IE920203A1

IE920203A1 - A method for measuring the mass flow of the different¹components of a multi-component flow

Info

Publication number: IE920203A1
Application number: IE920203A
Authority: IE
Original assignee: Europ Economic Community; Elf Aquitaine
Priority date: 1991-01-24
Filing date: 1992-01-23
Publication date: 1992-07-29
Also published as: EP0496330A2; EP0496330A3; LU87879A1

Abstract

The invention relates to a procedure for determining the mass flow of the individual components of a flow consisting of at least two liquids and a gas in a pipe. According to the invention, a sequence of high frequency magnetic pulses is applied to the flow, and the emission of the nuclear paramagnetic magnetisation, and thus the nuclear spin, is measured from the internal volume of the high frequency coil of a nuclear magnetic resonance spectrometer, giving the average speed and the volume fraction of the liquid phase; a free induction decrease in the flow is then produced and measured by means of a short high frequency single pulse, and its frequency dependence is analysed using a Fourier transform. Finally, the individual volume fractions of the liquid components are determined by computer analysis of the Fourier spectrum.

Description

A method for measuring the mass flow of the different components of a multi-component flow The invention relates to a method for measuring the mass flow of the different components of a flow composed of at least two liquids and a gas in a tube.

The invention was conceived in particular for the mass flow measurement in an oil conveying duct, in which the crude oil flows together with water and a gas.

A known solution for measuring the different components of a mass flow consists in separating these components from each other in a separator and then to measure them individually. A measurement in the course of the flow in an oil pipe is not possible with this method. This subsequent measurement is not accurate, firstly because of the non-ideal separation of the components, and secondly because of the temporal and spatial distance between the place where the flow measurement is of interest and the place where it is carried out.

The book Measuring Techniques in Gas-Liquid Two-Phase Flows, published after the IUTAM Symposium, Nancy, 1983, edited by J.M. Delhaye and C. Cognet, 1984 Springer Berlin-Heidelberg, on pages 435 to 454 comprises an essay by G.J. Kruger, J. Haupt and R. Weiss with the title A nuclear magnetic resonance method for the investigation of two-phase flow. Here, the efflux of the nuclear paramagnetic magnetization and thus the nuclear spin from the inner volume of the high frequency coil of a nuclear resonance spectrometer is measured by means of a Carr-Purcell-Meiboom-Gill pulse sequence. The direct measurement result is the nuclear spin echo sequence representing the flow curve and which fades to zero from an initial maximum value during the pulse sequence which determines the time period of the measurement. This curve - 2 contains the entire information of the two phase flow and is then processed. The results are the mean velocity and the volume part and thus the mass flow of the liquid phase. An information about the volume part and the velocity of the different components of the liquid phase cannot be obtained by this method.

It is thus the aim of the invention to propose a method for measuring the mass flow of the different components of a flow in a tube consisting of at least two liquids and a gas.

This aim is reached according to the invention by the method steps defined in claim 1.

The invention will now be explained more in detail with respect to the drawings.

Figure 1 shows diagrammatically a device for implementing the method according to the invention.

Figure 2 shows a measurement signal which appears during the first method step and Figures 3a to 3c show the frequency analysis of a measurement signal appearing in the second method step for different flow velocities.

The invention was conceived in particular for the real time measurement of material flows in the petrochemical. In an embodiment which is interesting for practical reasons, a mixture of gases, crude oil and water is to be measured, which flows through a tube of a diameter between 10 and 20 cm. The temperature of this mixture is to be variable between 25 and 130°C and the pressure is to lie between 2 and 80 bars.

The method according to the invention is based on nuclear - 3 magnetic resonance effects which are classically used for the examination of molecular structures (cf. for example the book by A. Abragam The principles of nuclear magnetism, Oxford, 1962).

Nuclear resonance methods have been applied most successfully for measuring the velocity of two-phase flows (see the above cited book of J.M. Delhaye and G. Cognet) or in medicine for tomography.

The device according to figure 1 has a polarization magnet 2 and a magnetic coil 3, which are disposed one behind the other in flow direction on a tube 1 through which the flow to be measured flows. The tube is of a non-ferromagnetic material at least in the measurement zone and conveys the mixture to be measured along an arrow 4. The polarization magnet 2 consists of a coil traversed by direct current and creates a magnetic field Bp in the direction of the arrow 4. In the same way, the magnetic coil 3 is equally traversed by a direct current and creates a magnetic measurement field B . o Furthermore, in the measurement field between the measurement coil and the tube 1, there is disposed a high frequency coil 5, which is connected to a transmitter 6 and to a receiver 7. The transmitter 6 is capable to feed a high frequency signal, for example a pulse modulated sinusoidal signal of a frequency of 10 MHz, into the high frequency coil 5, so that an additional high frequency field B f is created in the tube 1.

The method according to the invention consists of two partial measurements: The first measurement is a measuring method of the nuclear magnetic resonance for two phase flow, as it is described in the above cited document of Delhaye and Cognet. In this case, the flow of the nuclear paramagnetic magnetization and thus of the nuclear spin from the inner volume of the high frequency coil is measured in a nuclear resonance spectrometer. The direct - 4 result in the receiver 7, respectively a spectrometer 8 associated thereto, is the nuclear spin echo sequence representing the flow curve and which fades to zero from an initial maximum value during the pulse sequence which determines the time period of the measurement (see Figure 2). This efflux curve contains the entire information on the two-phase flow and is processed as described in the cited document. The result is the mean velocity and the volume part, and thus the mass flow of the liquid phase.

Thereafter, a nuclear resonance spectroscopy is carried out. For this, a free induction decay is created and measured by means of a high frequency pulse fed into the high frequency coil 5, which pulse is capable to rotate the nuclear magnetization by an angle of 90° or less. The Fourier transform of this measurement signal supplies a spectrum which can be analysed via a computer analysis according to the volume parts of the individual spin groups and velocities. The final measurement of the volume parts necessitates either calibration measurements or a reverse computation to the static case with velocity zero. Mean volume parts and velocities are thus respectively obtained which are integrated over the inner volume of the high frequency coil 5 and the measurement time.

It can be advisable to integrate also over longer time periods, when for example large gas bubbles are conveyed together with liquid plugs through the tube 1.

Sometimes it can be indicated to correct the free induction decay by means of the flow curve obtained in the first method step, for example when during the free induction decay, an important number of spins has already flown out of the high frequency coil.

Figure 2 shows the envelope curve 9 of the registered nuclear spin echo signal m/ιηθ as a function of the time t related to the efflux time T, if a CMPG pulse sequence has been fed into the - 5 high frequency coil 5. As mentioned above, the mean velocity and the mean volume part of the liquid phase can be computed from this signal.

Figures 3a, 3b and 3c show the theoretically computed frequency spectrum of the signal which results from the second method step. Here, a pure plug flow was taken as a basis in order to simplify the computation. Only the protons of the CH2 groups were taken into consideration for the oil part. The proton resonance frequency was assumed at 10 MHz and the homogenity of the magnetic field was assumed to lie at 1 ppm over the measurement volume. In Figures 3a to 3c the nuclear resonance line of the water protons and that of the CH2 protons were respectively drawn with the same amount of protons in both groups. Since the width of the spectral lines increases with the flow velocity, a comparison of Figure 3a, which corresponds to a velocity of 2 m/ε, with Figure 3b, which corresponds to a velocity of 4 m/s, and with Figure 3c, which corresponds to a flow verlocity of 8 m/s, shows that the spectral lines become blurred with increasing velocity. Possible influences of inhomogenity of the magnetic fields must be measured by a calibration experiment with velocity zero and must later be considered for the processing. The line shape of a proton group due to the flow is obtained by a Fourier transform of the mentioned efflux curve, i.e. the envelope 9. The nuclear resonance line of water, the line width of which is substantially given by the velocity of water in the mixture, must subsequently be deduced from the total spectrum of the liquid parts. An integration over the rest of the spectrum then delivers the oil volume part and the integration over the water line delivers the water volume part. If necessary, calibration measures with only one liquid component have to be made for comparison reasons, or a reverse computation to velocity zero has to be carried out by means of the known spin-grid-relaxation times. If oil and water have the same velocity, the measurement is thus achieved. If, on the contrary, oil and water have - 6 different velocities, the water efflux curve must be computed from the measured water line by a further Fourier transform back into time dependence, and must be subtracted from the initially measured mean efflux curve. The remaining efflux curve obtained in this way is to be attributed to the crude oil and permits the measurement of the volume part and the mean velocity of the oil.

If measurement accuracy is not too important, the polarization magnet 2 can be omitted, which substantially only ensures an improvement in the signal-noise ratio of the measurement signal. The transmitter 6 and the receiver 7 are conventionally associated with a computer-controlled nuclear spin resonance spectrometer 8, the computer optimizing the processing strategy, for example by back-reference to and comparison with memorized earlier analyses.

The method according to the invention can also be used for mass flow measurements, if more than two liquid components are present in the flow, as long as their parts in the spectrum can be sufficiently discriminated from each other.

Claims

1. A method for measuring the mass flow of the different components of a flow composed of at least two liquids and a gas in a tube, characterized in that a sequence of magnetic high frequency pulses is applied to the flow and the efflux of the nuclear paramagnetic magnetization and thus the nuclear spin are measured from the inner volume of the high frequency coil in a nuclear resonance spectrometer, from which result the mean velocity and the volume part of the liquid phase, that then by means of a single short high frequency pulse a free induction decay is created and measured in the flow, the time dependence of which is analysed by means of a Fourier transform and that finally by means of a computer analysis of the Fourier spectrum the individual volume parts of the liquid components are defined. 2. A method according to claim 1, characterized in that at least one of the measurements is integrated over selectable time periods. 3. A method for measuring the mass flow of the different components of a flow composed of a least two liquids and a gas in a tube, according to any preceding claim substantially as hereinbefore described. F. R. KELLY & CO. BY EXECUTIVE

2. 7 Clyde Road^^JIa-tTsbridge, Dublin 4 AGENTS FOR THE APPLICANTS