EP0895580A1

EP0895580A1 - Electromagnetic flowmeters

Info

Publication number: EP0895580A1
Application number: EP97919534A
Authority: EP
Inventors: Bernard John Patrick
Original assignee: Caledonian Control Technology Ltd
Current assignee: Caledonian Control Technology Ltd
Priority date: 1996-04-26
Filing date: 1997-04-23
Publication date: 1999-02-10
Also published as: GB9608668D0; WO1997041407A1; GB2312512A; JP2000509496A

Abstract

In an electromagnet for an electromagnetic flowmeter the ferromagnetic core (1) is in the shape of a closed figure (5, 6, 7) with confronting inwardly-directed poles (3, 4) between which a meter tube (10) is to be located and has opposite flat parallel surfaces and only a root part of one pole is surrounded by a coil (2). An electromagnetic flowmeter having such an electromagnet has a ferromagnetic and electrically conducting flange member (14) closely overlying flat surfaces of the ends of the poles and adjacent parts of the closed figure on each side of the core and screening the meter tube against magnetic fields directed at acute angles to the axis of the meter tube.

Description

ELECTROMAGNETIC FLOWMETERS

TECHNICAL FIELD.

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.

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 parts which hold the pole pieces in position and complete the magnetic circuit. 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 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 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 tube may be sheathed in metal for strength.

Electromagnetic flowmeters of lighter or simpler construction than that illustrated in the said British

Standard have been proposed, for example in British Patent Specifications Nos. 1,095,915 and 2 059 066 A, U.S. Patent Specifications Nos. 4,454,766 and 4,932,268 and European Patent Specifications Nos. 0045 646A1 and 0 255 275A1. In all of those proposals the construction is substantially symmetrical, with an electromagnet comprising two poles each surrounded by a coil. In British Patent Specification No. 2 269 443A there is described a flowmeter comprising a horseshoe-shaped core straddling the meter tube, with a single coil around the middle part of the core. The flow signal produced by an electromagnetic flowmeter, 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 contarninated by other signals which make up the electrode signal, including signals generated by spurious 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 the vicinity of the flowmeter or by eddy currents induced in the connections to the electromagnet.

The invention aims to eliminate or reduce the effect of such spurious magnetic fields.

DISCLOSURE OF THE INVENTION.

According to this invention, an electromagnet for an electromagnetic flowmeter 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 electricaliy- 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 around the core between the poles is 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.

By a "root part" of the pole is meant a part away from the tree 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 part of the pole extends free of the coil, for a purpose which will be described.

Preferably the substantially closed figure is rectangular and the opposite faces of the ferromagnetic core are common to the substantially closed figure and to the poles, which are of different lengths, the one not having a root part surrounded by the coil being the shorter. The substantially closed figure is preferably completely closed, to minimise flux losses. Whilst the core must be in at least two parts to provide for convenient assembly of the flowmeter, the parts should either fit closely together or be overlapped to provide a closed figure.

Further according to the invention an electromagnetic flowmeter comprises a meter tube including opposed electrodes in the inner wall thereof electrically insulated from each other and an electromagnet as described in the preceding paragraphs for producing a magnetic field across the meter tube. In such an electromagnetic flowmeter, flange members of ferromagnetic and electrically-conducting material, preferably circular, surround the axis of the meter tube, one closely adjacent to each of the said opposite faces of the ferromagnetic core, each of the flange members being of size such as to overlie the ends of the poles and adjacent parts of the substantially closed figure to screen the field area against spunous magnetic fields directed at acute angles to the axis of the meter tube. The flange members are preferably coaxial with the meter tube and may be pipe unions for connecting the meter tube into a pipeline carrying the liquid whose flow is to be metered.

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 parts and adjacent laminations may be divided on different lines so that the dividing unes are staggered when the core is assembled, ln an alternative construction each lamination is divided in similar positions and all the similar divisions are secured together. The divided surfaces of each secured group of divisions are accurately shaped to mate with each other so that when assembled any flux loss at the jomt is negligible. In yet another construction method the core may be moulded in fertile or a ferro-magnetic polymer composition.

By substantially surrounding the meter tube in the region containing the electrodes the core itself provides efficient screemng against spurious magnetic fields directed substantially perpendicular to the axis of the meter tube and the flange members provide screemng against magnetic fields directed at acute angles to that axis. Because of the non-symmetrical poles that axis is not in the centre oi the substantially closed figure as in previous flowmeters but is much closer to the part of the closed figure adjacent to the shorter pole than to the part adjacent to the longer pole. This enables moderately-sized flange members to overlie the above-mentioned parts of the ferromagnetic core. The meter tube preferably has end flanges which are precisely spaced to lie close to the opposite surfaces of the poles and adjacent parts of the substantially closed figure of the core and the flange members are clamped to the core with the flanges of the meter tube between the opposite surfaces of the core and the respective flange members. If necessary, liquid-ught sealing means may be provided between them. Because of the substantially flat surfaces and the precise spacing of the flanges of the meter tube, clamping the flange members to the core does not apply significant compressive force to the meter tube itself. This avoids the risk of mechanical damage to or distortion of the meter tube which has caused inaccurate performance in some previous electromagnetic flowmeters. Thus, m addition to providing efficient magnetic screening, the core provides mechanical protection for the meter tube. It is therefore possible to use a meter tube of electrically insulating material such as a ceramic, glass or plastics matenai and the need for reinforcement, such as a metal sheath, previously mentioned, is eliminated. Still further according to the invention an electromagnetic flowmeter comprising a meter tube of electrically insulating material including opposed electrodes in the inner wall thereof and a ferromagnetic core in the shape of a closed figure with confronting inwardly-directed poles defining between them a field area in which the meter tube is located, the closed figure and the confronting poles having common opposite flat surfaces substantially perpendicular to the axis of the meter tube 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 around the core between the poles is characterized in that the meter tube has outward flanges at its ends spaced so as to lie close to the said flat surfaces of the core and the poles are of different lengths, only a part towards the root of the longer one being surrounded by the coil to leave free a tip part of length at least equal to the radial dimension of the outward flanges of the meter tube, a ferromagnetic and electrically-conducting pipe union is mounted coaxially at each end of the meter tube for connecting it into a pipeline which is to carry a liquid to be metered, each pipe union having a flat end surface adjacent to the meter tube of size such as to overlie the ends of the poles and adjacent parts of the closed figure to screen the meter tube against magnetic fields directed at acute angles to the axis of the meter tube and clamping means is included for clamping the pipe unions to the core with the flanges of the meter tube between the flat surfaces of the respective unions and the core.

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 the 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 - 111 in Figure 2.

The electromagnetic flowmeter comprises an electromagnet having a ferromagnetic core 1 and a coil

2 with end connections 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 joined by two side limbs 7, with confronting poles 3, 4 directed inwardly from respective top and bottom limbs 5 and 6. The coil 2 surrounds a root part of the pole 3, which is longer than the pole 4 so as to accommodate the coil and leave a free tip part. When the coil 2 is energized a magnetic field is generated along the pole 3, across the field area defined between the confronting poles to the pole 4 and back along the top and bottom limbs 5 and 6 and the two side limbs 7. Due to the symmetrical shape of the core 1 with respect to the poles 3 and 4 the magnetic field will be divided substantially equally between the two side limbs 7. If those 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 mcluded 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 and thus the 5 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 cod each lamination must be divided mto two parts 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 define a field area between them in which a 0 meter tube 10 is snugly located and the opposite surfaces of the core 1 are flat and parallel at least in the region of the up part of the pole 3, the pole 4, the bottom limb 6 of the closed figure and the lower parts of the side limbs 7. The meter tube 10 is a true cylinder with outward end flanges 11 which are precisely spaced so that when the tube 10 is located in the field area the flanges 11 he close against those flat surfaces of the core, as shown in Figure 3, the free tip part of the longer pole 3 and the whole of the shorter pole 4 ^R fitting between the flanges 11. Two diametrically-opposed electrodes 12 are mounted m the middle of the wall of the true cylinder 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.

The meter tube 10 with flanges 11 is a unitary moulding in plastics such as polypropylene, with the 0 electrodes 12 posinoned as inserts m 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.

For mounting the flowmeter in a pipeline which is to carry the liquid to be metered a ferromagnetic and electrically-conductive metal pipe union 13 having a hexagonal flange 14 and an internally screw 5 threaded collar 15 is mounted coaxially at each end of the meter tube 10 with the flanges 11 and 14 confronting and an O-nng 16 providing a liquid-tight seal between them when the assembly is 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 meter tube itself, which might cause 0 distortion or damage to the true cylinder, leading to inaccuracy of measurements made by the flowmeter As shown most clearly in Figure 2, the true cylinder of the meter tube 10 is wholly enclosed within the core 1 and thus is not only magnetically screened by the core against spurious magnetic fields which may be directed substantially transversely to the meter tube, but is protected by the core against mechanical impact damage. Being conductive, the metal flanges 14 and collars 15 of the pipe unions 13 provide both electrostatic screening and screening against spurious magnetic fields which may be directed at acute angles to the axis of the meter tube 10 and the integrity of the assembly once 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.

Although a gap is shown in Figure 2 between the confronting ends of the poles 3, 4 and the meter tube 10, the dimensions may be such that there is no gap and the tube is securely held by the poles. For this purpose the tip part of the pole 3, extending beyond the coil 2, must be of length at least equal to the radial dimension of the flanges 11 of the meter tube 10.

In alternative constructions the meter tube may have no flange but be arranged for its ends to fit into coaxial flange members having flanges of such size, like the flanges 14, as to overlie the flat surfaces of the free portion of the pole 3, the pole 4 and adjacent parts 6 and 7 of the closed figure of the core 1, with the flanges snugly against the surfaces to provide screening against magnetic fields directed at acute angles to the axis of the meter tube. Sealing means such as the O-rings 16 may then be unnecessary. The flange members would be connected to pipes carrying the liquid to be metered.

Claims

1. An electromagnet for an electromagnetic flowmeter comprising a ferromagnetic core (1) in the shape of a closed figure (5, 6, 7) with confronting mwardly-directed poles (3, 4) defining between them a field area in which the meter tube (10) is to be located and at least one electrically-conducting coil (2) _c surrounding a part of the ferromagnetic core, the coil (2) havmg means for connection to an electric current source for generating a magnetic field in the field area and around the core (1) between the poles, characterized in that the opposite faces of the ferromagnetic core (1) are substantially flat and parallel and only a root part of one of the poles (3) is surrounded by a coil (2).

2. An electromagnet as claimed in Claim 1 characterized in that the substantially closed figure (5, 6, 7) is 0 rectangular.

3. An electromagnet as claimed in Claim 2 characterized in that the opposite faces of the ferromagnetic core (1) are common to the substantially closed figure (5, 6, 7) and to the poles (3, 4).

4. An electromagnet as claimed in Claim 3 characterized in that the poles (3, 4) of the core (1) are of different lengths, the one (4) not having a part surrounded by the coil (2) being the shorter.

5 5. An electromagnetic flowmeter comprising a meter tube (10) including opposed electrodes (12) in the inner wall thereof electrically insulated from each other and an electromagnet (1, 2) for producing a magnetic field across the meter tube (10), characterized in that the electromagnet (1, 2) is as claimed in any one of Claims 1 to 4.

6. An electromagnetic flowmeter comprising a meter tube (10) including opposed electrodes (12) in the 0 inner wall thereof electrically insulated from each other and an electromagnet (1, 2) for producing a magnetic field across the meter tube (10), the electromagnet (1, 2) comprismg a ferromagnetic core

(1) in the shape of a closed figure (5, 6, 7) with opposite faces substantially perpendicular to the axis of the meter tube (10) and confronting mwardly-directed poles (3, 4) defining between them a field area in which the meter tube (10) is located and at least one electπcally-conducting coil (2) 5 surrounding a part of the ferromagnetic core (1), the coil (2) having means for connection to an electπc current source for generating a magnetic field in the field area and around the core (1) between the poles (3, 4), characterized in that the opposite faces oi the ferromagnetic core (1) are substantially flat and only a root part of one of the poles (3) is surrounded by a coil (2), leaving free a tip part of the pole, flange members (13, 14) of ferromagnetic and electrically-conducting mateπal 0 surround the meter tube (10), one closely adjacent to each of the said opposite faces of the ferromagnetic core (1), each of the flange members (13, 14) being of size such as to overlie the ends of the poles (3, 4) and adjacent parts (6, 7) of the substantially closed figure to screen the field area against magnetic fields directed at acute angles to the axis of the meter tube (10). 7. An electromagnetic flowmeter as claimed in Claim 6 characterized in that the said opposite faces of the ferromagnetic core (1) are common to the substantially closed figure (5, 6,

7) and the poles (3, 4).

8. An electromagnetic flowmeter as claimed in Claim 7 characterized in that the poles (3, 4) of the core (1) are of different lengths, the one (3) surrounded by the coil (2) being the longer.

9. An electromagnetic flowmeter as claimed in Claim 7 characterized in that the meter tube (10) has a flange (11) at each end and its length between the flanges (11) is such that the flanges (11) lie snugly against the said opposite faces of the core (1), whereby the circular flange members (13) may be clamped to the core (1) without risk of compression damage to the meter tube (10).

10. An electromagnetic flowmeter as claimed in Claim 9 characterized in that the circular flange members ( 13 , 14) are pipe unions adapted for connecting the flowmeter into a pipeline which is to carry a liquid to be metered, clamping means (17) being provided for clamping the pipe unions (13) together with the ferromagnetic core (1) flanked by the flanges (11) of the meter tube (10) between them.

11. An electromagnetic flowmeter comprising a meter tube (10) of electrically-insulating material including opposed electrodes (12) in the inner wall thereof and a ferromagnetic core (1) in the shape of a closed figure (5, 6, 7) with confronting inwardly-directed poles (3, 4) defining between them a field area in which the meter tube (10) is located, the closed figure (5, 6, 7) and the confronting poles (3, 4) having common opposite flat surfaces substantially perpendicular to the axis of the meter tube (10) and at least one electrically-conducting coil (2) surrounding a part of the ferromagnetic core (1), the coil (2) having means for connection to an electric cuπent source for generating a magnetic field in the field area and around the core (1) between the poles (3, 4), characterized in that the meter tube

(10) has outward flanges (11) at its ends spaced so as to he close to the said flat surfaces of the core (1) and the poles (3, 4) are of different lengths, only a part towards the root of the longer one (3) being surrounded by a coil (2) to leave free a tip part of the pole of length at least equal to the radial dimension of the outward flanges (11) of the meter tube (10) so as to penetrate between the flanges (11) to reach the meter tube (10), a ferromagnetic and electrically-conducting pipe union (13) is mounted coaxially at each end of the meter tube (10) for connecting it into a pipeline which is to carry a liquid to be metered, each pipe union (13) having a flat end surface adjacent to the meter tube (10) of size such as to overlie the ends of the poles (3, 4) and adjacent parts (6, 7) of the closed figure to screen the meter tube (10) against magnetic fields directed at acute angles to the axis of the meter tube (10) and clamping means (17) is included for clamping the pipe unions (13) to the core (1) with the flanges (11) of the meter tube (10) between the flat surfaces of the respective unions (13) and the core (l).