WO1999036749A1

WO1999036749A1 - Electromagnetic flowmeters

Info

Publication number: WO1999036749A1
Application number: PCT/GB1999/000032
Authority: WO
Inventors: Bernard John Patrick
Original assignee: Caledonian Control Technology Limited
Priority date: 1998-01-17
Filing date: 1999-01-18
Publication date: 1999-07-22
Also published as: EP1000322A1; GB9800952D0; GB2333366A

Abstract

An electromagnetic flowmeter in which the electromagnet is substantially wholly enclosed in a casing of thermally moulded plastics material, and terminals outside the casing are electrically-connected through the casing to components to or from which electrical signals may be required to be supplied or received. The meter tube may be an integral part of the casing or included as an insert in the moulding of the casing or a separate part which closely fits the casing. The thermally moulded plastics material may be thermoplastic or thermosetting.

Description

ELECTROMAGNETIC FLOWMETERS

TECHNICAL FIELD

Electromagnetic flo meters, for measurement of conductive liquid flow in closed conduits, arr described in the specification of British Standard BS 5792 : Pan 1 : 1993 (ISO 6817 : 1992). hi thai

5 specification an electromagnetic fio meter is defined as a "fiowmeter which creates a magnetic field perpendicular to the flow, so enabling the flow -rate to be deduced from the induced electromotive force (e.in.f.) prfKiuced by the motion of a conducting liquid in the magnetic field. The electromagnetic fiowmeter consists of a primary device and one or more secondary devices." As described in the specification, the primary device usually comprises an elcctricaDy-iπsulated meter tube through which the conductive liquid to

: 0 be metered flows one or more pair, of electrodes, diametrically opposed, across which trie signal generated in the liquid Is measured, and an electromagnet for producing a magnetic field in the meter tube. The secondary device comprises circuitry for receiving and amplifying the "flow signal", that part of the potential difference between the electrodes (called the "electrode signal") that is generated by the movement of the liquid.

15 BACKGROUND ART.

A typical primary device, for example as illustrated in the said British Standard, is of massive construction with the electromagnet formed by an iron core comprising opposed pole pieces each surrounded by one or more saddle-shaped coils which seat upon the meter tube, the pole pieces being bolted to two arcuate core pans which hold the pole pieces in position and complete the magnetic circuit. The meter tube, 0 which must be non-magπεtic so as not to interfere with the magnetic field and electrically-insulating or insulated at least between the electrodes so as to avoid short-circuiting the electrode signal, may be made wholly of electrically-insulating material which will not be attacked by or react with the liquid to be etered such as glass, ceramic or plastics, or it may be made of non-magnetic metal such as aluminium with electrical insulation surrounding the electrodes. A non-metallic meter tube may be sheathed in metal for strength.

25 In our P.C.T. Patent Application No. PCT/GB97/01126 we have described an electromagnet for an electromagnetic fiowmeter comprising a ferromagnetic core in the shape of a substantially closed figure with confronting inwardly-directed poles defining between them a field area in which the meter tube is to be located and at least one electrically-conducting coil surrounding a part of the ferromagnetic core, the coil having means for connection to an electric current source for generating a magnetic field in the field area and

"0 around the core between the poles and the electromagnet being characterized in that the opposite faces of the ferromagnetic core are substantially flat and parallel and only a root part of one of the poles is surrounded by a coil. We explained in our said Application that by a "root pan" of the pole we mean a part away from the free end of the pole, towards its junction with the substantially closed figure. It is not essential that the coil extends right up to that junction, it is sufficient that a significant tip pan of the pole extends free of the coil. The other pole is preferably not surrounded by a coil, but if it is that should also be around a root part of the

- pole, leaving a significant tip part free. This enables a screening element or flange member to be positioned close to each of the said opposite faces of the core, surrounding the axis of the meter tube and of size such as to overlie the tip parts of the poles and adjacent parts of the substantially closed figure to screen the field area against spurious interfering fields, for example electromagnetic fields, directed at acute angles to the axis of the meter tube. These screening or flange members are preferably coaxial with the meter tube and may be

* 0 pipe unions for connecting the meter tube into a pipeline carrying the liquid whose fiow is to be metered or parts of such pipe unions . The ferromagnetic core itself screens the field area and the meter tube against spurious fields directed substantially perpendicular to the axis of the meter tube. The meter tube preferably has end flanges which are precisely spaced to fit against the opposite faces of the core and are overlain by he screening element-- or flange members. This enables the assembly to be clamped tightly together without 5 causing mechanical damage to or distortion of the meter tube.

The present invention is particularly but not exclusively applicable to electromagnetic flowmcters as described in the above-mentioned Application. Whilst the efficient screening of the field area and the meter tube against spurious interfering fields which is provided in those flowmeters as described above Is important for obtaining accurate fiow measurements, it is known that the accuracy of a magnetic fiowmeter can also be 20 affected by eddy currents in the core or cores of the electromagnet and by potential differences between parts of a pipeline in which it is connected. The present invention aims to reduce or elimiπare those sources of error and also provides an electromagnetic fiowmeter in unitary form, ready for mechanical connection in a pipeline or conduit in which the flow of a liquid is to be measured and electrical connection to indicating or recording means to provide an immediate indication of flow rate through the meter tube.

25 DISCLOSURE OF THE INVENTION,

According to this invention an electromagnet for an electromagnetic fiowmeter Is substantially wholly encapsulated in a casing of thermally moulded plastics material and has terminals outside the casing which are electrically connected through the casing to components to or from which electrical signals may be required to be supplied oi received.

^"*•*^"■ Further according to the invention a primary device for an electromagnetic fiowmeter comprises an electromagnet and a meter tube located between the poles of the electromagnet with electrodes in the meter tube and the primary device is substantially wholly encapsulated in a casing of thermally moulded plastics material and terminals outside the casing are electrically connected through the casing to components to or from which electrical signals may r required to be supplied or received. RELEVANT PRIOR ART.

Ir has previously been proposed to enclose the magnetic components of electromagnetic fiowmeter*., and sometimes also electrical components and connections, in moulded insulating material. Such proposals are described, for example, in the following Patent Specifications: GE 2289514A. GB 2059066A.

5 GB 1072521. GB 8^3 . US 4774844, s 4567^*775. US 4098118 and US 4065965. In most of these prior Specifications the insulating materials described are cast around the components of the Sowmeters when cold. Most commonly they are epo y resin-*- which were pourable when cold and cross-linked by catalytic action after pouring to form a solid body. For example US 4774844 mentions Palmer chocktite, 3M-5064 and Master Bond 36 SP A-3. GB 1072521 mentions cement, concrete, synthetic resin or cold pouring

10 rubber. An alternative approach described in GB 2289514 uses field coils which are embedded in a flexible liner formed for example of resin- impregnated fabric which is inserted into a pipe, expanded to fit the pipe and then hardened by heat or catalyst.

The cold cast bodies of insulating materials described in most of the aforementioned prior Specifications are enclosed in rigid casings, usually of metal, which serve as moulds for the insulting

;5 materials during casting. Only in US 4774844 are outer and/or inner moulds not present in the finished primary device and the cold cast body forms the outer surface of the primary device as well as the meter tube though which the liquid to be mesred is caused to flow. However- the Specification describes a problem of cracking due to differential expansion coefficients of the metal frame on which the coils and electrodes are mounted and the encapsulating insulating material. To avoid that problem the metal frame is coated with a

20 thin layer of elastomeric material before casting the insulating material. A prior coating with adhesive may be necessary.

The cold cast bodies suffer from several other defects. Limitations are imposed by the relatively iow duty temperatures of the epoxy resin insulating materials. The ability of such materials to be filled with beneficial materials such as mineral particles and fibres to impart useful characteristics is well-known but --^• due to the nature of the process it is difficult to produce consistent dispersal of the filling materials. Further disadvantages are the high cost of the resins: the long curing time, typically of several hours; the release of solvents during cross-linking and poor mechanical properties such as brittleπess and low strength.

MORE DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS.

In contrast, by using thermally moulded plastics materials for encapsulation, either thermoplastic c

-0 thermoseπing types, these problems are eliminated. The materials aie generally lower in cost than cold casting materials and more environmentally-friendly as no solvents are released and thermoplastic types can readily be recycled. Mechanical properties are generally superior, particularly with thcrmosεtting plastics. Production time is measured in seconds rather than hours: the dispersal of filling materials is uniform and due to the high moulding temperatures, typically around 200 degrees Celsius, thermal stability in service is

.^'5 far superior to tha^* of cold cast materialr- It would be very difficult or imposssible to use thermally moulded plastics materials with fiowmeter assemblies such as those described in the aforementioned prior Specifications because of the very high pressures and temperatures involved in the moulding processes and the velocity of the materials when injected into moulds, which would cause damage if special precautions were not taken.

The present invention enables a fiowmeter to be produced quickly by moulding with magnetic and electrical components located as inserts in the mould. The meter tube may be an integral part of the casing or a separate component. It can be an integral moulded part if the material of the casing is appropriate also to form the meter tube, which will depend inter alia on the kind of liquid which is to flow through the meter tube- Preferably it is a separate component which is located as an insert in the mould with other components so that when moulding is complete it fits the remainder of the casing without any significant gap. In a preferred construction the electromagnet and the meter tube are as described in our afore-mentioned Application, with end Ganges which fit close against the opposite flat faces of the poles and adjacent parts of the substantially closed figure and are substantially covered by parts of the casing. Those parts of the casing also preferably have substantially fiat and parallel opposite outer faces to be overlain by screening elements or flange members as previously mentioned when the meter is assembled in a pipeline or conduit. The casing is preferably moulded around the electromagnet and the meter tube in plastics material.

If the meter tube is formed integrally with the casing, electrodes which are to be exposed in its inner wall are arranged in the mould together with their electrical connections to the relevant terminals so as to be secured in position by the moulding operation.

The electrical connections from the terminals through the casing may include circuitry enclosed within the casing foi processing signals supplied to or received from components of the fiowmeter- Indeed, the whole secondary device may be enclosed within the casing, with only the terminals exposed ready foi connection to current source and recording and or indicating devices. To protect the connections from the electrodes to the terminals from damage during moulding they may be led through a bore in the ferromagnetic core.

When the casing has flax and parallel opposite faces over the ends of the meter tube, or if end flanges of the meter tube are exposed outside the casing, the fiowmeter can be connected into a pipeline or conduit in which liquid flow is to be measured by flanged pipe unions. If the flanges are of suitable material and overlie the tip parts of the poles and adjacent parts of the electromagnet they will provide the desired screening against spurious interfering fields, foi example electromagnetic fields- directed at acute angles to the axis of the merer tube. Otherwise separate screening or flange members may be required as previously described. The fiowmeter preferably has bores to receive bolts or other clamping means for securing it between the pipe unions or screening or flange members. If the bores also pass through the core of the electromagnet they are lined with portions of the non-jnagnetic electrically-insulating casing which are preferably integrally formed with the remainder of the casing by moulding so as to insulate the pipe unions and any other metallic components and metallic pipes connected to them from the elctromagnet. Sealing means a}' be interposed between the opposite faces of the fiowmeter and the pipe unions or screening or βange member.: if necessary.

f. In its preferred form the invention thus provides an economically produced electromagnetic ilowmeter which is an integral unit which can readily be connected into a pipeline or conduit in which liquid flow is to be measured- having continuous insulated surfaces of hot moulded plastics material from which project only terminals for connection to appropriate electrical supply and instrumentation to read anά'or record signals from the circuitry enclosed within the casing. The continuous insulated surfaces make the

- 0 Qowmetei most suitable for applications in which hygiene is essential, such as for monitoring the fiow of body fluids in medical equipment or controlling flows in food production plant or water supply pipelines.

Embodiments of the invention are illustrated by way of example by the accompanying drawings in which:

Figure 1 is an end elevation of z primary device for an electromagnetic fiowmeter viewed in the 15 direction of flow of a liquid to be meteied. with its encapsulating casing of thermally moulded plastics material sectioned to reveal details of the device- Figure 2 is a side elevation of the device- sectioned on the line II-II in Figure 1 , also showing flanges for connecting the fiowmeter in a pipeline and a bolt for securing it in position in relation to the flanges, and 0 Figure 3 is a view similar to Figure 1 showing a secondary device connected to a primary device.

Figure 3a is a scrap view showing a part of a primary device with a modified casing. Figure -' is an end elevation of another primary device, shown prior to encapsulation. Figure 5 i? a plsπ of the primary device shown in Figure 4 and

Figure.* 6 and ~ are a side view and plan respectively of the meter tube of the primary device --* shown in Figure 4.

The primary device shown in Figure 1 is mainly as described in our afore- entioned P.C.T. Patent Application, comprising an electromagnet having a ferromagnetic core 1 and a coil 2 with end terminals 3. 4 which can be connected to an electric current source for generating the required magnetic field. The core 1 is of a closed oblong rectangular shape comprising top and bottom limbs 5 and 6 of the rectangle which are ^"0 joined by two side limb.. ". with confronting poles 8, 9 directed inwardly from respective top and bottom limbs 5 and 6. The coil 2 surround-; a root part of the pole Z. which is longer than the pole 9 so as to accommodate the coil and leave z free tip pan. When the coil 2 is energized a magnetic field is generated along the pole 8. across the field ares defined between the confronting poles tc the pole 9 and back along thr :oρ and bottσπ, limbs 5 and 6 and the two side limbs ". The confronting rips of the poles 8. 9 arc curved tc define the cylindrical field area in which a meter tube 10 is located and the opposite faces of the core 1 are flat and parallel at least in the region of the tip part of the pole 8, the pole 9 and adjacent parts of the bottom limb 6 and the side limbs 7 of the closed figure. The meter tube 10 is a true cyhnder with outward end flanges 11 which are precisely spaced so that when the tube 10 is located in the field area they lie close 5 against those flat faces of the core, as shown in Figure 2, the free tip part of the longer pole 8 and the whole of the shorter pole 9 fitting between the flanges 11. Two diametrically-opposed electrodes 12 are mounted in the middle of the wall of the true cyhnder of the meter tube 10. When an electrically-conducting liquid flows along the meter tube and the coil 2 is energized, an e.m.f. is generated between the electrodes 12 which is proportional to the speed of flow.

10 The meter tube 10 with flanges 11 is a unitary moulding in plastics material such as polypropylene, with the electrodes 12 positioned as inserts in the mould. Alternative meter tubes could be of metal with the electrodes insulated from the metal and from each other. Such a meter tube could have an insulating lining penetrated by the electrodes. In yet another construction the meter tube could be an integral part of a thermally moulded plastics casing which substantially wholly encloses the primary device.

15 The primary device shown in Figures 1 and 2 includes a casing 13 which is moulded in a nonmagnetic electrically-insulating thermoplastics material and covers almost all the surfaces of the core 1, enclosing the coils 2 and the meter tube 10. As shown, the cylinder of the meter tube 10 is slightly smaller in diameter than the field area defined by the curved tips of the poles 8, 9 and is enclosed in a thin layer of the casing material which penetrates between the poles and the tube, but in other embodiments the tube may be a

20 precise fit between the poles so that no casing material penetrates there. The flanges 11 of the meter tube 10 which lie close agaiαst the opposite faces of the core 1 are also covered by parts of the casing 13 which have flat and parallel outer faces 14. For mounting the fiowmeter in a pipeline which is to cany the liquid to be metered. the core 1 has bores 15 perpendicular to the its faces and thus also perpendicular to the opposite faces 14. The bores are also lined with the thermoplastic material of the casing 13 and receive bolts 16

-5 which pass through flanges 17 of pipe unions 18 which are formed from electrically-conductive metal. The flanges overlie the poles 8, 9 and adjacent parts of the core 1 and screen the field area against spurious interfering fields, for example electromagnetic fields, directed at acute angles to the axis of the field area and the meter tube . O-ring seals 19 are located between the flat faces 14 of the casing and the flanges 17.

In other constructions the casing 13 does not extend over the flanges 11 of the meter tube 10 so the

^:'^Q O-ring seaLs are clamped directly between the flanges 11 and 17 or the seals may be omitted if the flanges are fitted to engage each other in sealing relation. If the meter tube has no end flanges the casing may extend ovei the whole of the opposite faces of the core. Such a meter tube may be inserted after the casing has been moulded on the electromagnet. Tie end teτ irι≥l< 3. -i an: electrically connected to the coil 2 through the casing and terminals 20. 21 zre elec-' ally connected through ihf casing respectively to the electrodes 12 and the core 1. Although insulated from the c rf 1 by the casing 1 , the pipe unions 8 are electrically connected together by the bolls

1 if those are of conductive material or may be separately connected by a conductor (not shown, bridging

- the primary device

The primary device shown in Figure - comprises most of the components described in relation tc Figures 1 and 2. which are indicated by corresponding reference numbers, bw differs in that ? secondary device, an electrica' circuit 22, is also enclosed in the casing 13. Conductors 23, 24 lead from the circuit 22 to the ends of the coil 2 and conductors 25 and 26. respectively from the electrodes 12 and the core 1 load to

10 the ci-cuit. 22 The conductors are al! inside the casino 1 and are wholly enclosed by it. Terminals 27 outside the casing arc connected through i< to the circuit 22 and can be connected to appropriate external circuitry foi providing the required electric power for the circuit 22 2nd for transferring electrical signals, for example indicating the liquid fiow rate in the tube 10, to an indicating or recording device. Figure 3 thus shows a complete fiowmeter almost wholly enclosed in the casing material which can be installed in a

15 pipeline between flanged pipe unions such as I f. which complete the screening of the field area. Alternatively, separate screening elements of suitable materi-al could be interposed between the casing 13 or the flanges 11 of the meter tube and pipe unions to overlie the tip parts of the poles 8, 9 and adjacent parts of the core 1. Such separate screening elements could be enclosed within the casing and may have electrical connections to the circuit 22. Figure 3a shows a part of a primary device which is identical in all respects to

?P that shown in Figure 3 except that instead of a separate meter tube 10 the part of the casing 13 between the poles 8, 9 forms an integral meter tube or passage 28 for the liquid to be metered. In the production of this embodiment the electrodes themselves are located as inserts in the mould before moulding.

The primary device shown in Figures 4 and 5 likewise comprises most of the components described in relation to Figures 1 and 2, which are indicated by corresponding reference numbers, but differs in that a

-5 central bore 29 is formed in the pole 8 and the conductors 25 from the electrodes 12 are led through this bore 29 instead of alongside the side limbs of the ferromagnetic core 1. This protects them from damage or displacement during the moulding process due to the high pressure and velocity with which the thermoplastic material is injected into the mould. Figures 6 and 7 show the path of the conductors 25 from the end of the pole 8 to the elertrodes 12 As shown ir. Figure 5. the other ends of these conductor- 25 are

"0 connected to pins 30 and the ends of the coil 2 are directly connected to pins 1. the pins 30 and 31 being conveniently trapped ir; the mould join? during the moulding process, further reducing the possibility of movement of components during moulding Those pins and an earth tab 21 are the only projections from the moulded outer surface of the finished Qowmetei.

In all the illustrated embodiments the electromagnetic core 1 is of substantial construction and fonns a .5 frame which rigidly supports the meter tube 10 with its electrode assembly and the coil 2, which is firrrdv mounted on the pole 8- which is a centre limb of the frame. The frame is made of a ferrous and highly durable material that can withstand the shock of moulding the casing 13 by injection, compression or transfer mould techniques. Thus in addition to acting as the main mechanical support holding al! of the components rigidly in correct positions prior to and during moulding also performs the usual role of magnetic guide for the electromagnetic field produced by the coil 2.

Claims

1. An electromagnet for an electromagnetic fiowmeter which is substantially wholly encapsulated in a casing of thermally moulded plastics material and has terminals outside the casing electrically- connected through the casing to components to or from which electrical signals may be required to be c supplied or received.

2. An electromagnet as claimed in Claim 1 wherein the casing covers the confronting faces of the poles,

3. An electromagnet as claimed in Claim 1 wherein the thermally moulded plastics material is a thermoplastic.

0 4. An electromagnet as claimed in Claim 1 wherein the thermally moulded plastics material is a thermosetting plastic.

5. A primary device for an electromagnetic fiowmeter comprising an electromagnet and a meter tube located between the poles of the electromagnet with electrodes in the meter tube and the primary device is substantially wholly enclosed in a casing of thermally moulded plastics material and 5 terminals outside the casing are electrically connected through the casing to components to or from which electrical signals may be required to be supplied or received.

6. A piimaiy device as claimed in Claim 5 wherein the meter tube is an integral part of the casing.

7. A primary device as claimed in Claim 5 wherein the meter tube is a separate component which is included as an insert in the moulding of the casing.

8. A primary device as claimed in Claim 5 wherein the meter tube is a separate component fitting between the poles of the electromagnet without any significant gap.

9 A piimaiy device a claimed in Claim 5 wherein the casing covers the confronting faces of the poles of the electromagnet and the meter tube fits closely to the casing between the poles without any significant gap.

10. A primary device as claimed in any of claims 5 to 9 wherein opposite faces of the casing surrounding the meter tube are substantially flat and parallel. -lull. An electromagnetic fiowmeter of which the outer surface is substantially wholly formed of thermally moulded plastics material.