GB2298716A - Electrode mounting for electromagnetic flowmeter - Google Patents

Description

FLOWMETERS This invention relates to flowmeters, for example of the type in which a magnetic field is established across a conductive fluid in a conduit and an induced voltage in the fluid is measured as indicative of its volume flow rate.

The conduit of such a magnetic flowmeter is lined with an electrically insulating material. Two or more electrodes, which are electrically conducting, penetrate the lining and the conduit. By this means electrical contact can be made between the liquid in the conduit and measuring equipment located elsewhere. The electrode must of course be insulated from the metal conduit.

Many materials such as various types of rubber or thermoplastics may be used for the lining. Since these magnetic flowmeters may be used to measure aggressive chemicals both the electrodes and the lining must be made from materials which are immune from attack. Many of the relatively inert materials which can be used as a lining have relatively poor mechanical properties and if subjected to even moderate levels of stress they undergo cold flow or creep, resulting in leakage around the electrode.

This invention is directed to this problem and thus provides a flowmeter having a conduit with an insulating lining, an electrode disposed through the insulating lining so as to contact conductive fluid in the conduit, and a fluid-tight seal between the electrode and the lining, characterised by the seal comprising means on the electrode for applying a local force to the material of the lining to achieve a contact pressure in excess of the elastic limit of the material, and means supporting the material in the vicinity of the local force to constrain the material in bulk compression against creep. This novel form of mechanical seal between the electrode and lining can be used in conditions where rubber seals such as 0 rings would be inappropriate because of the aggressive nature of the chemicals contained within the meter.

The seal between the lining and the electrode may be achieved by an arrangement whereby exceedingly high contact pressure (for example of the order of 3 times the ultimate tensile strength of the lining material) is generated over a small area of contact. The seal is arranged so that the lining material is constrained from moving and is put in bulk compression.

Thus, in a further aspect the invention provides a method of sealing an electrode in an insulating lining of a magnetic flowmeter conduit comprising applying a local force to the material of the lining to achieve a contact pressure in excess of its elastic limit by means of the electrode, and supporting the material locally in the vicinity of the applied force so that it is put in bulk compression and constrained against creep.

In a preferred form1 the electrode comprises a stem with an enlarged wedge portion.

The enlarged wedge portion may be at least partially of conical form.

The stem may be received within a bore in the lining and may deform the wall thereof.

The bore may be tapered.

The bore may pass through the wall of the conduit and the supporting means may be constituted by the thickness of said wall.

The enlarged portion of the stem may be aligned with the thickness of the conduit wall.

The supporting means may further comprise lining material defining those parts of the bore disposed on each side (axially of the stem) of the portion thereof aligned with the thickness of the conduit wall.

If desired, the material defining the bore may be further compressed by a force applied axially of the bore, for example by tensile loading applied to the electrode stem.

The invention will be described merely by way of example with reference to the accompanying drawings, wherein: Figure 1 shows a section through a conventional electrode sealing construction; and Figure 2 shows a section through a sealing construction according to the invention.

Referring to Figure 1, a known arrangement comprises a metal flowmeter conduit 10 having an insulating lining 12. The lining is of PTFE (polytetrafluoroethylene), PFA (perfluoroalkoxy), PVDF (polyvinylidenefluoride), polypropylene or polyethylene, as other suitable inert thermoplastic material, as known in the art. A bore 14 penetrates both the conduit and the lining1 to receive an electrode 16. A loose fitting insulating bush 17 is situated beneath the overlapping sections of the lining 12 such that there is a gap 15 around the periphery of the bush. The bush has an internal diameter greater than that of the electrode.

The electrode has a head 18 and stem 20, the end 21 of which is threaded. The underside of the head 18 is provided with a sharp rim 22. The electrode is disposed in the bore 14 with its head on the inner (fluid) side 24 of the conduit 10. A spring washer 25, nut 26 and washer 28 are fitted on the stem 20. Means not shown (eg. a further washer with a solder tag) are provided to permit electrical connection of a signal wire to receive the output of the electrode.

Tightening of the nut 26 draws the head 18 of the electrode down on to the surface of the lining, causing the rim 22 to compress the lining material and form a seal therewith. Small movements of the lining 12 during tightening of the nut are accommodated by movement of the bush 17 into the gap 15. Initially the seal is effective, but over time the lining material creeps (undergoes cold plastic fiow) away from the rim 22, under the influence of the compressive stress, as shown by arrows 30. The compression of the material thus is relieved, allowing the seal to leak.

Figure 2 shows a seal according to the invention, in which features already described have the same reference numerals as in Figure 1.

In this seal, the material of the lining 12 is moulded so as also to line completely the bore 14 through the conduit and to extend on the outside of the conduit sufficiently to form a pad 31 for the nut and washer 26,28.

The head 18 of the electrode 16 has a flat underside and so can be pulled-down flush against the inner surface 24 of the lining. The stem 20 has an enlarged portion 32 in the form of a circular-section wedge. The surface 34 of the wedge facing towards the end of the stem is tapered so as to be frustoconical in form. The surface 36 facing toward the head 18 is substantially perpendicular to the axis of the stem, and the two surfaces 34,36 intersect in a relatively sharp rim 38.

The bore 14 through the lining material is of sufficient diameter to admit the parallel part of the stem 20 as a relatively close sliding fit, but not the wedge 32. The degree of interference between the wedge and the bore, and the load-bearing capacity of the thread 21 are chosen so that the wedge can be drawn through the bore by tightening the nut 26. To form a seal the electrode stem is introduced into the bore 14 the nut fitted together with a plain washer 28, solder tag 29 and spring washer 30. The length of the stem is sufficient to project through the bore without the wedge 32 entering it). The stem is thus pulled through the bore by tightening the nut 26. The rim 38 engages the wall of the bore, deforming but not fracturing or severing the material thereof so as to pass through it, the nut 26 being tightened until the head of the electrode abuts the surface 24 of the lining.Then the rim 38 is aligned with the thickness 42 of the wall of the conduit 10.

In this position the wedge 32 subjects the material of the lining surrounding the bore to a considerable local force, such that the portion 40 of material contacted by the rim 38 is under a stress well in excess of its elastic limit, for example three times its ultimate tensile stress. The lining material is locally deformed by the rim 38 and wedge 32 to conform extremely closely thereto and form a fluid-tight seal.

The material 40 in this region is however backed or supported by the thickness 42 of the wall of the metal conduit 10. This support together with the support from adjacent portions 44,46 of material defining the bore ensures that the lining material 40 where under load from the rim 38 is maintained locally in bulk compression and thus does not creep or cold flow away from the rim, other than to creep slightly so that it fully fits any small radial clearance between the stem and the bore. The axial compression of the material is such that following the minimal radial creep the material is still in bulk compression and adequate sealing pressure is maintained, between the wedge 32 and the material in contact with it. The integrity of the seal thus is maintained.

The stress imparted to the material by the rim 38 reduces quickly with increasing distance from the point of contact, and therefore it is not in general necessary to support the portions 44,46 lining material against creep lengthwise of the bore 14. However, it is preferable to do so, and therefore the tension in the stem 20 is utilised to apply a compressive force axially of the bore between the electrode head 18 and the washer 28 to the lining material forming the bore.

Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features.

The appended abstract as filed herewith is included in the specification by reference.

Claims (11)

CLAIMS:

1. A flowmeter having a conduit with an insulating lining, an electrode disposed through the insulating lining so as to contact conductive fluid in the conduit, and a fluid-tight seal between the electrode and the lining1 characterised by the seal comprising means on the electrode for applying a local force to the material of the lining to achieve a contact pressure in excess of the elastic limit of the material, and means supporting the material in the vicinity of the applied force to constrain the material in bulk compression against creep.

2. A flowmeter as claimed in Claim 1, wherein means on the electrode comprises a stem with an enlarged wedge portion.

3. A flowmeter as claimed in Claim 2, wherein the enlarged wedge portion is at least partially of conical form.

4. A flowmeter as claimed in Claim 2 or 3, wherein the stem is received within a bore in the lining and deforms the wall thereof.

5. A flowmeter as claimed in Claim 4, wherein the bore is tapered.

6. A flowmeter as claimed in Claim 4 or 5, wherein the bore passes through the wall of the conduit and the supporting means is constituted by the thickness of said wall.

7. A flowmeter as claimed in Claim 6, wherein the enlarged portion of the stem is aligned with the thickness of the conduit wall.

8. A flowmeter as claimed in Claim 6 or 7, wherein the supporting means further comprises lining material defining those parts of the bore disposed on each side (axially of the stem) of the portion thereof aligned with the thickness of the conduit wall.

9. A flowmeter as claimed in Claim 8, wherein the material defining the bore is further compressed by a force applied axially of the bore.

10. A method of sealing an electrode in an insulating lining of a flowmeter conduit comprising applying a local force to the material of the lining to achieve a contact pressure in excess of its elastic limit by means of the electrode and supporting the material locally in the vicinity of the applied force so that it is put in bulk compression and constrained against creep.

11. A flowmeter or a method of sealing an electrode in an insulating lining of a flowmeter conduit substantially as herein described with reference to Figure 2 of the accompanying drawings.