GB2362220A

GB2362220A - Fluid flow rate measurement with tracer

Info

Publication number: GB2362220A
Application number: GB0025861A
Authority: GB
Inventors: Chris Harris; Christopher John Gillham; Edmund Sparks
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 2000-05-12
Filing date: 2000-10-23
Publication date: 2001-11-14
Also published as: GB0011387D0; GB0025861D0

Abstract

A non-contaminating, decaying tracer is introduced into a fluid flow such as water and the device includes means for generating the tracer, means for introducing a known quantity of the tracer into the fluid, means for mixing the tracer with the fluid, means for measuring the resultant concentration of the tracer, and means for calculating the flow rate of the fluid. The tracer does not pollute the fluid and may be in the form of a physical tracer such as an air suspension (bubbles) or heat or may be in the form of a chemical tracer such as ozone. The rate of decay of the tracer is calculated and used to derive the fluid flow rate. Independent claims refer to the use of multiple downstream tracer sensing devices, sensors to measure a downstream impulse response, or the use of a holding tank.

Description

2362220 METHOD AND APPARATUS FOR MEASURING FLOW RATES The present

invention relates to a method and apparatus for measuring flow rates of a fluid. More specifically the present invention relates to a method and apparatus for measuring the flow rate of a fluid, such as water, which avoid the need for artificially introducing substances into the fluid under examination.

For water provision and treatment authorities the measurement of flow in clear water streams or conduits is traditionally tackled by a variety of techniques. One of these is by means of the introduction of a tracer substance at a known delivery rate, with the water dilution of the tracer measured downstream. The concentration of the tracer downstream is the ratio of the known tracer volume input rate to the required stream volume flow. The main problem with this approach is that it is undesirable to introduce substances artificially into water courses, which would cause pollution of the water. The only exception to this is the introduction of certain substances for the purposes of making the water safe for public consumption.

The present invention therefore seeks to provide a method and apparatus for efficient flow rate measurements of fluids, which avoids the introduction of polluting tracer substances into the fluid under test.

Accordingly, the present invention provides a method and device for measuring fluid flow rates using a tracer, where the tracer leaves the fluid 2 unchanged and unpolluted, but where the tracer is detectable for at least a short period of time.

Accordingly, the present invention provides a method for measuring a flow 5 rate of a flow of a fluid, comprising the steps of. introducing a known quantity of a non-contaminant, decaying tracer into the flow; mixing the tracer into the flow; measuring the resultant concentration of the tracer; and calculating the required flow rate by comparing the resultant concentration of the tracer with the known quantity of the tracer.

The known quantity of tracer preferably comprises a known concentration of the tracer in a carrier supply of the fluid.

The tracer may comprise at least one selected from a group including 15 bubbles, ozone and heat. Where heat is used as the tracer, it may be introduced by direct heating of the flow of fluid. Alternatively, it may be introduced by heating a carrier supply of the fluid, and introducing the heated fluid into the flow.

The method may further comprise calculating the rate of decay of the tracer, and compensating the resultant concentration of the tracer to derive the required flow rate.

The step of compensating may comprise calculating a theoretical initial 25 resultant concentration po of the tracer, which would have resulted from instantaneous mixing of the tracer with the flow at the point of introduction of the tracer.

3 The step of calculating the rate of decay may comprise the steps of measuring a first resultant concentration p, at a distance X, from a point of introduction of the tracer; measuring a second resultant concentration P2 at a distance X2 from the point of introduction of the tracer; and calculating the decay rate according to the values Of PI, P2, X, and X2.

Preferably, the time taken for the flow to travel distance 22 is twice the time taken for it to travel distance X,. Preferably, the flow rate is assumed to be 10 constant. Preferably, X2= 2. X,.

The decay process may be assumed to be exponential, in which case PO may be calculated according to: PO =P12/ P2.

The method may further comprise a calibration phase for determining the input rate of the tracer.

The method may further comprise a calibration phase for determining the tracer decay by measuring dilution at a set of points at different distances 20 from the point of introduction of the tracer.

The tracer may be introduced continuously at a fixed rate during the measurement. An equilibrium is preferably reached at the point(s) of measurement.

4 Alternatively, the tracer may be introduced as an impulse. The resultant pulse shape may be measured at the point(s) of measurement, the area under the relevant pulses being measured and used for flow rate and decay calculations. In such an embodiment, the pulse width may be taken to be 5 constant, and only the pulse height is measured.

In such a method, the pulse shape measurement may be used to calculate mean fluid velocity and tracer decay, using only one downstream measurement point.

A carrier supply of the fluid may be collected, a quantity of tracer introduced, and then the carrier supply with tracer is released into the flow of fluid.

The concentration of the tracer in the carrier supply, and the flow rate of the carrier supply into the flow of fluid, may be measured to allow compensation for variations in tracer density and/or carrier flow rate during introduction.

Part of the fluid flow may be diverted into a holding tank, as a carrier supply. Such carrier supply may be held in the holding tank until it reaches a measurably different temperature from the fluid flow. The carrier supply may then be released, and the difference in temperature used as a heat tracer.

In certain embodiments of the invention, the decay rate may be assumed to be negligible.

The method may comprise initially measuring a background concentration of the tracer, initially present in the fluid before introduction of the tracer fluid.

The present invention also extends to a device for measuring a flow rate of a flow of fluid, comprising: means for generating a tracer fluid, which comprises a non-contaminating decaying tracer mixed into a supply of the fluid; means for introducing a known quantity of tracer into the flow; means for mixing the tracer with the flow; means for measuring the resultant concentration of the tracer; and means for calculating the flow rate from the resultant concentration and the known quantity.

The known quantity may be is a constant rate of introduction of a tracer; alternatively the known quantity may be an amount of tracer introduced as an impulse.

The tracer may be heat, in which case the means for generating a tracer fluid may be a heater or cooler for directly heating or cooling the flow. Alternatively, the means for generating a tracer fluid may be a holding tank arranged to provide a supply of heated or cooled fluid.

The device may further comprise means for measuring the decay rate of the tracer, and means for compensating the resultant concentration for the decay of the tracer.

In any of the described embodiments, the fluid may be water.

6 In addition, the present invention also provides the following aspects: water flow gauging by introducing non-contaminant, decaying tracers and measuring concentrations downstream; the use of multiple downstream tracer sensing devices to compensate for the decay rate of the tracer; the use of sensors to measure the downstream impulse response for a finite trace input event; the use of air bubble suspensions in water as a noncontaniinant tracer for water flow gauging; the use of ozone as a noncontammant naturally decaying tracer for water flow gauging; the use of water at a different temperature as a tracer for the purpose of flow gauging the use of a direct heating input into a stream for the purpose of gauging flow; the use of a holding tank for the purpose of carrying out any non-contaminant alteration of the condition of water over slow time, in order to use the reservoir as a tracer for stream gauging.

The above, and further, objects, advantages and characteristics of the present invention will become more apparent in reference to the following detailed description of certain embodiments, given by way of examples only.

The present invention provides a method and device for measuring fluid flow rates using a tracer, where the tracer leaves the fluid unchanged and unpolluted, but where the tracer is detectable for at least a short period of time. This is likely to be particularly acceptable, if, during its period of 7 detectability, the presence of the tracer represents a condition that is not an unusual state of the fluid. Two kinds of tracer are envisaged here - one which alters the state of the fluid physically, and one which alters the fluid chemically for a short period.

The following description will refer to the measurement of flow rates of water, by way of an example fluid, and the measurement of a stream, as an example of a flow of the example fluid.

A commonly observed physical condition of piped water is the air bubble suspension that causes milkiness. A glass of such milky water typically takes of the order of a few minutes to clear. During this period the physical property of turbidity would be measurable, to indicate the concentration of bubbles in the water. If milky water were to be mixed with clear water, the resultant turbidity would be a measure of dilution.

According to a certain embodiment of the invention, an air suspension (bubbles) in a carrier fluid (preferably also water, to avoid pollution) may be introduced into a stream at a constant rate. The suspension (bubbles) can then be thoroughly mixed with the stream (e.g. through the turbulence of a weir structure) and the turbidity measurement as measured at a point downstream of the weir represents a measurement of dilution of the air suspension, which in turn relates the clear water flow to the rate of input of the tracer.

In an alternative embodiment of the invention, a heated fluid (preferably the same fluid as that being measured, to avoid pollution) is introduced into the 8 flow. The heated fluid is then mixed with the fluid as discussed above, and the temperature of the resultant fluid is then measured downstream. In this embodiment, the applied heat is used as the tracer. The heated fluid is the carrier fluid.

In an example of such an embodiment, a heating element of known output power can be used directly in the stream flow.

Alternatively, where power consumption is an issue, the power required for 10 heating can be introduced in slower time (e.g. from a local renewable source like solar power, or by using a less powerful heating element) into a reservoir of water, which can be emptied into the stream (i.e. the water conduit) when the temperature difference between the holding tank and the stream is sufficiently different.

In either of the preceding examples, an appropriate cooling device could be used instead of the heating devices.

The holding tank principle can also be applicable even without a direct 20 heating device, relying on the environment to warm or cool the tank differently from the running stream.

According to a further embodiment of the present invention, ozone may be introduced into the stream as a tracer. Ozone is commonly applied to water in current water treatment techniques. Ozone has the advantage of being readily generated on site from the atmosphere, without recourse to shipped chemicals. Furthermore, with direct relevance to the present invention, 9 ozone has a natural decay, back to the atmospheric oxygen from whence it came. This is in contrast to chlorine, which is also known for use in water treatment processes, but which remains in the water as a polluting substance.

If the water under test has already undergone a treatment regime which is ozone-based, there may remain some of the treatment ozone in the water before the ozone tracer is added. This treatment ozone may need to be measured, and a correction applied to the calculation of flow rate. Some residual ozone may remain from the flow measurement for some time.

However, this will decay and is likely to be of low concentration. Since ozone is already permitted in water for human consumption and occurs naturally to some extent then it cannot be regarded as a pollutant. The concentrations of ozone required for flow measurement are likely to be very small. Whether or not ozone tracers can be used in any particular application will depend of the permissible levels of residual ozone in the water, and the level of residual ozone already present before the ozone tracer is applied.

The air suspension (bubble) and heat tracers may be considered physical 20 tracers, which only physically alter the water for a short period of time. The ozone tracer may be considered a chemical tracer, which adjusts the chemical composition of the water for a short period of time.

For tracers such as ozone, it is necessary to know the rate of input of ozone 25 gas into the stream, to allow the flow rate calculation to take place. Assuming the volume of the carrier fluid to be negligible, this could be calculated as the volume of gas per second absorbed into the carrier fluid, irrespective of the flow rate of the carrier fluid. Alternatively, the concentration of ozone in the carrier fluid may be measured along with the flow rate of the carrier fluid. In either case, the flow rate may then be calculated, for example, as: if 0.2g ozone introduced per second gives rise to a measured concentration of 0.02g/m, the flow rate is 10M3/second.

In the case of a heat tracer, it is necessary to know the rate of addition or subtraction of heat from the stream. This may be from knowledge of the power of the heater (e.g. 20W heater supplies 20J/second), or from the difference in temperature between the stream and the carrier supply, together with the flow rate of the carrier. For example, the carrier flow rate may be 0.001M 3 /sec at 290K, while the stream is at 280K. If the measured temperature rises to 280. 1 K, this would indicate a flow rate of 0. 1 m3/sec.

In the case of the air suspension (bubbles) tracer, the turbidity will give a measure of the concentration of bubbles in the carrier fluid. By measuring the turbidity of the stream after application of the carrier fluid, a ratio of concentrations can be determined, from which a flow rate can be deduced, provided that a flow rate of the carrier fluid is known. Supposing a carrier fluid of turbidity 20 (arbitrary units) is applied at a rate of 0. 001M3/second, and the resultant measurement of the stream is 0. 1 (arbitrary units), this would indicate a flow rate of the stream of 0.2M3 /second.

In each of the above examples, the possibility of a certain concentration of the relevant tracer being already present in the fluid under test is ignored.

However, for example, ozone may be present from pervious treatments (discussed earlier); the water may have a certain turbidity introduced by 11 weirs (for example), including the mixing structure. Such effects could be measured and corrected for in a correlation step performed either before or after the measurement of the tracer concentration in the stream. When a heat tracer is used, it will be necessary to measure the temperature of the stream 5 upstream of the introduction of the tracer.

In both types of tracer proposed here (physical and chemical), there are two dilution factors at work. Firstly there is the basic dilution which is the ratio of the tracer volume input rate to the clear water flow rate. Secondly the tracer decays: air disperses or is absorbed, ozone dissociates and heat is lost or gained from the ambient. If the tracer could be mixed, and the dilution measured, instantly at the point of injection of the tracer, the decay factor would be unimportant. The mixing of tracer with the clear water, however, cannot be achieved instantly and measurements of dilution need to be downstream of a mixing facility. This means that the decay rate of the tracer may become significant, as a certain time will elapse between injection of the tracer, mixing and measurement.

A single measurement place downstream, moreover, does not allow the distinction between dilution brought about by flow-rate and that brought about by decay rate. A compensatory process may be required.

In the example measurements given above, the effects of decay of the tracers are ignored, for the purposes of simplicity. Indeed, in practice, it may be sufficient to assume a zero rate of decay. The present invention includes the case of low tracer decay rate, where a single downstream measuring station would be adequate.

12 For the purposes of simplicity of the following explanation, it is assumed that a section of channel exists where the flow rate and mean velocity of the water is constant between in ection and measuring points. j At the point of tracer injection, let the volume input rate of tracer be dvT/dt. Let the clear water flow rate be dvwldt, which is assumed to be large compared with the tracer input rate. Assume a hypothetical instant mixing of the two, so that the ratio of the two rates is the volume concentration of the tracer in the water po. That is, the required flow rate dv,/dt = (dvT/dt) - p, A point distant,1 downstream of the point of introduction of the tracer will have a measurable tracer concentration p, different from p, Since the flow is constant, this difference arises from the decay rate of the tracer. Consider a second point distant 12=211 from the point of introduction of the tracer at which the measured tracer concentration is P2. It can be assumed that with a constant flow rate and velocity, the decay rate for the tracer is the same for the second section as for the first. Assuming also that the decay process follows an exponential form, then:

P2 P1 01 jo 0 111 by which the quantity we require, po, may be deduced from the measurements p, and P2.

13 Equation [11 represents the simplest equation for deriving po. It assumes that the point of measurement Of P2 is twice as far from the point of introduction of the tracer as the point of measurement of p,, and that the rate (m31second) and velocity (m/second) of flow is constant between the point of introduction and the point of measurement Of P2. However, since the decay of the tracer is dependent on time, not location, the equation [11 will hold whenever the point of measurement Of P2 is reached by the flow after a period of time t2 twice that taken to reach the point of measurement of p, (tl), each being from the point of introduction of the tracer. If the times do not fulfil this relationship, either due to the location of the measuring points of p, and P2. or due to a variation in the cross section of the stream, suitable adjustments may be made to the expression of equation [11.

If the tracer input rate is not easily determinable, the system would need a calibration phase. If tracer decay is not sufficiently approximate to exponential, the decay function could be determined once and for all by measuring dilution at points on a set of, values.

There are two versions of the process to be considered - the steady state and the impulse response. The steady state process is that in which the tracer is 14 introduced continuously at a fixed rate during the period of measurement and where equilibrium is reached at the point of measurement. The alternative is that the tracer is introduced over a short period of time, representing a pulse of information, which will travel downstream. In this case the pulse shape as it passes the measurement stations at X, and X2 can be measured. Depending on the flow rate and the nature of the tracer, the pulse shape will be more or less spread out. If it remains a reasonable approximation of the original impulse the peak measurements alone may be adequate and can be interpreted as for the equilibrium measurements described above. If the pulse becomes spread out (as, for example, heat measurements might under slow moving stream conditions), it is the area under the pulse that will constitute the basic measurement.

An advantage of the pulse shape measurement is that it can also give a direct 15 figure for the mean water velocity, which can be used to estimate any tracer decay, and to compensate for irregular values of.1 and X2.

The above methods could be implemented as follows.

The air bubble suspension tracer could be produced by pressurised in ection j of air into a closed chamber of water, after which the chamber contents are released as a steady stream into the water conduit (stream).

The ozonation of water is a standard process of water treatment units.

Heat input can be simply arranged with electrical heaters. Alternatively, where power supplies may be limited, the stream flow itself could be partially diverted into a small holding tank and some water held for a period.

It is very likely that such a tank would reach a measurably different temperature from that of the flowing stream, and indeed the tank could be designed to make this more likely. It does not matter whether the difference is positive or negative. Under steady state delivery the tank could be held at a constant head by a simple overflow facility and the water delivered from the bottom of the tank at a constant calibrated flow rate. Under impulse delivery the tank input can be stopped off and the tank output valve opened, allowing the whole volume to be delivered. In both cases a thermometer can be used in the output stream, either to measure the steady state input temperature of the tracer carrier fluid or to give a temperature profile of the emptying tank in the impulse delivery mode. Corrections would need to be 16 made (to relate temperature to heat input (in Joules) in the latter case, for the change in output flow rate as the tank head reduced.

The simplest approach to mixing the tracer with the fluid of the stream is to 5 employ the turbulence induced by a rough impeding structure in the channel, e.g. in the form of a weir.

The concentration of air bubble suspension can be measured by measuring the turbidity of the water, e.g. within a simple cell, the transniissivity of light from an illuminator lamp through the water, can be measured by the response of a suitably positioned photometer.

There are a number of commercial devices available for measuring the concentration of ozone in water. For cheapness it may be possible to simply use pH measurement - while many factors may affect pH, any changes in pH from the upstream value are likely to be due to the injection of ozone and its subsequent decay.

Small differences in temperature can be detected very readily by simple thermistor or resistance thermometers.

17 While the present invention has been specifically described in relation to the measurement of flow rates of water, the invention may equally be applied to measurement of flow rates of other fluids, both liquids and gases. The tracer used for any particular fluid must be carefully chosen. For example, a gas flow cannot be measured by introduction of bubbles, although ozone could be used in the measurement of air flow rates. Heat could conceivably be applied to any liquid or gas flow measurement, with suitable precautions being taken when applying a heat tracer to flammable gas.

18

Claims

1. A method for measuring a flow rate of a flow of a fluid, comprising the steps of:

- introducing a known quantity of a non-contaminant, decaying tracer into the flow; mixing the tracer into the flow; measuring the resultant concentration of the tracer; and calculating the required flow rate by comparing the resultant 10 concentration of the tracer with the known quantity of the tracer.

2. A method according to claim 1, wherein the known quantity of tracer comprises a known concentration of the tracer in a carrier supply of the fluid.

3. A method according to any preceding claim, wherein the tracer comprises at least one selected from a group including bubbles, ozone and heat.

4. A method according to any preceding claim, wherein heat is used as the tracer, and is introduced by direct heating of the flow of fluid.

5. A method according to any preceding claim, wherein heat is used as the tracer, and is introduced by heating a carrier supply of the fluid, and 25 introducing the heated fluid into the flow.

19 6. A method according to any preceding claim, further comprising calculating the rate of decay of the tracer, and compensating the resultant concentration of the tracer to derive the required flow rate.

7. A method according to claim 6 wherein the step of compensating comprises calculating a theoretical initial resultant concentration PO of the tracer, which would have resulted from instantaneous mixing of the tracer with the flow at the point of introduction of the tracer.

8. A method according to claim 6 or claim 7 wherein the step of calculating the rate of decay comprises the steps of. measuring a first resultant concentration p, at a distance X, from a point of introduction of the tracer; measuring a second resultant concentration P2 at a distance X2 from the point of introduction of the tracer; and calculating the decay rate according to the values Of P I, P2, ^41 and X2.

9. A method according to claim 8, wherein the time taken for the flow to travel distance 2L2 is twice the time taken for it to travel distance X,.

10. A method according to claim 9, wherein the flow rate is assumed to be constant, and X2 = 2. X,.

11. A method according to any of claims 9-10 wherein the decay process is assumed to be exponential, and PO is calculated according to:

PO =Pi 21 P2.

12. A method according to any preceding claim, further comprising a calibration phase for determining the input rate of the tracer.

13. A method according to any preceding claim, further comprising a calibration phase for determining the tracer decay by measuring dilution at a set of points at different distances from the point of introduction of the tracer.

14. A method according to any preceding claim, wherein the tracer is introduced continuously at a fixed rate during the measurement, and an equilibrium is reached at the point(s) of measurement.

15. A method according to any of claims 1-13, wherein the tracer is introduced as an impulse, and the resultant pulse shape is measured at the is point(s) of measurement, the area under the relevant pulses being measured and used for flow rate and decay calculations.

16. A method according to claim 15 wherein the pulse width is taken to be constant, and only the pulse height is measured.

17. A method according to any of claims 15-16, wherein the pulse shape measurement is used to calculate mean fluid velocity and tracer decay, using only one downstream measurement point.

18. A method according to any of claims 15-17 wherein a carrier supply of the fluid is collected, a quantity of tracer introduced, and then the carrier supply with tracer is released into the flow of fluid.

21 19.. A method according to claim 19 wherein the concentration of the tracer in the carrier supply, and the flow rate of the carrier supply into the flow of fluid, are measured to allow compensation for variations in tracer density and/or carrier flow rate during introduction.

20. A method according to claim 19 wherein: part of the fluid flow is diverted into a holding tank, as a carrier supply; the carrier supply is held in the holding tank until it reaches a measurably different temperature from the fluid flow, and the carrier supply is then released, and the difference in temperature is used as a heat tracer.

21. A method according to any of claims 1-5, 12, 14,-16 wherein the decay rate is assumed to be negligible.

22. A method fluid according to any preceding claim, further comprising initially measuring a background concentration of the tracer, initially present in the fluid before introduction of the tracer fluid.

23. A device for measuring a flow rate of a flow of fluid, comprising:

means for generating a tracer fluid, which comprises a non contaminating decaying tracer mixed into a supply of the fluid; means for introducing a known quantity of tracer into the flow; means for mixing the tracer with the flow; means for measuring the resultant concentration of the tracer; and means for calculating the flow rate from the resultant concentration and the known quantity.

22 24. A device according to claim 23 wherein the known quantity is a constant rate of introduction of a tracer.

25. A device according to claim 23 wherein the known quantity is an amount of tracer introduced as an impulse.

26. A device according to any of claims 23-25, wherein the tracer is heat, and the means for generating a tracer fluid is a heater or cooler for directly 10 heating or cooling the flow.

27. A device according to any of claims 23-26, wherein the tracer is heat, and the means for generating a tracer fluid is a holding tank arranged to provide a supply of heated or cooled fluid.

28. A device according to any of claims 23-27, further comprising means for measuring the decay rate of the tracer, and means for compensating the resultant concentration for the decay of the tracer.

29. A method or device according to any preceding claim which the fluid is water.

31. Water flow gauging by introducing non-contaminant, decaying tracers and measuring concentrations downstream.

23 32. The use of multiple downstream tracer sensing devices to compensate for the decay rate of the tracer.

33. The use of sensors to measure the downstream impulse response for a finite trace input event.

34. The use of air bubble suspensions in water as a non-contaminant tracer for water flow gauging.

35. The use of ozone as a non-contaminant naturally decaying tracer for water flow gauging.

36. The use of water at a different temperature as a tracer for the purpose 10 of flow gauging.

37. The use of a direct heating input into a stream for the purpose of gauging flow.

38. The use of a holding tank for the purpose of carrying out any noncontaminant alteration of the condition of water over slow time, in order to 15 use the reservoir as a tracer for stream gauging.

39. A method substantially as described.

40. A device substantially as described.

41. Use of non-contarninant, decaying tracers; or multiple downstream tracer sensing devices; or sensors; or air bubble suspensions; or ozone; or 24 water at a different temperature; or a direct heating input; or a holding tank, each substantially as described.