GB2109937A - Planar transducer for metal loss measurement - Google Patents

Abstract

A transducer for the measurement of material removed by corrosion or erosion from equipment (eg. plant, pipelines and vessels). The transducer comprises two electrically conducting elements 1,4 in close thermal contact formed from strips wound or bent into a compact form (eg a planar helix). The narrow face of one element (1) is exposed to the agressive environment, the other face and all surfaces of the second element are protected by encapsulation or otherwise. Measurement of the electrical resistances of the element allows calculation of the loss of material from the exposed element. The planar transducer can be constructed and position with its exposed element sensibly contiguous and co-planar with the surface of the equipment of interest. Furthermore the element can be constructed from material closely resembling that of the equipment. The corrosion/ erosion rate on the element is thus representative of that sustained by the equipment. <IMAGE>

Description

SPECIFICATION Planar transducer for metal loss measurement The present invention relates to the construction of transducers for the measurement of material removed, for instance, by corrosion and/or erosion, by the measurement of the thickness of a specimen of the said material as typifying the behaviour of the material as a whole.

By "transducer" we mean a device which senses a property of interest, but which is difficult to measure directly, and provides a signal which varies with the said property, and which may be measured conveniently. In the present instance, the property of interest is thickness or variations in thickness and the signal is a variation in electrical resistance.

Typically such a transducer comprises an electrically conductive element in physical contact with the environment, a second electrically conductive element electrically insulated from the first element and physically isolated from the environment, means for mounting the two elements and means for comparing the electrical resistance of the two elements.

The electrically conductive elements are typically made from drawn wire, rolled strip or drawn tubing in the form of unsupported loops projecting into the environment which is expected to remove metal.

Such elements have been found to possess certain disadvantages: 1. They cannot be fabricated from the materials actually used in the manufacture of pipes and vessels. Some, such as cast iron, cannot be imitated at all; but even those of the same nominal composition will possess different metallurgical states in thin wire, tapes and tubes and in thicker rolled plates and pipes. The differences engender different behaviour in certain corroding environments.

2. In order to possess reasonable strength and stiffness, the length of the unsupported loop has to be restricted severely. The sensitivity of measurement is restricted in consequence. Enhancing the sensitivity by using a thin-walled tube specimen renders it short-lived.

3. The hydrodynamic conditions surrounding a projecting specimen usually differ significantly from those experienced by the wall of a pipe or vessel. In many environments, the difference causes the behaviour of the projecting specimen to be atypical and sometimes misleading.

4. The operation known as "pigging" in which a robust obstruction is forced along a pipe e.g. to cleanse it or to separate fluids, damages projecting elements, or if they are robustly protected, the transit of the "pig" is obstructed.

Another known type of element is constructed from thin foil or ribbon, with some form of support, presenting the broad face of the foil or ribbon to the environment. Such thin elements are likely to have different properties to actual constructional materials. Furthermore they are frail, subject to mechanical damage and may be soon corroded through and expended.

To render the measurement of material loss by resistance signal sensitive it is necessary for nearly all conducting materials, but especially metals, to be made in the form of elements of which the length between the measurement terminals is at least an order of magnitude greater than the transverse directions. It is an object of the present invention to provide robust and sensitive transducers of which the overall dimensions are reduced to acceptable values by convoluting the element into a compact shape e.g. a planar grid or spiral.

It is a further object to provide an element which can be constructed from material in the form which would be used for actual constructional purposes. It is a feature of our invention that we form a robust convoluted element by winding or bending a strip of the relevant material or by other means and that the narrow face is exposed to the environment causing metal loss. Such construction maximisesthe length of element that can be included in a given area and hence optimises the sensitivity and accuracy of measurement. In addition since the removal of metal then takes place along the larger lateral dimension the useful working life is extended in comparison with elements exposing the larger lateral dimension to metal removal. Furthermore the construction allows firm encapsulation of the element.

According to the present invention there is provided a transducer for measuring the corrosion or erosion of an electrically conductive surface in an environment, comprising an electrically conductive element in physical contact with the environment, means for mounting the electrically conductive element electrically isolated from but sensibly contiguous and coplanar with a surface exposed to the environment, a second electrically conductive element electrically insulated from the first element and the surface and physically isolated from the environment, and means for comparing the electrical resistance of the said two electrically conductive elements, characterised in that the first electrically conductive element comprises an elongate strip of material normally but not necessarily of the same material as the said surface, a single surface of the elongate length of material being exposed to the environment and the elongate length of material being bent in the plane at right angles to the said single surface to form a compact shape.

The formed strip is embedded in a non-metallic encapsulant which is chosen to possess a combination of electrical, mechanical and chemical properties appropriate to the range of conditions for which the apparatus is designed. Examples of material suitable for known industrial environments are epoxy, bitumen/epoxy, phenol-formaldehyde, and furane resins.

The surfaces of the metal element are preferably given treatment to cause them to adhere to the encapsulant in such a manner that the resulting bonding action forbids the penetration of chemical attack along the interface. A variety of such treatments are known to those skilled in the art. Examples are grit-blasting, acid etching, chelating, and combinations of these.

In accordance with practice which is now well known, two formed elements are encapsulated closely together, so that in use their temperatures are practically equal. One face of one element is made bare by removing the encapsulant, so that the influences that remove metal may have access to it.

The three remaining faces of this element, the whole of the second element, the connecting wires, and the joints thereto are all protected from the attacking influences by being embedded in insulating material.

As a function of the apparatus is to simulate the behaviour of its metallic surroundings, it is important that the temperatures of the elements and of the surrounding metal should not be significantly different. The elements must be mounted in a container, and have mechanical fixing and electrical connections.

Elements of rectangular cross-section may be measured by a potentiometric measuring circuit giving response linear with the depth of metal removal. Other cross-sections may have the depth of metal-loss computed by a micro-processor in accordance with principles known to those skilled in the art.

It is part of this invention to minimize the flow of heat into the specimens or out of them, except from the front face, by promoting thermal resistance in their mounting and their electrical connections. This aspect has particular importance in exposure to gases and/or vapours.

The adequacy of the arrangements for equalising the temperatures of the specimens and of their surroundings is preferably monitored by including in the double assembly described above, means for measuring the temperature difference. Typical means are embedded thermocouples or resistance thermometers.

The invention is applicable only to material which are electrically conducting. Not all of these are metallic but in this application "metal" may be taken as referring also to non-metallic conducting materials.

Advantages of the invention over currently available transducer designs include: Elements can be fabricated from materials in the metallurgical form actually used in the construction of e.g. plant, equipment, vessels and pipelines.

Compact, sensitive, robust, planar transducers can be constructed and positioned sensibly contiguous and coplanar with a surface.

The useful lifetime of the element is greater for a given sensitivity.

The operation known as "pigging" of a pipeline can be carried out with the transducer in situ.

The transducer may be readily adapted to insertion into pipelines and pressure vessels and withdrawal therefrom by the means commonly known as "on-line taps".

In a preferred embodiment of the invention the element (1) is a flat spiral, the connection tabs (2) and (3) being forged from the strip and extending backwards from the flat face that will be subjected to the removal of metal. The protected companion element (4) is a similar flat spiral, with tabs (5) and (6).

The two spirals are assembled as a coaxial sandwich having a thin layer of electrical insulating material between (7). The front face of spiral (1) is installed sensibly coplanar with the metal surface from which the metal loss is to be imitated and measured.

The centre connecting tab (2) of spiral (1) projects through the centre of spiral (4) lying closely alongside its centre tab (5) to which it is welded.

Two thin cables of silver-coated copper wires (8) and (9) are brazed to the twin tabs (2) and (5), two more (10) and (11 ) to the outertab (3) of spiral (1) and two more (12) and (13) to the outer tab (6) of spiral (4). All six cables are twisted together and led through the hollow centre of an extension tube (16), thence to a fixed socket (17). A cable terminated in a matching plug connects the twin specimens to a suitable apparatus for measuring and recording the ratio of, or difference in, their respective electrical resistances.

Cup (15) sits in a recess (20) in a housing attached to the metal surface being simulated. Sealing and centring are achieved by two 0-rings, so that contact and thermal transfer are minimised. It will be evident to those skilled in the art that the cylindrical cup (15) can be extended to comprise also the stem (16), making the whole housing a uniform cylinder so that it may be installed in a pipeline or vessel and withdrawn again at will through the apparatus commonly known as an on-line tap.

CLAIMS (filed 9.11.82) 1. A metal loss transducer in which the measuring element comprises an elongated strip bent or formed into a substantially planar helix or other compact shape and mounted in such a manner as to expose one minor dimension of the cross section to attack by the environment, the remaining surfaces being protected.

2. A metal loss transducer comprising one or more elements as defined in Claim (1) and one or more additional elements totally protected from the environment and electrically isolated from the measuring element(s) but in close thermal proximity to them.

3. A metal loss transducer as described in Claim 1 and 2 comprising elements which have been surface treated to promote adhesion to the encapsulating material used to protect the element from the environment.

4. An apparatus for the measurement of surface loss, typically by corrosion, comprising a transducer as described in one or more of the previous claims mounted in an equipment item in such a manner as to expose the corroding face sensibly contiguous and sensibly coplanar with the equipment item surface under study.

5. A method of measuring material loss in which a transducer as defined in Claim 4 is mounted as defined in Claim 5 and connected to measuring equipment to provide indication and/or record as appropriate.

6. A method of measurement of surface loss in which the corroding element is made of material indentical of substantially identical to the materials of construction of the equipment item under test.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. are practically equal. One face of one element is made bare by removing the encapsulant, so that the influences that remove metal may have access to it. The three remaining faces of this element, the whole of the second element, the connecting wires, and the joints thereto are all protected from the attacking influences by being embedded in insulating material. As a function of the apparatus is to simulate the behaviour of its metallic surroundings, it is important that the temperatures of the elements and of the surrounding metal should not be significantly different. The elements must be mounted in a container, and have mechanical fixing and electrical connections. Elements of rectangular cross-section may be measured by a potentiometric measuring circuit giving response linear with the depth of metal removal. Other cross-sections may have the depth of metal-loss computed by a micro-processor in accordance with principles known to those skilled in the art. It is part of this invention to minimize the flow of heat into the specimens or out of them, except from the front face, by promoting thermal resistance in their mounting and their electrical connections. This aspect has particular importance in exposure to gases and/or vapours. The adequacy of the arrangements for equalising the temperatures of the specimens and of their surroundings is preferably monitored by including in the double assembly described above, means for measuring the temperature difference. Typical means are embedded thermocouples or resistance thermometers. The invention is applicable only to material which are electrically conducting. Not all of these are metallic but in this application "metal" may be taken as referring also to non-metallic conducting materials. Advantages of the invention over currently available transducer designs include: Elements can be fabricated from materials in the metallurgical form actually used in the construction of e.g. plant, equipment, vessels and pipelines. Compact, sensitive, robust, planar transducers can be constructed and positioned sensibly contiguous and coplanar with a surface. The useful lifetime of the element is greater for a given sensitivity. The operation known as "pigging" of a pipeline can be carried out with the transducer in situ. The transducer may be readily adapted to insertion into pipelines and pressure vessels and withdrawal therefrom by the means commonly known as "on-line taps". In a preferred embodiment of the invention the element (1) is a flat spiral, the connection tabs (2) and (3) being forged from the strip and extending backwards from the flat face that will be subjected to the removal of metal. The protected companion element (4) is a similar flat spiral, with tabs (5) and (6). The two spirals are assembled as a coaxial sandwich having a thin layer of electrical insulating material between (7). The front face of spiral (1) is installed sensibly coplanar with the metal surface from which the metal loss is to be imitated and measured. The centre connecting tab (2) of spiral (1) projects through the centre of spiral (4) lying closely alongside its centre tab (5) to which it is welded. Two thin cables of silver-coated copper wires (8) and (9) are brazed to the twin tabs (2) and (5), two more (10) and (11 ) to the outertab (3) of spiral (1) and two more (12) and (13) to the outer tab (6) of spiral (4). All six cables are twisted together and led through the hollow centre of an extension tube (16), thence to a fixed socket (17). A cable terminated in a matching plug connects the twin specimens to a suitable apparatus for measuring and recording the ratio of, or difference in, their respective electrical resistances. Cup (15) sits in a recess (20) in a housing attached to the metal surface being simulated. Sealing and centring are achieved by two 0-rings, so that contact and thermal transfer are minimised. It will be evident to those skilled in the art that the cylindrical cup (15) can be extended to comprise also the stem (16), making the whole housing a uniform cylinder so that it may be installed in a pipeline or vessel and withdrawn again at will through the apparatus commonly known as an on-line tap. CLAIMS (filed 9.11.82)

1. A metal loss transducer in which the measuring element comprises an elongated strip bent or formed into a substantially planar helix or other compact shape and mounted in such a manner as to expose one minor dimension of the cross section to attack by the environment, the remaining surfaces being protected.

2. A metal loss transducer comprising one or more elements as defined in Claim (1) and one or more additional elements totally protected from the environment and electrically isolated from the measuring element(s) but in close thermal proximity to them.

3. A metal loss transducer as described in Claim 1 and 2 comprising elements which have been surface treated to promote adhesion to the encapsulating material used to protect the element from the environment.

4. An apparatus for the measurement of surface loss, typically by corrosion, comprising a transducer as described in one or more of the previous claims mounted in an equipment item in such a manner as to expose the corroding face sensibly contiguous and sensibly coplanar with the equipment item surface under study.

5. A method of measuring material loss in which a transducer as defined in Claim 4 is mounted as defined in Claim 5 and connected to measuring equipment to provide indication and/or record as appropriate.

6. A method of measurement of surface loss in which the corroding element is made of material indentical of substantially identical to the materials of construction of the equipment item under test.

7. A metal loss transducer substantially as herein

before described with reference to the accompanying drawing.

8. A method of measuring metal loss from the surface of equipment using a metal loss transducer wherein the said method is substantially as herein before described with reference to the accompanying drawing.