Unitary electromagnetic flowmeter with sealed coils

This invention relates to electromagnetic flowmeters, and more particularly to a flangeless flowmeter having a cylindrical housing and whose components are integrated to form a highly compact, low-cost unit that may be readily installed in a flow line between the flanged ends of upstream and downstream pipes, the flowmeter including relatively thin electromagnet coils which conform to the inner surface of the housing.

Magnetic flowmeters such as those disclosed in U.S. Patents 3,695,104; 3,824,856; 3,783,687 and 3,965,738, are especially adapted to measure the volumetric flow rates of fluids which present difficult handling problems, such as corrosive acids, sewage and slurries. Because the instrument is free of flow obstructions, it does not tend to plug orfoul. In a magnetic flowmeter, an electromagnetic field is generated whose lines of flux are mutually perpendicular to the longitudinal axis of the flow tube through which the fluid to be metered is conducted and to the transverse axis along which the electrodes are located at diametrically-opposed positions with respect to the tube. The operating principles are based on Faraday's law of induction, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field will be proportional to the velocity of that conductor. The metered fluid effectively constitutes a series of fluid conductors moving through the magnetic field; the more rapid the rate of flow, the greater the instantaneous value of the voltage established at the electrodes.

The typical commercially-available magnetic flowmeter is provided with mounting flanges at either end thereof. The meter is interposed between the upstream and downstream pipes of a fluid line, each pipe having an end flange. The mounting flanges on the meter are bolted to the flanges of line pipes. It is, of course, essential that the circle of bolt holes on the mounting flanges of the meter match those on the pipe flanges.

In a magnetic flowmeter, the flow tube is subjected to the same fluid pressure as the line pipes. The flow tube must therefore be of a material and of a thickness sufficient to withstand this pressure, even though the strength of the flow tube is unrelated to its measuring function. This design factor contributes significantly to the cost of a standard meter. Existing meters are made up of components that must be assembled, are generally of substantial size and weight and quite expensive to manufacture.

In order to provide a compact and readily installable electromagnetic flowmeter whose weight and dimensions are substantially smaller than existing types, highly compact flangeless flowmeters have been proposed which, despite their reduced volume and weight, are capable of withstanding high fluid pressures, the flowmeters operating efficiently and reliably to measure flow rates accurately. The flangeless flowmeters disclosed in our earlier U.S. Patents No. 4181018 and No. 4098118 are interposable between the flanged ends of upstream and downstream line pipes to meter fluid passing through the line. In one preferred embodiment, the meter is constituted by a ferromagnetic ring within which a pair of electromagnet coils is supported at opposed positions along a diametrical axis normal to the longitudinal axis of the ring, the longitudinal axis passing through the central flow passage of an annular pressure vessel which is formed of high strength insulating material and is moulded within the ring to encapsulate the coils as well as a pair of electrodes disposed at diametrically-opposed positions with respect to the passage along a transverse axis at right angles to the coil axis to define a unitary structure. The unit is compressible between the end flanges of the pipes by bridging bolts that pass through bore holes in the pressure vessel which lie outside of the ring to encage the unit.

Our British Patent Specification No. 2059066 published 15 April 1981 discloses a flangeless flowmeter interposable between the flanged ends of upstream and downstream pipes in a fluid line for metering fluid passing therethrough, the meter including a non-magnetic metal spool of high mechanical strength which functions as a flow conduit and also renders the meter capable of withstanding high compressive forces as well as fluid pressure. This non-magnetic metal spool is surrounded by a ferromagnetic housing which acts as a mould for potting the inner chamber defined between the spool and the housing and thereby sealing the components contained therein. The housing also serves as the magnetic flux return path for the electromagnets supported thereby.

The said housing is formed by complementary half-pieces which include end plates that join the corresponding ends of the spool to define the innercham- ber. Integral with the half-pieces are two magnet cores which extend at diametrically opposed positions along an axis normal to the longitudinal axis of the cylindrical housing, the cores being surrounded by coils to define solenoid-type electromagnets. Mounted on the spool at diametrically opposed positions along a transverse axis at right angles to the core axis are two electrodes.

Thus, in this prior arrangement, the electromagnet field is produced by a pair of cored solenoids occupying diametrically-opposed positions with respect to the longitudinal axis of the flow conduit, the cores being magnetically coupled by the ferromagnetic housing. The air gap which exists between the pole faces of these cores depends on the diameter of the flow conduit: the larger the diameter, the bigger the gap. An arrangement of this type is appropriate to flowmeters having flow conduits of small diameter such as 2.54 cm (one inch); but, as will now be explained, it is unsuitable for larger diameters, that is, diameters of 5, 7.5 and 10 cm (two, three and four inches) and greater.

The reason why cored solenoids are inappropriate in larger diameter flowmeters will be clear from the basic flowmeter equation:<maths id="math0001" num=""><img file="EP0045646B2_D0001.tif" /></maths>wherein: <ul id="ul0001" list-style="none"><li>eflow is the flowmeter signal intensity;</li><li>B is the magnetic field strength;</li><li>L is the diameter of the flow conduit;</li><li>v is the flow velocity.</li></ul>

For a constant efloW/v relationship, B is inversely proportional to L. Hence, as the flow conduit diameter L increases with larger meters, one requires a reduction in magnetic field strength, this being accomplished by fewer ampere turns which results in a thinner electromagnet coil. The use of unduly thick coils for a given meter diameter would obviously be wasteful of power, resulting in excessive heat and adding unnecessarily to the cost of the meter.

With larger diameter flow conduits, the air gap becomes the predominant influence on the system permeability, and the effect of cores or pole pieces on this factor becomes less significant. Thus if a meter has a 12 inch (30 cm) diameter flow conduit, the appropriate coil therefor is only a quarter of an inch thick (0.635 cm). In this context, a one quarter inch thick pole piece would have a negligible effect on the permeability of the system, and is therefore unnecessary except possibly as a post on which to mount the coil.

The inner chamber is filled with a potting compound to seal the coils and the electrodes therein and thereby provides a highly stable structure, the housing serving as a mould for this purpose.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: <ul id="ul0002" list-style="none"><li>Figure 1 is a transverse section taken through a flangeless electromagnetic flowmeter in accordance with the invention, the spool thereof being omitted;</li><li>Figure 2 is a longitudinal section taken through the flowmeter, the position of the spool being indicated in dotted lines;</li><li>Figure 3 is a separate view of the spool, partly in section;</li><li>Figure 4 is a plan view of one of the electromagnet coils having a saddle-shaped form;</li><li>Figure 5 is a section taken through the coil;</li><li>Figure 6 is a perspective view of the assembled meter, partly cut away to expose the interior; and</li><li>Figure 7 is an elevational view of the unit under compression between the flanged ends of upstream and downstream pipes in a flow line.</li></ul>

Referring now to Figures 1, 2 and 3, there is illustrated a flangeless flowmeter unit in accordance with the invention, the unit including a non-magnetic metallic spool 10 of high strength, preferably fabricated of stainless steel, having end flanges 10A and 10B. Spool 10 is provided with an insulating liner 11 and acts as the flow conduit for the meter. The spool has the same or a smaller inner diameter as that of the adjacent piping. Spool 10 must be capable of withstanding not only the pressure of the fluid being metered but also the compressive force to which it is subjected. This force is generated by bolts which bridge the flanges of the upstream and downstream pipes between which the unit is interposed in a fluid line (see Figure 7).

Tubular liner 11 disposed within spool 10 is formed of synthetic plastics material or a fibre glass composition and is provided with end flanges 11A and 11B which overlie end flanges 10A and 10B of the spool. The liner is preferably injection moulded of a material such as KYNAR (polyvinylidene fluoride) and includes diametrically-opposed sockets S₁ and S₂ to accommodate the metal electrodes of the meter (not shown). The electrodes lie on an axis X which is perpendicular to the longitudinal flow axis Z.

Surrounding the lined metal spool 10 and concentric therewith is a split cylindrical housing or casing formed by complementary half-pieces 12_u and 12₁, the longitudinal edges of these pieces being seam welded or otherwise joined together to complete the housing. Housing 12,-12₁ is fabricated of carbon steel or other "soft" ferromagnetic material.

Integral with housing pieces 12_u and 12, are short magnet cores 13 and 14 formed of the same cast steel material, the cores being at diametrically-opposed positions along a coil axis Y normal to longitudinal flow axis Z which is also the axis of the cylindrical housing. Coil axis Y is at right angles to electrode axis X, so that the axes of the meter are mutually perpendicular. The cores have an arcuate formation which follows the curvature of the housing half-pieces.

Received over the cores are saddle-shaped coils 15 and 16, one of which is shown in Figures 4 and 5. These coils are pre-formed and preferably leadshiel- ded so that they can be slipped over the cores to form electromagnets. When excited, these electromagnets establish an electromagnetic field whose lines of flux are at right angles to the direction of fluid flow, whereby when the fluid passing through the spool intercepts this field, a voltage is induced therein which is picked up by the electrodes to generate a signal that is a function of flow rate.

The leads for the electrodes and for the electromagnets pass out of the unit in the associated converter assembly through bores 17 and 18 in the upper housing piece 12. Bores 17 and 18 register with the bores 19 and 20, respectively, in a bracket 21 welded to housing piece 12_u. The bracket serves to support a converter box (not shown) housing the electronic drive circuit for exciting the electromagnet, circuits for conditioning the signal derived from the electrodes and all other circuits normally associated with an electromagnetic flowmeter primary to provide an output signal suitable for transmission to a remote station, for indicating and recording the metered flow rate and for carrying out process control operations.

The opposite ends of the split housing are provided with annular closure plates 22 and 23 whose inner peripheries mate with the outer peripheries of end flanges 10A and 10B of the spool to define an enclosed inner chamber in the space between the spool and the housing.

All parts of the meter are properly located and symmetrically disposed with respect to the X-Y-Z axis, and are held within the inner chamber when assembling the unit. It then becomes possible to introduce a flowing potting compound of epoxy or other insulating material into the inner chamber through holes 17 or 18 in the housing, and thereby produce, when the compound is cured, a protected, moisture- free unit capable of functioning reliably and efficiently for an indefinite period.

Thus the split housing functions not only as the mechanical support for the converter assembly but it also affords the magnetic flux return path for the electromagnets. Moreover, it provides the means for locating the various sub-assemblies, spools, leads, magnet parts, etc. Not only does this housing afford a weatherproof enclosure for the meter, but it also acts as a mould for potting the inner chamber.

As shown in Figure 7, the flowmeter unit is interposed between the upstream and downstream pipes 24 and 25 of a fluid line. The pipes are provided with end flanges 24A and 25A having bolt angles therein to accommodate a set of bolts 26 which bridge the flanges and encage the unit. In order to effect a good fluid seal, the unit is subjected to a high compressive force by the bolt torque. But because of the high strength of the metal spool, the unit is capable of withstanding this force.

The electrodes used with the meter may be conventional small diameter wetted electrodes. Alternatively, large area insulated electrodes may be used, with or without shields. As pointed out previously, the permeability role played by the core of the electromagnet becomes less significant for thin coils in large diameter flow conduit meters. The thinness of the coil in large diameter meters may be such as to dispense with the need for cores, the coils in all instances lying adjacent the surface of the housing pieces and conforming to the curvature thereof.

While there has been shown and described a preferred embodiment of a unitary electromagnetic flowmeter with sealed coils in accordance with the invention, it will be appreciated that many changes and modifications may be made therein without, however, departing from the scope of the claims.

It is requested to replace lines 36, column 3 to line 6 (including) of column 4 of Patent Specification 45646 by the following:

GB-A-1 424 875 describes another category of flowmeters that can be readily installed or interposed between the pipe flanges. To enable that construction various conditions and charges over conventional flowmeters are prescribed:

Between pipe-flanges and flows tube a sealing contact is effected through the compressive force of bolts connecting the pipe flanges and encircling the flow meter. Therefore, the flow tube is designed and construted to be the principal structural element of the unit as to resist the compression forces and to carry all other elements (coils, electrodes and cover).

The present invention is set out in claim 1.

Preferably the spool diameter is at least 5 cm (2 inches).