GB2047409A

GB2047409A - Electrodes for electromagnetic flowmeters

Info

Publication number: GB2047409A
Application number: GB8008968A
Authority: GB
Original assignee: Davies P M
Current assignee: Davies P M
Priority date: 1979-03-21
Filing date: 1980-03-17
Publication date: 1980-11-26
Also published as: GB2047409B

Abstract

The invention relates to electrodes for electromagnetic flow measuring instruments in pipes lined with synthetic resin material. A hole is drilled in the pipe wall, and a plug (4) is welded into the hole so as to form a recess (8). After the liner (9) has been inserted in the pipe, an electrode is positioned with its mushroom-shaped head (13) with the pipe, and its stem (11) extending through a hole in the liner and a central hole in the plug. The outer end of the stem (11) is threaded, and the linear material is pulled up against the flat inner face of the plug (4) by means of a nut (15). Alternatively the liner may be pushed into the recess before passing the electrode stem through the holes. <IMAGE>

Description

SPECIFICATION Improvements in or relating to electromagnetic flow measuring instruments and methods of fitting electrodes in lined pipes This invention relates to electromagnetic flow measuring instruments and to methods of fitting electrodes for such instruments in lined pipes.

An electromagnetic flow measuring instrument includes a section of pipe having means for producing a magnetic field across the pipe, and a pair of electrodes mounted diametrically opposite to one another in such a way that, if electrically-conductive fluid is flowing through the pipe section, an electric potential difference is developed between the two electrodes, the magnitude of this potential difference being a measure of the rate of flow of the liquid.

When corrosive liquids are to be conveyed, it is common practice to use a metal pipe lined with rubber of a synthetic resin material, which term is used herein to include synthetic elastomers. For high-temperature liquids, in particular, it is usual to use polytetrafluorethylene (hereinafter referred to as PTFE) as the material for the liner. This material has a different coefficient of expansion from the metal used for the pipe and, accordingly, the liner is liable to move with respect to the pipe in which it is situated. An electrode for an electromagnetic flow measuring instrument usually includes a head located within the pipe and a stem passing through the pipe wall. Thus, in a lined pipe, the stem has to pass through the liner as well as the metal pipe wall.Consequently, if the liner moves with respect to the pipe, the stem will enlarge the hole in the liner through which it passes.

It is, of course, necessary for the electrode to be secured in such a way that it forms a fluidtight seal with the liner and this seal is liable to be broken if there is any movement between the liner and the electrode. This problem is particularly serious with PTFE because this material has very little resilience. The problem is also usually more serious in relatively small diameter pipes than in larger diameter pipes. It will be understood that the head of the electrode is normally provided with an annular sealing surface which has to engage with the cylindrical surface of the liner.In these circumstances, a seal can only be obtained if the annular sealing surface of the electrode penetrates into the liner surface at those parts which are located generally transversely of the pipe sufficiently for the parts of the annular sealing surface which are located generally longitudinally of the pipe to engage in fluid-tight relationship with the liner.

It is an object of the present invention to provide a method of fitting an electrode which does not suffer from at least some of the disadvantages experienced with known types of electrode when mounted in lined pipes.

It is a further object of the invention to provide an electromagnetic flow measuring instrument including an electrode fitted in a lined pipe by such method.

From one aspect the invention consists in a method of fitting an electrode having a head and a longitudinally extending stem in a metal pipe lined with synthetic resin material, wherein a hole, the diameter of which is greater than the diameter of the electrode head, is formed in the pipe wall, wherein a plug having a central hole therethrough is secured in said hole in the pipe wall with its inner substantially flat face spaced from the inner surface of the pipe wall to form a recess within the pipe wall, wherein the electrode is positioned with its stem passing through a hole in the liner and the central hole in said plug, and with its head in contact with the inner surface of the liner, and wherein the stem is drawn outwardly through the pipe wall until the head forces material of the liner into said recess.

From another aspect the invention consists in an electromagnetic flow measuring instrument including an electrode fitted in a pipe lined with synthetic resin material by a method in accordance with the preceding paragraph.

From yet another aspect the invention consists in an electromagnetic flow measuring instrument including an electrode fitted in a pipe lined with synthetic resin material, wherein said electrode has a head and a longitudinally extending stem, wherein said stem passes through a plug secured in a hole in said pipe to form a substantially flat-bottomed internal recess with the pipe wall, and wherein said head is in fluid-sealing contact with the liner and holds a portion of the liner in said recess.

The term "substantially flat" when used in connection with the inner face of the plug and the bottom of the recess is intended to indicate that the surface concerned does not follow the curvature of the pipe wall. The surface may be domed, conical, stepped, grooved or chamfered, for example, but at least one annular portion thereof will be planar.

It is, of course, to be understood that the electrode must be electrically insulated from the metal pipe wall. For this purpose, a bush of insulating material may be located between the stem of the electrode and the wall of the central hole in the plug. Preferably the outer end of the stem is threaded to receive a nut which is used to apply outward pressure on the stem. A spring washer or washers may be provided between the nut and the plug and again it is, of course, to be understood that an insulating washer or part of the insulating bush must be located between any-metal parts in contact with the stem and the plug.

Preferably the plug is welded in the pipe wall, and may be shaped to facilitate the welding process.

Means are, of course, provided to enable an electrical connection to be made to the outer end of the electrode. For this purpose, for example, the stem may be long enough to receive a second nut and an electrical connector may be secured between the two nuts.

Methods of performing the invention will now be described with reference to the accompanying diagrammatic drawings, in which: Figures 1 to 3 illustrate steps in a first embodiment of the invention; and Figures 4 and 5 illustrate steps in a second embodiment of the invention.

Referring to Fig. 1, the reference numeral 1 indicates a portion of the wall of a metal pipe section before the lining has been inserted. A hole 2 is drilled in the pipe wall, and an internal chamfer is formed as shown at 3. A metal plug 4, preferably consisting of the same material as the pipe wall, is inserted in the hole 2. The plug is formed with a central hole 5, and with a flat annular inner surface 7. As shown in Fig. 2, the plug 4 is welded to the pipe wall 1 as indicated at 6, thus forming a recess 8 within the pipe wall. A liner 9 consisting of PTFE is inserted in the pipe in the usual manner.

As shown in Fig. 3, a hole 10 is formed in the liner, and the stem 11 of an electrode is passed through the hole in the liner, and also through the hole 5 in the plug 4. Electrical contact between the electrode and the plug is prevented by means of a bush 1 2 consisting of insulating material. The electrode normally consists of a corrosion-resistant material such, for example, as stainless steel, silver, platinum, or tantalum. It will be seen that the electrode is formed with a generally mushroom-shaped head 1 3 having an annular sealing surface 14. The outer end of the stem 11 is threaded and the electrode is drawn outwardly of of the pipe by means of a nut 1 5. A pair of domed washers 1 6 are provided between the nut 1 5 and the outer surface of the bush 12.

It will be seen that, as the electrode is drawn outwardly, the sealing surface 14 pushes liner material into the recess 8 until the outside surface of the liner bears against the flat annular surface 7 of the plug 4. Thus, in this case, the portion of the liner material at which the seal is formed is held between two generally flat surfaces, namely the annular sealing surface 14 of the electrode head, and the flat inner face 7 of the plug 4. This is in contrast to the normal arrangement in which the liner is merely drawn against the cylindrical inner wall of the pipe.

It will be seen that the liner material is forced against the chamfered circular portion 3 of the hole 2 by the periphery of the electrode head 1 3 which is preferably shaped to form an additional seal in the area of contact.

It will be understood that, because a portion of the liner is held within the recess 8 by the head of the electrode, considerable resistance to any movement of the liner is provided without a large concentration of force on any portion of the liner. Thus, with the arrangement illustrated, the danger of tearing of the liner is substantially eliminated.

It is, of course, to be understood that arrangements not shown are provided for making an electrical connection to the outer end of the stem 11.

Fig. 4 is generally similar to Fig. 2 except that the liner 9 is not shown, and the inner face of the plug 4 differs from that of the plug shown in Fig. 2 in that the inner face is provided with an annular groove 1 7. As can be seen from Fig. 5, the annular sealing surface 1 8 on the head of the electrode forces material of the liner into the annular groove 17. This arrangement increases the number of changes of direction of the liner, and increases the bearing area between the liner and the pipe. Thus this engagement provides even greater resistance to movement of the liner within the pipe than the arrangement shown in Figs. 1 to 3.

In particular arrangement illustrated in Fig.

5, an electrical connector 1 9 is provided between the nut 1 5 and the washers 1 6.

Since the electrode frequently consists of a relatively expensive material and may be easily damaged, it is to be understood that a dummy electrode may be used to produce at least the initial deformation of the liner material. When the dummy electrode is removed, the liner material may recover to a certain extent, but there will be less strain on the actual electrode when it is inserted. Alternatively, or in addition, the nut used to draw the electrode or dummy electrode into position may be longer than a standard nut in order to distribute the forces over a larger number of threads then with a normal nut.

In an alternative method, the liner material is initially forced into the recess by means of an expandable tool inserted in the pipe and extending from one side of the pipe to the other. Such an expandable tool may be used to deform the recesses for both electrodes at the same time.

Accordingly, from yet another aspect the invention consists in a method of fitting an electrode having a head and a longitudinally extending stem in a metal pipe lined with synthetic resin material, wherein a hole, the diameter of which is greater than the diameter of the electrode head, is formed in the pipe wall, wherein a plug having a central hole therethrough is secured in said hole in the pipe wall with its inner substantially flat face spaced from the inner surface of the pipe wall to form a recess within the pipe wall, wherein material of the liner is forced into said recess, wherein the electrode is positioned with its stem passing through a hole in the liner, and wherein the stem is drawn outwardly through the pipe wall so that the head holds the liner material in said recess.

Claims

1. A method of fitting an electrode having a head and a longitudinally extending stem in a metal pipe lined with synthetic resin material, wherein a hole, the diameter of which is greater than the diameter of the electrode head, is formed in the pipe wall, wherein a plug having a central hole therethrough is secured in said hole in the pipe wall with its inner substantially flat face spaced from the inner surface of the pipe wall to form a recess within the pipe wall, wherein the electrode is positioned with its stem passing through a hole in the liner and the central hole in said plug, and with its head in contact with the inner surface of the liner, and wherein the stem is drawn outwardly through the pipe wall until the head forces material of the liner into said recess.

2. A method as claimed in Claim 1, wherein the inner periphery of the hole in the pipe wall is chamfered before the lining is fitted in the pipe.

3. A method as claimed in Claim 1 or Claim 2, wherein the plug is welded to the pipe wall.

4. A method as claimed in any of the preceding Claims, wherein the outer end of the electrode stem is threaded and the stem is drawn outwardly by tightening a nut on said threaded end.

5. A method as claimed in any of the preceding Claims, where the inner face of the plug is provided with an annular groove, and wherein liner material is forced into said annular groove by an annular sealing face on the electrode head.

6. An electromagnetic flow measuring instrument including an electrode fitted in a lined pipe by a method in accordance with any of the preceding Claims.

7. An electromagnetic flow measuring instrument including an electrode fitted in a pipe lined with synthetic resin material, wherein said electrode has a head and a longitudinally extending stem, wherein said stem passes through a plug secured in a hole in said pipe to form a substantially flat bottomed internal recess with the pipe wall, and wherein said head is in fluid-sealing contact with the liner and holds a portion of the liner in fluid-sealing contact with the bottom of said recess.

1 3. A method of fitting an electrode having a head and a longitudinally extending stem in a metal pipe lined with synthetic resin material, wherein a hole, the diameter of which is greater than the diameter of the electrode head, is formed in the pipe wall, wherein a plug having a central hole therethrough is secured in said hole in the pipe wall with its inner substantially flat face spaced from the inner surface of the pipe wall to form a recess with the pipe wall, wherein material of the liner is forced into said recess, wherein the electrode is positioned with its stem passing through a hole in the liner, and wherein the stem is drawn outwardly through the pipe wall so that the head holds the liner material in said recess with its outer surface bearing against the inner face of the plug.

7. An electromagnetic flow measuring instrument including an electrode fitted in a pipe lined with synthetic resin material, wherein said electrode has a head and a longitudinally extending stem, wherein said stem passes through a plug secured in a hole in said pipe to form a substantially flat bottomed internal recess within the pipe wall, and wherein said head is in fluid-sealing contact with the liner and holds a portion of the liner in said recess.

8. An instrument as claimed in Claim 7, wherein the synthetic resin material is PTFE.

9. An instrument as claimed in Claim 7 or Claim 8, wherein the electrode head is generally mushroom-shaped and is provided with an annular sealing surface.

10. An instrument as claimed in any of Claims 7 to 9, wherein the plug consists of the same material as the pipe.

11. An instrument as claimed in Claim 10, where the plug is welded to the pipe.

1 2. An instrument in any of Claims 7 to 11, wherein the inner face of the plug is provided with an annular groove.

1 3. A method of fitting an electrode having a head and a longitudinally extending stem in a metal pipe lined with synthetic resin material, wherein a hole, the diameter of which is greater than the diameter of the electrode head, is formed in the pipe wall, wherein a plug having a central hole therethrough is secured in said hole in the pipe wall with its inner substantially flat face spaced from the inner surface of the pipe wall to form a recess within the pipe wall, wherein material of the liner is forced into said recess, wherein the electrode is positioned with its stem passing through a hole in the liner, and wherein the stem is drawn outwardly through the pipe wall so that the head holds the liner material in said recess.

14. A method of fitting an electrode in a lined metal pipe substantially as hereinbefore described with reference to Fig. 1 to 3, or Fig.

4 and 5 of the accompanying diagrammatic drawings.

1 5. An electromagnetic flow measuring instrument including an electrode fitted in a lined pipe substantially as hereinbefore described, and as illustrated in Fig. 3 or Fig. 5 of the accompanying diagrammatic drawings.

1 6. Any features of novelty, taken singly or in combination, of the method and electrode arrangement hereinbefore described with reference to the accompanying diagrammatic drawings.

CLAIMS (21 Aug 1980)

1. A method of fitting an electrode having a head and a longitudinally extending stem in a metal pipe lined with synthetic resin material, wherein a hole, the diameter of which is greater than the diameter of the electrode head, is formed in the pipe wall, wherein a plug having a central hole therethrough is secured in said hole in the pipe wall with its inner substantially flat face spaced from the inner surface of the pipe wall to form a recess with the pipe wall, wherein the electrode is positioned with its stem passing through a hole in the liner and the central hole in said plug, and with its head in contact with the inner surface of the liner, and wherein the stem is drawn outwardly through the pipe wall until the head forces the outer surface of the liner against the inner face of the plug.