The present invention relates to a differential pressure flow measurement device.

There are many known devices for measuring the rate of flow of fluid through a pipe. The known devices use various techniques to measure flow, one of those techniques being differential pressure flow measurement. Differential pressure flow measurement devices in common use include orifice plate meters and venturi meters.

A venturi meter comprises a length of pipe formed with a constriction intermediate its ends. The pipe has two small tappings, one at the entrance to the constriction upstream of the fluid flow, and one at the narrowest point of the constriction. The two tappings are connected to a suitable pressure measuring device so that the static flow pressure can be measured at these two points. The static pressure of a flowing fluid is proportional to its flow velocity. Fluid flowing through the venturi will increase in velocity in the region of the constriction and hence there will be a difference in the pressure measured by the two tappings. This pressure difference can be used to calculate the rate of fluid flow through the pipe using a standard fluid flow formula:

Q = C _d A ₀ /(2 p/ _β ) / yU-CA./A,) ² )

Q = fluid flow rate

C _d = discharge coefficient which takes into account energy dissipation p = pressure difference between the two measurement points A _x = the cross sectional area of the entrance to the venturi meter

A ₀ = the cross sectional area of the constriction. = the fluid density

Venturi meters are very suited to applications such as measuring the rate of flow in large water mains, but have the disadvantage that they have to be very precisely machined and are therefore difficult to manufacture.

SUBSTITUTE SHEET

An orifice plate meter generally comprises a centrally orificed plate which is positioned normally to the fluid flow through a pipe. The plate has a diameter greater than that of the pipe in which it is intended to be installed and is located between flanges at the ends of two lengths of pipe. The central orifice has a diameter less than that of the pipe and restricts the fluid flow in a similar manner to the constriction of the venturi meter described above with a similar effect on the velocity and pressure of the flowing fluid. Pipe wall tappings are provided both upstream and downstream of the orifice plate and used to measure the differential pressure across the plate. This pressure difference is used to calculate the fluid flow using a mathematical relationship of the type shown above.

There are three main types of orifice plate meter in common use, distinguished by the positioning of the pressure tappings. The first has the tappings through the pipe wall, the upstream tapping being positioned at a distance from the orifice plate equal to the diameter of the pipe and the downstream pressure tapping being located at a distance from the orifice plate equal to half the diameter of the pipe. The other two common types of orifice plate have tappings through the pipe flanges. In one type the tappings are perpendicular to the pipe wall, and in the other type the tappings are at an angle to the pipe wall and open on the internal surface of the pipe at the corner formed between the pipe wall and the orifice plate. Tappings of the first type are known as flange tappings, whereas tappings of the second type are known as corner tappings.

Orifice plate meters of the general type described above have a considerable advantage over venturi meters in that they are relatively simple to manufacture. However, known orifice meters present installation difficulties in that existing pipework must be altered to provide the necessary pressure tappings. In addition, if they are to be used in anything other than "ideal" flow conditions they must be carefully calibrated to provide an accurate value for C _d . Further, known orifice plates are sensitive to flow distortions which are usually an inherent feature of most installations, and therefore usually require a relatively long length of straight pipe upstream of the meter in order to significantly reduce the installation effects.

An alternative form of orifice plate meter which is not very widely used is the annular orifice meter. The annular orifice meter comprises a circular plate of a diameter less than that of the pipe in which it is to be installed. The plate is supported normally to the fluid flow within the pipe on a hollow spindle which is itself supported co-axially within the pipe by means of a spider at each end thereof. Fluid flows around the plate between the edge of the plate and the inner surface of the pipe with a resultant change in flow velocity and pressure. The plate has a ring of pressure tapping holes on both its upstream and its downstream faces which communicate with the hollow supporting spindle. The section of the spindle connected to the pressure tappings on the upstream surface of the plate is sealed from the section of the spindle connected to the pressure tappings on the downstream surface. Both sections of the spindle are connected to tappings through the pipe wall enabling the respective pressures to be measured. The difference in the pressure registered on the upstream and downstream surfaces of the plate is then used to calculate the fluid flow using standard mathematics similar to that above. Annular orifice meters are difficult to install and maintain and have the additional disadvantage of their dependence on pipe dimensions to define a flow area. Thus they have not been widely used.

It is an object of the present invention to provide an orifice plate meter that obviates or mitigates the above disadvantages.

According to the present invention there is provided a differential pressure flow measurement device comprising an apertured plate intended to be secured across a flow passageway, at least two tapping ducts being defined within the body of the plate so as to communicate with respective tapping openings on opposite sides of the plate to enable the differential pressure across the plate to be measured.

The plate may define a single centrally located circular aperture, or may define a plurality of apertures, for example a central circular aperture and one or more rings of apertures disposed therearound. Thus the plate may serve as a flow conditioner in addition to deriving pressure information related to flow rate.

Preferably the plate is provided with a first duct having two interconnected portions, a first portion communicating with a tapping opening on one surface of the plate and a second portion communicating with a pressure transducer, and a second duct having two interconnecting portions, the first portion communicating with a tapping opening on the other surface of the plate and a second portion communicating with a pressure transducer.

The transducer may be located within the body of the plate. Alternatively the said second portions of the first and second ducts may communicate with the edge of the plate and may then be coupled to a transducer external to the plate. The said second portions of the ducts may extend radially from the edge of the plate to a transducer remote from the plate.

Preferably each of the said first and second ducts has at least four such first portions. Preferably the first portions of the ducts communicate with tapping openings in the form of small circular apertures on respective surfaces of the plate, the apertures preferably being arranged in a circular array concentric with the centre of the plate. Preferably the positions of the apertures defined by the first portions of the first duct on one surface of the plate correspond with the positions of the apertures formed by the first portions of the second duct on the other surface of the plate. The first portions of each of the first and second ducts may open into annular grooves formed on respective surfaces of the plate. Preferably the plate is provided with a first internal annular channel interconnecting the first portions of the first duct, and a second internal annular channel interconnecting the first portions of the second duct. Preferably each of the first and second ducts then have only a single second portion which interconnects with the respective annular channel.

Embodiments of the present invention will now be described, by way of example, with reference to accompanying drawings, in which;

Fig. 1 is a front view of an orifice plate meter according to a first embodiment of the present invention;

Fig. 2 is a cross-section taken on line A-A of Fig.l; Fig. 3 is a cross-section illustrating the embodiment of Fig.l installed in a pipe;

Fig. 4 is a front view of a modified orifice plate meter of Fig.l according to a second embodiment of the present invention;

Fig. 5 is a front elevation of a combined orifice plate meter and flow conditioner according to a third embodiment of the present invention;

Figs. 6 to 15 show the results of the calibration of various embodiments of the invention under different test conditions;

Figs. 16 to 18 show the results of the calibration of a standard orifice plate meter under various test conditions; and,

Figs. 19 to 21 show the results of the calibration of a further embodiment of the present invention as a combined flow meter/flow conditioner.

Referring to Figures 1 and 2 of the drawings, the device comprises a circular plate 1 which has a central circular orifice 2. Each surface of the plate 1 is provided with eight pressure sensing tapping openings or apertures 3 arranged in a circular array concentric with the orifice 2. The apertures 3 lead in to internal ducts which have a portion 4 normal to the surface of the plate 1 and a radial portion 5. The radial portions 5 of the ducts . from one surface of the plate 1 interconnect with an internal annular channel 6, and the radial portions 5 of the ducts from the other surface of the plate interconnect with an internal annular channel 7. Each of the channels 6 and 7 has a single pressure tapping, 8 and 9, which extends radially from the outer edge of the plate 1. A support member 10 is provided on the edge of the plate 1 to support the tappings 8 and 9 where they extend from the plate 1. The plate 1 is constructed from two concentric portions to facilitate formation of the various ducts and channels, the join between the two portions being indicated in Fig.l by the line at the radially outer end of the duct portions 5.

Referring to Figure 3, in use the plate is installed in a pipe system between flanges 11 at the ends of two lengths of pipe 12 such that it is disposed normally to the direction of fluid flow through the pipe 12. The direction of fluid flow is indicated by an arrow. The diameter of the plate 1 is chosen according to the diameter of the pipe 12 in which it is to be installed. It must be greater than the diameter of the pipe so that it can be fixed between the blank flanges 11. The diameter of the orifice 2 must be less than the internal

diameter of the pipe 12. The radial position of the apertures 3 is approximately midway between the internal wall of the pipe 12 and the edge of the orifice 2. The pressure tappings 8 and 9 extend beyond the pipe wall and are connected to suitable pressure measuring instruments (not shown).

The pressure of the fluid flowing through the pipe changes as it flows through the orifice plate 1. The fluid pressure at the location of each of the pressure sensing apertures 3 on the upstream surface of the plate 1 is transmitted via the ducts 5 to the channel 6. Thus the pressure measured through the pressure tapping 8 is the mean fluid pressure registered by the pressure sensing apertures 3 on the upstream surface of the plate 1. Similarly the mean static fluid pressure measured at the pressure sensing apertures 3 on the downstream surface of the plate 1 is measured through the pressure tapping 9. The measured pressure difference across the plate 1 can be used to calculate the rate of fluid flow through the pipe 12 at the location of the plate 1 using the method described below.

Applying the standard fluid flow continuity equation (ref. Experimental Fluid Flow Mechanics, P.A. Bradshaw, Pergamen, page 60) between a nominal upstream position X far upstream of the plate 1, and at the downstream face position, P, of the plate 1, yields the expression;

9 χ"χAχ ⁼ e _p UpA _p where u is the mean velocity taken across a section of pipe orthogonal to the flow.

This can be rearranged to give:

ύ _χ = P _p /&• A _p /.A _X . u _p

Assuming that the pressure sensing apertures 3 on the upstream surface of plate 1 register the mean total local stagnation pressure P ₀ , and those on the downstream surface of plate 1 register a mean local static pressure P, then the measured pressure difference across the plate Δ P _p can be used to calculate the local velocity of fluid at the

plate 1 from Bernoulli's expression (ref. Experimental Fluid Mechanics, PA Bradshaw, Pergamen, page 19);

where u is the localised velocity at the downstream face of the plate.

Given that: where ot would be expected to be less than 1, because the localised flow rate at the surface of the plate is expected to be less than the mean velocity, then;

thus:

The above expression therefore can be used to calculate the rate of fluid flow at the location of the plate 1 from the known fluid density, , and the mean differential pressure across the plate, P .

For meaningful results the pressure sensing apertures 3 on the upstream and downstream surfaces of the plate must be located at corresponding positions so that the upstream and downstream pressures are measured at the same annular and radial locations within the pipe.

The more pressure sensing apertures 3 that are provided on each surface of the plate 1, the closer the measured pressures will be to the actual mean pressures over the respective surfaces of the plate 1. It is recommended that a minimum of four such pressure sensing apertures 3 are provided on each surface of the plate 1.

Fig. 4 illustrates a modification of the orifice plate described above, in which the plate 1 defines an annular groove 13 on each surface thereof concentric with the orifice 2. The annular groove 13 effectively replaces the pressure sensing apertures 3 such that the duct portions 4 break into the side of the groove 13.

In the embodiment of the invention shown in Fig. 5, the plate 1 defines a plurality of circular apertures 14 in addition to the central

orifice 2. The apertures 14 enable the plate 1 to function as a flow conditioner as well as a fluid flow meter. Flow conditioners comprising orifice plates are well known in the prior art. The apertures 14 can be disposed in any suitable arrangement to give the desired flow conditioning effect. The arrangement shown in Fig. 5 is that recommended in British Patent Application No. 2,235 064. The illustrated plate has twelve pressure sensing apertures 3 on each face but it will be appreciated that the apertures 3 could be replaced by an annular groove as shown in Fig. 4.

A series of tests have been conducted to assess the performance of embodiments of the present invention in comparison with the performance of a standard orifice plate meter. For each of the tests conducted, embodiments of the invention having different ratios of orifice diameter, (d), to internal pipe diameter, (D), were positioned at various locations in a water pipe downstream of a butterfly valve. Measurements were taken with the butterfly valve in three different positions - fully open, +45° closed, and -45° closed, and for a range of fluid velocities. The results obtained were used to calculate the fluid velocity at the orifice plate using equation 1 above. The calculated values were then plotted against measured values of the velocity obtained using a vane anemometer of standard construction positioned a sufficient distance downstream of the plate installation to allow normal fluid flow conditions to resume. The results of the various tests are shown in Figs. 6-21 and are discussed below. On all of the figures the symbol is used to denote results for the case when the butterfly valve was set fully open, and the symbols and + denote the cases for which the butterfly valve was at +45° and -45° closed respectively.

Test A - Single Hole Plate with d/D = 0.6

For this test measurements were taken and results obtained for a plate located at various positions between a distance of 4D and 14D downstream of the butterfly valve. The distance of the plate from the butterfly valve will hereinafter be denoted (z). Fig. 6 is a plot showing the results for each of the three valve settings with the plate positioned at z = 4D. Similarly, Figs. 7-9 show the results for z = 7D, z = 10D and z = 14D respectively. Fig. 10 is a plot combining

with data of Figs. 6-9. It will be evident from Figs. 6-10 that the data calculated from the differential pressure measurements varied very little from the straight line fit which corresponds to agreement between the calculated and measured velocity values (measured values obtained from the vane anemometer). This shows that operation of the orifice plate was not significantly affected by the swirl and flow asymmetry introduced by the partially closed butterfly valve. The straight line fit between the calculated and measured velocity values was obtained with a value of c* = 0.6329.

Test B - Single Hole Plate with d/D = 0.4 and 0.5.

In order to check the general applicability of this method of measurement a first insert was fitted to the plate which reduced the size of the orifice yielding a ratio d/D = 0.5 and a second insert which further reduced the size of the orifice to give d/D = 0.4. Measurements were taken at z = 7D, and the results for d/D = 0.5 and d/D = 0.4 are shown in Figs. 11 and 12 respectively. The value of*< used for d/D = 0.5 was 0.375, and for d/D = 0.4 was 0.513. Here again the results show that the measured values were not substantially affected by the swirl and flow asymmetry caused by the butterfly valve.

Test C - Grooved Plate with d/D = 0.6.

Using the grooved plate of Fig. 4, measurements were taken at z = 4D and z = 7D. The results are shown in Figs. 13 and Figs. 14 respectively. The value of &<- used was 0.645. Fig. 15 is a plot combining the data of Figs. 13 and 14 Here again the changes in the valve settings show very little effect on the velocity values obtained from the differential pressure measurements as compared with those registered by the vane anemometer.

Test D - Orifice Plate

For this test a standard orifice plate constructed according to ISO 5167 with pressure tappings through the pipe wall at positions of D and D2 upstream and downstream from the plate respectively, was used. Figs. 16 and 17 show the results for z = 4D and z = 10D

respectively, and Fig. 18 shows the combined data points. The illustrated plots show a clear deviation from the straight line showing that the standard orifice plate was much more affected by the asymmetries in the fluid flow produced by the butterfly valve than were the orifice plates according to the present invention.

Test E - Combined Differential Pressure Meter/Flow Conditioner

For this test measurements were taken for a multi-hole plate of the type shown in Fig. 5. The hole diameters of the centre hole and inner and outer ring holes are denoted d ₁₅ d ₂ and d ₃ respectively and are related to the plate porosity B by the relationships: d _χ = 0.2679 (B) . D d ₂ = 0.6238 (B/n) . D d ₃ = 0.7341 (B/m) . D where n and m are the number of orifices in the inner and outer rings respectively. For this test the plate used had a porosity of B = 50% and d _x = 0.1894D, d ₂ = 0.1800D and d ₃ = 0.1498D. The pitch circle diameters for the inner and outer ring of orifices are 0.4616D and 0.8436D respectively. In the example illustrated twelve pressure sensing apertures with an annular separation of 30° are provided on the upstream and downstream faces of the plate on a pitch circle diameter of 0.667D which is approximately mid-way between the outer edges of the inner ring of holes and the inner edges of the outer ring of holes.

Results for this plate are shown if Figs. 19-21. Fig. 19 shows the results obtained at z = 7D, Fig. 20 shows the results at z = 9D, and Fig. 21 shows the combined results. This data has been processed with the value of &- = 0.665. Again the results obtained are not significantly affected by the swirl and asymmetry introduced into the fluid flow as the butterfly valve is partially closed.

The invention therefore provides a differential pressure fluid flow meter which is relatively insensitive to flow asymmetry and swirl caused by pipe installations. This is a result of the number of the relatively large number of measuring points that can be provided and of the method use to average the measured values.

The invention provides a self contained unit which is relatively simple to manufacture and install. It may be installed between any pair of flanges in a pipe system without requiring any alteration of the existing pipework.

Further the invention works well in its embodiment of dual flow meter/flow conditioner. It can therefore be used to provide back up measurement to a more precise and expensive instrument, such as a turbine meter, which requires a conditioned flow for accurate measurement.

In addition, because of the symmetry of the plate it is possible to test the soundness of the pressure sensing points and interconnections to the pressure tappings by reversing the plate in its flanges and checking for repeatable results.

In a further possible embodiment of the invention the plate may contain an active pressure transducer embedded within it. If this is an electrical transducer, for example, the interface from the edge of the plate would comprise electrical feed through. The resultant absence of any fluid pressure tappings to the outside of the plate could be very advantageous in, for example, high pressure applications and applications involving dangerous fluids.