GB2312512A

GB2312512A - Electromagnetic flowmeters

Info

Publication number: GB2312512A
Application number: GB9608668A
Authority: GB
Inventors: Bernard John Patrick
Original assignee: CALEDONIAN CONTROL TECHNOLOGY
Current assignee: CALEDONIAN CONTROL TECHNOLOGY
Priority date: 1996-04-26
Filing date: 1996-04-26
Publication date: 1997-10-29
Also published as: GB9608668D0; JP2000509496A; EP0895580A1; WO1997041407A1

Abstract

An electromagnet for an electromagnetic flowmeter comprises a ferromagnetic core 1 in the shape of a substantially closed figure with confronting inwardly-directed poles 3,4 defining between them a field area in which the meter tube 10 may be accommodated so that the core will substantially surround and screen the meter tube against spurious magnetic fields directed transversely to the axis of the meter tube and protect it against mechanical damage. An electromagnetic flowmeter having such an electromagnet may have a metal pipe union joining the meter tube on each side of the electromagnet and screening it against spurious magnetic fields directed at acute angles to the axis of the meter tube.

Description

ELECTROMAGNETIC FLOWMETERS Electromagnetic flowmeters, for measurement of conductive liquid flow in closed conduits, are described in the specification of British Standard BS 5792 : Part 1 @ 1993 (ISO 6817 : 1992). In that specification an electromagnetic flowmeter is defined as a "flowmeter which creates a magnetic field perpendicular to the flow, so enabling the flow-rate to be deduced from the induced electromotive force (e.m.f.) produced by the motion of a conducting liquid in the magnetic field. The electromagnetic flowmeter consists of a primary device and one or more secondary devices." As described in the specification, the primary device usually comprises an electrically-insulated meter tube through which the conductive liquid to be metered flows, one or more pairs of electrodes, diametrically opposed, across which the 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.

In the primary device the meter tube, which must be non-magnetic so as not to interfere with the magnetic field and electrically-insulating or insulated at least between the elctrodes so as to avoid shortcircuiting the electrode signal, may be made wholly of electrically-insulating material which will not be attacked by or react with the liquid to be metered 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 tobe may be sheathed in metal for strength.

The flow signal, which is proportional to the speed of flow and the strength of the magnetic field across the meter tube is usually only of the order of microvolts and it is usually contaminated by other signals which malte up the electrode signal, including signals generated by specious magnetic fields crossing the meter tube. This contamination interferes with the performance of an electromagnetic flowmeter and necessitates the use of complex and powerful amplifier and filter circuits in the secondary device, complicating and increasing the cost of manufacture. Such spurious magnetic fields may be generated, for example, by switching in nearby electrical circuits or by radio or electro-magnetic communications equipment in die vicinity of the flowmeter. The invention aims to eliminate or reduce the effect of such spurious magnetic fields.

According to this invention, an electromagnet for an electromagnetic flowmeter comprises 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 may be accommodated, whereby the core will substantially surround and screen the meter tube against spurious magnetic fields, and a coil around at least one of the poles for connection to an electric current source for generating a magnetic field in the field area.

The substantially closed figure is preferably closed or continuous, providing two parallel paths for magnetic flux between the poles, but it may have a narrow air gap. It is preSably a rectangle or a circle, but it may be of any convenient shape.

Further according to the invention, in an electromagnetic flowmeter the meter tube is substantially surrounded in the region containing the electrodes by the ferromagnetic core of the electromagnet, the core including confronting poles defining a field area between them in which the meter tube is located, at least one of which is encircled by a coil for connection to aa electric current source for generating a magnetic field in the field area.

The electromagnet is preferably as just described.

To improve the magnetic performance the core is preferably assembled from a number of thin sheets or laminations, for example of iron, as in the conventional construction of a transformer core. To enable the coil and perhaps also the meter tube to be assembled easily with the core, each lamination may be divided into two piti and adjacent 1 laminations may be divided on different lines so that the dividing lines are staggered when the core is assembled. Alternatively the core may be moulded in ferrite or a ferro-magnetic polymer composition.

By substantially surrounding the meter tube in the region containing the electrodes the core provides efficient screening against spurious magnetic fields directed substantially perpendicular to the axis of the tube in tin region, but the ilow signal may still be contaminated by spurious magnetic fields directed at an an angle to tbat axis. To provide mote complete screening, the meter tube per- has end flanges which lie close to the opposite faces of the poles and of the substantially closed figure of the core and a metal pipe connector or union is provided on each side of the flowmeter for connecting it into a pipeline which is to carry the liquid to be metered, for example water. Each union has a flange complementary to the end flanges of tile meter tube and the two unions are damped to the meter tube with their flanges confronting the flanges of the meter tube with liquid-tight sealing means between them.

In addition to providing efficient magnetic screening, tile core provides mechanical protection for the meter tube which may be augmented by clamping the flanges of the pipe unions together with the sealing means, the flanges of the meter tube and the core compressed together between them, tor example by bolts passing through the assembly. The meter tube is then completly protected against mechanical damage and the need for a metal sheath, previously mentioned, is eliminated.

An embodiment of the invention is illustrated by way of example by the accompanying drawing in which: Figure 1 is an end elevation of an electromagnetic flowmeter viewed in the direction of flow of a liquid to be metered.

Figure 2 is a view similar to Figure 1 with a pipe union removed and tbe meter tube sectioned to reveal the electrodes and Figure 3 is an exploded side elevation of the flowmeter with the core sectioned on the line III - III in Figure 2.

The electromagnetic flowmeter comprises an electromagnet having a ferromagnetic core 1 and a coil 2 through which an electric current can be passed to generate the required magnetic field. The core 1 is of oblong rectangular shape with confronting poles 3, 4 directed inwardly from top and bottom limbs 5 and 6 which are pined by two side limbs 7. The coil 2 surrounds the pole 3 so when it is energized a magnetic field is gelerated along tile pole 3, across tile field area between the poles 3 and 4 and bad: along tbe two side limbs 7. Witb a symmetrically-shaped core with substantially equal aide limbs as shown tile magnetic field will be divided substantially equally between the two limbs. If the limbs were of different lengths or shapes the division would be unequal, but the performance of the flowmeter would not be affected significantly although its calibration would be different. Likewise if one side limb or top or bottom limb included an air gap that would greatly reduce the magnetic field through that part of the core, but the flux through the field area between the poles ad thus the performance of the flowmeter would not be affected significantly.

The core 1 is assembled from a number of thin sheets or laminations in the manner of a transformer core. In order to be able to assemble the core with the cobl each lamination be be divided into two pans and in order to secure magnetic and mechanical integrity the divisions 8, 9 in adjacent laminations are staggered.

The confronting ends of the poles 3, 4 are curved to fir snugly around a meter tube 11 which is located in the field area between the poles. The meter tobe 10 is a true cylinder with outward end flanges 11 and its length is such that when it is located in the field area the flanges 11 lie close against the faces of the core, as shown in Figure 3. Two diametrically-opposed electrodes 12 are mounted in tile middle of tile will of the true cylinder of tile 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.

The meter tube 10 with flanges 11 is a unitary moulding in plittics soch as polypropylene, with the electrodes 12 positioned as inserts in the mould For mounting the flowmeter in a pipeline which is to carry the liquid to be metered a metal pipe union 13, preferably ferromagnetic, having a hexagonal flange 14 and an internally screw-threaded collar 15 is mounted at each end of the meter tube 10 with the flanges 11 and 14 confronting and an O-ring 16 providing a liquid-tight seal berween them wben the assembly 15 secured by clamping bolts 17. The assembly is mechanically strong because the flanges 11 of the metering tube 10 are firmly clamped against the faces of the core 1, yet correct dimensioning of the metering tube ensures that there is no significant compressive force on the true cylinder of the tube itself, which might cause distortion of the true cylinder, leading to inaccuccy of measurements made by the As shown most dearly in Figure 2, the tme cylinder of the meter tobe is wholly enclosed within the core 1 and thus is not only magnetically screened against external magnetic fields which may be directed substantially transversely to the meter tube, but is protected by the core against mechanical impact damage.

The metal flanges 14 and collars 15 of the pipe unions 13 provide screening against external fields which may be directed at acute angles to the axis of the meter tube and the integrity of tbe ably onoe the clamping bolts are tightened provides substantial strength and in particular protects the meter tube from torsional damage which is sometimes caused when electromagnetic flowmeters are carelessly fitted in pipelines. Thus the region between the electrodes 12 where the flow signal is to be generated is securely screened against spurious magnetic fields which might otherwise contaminate the flow signal and the meter tube itself is mechanically protected against damage. The strengthening sheath used in some prior flowmeters to protect the meter tube is therefore unnecessary.

Claims

1 An electromagnet for an electromagnetic flowmeter comprising a ferromagnetic core in the shape of a substantially closed flgure with confronting inwardly-directed poles defining between them a field area in which the meter tube may be accommodated, whereby the core will substantially surround and screen the meter tube against spurious magnetic fields directed transversely to the meter tube, and a coil around at least one of the poles for connection to an electric current source for generating a magnetic field in the field area.

2. An electromagnet as claimed in Claim 1 wherein the substantially closed figure is oblong rectangular.

3. An electromagnetic flowmeter wherein the meter tube is substantially surrounded in the region containing the electrodes by the ferromagnetic core of the electromagnet.

4. An electromagnetic flowmeter as claimed in Claim 3 wherein the electromagnet is as claimed in Claim 1 or Claim 2.

5. An electromagnetic flowmeter as claimed in Claim 4 including a metal pipe union mounted at each end of the meter tube for connecting it into a pipeline which is to carry a liquid to be metered, the union including an end part of size such as to overlie the ends of the poles and adjacent parts of the ferromagnetic core to screen the meter tube against spurious magnetic fields directed at acute angles to tbe aids of die meter tube.

6. An electromagnetic flowmeter as claimed in Claim 5 wherein the meter tube has end flanges which lie close to the opposite faces of the poles and adjacent parts of the ferromagnetic core and the end part of each metal pipe union is a complementary flange, clamping means being provided for clamping the complementary flanges of the meter tube and the pipe unions together with sealing means between them.

7. An electromagnetic flowmeter as claimed in Claim 5 or Claim 6 wherein the metal pipe unions are of ferromagnetic metal.

8. An electromagnet for an electromagnetic flowmeter substantially as hereinbefore described with reference to and as illustrated by the accompanying drawing.

9. An electromagnetic flowmeter substantially as hereinbefore described with reference to and as illustrated by the accompanying drawing.