GB2312511A

GB2312511A - Triboelectric flowmeter

Info

Publication number: GB2312511A
Application number: GB9608619A
Authority: GB
Inventors: Philip Head
Original assignee: Individual
Current assignee: Individual
Priority date: 1996-04-24
Filing date: 1996-04-24
Publication date: 1997-10-29
Also published as: GB9608619D0

Abstract

A flow meter for measuring the rate of flow of a fluid in a pipe 10 comprises two pairs of substantially parallel electrodes X1, Y1; X2, Y2 with an earth electrode E located between the two pairs. The electrodes of each pair are axially separated from each other and are in the form of circular electrodes embedded in an inner layer 12 on the inside wall of the pipe. The earth electrode is also circular, but is in contact with the fluid. Tribo-electric effects in the flowing fluid randomly generate electrostatic charges which give rise to a randomly variable output voltage from the two electrodes, often referred to as tribo-electric noise. Because the two electrodes are axially separated the two signals from the electrodes are temporally separated. By connecting the earth electrode to ground, the noise is substantially reduced so that the output from the electrodes becomes substantially constant. The temporal separation between the points at which the two output signals change from being randomly variable to substantially constant is determined, and from this the fluid velocity and flow rate can be calculated.

Description

A FLOW METER The present invention relates to a flow meter for measuring the flow rate of a fluid.

It is very often necessary, or advisable, to be able to measure the flow rate of a fluid. An example of this is in the oil and gas industry where, for example, in acidfracking, hydrochloric acid is pumped, at pressures that may exceed 100 MPa, through pipes of diameter around 5 to 10 cm, at flow rates of O to 25 m/s. In such circumstances, it necessary to be able to measure the flow rate with precision.

There are a number of devices for measuring flow rates. A known flow meter measures flow rate using the principle of cross-correlation of triboelectric noise.

A known tribo-electric noise cross-correlation flow meter is described with the aid of Figure 1.

The flow meter comprises two sensors 1,2 axially separated by a distance d along the pipe 3 through which the fluid, whose flow rate is to be measured, is flowing. The basis of the measurement is to estimate the time To for perturbations in the flow to travel between the two sensors 1,2. The fluid velocity v is given by v = d/To. As the fluid flows past the first sensor 1, an output voltage V1 is output by the sensor 1 as a result of the perturbations in the fluid as it flows along the pipe 3 past the first sensor 1. Similarly, a second output voltage V2, is output by the second sensor 2 as the fluid flows by the second sensor 2.

The two sensors 1,2 are in the form of circular electrodes provided around the pipe 3. V1 and V2 are functions of time and are random in nature because of the turbulent nature of the fluid flow. These two voltages V1 and V2 are illustrated schematically in Figure 2. Because the fluid perturbations alter as the fluid flows between the two sensors 1,2 the two voltage signals V1, V2 are not the same and therefore to be able to deduce the time delay T,, a known signal processing technique called cross-correlation is used. The cross-correlation as a function against time is illustrated schematically in Figure 3. A value for the time delay To is taken as that value for the maximum of the cross-correlation function The fluid flow rate Q is therefore given by the function Q - (d/To).A, where A is the cross-sectional area of the pipe 3.

Such known flow meters have the problems that the need to carry out the complex signal processing of cross-correlation requires extra circuitry, which is expensive and complex. In addition, by its very nature, the signal processing cannot give a precise measurement, particularly in this case, where the second output voltage V2 contains an element of noise N compared to the first signal V1 i.e. V2 = V1 + N and if the noise N is correlated with the two output voltages, or if the noise N is great compared to these two voltages, then the measurement may be biased.

According to the present invention, there is provided a flow meter for measuring the rate of flow of a fluid in a pipe, the flow meter comprising at least a first pair of first and second sensing means for providing respective first and second randomly variable signals in response to randomly generated perturbations in the fluid flow, the first and second sensing means being axially separated from each other along the pipe such that the first and second signals are temporally separated, means operable to switch the first and second signals between their randomly variable state and a substantially constant state, signal processing means for determining the temporal separation between the point at which the first signal switches between the randomly variable and substantially constant states and the point at which the second signal switches between the randomly variable and the substantially constant states, and means for determining the flow rate from the predetermined temporal separation. Because the point at which the two signals change from being randomly variable to substantially constant is easy to measure, there is no need for any cross-correlation to be carried out, which has the advantage of providing a flow meter which provides a more accurate measurement, but with less signal processing, and therefore, less electronic circuitry, with all its attendant advantages.

A second pair of first and second electrodes may be provided so that the first pair can be used to determine the flow rate in one direction along the pipe, while the second pair can be used to determine the flow rate in the opposite direction. This has the advantage of providing bi-directional flow meter.

According to a further embodiment of the invention a voltage is induced in the electrodes in order to induce a perceptible signal for slow flow rates or other circumstances resulting in a poor signal.

The invention will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 is a cut-away perspective view illustrating a known flow meter; Figure 2 is a graph illustrating the output voltages from the transducers of Figure 1, as a function of time; Figure 3 is graph illustrating the cross-correlation function of the two signals of Figure 2; Figure 4 is a cut-away perspective view illustrating a flow meter in accordance with the invention; Figure 5 is a cross-section along the length of the pipe of Figure 4; Figure 6 is cross-section along the line A-A of Figure 5; Figure 7 is across-section along the line B-B of Figure 5; and Figures 8 and 9 are graphs of output voltage against time for the two transducers of the flow meter of Figure 4.

A pipe 10, through which a fluid, whose flow rate is to be measured, has two pairs of circular electrodes X1, Y1; X2, Y2 surrounding the bore 11 of the pipe 10. Each electrode is arranged substantially parallel to the other electrode of its pair and at an axial distance D therefrom. The electrodes X1, Y1; X2, Y2 may be of, for example, copper or any other suitably conducting material. The inside wall of the pipe 10 is covered by a uniform thickness inner layer 12 in which the electrodes X1, Y1; X2, Y2 are embedded, as illustrated in Figure 6, to isolate the electrodes X1, Y1; X2, Y2 from the fluid flowing through the bore 12. Each electrode X1, Y1; X2, Y2 is coupled, in a known manner, to external circuitry for processing the signals output from the electrodes X1, Y1; X2, Y2, by means of wires 13,14,15,16 respectively.

One of the pairs of electrodes is used to measure the flow rate in one direction down the pipe 10, while the other pair is used to measure the flow rate in the other direction. Thus, for example, the electrodes X1 and Y1 are used to measure fluid flowing in the direction of arrow F in Figure 5 and electrodes X2 and Y2 are used to measure fluid flow in the direction of arrow G in Figure 5. As each pair of electrodes X1, Y1; X2, Y2 act in substantially the same way, only on pair of electrodes, namely X1 and Y1, will be described herein, for ease of understanding However, it will be understood that the description given below will apply equally to the second pair of electrodes X2, Y2.

Located along the pipe 10 between the first electrodes X1, X2 of each pair, and at a predetermined distance therefrom is an earth electrode E.

The distance will be not too large for new static charging to build up and long enough for a sufficient step to occur. The earth electrode E is a circular electrode located around the wall of the bore 11 i.e. against the inner layer 12 so that it is contact with the fluid flowing through the bore 11. The earth electrode E is also coupled to external circuitry by means of a wire 17.

Consider fluid flowing through the pipe 10 in the direction of arrow F of Figure 5. As the fluid flows through the pipe 10, electrostatic charges are naturally generated, within the pipe, by tribo-electric effects, in a number of ways. Under normal circumstances, without the earth electrode E being operable, the two electrodes X1 and Y1 operate as in a known cross-correlation flow meter to provide output voltages Vl(t) and V2(t) respectively as the fluid flows past the electrodes X1 and Y1. The electrodes X1, Y1 act as sensors detecting the presence of the electrostatic charges and providing the output voltages Vl(t) and V2(t).

The values of the output voltages V1(t),V2(t) vary with the generated electrostatic charges present in the fluid as it flows past the electrode X1, Y1. Because the electrostatic charges are generated randomly, and are scattered randomly by the turbulence of the fluid flow, the two output voltages V1(t) and V2(t) are randomly variable over time. This randomly variable voltage is often referred to as tribo-electric noise.

Figure 8 illustrates the outputs from the two electrodes. The unbroken line indicates the output Vi (t) from the first electrode X1, while the dotted line indicates the voltage V2(t) from the second electrode Y1. It can be clearly seen that this is similar to the sort of trace obtained with a known flow meter, and illustrated in Figure 2. In a known flow meter, these two signals Vl(t) and V2(t) would be cross-correlated, and an average value for the time delay for fluid to flow between the two electrodes deduced from the maximum of the cross-correlation function.

However, in the present invention, at predetermined intervals, the earth electrode E is "switched on" i.e. it is coupled to ground. At this point, the electrostatic charges generated within the fluid are allowed to flow to earth through the earth electrode E, with the result that the signal output from the electrodes X1, Y1 remains essentially constant. This is shown clearly in Figure 9. At a time T1, the earth electrode E is coupled to ground, and ,consequently, the signal Vi (t) from the first electrode X1 becomes essentially constant. At a time T2, the output voltage V2(t) also becomes essentially constant. The time delay T' between T1 and T2 is the time taken for the fluid to flow between the two electrodes X1 and Y1, and the fluid velocity v can be measured simply using the known formula v = D/T', and the flow rate Q can be deduced from the formula Q = (D;T').A, where A is the cross-sectional area of the bore 11.

Because the output voltages Vl(t) and V2(t) change from being randomly variable to substantially constant, this point is easy to measure and, therefore an easy datum is provided between which to measure the time delay T'.

When the earth electrode is "switched off" i.e. de-coupled from ground at time T3, then signals from the two electrodes X1 and Y1 become once more randomly variable at times T3 and T4 respectively, as illustrated in Figure 9. The time delay T" between T3 and T4 may be measured again to give a further value of the fluid velocity v and, consequently, the fluid flow rate.

This operation can be repeated as often as required.

The earth electrode may coupled and de-coupled from ground in any known manner. Similarly, the measurement of the voltages from the electrodes X1 and Y1 and the measurement of the time delay T', T" may be accomplished by any known method.

A known voltage may placed across the electrodes in order to induce a perceptible signal for slow flow rates or other circumstances resulting in a poor signal.

Typically, the pipe 10 may be made of steel or other suitably strong metal or plastic, and the inner layer 12 of preferably polyurethane.

The earth electrode may be of any suitably conducting material.

It will be understood to a person skilled in the art that various modifications are possible within the scope of the present invention. For example, other shaped electrodes may be used. The pipe may be noncircular in cross-section, and the electrodes of each pair may have different separations. There may be only one pair of electrodes or there may be more than two depending upon the requirements.

Claims

1. A flow meter for measuring the rate of flow of a fluid in a pipe, the flow meter comprising: at least a first and a second sensing means for providing respective first and second randomly variable signals in response to randomly generated perturbations in the fluid flow, the first and second sensing means being axially separated from each other along the pipe such that the first and second signals are temporally separated; means operable to switch the first and second signals between their randomly variable state and a substantially constant state; signal processing means for determining the temporal separation between the point at which the first signal switches between the randomly variable and substantially constant states and the point at which the second signal switches between the randomly variable and the substantially constant states; and means for determining the flow rate from the predetermined temporal separation.

2. A flow meter according to claim 1, wherein the first and second sensing means are operable to provide randomly variable output voltages in response to randomly generated electrostatic charges in the fluid, and the switching means is operable, in a first mode, to provide means for allowing the electrostatic charges to flow to ground such that the output voltages are no longer randomly variable, and, in a second mode, to be de-coupled from ground such that the output voltages remain randomly variable.

3. A flow meter according to claim 2, wherein the switching means is an electrode selectively coupled to ground, and in contact with the fluid.

4. A flow meter according to claim 3, wherein the electrode is a circular electrode provided around the bore of the pipe.

5. A flow meter according to any of claims 2 to 4, wherein the first and second sensing means are circular electrodes embedded in an inner layer provided around the inner wall of the pipe.

6. A flow meter according to any preceding claim, further comprising a second pair of first and second sensing means, the first pair being operable to determine the flow rate of a fluid flowing in a first direction along the pipe, and the second pair being operable to determine the flow rate of a fluid flowing in the opposite direction along the pipe.

7. A flow meter according to any preceding claim, characterising in that a voltage is induced in the first and second sensing means in order to induce a perceptible signal for slow flow rates or other circumstances resulting in a poor signal.

8. A flow meter as hereinbefore described with reference to the accompanying Figure 4 to

9.

8. A method for determining the flow rate of a fluid in a pipe, the method comprising the steps of: providing first and second temporally spaced randomly variable signals in response to randomly generated perturbations in the fluid; at regular intervals, switching the first and second signals from the randomly variable state and a substantially constant state, and determining the temporal separation between the points in the two signals where they switch between the two states; and determining the flow rate from the predetermined temporal separation.