device is arranged in the first section and a second pressure measuring device in the second section. A differential pressure between the first and second sections can be determined by means of the pressure measuring devices. Such a differential pressure sensor is already known from German patent applications DE 10 2017 001 049 A1 and US patent 2022 / 178590 A1.

[0003] Flow meters for gaseous and liquid fluids that rely on differential pressure have been used for decades. They utilize a pressure difference across constrictions in pipes or containers to determine the flow rate. Constrictions can be created using nozzles, orifices, Venturi throttles, and baffles such as cones, baffles, or wedges. Due to the constriction, the velocity of the fluid increases while the pressure drops between two defined points where pressure sensors are positioned. The pressure difference is proportional to the square of the flow velocity. With evaluation electronics, the volumetric flow rate per unit time can be determined, and, knowing the density of the flowing medium, the mass flow rate can be calculated. Details are defined in the ISO 5167-1 to 6 series of standards, specific to the geometry and application method.Advantages of the method include its simple design, long-standing operational experience, and wide range of applications in various industrial sectors with diverse fluids. Disadvantages include limited measurement dynamics and sensitivity to pressure and density fluctuations, as well as geometric changes due to wear, abrasion, contamination, erosion, corrosion, and changing pipe or vessel roughness during operation.

[0004] Typical application areas where differential pressure methods are used for flow measurement include, among others... High-pressure natural gas measuring systems in transport infrastructure; measurement of fluids at high temperatures (steam in power plants); CO2 transport systems for determining CO2 flows in gaseous, liquid and supercritical states; determination of compressed air flows

[0005] Differential pressure transducers determine flows at flow velocities up to 90 m / s. High flow velocities lead to higher differential pressures, resulting in more precise measurements. However, the associated permanent pressure loss negatively impacts the cost-effectiveness of the measurement. Therefore, it is understandable that designs are preferred that result in a low permanent pressure loss while maintaining an acceptable differential pressure. The differential pressure transducer described in the aforementioned document DE 10 2017 001 049 A1 represents such a design.

[0006] From a metrological perspective, the focus of all measurement processes is on the measurement deviations of the results. These deviations consist of systematic and random deviations. Systematic deviations can be detected on test benches. In such cases, the measured value is corrected for the known systematic deviations. If necessary, gas meters are readjusted on test benches, and their remaining deviation is verified with further test bench measurements (part of the calibration process). In commercial and official transactions, measurement results must fall within permissible deviations according to the European Measuring Instruments Directive. These limits, also called error limits, are defined in the Measuring Instruments Directive and its national implementations—in Germany, in the Measurement and Calibration Ordinance (MessEV). These are device-specific parameters that must be demonstrably adhered to during operation.

[0007] The measurement result is expressed by a measured value and its associated measurement uncertainty. This uncertainty can be positive or negative. Measurement uncertainty represents random influences on the measurement process; for example, randomly occurring disturbances such as particles in the fluid flow, pressure surges, temperature fluctuations, and variations in the medium's properties manifest as temporary measurement deviations. These are not instrument-specific and are therefore assigned to the measured value in the form of an uncertainty value. If the measurement uncertainty increases, repeated measurements become less precise. As a guideline, the International Organization of Legal Metrology (OIML) requires that the measurement uncertainty not exceed one-third of the instrument's limit of error. Measurement uncertainties are documented in the calibration certificate.

[0008] High flow velocities in measuring instruments necessitate a minimum of accompanying substances in the flow to prevent abrasion, erosion, and corrosion. Recent simulations of particle and droplet entrainment in fuel gas flows show that particles and droplets are stirred up and carried along in natural gas at average flow velocities above 3 m / s and in hydrogen at approximately 10 m / s. English-language textbooks for pipeline and related system designers recommend not exceeding a flow velocity of 4 m / s for liquid media to avoid wear, abrasion, and erosion caused by solid accompanying substances. For gases, a limit of 5 m / s is suggested. Furthermore, empirically determined approximation equations are provided to estimate maximum flow velocities depending on factors such as mass and temperature.

[0009] To minimize abrasion, erosion, etc. caused by the flow medium, maximum average flow velocities of 10 m / s for natural gas and 40 m / s for hydrogen are recommended.

[0010] Furthermore, minimum flow rates specific to the medium are specified to reliably prevent water from condensing in moist media. Since process-related downtime cannot be ruled out, even low moisture content can lead to water droplets in the pipeline, connected equipment, or measuring instruments, and consequently, corrosion. Abrasion, erosion, or corrosion have a significant detrimental effect on measurement accuracy; the measurement stability required by the European Measuring Instruments Directive (MID) and the German Measurement and Calibration Ordinance (MessEV) may no longer be met.

[0011] Abrasive (scratching, scraping, micro-machining), erosive (removal of surface material), or corrosive (reactive decomposition) wear of the flow-contacting inner surface of the diffuser and confuser of the differential pressure sensor leads to a systematic measurement deviation (measurement accuracy, formerly measurement error) of the measuring device. This deviation gradually increases over the device's service life and can reach values ​​that are metrologically unacceptable. Traditionally, this phenomenon can only be detected through repeated calibrations and taken into account when determining the measured value. Unfortunately, whether a recalibration is necessary or can be postponed further can only be determined with the calibration results. If this is no longer possible due to wear, the measuring device must be replaced.

[0012] Experience shows that the accuracy requirements of commercial and official transactions are also adopted analogously for purely operational measurements. This is evident in operational measurements to ensure technical operational safety or in process controls to guarantee product quality. Summary

[0013] The object of the present invention is to further develop a differential pressure transmitter in such a way that measurement inaccuracies caused by abrasion, erosion and corrosion can be compensated for in the measurement process and a falsification of the measurement result can be prevented.

[0014] The problem is solved by the features of the independent claims. Advantageous embodiments are specified in the dependent claims.

[0015] Accordingly, it is provided that a source for emitting electromagnetic radiation, for example bundled light, a reflective surface and at least one photocell are arranged in the pipe body, which are aligned to each other in such a way that the radiation emitted by the source falls on the reflective surface and is reflected by it either directed or diffusely onto the photocell, wherein the differential pressure transmitter also has an evaluation unit by means of which a decrease and / or increase in the radiation intensity detected by the photocell can be determined.

[0016] The present invention can compensate for measurement inaccuracies caused by erosion and corrosion during measurement acquisition, thereby preventing falsification of the measurement result. This is achieved by 1. an optical device for the continuous determination of abrasion, erosion or corrosion; 2. an electronic assessment of the extent of this influence and, if permissible limits are exceeded, 3. the computational consideration of the influence on the measurement process.

[0017] The differential pressure sensor can receive information from a sensor within the measuring device that detects wear caused by abrasion, erosion, and corrosion. The influence of wear on the measured value can be determined in advance on certified test benches or with test equipment in accredited testing laboratories using test equipment or material samples, depending on the medium, its properties, and the operating conditions (if unknown). This information can then be recorded in operating manuals or stored electronically within the device itself. Based on the stored information, the measuring device can automatically decide to what extent the measured value should be corrected or, if the wear is still within acceptable limits, disregarded. This reduces or even eliminates the need for time-consuming intermediate tests, which are typically required for differential pressure sensors.The invention avoids questions regarding the influence of wear and tear and increases the confidence of the trading partners involved in the measurement result. This applies particularly to commercial or official transactions. Furthermore, the correction device integrated into the measuring instrument is considerably more cost-effective than removing the measuring instrument, operating a replacement measuring instrument, and / or (re-)calibrating it on a certified and accredited test bench for the relevant medium.

[0018] To determine the wear condition in the differential pressure sensor, the invention utilizes the different behavior of specular and diffuse reflection. An electromagnetic wave transparent to the process fluid, for example, a light signal, is directed to the location in the differential pressure sensor where, for example, the greatest abrasion, erosion, or corrosion is expected. This is typically the point with the highest flow velocity at the end of the confounder section. This location is made of a metallic, reflective surface. The reflection of the wave or light beam from the reflective inner surface can be described by the law of reflection (angle of incidence equals angle of reflection). If the reflective surface becomes rougher due to abrasion, erosion, or corrosion, specular reflection transitions into diffuse reflection. The reflecting surface develops many scattering centers, which, in total, result in the main backscattering occurring independently of the law of reflection.It then occurs perpendicular to the material. The diffuse scattering follows Lambert's law.

[0019] In its new state, the electromagnetic beam, particularly the light beam, is almost completely detected by the first photocell, which is positioned obliquely opposite the reflecting surface. The second photocell, positioned perpendicular to the reflecting surface, receives virtually no portion of the light wave scattered by the surface. During operation, the reflective surface becomes rougher, and the specular reflection increasingly transitions into diffuse reflection. This is detected by the second photocell, whose signal increases at the expense of the light incident on the photocell. Consequently, the ratio of the photocells' light output can be correlated with laboratory or test bench results regarding the wear of the reflective metallic surface. This allows the differential pressure sensor to determine permissible and impermissible wear values.Automatically correct the relationship between wear progress and measurement error or the size of the measurement accuracy based on stored test bench information.

[0020] The reflecting surface can be a mirror. Alternatively, it can be a polished metallic surface inside the tube. Any curvature due to the design is irrelevant to the reflection. Directional reflections also occur at concave mirrors, such as shaving mirrors or astronomical reflecting telescopes, or at convexly curved exterior mirrors of cars, i.e., also at a reflective metallic surface on the inner wall of the differential pressure sensor.

[0021] It may be possible to determine and correct the resulting systematic measurement error, or more accurately, the systematic measurement deviation, based on knowledge of abrasion, erosion, and corrosion on the inner surface of the differential pressure sensor, or the changes in the resulting wear. Systematic measurement deviations can be evaluated and quantified on test benches. It may be possible to experimentally record corresponding values ​​before field use and store them, for example, in tabular form in the differential pressure sensor's evaluation unit. This table can then be used to correct the measurement result during operation. For this purpose, a threshold value of a sensor in the differential pressure sensor can be monitored during operation. When the threshold value(s) are reached, subsequent measured values ​​are automatically corrected. As long as the threshold value is not exceeded, additional measurement deviations are accepted and disregarded.

[0022] It can be provided that at least one photocell is arranged symmetrically to the source with respect to the reflecting surface, such that the directed beam reflected by the reflecting surface strikes the photocell. The evaluation unit then detects when the radiation intensity detected by the photocell falls below a predetermined threshold. The decrease in the radiation intensity of the directed reflection detected by the diagonally opposite photocell corresponds to increasing wear of the reflecting surface, and the proportion of diffuse reflection increases.

[0023] The angle of incidence or reflection of the light beam can preferably be chosen so that as little scattered light from the directional reflection as possible falls into the second photocell. This means choosing the largest possible angles. However, the angles are structurally limited by the maximum length of the downflow section of the pipe body and are then calculated purely mathematically from the cotangent of the ratio of the downflow section length to the downflow section diameter.

[0024] Furthermore, it can be provided that at least one photocell is arranged perpendicular to the reflecting surface such that the main backscatter of the diffuse reflection reflected by the reflecting surface strikes the photocell, with the evaluation unit detecting when a predetermined threshold of the radiation intensity detected by the photocell is exceeded. The increase in the radiation intensity of the diffuse reflection detected by the photocell perpendicular to the reflecting surface is accompanied by increasing wear of the reflecting surface; the proportion of specular reflection decreases.

[0025] It is conceivable that the reflective surface is positioned between the first and second pressure measuring devices. Furthermore, the reflective surface can be located at a gravitational low point of the pipe body, relative to its circumference. This can be advantageous if the influence of contamination, e.g., from oils on the inner wall of the measuring device, is also to be taken into account. In principle, however, the reflective surface can also be positioned at other locations. The entire sensor assembly would then, figuratively speaking, be rotated and positioned symmetrically.

[0026] The reflective surface can be located in the pipe body, for example at the point with the highest flow velocity, for example in the second section with a reduced diameter compared to the first section, with respect to the flow direction.

[0027] Furthermore, the source of electromagnetic radiation can be located upstream of the first pressure measuring device. The photocell, positioned obliquely opposite the reflecting surface, can be located downstream of the second pressure measuring device. Depending on the design and dimensions, however, the photocell can also be positioned in front of or behind the pressure sensor.

[0028] It may be provided that if a predetermined threshold value of the radiation intensity detected by the photocell is exceeded and / or fallen below, a predetermined correction of the pressure difference or the determined flow rate determined by the differential pressure transmitter is made.

[0029] Furthermore, the reflective surface can be provided with a coating designed to determine the influence of a specific accompanying substance, such as moisture, on the measurement accuracy. Advantages include the ability to tailor abrasion, erosion, or corrosion to the flowing medium or accompanying substances. For example, precipitated moisture could cloud the coating, resulting in a more targeted and / or faster transition from specular to diffuse reflection.

[0030] The invention further relates to a differential pressure sensor for measuring the flow of gaseous and liquid fluids, comprising a pipe body arranged between a first and a second connection element, wherein a fluid flows into the pipe body through the first connection element and out through the second connection element, wherein the pipe body has a first section with a first diameter and, downstream of this, a second section with a second diameter, which is smaller than the first diameter, wherein a first pressure measuring device is arranged in the first section and a second pressure measuring device is arranged in the second section, wherein a differential pressure between the first and second sections can be determined by means of the pressure measuring devices, wherein a plurality of pressure sensors for detecting the local fluid pressure are arranged radially distributed around the circumference of the pipe in the inflow section or in the compensating pipe.which are connected to the evaluation unit, wherein the evaluation unit is configured to detect the swirl of the incoming fluid based on the pressure differences determined by the multiple pressure sensors or to correct the measurement result accordingly. Alternatively, it can be provided that an inflow section is arranged upstream of the first section, around the circumference of which the multiple pressure sensors for detecting the local fluid pressure are arranged radially distributed, which are connected to the evaluation unit, wherein the evaluation unit is configured to detect the swirl of the incoming fluid based on the pressure differences determined by the multiple pressure sensors around the circumference of the inflow section.

[0031] Separating the pressure sensors for measuring the differential pressure and those for detecting turbulence (swirl), which negatively affects the measurement result, is advantageous because the differential pressure and the pressure changes caused by swirl or turbulence can occur in different ranges or scales. This separation of the pressure sensors allows for both technical and economic optimization of the pressure sensors.

[0032] Pressure sensors improve measurement accuracy by detecting swirl in the process fluid, which can be caused by inlet disturbances (e.g., pipe bends or components in the connected pipes upstream of the measuring device). Arranging two or more sensors, such as pressure sensors, distributed around the circumference of the measuring tube results in dynamic data acquisition via multiple paths and increases the accuracy of determining flow disturbances caused by turbulence and swirl.

[0033] Furthermore, the use of two or more sensors allows for the optimization and verification of the automatic zero-point correction process. The current state of the art involves zero-point correction of the differential pressure transmitter while it is installed, achieved by isolating it from the process and opening a compensating line via a manual valve block. With two or more sensors, this correction can be performed when the medium is stationary. During the zero-point correction of the other sensor, one sensor monitors whether the process fluid is moving. Therefore, manual intervention is no longer necessary.

[0034] The invention further relates to a differential pressure sensor for measuring the flow of gaseous and liquid fluids, comprising a pipe body arranged between a first and a second connection element, wherein a fluid flows into the pipe body through the first connection element and out through the second connection element, the pipe body having a first section with a first diameter and, downstream of this, a second section with a second diameter, which is smaller than the first diameter, wherein a first pressure measuring device and a second pressure measuring device are arranged in the first section, and wherein a differential pressure between the first and second sections can be determined by means of the pressure measuring devices.wherein a measuring device for determining density fluctuations of the process fluid is arranged in an inflow section between the first connection element and the first section or in an outflow section between the second section and the second connection element of the differential pressure transmitter, comprising an ultrasonic transmitter and an ultrasonic receiver which are aligned to each other such that an ultrasonic pulse emitted by the ultrasonic transmitter through the process fluid can be received by the ultrasonic receiver, wherein the measuring device is connected to the evaluation unit which is configured to detect density fluctuations in the process fluid based on changes in the transit time of the emitted ultrasonic pulses.

[0035] In line with the requirements for a climate-neutral supply of fuel gases to the general public, the increased distribution of biogas and, in particular, hydrogen is being demanded. The supply of hydrogen to industry, both as a material and energy source, plays an increasingly important, and soon dominant, role in this. This is taken into account in the European and German hydrogen strategies. In the case of pipeline-based supply to the general public, this leads to an increased injection of hydrogen into the German and European gas networks. The production of renewable or climate-neutral hydrogen follows the supply of renewable electricity from solar and wind power plants. This supply is volatile and fluctuates considerably. Consequently, the composition of the fuel gas supply will also fluctuate. Depending on the hydrogen content in the fuel gas, the gas density will therefore vary, which in turn significantly influences the measurement result of a differential pressure gauge.This influence is not systematic but random, as it depends on the amount of hydrogen injected and / or on wind and solar energy. Such influences are random disturbances that significantly increase the measurement uncertainty of the differential pressure gauge and impair confidence in the measurement result. To avoid this, network operators use legally approved gas quality measuring instruments such as process gas chromatographs or perform complex calculations of the gas quality using gas quality monitoring systems, which must also be legally approved.

[0036] The present invention registers the increase or drift of measurement uncertainty due to density fluctuations by means of an additional sensor device and takes this into account during measurement calculation according to the calculation method in ISO 5167-1 to 6. In the case of impermissible measurement uncertainties, the measurement calculation can be suppressed and this suppression can be displayed as a fault on the counter / index or made available at electronic interfaces.

[0037] Differential pressure sensors measure the flow of fluids whose properties can change. In complex media such as natural gas, shifts in the composition of the fluids, sometimes reaching into the percentage range, are common. This occurs, for example, in complex network structures with multiple feed-in points. A typical example is a municipal distribution network with natural gas as its main component, into which biogas and / or hydrogen are injected at various points. Due to the nature of the process, however, it is virtually impossible for the measuring sections installed in the network to be exposed to completely different media in succession – for example, gas followed by liquids. This means that the physicochemical parameters of the fluid flow can change or fluctuate continuously, but they do not jump by orders of magnitude to completely different values.

[0038] As explained above, variations in composition act as a disturbance variable that contributes to measurement uncertainty. With increasing feed-in of renewable gases such as hydrogen, this will be the primary disturbance variable leading to larger measurement uncertainties.

[0039] Physical variations in material properties manifest themselves as density variations. The present invention utilizes this relationship to detect increasing measurement uncertainties and to electronically compensate for them in the differential pressure sensor.

[0040] Accordingly, one object of the invention is to improve a differential pressure transmitter in such a way that it can take into account density fluctuations of the process fluid when determining the flow rate.

[0041] Density fluctuations correspond directly to fluctuations in the speed of sound within the medium. One method for detecting such fluctuations is through time-of-flight measurements of acoustic waves, preferably ultrasonic signals, traveling through the medium. For this purpose, a pair of ultrasonic sensors is positioned in the compensating tube so that the emitted ultrasonic pulse travels from the first sensor through the medium to the second receiving sensor. The required transmitter or transducer can be integrated into the electronics of the differential pressure sensor.

[0042] According to the invention, an ultrasonic transmitter and receiver are located in the compensating pipe or in the outflow area. The invention will be discussed further below only with regard to the positioning of the sensor pair in the compensating pipe.

[0043] To avoid interference from dirt and deposits on the transit-time measurement, the sensors can preferably be positioned at the 3 or 9 o'clock position of the compensating pipe, instead of at the lowest point. Furthermore, the sensors can be recessed slightly into the wall of the compensating pipe, allowing a sufficiently strong ultrasonic pulse to be emitted into the pipe while simultaneously minimizing abrasion or erosion of the sensor by the flow. The pulse from the transmitting sensor can be radiated directly to the receiver, for example, from the 3 or 9 o'clock position. Alternatively, instead of direct transmission of the acoustic signal from the transmitter to the receiver, the acoustic signal can also be transmitted by reflection off the inner wall of the compensating pipe. This allows for greater flexibility in the positioning of the transmitter and receiver.Positioning the transmitter and receiver away from low points in the pipe is advantageous; for example, positioning them at the 3 o'clock position and reflecting the signal at the 9 o'clock position. In all cases, the travel time is calculated by subtracting the time of transmission from the time of the acoustic signal's arrival.

[0044] Changes in transit time are a strong indicator of density fluctuations in the medium, provided it can be assumed that the medium does not fundamentally change, which is the case with piped gas supplies to the public, as explained above. In principle, the increase in measurement uncertainty can be calculated from the frequency of the density fluctuations, and the measurement result (measured value and measurement uncertainty) can be determined on this basis, or the drift in measurement uncertainty can be electronically compensated in the differential pressure sensor. However, according to the invention, it is more advantageous to determine the relationship between density fluctuation and measurement uncertainty on accredited test benches or in recognized testing laboratories, depending on the medium and its dimensions. The test bench results can be stored in the evaluation unit and used to compensate for the measurement uncertainty caused by changes in the medium's properties, depending on the density fluctuation measured with the invention.Such a combination of "measurement-based rather than theoretical (calculated) testing" with the inventive device for the purpose of compensation can be considered a quality feature of the respective differential pressure sensor placed on the market. It can be carried out as part of the calibration or verification that is generally required and significantly increases the acceptance of the inventive compensation of increased or drifting measurement uncertainties in the differential pressure sensor.

[0045] It is possible for the ultrasonic transmitter and the ultrasonic receiver to be arranged opposite each other and, with respect to the pipe circumference, to each be located outside the lowest point of gravity within the pipe body. Furthermore, it is possible for the ultrasonic transmitter and the ultrasonic receiver to be arranged one behind the other in the flow direction, either in the inflow or outflow section, and to be aligned with each other such that an ultrasonic pulse emitted by the ultrasonic transmitter is reflected back from the opposite inner wall of the pipe and from there to the ultrasonic receiver.

[0046] Furthermore, it is conceivable that the ultrasonic transmitter and receiver are recessed into the wall of the pipe body. For example, the ultrasonic transmitter and receiver can be arranged behind the inner pipe wall, so that they are shielded from the process fluid and thus from any wear-promoting elements contained therein.

[0047] The invention further relates to a method for determining the measuring accuracy of a differential pressure sensor, in particular according to one of the preceding claims, comprising the steps of: passing a process fluid through the differential pressure sensor; determining a first pressure in a first section of a tube body of the differential pressure sensor and determining a second pressure in a second section of the tube body, wherein the inner diameter of the tube body in the first section is larger than in the second section; determining the pressure difference between the first and second pressures and determining a flow rate of the process fluid passed through; emitting an electromagnetic wave from a source arranged on or in an inner wall of the tube body onto a reflecting surface which is arranged on an inner wall section of the tube body opposite the source, wherein the electromagnetic wave does not strike the reflecting surface perpendicularly;Directed reflection of the electromagnetic wave from the reflecting surface onto a photocell obliquely opposite the reflecting surface and / or diffuse reflection onto a photocell perpendicularly opposite the reflecting surface; determination of a decrease in the radiation intensity detected by the obliquely opposite photocell and / or determination of an increase in the radiation intensity detected by the perpendicularly opposite photocell; and making a predetermined correction to the pressure difference determined by the differential pressure sensor when a predetermined threshold value of the radiation intensity detected by the photocell is exceeded and / or fallen below.

[0048] By taking the measured wear into account when determining the measured value, the measured value can be electronically corrected for the systematic measurement deviation caused by the wear (deviation compensation).

[0049] Furthermore, the predetermined threshold value can be displayed as a warning on the index (counter) of the differential pressure sensor or signaled to the automation system. Additionally, an alarm can be communicated to the operator when another threshold value is reached.

[0050] Viewed in the direction of fluid flow, the pipe body can initially have an inlet section with a first flow cross-section. Subsequently, the pipe body can have a diffuser section with a flow cross-section that continuously increases until it reaches a second flow cross-section. This can be followed by a confusing section with a flow cross-section that continuously decreases until it reaches the first flow cross-section. Finally, an outlet section with the first flow cross-section can be arranged. The first pressure measuring device can be located in the inlet section or the outlet section. The second pressure measuring device can be located in the region of the second flow cross-section.It may be provided that a compensating pipe is arranged between the first connection element and the inflow section, and that the ratio between the length of the compensating pipe and the internal dimension of the compensating pipe is at least 2.5.

[0051] It may be provided that at least one third pressure measuring device is arranged in the inflow section or in the outflow section, so that the inflow section and the outflow section have either at least one second or at least one third pressure measuring device.

[0052] It is conceivable that the quotient of a second internal dimension of the pipe body to a first internal dimension of the inflow section is at least 1.2 and at most 2.5, but ideally 1.5.

[0053] Furthermore, it is conceivable that the ratio of the length of the diffuser section to the length of the confuser section, viewed in the direction of flow, is at least 1 and at most 5, preferably 3.7.

[0054] The ratio of the second internal dimension of the tube body to the length of the diffuser section can be at least 0.3 and at most 0.5.

[0055] A measuring section may be inserted between the diffuser section and the confuser section, which has the second internal dimension and a measuring section length whose maximum length corresponds to the value of half the first internal dimension.

[0056] It may be provided that at least one of the pressure measuring devices is designed as a coupling recess in such a way that it is suitable for connecting a coupling element or for introducing sensors for visual inspection, geometric measurement or calibration of the metrologically relevant area of ​​the inside of the pipe body.

[0057] The at least one coupling recess can interact with a coupling element connected to the pipe body, which has a first and / or a second connection recess for a pressure sensor or a differential pressure line of a pressure measuring device, or is suitable for connecting or inserting sensors.

[0058] It may be provided that a connection recess can be closed, glued, or welded by means of an external threaded cap, an internal threaded plug, a flange cover, a clamp cover, or another closing element.

[0059] It may also be provided that the connecting element is connected to a compensating device with which the free volume in the differential pressure line can be changed.

[0060] The connection element can be connected to a cleaning device with which the pressure squeezing recess connected to the connection element can be cleaned as needed. Description of exemplary implementations

[0061] Further examples of implementation are explained in more detail below with reference to figures in a drawing. These show: Fig. 1 a cross-sectional view of an embodiment of the differential pressure sensor according to the invention; Fig. 2 a perspective view of a differential pressure sensor according to the invention with ultrasonic measurement; Fig. 3 a side view of a compensating tube of a differential pressure sensor according to the invention with ultrasonic measurement; Fig. 4a an exemplary view of a non-axisymmetrical flow profile; Fig. 4b an exemplary representation of a flow exhibiting swirl; Fig. 5 a longitudinal sectional view of a differential pressure sensor according to the invention with pressure sensors arranged radially distributed around the circumference in the inflow or outflow area; Fig. 6 exemplary procedures for measurement correction and zero point adjustment.

[0062] Fig. 1Figure 1 shows an embodiment of the differential pressure sensor 10 according to the invention with a circular flow cross-section in a sectional view through its longitudinal extent. Therefore, the internal dimensions or pipe internal dimensions are always referred to as diameters in the following. The section is drawn through a central axis 12 of the first differential pressure sensor 10, and the flow direction of a fluid through the first differential pressure sensor 10 is indicated by an inflow arrow 14 and an outflow arrow 16. Viewed in the direction of flow, a fluid initially flows through a first connection element 18, which has a tubular inner region with a constant inner diameter 19 and is designed as a screw flange in the example shown. However, a variety of connection options are conceivable, for example, a weld flange or an adhesive connection element.The first connection element 18 is connected downstream to a first end of a compensating tube 20, for example by welding, which has a constant inner diameter 21 and a length 23. A second end of the compensating tube 20 is connected to one end of the pipe body 22, the other end of which is connected to a second connection element 24. The second connection element 24 is also designed as a screw flange, but can also be designed as a different type of connection, like the first connection element 18. In the selected example, the first connection element 18, the compensating tube 20, the pipe body 22, and the second connection element 24 are made of steel, preferably the same material as the pipeline in which the differential pressure sensor 10 is installed.The choice of material can also depend on the type of fluid that will flow through the differential pressure sensor, for example, whether it is corrosive or contaminated with substances that have an abrasive effect. In such cases, a material with particularly low wear resistance must be selected. This ensures that the internal geometry of the differential pressure sensor 10 undergoes only minor changes during its operating period.

[0063] The pipe body 22 can be divided into different sections along its length. On the side where the fluid flows into the pipe body 22, there is an inflow section 26, which has a constant first inner diameter 28. Corresponding to the first inner diameter 28, the inflow section 26 has a first flow cross-section.

[0064] Downstream of the inlet section 26, a diffuser section 30 follows, the circular inner diameter of which increases continuously in the flow direction until a second inner diameter 32 is reached. Corresponding to the increase in the inner diameter in the diffuser section 30, the flow cross-section for the fluid increases continuously from the first flow cross-section to a second flow cross-section, which is reached at the second inner diameter 32. The second inner diameter 32 is also the maximum inner diameter of the pipe body. Accordingly, the velocity of the medium is slowed in the flow direction, and its static pressure component is increased at this point compared to the region of the first inner diameter 28.Downstream of the diffuser section 28, the pipe body 22 has a confuser section 34, also called a nozzle section, whose inner diameter continuously decreases from the second inner diameter 32 back to the first inner diameter 28. Accordingly, in the outflow section 34 following the confuser section, with its first inner diameter 28, the first flow cross-section again exhibits the same flow velocities as in the inflow section 26.

[0065] In the chosen example, the ratio between the second inner diameter (32) and the first inner diameter (28) is approximately 1.4, which is within a typical range of 1.2 to 2.5. However, depending on the fluid, flow velocity, pipe diameter, operating pressure, and other boundary conditions, this ratio can vary within the specified range.

[0066] In order to obtain advantageous flow conditions in the diffuser section 30, it is provided that the ratio of the second inner diameter 32 and the length of the diffuser section 30, viewed in its longitudinal extent, is between 0.3 and 0.5.

[0067] Furthermore, it has been found that the ratio between the length of the diffuser section 30 and the confuser section 34 also influences the pressure measurement. A ratio between 0.3 and 0.5 has proven advantageous.

[0068] For example, a DN 100 inlet flange 19 results in a nominal diameter of approximately 100 mm at the inlet of the diffuser 28. This means that the maximum nominal diameter of the diffuser 28 at position 32 reaches approximately 140 mm as an optimum. According to German patent application DE 10 2017 001 049 A1, the nominal diameter can, according to the invention, range between 120 and 250 mm. Let us consider the optimum nominal diameter of 140 mm at position 32 according to the invention. The ratio of the inner diameter at position 32 to the length of the diffuser section 30 should be between 0.3 and 0.5. Therefore, the optimal length of the diffuser 30 can vary between 280 and 467 mm.

[0069] The ratio of the length of the confuser section 34 to the length of the diffuser section 30 should also preferably be between 0.3 and 0.5. Therefore, in the example DN 100 inlet flange, the confuser should preferably be dimensioned with a length between approximately 84 and 234 mm.

[0070] In the outflow section 36 of the pipe body 22, a second coupling recess 44 is shown as a bore through the pipe body 22. A second coupling element 40 is attached to the outside of the second coupling recess 38. The through-bore of this element corresponds on one side to the second coupling recess 38 and on the other side has a connection element that is connected to a first impulse line 42 or differential pressure line of a pressure gauge or flow meter (not shown in the figure). This allows the pressure measurement to be taken directly on the inner surface of the pipe body 22 in the outflow section 36 via the coupling recess 38. A variety of pressure sensors are suitable for pressure measurement, for example, a hydraulic U-tube manometer, a single pressure sensor, a differential pressure sensor, or a combined differential pressure sensor with integrated separate pressure measurement.A pressure measuring point in the region of the first inner diameter 32 has a comparable design. There, a first coupling recess 44 is arranged in the pipe body 22, above which a first coupling element 46 is mounted. The through-bore of this coupling element corresponds on one side to the first coupling recess 44 and on the other side has a further connection element to which a second impulse line 48 is connected. Here, too, the pressure in the region of the first inner diameter 32 is measured directly, without the need for an intermediate pressure equalization element, such as a pressure equalization annular chamber, which would otherwise be necessary. Furthermore, measuring the pressure at only one point on the circumference of the pipe body 22 in the region of the first inner diameter 32 or in the outflow section 36 is entirely sufficient for measuring or calculating pressures or pressure differences.For certain process-related reasons, it may be advantageous to arrange one, two, and / or three additional coupling recesses around the circumference of the pipe body 22 at locations of the first 38 or second coupling recess 44. This makes it particularly easy to implement metrological redundancy, for example, 1 out of 2, 2 out of 3, 2 out of 4, or 3 out of 4 pressure measurements.

[0071] Another important factor influencing the flow in the differential pressure sensor 10 is the ratio of the length of the compensating tube 20 to its tube diameter 21, which corresponds to the second inner diameter 28. In the chosen example, this ratio is approximately 4.2, which is within a range that allows for advantageous flow conditions in the differential pressure sensor 10. For a given diameter, the minimum length of the compensating tube 20 should have a ratio of at least 2.5 to ensure favorable flow conditions for pressure measurement in the differential pressure sensor. It is taken into account that the length of the first connection element 18 and the length of the first inlet section 26 are added to the length of the compensating tube 20 to affect the flow conditions in the differential pressure sensor. Typically, the ratio of the total length to the tube diameter 21 will therefore be at least 3.Only downstream of the inflow section 26 does the deceleration of the fluid velocity begin due to the increase in the inner diameter in the diffuser section 30.

[0072] To determine the measurement accuracy of the differential pressure sensor, the invention utilizes the different behavior of specular and diffuse reflection. By understanding abrasion, erosion, and corrosion on the inner surface of the differential pressure sensor 10, or the changes in the resulting wear, the systematic measurement error, or more precisely, the systematic measurement deviation, can be determined and corrected. Systematic measurement deviations can be evaluated (quantified) on test benches. This can also be experimentally recorded before field use and, for example, stored in tabular form in the evaluation electronics of the differential pressure sensor 10. The table then serves to correct the measurement result during operation. For this purpose, a threshold value of a sensor in the differential pressure sensor 10 is monitored during operation. When the threshold value(s) are reached, the subsequent measured values ​​are automatically corrected.Below the threshold value, additional measurement deviations are accepted and neglected. An electromagnetic wave, preferably a light signal, transparent to the medium, is directed towards a location in the differential pressure sensor 10 where the greatest erosion or corrosion is expected. This is typically the point with the highest flow velocity at the end of the confounder section 34. This location is made of a metallic mirror or a mirror surface 80 is arranged at this point. The reflection of the wave or light beam from the mirrored inner surface can be described by the law of reflection (angle of incidence equals angle of reflection). If the mirror surface 80 becomes rougher due to abrasion, erosion, or corrosion, specular reflection transitions into diffuse reflection. The mirror 80 thus acquires many scattering centers, which, in sum, result in the main backscattering occurring independently of the law of reflection.It then occurs perpendicular to the material. Diffuse scattering follows Lambert's law. Typical examples of diffuse reflection are light scattering by media such as milk, paper, or wall paint. Light falling on these media is scattered in all directions. For example, to an observer, an illuminated piece of white paper appears almost equally bright from all directions. The reflective surface, or mirror 80, can be made, for example, with a surface coating. Advantages include the ability to tailor erosion or corrosion to the flowing medium or accompanying substances. For instance, precipitated moisture could cloud the coating, allowing the transition from specular to diffuse reflection to occur more precisely and / or quickly.

[0073] The invention thus utilizes the transition from specular to diffuse scattering. For this purpose, a light source 81, ideally a light-emitting diode, is integrated into the differential pressure sensor in such a way that it directs a specular beam 82 onto the point where the highest erosion or corrosion is expected. Due to the manufacturing process, the surface at the point where the light beam strikes behaves like a mirror. The specular reflection 85 is captured by a photocell 83. Its position is calculated according to the law of reflection. In addition, a second photocell 84 is arranged vertically above the reflecting surface. This direction corresponds to the main backscatter 86 of the diffuse reflection. In the new state of the reflecting surface 80, the light beam 82 is detected almost completely by the first photocell 83. The second photocell 84 receives virtually no portion of the light wave scattered by the surface 80.During operation, the reflective surface 80 becomes rougher, and the directed reflection 85 increasingly transitions into diffuse reflection 86. This is detected by the second photocell 84, whose signal increases at the expense of the light incident on the first photocell 83. The ratio of the light output of the first and second photocells 83 and 84 can therefore be correlated. This provides the differential pressure sensor 10 with a value to determine permissible or impermissible wear values, or, if test bench information is stored, to automatically correct the relationship between wear progression and measurement deviation (measurement error) or the degree of measurement accuracy. This device can be used, for example, in commercial and official applications to compensate for measurement inaccuracies, provided it has an EU type-examination certificate in accordance with the Measuring Instruments Directive and has been calibrated or verified on accredited test benches.If the differential pressure sensor is to be designed in such a way that the influence of contamination, e.g., by oils, on the inner wall of the measuring device is also detected, it is recommended to position the reflective surface 80 at the lowest point. In principle, however, the reflective surface can also be attached at other positions. The entire sensor assembly would then be rotated and positioned symmetrically, figuratively speaking. Any possible curvature of the mirror position due to the design is irrelevant for the reflection. Directional reflections also occur at concave mirrors such as a shaving mirror or astronomical reflecting telescopes, or even at convexly curved exterior mirrors of cars, i.e., also at a reflective metallic surface on the inner wall of the differential pressure sensor 10 according to the invention. The determination of the inlet or outlet...The angle of incidence of the light beam should preferably be chosen such that, in the initial state, as little scattered light as possible from the directional reflection falls into the second photocell 84. This means choosing the largest possible angles. The angles are structurally limited by the maximum length of the outflow region 36 and are then calculated purely mathematically from the cotangent of the ratio of the length to the diameter of the outflow region 36. Analytically, the following relationship is obtained: . tan Ausfallwinkel = L ä nge des Abstr ö mabschnittes 36 Nennweite des Anschlusselements 24

[0074] The practical maximum length of the outflow section 36 results from the bidirectional variant of the differential pressure sensor. In this case, the length of the outflow section 36 corresponds to the length of the compensating pipe 23. According to main claim 1 of German patent application DE 10 2017 001 049 A1, the length of the compensating pipe 23 is at least 2.5 times the internal diameter of the compensating pipe 20. In the example above with a DN 100 inlet flange 19, the length of the outflow section 23 is then approximately 250 mm at most, and, according to the law of reflection, the outlet angle is approximately 68°. Experience has shown that it should be at least 30° to be able to sufficiently distinguish between directed and diffuse reflection.

[0075] In principle, wear can be determined using only one photocell, either 83 or 84. The weakening of the signal from photocell 83 or the increasing light output of photocell 84 up to the threshold values ​​is, in principle, sufficient for wear determination. However, it should be noted that using both signals from photocells 83 and 84 significantly increases the reliability of the wear determination and eliminates measurement errors, for example, due to a weakening of the light output caused by soiling of the mirror. According to the invention, this can be achieved, for example, by calculating the ratio of the light output of both photocells. This ratio remains constant even if the reflective surface is soiled. If only the light output of, for example, photocell 83 were evaluated, the reduced light output due to soiling alone could lead to the threshold value being reached. This would occur regardless of whether any wear is actually present.The result would be a false alarm, which must be avoided.

[0076] Fig. 2 and 3 Figure 1 shows a differential pressure sensor 10 which, on the inlet side, has an ultrasonic transmitter 90 and an ultrasonic receiver 91 in a compensating tube 20 between the first connection element 18 and the diffuser section 30. The ultrasonic sensor pair 90, 91 is positioned in the compensating tube 20 such that the emitted ultrasonic pulse travels from the first sensor through the medium to the second receiving ultrasonic sensor. The required transmitter or transducer can be integrated into the electronics of the differential pressure sensor or connected to the evaluation unit.

[0077] To prevent wear, contamination, and deposits, sensors 90 and 91 are preferably located at the 3 and 9 o'clock positions of the compensating tube 20. Furthermore, the sensors are recessed slightly into the wall of the compensating tube 20 such that a sufficiently strong ultrasonic pulse can be emitted into the compensating tube 20, minimizing erosion of the sensor by the flow. The pulse from the emitting sensor can be radiated directly to the receiver at the 9 o'clock position. Alternatively, the acoustic signal can also be reflected off the inner wall of the compensating tube and radiated to a receiver downstream at the 3 o'clock position. In all cases, the transit time is calculated by subtracting the time of the emitted signal from the time of the acoustic signal's arrival.

[0078] Figs. 4a and 4b They show, firstly, an exemplary view of a non-axially symmetrical flow profile and Fig. 4bsecondly, an exemplary representation of a swirling flow, which is characterized by a flow as in Fig. 4a The pipe routing shown can cause this. Pipe bends, valves, and other pipe fittings create inlet disturbances upstream of the measuring point. A simple pipe bend changes a symmetrical flow profile into a non-symmetrical one, see [reference]. Fig. 4a .

[0079] Multiple running-in problems can also cause a twist, which can change its profile depending on the speed, as in Fig. 4b depicted.

[0080] Fig. 5Figure 1 shows an arrangement by which inlet disturbances present upstream of the measuring point can be factored out of the measurement result. The view shows the positioning of the sensors 42, 48 for acquiring the differential pressure measurement around the circumference of the pipe. The arrangement of two or more sensors 42, 48 distributed around the circumference of the measuring pipe results in dynamic measurement acquisition via multiple paths. This is a major cause of measurement inaccuracies in differential pressure measurements.

[0081] The data field generated by the arrangement is then evaluated for dynamic adjustment of the measurement correction and thus verifies the measurement accuracy, as described in Fig. 6a is shown

[0082] Furthermore, the automatic zero-point correction process can be verified by using two or more sensors 42, 48. In the prior art, zero-point correction of the differential pressure transmitter is performed in its installed state by disconnecting it from the process and opening the compensating line via the manual valve block. With the proposed arrangement, which incorporates two or more sensors, this correction can be easily performed when the process fluid is stationary. During the zero-point correction of the other sensor, one sensor monitors whether the process fluid is moving. Manual intervention is no longer necessary. Fig. 6b This shows an example of a corresponding procedure.

[0083] The features disclosed in the foregoing description, the claims and the drawing can be important for the realization of the various embodiments of the invention, both individually and in any combination.

Claims

1. An differential pressure transducer (10) for measuring flows of gaseous and liquid fluids, in particular combustion gases such as natural gases, biogases, hydrogen-containing combustion gases and H2, gaseous, liquid and dense, for example supercritical carbon dioxide or steam, with a pipe body (22) which is arranged between a first (18) and a second connection element (24), wherein it is provided that a fluid flows through the first connection element (18) to the pipe body (22) and flows out through the second connection element (24), wherein the pipe body (22) has a first section with a first diameter and, arranged downstream thereof, a second section with a second diameter which becomes smaller in the flow direction and which is smaller than the largest diameter in the first section, wherein a first pressure measuring device (48) is arranged in the first section and a second pressure measuring device (42) is arranged in the second section, wherein a differential pressure between the first and second section can be determined by means of the pressure measuring devices (42, 48), wherein a source (81) for emitting electromagnetic radiation, for example collimated light, and at least one photocell (83, 84) are further arranged in the pipe body (22), wherein the differential pressure transducer (10) further comprises an evaluation unit by means of which a decrease and / or increase in the radiation intensity detected by the photocell (83, 84) can be determined, characterized in that the pipe body (22) has a first section with a first diameter which becomes larger in the flow direction, and in that a reflecting surface (80) is arranged in the pipe body (22), which is oriented with respect to the source (81) and the photocell (83, 84) such that the radiation (82) emitted by the source (81) impinges on the reflecting surface (80) and is reflected by the latter in a directed (85) or diffuse (86) manner onto the photocell (83, 84).

2. Differential pressure transducer (10) according to claim 1, in which the at least one photocell (83), in relation to the reflecting surface (80), is arranged symmetrically with respect to the source (81) such that the directed beam reflected by the reflecting surface (80) impinges on the photocell (83), wherein the radiation intensity detected by the photocell (83) falling below a predetermined threshold value is determined by means of the evaluation unit.

3. Differential pressure transducer (10) according to claim 1 or 2, in which the at least one photocell (84) is arranged perpendicularly opposite the reflecting surface (80) such that the main backscatter (86) of the diffuse reflection reflected by the reflecting surface (80) impinges on the photocell (84), wherein the radiation intensity detected by the photocell (84) exceeding a predetermined threshold value is determined by means of the evaluation unit.

4. Differential pressure transducer (10) according to one of the preceding claims, in which the reflecting surface (80) is arranged between the first and the second pressure measuring device.

5. Differential pressure transducer (10) according to one of the preceding claims, in which the reflecting surface (80) is arranged at a gravitational lowest point of the pipe body (22) with respect to the pipe circumference.

6. Differential pressure transducer (10) according to one of the preceding claims, wherein the reflecting surface (10) is arranged in the pipe body (22) at the point with the highest flow velocity with respect to the flow direction, for example in the second section with a diameter which is reduced with respect to the first section.

7. Differential pressure transducer (10) according to one of the preceding claims, in which the source (81) of electromagnetic radiation is arranged upstream of the first pressure measuring device.

8. Differential pressure transducer (10) according to one of the preceding claims, in which the at least one photocell (83) is arranged downstream of the second pressure measuring device (42).

9. Differential pressure transducer (10) according to one of the preceding claims, wherein, when the radiation intensity detected by the photocell (83, 84) exceeds and / or falls below a predetermined threshold value, a predetermined correction of the pressure difference determined by the differential pressure transducer (10) or of the determined flow rate is carried out.

10. Differential pressure transducer (10) according to one of the preceding claims, wherein the reflecting surface (80) has a coating which is configured to be able to determine the influence of a specific accompanying substance such as condensates, such as, for example, moisture, on the measurement accuracy.

11. Differential pressure transducer (10) according to one of the preceding claims, which has, in the compensation pipe or in the inflow section (20) or upstream thereof, distributed radially over the pipe circumference, a plurality of pressure sensors for detecting the local fluid pressure, which pressure sensors are connected to the evaluation unit, wherein the evaluation unit is configured to detect the swirl or vortices of the inflowing fluid on the basis of the pressure differences determined by means of the plurality of pressure sensors or to compensate the measurement result accordingly.

12. Differential pressure transducer (10) according to one of the preceding claims, wherein a measuring device for determining density fluctuations of the process fluid is arranged in an inflow section (20) between the first connection element (18) and the first section or in an outflow section (36) between the second section and the second connection element (20) of the differential pressure transducer (10), which measuring device comprises an ultrasonic transmitter (90) and an ultrasonic receiver (91) which are oriented with respect to one another such that an ultrasonic pulse emitted through the process fluid by the ultrasonic transmitter (90) can be received by the ultrasonic receiver (91), wherein the measuring device is connected to the evaluation unit which is configured to detect density fluctuations in the process fluid on the basis of transit time changes of the emitted ultrasonic pulses.

13. Differential pressure transducer (10) according to claim 12, wherein the ultrasonic transmitter (90) and the ultrasonic receiver (91) are arranged opposite one another and are each located outside the gravitational lowest point of the pipe body (22) with respect to the pipe circumference.

14. Differential pressure transducer (10) according to claim 12 or 13, wherein the ultrasonic transmitter and the ultrasonic receiver are recessed into the wall of the pipe body (22).

15. Method for determining the measurement accuracy of a differential pressure transducer (10) according to one of the preceding claims, comprising the steps: - passing a process fluid through the differential pressure transducer (10); - determining a first pressure in a first section of a pipe body (22) of the differential pressure transducer (10) and determining a second pressure in a second section of the pipe body (22), wherein the inner diameter (32, 28) of the pipe body (22) is larger in the first section than in the second section; - determining the pressure difference between the first and second pressures and determining a flow rate of the process fluid passed through; - emitting an electromagnetic wave (82) from a source (81) arranged on or in an inner wall of the pipe body (22) onto a reflecting surface (80) which is arranged on an inner wall section of the pipe body (22) opposite the source (81), wherein the electromagnetic wave (82) does not impinge perpendicularly on the reflecting surface (80); - directly reflecting (85) the electromagnetic wave (82) from the reflecting surface (80) onto a photocell (83) obliquely opposite the reflecting surface (80) and / or diffusely reflecting (86) onto a photocell (84) perpendicularly opposite the reflecting surface (80); - determining a decrease in the radiation intensity detected by the obliquely opposite photocell (83) and / or determining an increase in the radiation intensity detected by the perpendicularly opposite photocell (84); - carrying out a predetermined correction of the pressure difference determined by the differential pressure transducer when the radiation intensity detected by the photocell (83, 84) exceeds and / or falls below a predetermined threshold value.