GB2452721A

GB2452721A - A powered flow meter

Info

Publication number: GB2452721A
Application number: GB0717700A
Authority: GB
Inventors: Colin Edwin Turner; Charles Henderson; Thomas Neudeck
Original assignee: PA Knowledge Ltd
Current assignee: PA Knowledge Ltd
Priority date: 2007-09-11
Filing date: 2007-09-11
Publication date: 2009-03-18
Also published as: GB0717700D0; WO2009034290A1

Abstract

A flow meter for determining a flow rate of a flow of fluid 16 flowing within a channel or pipe 14, comprises: a moveable member, e.g. a turbine 2, for immersion in the flow of fluid 16; a driving means consisting of a power supply 6 and a motor 12 for driving movement of the moveable member 2; and a control system 4 for varying the power supplied to the moveable member 2 by the driving means 6, 12 in response to signals received from pressure sensors 8A, 8B. The flow meter is configured to determine the flow rate by monitoring the operation of at least one of the moveable member 2 and driving means 6, 12. The control system 4 may vary the power until a disturbance caused to the flow by the meter is substantially minimized, with the flow meter using the characteristics of that state to determine a flow rate.

Description

Powered Flow Meter The present invention relates to a meter for measuring the flow rate of a fluid.

Flow meters using turbines are known for this purpose and can achieve high accuracy. Typically, such meters are arranged so that the fluid engages with a multi-bladed turbine, generally mounted on a free-running bearing, the flow causing the turbine to rotate at an angular velocity proportional to the flow rate. The speed of movement can be detected by solid state devices (e.g. inductive, capacitive, optical or magnetic detection) or mechanical gears and processed to give the rate of volumetric flow.

It is an object of the present invention to provide a flow meter that overcomes or at least reduces shortcomings with conventional turbine flow meters, which limit their usability and range of operation.

According to an aspect of the invention, there is provided a flow meter for determining a flow rate of a flow of fluid, comprising: a moveable member for immersion in the flow of fluid; a driving means for driving movement of the moveable member; and a control system for varying the power supplied to the moveable member by the driving means, wherein: the flow meter is configured to determine the flow rate by monitoring the operation of at least one of the moveable member and driving means.

Compared with prior art systems, actively driving movement of the moveable member can reduce the disruption to the fluid flow caused by the flow meter by reducing the extent to which the flow itself powers the movement. Additionally, the presence of the driving means can be exploited to determine the flow rate without the need for separate sensors (e.g. to measure the speed of movement of the moveable member) by monitoring characteristics of the driving means that are affected by the flow (e.g. the shape of the applied voltage-current characteristic).

In contrast to apparatus such as pumping equipment, which may be provided with power-driven elements intended for immersion in a flow of fluid, the present embodiment relates to a flow meter, which has means for determining a flow rate by varying a power supplied to the moveable member in a controlled way. There are a number of different ways in which the power supplied to the moveable member may be varied in order to derive the fluid flow.

For example, the control system may be configured to vary the power supplied to the moveable member by the driving means until the flow meter reaches a state in which a disruption to the fluid flow caused by the flow meter is substantially minimized, with the flow meter being configured in this case to determine a flow rate of the fluid using information about how at least one of the moveable member and driving means are operating while the flow meter is in said state or while the flow meter is being driven towards that state by said control system.

In other words, in this embodiment, the control system adjusts the speed of movement of the immersed moveable member until the disturbance caused to the flow (which may change the flow rate, pattern or add a pressure drop) is substantially minimized and then uses the characteristics of this state (such as the particular input power or the speed of movement of the moveable member at which the minimization is achieved) or the characteristics of how this state was achieved (such as the shape of the applied voltage versus current curve over the period while the flow meter is being driven towards the equilibrium state), to deduce the flow rate. This arrangement provides a simple measure of a flow rate while minimizing disruption to the flow.

The extent to which the interaction can actually be minimized in practice will depend on the nature of the flow (for example, how uniform the flow is and whether any turbulence is present in the flow). It will also depend on the particular design of the moveable member.

Where the moveable member is a high quality turbine and the fluid flow is highly uniform, one should expect the minimum interaction to approach zero.

Systems that allow a flow rate to be measured with a reduced disruption to the flow are particularly advantageous where low volume flows are involved or where the flow volume needs to be controlled with high accuracy. For example, embodiments of the present invention may be suitable for accurate measurement of flow rates that are too small for conventional turbine flow meters.

Reducing disruption to the fluid flow generally means reducing the pressure drop across the turbine, which in turn prevents "flashing" (creation of vapour bubbles in a liquid flow) and subsequent cavitation, which can affect accuracy and/or damage the meter.

The range of flow rates that can be accurately measured, i.e. turndown ratio (ratio of maximum measurable flow rate to minimum measurable flow rate), may also be larger than conventional turbines with a single blade.

Wear and tear on mechanical components (e.g. increase in bearing friction over time) and the resulting change in performance of the moveable member can be compensated by adjusting relevant operational parameters of the driving means (e.g. voltage, current, mechanical energy in, etc.). The performance of the flow meter can thus be maintained within acceptable bounds for longer periods of time compared with a conventional system, in which such wear and tear will have a direct negative effect on the efficiency of the flow meter (the fluid flow will have to compensate the increased drag, and will thus be more severely disrupted by the presence of the meter).

More generally because drag factors (such as losses in bearings, losses when measuring the speed of movement of the moveable member, friction between the moveable member and the fluid, etc.) no longer have to be compensated by extracting energy from the fluid, a given design of moveable member will be usable in a wider variety of situations.

Embodiments of the present invention are thus more easily applicable to a wide range of different flow rates and require fewer design features to address issues like drag or a limited turndown ratio. This facilitates standardization and thus potentially reduces the cost of mass production.

The fluid may be a liquid or a gas. The moveable member may be moveable rotationally (e.g. about an axis parallel to the fluid flow), translationally, or otherwise.

A pressure difference measuring device may be provided for measuring a difference in pressure in the fluid flow between at least one point located upstream of the moveable member and at least one point located downstream of the moveable member, and the control system may be arranged in this case to use an output from the pressure difference measuring device to determine when the flow meter reaches the state in which disruption to the fluid flow caused by the flow meter is substantially minimized.

The pressure difference before and after the flow meter provides a measure of the extent to which the flow meter interferes with the flow andlor changes the flow rate. In general, the larger the drop in pressure across the flow meter the larger the change in flow rate caused by the flow meter. In order to minimize the interaction between the fluid and the flow meter the control system will generally vary the power to the movable member until the pressure drop is minimized.

The use of sensors separate from the moveable member allows the interaction between the moveable member and the fluid to be determined without interfering with the moveable member.

An interaction sensor may be provided, positioned on the moveable member and configured to measure an interaction between the fluid and the moveable member, and the control system may be arranged in this case to use an output from the interaction sensor to determine when the flow meter reaches the state in which disruption to the fluid flow caused by the flow meter is substantially minimized. For example, where the moveable member is a turbine, sensors that measure the pressure exerted on the blades may be used.

The use of sensors on the moveable member may be advantageous because it provides a direct and thus potentially highly accurate measure of the interaction between the moveable member and the fluid, which is indicative of the flow rate drop across the meter. The disruption to the fluid flow caused by the sensors themselves may also easily be minimized in this arrangement (for example by making the sensors flush with the surface of the blades).

A monitoring device may be provided for monitoring one or more operational parameters of the driving means, the control system being arranged to use an output from the monitoring device to determine when the flow meter reaches the state in which disruption to the fluid flow caused by the flow meter is substantially minimized.

This approach allows the point of minimum change in flow rate to be determined without requiring any additional sensors either in the fluid or on the moveable member. This arrangement is thus potentially simple and cheap to implement, with a minimum of disruption both to the fluid and the operation of the moveable member.

A speed of movement measurement device may be provided for measuring the speed of movement of the moveable member, the flow meter being configured to determine a flow rate on the basis of an output from the speed of movement measurement device.

The flow rate determining device may use a predetermined relationship between the (angular) speed of movement of the moveable member and the fluid flow to derive the flow rate from the movement speed. The predetermined relationship may be based on a theoretical model of the fluid flow and the moveable member and/or on calibration data.

A monitoring means may be provided which is configured to monitor one or more operational parameters of the driving means during a time period while the power supplied to the moveable member by the driving means is varied by the control system, and the flow meter may be configured to determine a flow rate by reference to how the monitored one or more operational parameters vary during the time period.

According to this arrangement the flow rate can be determined without any additional sensors or with a minimum of additional sensors.

For example, the monitoring device may be configured to monitor the voltage and current applied by the driving means during a time period while the power is varied by the control system, with the flow meter being configured in this case to determine a flow rate by reference to the shape of the curve of voltage against current during the time period.

Alternatively, a speed of movement measurement device for measuring the speed of movement of the moveable member may be provided and the flow meter configured to determine a flow rate by reference to how the monitored one or more operational parameters and the measured speed of movement vary during the time period.

The time period may be chosen so that the power to the moveable member is varied around a point (i.e. a particular power) at which the disruption to the flow is minimized.

Alternatively, the time period may be chosen between when the power starts to be varied and the point at which the disruption to the flow is minimized.

According to a further embodiment, the moveable member may be rotatable, in particular a turbine. Alternatively, the moveable member may operate linearly in the manner of a linear pump.

According to a further embodiment, the flow rate determining device is configured to determine the flow rate using also information from additional sensors, such as pressure sensors in the fluid.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: Figure 1 is a schematic side-view of a flow meter with upstream and downstream pressure sensors according to an embodiment of the invention; Figure 2 is a schematic side-view of a flow meter with interaction sensors on the moveable member according to an embodiment of the present invention; Figure 3 is a schematic side-view of a flow meter with monitoring means according to an embodiment of the invention; and Figure 4 is a graph showing performance data of a flow meter according to an embodiment of the invention.

The flow meter of Figure 1 is arranged to measure a flow rate of a fluid 16 flowing within a channel or pipe 14. The direction of flow is indicated by arrows 18. According to this embodiment, the flow meter comprises a turbine 2 (moveable member) mounted within the fluid 16 so as to be moveable about an axis substantially parallel to the direction of the fluid flow. The turbine 2 can be driven (rotated) by power supply 6 and motor 12 (together an example of a "driving means").

In the arrangement shown, the motor 12 is located within the channel 14 while the power supply 6 is located outside of the channel 14 (with electrical power leads connecting the motor 12 to the power supply 6). However, as an alternative, the power supply 6 and motor 12 could both be located within the channel 14. In this case, it would be particularly advantageous to use a power supply 6 that does not need an electrical connection to the outside (for example, a battery pack) so as to avoid interference to the fluid flow from electrical connections. The motor 12 is electrically powered (DC or AC) in the example embodiment. However, other types of "driving means" could be used, such as hydaulic or pneumatic based systems.

As will be described in more detail below, a control system 4 may be provided, which in certain embodiments is configured to minimize the size of an interaction between the rotating turbine and the fluid. In other words, the disturbance caused by the turbine 2 is minimized, or the change in fluid flow pattern across the turbine is minimized.

According to the arrangement of Figure 1, the size of the interaction is determined using pressure sensors 8A and 8B, respectively upstream and downstream of the turbine 2. In general, the greater the interaction between the turbine 2 and the fluid 16, the greater the change in pressure across the turbine 2, which is reflected in a larger difference between the readings of sensors 8A and 8B. The situation may be complicated where the fluid flow is not substantially uniform. In this case, the pressure sensors 8A and 8B may be configured to take an average pressure over a predetermined period of time (which may be chosen as a function of the extent to p which the flow is found to be non-uniform, for example), a difference between such average pressures upstream and downstream of the turbine 2 being taken to indicate the size of an interaction between the turbine 2 and the fluid 16 and thus the amount of reduction in flow rate caused by the presence of the turbine. Alternatively, a plurality of pressure sensors may be provided upstream and/or a plurality of sensors may be provided downstream and an average value of the readings from the plurality of sensors may be used to determine the size of the interaction. A combination of averaging over time and between different sensors may also be used.

Figure 2 shows an alternative arrangement for determining the size of the interaction between the turbine 2 and the fluid 16 or the amount of flow disruption across the meter, which uses interaction sensors 20A and 20B, integral with or attached to the turbine 2. The interaction sensors 20A and 20B provide a direct measure of the pressure exerted by the fluid at one or more points on the turbine 2. The output from the interaction sensors 20A and 20B can be used to determine the total amount of work done by the fluid 16 on the turbine 2 and by the turbine 2 on the fluid 16. A plurality of sensors distributed between different blades of the turbine 2 and at different points on the blades may be used. When all of the sensor readings are minimized, this may be taken to mean that the overall interaction between the turbine 2 and the fluid 16 is minimized, but the situation may be more complicated than this for complex flows and/or complex designs for the turbine 2. Calibration measurements may be carried out to determine the relationship between the forces measured by the sensors 20A and 20B and the interaction between the turbine 2 and the fluid 16 and/or change in flow rate. Fluid pressure sensors upstream and downstream of the turbine 2 may be used to correlate readings from the sensors 20A and 20B with states in which the interaction and/or change in flow rate is minimized. The calibration measurements may be made at different flow rates and with different flow types (e.g. varying the degree of turbulence and/or other types of flow non-uniformity).

The interaction sensors may optionally be used in conjunction with fluid pressure sensors such as those discussed with reference to Figure 1 above when measuring a flow rate.

Alternative or additionally, the performance characteristics of the driving means 6/12 may be analysed to determine when the interaction between the fluid 16 and the turbine 2 and/or the change in flow rate across the meter is minimized. For example, additional torque on the p turbine 2 caused by the fluid flow may affect the relationship between the rotational speed and the electrical power required to drive the turbine at this speed. One approach for determining the interaction minimum would be to adjust relevant parameters of the driving means 6/12 (for example, in continuous sweeps) until they correlate with a stored running profile where the work on the turbine 2 due to the external flow is known to be minimized (for example, because independent measurements of a pressure differential across the turbine 2 showed this to be the case).

Figure 3 shows an embodiment of this type, comprising a monitoring device 24 for monitoring one or more operational parameters of the driving means 6/12. Operational parameters could include voltage, current, and/or power delivered to the turbine motor. These could additionally be compared to the rotational speed achieved.

The embodiments of Figures 1, 2 and 3 comprise a control system 4,which is arranged to receive input data from the fluid pressure sensors 8A18B, interaction sensors 20A/20B and/or monitoring device 24. Other sensors capable of providing a measure of the size of the interaction between fluid 16 and turbine 2 could also be used. The control system 4 may be arranged to use the input data to determine a suitable power to supply to the driving means 6/12 for the turbine 2 to minimize the interaction. In practice, this effectively involves varying the speed of the turbine 2 until it is "matched" with the flow speed of the fluid.

A flow rate determining means 22 is provided, which may be configured to determine the flow rate based on the characteristics of the turbine 2 (e.g. speed) and/or driving means 6/12 (e.g. applied voltage, current, input power) in the state where the interaction and/or change in flow pattern is minimized. The relationship between the flow rate and the various possible parameters may be determined theoretically or from calibration measurements.

Optionally, the flow rate determining means 22 may use information also from additional sensors, such as pressure sensors, in combination with the characteristics of the turbine 2 and/or driving means 6/12 in the state where the interaction and/or change in flow pattern is minimized.

Alternatively or additionally, the monitoring device 24 may be configured to monitor one or more operational parameters of the driving means 6/12 during a time period while the power supplied to the turbine 2 by the driving means 6/12 is varied by the control system 4, with the flow rate determining means 22 being configured in this case to determine a flow rate by reference to how the monitored one or more operational parameters vary during the time period.

For example, the control system 4 may be configured to vary the supplied power in sweeps in order to determine the flow rate from variations in the supplied voltage and current during the sweeps.

According to this latter embodiment, it is not necessary to minimize the interaction completely, although it is preferable to arrange the sweeps to be in close proximity to the point of minimum interaction or to envelope the point of minimum interaction (i.e. so that the minimum power in the sweep is below the power at which the interaction is minimized and the maximum power in the sweep is above the power at which the interaction is minimized).

More specifically, where a DC motor is used as driving means, the characteristic curve of applied voltage against current will in general be a function of the flow rate. Thus, calibration data could be derived by measuring curves of voltage against current at a variety of different flow rates. During use, measurements of the applied voltage and current over a time period, such as when the control system 4 is adjusting the power supplied to the moveable member towards the state in which fluid flow disruption is minimized, could then be used to determine the flow rate from the calibration data.

Alternatively, calibration data could be derived by using current measurements to calculate the amount of electrical power delivered to the motor for a plurality of voltage settings, whilst simultaneously measuring the speed of rotation of the turbine 2 and the flow rate. During use, measurements of the current and speed of rotation of the turbine over a time period, such as when the control system 4 is adjusting the power supplied to the moveable member towards the state in which fluid flow dismption is minimized, could then be used to determine the flow rate from the calibration data.

In the embodiments shown, the turbine speed is provided independently by a turbine speed measuring device 10. Means may be provided, for example, to sense the passing of individual blades of the turbine 2 and the time interval between the passing of adjacent blades or of the same blade, for example, may be used to deduce the speed of rotation. Sensing of the blades can be carried out by solid state devices (e.g. inductive, capacitive, optical or magnetic detection), mechanical gears, or as an integral part of a brushless DC motor drive. Any direct measurement of this kind will involve some interaction with the turbine 2 and will therefore represent drag. This is a problem with prior art systems and careful design is necessary to minimize the interaction. Where the turbine 2 is powered, however, the interaction is less serious as it can be compensated by the driving means 6/12, and the turbine speed measuring device 10 can thus easily be designed to provide high accuracy measurements at reasonable cost.

In the embodiments shown, connections from outside of the fluid channel 14 to the fluid pressure sensors 8A18B, interaction sensors 20A120B, and motor 12 are implemented using physical connections that pass through the fluid flow. However, wireless connections may also be used to the same effect, with the advantage of minimizing disruption to the fluid flow.

Alternatively or additionally, connections based on pneumatic andlor hydraulic means may also be used. Pneumatic or hydraulic means may be particularly appropriate for the pressure sensors

8A/8B for example.

In all of the embodiments discussed above, a turbine was used as the moveable member. However, the moveable member does not have to be a turbine. Any member whose interaction with a flowing fluid varies according to the speed of movement of the member when immersed in the fluid may in principle be used without departing from the scope of the invention.

Figure 4 shows a graph of performance data for a flow meter according to an embodiment of the invention in which a turbine speed is adjusted until the pressure drop across the turbine is minimum (i.e. "zero"), the flow rate being then deducible from the particular turbine speed at which this occurs (as described above). The vertical axis gives the fan (turbine) speed, i.e. the raw output from the flow meter, at "zero" pressure drop in the units of revolutions per minute (rpm). The horizontal axis gives the actual corresponding flow rate in the units of litres per minute (1/mm). The plotted data and superimposed best-fit show how linear performance is achieved over a turndown ratio of 60:1. The performance range plotted is limited by the equipment that was available to generate the flow and not by the flow meter itself.

Nevertheless, it is noted that even 60:1 is significantly above what can be achieved using normal turbine meters. In fact, it is thought that there is no upper limit in flow rate for the flow meter according to this embodiment until the flow rate goes supersonic. Therefore, linear behaviour over substantially larger turndown ratios should be possible. p

Claims

1. A flow meter for determining a flow rate of a flow of fluid, comprising: a moveable member for immersion in the flow of fluid; a driving means for driving movement of the moveable member; and a control system for varying the power supplied to the moveable member by the driving means, wherein: the flow meter is configured to determine the flow rate by monitoring the operation of at least one of the moveable member and driving means.

2. A flow meter according to claim 1, wherein: the control system is configured to vary the power supplied to the moveable member by the driving means until the flow meter reaches a state in which a disruption to the fluid flow caused by the flow meter is substantially minimized; and the flow meter is configured to determine a flow rate of the fluid by monitoring the operation of at least one of the moveable member and driving means while the flow meter is in said state.

3. A flow meter according to claim 1, wherein: the control system is configured to vary the power supplied to the moveable member by the driving means until the flow meter reaches a state in which a disruption to the fluid flow caused by the flow meter is substantially minimized; and the flow meter is configured to determine a flow rate of the fluid by monitoring the operation of the moveable member and driving means while the flow meter is being driven towards said state by said control system.

4. A flow meter according to claim 2 or 3, wherein: the control system is configured to vary the power supplied to the moveable member by the driving means until the flow meter reaches said state by substantially minimizing at least p one of the following: a change in the flow rate, a change in the flow pattern, and a pressure change in the fluid.

5. A flow meter according to claim 2, 3 or 4, wherein: the flow meter is configured to determine a flow rate of the fluid by monitoring at least one of the following: the speed of movement of the moveable member, the mechanical power supplied by the driving means, the torque supplied by the driving means, the electrical power supplied to the driving means, the voltage supplied to the driving means, the current supplied to the driving means, the frequency of voltage supplied to the driving means, and the power factor supplied to the driving means.

6. A flow meter according to any one of claims 2 to 5, further comprising: a pressure difference measuring device for measuring a difference in pressure in the fluid flow between at least one point located upstream of the moveable member and at least one point located downstream of the moveable member, the control system being arranged to use an output from the pressure difference measuring device to determine when the flow meter reaches the state in which disruption to the fluid flow caused by the flow meter is substantially minimized.

7. A flow meter according to any one of claims 2 to 6, further comprising: an interaction sensor positioned on the moveable member and configured to measure an interaction between the fluid and the moveable member, the control system being arranged to use an output from the interaction sensor to determine when the flow meter reaches the state in which disruption to the fluid flow caused by the flow meter is substantially minimized.

8. A flow meter according to any one of claims 2 to 7, further comprising: a monitoring device for monitoring one or more operational parameters of the driving means, the control system being arranged to use an output from the monitoring device to determine when the flow meter reaches the state in which disruption to the fluid flow caused by the flow meter is substantially minimized.

9. A flow meter according to any one of the preceding claims, further comprising: a speed of movement measurement device for measuring the speed of movement of the moveable member, the flow meter being configured to determine a flow rate on the basis of an output from the speed of movement measurement device.

10. A flow meter according to claim 1, further comprising: a monitoring device configured to monitor one or more operational parameters of the driving means during a time period while the power supplied to the moveable member by the driving means is varied by the control system, the flow meter being configured to determine a flow rate by reference to how the monitored one or more operational parameters vary during the time period.

11. A flow meter according to claim 10, wherein: the monitoring device is configured to monitor the voltage and current applied by the driving means during a time period while the power is varied by the control system, the flow meter being configured to determine a flow rate by reference to the curve of voltage against current during the time period.

12. A flow meter according to claim 10 or 11, further comprising: a speed of movement measurement device for measuring the speed of movement of the moveable member; and wherein: the flow meter is configured to determine a flow rate by reference to how the monitored one or more operational parameters and the measured speed of movement vary during the time period.

13. A flow meter according to claim 10, II or 12, wherein the control system is configured to vary the power supplied to the moveable member by the driving means until the flow meter reaches a state in which a disruption to the fluid flow caused by the flow meter is substantially minimized; and said time period is from when the control system begins to vary the power supplied to the moveable member to when the flow meter reaches said state.

14. A flow meter according to any one of the preceding claims, wherein: the moveable member is rotatable.

15. A flow meter according to any one of the preceding claims, wherein: the moveable member is a turbine.

16. A flow meter according to any one of claims Ito 13, wherein: the moveable member is configured to operate via a linear pump motion.

17. A method for determining a flow rate of a flow of fluid, comprising: providing a moveable member for immersion in the flow of fluid; providing a driving means for driving movement of the moveable member; varying the power supplied to the moveable member by the driving means; and monitoring operation of at least one of the moveable member and driving means in order to determine the flow rate.

18. A method according to claim 17, wherein: said varying step includes varying the power until the flow meter reaches a state in which disruption to the fluid flow caused by the flow meter is substantially minimized; and further comprising: determining a flow rate of the fluid using information about how at least one of the moveable member and driving means are operating while the flow meter is in said state.

19. A method according to claim 17, wherein: said varying step includes varying the power until the flow meter reaches a state in which disruption to the fluid flow caused by the flow meter is substantially minimized; and further comprising: determining a flow rate of the fluid using information about how at least one of the moveable member and driving means are operating while the flow meter is being driven towards said state.

20. A flow meter constructed and arranged to operate substantially as hereinbefore described with reference to and/or as illustrated by the accompanying drawings.

21. A method of determining a flow rate substantially as hereinbefore described with reference to and/or as illustrated by the accompanying drawings.