HK1015873B - Process for manufacturing a measuring tube of an electromagnetic flow sensor - Google Patents

Description

Method for producing measuring tube of electromagnetic flow sensor

The invention relates to a method for producing a measuring tube for an electromagnetic flow sensor, comprising an insulating body formed on the inner surface thereof and a metal tube having a metal flange at each end.

Electromagnetic flow meters are known for measuring the volumetric flow rate of a conductive liquid flowing through a measuring tube. The parts of the measuring tube which are in contact with the fluid are electrically non-conductive, so that the induced voltage generated by the magnetic field does not produce a short circuit, even if the measuring tube is entirely composed of metal, i.e. even if the liquid is in contact with metal, according to the faraday's law of electromagnetic induction. The magnetic field direction intersects the measuring tube, generally perpendicular to the longitudinal direction of the measuring tube.

The metallic measuring tube must not, of course, be a magnet body, which would otherwise disturb the magnetic field, and is usually provided on its inner surface with a non-conductive layer serving as an insulator, which is not necessary when the measuring tube is made of plastic or ceramic.

The induced voltage is picked up by a galvanic or capacitive electrode. Galvanic electrodes extend through the measurement vessel wall so that they are in contact with the liquid, while capacitive electrodes are mounted in, out of contact with, or on the measurement vessel wall.

Us patent No. 5,403,533 discloses a method for manufacturing a metal measuring tube with an insulator on the inner surface thereof and no metal flange, comprising the steps of:

integrally forming a tube thickening with external cuts on the measurement tube at each end of the measurement tube;

embedding a tubular insulating bush with the length larger than that of the measuring pipe into a cavity of the measuring pipe;

the projecting end portions of the measuring tubes are pulled down, the thickened portions of the measuring tubes are hidden and brought into close contact with the notched portions, respectively, and if necessary, they are firmly fastened together by the ring-shaped mounting member.

In JP-B5-48846, which is based on the japanese JP-a 61-294316 patent, a metal measuring tube of an electromagnetic flow sensor is disclosed, which measuring tube is provided with an internal insulator and a metal flange, and,

the metal flanges are integrally formed at each end of the measuring pipe, respectively;

the front side of the metal flange is provided with a flange and/or a groove;

an insulator is filled in the groove;

the flange is enclosed in the insulator.

A similar design is shown in german patent DE-a 3101562, but in addition a groove is provided on the inner side of each flange, which is a continuation of the inner surface of the measuring tube.

British patent No. 1,111,981 discloses a highly conductive liquid electromagnetic flowmeter having a metal measuring tube with a steel lining having conical ends which are placed on the end faces of flanges, respectively.

It has been shown that the changes made to the first of the three prior art techniques described above for embedding or fixing the insulator in or to the end of the measuring tube do not satisfy the practical requirements of the tightness of the connection between the metal tube and the insulator, in particular in the flange region, nor the requirements of simplicity and cost performance of the manufacturing process.

Accordingly, an object of the present invention is to solve the above problems by providing a method of manufacturing a measuring tube of an electromagnetic flow sensor, the measuring tube including an insulator formed at an inner surface thereof with a predetermined thickness and a metal tube with a metal flange at each end thereof, the method comprising the steps of:

respectively sliding the metal flanges at each end of the metal pipe outside the metal pipe in the following manner: the front sides of the metal flanges extend beyond the ends of the metal tubes, respectively, by an amount at least equal to the thickness of the insulator;

fixedly connecting the rear sides of the metal flanges to the outside of the metal pipe firmly;

before sliding the metal flanges on the metal pipe, respectively forming a groove from the front side of each metal flange, and forming a first annular groove at the inner end of each groove;

inserting a mandrel into the cavity of the metal tube, said mandrel having a diameter determined by the predetermined thickness of the insulator and having a fill port and two end flanges, the flanges at each end covering a respective groove in the front side of said flanges;

filling the insulating material of the insulator from the filling port in a liquid state;

the insulating material is cured and the mandrel is removed.

In the preferred embodiment of the invention, a second annular groove is formed in the central region of each groove, a seal ring is disposed in each second annular groove, and an end flange at each end covers each groove and seal ring, respectively, by means of a mandrel with an end flange at each end.

In another embodiment of the invention, the metal tube is provided with means for mechanically fixing the insulation, preferably an expanded metal lattice, before embedding the insulation.

In another preferred embodiment of the invention, the insulating material is filled using a conventional transfer molding process.

One of the advantages of the present invention is that no further process steps are required to manufacture the metal tube or the insulator after the insulating material has cured.

The invention is described in detail below with reference to a drawing which is a cross-sectional view of one embodiment of a measuring tube made by the method of the invention.

As shown in the figure, the measuring tube 1 of the electromagnetic flow sensor, which is shown in longitudinal section, is mounted in a pipe of a defined diameter, through which the liquid flowing is measured, but the pipe is not shown for the sake of clarity. Also, the electrodes are not shown because they are installed after the method of the present invention is completed, and thus, fall outside the scope of the present invention.

The measuring tube 1 comprises a metal tube 2, the inner surface of which metal tube 2 is covered with an insulator 3 made with a desired or predetermined thickness and provided with a metal flange 4, 5 at each end thereof. In the preferred embodiment of the invention, means 20 for mechanically fixing the insulator 3 are provided inside the metal tube 2, the means 20 preferably being an expanded metal lattice.

The cross-sectional shapes of the metal pipe 2 and the metal flanges 4, 5 are made into a ring shape. The front side 41, 51 of each metal flange 4, 5 extends outside the respective end 21, 22 of the metal tube 2 by an amount at least equal to the predetermined thickness of the insulator 3.

The space 6 formed by the metal flanges 4, 5 and the metal tube 2 is closed by a sheet metal part 7. Usually the space 6 is used for mounting coils and other elements generating the above-mentioned magnetic field. If the thin sheet metal part 7 is part of the magnetic circuit of the magnetic field generated by the coil, this part 7 is ferromagnetic.

Before the beginning of the production of the measuring tube 1, a groove 42, 52 is formed from the front side 41, 51 of each metal flange 4, 5, respectively, the inner end of the groove 42, 52 being provided with an annular groove 43, 53. According to the preferred embodiment described above, a second annular groove 44, 54 is also provided in the central region of each groove 42, 52 for receiving the sealing rings 8, 9.

In the manufacture of the measuring tube 1, a metal tube 2 of a predetermined length is first formed and the metal flanges 4, 5 are prepared in the manner described above (forming the grooves 42, 52; the first annular grooves 43, 53; and possibly the second annular grooves 44, 54).

Then, at each end 21, 22 of the metal tube 2, each of the metal flanges 4, 5 is slid over the outer portion 23 of the metal tube 2, respectively, in the manner described above, with the front sides 41, 51 of the metal flanges 4, 5, respectively, protruding beyond the ends of the metal tube by an amount at least equal to the predetermined thickness of the insulator 3.

The rear sides 45, 55 of the metal flanges 4, 5, respectively, are then tightly fixedly connected to the outer part 23 of the metal tube 2. This can be achieved by soldering, brazing or welding, forming the soldered, brazed or welded connections 46, 56. Thereafter, if annular grooves 44, 54 are also provided, sealing rings 8, 9, for example suitable O-ring seals, are placed in the second annular grooves 44, 54, respectively.

Next, a mandrel (not shown) is inserted into the cavity of the metal tube 2. The diameter of the mandrel is determined by the predetermined thickness of the insulator 3 and the diameter of the cavity. Thus, if the inner diameter of the metal tube 2 is 100mm and the thickness of the insulator 3 is 2mm, the mandrel must have a diameter of 96 mm.

Each end of the mandrel is provided with an end flange covering the groove 42, 52 on the front side of the respective flange 4, 5, i.e. resting against the front surface 41, 51 of the metal flange 4, 5, respectively. If the sealing rings 8, 9 and the second annular grooves 44, 54 are present, each end of the mandrel must also cover the respective groove and the respective sealing ring.

Thereafter, the liquid insulating material 3 is poured from the filling opening of the mandrel, which in this connection can be achieved by means of a conventional transfer moulding method: first, the insulating material formed of thermoplastic material is liquefied and then pressed under pressure into the space between the metal tube 2 and the metal flanges 4, 5 on the one hand and the metal tube 2 and the mandrel on the other hand, and allowed to solidify in said space.

Instead of transfer moulding, it is also possible to fill the initial liquid insulating material, i.e. a normally suitable plastic, by other methods and then to cure it. After the insulating material is cured, the mandrel is then withdrawn.

After the mandrel has been extracted, the insulator 3 covers not only the entire inner surface of the metal tube 2, but also the end portions 21, 22 of the metal tube 2 and completely fills the respective grooves 42, 52 and the respective first annular grooves 43, 53. Further, the insulator 3 also covers the sealing rings 8, 9, if present, and holds the sealing rings 8, 9 in such a position that they increase the tightness of the connection between the insulator 3 and the respective flange 4, 5.

The method of the invention is particularly suitable for the production of measuring tubes with a nominal diameter in the range between 10mm and 200 mm.

Claims (5)

1. A method of manufacturing a measuring tube for an electromagnetic flow sensor, said measuring tube comprising an insulator formed on an inner surface thereof in a predetermined thickness and a metal tube having a metal flange at each end thereof, said method comprising the steps of:

a. before sliding the metal flanges on the metal pipe, respectively forming a groove from the front side of each metal flange, and forming a first annular groove at the inner end of each groove;

b. respectively sliding the metal flanges at each end of the metal pipe outside the metal pipe in the following manner: the front sides of the metal flanges extend beyond the ends of the metal tubes, respectively, by an amount at least equal to the thickness of the insulator;

c. fixedly connecting the rear sides of the metal flanges to the outside of the metal pipe firmly;

d. inserting a mandrel into the cavity of the metal tube, said mandrel having a diameter determined by the predetermined thickness of the insulator and having a fill port and two end flanges, the end flanges at each end covering a respective groove in the front side of said flanges;

e. filling the insulating material of the insulator from the filling port in a liquid state;

f. the insulating material is cured and the mandrel is removed.

2. The method of claim 1,

in step a, a second annular groove is formed in the central area of each groove;

between steps c and d, a sealing ring is respectively placed in each second annular groove;

each end of the mandrel is provided with an end flange, and the end flange at each end covers each groove and the sealing ring respectively.

3. A method according to claim 1, characterized in that means for mechanically fixing the insulation are arranged in the metal tube before the insertion of the mandrel and the filling with the insulating material.

4. The method of claim 3 wherein the device is an expanded metal lattice.

5. The method of claim 1, wherein the insulating material is filled using a conventional transfer molding process.