FLOW METER FOR TWO-PHASE FLUID

Description

[0001] The present invention relates to the field of flow meters for two-phase gas / liquid cryogenic fluids.

[0002] Measuring the flow rate of a two-phase fluid composed of a liquid and a gas is a difficult operation when trying to measure a mass flow rate. Indeed, all sensors measuring a flow rate are hampered when they are placed in the presence of a two-phase liquid whose density changes at any time. This is particularly true for measuring the flow rate of cryogenic fluids such as liquid nitrogen.

[0003] Some flow meters listed in the literature are based on measuring fluid velocity. These include, for example: Turbine flow meters: a turbine is installed in the moving fluid and the rotational speed of the turbine gives an image of the fluid's velocity. Pitot tube flow meters: two tubes are installed in the moving fluid to be measured. One tube is installed perpendicular to the flow and gives the static pressure, the other is installed parallel to the flow and gives the total dynamic pressure. The difference in dynamic pressure between these two measurements is used to calculate the flow rate. Ultrasonic flow meters: some use the Doppler effect (analysis of the frequency reflected by the particles of the fluid which gives an image of the velocity of the particle and therefore of the fluid) while others measure a difference in travel time of an ultrasonic wave from upstream to downstream and from downstream to upstream (image of the fluid velocity).

[0004] In all these cases, when the density of the fluid varies continuously, the transition from volume flow to mass flow is difficult to achieve precisely.

[0005] Other systems use pressure drop measurement to deduce flow rate. These include, for example, calibrated orifice flow meters, which measure pressure drop upstream and downstream of a calibrated orifice placed in the moving fluid. The measurement of these devices is very disturbed when the fluid does not have a constant density and when the gas content in the liquid increases.

[0006] Electromagnetic flow meters, applicable only to fluids with sufficient electrical conductivity, use the principle of electromagnetic induction: An electromagnetic field is applied to the fluid and the electromotive force created (force proportional to the fluid flow rate) is measured. In the case of flow measurement of cryogenic (non-conductive) fluids such as liquid nitrogen, this principle is not applicable.

[0007] Vortex flow meters are based on the phenomenon of vortex generation observed behind a fixed, non-profiled body placed in a moving fluid (Karman effect). Measuring the pressure variations created by these vortices gives the vortex frequency, which is proportional to the fluid velocity when the fluid maintains constant properties. When the fluid density varies, the measurement is distorted.

[0008] Thermal flow meters are based on measuring the temperature increase created by a constant supply of energy. A system with two temperature probes measures the temperature difference between the flow entering and leaving the flow meter. Between these two probes, a resistor provides a known amount of energy. When the heat capacity of the moving fluid is known, the flow rate can be calculated from these measurements. However, this principle is not applicable to two-phase liquids whose thermal behavior (vaporization of the liquid) is completely different from single-phase liquids.

[0009] Only the Coriolis mass flow meter provides an accurate measurement of the mass flow rate of a fluid. The flow meter consists of a U-shaped or omega-shaped or curved tube, through which the fluid circulates. The U-shaped tube is subject to lateral oscillation and measuring the phase shift of the vibrations between the two branches of the U-shaped tube provides an image of the mass flow rate. However, its cost is quite high and when used at very low temperatures (liquid nitrogen at -196°C for example) and with a fluid whose density varies enormously and includes a significant portion in the gas phase, it is necessary to strongly isolate the system (high-performance insulation such as vacuum insulation for example) and despite this, the measurements are distorted when the gas content exceeds a few percent by mass. It should also be noted that the measurement is often made impossible when the fluid velocity is low or zero (in the first half of the measurement range).

[0010] As can be seen, measuring the flow rate of a two-phase liquid and in particular measuring the flow rate of a cryogenic fluid with acceptable accuracy is not easy to achieve with the devices currently available on the market.

[0011] It can nevertheless be noted that systems are currently being marketed.

[0012] For example, we can cite the case of systems based on the principle of measuring the level of a liquid flowing in a channel just before a restriction of the passage section. This system, described in document US- 5,679,905, operates essentially as follows: the two-phase fluid is first separated into a gas phase which is not measured and a liquid phase whose flow rate is measured. This liquid passes into a channel which has a reduction in section at its outlet. The greater the flow rate, the greater the liquid level in the channel and a level measurement in this channel then makes it possible to deduce the instantaneous flow rate. As can be seen, this system does not take into account the gas flow rate which in certain applications is negligible. On the other hand, this system makes it possible to measure the liquid flow rate with relatively good accuracy without being disturbed by the gas level, which is the desired goal.

[0013] It should be noted in passing that for this system to function properly, it must be well insulated from heat inputs that could vaporize part of the isolated liquid and thus disrupt the level measurement. This is why vacuum insulation is used in this system.

[0014] It should also be noted that for the system to work, there must be two phases in the flow meter, which prevents it from operating with a sub-cooled liquid (clear liquid without a gas phase).

[0015] It should also be noted that this document uses a V-shaped slot, which has the disadvantage of being difficult to produce with great precision. A 5% variation in the slot width has very serious consequences because it makes the measurement inaccurate in the same proportions.

[0016] We can also cite the case of phase separator flow meters.

[0017] In fact, in the case where the measurement of liquid and gas flow rates is necessary, a system is sometimes used which uses the same principle of phase separation before the flow measurement.

[0018] Thus, commercially available devices have the following feature: The two-phase liquid first passes through a phase separator which separates the liquid phase from the gas phase; The gas phase is directed to a volume flow meter (turbine type for example) with temperature compensation; The liquid phase is also directed to a volume flow meter (turbine type for example); These two flow measurements are then converted into mass measurement and added.

[0019] A priori, this device is more expensive than the previous one, we can think that it will be very precise. In practice, we note that the measurement of the liquid flow is affected by errors which fluctuate according to the pressure and temperature conditions of the liquid entering the flow meter. These measurement errors are due to the presence of gas in the liquid phase which passes through the flow meter. Indeed, when the liquid leaves the phase separator to go towards the flow meter, a part of the liquid vaporizes, either because of heat inputs or because of the pressure drop due to a rise of the liquid, or because of a pressure drop due to the pressure loss created by the flow meter itself.

[0020] Finally, to measure the flow rate of a cryogenic liquid, we can also overcome the problems mentioned above by creating pressure and temperature conditions different from the equilibrium pressure (boiling limit). In this area, the most commonly used method is, for example, a flow meter at the outlet of a cryogenic pump (high pressure side). In this case, the liquid is, for example, pumped into a tank where it is at equilibrium and its pressure is increased by the pump, with almost no increase in temperature. The pipes and the flow meter that follow can then create a pressure drop; this will not result in vaporizing the liquid, provided that the pressure drop is significantly lower than the increase in pressure created by the pump.

[0021] In this case, a classic vortex, turbine or other type flow meter can be used as long as it can withstand low temperatures.

[0022] This technique is, for example, perfectly suited to measuring the flow rate of nitrogen delivery trucks. It is reliable and cost-effective, as the cryogenic pump is required for other reasons.

[0023] On the other hand, when it is necessary to measure the flow of liquid nitrogen at a point where there is no cryogenic pump, then this technique is no longer interesting.

[0024] We also know a solution for simultaneous or alternating measurement of the liquid and gaseous phases, as described in document FR-3 013 446 in the name of the Applicant, based on the following principle: the fluid arrives in a tank acting as a phase separator; the gas phase is evacuated from the top of the tank via a flow meter operating on a pure gas phase; the liquid phase is evacuated from the bottom of the tank via a flow meter operating on a pure liquid phase; the two phases are then combined at a three-way valve and continue on their way; equipped with the two measured flow rates as well as the pressure and temperature of the fluid, the system can calculate the mass flow rate of the fluid passing through the flow meter.

[0025] This system is accurate and works regardless of the two-phase ratio present in the fluid. It works accurately when the fluid is completely gaseous or when it is completely liquid or subcooled, but it also works in all intermediate situations.

[0026] However, this system is penalized by the fact that it is relatively expensive and its installation is relatively complex.

[0027] It must be installed horizontally and its size is quite large (typically 1 meter wide, 1 meter long, 2 meters high).

[0028] The present invention therefore aims to propose a new, simple and reliable solution for measuring the flow rate of cryogenic gas / liquid two-phase fluids, making it possible to solve all or part of the technical problems mentioned above.

[0029] As will be seen in more detail below, the solution proposed here is based on the implementation of the following measures: 1. Flow measurement of the non-subcooled liquid phase using a weir system: a flow measurement of the non-subcooled liquid phase with a so-called "weir" system. For this, the two-phase fluid is first naturally separated into a gas phase which is not measured and a liquid phase whose flow rate is measured.

[0030] The principle of the spillway is as follows: An obstacle (partition perforated with one or more slots) is installed in the liquid passage, it slows the flow of the liquid. The greater the flow rate, the higher the level upstream of the obstacle will rise. With a calibrated obstacle, it is then possible to calculate the flow rate based on the liquid level measured upstream of the obstacle.

[0031] Differential pressure measurement is used to measure the height of the liquid upstream of the obstacle. Among the differential pressure sensors available on the market, sensors that can measure low pressure values ​​are particularly suitable.

[0032] However, to achieve this pressure level with a cryogenic fluid height, a height of around 300 mm must be obtained. For this reason, it is advantageous to orient the weir in the vertical direction so that it allows the creation of a high liquid height, and the differential pressure measurement is therefore also quite high. 2. Measurement of the flow rate of the subcooled liquid phase with a calibrated orifice: when the cryogenic fluid is subcooled, then the flow rate measurement is carried out using a calibrated orifice located upstream or downstream of the weir.

[0033] The liquid passes through the calibrated orifice and generates a pressure difference. By calculation, it is then possible to obtain the flow rate of the subcooled cryogenic fluid.

[0034] It should be noted here that this flow measurement system does not work when the fluid is not subcooled. When it is saturated (or at equilibrium), the presence of gas in the liquid distorts the measurement, the variation in measured pressure generates more two-phase flow. The measured pressure difference is not representative of the quantity of cryogenic fluid passing through the calibrated orifice. It is therefore necessary to know at all times the state of the cryogenic fluid, subcooled or not. 3. Measurement of the subcooling state of the cryogenic fluid (determination of the state of the cryogenic fluid: subcooled or gas-liquid equilibrium): to know the state of the cryogenic fluid being evaluated, the subcooling state of the cryogenic fluid whose flow rate is to be measured is measured.

[0035] To this end, the liquid level downstream of the spillway is measured. When this level is zero, the cryogenic fluid is not undercooled, whereas in the opposite case, the cryogenic fluid is undercooled: Case No. 1: If the sensor measuring the liquid height in the device (delta P2, downstream of the weir) indicates a near-zero value, the fluid therefore has a gas phase and a liquid phase. The sensor measuring the pressure difference across the calibrated orifice (Delta P3) will also indicate a non-zero value (gas + liquid). Case No. 2: If the sensor measuring the liquid height in the device (ΔP2, downstream of the weir) indicates a non-zero value, the fluid therefore has only a liquid phase. The sensor measuring the pressure difference across the calibrated orifice (ΔP3) will indicate a value representative of the pure liquid flow rate.

[0036] To reliably detect whether the fluid is in case No. 1 or in case No. 2, a pressure difference measurement is carried out in a second volume downstream of the spillway (ΔP2) as seen previously. This volume is either filled with gas (case No. 1) or filled with liquid (case No. 2), due to the difference in density between the liquid and gas phases, and thus makes it possible to define whether the fluid is pure liquid or a two-phase fluid.

[0037] And when ΔP3 is negative, this means that the fluid flow is reversed (from downstream to upstream): In this case, this flow is neutralized and not taken into account by the flow meter. The error is thus minimized. 4. According to an advantageous embodiment of the invention, what can be called a “multi-slot weir” is implemented.

[0038] As mentioned above in the description of the prior art (US-5,679,905), one of the critical points of such prior weirs lies in the manufacturing precision of their V-shaped slot. For the measurement to be precise, the geometry of this V-shaped slot must be perfectly controlled. In the case of a slot with a fixed width (not V-shaped), it is important that this slot maintains a constant width over its entire length. In practice, with the constraints linked to construction techniques, it is very difficult to guarantee that this slot has a constant width.

[0039] In this context, it is proposed according to the present invention to produce this slot in several portions, thus in a way implementing a spillway with several slots, each segment having a short length making it possible to maintain a constant width more reliably.

[0040] The combination of all these slot sections forms the equivalent of a slot with a constant width over the entire height. The high point of one slot corresponds exactly to the low point of the next. There is no overlap. 5. Calculation of an estimate of the two-phase rate: thanks to the system described above, we can, as we have understood, measure the liquid phase whatever the conditions, but we can go further and carry out calculations based on the two measurements (via weir and calibrated orifice) and then estimate the two-phase rate. This rate makes it possible to determine the gas phase and thus refine the measurement of total fluid flow rate.

[0041] The advantages of the flow meter proposed according to the present invention can be summarized as follows: moderate cost; a measurement offering very good accuracy (typically 2%); a system that cannot give rise to an erroneous measurement when the flow rate is zero or slightly negative and the cryogenic fluid is boiling in the flow meter; ease of installation; it provides reliable measurements when the gas rate in the fluid varies from 0 to 100%, the system even allowing the flow rate to be measured when the liquid is undercooled.

[0042] In summary, the flow meter proposed here operates according to the following operating principle, which is explained in connection with the [ Figure 1 ] annexed: use of a flow measurement using a weir for the liquid phase when the fluid is two-phase, the level being measured using the differential pressure ΔP1, where ΔP1 is the measurement of the pressure difference between the bottom of the tank 5 and the internal atmosphere of the device. This allows us to deduce the height of liquid in the tank and subsequently the flow rate 7 passing through the slots 6. use of a flow measurement by a calibrated orifice (8) and a differential pressure measurement ΔP3 for the gas phase when the fluid is 100% gaseous, for the liquid phase when the fluid is 100% liquid (undercooled) and to correct the liquid flow rate measured by the weir when the fluid is two-phase. In other words, ΔP3 is measured on each side of a calibrated orifice, which allows a quality measurement of the flow rate when the cryogen is 100% gaseous or 100% liquid (subcooled).we can see it in the figure, the system is composed of a tank 5 which is equipped with a segmented slot (several segments of slots 6). These slots represent the spillway. we consider a differential pressure measurement ΔP2, to deduce if there is presence of liquid downstream of the spillway in order to determine the state of the fluid (gaseous, two-phase or sub-cooled). This measurement indicates if there is liquid invasion downstream of the spillway or not, this invasion being able to be due to the sub-cooled state of the cryogenic fluid or to the existence of a flow in the opposite direction. Indeed, in normal operation, this ΔP2 makes a measurement which is close to zero, in other words there is almost no pressure difference between these two points in the gas phase. On the other hand, when the liquid cryogen (for example liquid nitrogen) arrives sub-cooled, the entire system will fill with liquid, we will then have liquid in the entire device (9). In this last case, according to the invention, the flow calculation algorithm will switch to the measurement using the calibrated orifice (ΔP3) which will function completely correctly in the case of a sub-cooled liquid. In this case, we will detect this situation by the appearance of a significant ΔP2 value, and thus when ΔP2 moves away from zero, we switch to the flow measurement mode using the calibrated orifice (ΔP3) and vice versa.

[0043] As will be developed later, in summary, in the case of a two-phase liquid, formulas are used to calculate the gas rate with ΔP1 and ΔP3 as input data. Subsequently, this gas rate is used to correct the flow measurement obtained by the spillway.

[0044] It is also advantageous to measure the fluid temperature in order to deduce its density. Using this density, it is then possible to refine the flow rate calculations linked to the ΔP measurements: ΔP1 of the spillway and ΔP3 of the calibrated orifice.

[0045] Let's recap the nomenclature of the elements present on the [ Figure 1 ] illustrating one of the embodiments of the invention: 1: the flow meter for cryogenic fluids 2: the fluid entering the device, the flow rate of which must be measured 3: the fluid leaving the device and heading downstream 4: device, boiler assembly 5: the tank upstream of the spillway 6: the slots in a vertical wall of the tank 5 (spillway) 7: the flow of liquid escaping from the slots 6 8: a calibrated orifice (providing the pressure difference ΔP3) 9: volume internal to the device

[0046] The embodiment presented in [ Figure 1 ] attached was used to implement the tests explained below, which gave very positive results. Implementation with liquid nitrogen, tests carried out under different pressure conditions (from 1 to 3 barg) and different two-phase rates (from 0 to 100% gas).

[0047] A test was carried out with subcooled liquid nitrogen.

[0048] Of course, the flow meter can easily be adapted to other cryogenic fluids, simply by modifying the section of the slots and the diameter of the calibrated orifice. Overall, these tests show an accuracy of around + / -2% of the full measurement scale, which amounts to 3000 kg / h.

[0049] The experimental results are collected in [Tables] 1 and 2 below.

[0050] [Table 1] shows the calculation logic used here.

[0051] [Table 2] presents the different situations that can be encountered in flow measurement, and by applying the logic stated in Table 1, a consistent flow measurement is obtained.

[0052] All pressures in the Table are ΔP.

[0053] The formulas of the fi functions used are as follows: formula for f1: formula for f2 liquid phase: with liquid height in the spillway = ΔP1 / density of the liquid gas / g (9.81 gravity of the earth) k being a fixed coefficient specific to each device. formula for f2 total (gas and liquid phases of the two-phase gas) = ​​mathematical resolution of the following equations debit massique liquide = f 2 phase liquide calcule ci - dessus debit total = f 2 debit massique liquide + f 2 debit massique gaz with formula for f4: To calculate f4, we perform a mathematical resolution of the equations presented for the calculation of f2. [Table 1] Condition 1 Condition 2 Condition 3 flow measurement Two-phase rate If ΔP3>=0 If ΔP2>0 f3(ΔP3) (flow rate of a calibrated orifice with pure liquid) 0 If ΔP2=<0 If ΔP1>0 f2(ΔP1, f4) (flow rate of a weir with pure liquid + correction for the gas phase) f4(ΔP1, ΔP3) (two-phase rate calculated with ΔP1 and ΔP3) If ΔP1=<0 f1(ΔP3) (flow rate of an orifice calibrated with pure gas) 100 If ΔP3<0 0 0 [Table 2] Direction of flow Cryo fluid condition % gas Delta P1 Delta P2 Delta P3 flow measurement Estimation of the two-phase rate Normal Gaseous 100 0 0 >0 f1(ΔP3) 100% Normal two-phase 0 à 100 >0 0 >0 f2(L1P1, f4) f4(ΔP1, ΔP3) Normal undercooled 0 0 >0 >0 f3(ΔP3) 0% Reverse Gaseous 100 0 0 <0 0 measurement error 0% measurement error Reverse two-phase 0 à 100 0 >0 <0 0 measurement error 0% measurement error reverse undercooled 0 0 >0 <0 0 measurement error 0%

[0054] The examples developed below are based on measurements carried out on equipment conforming to that described in the context of the figure 1 , with a flow meter equipped with a weir with a 2 mm wide slot and a calibrated orifice 8 of 20 mm diameter. The fluid used was liquid nitrogen at a pressure of 2 barg. Situation #1: No gas circulation:

[0055] ΔP1 = 0 ΔP2 = 0 ΔP3 = 0 Condition 1 Condition 2 Condition 3 flow measurement Two-phase rate If ΔP3>=0 If ΔP2>0 f3(ΔP3) (flow rate of a calibrated orifice with pure liquid) 0 If ΔP2=<0 If ΔP1>0 f2(ΔP1, f4) (flow rate of a weir with pure liquid + correction for the gas phase) f4(ΔP1, ΔP3) (two-phase rate calculated with ΔP1 and ΔP3) If AP1=<0 f1(ΔP3) (flow rate of a calibrated orifice with pure gas) 0 kg / h 100% gas If ΔP3<0 0 0 Situation No. 2: pure gas during the cooling of the device:

[0056] ΔP1 = 0 ΔP2 = 0 ΔP3 = 52 mbar Condition 1 Condition 2 Condition 3 flow measurement Two-phase rate If ΔP3>=0 If ΔP2>0 f3(ΔP3) (flow rate of a calibrated orifice with pure liquid) 0 If ΔP2=<0 If ΔP1>0 f2(ΔP1, f4) (flow rate of a weir with pure liquid + correction for the gas phase) f4(ΔP1, ΔP3) (two-phase rate calculated with ΔP1 and ΔP3) If ΔP1=<0 f1(ΔP3) (flow rate of a calibrated orifice with pure gas) 301 kg / h 100% gas If ΔP3<0 0 0 Situation No. 3: Two-phase gas during standard operation:

[0057] ΔP1 = 16 ΔP2 = 0 ΔP3 = 18 mbar Condition 1 Condition 2 Condition 3 flow measurement Two-phase rate If ΔP3>=0 If ΔP2>0 f3(ΔP3) (flow rate of a calibrated orifice with pure liquid) 0 If ΔP2=<0 If ΔP1>0 2(ΔP1, f4) (flow rate of a weir with pure liquid + correction for the gas phase) 863 kg / h f4(ΔP1, ΔP3) (two-phase rate calculated with ΔP1 and ΔP3) 2% gas If ΔP1=<0 f1(ΔP3) (flow rate of an orifice calibrated with pure gas) 100 If ΔP3<0 0 0 Situation No. 4: subcooled liquid gas:

[0058] ΔP1 = 16 ΔP2 = 5 ΔP3 = 15 mbar Condition 1 Condition 2 Condition 3 flow measurement Two-phase rate If ΔP3>=0 If ΔP2>0 f3(ΔP3) (flow rate of a calibrated orifice with pure liquid) 1205 kg / h 0% gas If ΔP2=<0 If ΔP1>0 f2(ΔP1, f4) (flow rate of a weir with pure liquid + correction for the gas phase) f4(ΔP1, ΔP3) (two-phase rate calculated with ΔP1 and ΔP3) If ΔP1=<0 f1(ΔP3) (flow rate of an orifice calibrated with pure gas) 100 If ΔP3<0 0 0 Situation No. 5: Gas flow in the opposite direction:

[0059] ΔP1 = 1 ΔP2 = 8 ΔP3 = -11 (négatif) Condition 1 Condition 2 Condition 3 flow measurement Two-phase rate If ΔP3>=0 If ΔP2>0 f3(ΔP3) (flow rate of a calibrated orifice with pure liquid) 0 If ΔP2=<0 If ΔP1>0 f2(ΔP1, f4) (flow rate of a weir with pure liquid + correction for the gas phase) f4(ΔP1, ΔP3) (two-phase rate calculated with ΔP1 and ΔP3) If ΔP1=<0 f1 (ΔP3) (flow rate from an orifice calibrated with pure gas) 100 If ΔP3<0 0 kg / h 0% gas

[0060] The present invention then relates to (the numerical references can be found in the attached Figure) a flow meter for cryogenic liquid / gas two-phase fluids, comprising: An inlet pipe for the fluid whose flow rate is to be measured, in the flow meter, pipe provided with a calibrated orifice; A vertical tank (5), tank surrounded by a device (4), tank into which said pipe opens, the wall of the tank being provided with several fluid discharge slots (6 - "overflow" system), from the tank (5) to the interior space (9) to the device surrounding the tank (5); Pressure sensors for measuring the following pressure differences: A pressure difference (ΔP3) between upstream and downstream of the calibrated orifice A pressure difference (ΔP1) between the bottom of the tank (5) and the atmosphere surrounding the tank (9) within the interior space (9) within the device;A pressure difference (ΔP2) existing between two points located in the space surrounding the tank (5) within the interior space (9), making it possible to deduce the height of liquid downstream of the tank, in order to determine the state of the fluid (gaseous, two-phase or sub-cooled), and giving the level of liquid invasion downstream of the tank. a data acquisition and processing system, capable of carrying out the following evaluations: a. a determination of the information on the state of the incoming fluid: gaseous, two-phase or sub-cooled, from the pressure difference data ΔP2; b. depending on this state information, the determination of said flow rate using either the pressure differential ΔP3 when the fluid is 100% gaseous or 100% liquid (sub-cooled), or the pressure differential ΔP1 which makes it possible to deduce the height of liquid in the tank and subsequently the flow rate of fluid passing through the slots.;

Claims

1. Flowmeter (1) for liquid / gas cryogenic two-phase fluids, comprising: - An inlet pipe (2) for the fluid of which the flow rate is to be measured, in the flowmeter, which pipe is provided with a calibrated orifice (8); - A vertical reservoir (5), which reservoir is surrounded by an apparatus (4), into which reservoir said pipe opens, the wall of the reservoir being provided with a plurality of slots for outflow of the fluid (6) forming a "spillway" system, from the reservoir (5) towards the interior space (9) inside the apparatus surrounding the reservoir (5); - Pressure sensors making it possible to measure the following pressure differences: • A pressure difference (ΔP3) between upstream and downstream of the calibrated orifice • A pressure difference (ΔP1) between the bottom of the reservoir (5) and the atmosphere surrounding the reservoir (9) within the interior space (9) within the apparatus; • A pressure difference (ΔP2) existing between two points located in the space surrounding the reservoir (5) within said interior space (9), making it possible to deduce the height of liquid downstream of the reservoir, in order to determine the state of the fluid, gaseous, two-phase or subcooled, and giving the level of liquid flooding downstream of the reservoir. - a data acquisition and processing system, which is able to carry out the following evaluations: a. determination of the information on the state of the incoming fluid: gaseous, two-phase or subcooled, from the pressure difference datum ΔP2; b. depending on this state information, the determination of said flow rate using either the pressure differential ΔP3 when the fluid is 100% gaseous or 100% liquid (subcooled), or the pressure differential ΔP1 that makes it possible to deduce the height of liquid in the reservoir and subsequently the flow rate of fluid passing through the slots.