Differential pressure flow meter

Abstract

Disclosed herein is a flow meter 100 for determining a flow rate, in different flow rate ranges, of a fluid through a pipe 101a, 101b. The flow meter comprises: a conduit 102 in fluid communication with the pipe 101a, 101b arranged to provide a flow path for the fluid, wherein the diameter of the conduit varies along the flow path; a pressure sensing apparatus 114 configured to sense a first differential pressure of the fluid between a first pair of sensing positions, and a second differential pressure between a second pair of sensing positions, wherein the position along the conduit of at least one of the first pair of sensing positions is different from at least one of the second pair; a flow calculator 116 configured: to determine a mass flow rate in a first range based on the first differential pressure; and to determine a mass flow rate in a second, different, range based on the second differential pressure.

Description

Technical Field The present invention relates to flow meters, and more particularly to differential pressure flowmeters used to determine the rate of flow of a fluid through a pipe based on a sensed pressure difference. It also relates to flow measurements for multi-phase fluid flows. Background Fluids such as oil, gas and water may be transported from a source to a user population or a storage facility using a pipe or pipeline. There is a need to determine the rate of flow of fluid through these pipes, for example to quantify the amount of substance being transported or to determine the presence of leaks. In addition, these pipes may be located in difficult to access locations, or in harsh environmental conditions, such as subsea. Subsea pipes are normally situated in remote locations, and are surrounded by seawater. Therefore, there are various technical challenges associated with accurately and cost effectively measuring the flow rate of fluid flowing through them. Various classes of flow meters are available for measuring fluid flow rates. Some flow meters provide regions of different fluid pressures, e.g. by providing a constriction in the flow path, and sensing the difference in pressure between the two regions in order to determine the rate of flow of the fluid in a connected pipe. Flow meters have an associated turndown ratio (also known as rangeability), which indicates the capacity of the flow meter to accurately measure a range of flow rates. In particular, the turndown ratio is defined as the ratio of maximum flow rate which can be measured to the minimum flow rate which can be measured. If the flow rate is outside of the operating range of the flow meter, the readings obtained by the flowmeter may be inaccurate. Inaccuracies in flow rate calculations may also be derived fromthe fluid itself. For example, in fluids consisting of a mixture of multiple fluid phases, each fluid phase may flow at different rates. One example of a multiphase flow is wet gas, in which some amount of liquid is present in a gas flow such as a flow of hydrocarbon gas being transported from a natural gas or gas condensate well. In such examples, the gas flow may include some amount of liquid hydrocarbon condensate and / or liquid water. The presence of these liquids may cause inaccuracies in the calculation of the gas flow rate, typically in the form of an over-re ad. Summary of Invention Aspects of the invention are as set out in the independent claims and optional features are set out in the dependent claims. Aspects of the invention may be provided in conjunction with each other and features of one aspect may be applied to other aspects. In an aspect there is provided a flow meter for determining a flow rate of a flow of fluid through a pipe, the flow meter comprising: a conduit in fluid communication with the pipe and arranged to provide a flow path forthe fluid, wherein the diameter of the conduit varies along the flow path; a pressure sensing apparatus configured to sense a first differential pressure of the fluid between a first pair of sensing positions along the conduit, and a second differential pressure between a second pair of sensing positions along the conduit, wherein the position along the conduit of at least one of the first pair of sensing positions is different from at least one of the second pair of sensing positions; a flow calculator configured: to determine a mass flow rate in a first range based on the first differential pressure; and to determine a mass flow rate in a second range based on the second differential pressure; wherein the second range is different from the first range. The flow calculator may be configured to provide the determination of the mass flow rate in the second range in response to the sensed differential pressure falling below a first threshold value. The flow calculator may be configured to provide the determination of the mass flow rate in the first range in response to the sensed differential pressure exceeding a second threshold value. The first threshold value may be the same as the second threshold value. The first threshold value may be greater than the second threshold value. The first threshold value may be less than the second threshold value. The flow meter may be configured to determine the mass flow rate in the first range in a first mode, and to determine the mass flow rate in the second range in a second mode. The flow meter may be configured to switch from the first mode to the second mode, e.g. in response to the sensed differential pressure falling below the first threshold value and / or to switch from the second mode to the first mode, e.g. in response to the sensed differential pressure exceeding the second threshold value. One of the first pair of sensing positions may be the same as one of the second pair of sensing positions. Alternatively, both of the second pair of sensing positions may be different from both of the first pair. Each pair of sensing positions may comprise a first sensing position at which the diameter of the conduit is greater than at a second sensing position of the pair. The larger diameter position of each pair may be upstream of the smaller diameter position of that pair. Alternatively, the larger diameter position of each pair may downstream of the smaller diameter position of that pair. The larger diameter position of the first pair may be upstream of the smaller diameter position, and the larger diameter position of the second pair may be downstream of the smaller diameter position. Alternatively, one or more of the pairs of sensing positions may comprise a pair of positions at different points along the conduit of the same diameter, e.g. with a section of the conduit with a smaller diameter arranged between the pair of positions. For example the pair of positions may be arranged either side of the throat portion of a Venturi tube. The difference in the diameter of the conduit between the first pair of sensing positions may be greater than between the second pair of sensing positions. One of the sensing positions of one or both pairs may be arranged at the narrowest portion of the conduit. The flow calculator may be configured in the first range to determine the mass flow rate based on a ratio of the conduit diameters at the first pair of sensing positions, and in the second range to determine the mass flow rate based on a ratio of the conduit diameters at the second pair of sensing positions. The flow meter may be configured to output the determined mass flow rate to a resource, e.g. a user device, terminal, or computer. The conduit may be a tube or may be provided by a tube. For example the conduit may be a Venturi tube or may be provided by a Venturi tube. The flow meter may be a Venturi flow meter. The conduit may be provided within a single solid body. The solid body may comprise a tubular cavity which extends from one end of the solid body to an opposite end of the solid body. The conduit may be provided by the tubular cavity. The solid body may be configured for attachment to a section of pipe, e.g. each end of the solid body may be configured for attachment to a corresponding section of the pipe. The conduit may comprise a plurality of tapered portions along its longitudinal length. Each of the tapered portions may be separated by a plurality of portions of constant diameter. The conduit may have an inlet portion, an outlet portion, and a throat portion arranged between the inlet portion and the outlet portion. The diameter of the conduit at the throat portion may be less than the diameter of the conduit at the inlet portion and at the outlet portion. The conduit may taper inwards (e.g. decrease in diameter) from the inlet portion to the throat portion, and may taper outwards (e.g. increase in diameter) from the throat portion to the outlet portion. The inlet portion, the throat portion and / or the outlet portion may be one of the portions of constant diameter. One or more of the first or second pair of positions may be provided at the inlet portion, the throat portion and / or the outlet portion. Each sensing position may be arranged in a corresponding one of the portions of constant diameter. At least one of the sections of constant diameter may have a longitudinal length different from at least one other of the portions of constant diameter. The flow meter may further comprise a temperature sensor arranged to sense the temperature of the fluid in the conduit. The flow calculator may be configured to determine the mass flow rate based on the sensed temperature. The flow calculator may be configured to determine a first pressure loss ratio, based on the first differential pressure, and a second pressure loss ratio, based on the second differential pressure. In the first range, the flow calculator may be configured to determine flow rates for each of the phases of a multiphase fluid flow based on the first pressure loss ratio. In the second range, the flow calculator may be configured to determine flow rates for each of the phases of a multiphase fluid flow based on the second pressure loss ratio. The flow meter may be configured for coupling to the pipe. The flow meter may have a first end configured to couple to a first section of pipe and a second end configured to couple to a second section of pipe. The conduit may be arranged to provide a flow path for the fluid between the first section of pipe and the second section of pipe. In another aspect there is provided a method of determining a flow rate of a flow of fluid through a pipe, the method comprising: sensing a first differential pressure of the fluid between a first pair of positions in a conduit, said conduit in fluid communication with the pipe; sensing a second differential pressure between a second pair of positions along the conduit; determining a mass flow rate in a first range based on the sensed differential pressure of the fluid between the first pair of positions; and determining a mass flow rate in a second range based on the sensed differential pressure of the fluid between the second pair of positions; wherein the second range is different from the first range. The method may further comprise determining the mass flow rate in the second range in response to the sensed differential pressure falling below a first threshold value. The method may comprise determining the mass flow rate in the first range in response to the sensed differential pressure exceeding a second threshold value. The method may comprise determining the mass flow rate in the first range in a first mode, and determining the mass flow rate in the second range in a second mode. The method may comprise switching from the first mode to the second mode, e.g. in response to the sensed differential pressure falling below the firstthreshold value and / or switching from the second mode to the first mode, e.g. in response to the sensed differential pressure exceeding the second threshold value. The method may further comprise outputting the determined mass flow rate to a resource, e.g. a user device, terminal, or computer. In another aspect there is provided a method of determining flow rates for one or more of the phases of a multiphase fluid flow, the method comprising: sensing, in a conduit which has a diameter that varies along its length, a plurality of differential pressures between pairs of positions along the length of the conduit; determining, based on the sensed differential pressures: a mass flow rate; a first pressure loss ratio; a second pressure loss ratio; wherein the first pressure loss ratio is based on a first differential pressure sensed between a first pair of positions along the length of the conduit; wherein the second pressure loss ratio is based on a second differential pressure sensed between a second pair of positions along the length of the conduit; wherein the diameter of the conduit at one or both of the first pair of positions is different from the diameter of the conduit at both of the second pair of positions; determining the flow rate of at least one of the phases of the flow, based on the mass flow rate, the first pressure loss ratio, and the second pressure loss ratio. Determining the flow rate of at least one of the phases of the flow may comprise determining the flow rate of each of the phases of the flow. Determining the flow rate of at least one of the phases of the flow may comprise determining one or more phase fractions of the flow. The phase fractions may comprise one or more of a liquid volume fraction or a gas volume fraction. The phase fractions may comprise a hydrocarbon condensate volume fraction, and / or a liquid water volume fraction. In one example determining the flow rate of at least one of the phases of the flow may comprise determining a hydrocarbon gas volume fraction, a hydrocarbon condensate volume fraction, and a liquid water volume fraction. Additionally or alternatively the phase fractions may comprise mass fractions, for example Liquid Mass Fraction (LMF)and Gas Mass Fraction (GMF) (sometimes referred to as gas quality). Mass fractions corresponding to any of the volume fractions described herein may alternatively be determined. Additionally or alternatively, the phase fractions may comprise Liquid to Gas Volume Ratio (LGVR), Liquid to Gas Mass Ratio (LGMR), or Lockhart-Martinelli parameter. The conduit may have an inlet portion, an outlet portion, and a throat portion arranged between the inlet portion and the outlet portion, wherein the diameter of the conduit at the throat portion is less than the diameter of the conduit at the inlet portion and at the outlet portion. The conduit may be a Venturi tube. The fluid flow may comprise flow of wet gas. The fluid flow may comprise a mixture of gas and liquid, for example the fluid flow may comprise a gas (e.g. natural gas), liquid hydrocarbon condensate, and liquid water. Determining may comprise determining the flow rate of three phases of a flow, e.g. natural gas, liquid hydrocarbon condensate, and liquid water. The first pressure loss ratio may be based on a ratio of a third differential pressure to the first differential pressure (or vice versa). The second pressure loss ratio may be based on a ratio of a fourth differential pressure to the second differential pressure (or vice versa). The third differential pressure and / or the fourth differential pressure may correspond to and / or indicate a recovered pressure. Determining the mass flow rate may be based on the first differential pressure or the second differential pressure. The method may further comprise determining a density of the flow. Determining the fluid flow rate of one or more of the phases of the flow may be further based on the determined density. The method may further comprise sensing a temperature of the fluid in the flow. The method may further comprise sensing a line pressure at an input of the conduit. The line pressure may indicate the static (e.g. absolute) pressure of the fluid in the pipe. Determining the density may be based on the sensed temperature and line pressure. The line pressure may be sensed at the inlet portion. The line pressure may be sensed at one or more of the first pair of positions and / or one or more of the second pair of positions. The method may further comprise determining a third pressure loss ratio based on a third differential pressure sensed between a third pair of positions along the length of the conduit. The diameter of the conduit at one or both of the third pair of positions may be different from the diameter of the conduit at both of the first pair of positions and both of the second pair of positions. In another aspect there is provided a flow meter for determining flow rates for one or more of the phases of a multiphase fluid flow, the flow meter comprising: a conduit arranged to provide a flow path for the fluid, wherein the diameter of the conduit varies along the flow path; a pressure sensing apparatus configured to sense a first differential pressure between a first pair of positions along the length of the tube, and a second differential pressure between a second pair of positions along the length of the tube, wherein the diameter of the conduit at both of the first pair of positions is different from the diameter of the conduit at one or both of the second pair of positions, a flow calculator configured to determine: a mass flow rate; a first pressure loss ratio based on the first differential pressure; a second pressure loss ratio based on the second differential pressure; and further configured to determine the flow rate of at least one of the phases of the flow, based on the mass flow rate, the first pressure loss ratio, and the second pressure loss ratio. The multiphase fluid flow may be a wet gas flow. The multiphase fluid flow may be a three phase flow, forexample comprising hydrocarbon gas, liquid hydrocarbon condensate, and liquid water. The flow calculator may be configured to determine the flow rate of each of the phases of the flow. The pressure sensing apparatus may be configured to sense a third differential pressure between a third pair of positions along the length of the conduit. The diameter of the conduit at at least one of the third pair of positions may be differentfromthe diameter of the conduit at both of the first pair of positions and the diameter of the conduit at both of the second pair of positions. The flow calculator may be further configured to: determine a third pressure loss ratio based on the third differential pressure; and determine the flow rate of one or more of the phases of the flow further based on the third pressure loss ratio. In another aspect there is provided a flow meter for determining at least one phase fraction of a multiphase fluid flow, the flow meter comprising: a conduit arranged to provide a flow path for the fluid, wherein the diameter of the conduit varies along the flow path; a pressure sensing apparatus configured to sense a first differential pressure (x) between a first pair of positions along the length of the conduit, and a second differential pressure (y) between a second pair of positions along the length of the conduit, wherein the diameter of the conduit at both of the first pair of positions is differentfromthe diameter of the conduit at one or both of the second pair of positions, a flow calculator configured to determine: a mass flow rate; a first pressure loss ratio based on the first differential pressure; a second pressure loss ratio based on the second differential pressure; determine at least one phase fraction based on the first pressure loss ratio and the second pressure loss ratio; The flow calculator may be configured to determine a flow rate for one or more of the phases of the multiphase flow, based on the at least one phase fraction. The at least one phase fraction may comprise a volume fraction for example a liquid volume fraction and / or a gas volume fraction. The flow calculator may be configured to determine a plurality of phase fractions based on the first pressure loss ratio and the second pressure loss ratio. For example the flow calculator may be configured to determine a first phase fraction based on the first pressure loss ratio, and a second phase fraction based on the second pressure loss ratio The at least one phase fraction may comprise three phase fractions. In one example the three phase fractions may be hydrocarbon gas, hydrocarbon condensate, and liquid water. In another aspect there is provided a method for determining at least one phase fraction of a multiphase fluid flow, the method comprising: sensing, in a conduit which has a diameter that varies along its length, a plurality of differential pressures between pairs of positions along the length of the conduit; determining, based on the sensed differential pressures: a mass flow rate, a first pressure loss ratio, and a second pressure loss ratio, wherein the first pressure loss ratio is based on a first differential pressure sensed between a first pair of positions along the length of the conduit; wherein the second pressure loss ratio is based on a second differential pressure sensed between a second pair of positions along the length of the conduit; wherein the diameter of the conduit at one or both of the first pair of positions is different from the diameter of the conduit at both of the second pair of positions; and determining the at least one phase fraction based on the first pressure loss ratio and the second pressure loss ratio. Any of the flow meters described above may be configured to perform any of the methods described above. Brief Description of Figures Some examples of the present disclosure will now be described, byway of example only, with reference to the figures, in which: Figure 1 shows a schematic cross-sectional view of an example flow meter; Figure 2 is a flow chart illustrating a method which may be performed by a flow meter; Figure 3a illustrates a method of determining flow rates of the component parts of a multiphase flow; Figure 3b shows a schematic cross section of a portion of a flow meter; Figure 3c is a graph illustrating how pressure loss ratio may vary with liquid volume fraction of a multiphase flow. In the drawings like reference numerals are used to indicate like elements. Specific Description The present disclosure relates to flow meters which may be arranged between adjacent sections of a pipe transporting fluid. The flow meter has a conduit of varying diameter along its length (e.g. a Venturi tube) which causes a variation in the pressure of the fluid along the length of the conduit. The flow meter senses the differential pressure of the fluid in the flow between pairs of points along the conduit, and determines the flow rate of the fluid passing through the pipe based on the sensed differential pressures. The flow meter includes multiple tapping points at which the pressure can be sensed, so that a plurality of differential pressures can be sensed from different pairs of positions along the length of the conduit. In this way the rangeability or turndown ratio of the flowmeter may be increased. For example, when the flow rate is in a certain range, a first differential pressure may be sensed from a first pair of tapping points along the conduit, and when the flow rate is in a different range, a second differential pressure may be sensed from a different pair of tapping points along the conduit which provides a more accurate reading for flow rates in that range in comparison to the first pair of tapping points. Figure 1 shows an example flow meter 100, arranged between two adjacent sections of pipe 101a, b. The flow meter 100 includes a conduit 102 provided within a solid body 104. The conduit 102 extends from a first section of the pipe 101a to a second section of the pipe 101b, such that fluid (which may be a gas or a liquid, or a mixture of both) flows from the first section of pipe 101a to the second section of pipe 101b via the conduit 102. As shown, the conduit 102 has a varying diameter along its length. In particular, the conduit 102 in the example of Figure 1 is a Venturi tube. The diameter of the conduit 102 at the inlet and outlet ends which connect to the pipe sections 101a, 101b is the same as the diameter of the conduit of the pipe. The conduit 102 tapers from each end to a central portion of smallest diameter 110. The change in diameter between the inlet end and the central portion 110 causes the pressure of the fluid flowing in the conduit 102 to drop between the inlet and the central portion 110. Between each end of the conduit 102 and the central portion 110, the conduit comprises a series of alternating portions of constant diameter 106a-f and tapering portions 108a-f, adjacent to one another. Each of the portions of constant diameter 106a-f and the central portion 110 comprise a tapping point 112a-g at which the pressure of the fluid in the conduit in that portion, and / or the differential pressure of the fluid in the conduit between two such points, can be sensed. The portions of constant diameter 106a-f may enable a more stable reading of differential pressure to be obtained in comparison to a continuously tapering conduit. The flow meter 100 further comprises a pressure sensing apparatus 114. the pressure sensing apparatus comprises a plurality of differential pressure sensors each configured to sense the differential pressure of the fluid between a given pair of the tapping points 112a-g, such that the differential pressure between any of the possible pairs of tapping points 112a-g can be sensed. The pressure sensing apparatus 114, (for example each of the differential pressure sensors) may also comprise a pressure transmitter configured to sense the static line pressure of the fluid at any of the tapping points 112a-g. The pressure sensing apparatus 114 is electrically coupled to the flow calculator 116 and is configured to provide a signal indicating the sensed differential pressure to the flow calculator 116. The flow meter 100 further comprises a temperature sensor 118. The temperature sensor 118 is configured to sense a temperature of the fluid in the flow. The temperature sensor 118 may be arranged within the conduit 102 or within the solid body 104. Alternatively the temperature sensor may be arranged upstream or downstream of the conduit 102. The temperature sensor 119 is electrically coupled to the flow calculator 116 and is configured to provide a signal indicating the sensed temperature to the flow calculator 116. The flow meter 100 further comprises the flow calculator 116. The flow calculator 116 is configured to obtain the signals indicating the sensed differential pressure and temperature, and determine a flow rate of the fluid in the pipe based on the obtained pressure and temperature signals. It will be appreciated that although shown separately in Figure 1, in some examples, the pressure sensing apparatus 114 and the flow calculator 116 may be integrated into a single unit. For example, the pressure sensing apparatus 114 may comprise the flow calculator 116 (e.g. the flow calculator 116 may be a processor of the pressure sensing apparatus 114) or vice versa, and / or the pressure sensing apparatus 114 and flow calculator 116 may be provided within a common housing. The flow calculator 116 is configured to use the obtained temperature and differential pressure values to calculate the mass flow rate of the fluid in the pipe. For a given pair of tapping points 112a-g (e.g. between 112a and 112d), the restriction of the flow caused by the decrease of the diameter of the conduit between the two points provides a drop in pressure in the fluid. The length of the conduit 102, e.g. Venturi tube, between the given pair of tapping points can therefore be considered as a pressure drop provider. The sensed differential pressure is related to the mass flow rate according to the equation below. In particular, the flow calculator 116 may be configured to use the obtained values, and stored values, to calculate the flow rate through the pipe to which the flow meter 100 is coupled according to the equation: C 7T ,-------------- 9™ = where: • p is the fluid density. • AP is the measured differential pressure between the pair of points along the conduit. • qm is the mass flow rate to be determined. • C is the discharge coefficient of the pressure drop provider (i.e. of the section of the conduit 102 between the two tapping points). • iS the velocity of approach factor, where • p is the ratio of the diameter of the conduit at the smaller-diameter tapping point of the pair, d, to the diameter of the conduit at the larger-diameter tapping point of the pair, D. • d is the diameter of the conduit at the smaller-diameter tapping point of the pair. • e is the expansibility section of the conduit 102 between the two tapping points, where: and k is the isentropic exponent of the fluid. 1 — \ / 1 — - 1 / \ 1 — T J P — Ap T “ P It will be appreciated that for each of the possible pairs of tapping positions 112a-g from which the differential pressure can be measured, each of the parameters above may have different values.. A differential pressure &P may be sensed between any given pair of the tapping points 112a-g along the conduit 102. As the diameter of the conduit 102 at each tapping point 112a-g is known, the p ratio for each pair can be calculated and / or may be known and stored. It is thus possible to determine the mass flow rate based on a first differential pressure between a first pair of positions (e.g. between point 112a and point 112d shown in Figure 1) and the associated p ratio of that pair, and to also determine the mass flow rate based on a second pair of positions (e.g. point 112b and point 112e) and the associated p ratio of that pair. Different pairs of tapping points are preferred for different ranges of fluid flow rates. For example, if the fluid flow rate is too high fluid the differential pressure between one pair of tapping points (e.g. 112c &112d) may be too high to be accurately measured, and so instead the flow meter 100 determines the flow rate based on the differential pressure between another pair of tapping points (e.g. 112b &112d, 112a &112e). The same is also true if the differential pressure between a given pair of tapping points is too low to be accurately measured. It will be appreciated that the particular pairs mentioned above are merely exemplary, and that the flow rate may be determined based on the sensed differential pressure between any pair of tapping points which provided a differential pressure in a suitable range. In this way the turndown ratio of the flowmeter 100 may be improved. In some examples, the flow meter 100 may switch between operating in one mode in which the mass flow rate in a first range is determined based on the differential pressure between a first pair of positions, and a second mode in which the mass flow rate in a second range is determined based on the differential pressure between a second pair of positions. Said switching may occur in response to the sensed differential pressure falling below a threshold value, or exceeding a threshold value. One of the tapping points of a given second pair may be the same as one of the tapping points of a first pair, or alternatively both of the tapping points of the second pair may be diff erentfrom the tapping points of the first pair. Figure 2 illustrates an example method 200 for determining a flow rate of a flow fluid through a pipe, which for example may be a method of operation of the flow meter 100. The first step 202 comprises sensing a first differential pressure of the fluid between a first pair of positions in a conduit, e.g. between tapping points a and d of the conduit 102 of the flow meter 100 discussed above. The method 200 further comprises at step 204 sensing a second differential pressure between a second pair of positions in the conduit (e.g. positions 112b and 112e). The method further comprises 206 determining a mass flow rate in a first range based on the sensed differential pressure of the fluid between the firstand the second position, and at step 208 determining a mass flow rate in a second range based on the sensed differential pressure of the fluid between the first and the third position, wherein the second range is different from the first range. Flow meters such as flowmeter 100 described above may also be used to determine flow rates for each of the phases of a multiphase fluid flow, in addition to determining a mass flow rate as described above. Figure 3a helps to illustrate an example process which may be used for determining the flow rates of a multiphase fluid. In particular, as discussed above, when transporting fluid such as natural gas, certain amounts of other materials, e.g. liquid hydrocarbon condensate, and liquid water, may be present in the gas flow. The presence of these other fluid phases may affect the determination of the flow rate of the gas and lead to inaccuracies. Some flow meters according to the present disclosure are configured to determine, based on the sensed differential pressures from multiple pairs of positions along a conduit, a flow rate for each of the phases of a multiphase flow. As described above, a sensed differential pressure may be used to determine a mass flow rate. However, in addition, a further differential pressure may be sensed which corresponds to a ‘recovered pressure’ that can be used to determine the ‘pressure loss ratio’ of flow meter, e.g. of the Venturi tube shown in Figure 1. The ‘pressure loss ratio’ is typically defined as the ratio of the differential pressure between the inflow end and the outflow end of the conduit, to the differential pressure between the inflow end of the conduit and its narrowest portion (e.g. the sensed differential pressure used to determine the mass flow rate). For example, with reference to Figure 1, a ‘recovered pressure’ may be sensed between tapping points 112a and 112f, and a differential pressure based on the pressure drop provided by the flow restrictor (e.g. the Venturi tube in Fig. 1) may be sensed e.g. between tapping points 112a and 112d. Based on these readings, and predetermined calibration data and pressure loss functions, individual flow rates for both fluids in a two-phase flow may be determined. However, in some circumstances, it may be necessary or desirable to determine the flow rates of three or more fluids in a multiphase flow. One such example is a natural (e.g. hydrocarbon) gas flow which contains liquid hydrocarbon condensate and liquid water. The presence of these liquids in the gas flow may cause an overread in the standard Venturi sensed differential pressure, such that the mass flow rate determined using this sensed differential pressure does not correspond to the flow rate of the gas. It is believed that the pressure loss from a Venturi flow meter in wet gas service that can create the differential pressure over-read is in part a function of p (discussed above), inlet diameter and gas Froude number (where the Froude number is based on the density and the velocity of the flow). Due to the geometry of the flow meter, and pressure losses at different diameters, different values of gas density and velocity occur at different points along the conduit (e.g. conduit 102). Embodiments of the present disclosure, for example the flow meter 100 described, may be used to sense multiple “recovered pressures” from different points along the conduit, and thereby enable the calculation of multiple “pressure loss ratios”. Based on the determined mass flow rate (which may be determined using the method described above with reference to figures 1 and 2) and these two (or more) determined pressure loss ratios, a value for the flow rates of each of the component phases of a multiphase (e.g. 3 or more)flow may be obtained. Figure 3a illustrates one example of this process 300. Such a process may be performed by a computer or processor, for example by a flow calculator such as the flow calculator 116 shown in Figure 1. As shown in Figure 2, at a first step 301, a plurality of input data is obtained. These inputs correspond to parameters which are sensed by the flow meter, e.g. the flow meter 100 shown in Figure 1. In particular, a static line pressure, a temperature, and multiple (e.g. 4) differential pressures (including ‘recovered’ pressures) are obtained. A value for the temperature of the fluid is obtained, e.g. based on a temperature reading from the temperature sensor 118 shown in Figure 1. In addition, a plurality of differential pressure values are obtained from the conduit 102, e.g. via the differential pressure sensors of the pressure sensing apparatus 114. To help illustrate this, figure 3b is a simplified version of a portion of the flowmeter 100 of Figure 1, and shows an example of the sensed differential pressures used in the method illustrated in Figure 3a. In particular, a first differential pressure (x) is obtained between tapping points 112a and 112d, and a second differential pressure (y) may be obtained between tapping points 112b and 112c. Further differential pressure values are also obtained, which correspond to ‘recovered pressure’. In particular, a first recovered pressure Zi is obtained between tapping points 112a and 112g, and a second recovered pressure Z2 is obtained between tapping points 112b and 112g. The flow meter 100 is also configured to sense a static line pressure of the fluid in the pipe, e.g. at point 112a. At step 302, the obtained data is used to determine further parameters. In particular, a pair of pressure loss ratios PLR 1, PLR 2, are calculated based on the obtained differential pressure measurements. A first pressure loss ratio PLR 1 may be defined, with reference to Figure 3b, as the ratio of differential pressure Z1 to differential pressure X, and a second pressure loss ratio PLR 2 may be defined as the ratio of differential pressure Z2 to differential pressure Y. In addition, a mass flow rate for the flow may be determined using differential pressure X and / or Y. The mass flow rate may be determined using the mass flow rate equation set out above, based on the known diameters of the conduit and p ratios at the pair of positions X and / or Y, and the sensed differential pressures. In addition, a density of the fluid is determined based on the obtained temperature and static line pressure values. In particular, based on the sensed line pressure and temperature, and the known volume of the conduit (or a portion thereof), both the density of each phase of the fluid, and the condensate to gas mass ratio (CGMR) may be calculated, e.g. based on known equations of state and the fluid composition. As shown in Fig. 3a the calculated density of the gas phase is also used in the calculation of mass flow rate, e.g. according to the mass flow equation outlined above. At step 303 the values calculated at step 302 are used to determine a “wet gas correction”. In particular, the determined mass flow rate is adjusted based on the determined pressure loss ratios and the phase density values, in order to determine the mass flow rate for one or more of the individual phases which make up the multi-phase flow. In the case of a three-phase flow comprising gas, liquid water, and liquid hydrocarbon condensate, it is possible to determine the mass flow rate for all three phases, from any two parameters which provide non-correlated information about the condensate and water fractions, and thus determine the gas mass fraction and gas volume fraction of the flow. In particular, the pressure loss ratio fora given differential pressure and corresponding recovered pressure (e.g. X and Zi shown in figure 3b) may vary with liquid volume fraction (the ratio of the liquid volumetric flow rate to the total volumetric flow rate) according to a known relationship. The pressure loss ratio for another differential pressure and corresponding recovered pressure (e.g. Y and Z2) may vary with liquid volume fraction according to another known relationship. These known relationships may have been determined based on calibration data. An example of this is illustrated in Figure 3c, which shows how the pressure loss ratio varies with liquid volume fraction as measured at different pairs of tapping points. Therefore, a first determined pressure loss ratio (e.g. PLR 1) may be used to determine a first liquid volume fraction (LVF 1)and a second determined pressure loss ratio (e.g. PLR 2) may be used to determine a second liquid volume fraction (LVF 2). Based on these determined liquid volume fraction values, and the determined mass flow rate and densities, a value for the flow rate of the gas phase of the flow, or indeed values for the flow rates of each of the multiple (e.g. 3) phases in the flow, such as gas, condensate, and water, may be obtained via an iterative process. As one example, Figure 3c illustrates that a liquid volume fraction (LVF) may be determined from each of the measured and determined pressure loss ratios (e.g. — or—). x y For a given flow, a difference may be observed between the determined liquid volume fractions derived from different pairs of sensors. This may be accounted for due to the composition of the liquid, and the relative proportion of the flow which is water and which is condensate. Set out below are three worked examples of methods which may be used to calculate the mass flow rates, and mass and volume fractions, of the individual phases of a three-phase flow, based on a pair of determined pressure loss ratios PLR1, PLR2. PLR1 and PLR2 may be determined as discussed above using differential pressure readings obtained from different pairs of positions along the conduit. In these examples, the densities of each of the component parts of the fluid are known, and the flow meter and fluid have the following exemplary properties: Line Pressure (bar) 70 Isentropic Exponent 1.35 Gas Density (kg / m3) 63.000 Condensate Density (kg / m3) 650 Water Density (kg / m3) 1000 A given first differential pressure pair may have or provide the following values: Discharge Coefficient 1.010 Differential Pressure (mbar) 1000 Beta 0.5500 Throat diameter (mm) 68.409 Tau 0.985714 Expansibility Coefficient 0.990982 From which the following indicated (i.e. uncorrected) mass and volume flow rates may be obtained: Indicated Mass Flow Rate (kg / s) 13.700 Indicated Volume Flow Rate (m3 / h) 782.870 Where the indicated mass flow rate is calculated according to the equation discussed above: Qm E^d2-j2 * fluid density And, the indicated volume flow rate is calculating according to: _____Qm____ gas density*3600 A first and second pressure loss ratio (PLR 1, PLR 2) may then be obtained according to the process discussed above (e.g. from the pairs x, zi, y, Z2), from which two noncorrelated parameters relating the condensate and water fractions, can be obtained according to an iterative process. For example, any of the parameters described herein as being determined iteratively, may derived by an iterative method such as any of the numerical methods described for solving optimisation and simulation problems in Numerical Recipes in C: The Art of Scientific Computing, Second Edition (1992). In a first example, condensate volume fraction and water volume fraction values are derived from determined values of PLR1 and PLR2: Condensate Volume Fraction (CVF) 0.025 Water Volume Fraction (WVF) 0.010 Volumetric flow rates for each of the individual phases of the fluid in the flow can then be obtained: Gas volumetric flow rate (m3 / h) 755.470 Condensate volumetric flow rate (m3 / h) 19.572 Water volumetric flow rate (m3 / h) 7.829 Total volumetric flow rate (m3 / h) 782.870 Where the individual volumetric flow rates are calculated by: qv(gas) = Indicated Volume Flow Rate * (1 — CVF — WVF) qv(condensate) = Indicated Volume Flow Rate * CVF qv(water) = Indicated Volume Flow Rate * WVF Values for the mass flow rate of each phase can then be obtained by dividing the volumetric flow rate of that phase by its known density: Gas mass flow rate (kg / s) 13.221 Condensate mass flow rate (kg / s) 3.534 Water mass flow rate (kg / s) 2.175 Total mass flow rate (kg / s) 18.929 Obtaining these values also allow the derivation of other parameters. For example, the overall density of liquid in the flow (i.e. water and condensate combined), according to: Condensate mass flow rate + Water mass flow rate liquid density = -— -----------—-------—---------— ----- Condensate volumetric flow rate + Water volumetric flow rate = 750.00 kg / m3 The gas mass fraction can also be determined according to: Gas mass flow rate GMF = -—;----------= 69.8% Total mass now rate and the gas volume fraction according to: Gas volume flow rate GVF = -----------------= 96.5% Total volume now rate Each of the above calculated values may be provided to a user for review / analysis, e.g. at a user terminal. In a second example, values for the liquid volume fraction (LVF) and the water-to-liquid volume ratio (WLVR) may be obtained iteratively from PLR1 &PLR2 instead of the condensate and water volume fractions as discussed above. liquid volume fraction (LVF) 0.035 water-to-liquid volume ratio (WLVR) 29% In this case the individual volumetric flow rates are calculated according to: qv(gas) = Indicated Volume Flow Rate * (1 — LVF) qv(condensate) = Indicated Volume Flow Rate * LVF * (1 — WLVR) qv(water) = Indicated Volume Flow Rate * LVF * WLVR Thereby arriving at the same values as in the first example. The mass flow rates for each individual phase, as well as the liquid density, gas mass fraction and gas volume fraction can then also be calculated in the same way as in the first example. In a third example, a condensate-to-gas mass ratio and a water-to-liquid volume ratio (WLVR) may be derived iteratively from PLR1 and PLR2 and used to determine the same parameters. In this case, the mass flow rates for each individual phase, as well as the liquid density, gas mass fraction and gas volume fraction, may be calculated according to the de Leeuw wet gas correction formula, which is set out for example in standards document ISO / TR 12748:2015 section 6.4.3. In other examples, the mass flow rate of the constituent parts of a wet gas flow may be determined based on a calculation of the “Lockhart-Martinelli parameter”, as set out for example in BS Standards Publication PD ISO / TR 11583:2012 “Measurement of wet gas flow by means of pressure differential devices inserted in circular cross-section conduits”. However, it will be understood that this and the other examples above are merely exemplary and that other methods of determining flow rates of the constituent parts of a multi-phase flow will be apparent to the skilled reader. The calculated values are provided as an output at step 304 of Fig. 3a. The outputs 304 may be provided from the flow calculator to a resource such as a user terminal, computer, server or other device. Inaddition to the calculated mass flowrates, any of the other values measured or determined throughoutthe processes mentioned above may also be provided from the flow meter to such a resource in this way. In addition, the output of line volume or base volume (at stock tank conditions) may also be output from the meter. It will be appreciated that the pairs of tapping points shown in Figure 3b are merely exemplary, and that differential pressures, including recovered pressures, may be sensed between any pair of tapping points and used in the method of figure 3a. Although the recovered pressures Zi, Z2 are shown as being sensed between the output end of the conduit 112g, this is merely exemplary, and it is possible to sense recovered pressures from points along the conduit other than the output region of maximum diameter. Although flowmeter 100 in Figure 1 is shown to have seven tapping points between which the differential pressure can be sensed, it will be appreciated that this is merely exemplary, and that flow meters may have more than or less than this number of tapping points (e.g. as few as three such tapping points). It will be appreciated from the discussion above that the examples shown in the figures are merely exemplary, and include features which may be generalised, removed or replaced as described herein and as set out in the claims. With reference to the drawings in general, it will be appreciated that schematic functional block diagrams are used to indicate functionality of systems and apparatus described herein. In addition, the processing functionality may also be provided by devices which are supported by an electronic device. It will be appreciated however that the functionality need not be divided in this way, and should not be taken to imply any particular structure of hardware other than that described and claimed below. The function of one or more of the elements shown in the drawings may be further subdivided, and / or distributed throughout apparatus of the disclosure. In some examples the function of one or more elements shown in the drawings may be integrated into a single functional unit, or alternatively further divided into subunits. As will be appreciated by the skilled reader in the context of the present disclosure, each of the examples described herein may be implemented in a variety of different ways. Any feature of any aspects of the disclosure may be combined with any of the other aspects of the disclosure. For example, method aspects may be combined with apparatus aspects, and features described with reference to the operation of particular elements of apparatus may be provided in methods which do not use those particular types of apparatus. In addition, each of the features of each of the examples is intended to be separable from the features which it is described in combination with, unless it is expressly stated that some other feature is essential to its operation. Each of these separable features may of course be combined with any of the other features of the examples in which it is described, or with any of the other features or combination of features of any of the other examples described herein. Furthermore, equivalents and modifications not described above may also be employed without departing from the invention. Certain features of the methods described herein may be implemented in hardware, and one or more functions of the apparatus may be implemented in method steps. It will also be appreciated in the context of the present disclosure that the methods described herein need not be performed in the order in which they are described, nor necessarily in the order in which they are depicted in the drawings. Accordingly, aspects of the disclosure which are described with reference to products or apparatus are also intended to be implemented as methods and vice versa. The methods described herein may be implemented in computer programs, or in hardware or in any combination thereof. Computer programs include software, middleware, firmware, and any combination thereof. 5 Such programs may be provided as signals or network messages and may be recorded on computer readable media such as tangible computer readable media which may store the computer programs in non-transitory form. Hardware includes computers, handheld devices, programmable processors, general purpose processors, application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), and arrays of logic 10 gates. Other examples and variations of the disclosure will be apparent to the skilled addressee in the context of the present disclosure.

Claims

1. A flow meter for determining a flow rate of a flow of fluid through a pipe, the flow meter comprising:a conduit in fluid communication with the pipe and arranged to provide a flow path for the fluid, wherein the diameter of the conduit varies along the flow path;a pressure sensing apparatus configured to sense a first differential pressure of the fluid between a first pair of sensing positions along the conduit, and a second differential pressure between a second pair of sensing positions along the conduit, wherein the position along the conduit of at least one of the first pair of sensing positions is different from at least one of the second pair of sensing positions;a flow calculator configured:to determine a mass flow rate in a first range based on the first differential pressure; andto determine a mass flow rate in a second range based on the second differential pressure;wherein the second range is different from the first range.

2. The flow meter of claim 1, wherein the flow calculator is configured to provide the determination of the mass flow rate in the second range in response to the sensed differential pressure falling below a first threshold value.

3. The flow meter of claim 1 or 2, wherein the flow calculator is configured to provide the determination of the mass flow rate in the first range in response to the sensed differential pressure exceeding a second threshold value.

4. The flow meter of any preceding claim, wherein one of the first pair of sensing positions is the same as one of the second pair of sensing positions.

5. The flow meter of any preceding claim, wherein each pair of sensing positions comprise a first sensing position at which the diameter of the conduit is greater than at a second sensing position of the pair.

6. The flow meter of claim 5, wherein the larger diameter position of each pair isupstream of the smaller diameter position of that pair.

7. The flow meter of claim 5, wherein the larger diameter position of each pair is downstream of the smaller diameter position of that pair.

8. The flow meter of claim 5, wherein the larger diameter position of the first pair is upstream of the smaller diameter position, and the larger diameter position of the second pair is downstream of the smaller diameter position.

9. The flow meter of any preceding claim, wherein the difference in the diameter of the conduit between the first pair of sensing positions is greater than between the second pair of sensing positions.

10. The flow meter of any preceding claim, wherein in the flow calculator is configured in the first range to determine the mass flow rate based on a ratio of the conduit diameters at the first pair of sensing positions, and in the second range to determine the mass flow rate based on a ratio of the conduit diameters at the second pair of sensing positions.

11. The flow meter of any preceding claim, wherein the conduit comprises a plurality of tapered portions along its longitudinal length, separated by a plurality of portions of constant diameter, for example wherein each sensing position is arranged in a corresponding one of the portions of constant diameter for example, wherein at least one of the sections of constant diameter has longitudinal length different from at least one other of the portions of constant diameter.

12. The flow meter of any preceding claim, further comprising a temperature sensor arranged to sense the temperature of the fluid in the conduit, wherein the flow calculator is configured to determine the mass flow rate based on the sensed temperature.

13. A method of determining a flow rate of a flow of fluid through a pipe, the method comprising:sensing a first differential pressure of the fluid between a first pair of positions in aconduit, said conduit in fluid communication with the pipe;sensing a second differential pressure between a second pair of positions along the conduit;determining a mass flow rate in a first range based on the sensed differential pressure of the fluid between the first pair of positions; anddetermining a mass flow rate in a second range based on the sensed differential pressure of the fluid between the second pair of positions;wherein the second range is different from the first range.

14. The method of claim 13 comprising determining the mass flow rate in the second range in response to the sensed differential pressure falling below a first threshold value.

15. The method of claim 13 or 14 comprising determining the mass flow rate in the first range in response to the sensed differential pressure exceeding a second threshold value.

16. A method of determining flow rates for one or more of the phases of a multiphase fluid flow, the method comprising:sensing, in a conduit which has a diameter that varies along its length, a plurality of differential pressures between pairs of positions along the length of the conduit;determining, based on the sensed differential pressures:a mass flow rate;a first pressure loss ratio;a second pressure loss ratio;wherein the first pressure loss ratio is based on a first differential pressure (x) sensed between a first pair of positions along the length of the conduit;wherein the second pressure loss ratio is based on a second differential pressure (y) sensed between a second pair of positions along the length of the conduit;wherein the diameter of the conduit at one or both of the first pair of positions is different from the diameter of the conduit at both of the second pair of positions;determining the flow rate of at least one of the phases of the flow, based on the mass flow rate, the first pressure loss ratio, and the second pressure loss ratio.

17. The method of claim 16, wherein the conduit has an inlet portion, an outlet portion, and a throat portion arranged between the inlet portion and the outlet portion, wherein the diameter of the conduit at the throat portion is less than the diameter of conduit at the inlet portion and at the outlet portion.

18. The method of claim 16 or 17, a third differential pressure to the first differential pressure.

19. The method of any of claims 16 to 18, wherein the second pressure loss ratio is based on a ratio of a fourth differential pressure to the second differential pressure.

20. The method of any of claims 16 to 19, further comprising determining a density of the flow, wherein determining the fluid flow rate of each of the phases of the flow is further based on the determined density, for example further comprising, sensing a temperature of the fluid in the flow; and sensing a line pressure at an input of the conduit; wherein determining the density is based on the sensed temperature and line pressure.

21. The method of any of claims 16 to 20, further comprising determining a third pressure loss ratio based on a third differential pressure sensed between a third pair of positions along the length of the conduit, wherein the diameter of the conduit at one or both of the third pair of positions is different from the diameter of the conduit at both of the first pair of positions and both of the second pair of positions.

22. A flow meter configured to perform the method of any of claims 16 to 21.

23. The flow meter of any of claims 1 to 12, configured to perform the method of any of claims 13 to 21.

24. A flow meter for determining flow rates for one or more of the phases of a multiphase fluid flow, the flow meter comprising:a conduit arranged to provide a flow path for the fluid, wherein the diameter of the conduit varies along the flow path;a pressure sensing apparatus configured to sense a first differential pressure (x) between a first pair of positions along the length of the conduit, and a second differentialpressure (y) between a second pair of positions along the length of the conduit, wherein the diameter of the conduit at both of the first pair of positions is different from the diameter of the conduit at one or both of the second pair of positions,a flow calculator configured to determine:a mass flow rate;a first pressure loss ratio based on the first differential pressure;a second pressure loss ratio based on the second differential pressure;and further configured to determine the flow rate of at least one of the phases of the flow, based on the mass flow rate, the first pressure loss ratio, and the second pressure loss ratio.

25. The flow meter of claim 24, wherein the pressure sensing apparatus is configured to sense a third differential pressure between a third pair of positions along the length of the conduit, wherein the diameter of the conduit at at least one of the third pair of positions is different from the diameter of the conduit at both of the first pair of positions and the diameter of the conduit at both of the second pair of positions;wherein the flow calculator is further configured to:determine a third pressure loss ratio based on the third differential pressure;and determine the flow rate of at least one of the phases of the flow further based on the third pressure loss ratio.

Images