GB2256490A - Flow meter - Google Patents

Abstract

A flow meter comprises a body 1 for location in a flowing fluid typically inside a pipe 6, the body 1 including a magnetic field generator 2 capable of generating a magnetic field eg using an electromagnet or permanent magnet, at least part of which field passes through the fluid which may be oil and/or water, and at least two electrodes 4a, b, c, d in electrical contact with the fluid and connected by a relatively high impedance voltage sensor 5 to measure a voltage induced therebetween. <IMAGE>

Description

FLOW METER The present invention relates to an apparatus which can be used to measure the flow of a fluid. The invention is particularly useful for measuring flow in confined spaces such as pipelines but may also find application in unconfined flowing liquids.

It has been previously proposed to measure the flow of a fluid in a pipeline by electromagnetic means. A prior art arrangement is shown in Figure 1 and comprises a pipeline 10 having a fluid flow U therethrough. A pair of electrodes 12, 14 are provided on a diameter of the pipeline 10 in contact with the fluid, both being connected to a voltage sensor 16. A magnetic field B is imposed on the pipeline which is perpendicular to both the flow axis and the diameter such that the flow of the fluid induces an electric field E between the electrodes which can be sensed at the voltage sensor 16. The measured voltage is given by the equation: V=k(e)UB (1) where k(e) is a constant corresponding the the electric field distribution and the electrical properties of the fluid.

The arrangement shown in Figure 1 has the disadvantage that it is not moveable easily to different sections of pipeline and that it is difficult to implement when access to the pipeline is restricted, for example when the pipeline is buried below ground.

It is an object of this invention to obviate or mitigate these disadvantages and the object is achieved by placing both the magnetic field generator and the electrodes in the flowing fluid.

In accordance with the present invention there is provided a flow meter comprising: a body for location in a flowing fluid, said body comprising a magnetic field generator arranged to generate a magnetic field, at least part of which passes through the fluid; and at least two electrodes in electrical contact with the fluid and connected by a voltage sensor to measure the voltage induced therebetween.

By providing all of the component parts of the sensor in a single body, the sensor can be placed inside a pipeline and so can be used where access to the outside is restricted. Furthermore, the sensor can be retrieved and used in different locations if required.

The magnetic field generator can comprise a high permeability core and a winding wound around it (an electromagnet) or a permanent bar magnet. Improved results can be obtained if the magnetic field is a time varying field near the electrodes preferably one which oscillates through 1800. Where an electromagnet is used, this can be achieved by oscillating the direction of current flow causing the magnetic field. Where a permanent bar magnet is used, this can be physically rotated or oscillated to achieve a similar effect.

The present invention is particularly suitable for use in pipes made of highly magnetically permeable material as this serves to concentrate the magnetic field between the generator and the walls of the pipe thus improving the induced signal.

The voltage sensor must have a sufficiently high impedance with respect to the fluid that there is no significant distortion to the measured signal. Thus for substantially non-conducting fluids the impedance must be very high.

In certain circumstances several pairs of electrodes can be provided around the body and this can be useful in deviated pipes where the flowing fluid comprises more than one distinct phase. In this case, one set of electrodes can measure the flow in one phase while another set can measure the flow in the other phase.

The invention will now be described by way of example with reference to the accompanying drawings. In the drawings: - Figure 1 shows a prior art arrangement; - Figure 2 shows a cross section through a first embodiment of the invention; - Figure 3 shows a cross section through a second embodiment of the invention; - Figure 4 shows a detector and amplifier arrangement used in Figure 3; - Figure 5 shows a third embodiment of the invention; - Figure 6 shows a switch and sensor arrangement used in Figure 5; - Figure 7 shows a diagramatic representation of two phase flow in a deviated pipe; - Figure 8 shows a fourth embodiment of the invention; - Figure 9 shows a switch and sensor arrangement used in Figure 8; and - Figures 10 and 11 show a fifth embodiment of the invention.

Referring now to the drawings, Figure 1 has already been described in the introduction to this application. Figure 2 shows a first embodiment of the invention comprising a body 1 which can be located inside a pipe 6 by some appropriate means (not shown). The body 1 can be permanently attached to the pipe 6 or removable therefrom. A magnetic field generator comprises a magnetic core 2 encased in a magnetic sheath 2a, an electric coil 3 being wound around the core 2 and connected to a current supply I. The sheath 2a is open at the ends such that the magnetic field generated is confined to those regions near the body 1. Two pairs of electrodes 4a-d are mounted on the surface of the body 1, an electrode being positioned on either side of the ends of the field generator on a radial plane.Electrodes 4a and 4b are shorted together and electrodes 4c and 4d are both connected to a high impedance voltage sensor 5. Power for the current supply I and storage of the voltage output from 5 can be internal or external depending upon circumstances.

By arranging the core 2 and electrodes 4a-d in a radial plane, a magnetic field B is generated at perpendicular to the flow U of the fluid in the pipe 6. Thus electric fields E, E' are induced and the potential generated by these electric fields can be measured between the electrodes 4c-4d. Because of the dipolar nature of the core 2, the magnetic field B at one end has the opposite direction to the field at the other end. Consequently the electric fields E, E' will have opposite direction. By connecting the electrodes 4a-d in the manner shown in Figure 2, a single measurement of V can be made at 5.

Figures 3 and 4 show a second embodiment of the invention. In this case, an insulating layer 11 separates the body and the electrodes 14a-d from direct contact with the fluid in the pipe 15. However, the insulating layer 11 and electrodes 14a-d are chosen such that the electric fields E, E' can still be detected. This embodiment also differs from Figure 2 in that the electrodes are connected in opposite pairs, ie 14a-14c and 14b-14d, which are connected at X and Y to the detector/amplifier arrangement of Figure 4. Here the outputs X, Y are connected to respective high impedance voltage detectors 16a, 16b, the output of which are amplified by a differential amplifier 17.

This voltage detecting arrangement senses the voltage change of the electrodes 14a-14d caused by the induced electric field as given by equation (1). Extremely high impedance measurement of the voltage change is possible in extremely low conductivity liquids such as oil.

A third embodiment of the invention is shown in Figures 5 and 6. The magnetic field generator 22 is rotatable about the pipe axis relative to the body 21 and the electrodes 24a-d are connected to each other and to voltage sensors 25a, b and switches SW1, SW2 as shown in Figure 6. The switches SW1, SW2 are operated in accordance with the rotational position of the magnetic field generator 22. Measurements are made at 25a (V1) and 25b (V2) in accordance with the following scheme (the position shown in Figure 5 corresponds to 8 = 00, anticlockwise rotation): V1 is measured with SW1 closed and SW2 open for 0=00 and 1800 V2 is measured with SW1 open and SWIRL closed for 8 = 900 and 2700 This arrangement is particularly useful for measuring the flow of two liquid phases in a non-vertical pipe (see Figure 7). In cases where the fluid comprises oil and water, the oil phase generally sits on top of the water phase and flows faster. The measured voltage for a single phase is given by: V = AZBUD (2) wherein A is a constant related to the cross sectional geometry of the pipe; Z is the fluid impedance and D is the diameter of the pipe.Typical values of Z are 1 for water (Zwater) and 0.5 for oil (Zoil).

In the case of Figure 7 where the pipe is deviated at an angle a to the horizontal H and if the oil fraction is not large compared to the water fraction, the measured voltage V1 represents the water velocity (Uwater): V1 = AZwater BUwater (3) and the voltage V2 represents the oil velocity (Uoil) multiplied by Lhe fraction of oil Fv in the vertical phase, as: V2 = AZoil B Uoil Fv (4) division of the two voltages gives: V2 Zoil Uoil Fv Vl Zwater Uwater 2Voil Fv 5 - Vwater ( The volume fraction Fv defined in one dimensional direction is related to the true volume fraction Ft and the cross sectional geometry Kv of the pipe by:: Fv=KvFt (6) If V1 has been calibrated for the water velocity as given in (3), the oil volume velocity Uoil Ft can be obtained from (5) and (6).

Figures 8 and 9 show a fourth embodiment of the invention which is similar to that shown in Figures 5 and 6. However, in this case the electrodes 34a-h are provided in pairs and extend into the fluid. These electrodes serve to reduce the output impedance of the signal and to stabilise the flow around the sensor. As before, the magnetic fluid generator 32 rotates. The two pairs of electrodes 34a,d, 34e,h lying in the direction of the magnetic field B measure the induced voltage and the other two pairs 34b,c, 34f,g which are at right angles to the field B provide correction voltage measurements. The electrodes 34a-h are connected via switches SW1 and 5W2 and voltage sensors 35a, 35b as shown in Figure 9.During measurement, the switches are set as follows: V1 measurement - SW1 closed, SW2 open V2 measurement - SW1 open, SW2 closed With the magnetic flux generator in the position shown in Figure 8, V1 provides the flow signal and V2 gives a signal due to noise generated by friction within the flow.

Friction noise is also present in the V1 measurement so by subtraction of V2 this can be removed to obtain a better signal.

Figures 10 and 11 show a fifth embodiment of the present invention. There are two sets of magnetic field generators 42a, b, electrode pairs 44a-d and 44a'-d' and voltage measurement means 45a,b. The two magnetic field generators, generate the fields in reverse directions and therefore, the detected voltages are: V1 =Vind+Sn V2 = - Vind + Sn where Vind is the induced voltage by the liquid flow and Sn is the noise signal. The subtraction of these voltages can eliminate the noise signal as: V1-V2=2Vind By this method the signal to noise ratio can be improved.

A typical configuration of the present invention shown in Figure 2 may have the following parameters: inductance of the magnetic field generator - 50 mH current to magnetic field generator - 0.1 A magnetic field at the generator surface - 6 Gauss The velocity of water can be measured by the voltage difference induced at the electrodes denoted as 4c and 4d in the order of 10-9 V / meter / sec.

This voltage can be amplified before it is measured by an AC volt meter. For the dual phase flow for water and oil, the oil volume velocity can be typically measured up to 15% of the total flow.

The figures show the magnetic field generator as being an electromagnet which is rotated in certain circumstances to provide a time varying magnetic field. In some cases a permanent bar magnet may be more appropriate as this can provide a greater field intensity and is more easily rotated. Furthennore, no power supply would be required to produce the field. Where an electromagnet is used, it may be more appropriate to periodically reverse the current flow in the coil windings to provide the time varying field.

Where a rotating magnet is required, this could be achieved by providing an impeller on the body which drives the magnet from the fluid flow. This has the advantage that no power needs to be supplied for this purpose and the rate of rotation would be dependent upon the flow such that in high flow situations the magnetic field would vary at a greater frequency giving improved readings.

Claims (18)

1 A flow meter comprising a body for location in a flowing fluid, said body including a magnetic field generator capable of generating a magnetic field, at least part of which passes through the fluid, and at least two electrodes in electrical contact with the fluid and connected by a voltage sensor to measure a voltage induced therebetween.

2 A flow meter as claimed in claim 1, wherein means are provided for locating the body within a pipeline containing the fluid.

3 A flow meter as claimed in claim 1 or 2, wherein two pairs of electrodes are provided.

4 A flow meter as claimed in claim 1,2 or 3, wherein the electrodes are in physical contact with the fluid.

5 A flow meter as claimed in claim 1,2 or 3, wherein the electrodes are separated from the fluid by an insulating layer.

6 A flow meter as claimed in any preceding claim, wherein the magnetic field is a time varying field.

7 A flow meter as claimed in any preceding claim wherein the magnetic field generator includes a permanent magnet.

8 A flow meter as claimed in claim 7, wherein the permanent magnet is oscillated or rotated within the body.

9 A flow meter as claimed in any of claims 1-6, wherein the magnetic field generator includes an electromagnet.

10 A flow meter as claimed in claim 9, wherein the current in the electromagnet is periodically reversed.

11 A flow meter as claimed in any preceding claim, wherein the electrodes are arranged in a plane which is perpendicular to the flow of the fluid and to the direction of the magnetic field.

12 A flow meter as claimed in any of claims 3-11, whereas the electrodes are connected together and to the voltage sensor by means of one or more switches which enable the voltage induced between different ones of the electrodes to be measured.

13 A flow meter as claimed in claim 12, wherein the one or more switches are arranged to allow voltages induced in different directions across the body to be measured.

14 A flow meter as claimed in claim 12 or 13, wherein more than one voltage sensor is provided.

15 A flow meter as claimed in any preceding claim, wherein the electrodes are arranged so as to be in electrical contact with fluid in two distinct phases so as to measure the flow of each phase separately.

16 A flow meter as claimed in any of claims 3-15, wherein a pair of electrodes are provided to measure the electrical noise generated by the flowing fluid.

17 A flow meter as claimed in any preceding claim, wherein two separate bodies are provided.

18 A flow meter which is substantially as herein described in relation to the accompanying drawings.