GB2043916A - Fluid Flow Measurement - Google Patents

Abstract

A device (10) for diverting a relatively small but representative flow sample out of a large diameter conduit (12), through a flow meter (15), and back into the large diameter conduit comprises a generally flat body (11) extending diametrically across the conduit and having in its upstream face a narrow flow intake slot (16) whose length is substantially equal to the diameter of the conduit. A deflector plate (18) inside the body divides the flow sampled by the intake slot into two flow paths, which then recombine and lead into a meter inlet conduit (13) projecting exteriorly of the large diameter conduit. Fluid returns to the conduit via an orifice (24) disposed on the conduit axis. <IMAGE>

Description

SPECIFICATION Fluid Flow Sensors The present invention relates to fluid flow sensors, particularly for use in large diameter conduits. Such a sensor is intended to supply a fluid meter, or a flow meter, mounted in parallel with the large diameter (or principal) conduit, such that this meter passes a partial flow varying linearly with the flow in the principal conduit.

The measurement of the flow of fluid, for example water, in conduits of large diameter, i.e.

larger than about 1 metre, cannot be directly effected by means of a normal meter, e.g. the volumetric or flow velocity type, passing the total flow of the fluid to be metered, because of the dimensions and cost of such meter of this size.

The use of meters of the electro-magnetic or ultrasonic type provides a satisfactory solution from the point of view of dynamic measurement range and accuracy, but meters of this type normally require the presence of a source of electricity supply, and are also rather expensive.

As an alternative, small bore proportionai meters can be used, mounted in a small diameter conduit connected in parallel with the large diameter principal conduit in such a manner as to pass only a fraction of the flow passing in the principal conduit. The major disadvantage of this type of meter is that the secondary flow which passes through the meter does not remain constantly representative of the flow in the principal conduit.In fact, if the secondary flow is derived from a quasi-point flow intake, or a point too localised with respect to the full extent of the section of the principal conduit, this flow intake correctly transduces the changes of the flow in the principal conduit according to a determined law only when this flow is always produced under the same conditions, for example by calibrating the meter in the absence of turbulences and disturbances in the conduit. But when the meter is mounted in use on site, it is found that the results of measurement can differ very significantly from the law determined under the ideal calibration conditions.

It is also known to introduce a restriction of the area of the conduit, e.g. by means of a venturi or a diaphragm, thus producing a pressure drop across the restriction, and to measure the resulting pressure drop with a differential pressure sensor, from which the flow to be measured is finally inferred or calculated. This solution also has several disadvantages: besides the introduction of a significant pressure drop, and a blockage in the case of venturis, it provides a very small dynamic range of flow measurement, of the order of 5, due to the fact that since the law relating the flow to the variation of differential pressure is quadratic, the differential pressure sensor needs to have good precision over a measurement range equal to the square of the dynamic flow range.Such meters having the required precision are not generally used in a dynamic measurement range going beyond 5, and they are also particularly expensive.

Yet another solution consists of using a hollow tube projecting diametrically across the conduit, and pierced with several orifices distributed symmetrically with respect to the axis of the conduit. The flow penetrating through these orifices creates an over-pressure with respect to a pressure intake situated in the conduit downstream of the projecting tube. This overpressure is measured at the mid-point of the axis of the tube and corresponds to a mean value of the flow in the conduit, taken across the diameter of the tube. This solution, more satisfactory with respect to the small pressure drop introduced by the sensing element, is still based on a measurement of differential pressure and presents, as in the preceding case, the same disadvantage of a dynamic measurement range limited by the differential pressure sensor.

It is an object of the invention to provide a fluid flow sensor which enables at least some of the disadvantages of the above mentioned solutions to the problems of flow metering in large diameter conduits to be avoided or alleviated.

According to the invention, there is provided a fluid flow sensor for use in a cylindrical conduit traversed by a fluid, for sampling a partial flow representative of the fluid flow in the conduit, the sensor comprising an elongated body adapted to be mounted in the conduit so as to extend substantially diametrically thereacross, the body having a generally flat form and having an inlet orifice in the form of a slot whose length is substantially equal to the diameter of the conduit, an outlet orifice, and two coupling conduits for coupling a fluid meter to the sensor such that the meter is connected in a flow path between the slot and the outlet orifice.

Since the sensor has a generally flat shape, it presents a transverse cross-sectional area which is relatively small with respect to that of the conduit, thus producing only a relatively small pressure drop. On the other hand, since the sampling slot is disposed along a diameter of the conduit, the sampling obtained of the flow in the conduit represents a faithful sampling of the profile of the flow velocities along this diameter and, is thus substantially correct in the totality of the cross-sectional area of the conduit. It follows that the partial flow transmitted to the meter connected in parallel with the principal conduit is truly representative of the flow in the principal conduit.

Other constructional arrangements which will be explained hereinafter, ensure that the diversion and measurement of the partial flow do not disturb this flow velocity profile in the sampling zone and do not destroy its axial symmetry. The flow sensor according to the invention thus assists in the realisation of a proportional meter providing relatively cheaply results as good as those obtained with meters connected directly in the conduit, for example meters of the much more expensive electro-magnetic or ultrasonic type.

The invention will now be described, by way of non-limitative example only, with reference to the accompanying drawings, of which: Figure 1 shows schematically a perspective view of a sensor according to the invention; and Figures 2 and 3 are respectively a transverse cross-sectional view and a front view of this sensor, fixed in a pipe.

As can be seen in the drawings, the flow sensor is indicated generally at 10 and comprises a body 11 with parallel faces 1 A and 1 B spaced apart by a small distance, the median plane of the body 11 extending diametrically of a cylindrical conduit 12. The body 11 has threaded inlet and outlet coupling conduits 13 and 14, which are intended for coupling to a partial flow meter, indicated at 15, connected in parallel with the conduit 1 2. The conduit 12 is for example constituted by a pipe provided at its ends with flanges, for connecting it in series with the conduit through which the fluid whose flow is to be measured is flowing, and a sensor 10 is for example welded in a sealed manner in openings provided for this purpose in the pipe.

The direction of flow of the fluid being indicated by the arrow F, the sensor has an upstream face 1 7 of elongated rectangular profile, whose length is equal to the diameter of the conduit 12.

In the face 1 7 is cut a diametral slot 16, of rectangular form and of length substantially equal to the diameter of the conduit. Behind the slot 1 6, a deflector plate 1 8 is disposed transversally and symmetrically in the body 11, separating the flow entering via the slot into two equal flows passing at each extremity of the diameter under consideration. These two flows join up again in an orifice 1 9 of rectangular section at the inlet of an internal conduit 20, also of rectangular section, connected to the conduit 1 3.

The deflector plate 1 8 is intended to define two symmetrical paths, permitting the flow sampled from the conduit 1 2 to retain its symmetrical flow velocity profile with respect to the axis of the conduit despite disturbances which may be introduced downstream by the pressure drop due to the passage of the partial flow in the elements 13, 1 5 and 14.

The conduit 20 and an internal outlet conduit 21 which is connected to the conduit 14 are separated in the body 11 by a transversely disposed partition 22, and are bounded by a wall 23 of the body 11 parallel to the axis of the conduit. The diverted flow leaving the meter 1 5 by the conduit 14 returns into the conduit 12 via an outlet orifice 24, whose dimensions are equal to those of the orifice 1 9 and which opens into the conduit 12, symmetrically about the axis thereof.

The various parts of the sensor 10 are dimensioned such that the area of the orifice 1 9 is substantially equal to the area of the slot 16, and such that the two paths defined by the deflector plate 1 8 each have a cross-sectional area equal to half the area of the slot 1 6. The cross-sectional areas of the conduits 20, 13 and 14, 22 are equal to the area of the slot 16.

To the extent that the flow area of the fluid sampled by the sensor in said conduits is, as described in the example, of the same order of magnitude as the area of the slot 1 6, as well as of the same order of magnitude as the area of the inlet of the meter 1 5, it can be estimated that the pressure drop, and as a result the linearity of the response of the meter 1 5, depend only on the sensor 1 0. As a result, small variations of the area of the slot 1 6 by obstruction or by the building up of deposits for example, do not affect the accuracy of the metering.

Further, the same type of sensor, connected to a similar by-pass connected meter, can be connected in conduits of different diameters (of course with diametral slots of suitably adapted length) while retaining the same diameter of measurement, if the corresponding flow remains in the same range of variation of mean speeds of the fluid, for example between 10 and 200 cm/s.

The elements 13, 1 5, 14 exterior to the conduit form in this case a common unitary assembly which can be associated with different fluid flow sensors proper, adapted to the diameter of the conduit under consideration.

By way of example, for a conduit of 1 meter in diameter, the slot has a width of 2.5 mm, the area of the orifices and conduits such as 19, 20, 13, 14, 21 and 24 is of the same order of magnitude, namely 25 square centimeters, and the meter is chosne to operate with a given precision in a flow measurement range extending from 1 50 I/h to 40 m3/h.

Instead of being determined by the slot, the pressure drop can also be determined by the area of the downstream conduits, if this is less than that of the slot. This permits the use of a meter of smaller bore, the law representing the flow in the conduit 12 as a function of the flow read on the by-pass connected meter having in this case a more accentuated slope than in the preceding case. Still for a conduit of 1 meter in diameter, and retaining the same slot of 2.5 mm in width, an area of only 4 square centimeters for the downstream conduits permits the use of a meter having a measurement range of 10 I/h to 3 m3/h.

In the implementation hereinbefore described by way of example, the body of the sensor presents, in a plane perpendicular to the axis of the conduit and in a plane perpendicular to the plane of the slot, generally rectangular forms.

However, the body may if desired be surrounded by a streamlined fairing giving the sensor a more aerodynamic external form, this fairing playing no role with respect to the sampling and the circulation of the fluid inside the sensor.

Similarly, the mounting of the sensor in a pipe implies an interruption in the conduit and the cessation of its use in order to permit the connection of this pipe. To avoid this disadvantage, it can be useful to give to the fairing of the sensor a cylindrical form of circular section so as to permit its direct mounting in the conduit, for example according to a technique of jointing under pressure, the other extremity of the sensor coming into abuttment with the bottom of the conduit.

Claims (10)

Claims

1. A fluid flow sensor for use in a cylindrical conduit traversed by a fluid, for sampling a partial flow representative of the fluid flow in the conduit, the sensor comprising an elongated body adapted to be mounted in the conduit so as to extend substantially diametrically thereacross, the body having a generally flat form and having an inlet orifice in the form of a slot, whose length is substantially equal to the diameter of the conduit, an outlet orifice, and two coupling conduits for coupling a fluid meter to the sensor such that the meter is connected in a flow path between the slot and the outlet orifice.

2. A sensor according to claim 1, wherein said body comprises an elongated deflector plate disposed symmetrically behind the slot and defining two symmetrical paths each having a flow area substantially equal to half the area of the slot.

3. A sensor according to claim 1 or claim 2, wherein the respective flow areas of the coupling conduits and the area of the outlet orifice are at least equal to the area of the slot.

4. A sensor according to any one of the preceding claims, wherein the outlet orifice is substantially centred on the axis of the conduit.

5. A sensor according to any one of the preceding claims, wherein the inlet of the first coupling conduit is formed by an orifice which is substantially centred on the axis of the conduit and of the same area as the outlet orifice.

6. A sensor according to any one of the preceding claims, wherein the body is streamlined.

7. A sensor according to any one of the preceding claims, mounted in a cylindrical pipe of the same flow area as, and adapted to be connected in flow series with, said conduit.

8. A sensor according to any one of claims 1 to 6, said sensor being adapted to be mounted in said conduit by a technique of joining under pressure.

9. A fluid flow sensor substantially as hereinbefore described with reference to the accompanying drawings.

10. The combination of a sensor as claimed in any preceding claim with a fluid flow meter having an inlet and an outlet coupled to respective ones of the coupling conduits of the i sensor.