GB1574702A

GB1574702A - Fluid flow measuring assembly

Info

Publication number: GB1574702A
Application number: GB4637475A
Authority: GB
Original assignee: TEKFLO Ltd
Current assignee: TEKFLO Ltd
Priority date: 1977-01-10
Filing date: 1977-01-10
Publication date: 1980-09-10

Description

(54) FLUID FLOW MEASURING ASSEMBLY (71) We, T@KILO LIMITED, a British Company, of Kent Close, Granby Industrial Estate, Weymouth, Dorset, DT4 9TF, do hereby declare tite invention for which we pray tliat a patent may he granted to us. and tlie method by which it is to to performed,t to toe particularly described in and by the following statement: The present invention relates to the measurement of fluid flow in a fluid-carrying conduit.

In one known technique of measuring fluid flow, a probe having an opening therein is inserted in the conduit to interrupt the fluid flow with the opening exposed to the impact of the fluid stream. Because of the moving stream of mid the pressure in lie vicinity to the probe opening will differ from the so-called "stafic" fluid pressure immediately downstream of the opening, and this pressure difference is a function of the fluid velocity. This pressure difference is measured, and fed to a transducer to provide a suitable indication of the fluid flow in the conduit.

One fluid flow measuring device consists essentially of a plate having an orifice therein.

This plate is located in the conduit and the fluid pressure drop is measured in the vicinity of the orifice.

Another suitable probe is a pitot tube device having a pair tot' openings therein The pitot tube is inserted in tlie conduit with the first opening exposed to the impact of the fluid stream, and the second opening opposite to the first opening and facing downstream. The fluid pressure is measured at both these openings.

Under operational conditions tloe pressure difference is proportional to the fluid velocity.

In this specification any type of probe adapted to be exposed to the impact of the fluid stream will be referred to as an impact probe. and the pressure measured by this probe will be referred to as the impact pressure. Any probe to measure the pressure downstream of the impact probe will be referred to as a static probe, and the pressure measured by this probe will be referred to as the static or suction pressure.

In the previously proposed fluid flow measuring devices, the fluid pressure difference between the impact pressure and the static pressure is transmitted from the measuring points in the fluid to a transducer located outside the conduit. It has been found that this transmission of the fluid pressure difference can give rise to inaccuracies in the fluid flow indication.

The fluid flow may vary significantly at different transversely spaced points across the titow conduit to provide a distinct fluid fltow profile. As previously mentioned, a pitot tube device measures the pressure differential, and hence the fluid flow, only in the immediate vicinity of the tube openings. In order to provide an accurate measurement of fluid flow in the conduit from such a "single point measurement it is necessary to be able to predict the flow profile in tlie conduit, and usually this is possible only if tlie fluid now profile is symmetrical about tlie Itongitudinal axis of the conduit. It will be appreciated that this requirement limits the scope of any flow measuring device which indicates tile fluid pressure difference only at one point in the conduit.

Attempts have been made to measure fluid tlow at several transversely spaced points across the flow conduit and then to obtain an average value of the measured flow values. With these known averaging devices the indicated fluid pressures or pressure differentials are trans mitted from the measuring points in the fluid to a suitable transducer located outside the conduit. It has been found that inaccuracies may occur in these known devices if the flow profile is not axis-symmetrical, or if the flow profile changes froim streamline to turbulent flow, or develops swirling characteristics, particularly in regions adjacent to any changes of direction in the flow conduit.

It is an aim of the invention to at least reduce the magnitude of tlie previously mentioned inaccuracies in fluid flow measurement.

According to the invention there is provided an assembly to measure fluid flow in a conduit, said assembly comprising a plurality of impact probes adapted to sense the impact fluid pressure at transversely spaced points across tlle conduit, and an impact pressure averaging chamber to provide an average of the impact pressures at each impact probe, a static probe to sense the static fluid pressure downstream of the impact probes, a transducer to provide an electric signal proportional to the pressure difference between the pressure reading of the static probe and the pressure in the averaging chamber, in which the transducer is adjacent to the probes and in operation is located in the conduit.

Another measuring assembly of the invention may include a plurality of static probes adapted to sense the static fluid pressure at transversely spaced points across the conduit.

A static pressure averaging chamber may be included to provide an average of the static pressures at each probe, and the transducer may be adapted to provide an electric signal proportional to the pressure difference between the pressure in the static pressure averaging chamber and either the pressure in the impact pressure averaging chamber or the pressure reading of the sole impact probe.

Another measuring assembly of the invention includes a plurality of impact probes to sense the impact fluid pressure at transversely spaced points across the conduit, a single static probe to sense the static fluid pressure downstream of the impact probes, and a purality of transducers, each being adapted to provide an electric signal proportional to the pressure difference between the pressure reading of the static probe and its associated impact probe or probes, and a means to provide an average of the impact pressures at those impact probes associated with the same transducer. If desired, the structure of this assembly can be effectively reversed so that there is a plurality ol' static probes to sense the static fluid pressure at transversely spaced points across the conduit. and a single impact probe to sense the impact fluid pressure upstream ol the static probes.

Another assembly tot' tlie invention, iiicludes a plurality of impact probes and static probes to sense respectively the tilt pact fluid pressure and the static fluid pressure at transversely spaced points across the conduit, and a plurality of transducers each being adapted to provide an electric signal proportional to the pressure difference between the pressure readings of its associated impact probes and static probes. Means can be included to provide an average of the impact pressures and static pressures of those impact probes and static probes associated with the same transducer.

The measuring assembly of the invention may comprise a pitot tube having therein the appropriate impact probe and static probe openings.

In all the abovementioned measuring assemblies of the invention, the or each transducer is adjacent to tlie probes and in operation is located in tlie conduit.

Embodiments of the invention will now bedescribed by way of example with reference to the accompanying illustrative drawings, in which : FIGURE 1 is a diagrammatic side elevation of one fluid flow measuring device located in a fluid carrying conduit.

FIGURE 2 is a diagrammatic side elevation of fluid flow measuring device of the invention located in a fluid carrying conduit: FIGURES 3 and 4 are diagrammatic side elevations of two similar further fluid flow measuring devices of the invention located in a fluid carrying conduit; FIGURE 5 is a circuit diagram of part of the electronic circuitry associated with the device of Figure 3; and FIGURE 6 is a diagrammatic view of an array of devices of Figures 3 and 4 located in a transverse plane of a fluid carrying conduit.

Figure 1 illustrates a part of a fluid flow conduit 2 in which the fluid is flowing from left to right. A pitot tube device 4 is mounted in an opening in the wall of the conduit 2 so as to extend into the conduit. The device 4 includes an impact probe having an opening 6, and a static probe having an opening 8 diametrically opposite to the opening 6. The device 4 is arranged in the conduit 2 so that the two openings are on or adjacent to the longitudinal axis of the conduit 2 with the opening 6 facing upstream and the opening 8 facing downstream. It will be appreciated that the opening 6 is exposed to the impact of the fluid stream in order that the impact probe may provide an indication of the impact pressure, and the opening 8 is directed so that the static probe may provide an indication of the suction pressure which will be less than the impact pressure.

The impact pressure and suction pressure values are transmitted along passages 10 and 12 to a transducer 14 located in the device 4 between the two openings 6 and 8. This transducer 14 provides an electric output signal which is proportional to tlic pressure difference between the determined impact pressure and suction pressure; This pressure difference being a function of the fluid flow in the conduit 2.

This electric signal is passed to a suitable indicating device located outside the conduit to provide an indication of the fluid flow.

An advantage of this flow measuring device is that the output signal from tloe transducer 14 can be modified by known electronic circuitry to compensate for any deviation of the fluid flow profile from a theoretical correct value, It will be appreciated that this circuitry can be located in a convenient position outside the conduit 2, Another advantage of this device is that it is not necessary to pass fluid at accurately controlled pressures to an indicating device outside the conduit. with the consequent risk of inaccuracy in measurement.

As previously mentioned, the fluid flow velocity may vary significantly at different transversely spaced points across the conduit cross-section. A prior measuring device includes a tube having a plurality of longitudinally spaced openings in it. In operation this tube is mounted in the conduit wall to extend into the conduit with the openings located at dilierent points across the conduit cross-section. These openings are in communication with a single "impact" pressure chamber and the intention is that tloc internal pressure in this chamber is an average ol tile individual pressures generated at the respective openings. This average pressure is then compared to the suction pressure generated at a downstream facing opening, and the pressure differential is used to provide an indication of the average fluid titow in the conduit. However, this pressure differential is transmitted to apparatus outside the flow conduit by means tot' a fltiitl flow system, , and difficulties can arise in attempting to compensate for measurement inaccuracies caused by irregularities in tlie fluid litow profile in tlic conduit.

A similar known device operating on I the same general principle, uses a single elongate slit in the tube in place of the plurality of longi tudinaliy spaced impact probe openings.

One flow measuriiig device of the invention illustrated in Figure 2 has been designed to reduce the effect of the above-mentiolled difliculties. Referring to Figure 2, a pitot tube device 16 is mounted in the opening of a fluid now conduit 18 to extend into the conduit, Four colinear longitudinally spaced openings 20 are located in the device 16 to constitute four impact probes, and a single opening 22 constituting a static probe is located in the device 16 diametrically opposite to the openings 20 and mid-way along the length of these four openings 20 feed into a single primary averaging pressure chamber 24 which is connected by longitudinally spaced openings 26 to a single secondary averaging pressure chamber 28 having an outlet opening 30. The openings 22 and 30 communicate by conduits 32 and 34 to a differential pressure transducer 36.

In operation, the device is located in the conduit 18 with the openings 20 facing upstream to sense the fluid impact pressure, and the opening 22 facing downstream to sense the fluid suction pressure. An average value of the impact pressure as sensed at the four openings 20 is passed through the primary and secondary averaging chambers 24 and 28 and the conduit 34 to the pressure transducer 36, and the suction pressure sensed at the opening 22 is passed to this transducer 36 by tloe conduit 32.

The transducer 36 produces an electric output signal proportional to the pressure difference between the average impact pressure and the suction pressure, and this signal is fed to a suitable indicating device located outside the conduit, As with the previously described device, any irregularities in the conduit flow profile can he compensated by suitable standard electronic circuitry located in a convenient position outside the conduit 18.

Figures 3 and 4 illustrate measuring devices similar to the device ot Figure 2, and l'or clarity corresponding components will be given the same teference numerals. The principal difference between the devices of Figures 2,3 311d 4 is that the devices of Figures 3 and 4 do not include a pressure averaging chamber. Each impasct probe opening 20 communicates directly witch its respective pressure transducer 38. I In the device of Figure 3, tile four transducers 38 communicate via conduits 40 and 42 to the single static probe opening 22. In the assembly of Figure 4, the four transducers 38 assembly of F via their respective individual conduits 40(a), 40(b), 42(a) and 42(b) to their respective individual static probe openings 22.

-'iclt opening 22 is apposite to its respective 1100 pact probe topeloing 20.

Immediately before the device of Figure 3 Or Figure 4 is used to take at fluid tlow measurement - tie tlevice is repalced in the conduit 18 by tile single point pitot tube device 4 illustrated in Figure 1. This device is adjusted in the conduit I 8 so that its impact probe opening 6 coincides with the intended position of tile uppermost impact probe opening 20 of the device of Figure 3 or Figure 4. The device 4 is then operated to provide an indication of the fluid flow at that region.

The device 4 is then lowered in the conduit until its impact probe opening 6 corresponds to the intendetl position of the next lower impact probe opening 20 of the device of Figure 3 or Figure 4. and an indication is obtained of the fluid flow at that region.

This procedure is repeated with the impact probe opening 6 at the position of the two lowest impact probe opening 20 so that values have been obtained of the fluid flow conditions at each of the four impact probe openings 0.

The measuring device tof Figure 3 or Figure 4 is then reinserted in the conduit 18, and the four pressure transducers 38 are electrically connected through respective variable resistors R1, R2, R3 and R4 to the input of an "adding amplifier" 41 designed to provide an output signal proportional to the average value of the four electric input signals from the transducers.

The amplification and operational characteristics of the four input circuits to the amplifier 41 are then individually adjusted in accordance with the previously obtained values for the fluid flow in the regions of the four impact probe openings 20.

In operation, the output signals from each of the four pressure transducers 38 are passed through the respective variable resistor R1, R2, R3 and R4 to the input of the adding amplifier 41 which provides an output signal proportional to the average value of the four electric input signals from the transducers. In this way, an accurate average value can be obtained of tile fluid flow without having to transmit fluid pressure ftor any discrete distance either inside or out side the conduit.

It will be appreciated that the greater the number of impact pressure and static pressure readings that can be taken over a transverse plane of the conduit, the greater the accuracy of the obtamed average value of the fluid flow.

Figure 6 illustrates an array of four equally spaced measuring devices 1() tol' Figure 3 Lot Figure 4 which extend diametrically over a transverse plane of the conduit IX. Tile impact probe and static probe openings of the devices 16 face upstream and downstream respectively, and the electric output signals of the devices are connected via suitable electronic control to a fluid flow indicator.

By suitably adjusting the electronic circuitry this measuring device assembly can provide an accurate indication of fluid flow along the conduit 18 for virtually any fluid flow profile, A basic feature of tlte invention is the to- cation of the pressure transducer in the conduit so that it is not necessary to transmit fluid pressure from the measuring region in the fluid flow stream to the exterior of the fluid flow conduit. Although ilte above described measuring devices are of the pitor tube type, the invention is equally applicable to other types of flow measuring devices such as a venturi or dall tube or an orilice plate device.

It is to be emphasized that the invention is not limited to the illustrated constructions, and the invention covers a flow measuring device having any combination of impact probes and static probes. For example, the measuring device illustrated in Figure 2 may be modified to have a single impact probe opening 20, and a plurality of static probe openings 22. These static probe openings feed into a primary averaging pressure chamber which communicates with the transducer 36 either directly or through a secondary averaging pressure chamber.

Another flow measuring device of the invention is similar to the device illustrated in Figure 2 except that it has a plurality of impact probe openings 20 and static probe openings 22. These static probe openings 22 feed into a primary averaging pressure chamber similar to the chamber 24 whiclo communicates with the transducer either directly or through a secondary averaging pressure chamber similar to the chamber 28.

The flow measuring device illustrated in Figure 3 may be modified to have two or more transducers 38; each transducer communicating with any number of associated impact probe openings 20. If a transducer 38 is associated with a plurality of impact probe openings 20, then these probe openings 20 communicate with their associated transducer through a pressure averaging means such as an averaging pressure chamber.

The measuring device of Figure 3 may be further modified to have a single impact probe opening 20 and a plurality of static probe openings 22. Each transducer 38 is associated with any preselected number of associated static probe openings 22. II' at plurality of static probe openings 22 are associated with a transducer, then these openings 22 communicate with their associated transducer via a pressure averaging means such as an averaging pressure chamber.

The measuring device of Figure 4 may be totodified to have any desired nulllber of impact probe openings 20 and static probe openings 22, these openings 20 and 22 communicating with their associated transducers 38. If a plurality of impact probe openings 20 or static probe openings 22 communicate with an associated transducer 38, then this communication is effected via a pressure averaging means socio as an averaging pressure chamber.

Claims

WHAT WE CLAIM IS: . An An assembly to measure fluid tlow in a conduit, said assembly comprising a plurality of impact probes adapted to sense the impact of fluid pressure at transversely spaced points across the conduit. an tilopact pressure averaging chamber to provide an average of the impact pressures at each impact probe, a static probe to sense the static fluid pressure downstream to the impact probes. a transducer to provide an electric signal proportional to the pressure difference between the pressure reading of the static probe and the pressure in tile averaging chamber, in which the transducer is adjacent to the probes and in operation is located in the conduit.
2. An assembly as claimed in Claim 1, in which the said averaging chamber communicates with the transducer through a second pressure averaging chamber.
3. An assembly as claimed in either preceding claim. further comprising a plurality of static probes adapted to sense the static fluid pressure at transversely spaced points across the conduit, a static pressure averaging chamber to provide an average of the static pressures al each probe, in which the transducer is adapted an electric signal proportional to the pressure difference between the pressure in the static pressure averaging chamber and either the pressure in the impact pressure averaging chamber or the pressure reading of the sole impact probe.
4. An assembly to measure fluid flow in a conduit. said assembly comprising a plurality of impact probes adapted to sense the impact fluid pressure at transversely spaced points across the conduit, a single static probe adapted to sense the static fluid pressure downstream of the impact probe and a plurality of transducers, each being adapted to provide an electric signal proportional to the pressure difference between tlie pressure readings of the static probe and its associated impact probe or probes, and means to provide an average of the impact pressures at those impact probes associated with the same transducer, in which the transducers are adjacent to the probes and in operation are located in the conduit.
5. An assembly as claimed in Claim 4, in which there is a separate transducer for each impact probe to provide an electric signal pro- portional to the pressure difference between the pressure readings of its respective impact probe and the static probe.
6. An assembly to measure fluid flow in a conduit; said assembly comprising a plurality of static probes adapted to sense tlic slat it; Iluid pressure at transversely spaced points across the conduit, a single impact probe to sense the impact fluid pressure upstream of the static probes, and a plurality of transducers, each being adapted to provide an electric signal proportional to the pressure difference between the pressure readings of the impact probe and its associated static probe or probes, and means to provide an average of the static pressures at those static probes associated with tlie satire transducer. ill which tlie transducers are adjacent to the probes and in operation are located in the conduit.
7. An assembly as claimed in Claim 6, in which there is a separate transducer for each static probe to provide an electric signal proportional to the pressure difference between the pressure readings of its respective static probe and the impact probe.
8. An assembly to measure fluid flow in a conduit: said assembly comprising a plurality of impact probes and static probes adapted to sense respectively the impact fluid pressures and the static fluid pressures at transversely spaced points across the conduit, a plurality of transducers, each being adapted to provide an electric signal proportional to the pressure difference between the pressure readings of its associated impact probes and static probes, and means to provide an average of the impact pressure and static pressure of those impact probes and static probes associated with the same transducer, in which the transducers are adjacent to the probes and in operation are located in the conduit.
9. An assembly as claimed in Claim 8, in whicli there is only one impact probe and only one static probe associated with each transducer.
10. An assembly as claimed in any one tot Claims 4 to 9, in which said means to provide an average of the impact or static fluid pressure is an averaging pressure chamber.
11. An assembly as claimed in any preceding claim, in which the assembly comprises a pitot tube having therein the appropriate impact probe and static probe openings.
12. An assembly as claimed in any one of Claims 4 to 10, and in Claim 11 as dependent upon any one of Claims 4 to 10, including a circuit to modify if necessary the electric signals from each transducer to compensate for any deviation of the fluid flow profile fro ma theoretical correct value.
13. An assembly as claimed in Claim 12, in which said circuit comprises a separate variable resistor for each transducer signal. and an amplifier to provide an output signal proportional to the average of the transducer signals from saitl resistors.
14. An array of assemblies as claimed in any one of Claims 4 to 11v secured to one another at their central region and extending in different directions from one another in the same general plane; saitl array being adapted to be arranged in the conduit with tloe assemblies extending along different diameters of the conduit.
15. An assembly or an array of assemblies to measure fluid flow in a conduit, substantially as herein described and shown in Figures 2 to 6 of the accompanying drawings.