TRANSDUCER FOR FLUID FLOW

Abstract: A flow transducer for producing a pressure differential responsive to rate of fluid flow comprises:- a housing (10) forming a flow passage (11a, 11b) for the fluid; said passage (11a, 11b) having disposed therein a parallel throat (13) which enlarges in the downstream direction into a chamber (14); a closely fitting yet freely sliding piston (16) disposed within the chamber and having an axial opening (18) for the flow of fluid; a valve member (19) of substantially conical form located and supported in the chamber (14) for obturating the upstream end of the said axial opening (18) when there is no flow; the said conical valve member (19) extending the the downstream direction nose first into the said axial opening (18) to provide a variable annular orifice (21) for fluid flow, the area of said orifice (21) increasing progressively with increasing axial displacement of the said sliding piston (16) in the downstream direction; an axially extending coil spring (23) for biasing the sliding piston (16) in an upstream direction to tend to reduce the area of the variable annular orifice (21) formed between the piston (16) and the valve memeber (19); the housing (10) having two pressure tappings (24a, 24b) therein at positions respectively upstream and downstream of the variable annular orifice (21) for the detection of a pressure differential induced by fluid flow; the said upstream pressure tapping (24a) communicating with an annular chamber (25) surrounding the throat (13); a plurality of radial holes (26) in an annular arrangement opening from the annular chamber (25) into the bore (13) of the throat for the detection of the upstream component of the said pressure differential, the said radial holes (26) being all of the same diameter and in the same axial position relative to the throat (13). The housing (10) is made in one piece to minimize cost and to avoid possible leakage and fatigue problems such as are commonly encountered in high pressure hydraulic service. Connecting means (28, 29) are provided at the upstream and downstream ends of the housing in the form of internal screw threads. These threads are of standard size having regard to the maximum flow capacity of the transducer and of the system in which it is fitted; making them internal (as opposed to external) screw threads maximizes the possible internal bore size of the housing for the containment of the working parts of the transducer mechanism and allows of the provision of flow passages of optimum size. The valve member (19) is of simple frustro-conical form which has the effect of producing a roughly linear relationship between rate of fluid flow and pressure differential. If it should be necessary to produce some other relationship between these variables the longitudinal profile of the valve member (19) may be suitably modified; such a modification might be required were a more exactly linear relationship be necessary for instance.

Description: This invention relates to a flow transducer for producing a pressure differential responsive to rate of fluid flow. DE-A-1773073 discloses three different designs of fluid flow transducers, none of which is very practical, each comprising a housing having a through passage with an inlet and an outlet and having two pressure tappings, one pressure tapping being upstream of the other. A disk or the like is mounted within the chamber for axial movement and is biased by a spring in the upstream direction to tend to close a variable width orifice between the disk and a throat formed by the housing. Fluid flow displaces the disk or the like in the downstream direction to an extent depending upon the flow rate, and produces a pressure differential between the two pressure tappings. FR-A-2209925 discloses a fluid flow meter comprising a tubular body of non-magnetic material with an inlet opening, an outlet opening and a tubular chamber between said openings. An annular piston is slidably disposed in the chamber and has an opening therethrough for fluid flow, the piston being spring-biased in the upstream direction. An elongate conical piece is supported in said body and disposed in the opening of the piston for movement of the piston axially of, and along, the conical piece. A magnet is disposed within the body and connected to the piston for movement therewith. A cylindrical indicator of magnetically attractive material, is arranged for movement in a space between the body and a tubular piece of transparent material bearing a scale, the indicator moving in unison with the magnet. There are no pressure tappings. DE-A-2919192 discloses another fluid-flow responsive device comprising a hollow bobbin which moves within a constriction member thus providing a variable orifice. It is an object of the invention to provide a compact flow transducer of simple and inexpensive construction which will measure with consistent accuracy over a wide flow range. A further object of the invention is to provide a flow transducer which is particularly suitable for incorporation into a high pressure hydraulic system. A further object of the invention is to provide a flow transducer which can be serviced in the field by operatives of limited skill and experience without consequent loss of calibration accuracy. The present invention provides a flow transducer claimed in each of the claims, to which reference is directed. Preferably the housing is made in one piece to minimize cost and to avoid possible leakage and fatigue problems such as are commonly encountered in high pressure hydraulic service. Preferably connecting means are provided at the upstream and downstream ends of the housing in the form of internal screw threads. If these threads are of standard size having regard to the maximum flow capacity of the transducer and of the system in which it is fitted, making them internal (as opposed to external) screw threads maximizes the possible internal bore size of the housing for the containment of the working parts of the transducer mechanism and allows of the provision of flow passages of optimum size. Preferably the valve member is of simple frustro-conical form which has the effect of producing a roughly linear relationship between rate of fluid flow and pressure differential. If it should be necessary to produce some other relationship between these variables the longitudinal profile of the valve member may be suitably modified; such a modification might be required were a more exactly linear relationship be necessary for instance. Preferably means are provided for the convenient fine adjustment of the initial loading of the piston biasing spring during manufacturing calibration, together with facilities for subsequent disassembly and reassembly of the transducer mechanism in the field without loss of the original calibration. The invention will be further described by way of example with reference to the accompanying drawings wherein: Fig. 1 is a longitudinal section through a flow transducer embodying the invention. The figure shows, above the centreline, the positions of the moving parts at maximum flow and, below the centreline, their positions when little or no flow is taking place; and Fig. 2 is a section along the line A-A of Fig. 1. Referring to the drawings, the illustrated flow transducer for producing a pressure differential responsive to rate of fluid flow comprises a one-piece housing 10 forming a flow passage 11 for the fluid (the fluid normally entering at the end 11a and leaving at the end 11b). The passage has inserted therein a throat member 12, the parallel bore of which forms a throat 13 leading in the downstream direction into a chamber 14, of greater diameter than throat 13, via a number of fixed apertures 15 formed at the downstream end of the throat member 12. A piston 16 is in closely sliding engagement with part 17 of the interior of the chamber 14 and has an axial passage 18 therethrough for the flow of fluid. A valve member 19 of substantially conical form is located and supported coaxially with the chamber 14 by an extension 20 of the throat member 12, and extends in the downstream direction (from 11a to 11b), nose first, into the axial passage 18. The valve member 19 obturates the axial passage 18 when there is little or no flow taking place. An annular orifice 21 for fluid flow is presented between the conical surface of valve member 19 and the upstream edge 22 of the axial passage 18, the total cross-sectional area of which varies progressively according to the axial position of the piston 16 relative to the valve member. An axially extending coil compression spring 23 is arranged between the piston 16 and a spring seating 31 (see below) for biasing the piston in the upstream direction to tend to close the annular orifice 21. The housing has two pressure tappings 24a, 24b through the housing wall, situated respectively upstream and downstream of the annular orifice 21, for detecting a pressure differential induced by fluid flow. The upstream tapping 24a communicates with the throat 13 of the throat member 12 via an annular chamber 25 of intermediate capacity formed between the throat member 12 and the wall of the housing 10 and extending right round a short length of the throat 13, and via a number of relatively small radial holes 26, all of the same diameter and all at the same axial distance along the throat 13, in an annular arrangement, which connect the annular chamber 25 in turn to the parallel bore of the throat member 12. The effect of the annular chamber 25 is to average the individual and often slightly varying pressures detected by each of the holes 26 and thus compensate in part for any distortions in the transverse velocity profile of the fluid entering the throat member 12 which may have been caused by disturbances in the flow conduits leading to the transducer. A groove 27 containing an elastomeric O-ring contributes to the proper sealing of the annular chamber 25, as does the use of anaerobic sealant between screw-threads 28, 30 ― see below. The downstream pressure tapping 24b opens into the housing passage 11, 11a; 11b at a position which is slightly downstream of the effective downstream end of the piston 16 when the piston 16 is in its most extreme downstream position. Connecting means are provided at each end of the housing 10 in the form of internal screw threads 28 and 29 by means of which the transducer can be incorporated into a hydraulic system (not shown). As fluid flow through the flow passage from the upstream end 11a to the downstream end 11b increases, the piston 16 moves progressively in a downstream direction to present an increasing annular orifice 21 to fluid flow, at the same time compressing the spring 23. A varying differential pressure is thus induced across the annular orifice 21, according to the load rate of the spring 23 and the longitudinal profile of the valve member 19. This differential pressure is detected by tappings 24a and 24b, in the manner previously described herein, and is led away to a suitable measuring device. For example, a suitable differential pressure transducer for this purpose is disclosed in European Patent Specification No. 0119038 and United States Patent Specification No. 4523476. The shape of the valve member is, as shown in the drawings, conveniently frustro-conical which results in a relationship between flow and pressure differential which is roughly linear. This relationship is adequate where the transducer is used in conjunction with an analogue type readout device (such as a differential pressure dial gauge for instance) which can be calibrated to match. By varying the exact longitudinal profile of the valve member 19, different relationships can be brought about; one such variation would, for instance, result in a more precisely linear relationship such as would be necessary for use in conjunction with a digital readout device. Fine adjustment of the initial loading of the piston biasing spring 23 during calibration is effected by appropriate axial positioning of the throat member 12, which has male screw threads formed on part 30 of its exterior for engagement with an extension of the internal connecting thread 28. After such adjustment the throat member is locked and sealed in position by, for instance, the use of an anaerobic locking sealant applied to the threaded part 30 of the throat member. The downstream end of spring 23 bears upon the above-mentioned spring seating 31, which is itself retained by a circlip 32. By removal of this circlip at any time the working parts of the device may be withdrawn from the housing for cleaning and inspection; subsequent reassembly of the parts and replacement of the circlip restores the original calibration conditions within the transducer. It has been determined by flow analysis that the maximum diameter of the valve member 19 can be reduced relative to that of the axial opening 18 by an amount which will permit the passage of full rated reverse flow whilst inducing a pressure drop of about 4 MPa (40 bar), without the proportional operating flow range of the transducer being reduced below about 30: 1 which is quite acceptable for most industrial applications. In this way, inadvertent reverse flow can be accommodated in a simple manner without destructive effects to the interior parts of the device; it will be appreciated that system operating pressures in this type of application often rise as high as 42 MPa (420 bar) or more, and, were no such provision made to accommodate reverse flow, the mechanism would be destroyed. In a possible general modification, the screw threads 28 and 29 may be replaced by flanges (not shown) for purpose of connecting the transducer into a hydraulic system.

Claims: 1. A flow transducer for producing a pressure differential responsive to rate of fluid flow, comprising: a housing (10) forming a passage (11, 11a, 11b) for the fluid; a throat member (12) disposed in said passage (11, 11a, 11b); the throat member (12) having a bore which is parallel to said passage (11, 11a, 11b) and which defines a throat (13) leading downstream into a chamber (14) which has a larger diameter than that of said throat (13); a closely fitting yet freely sliding piston (16) disposed within said chamber (14); the piston (16) having an axial passage (18) therethrough for the flow of fluid; a valve member (19) of substantially conical form, located and supported in a fixed position in the chamber (14) for obturating the upstream end of said axial passage (18) through the piston (16) when there is no flow; the said substantially conical valve member (19) extending in the downstream direction, nose first, into the axial passage (18) of the piston (16), to provide a variable annular orifice (21) for fluid flow; the area of said orifice (21) increasing progressively with increasing axial displacement of the piston (16) in the downstream direction; an axially extending coil spring (23) biasing the piston (16) in an upstream direction to tend to reduce the area of the variable annular orifice (21) which is formed between the piston (16) and the valve member (19); the housing (10) having two pressure tappings (24a, 24b) at positions respectively upstream and downstream of the variable annular orifice (21), for the detection of a pressure differential induced by fluid flow through the transducer; the said upstream pressure tapping (24a) communicating with an annular chamber (25) which is formed between an outside surface of the said throat member (12) and an inside surface of the housing (10) and which extends right round a short length of the throat (13); and a plurality of radial holes (26), all of the same diameter, each connecting the annular chamber (25) to the throat (13); the said radial holes being all at the same axial distance along the throat (13), in an annular arrangement, for the detection at the upstream tapping (24a) of the upstream component of the said pressure differential; the said downstream pressure tapping (24b) opening into the housing passage (11, 11a, 11b) at a position which is slightly downstream of the position of the effective downstream end of the piston (16) when the piston (16) is in its most extreme downstream position. 2. A flow transducer as claimed in claim 1 wherein the housing (10) is made in one piece. 3. A flow transducer as claimed in claim 1 or 2 wherein connecting means are provided at the upstream and downstream ends of the housing (10) in the form of internal screw threads (28, 29). 4. A flow transducer as claimed in claim 1, 2 or 3 wherein the valve member (19) is of simple frustro-conical form which has the effect of producing a roughly linear relationship between rate of fluid flow and pressure differential. 5. A flow transducer as claimed in any preceding claim wherein an internal screw thread (28) is provided at the upstream end of the housing (10) and is engaged by an external screw thread (30) on said throat member (12) to enable a longitudinal adjustment of the throat member (12) and thereby a fine adjustment of the initial loading of the piston-biasing spring (23) during manufacturing calibration, and wherein removable retaining means (32) is provided to hold a removable spring seat (31) in position during use, removal of the retaining means (32) and spring seat (31) when the transducer is not in use enabling removal and replacement of the spring (23) and piston (16) without loss of said calibration.

Inventors: LARNER, DONALD ALEXANDER
Applicants: FLUID DEVICES LIMITED
Application year: 1988
Main classification: G01F1
IPC codes: G01F1