WO2017077287A1

WO2017077287A1 - Ultrasonic thickness gauge to be used in a high temperature environment and process for attaching it

Info

Publication number: WO2017077287A1
Application number: PCT/GB2016/053391
Authority: WO
Inventors: Adrian Robert Bowles
Original assignee: 3-Sci Ltd
Priority date: 2015-11-06
Filing date: 2016-11-02
Publication date: 2017-05-11
Also published as: GB2544108B; GB2544108A; GB201519675D0

Abstract

An ultrasonic thickness gauge is provided for measuring the thickness of a hot substrate and configured to be permanently installed on the substrate, the ultrasonic thickness gauge comprising: (a) a transducer, the transducer comprising a piezoelectric element for transmitting pulses of ultrasonic vibrations and for receiving pulses of reflected ultrasonic vibrations; (b) a delay line having a first surface attached to the piezoelectric element and a second surface configured to be attached to the substrate; wherein the delay line has a coefficient of thermal expansion (CTE) at 200°C from 4x 10^-6/°C to 11 x 10^-6/°C, preferably from 5 x 10^-6/°C to 11 x 10^-6/°C, more preferably from 6 x 10^-6/°C to 10 x 10^-6/°C.

Description

ULTRASONIC THICKNESS GAUGE TO BE USED IN A HIGH TEMPERATURE ENVIRONMENT AND PROCESS FOR

ATTACHING IT

TECHNICAL FIELD

The present invention relates to an ultrasonic thickness gauge suitable to be permanently installed on chemical or refinery plant, such as pipelines, tanks and vessels, especially in a high temperature environment.

BACKGROUND

It is known to employ ultrasonic thickness gauges to monitor the wall thickness of chemical or refinery plant, such as pipelines, tanks and vessels (generically referred to herein as a "substrate"), in order, for example, to measure the effects of corrosion and erosion. More specifically, a substrate's wall thickness is typically calculated using a pulse-echo technique from an ultrasonic transducer, as follows: an electronic pulser-receiver generates an electrical pulse which is transmitted to the transducer. The transducer converts this electrical pulse into an ultrasonic acoustic pulse and is configured then to transmit the acoustic pulse so that it impinges upon the substrate under test. A very short period of time after transmission (typically a few

microsecondsseconds), an acoustic reflection occurs at the interface between the end of the transducer and the front wall of the substrate. This returning pulse may be referred to as the "front wall echo" (FWE). A further short time period later (a few microseconds again), another reflection occurs at the back wall of the substrate. This second returning pulse may be referred to as the "back wall echo (BWE). Both FWE and BWE are converted back into electrical signals by the transducer and

transmitted back to the pulser-receiver. The difference in time between the arrival of the BWE and the FWE yields the thickness of the substrate, providing that the speed of sound in the material of the substrate is known at the relevant temperature.

Refinements and variations of this method are known in the art.

The majority of transducers comprise a piezoelectric crystal. Although other materials also have the ability to convert acoustic signals to electrical signals and vice versa, piezoelectric materials are usually employed as the active element in ultrasonic inspection equipment, because they perform well at the required acoustic frequencies and because of the ease with which they may be incorporated into a transducer.

Traditionally, such ultrasonic acoustic thickness gauges have been hand-held devices and the thickness measurements have been performed by workers who visit the inspection site in person and briefly couple the hand-held device to the substrate under evaluation using a water-based or polyol-based (commonly employed polyols include propylene glycol or glycerine) couplant in order to gather the thickness measurement data.

There are considerable logistical drawbacks to manual inspections. Accessing the inspection site during plant operation may be challenging in the first place and may be dangerous as a result of explosive, high temperature and/or elevated locations. Inspecting a plant during a planned shutdown may not always be a suitable alternative, because more frequent inspection may be required. In addition, manual measurements are generally considered to be inaccurate, because of differences between the methods of different inspection workers. The workers do not always test exactly the same inspection site each time and there is usually no control of the measurement conditions. These factors may limit the usefulness of thickness data produced in this way.

A permanently installed transducer could eradicate the limitations affecting

measurement accuracy. Permanent installation may, however, give rise to its own challenges. For example, many industrial processes operate at high temperatures and/or over a widely varying temperature range, such that, in normal operation, the components of a permanently installed transducer may be subject to rapid heating and/or cooling of hundreds of degrees centigrade. Temperatures may fluctuate between -20C and 200^°C and, in some operations, even outside that range. Neither the transducer nor the water-based couplants generally employed with manual devices are designed to operate for more than a few seconds under such conditions. Conventional transducers may become permanently damaged if exposed to temperatures above 60°C for more than a few seconds. Differing magnitudes of thermal expansion of the component parts may, for example, cause parts of the device to rupture and fail. Failure at the interface with the transducer and failure of the piezoelectric crystal itself may be cited as examples. Furthermore, the water- based couplants evaporate quickly when warm. Some piezoelectric materials may also depolarise at high temperatures.

In order to address this problem, a system is proposed in a paper entitled "High- temperature (>500°C) wall thickness monitoring using dry-coupled ultrasonic waveguide transducers." by Cegla FB., Cawley P., Allin J. and Davies J. in IEEE Trans Ultrason Ferroelectr Freq Control. 201 1 Jan; 58(1 ): 156-67 (referred to herein as the "Cegla Paper"). That system employs a long, thin, steel waveguide (an exemplified waveguide has the following dimensions: 300mm length, 15mm width and 1 mm depth) to isolate the vulnerable transducer and the piezoelectric crystal within it from the high-temperature measurement zone. The waveguide is "dry- coupled" to the substrate (specifically, a clamp is welded to the substrate and the wave guide is clamped thereto). According to this paper, the "use of thin and long waveguides of rectangular cross section allows large temperature gradients to be sustained over short distances without the need for additional cooling equipment".

A long waveguide of the type disclosed in the Cegla Paper may have the disadvantage that it cannot be installed in confined spaces or spaces which would bring the ultrasonic thickness gauge at the end of the waveguide into the vicinity of another region of high temperature. In addition, long structures are more prone to knock damage and the positioning of a heavy transducer on a long thin structure generates a significant turning moment, requiring a very strong method of attachment to the substrate (hence the welding of a clamp). Finally, dry coupling provides poor acoustic coupling, as accepted in the Cegla paper itself, which refers to the dominance of the end reflection echo.

It would be desirable if the waveguide could be configured to have a smaller length than proposed in the Cegla Paper and still avoid the disadvantages arising from differential thermal expansion at high temperature. SUMMARY OF THE INVENTION

According to a first aspect of the invention, an ultrasonic thickness gauge is provided for measuring the thickness of a hot substrate and configured to be permanently installed on the substrate, the ultrasonic thickness gauge comprising:

(a) a transducer, the transducer comprising a piezoelectric element for transmitting pulses of ultrasonic vibrations and for receiving pulses of reflected ultrasonic vibrations;

(b) a delay line having a first surface attached to the piezoelectric element and a second surface configured to be attached to the substrate;

wherein the delay line has a coefficient of thermal expansion (CTE) at 200°C from 4 x 10^"6/°C to 1 1 x 10^"6/°C, preferably from 5 x 10^"6/°C to 1 1 x 10^"6/°C, more preferably from 6 x 10^"6/°C to 10 x 10^"6/°C.

As used herein, the term "delay line" refers to the element which is part of the ultrasonic thickness gauge and which, in use, connects the transducer to the substrate such that it is disposed between the transducer and the substrate. Delay lines are known in the art. With reference to the Cegla Paper, discussed above, a delay line may also function to some extent as a "wave guide", but its primary function according to the invention is to delay the return of reflected ultrasonic pulses so that they are not masked by the initial excitation pulse of ultrasonic energy, hence the term "delay line".

As used herein, the "coefficient of thermal expansion" (CTE) is measured according to ASTM E289-04 entitled "Standard Test Method for Linear Thermal Expansion of Rigid Solids with Interferometry". The ASTM E289-04 test method is applicable to the temperature range -150 to 700°C and is applicable to all elements of the ultrasonic thickness gauge according to the invention, including to measurements of short specimens such as thin piezoelectric discs.

As used herein, the term "cure" or "curing" when used in relation to adhesives, means conversion of a liquid or paste to a solid and the "curing temperature" is the temperature at and above which curing is considered to commence. BRIEF DESCRIPTION OF THE DRAWING

Figure 1 illustrates one embodiment of an ultrasonic thickness gauge according to the invention.

DETAILED DESCRIPTION

The present invention provides an ultrasonic thickness gauge which overcomes the problems of prior art thickness gauges.

Many modifications may be made to the above examples without departing from the scope of the present invention as defined in the accompanying claims.

Further aspects of the present invention are set out hereinbelow:

The delay line of the present invention has a first surface attached to the

piezoelectric element and a second surface which is configured to be attached to the substrate whose thickness is to be measured, such that the delay line is disposed between the piezoelectric element and that substrate. In order to minimise the stress at an interface between two different elements, the difference between the

coefficients of thermal expansion (CTE) of the materials of the two elements should be minimised, since then they will expand at similar rates and the stresses on the interfacial connection will also be minimised. In the present case, however, there are three elements connected in series and the materials of the elements at each end of the series have widely different CTEs, one typically being a metal, usually steel, substrate having a CTE measured at 200°C of about 14, and the other being a piezoelectric material, typically having a CTE measured at 200°C of 1 -2. Selecting a delay line material with a CTE closely matched to that of the substrate, would minimise stresses due to large temperature excursions at the interface between the substrate and the delay line, but would lead to a high CTE-difference and thus high stresses in the face of large temperature excursions at the interface between the delay line and the piezoelectric element. On the other hand, selecting a delay line material with a CTE closely matched to that of the piezoelectric element, would minimise stresses when confronted with large excursions at the interface between the piezoelectric element and the delay line, but would lead to a high CTE-difference and thus high stresses when faced with significant temperature excursions at the interface between the delay line and the substrate. The present inventors have established that, if the CTE of the delay line is selected to have a value in the claimed range between the CTE of the substrate and the CTE of the piezoelectric element, then stresses do not lead to rupture and failure of the device at

temperatures in the range -20°C to 200°C and the delay line may, if so desired, be configured to have a length which is far shorter than the wave guide defined in the Cegla Paper. More specifically, the delay line according to the invention has a coefficient of thermal expansion (CTE) from 4 x 10^"6/°C to 1 1 x 10^"6/°C.

Preferably, the CTE is from 5 x 10^"6/°C to 1 1 x 10^"6/°C, more preferably from 6 x 10^" ⁶/°C to 10 x 10^"6/°C. The preferred range is skewed towards the CTE of the substrate, which is typically steel, because the delay line is typically attached to the substrate in situ, often outside, exposed to the prevailing weather and so the bond may be of a lower quality than the attachment between the piezoelectric element and the delay line, which tends to be stronger, higher quality attachment, since it is made inside, under controlled, factory conditions. In addition, as explained below, the piezoelectric element may be pre-adhered to the delay line at high temperature using an adhesive that cures at that high temperature, thereby minimising tensile stresses in the piezoelectric element at temperatures up to 200°C. As a result, it is preferred for the CTE of the delay line material to be more closely matched to the CTE of the substrate material than to the CTE of the piezoelectric element.

CTE changes with temperature, so it is necessary to specify the temperature of measurement. The present inventors have measured CTE at 200°C, because that is the highest temperature at which the present applicants wish their ultrasonic thickness gauge to operate and is the temperature at which the difference between the CTE of the substrate material and the delay line material, on the one hand, and the difference between the CTE of the delay line material and the piezoelectric element material, on the other hand, is greatest. It is thus the temperature at which the greatest stresses will occur and at which the greatest likelihood of rupture and failure at the respective interfaces.

As discussed below, the transducer comprises materials which function at the intended temperature of operation of the ultrasonic thickness gauge. It is known, however, that the reaction rate of chemical processes speeds up by a factor of two for every increase in temperature of 10-20°C (this generalisation is supported by the Arrhenius Equation). It is therefore advantageous to reduce the temperature extremes experienced by the transducer in order to extend its lifetime. In

consequence, the delay line advantageously comprises and preferably consists only of a material that is thermally insulating.

Advantageously, the material of the delay line also comprises and preferably consists only of an electrically insulating material, so as to isolate any electrical energy which builds up in the piezoelectric element from being discharged to the potentially explosive atmosphere of the surrounding plant.

The material of the delay line may suitably comprise glass, ceramic or mixtures thereof, provided that the material meets the CTE criterion according to the invention. Preferably, the material of the delay line comprises fluorophlogopite mica, borosilicate or mixtures thereof.

Advantageously, the material of the delay line may be readily machined in order to profile the second surface (the surface configured to be attached to the substrate) to the shape of the substrate surface to which it is to be attached, thereby maximising the area for the adhesive to bond and thus the bond strength. Preferably, the material of the delay line comprises Macor®, manufactured and sold by Corning® Inc., which comprises fluorophlogopite mica in a borosilicate matrix and which is thermally and electrically insulating and may be readily machined. Macor® has a CTE at 200°C of about 9 x 10^"6/°C.

The presence of a delay line may give rise to disruptive and interfering acoustic signals that can cause problems when trying to identify and ascertain the position of the BWE. In some cases, these interfering signals may be formed from guided wave packets that can be comparable in size to the FWE or BWE, making identification of the FWE and BWE challenging. This problem may arise in use of the thin wave guide defined in the Cegla Paper. The problem may, however, be reduced or eliminated by an appropriately dimensioned delay line.

The delay line may have any suitable cross-sectional shape, but is advantageously circular in cross-section.

The present inventors have established that the peak amplitude of the guided wave packets increases in inverse proportion to the delay line diameter, to the extent that below a certain diameter it even manifests a similar amplitude to the FWE or BWE, making the signals hard to tell apart. Advantageously, for piezoelectric crystals of the dimensions commonly used in the field of ultrasonic thickness measurement, the delay line has a diameter in the range 10mm to 45mm, preferably 15mm to 40mm.

In addition, the delay line according to the invention advantageously has a length, L, being the distance between the first and second surfaces, which is more than 10mm and less than 100mm, preferably less than 75mm, more preferably less than 50mm.

The piezoelectric element may comprise any piezoelectric material which remains stable and polarised, that is which does not depolarise, at the desired operating temperature, which is advantageously from -20°C to 200°C. Such materials are known to the person skilled in this art. Advantageously, the piezoelectric element comprises a ceramic material. Suitable materials include lead zirconate-titanate (also known as "PZT"), bismuth titanate, lithium niobate, lead titanate, lead metaniobate and mixtures thereof. Advantageously, the piezoelectric element comprises lead metaniobate.

The piezoelectric element is operable at a frequency appropriate for measuring the thickness of the material of the substrate. A significant proportion of the substrates in question will be elements of chemical or refinery plant and will be manufactured from steel. In steels, the attenuation of an acoustic ultrasound signal depends upon the signal frequency. A suitable frequency range for a steel which is less than or equal to 25mm in thickness is from 1 MHz to 10MHz, preferably from 4MHz to 7MHz, more preferably from 4MHz to 6MHz and more preferably still 5MHz.

The ceramic materials of piezoelectric elements exhibit a low tensile strength

(generally on the order of tens of MPa), but a high compressive strength (generally on the order of hundreds of MPa) and the piezoelectric element will also generally have a lower CTE than the other elements of the transducer within which it is retained. As a result, in order to avoid the piezoelectric element being put under excessive tension and potentially rupturing and failing as a result of the higher rates of thermal expansion of the surrounding elements, it is advantageous to attach the piezoelectric element to the delay line at high temperature by means of a first adhesive layer. Advantageously, the first adhesive layer comprises a thermosetting adhesive that cures at that high temperature. Ideally, the curing temperature would be about 200^°C so that in operation at up to 200^°C, the combination of the

piezoelectric element, the first adhesive layer binding it to the delay line and the delay line would not subject the piezoelectric element to any tension as a result of differing rates of thermal expansion. In practice, lower curing temperatures than 200C may be tolerated. Applicants have established that the use of a thermosetting adhesive with a curing temperature in the range from greater than or equal to 140^°C to less than or equal to about 200^°C, preferably from greater than or equal to 150^°C to less than or equal to about 200^°C can be suitable and that tension arising as a result of differing rates of thermal expansion above the curing temperature, when it is less than 200^°C, may be tolerated. For completeness, when the combination of

piezoelectric element, thermoset adhesive layer and delay line is maintained at a temperature of less than the temperature at which the adhesive has been cured, then, as a result of the differing coefficients of thermal expansion, the piezoelectric element will typically be retained under compression, but that is acceptable because, as stated above, the ceramic materials of which they are made have a high compressive strength.

Advantageously, thermosetting adhesive comprised within the first adhesive layer has a glass transition temperature, Tg, which is greater than the maximum intended operating temperature of the substrate, TMAX- The reasons for this are explained hereinbelow in relation to the second adhesive layer. In practice, to minimise the number of variants of ultrasonic transducer that have to be manufactured, the first adhesive layer usually comprises thermosetting adhesive having a Tg which is greater than 200°C and is advantageously greater than 210°C.

Preferably, the first adhesive layer is thin in order to minimise acoustic reflection and absorption by the first adhesive layer, but it may not be too thin or excessive stresses may build up within the first adhesive layer and a sufficiently strong adhesive bond may not be achieved. The first adhesive layer advantageously has a thickness of less than or equal to 100 microns, preferably less than or equal to 50 microns, but more than or equal to 3 microns.

The skilled person is aware of adhesives, especially epoxy adhesives, suitable to be comprised within the first adhesive layer and which fulfil the above conditions.

The transducer comprises the piezoelectric element and standard electric circuitry, usually on a printed circuit board (PCB). All elements of the transducer are configured to be operable at the intended temperature of operation of the ultrasonic thickness gauge, preferably from -20°C to 200°C.

Advantageously, the volume within the transducer not occupied by functional elements, such as the piezoelectric element, the PCB and connecting wires, is filled with an encapsulant, to prevent the ingress of explosive materials during use. Such materials are known to the person skilled in the art and may include thermoset urethane, epoxy or silicone polymers. Optionally, a small volume within the transducer is not filled with encapsulant, to allow for thermal expansion of the contents of the transducer.

Dry coupling of the waveguide via a clamp which is brazed or welded to the substrate, as the authors of the Cegla Paper propose, may result in ineffective acoustic coupling, as a result of the degrading amplitude of the acoustic signal due to a change in the external properties of the interface between the waveguide and the pipe which is exposed to the elements. This may confuse automated measurement systems. Furthermore, welding is a dangerous activity when carried out in an explosive environment, such as a refinery; it requires time-consuming and costly safety measures and monitoring procedures to be followed and, in extreme cases, may even require a plant shutdown, which is extremely expensive.

Advantageously, the delay line is attached to the substrate by means of a second adhesive layer. More advantageously, adhesive comprised within the second adhesive layer is thermosetting adhesive which cures in situ as a result of the innate temperature of the substrate, thereby avoiding further time-consuming and costly measures to effect attachment.

As the temperature of an adhesive increases to its glass transition temperature (Tg) the adhesive generally changes from a high modulus, glassy state to that of a soft, rubbery state. In the high modulus, glassy state, the speed of sound in the adhesive is generally about 3000m/s, but that can drop to well below 1000m/s in its rubbery state. Assuming that the adhesive layer is acceptably thin, as discussed above, then, if it is in its glassy state, there is minimal acoustic reflection or absorption from the extremely thin adhesive layer. As the adhesive is heated above its Tg, however, the reflection and absorption of the acoustic signal increases sharply and a reduced amount of acoustic energy is able to enter the substrate, reducing the size of the BWE and increasing the relative magnitude of the noise detected by the

measurement system. Furthermore, at the glass transition temperature the CTE of an adhesive increases rapidly, creating high internal stresses within the adhesive joint, which may lead to cohesive or adhesive bond failures.

As a result, it is preferred that adhesive comprised within the second adhesive layer has a high Tg and to ensure that adhesive is not exposed to temperatures above its glass transition temperature. The Tg of adhesive comprised within the second adhesive layer should always be selected to be above the maximum operating temperature, TMAX, of the substrate. Advantageously, the Tg of the adhesive comprised within the second adhesive layer is above 150^°C, preferably above 175C and more preferably above 200^°C. The skilled person is aware of adhesives, especially epoxy adhesives, suitable to be comprised within the second adhesive layer and which fulfil the above conditions.

Preferably, the second adhesive layer is thin in order to minimise acoustic reflection and absorption by the second adhesive layer, but it may not be too thin or excessive stresses may build up within the second adhesive layer and a sufficiently strong adhesive bond may not be achieved. The second adhesive layer advantageously has a thickness of less than or equal to 100 microns, preferably less than or equal to 50 microns, but more than or equal to 3 microns.

Many high performance adhesives comprise a particulate filler to modify the physical and/or chemical properties of the adhesive. Examples of such particulates include glass bubbles, milled carbon fibre and aluminium trihydrate. Measurements have shown, however, that the presence of such particulates may attenuate or reflect acoustic energy in the frequencies of interest, thus lowering the amplitude of the BWE and decreasing the signal to noise ratio of the system. Additionally, the typical filler particle sizes may hinder or prevent the attainment of an acceptably thin bond layer (usually on the order of tens of microns, as mentioned above). A thicker adhesive layer may further increase unwanted absorption and reflection of acoustic energy. It is therefore preferred to employ adhesives which do not comprise particulates, such as a filler, especially filler comprising glass bubbles, milled carbon fibre and aluminium trihydrate.

In order to avoid the high cost of plant shutdown, the ultrasonic thickness gauge of the invention should advantageously be attachable to the substrate in question while the plant is running. High glass transition temperature adhesives require exposure to high temperatures to cure the adhesive, preferably temperatures not significantly less than that of the glass transition temperature. Achieving this on cool plant, such as industrial pipework, is challenging, because the high thermal conductivity and high thermal mass of the pipework renders it difficult to bring to the appropriate temperature. Additionally, operators want to be able to install many tens of units per man-day, requiring that the thickness gauges bond onto the pipe rapidly. Use of high glass transition temperature adhesives advantageously allows bonding of the ultrasonic thickness gauge to the substrate, such as pipelines tanks and vessels, when the plant is operational and at the plant's operational temperature, avoiding the cost of shutdown.

The skilled person is aware of adhesives, especially epoxy adhesives, suitable to be comprised within the second adhesive layer and which fulfil the above conditions.

The ultrasonic thickness gauge according to the first aspect of the invention is configured to be permanently attached to a substrate in a remote location, such as in a refinery or chemical plant. Advantageously, the gauge additionally comprises a wireless transmitter/receiver (transceiver) configured to receive data and instructions and to transmit data wirelessly. The details of suitable wireless systems are known to the skilled person. For industrial applications, a suitable wireless system would beneficially have the following features: a self-organising 'ad-hoc' network (to overcome obstacles and dead-spots caused by large metallic structures), low power (long battery life), operate in a licence free band (commonly termed an ISM

(Industrial, Scientific and Medical) band, encrypt the data (for privacy), be secure (so the network cannot be hacked), have transmission ranges compatible with the likely transceiver spacing and distance to the gateway (base station).

The invention will now be described, by way of example only, with reference to the accompanying Figure 1. Figure 1 illustrates a transducer (1 ) comprising a piezoelectric element (2), a printed circuit board (PCB) (3) connected to the piezoelectric element (2) by means of wires (4). The piezoelectric element (2) is adhered to the first surface of a delay line (5) by means of a layer of adhesive (6) and the second surface of the delay line (5) is adhered to a substrate (7) by means of a further layer of adhesive (8). A wireless transmitter/receiver (9) is additionally shown for wirelessly receiving data and instructions and for wirelessly transmitting data. It is connected to the transducer (1 ) via cable (10).

According to a second aspect of the invention, an arrangement comprising a substrate, especially a pipeline, tank or vessel, and an ultrasonic thickness gauge according to the first aspect of the invention is provided, wherein the ultrasonic thickness gauge is adhered to the substrate by means of a second adhesive layer.

According to a third aspect of the invention, an arrangement comprising a substrate, especially a pipeline, tank or vessel, suitable to be operated at a maximum operating temperature, TMAX, and an ultrasonic thickness gauge is provided, the ultrasonic thickness gauge comprising:

(b) a delay line having a first surface attached to the piezoelectric element and a second surface attached to the substrate;

wherein the second surface is adhered to the substrate by means of an adhesive layer and wherein the adhesive layer comprises thermosetting adhesive having a glass transition temperature which is greater than TMAX-

Advantageously, according to the third aspect of the invention, the material of the delay line comprises ceramic, glass or mixtures thereof and preferably comprises borosilicate glass, fluorophlogopite mica, or mixtures thereof.

Preferably, according to the third aspect of the invention, the delay line has a circular cross-section with a diameter in the range from 10mm to 45mm, preferably 15mm to 40mm, and a length, L, which is the distance between the first and second surfaces, which is more than 10mm and less than 100mm, preferably less than 75mm, more preferably less than 50mm.

Highly, advantageously, according to the third aspect of the invention, the thermosetting adhesive has a glass transition temperature which is greater than or equal to 150^°C, preferably greater than or equal to 175C, more preferably greater than or equal to 200^°C and the adhesive layer has a thickness of more than or equal to 3 microns and less than or equal to 100 microns, preferably less than or equal to 50 microns. According to a fourth aspect of the invention, a kit comprising an ultrasonic thickness gauge according to the first aspect of the invention and a thermosetting adhesive are provided, the thermosetting adhesive having a glass transition temperature which is greater than or equal to 150^°C, preferably greater than or equal to 175C, more preferably greater than or equal to 200^°C.

According to a fifth aspect of the invention, a process is provided for attaching the ultrasonic thickness gauge of the first aspect of the invention to a substrate having a maximum operational temperature, TMAX, the process comprising the steps of:

(a) applying a second adhesive layer to the substrate the second adhesive layer comprising adhesive having a glass transition temperature, Tg, which is greater than T AX;

(b) bringing the ultrasonic thickness gauge into contact with the second adhesive layer so as to cure the adhesive and adhere the ultrasonic thickness gauge thereto.

Example

An exemplary, non-limiting example of an ultrasonic thickness gauge comprises:

(a) A transducer comprising a piezoelectric element made of lead metaniobiate and having a diameter of 10-13mm;

(b) A delay line adhered to the piezoelectric element by means of first adhesive layer which is from 3 to 50 microns in thickness comprising a thermosetting, epoxy adhesive having a Tg of greater than 200°C (suitable adhesives are known to the skilled person), the delay line being made of a rod of Macor® having a constant, circular cross-section with a diameter of 25mm and a length of 27mm.

The ultrasonic thickness gauge is attached to a steel pipe with a wall thickness of 25mm by means of a second adhesive layer comprising a thermosetting, epoxy adhesive having a Tg between 200 and 210°C which is from 3 to 50 microns in thickness. In use for 1 year, at temperatures of 200°C and at an excitation frequency of 5MHz, no damage to the piezoelectric crystal is observed and there is no rupture at the interface between the substrate and the delay line or at the interface between the delay line and the piezoelectric crystal. Furthermore, the guided wave peak amplitude observed is less than 5% of the magnitude of the FWE.

Claims

1 . An ultrasonic thickness gauge for measuring the thickness of a hot substrate and configured to be permanently installed on the substrate, the ultrasonic thickness gauge comprising:

2. The ultrasonic thickness gauge of claim 1 , wherein the material of the delay line comprises ceramic, glass or mixtures thereof and preferably comprises borosilicate glass, fluorophlogopite mica, or mixtures thereof.

3. The ultrasonic thickness gauge of claim 1 or 2, wherein the delay line is attached to the transducer by means of first adhesive layer comprising a thermosetting adhesive.

4. The ultrasonic thickness gauge of claim 3, wherein the thermosetting adhesive has a curing temperature which is greater than or equal to 140°C, preferably greater than or equal to 150°C and less than or equal to 200°C.

5. The ultrasonic thickness gauge of claim 4, wherein the piezoelectric element is retained under compression at temperatures below the curing temperature of the adhesive comprised within the first adhesive layer.

6. The ultrasonic thickness gauge of any one of claims 3 to 5, wherein the thermosetting adhesive has a glass transition temperature, Tg, which is greater than the maximum intended operating temperature of the substrate, TMAX, and wherein Tg is preferably greater than 200°C.

7. The ultrasonic thickness gauge of any preceding claim, wherein the delay line has a circular cross-section with a diameter in the range from 10mm to

45mm, preferably 15mm to 40mm.

8. The ultrasonic thickness gauge of any preceding claim, wherein the delay line has a length, L, which is the distance between the first and second surfaces, which is more than 10mm and less than 100mm, preferably less than 75mm, more preferably less than 50mm.

9. The ultrasonic thickness gauge of any preceding claim, wherein the piezoelectric element comprises ceramic material which remains stably polarised at temperatures from -20C to 200^°C.

10. The ultrasonic thickness gauge of any preceding claim, wherein the piezoelectric element comprises lead metaniobate.

1 1 . The ultrasonic thickness gauge of any preceding claims, additionally comprising a wireless transceiver connected to the transducer and configured to receive instructions via a wireless network and transmit signals to the transducer and to receive signals from the transducer and to transmit the data to a receiver elsewhere in the wireless network.

12. An arrangement comprising a substrate, especially a pipeline, tank or vessel, suitable to be operated at a maximum operating temperature, TMAX, and an ultrasonic thickness gauge according to any one of the preceding claims, wherein the ultrasonic thickness gauge is adhered to the substrate by means of a second adhesive layer.

13. The arrangement of claim 12, wherein the second adhesive layer comprises thermosetting adhesive having a glass transition temperature, Tg, which is greater than TMAX-

14. The arrangement of claim 13, wherein Tg is greater than or equal to 150^°C, preferably greater than or equal to 175C, more preferably greater than or equal to 200^°C.

15. The arrangement of any one of claims 12 to 14, wherein the second adhesive layer has a thickness of more than or equal to 3 microns and less than or equal to 100 microns, preferably less than or equal to 50 microns.

16. An arrangement comprising a substrate, especially a pipeline, tank or vessel, suitable to be operated at a maximum operating temperature, TMAX, and an ultrasonic thickness gauge, the ultrasonic thickness gauge comprising:

17. The arrangement of claim 16, wherein the material of the delay line comprises ceramic, glass or mixtures thereof, and preferably comprises borosilicate glass, fluorophlogopite mica, or mixtures thereof.

18. The arrangement of claim 16 or 17, wherein the delay line has a circular cross-section with a diameter in the range from 10mm to 45mm, preferably 15mm to 40mm.

19. The arrangement of any of claims 16 to 18, wherein the delay line has a length, L, which is the distance between the first and second surfaces, which is more than 10mm and less than 100mm, preferably less than 75mm, more preferably less than 50mm.

20. The arrangement of any of claims 16 to 19, wherein the thermosetting adhesive has a glass transition temperature which is greater than or equal to 150^°C, preferably greater than or equal to 175C, more preferably greater than or equal to 200^°C.

21 . The arrangement of any of claims 16 to 20, wherein the adhesive layer has a thickness of more than or equal to 3 microns and less than or equal to 100 microns, preferably less than or equal to 50 microns.

22. A kit comprising an ultrasonic thickness gauge according to any one of claims 1 to 1 1 and a thermosetting adhesive having a glass transition temperature which is greater than or equal to 150^°C, preferably greater than or equal to 175C, more preferably greater than or equal to 200^°C.

23. The kit of claim 22, additionally comprising an instruction to apply a layer of the adhesive having a thickness of 3 microns or more and less than or equal to 100 microns, preferably less than or equal to 50 microns, to the substrate, then to adhere the ultrasonic thickness gauge thereto.

24. A process for attaching the ultrasonic thickness gauge of claims 1 to 1 1 to a high temperature substrate having a maximum operating temperature, TMAX, comprising the steps of:

(a) applying a second adhesive layer to the substrate the second adhesive layer comprising thermosetting adhesive having a glass transition temperature, which is greater than TMAX_, the temperature of the substrate being sufficient to cure the thermosetting adhesive; (b) rapidly bringing the ultrasonic thickness gauge into contact with the second adhesive layer so as to cause adhesion of the ultrasonic thickness gauge thereto as the thermosetting adhesive cures.

25. The process of claim 24, wherein the layer of adhesive has a thickness of greater than or equal to 3 microns and less than or equal to 100 microns, preferably less than or equal to 50 microns.