WO1993004504A1

WO1993004504A1 - Thermocouple temperature sensor

Info

Publication number: WO1993004504A1
Application number: PCT/AU1992/000382
Authority: WO
Inventors: Noel Arthur Burley
Original assignee: Nicrobell Pty. Limited
Priority date: 1991-08-16
Filing date: 1992-07-27
Publication date: 1993-03-04

Abstract

A thermocouple suitable for use at high temperatures in a hostile environment such as molten glass, comprising thermocouple conductors held within a thermocouple sheath, is contained within a protection tube which comprises a ceramic compound which may be a conventional ceramic compound or a fine structural ceramic compound. The protection tube may be coated with a layer of a protective material selected from one or more of the group consisting of a rare metal, a rare metal alloy, a refractory metal oxide, and a metal aluminide.

Description

THERMOCOUPLE TEMPERATURE SENSOR

This invention relates to thermocouples, thermocouple structures, thermocouple sheathing, and to protection devices containing a thermocouple. In one embodiment this invention relates to a novel improved design and structure of a thermocouple intended for application at high temperatures above, say 1000°C. In another embodiment this invention relates to high-temperature structural fine ceramics for use in the manufacture of thermocouple structures. In a further embodiment this invention relates to novel improved thermocouple structures intended specifically for use in the measurement of the temperatures of hot molten glass in the manufacture of a wide range of glass materials, products and components. The thermocouples of the invention are primarily of the mineral-insulated integrally metal-sheathed (MIMS) structure.

Brief Description of the Figures The invention is illustrated by reference to the drawings, in which:

Figure 1 represents a longitudinal section of a typical prior art thermocouple of the Pt-Rh/Pt type;

Figure 2 represents a perspective view of a conventional MIMS-type thermocouple;

Figure 3 is a bar graph comparing modulus of rupture for four different ceramic materials; and

Figure 4 is a diagrammatic representation of a longitudinal section of a thermocouple according to this invention.

Background of the Invention

The measurement of the temperature of hot molten glass presents singular technical and economic difficulties due to a range of diverse factors. These factors include the very high industrial temperatures involved, the high viscosity and abrasiveness of flowing molten glass at these temperatures, the chemical reactivity of both the glass itself and also of the combustion atmosphere in which it is heated, and the high cost of the rare-metal materials of construction of the conventional thermocouple sensor structures presently employed.

No satisfactory solution to this temperature measurement problem, which at the same time is both technically feasible and economically acceptable, has hitherto been found and proved reliable in extended practice.

The term "glass" can be used to describe many substances which possess the physical characteristics of a liquid but the rigidity of a solid. Glass, like most liquids, has a random molecular structure particularly when hot. When most liquids solidify or "freeze" these molecules normally are regimented into precise crystallographic arrays. In the manufacture of glass, during subsequent cooling, this freezing does not take place; the viscosity simply rises with falling temperature to a stage where the molecules of the super-cooled liquid cannot move to form a regular crystal structure. Even when still very hot, say at 1200°C, the viscosity of the flowing molten mass remains relatively quite high, and it can exert a significant force on any object located in its path. This includes elongated cylindrical objects such as sheaths housing thermocouples. Thus a thermocouple sheath material capable of resisting the high bending moment forces exerted by the hot flowing glass must exhibit high values of strength, toughness and creep resistance.

In all sectors of the glass manufacturing industry, temperature measurement and control is of the utmost importance. With temperature change, glass viscosity and hence flow rate can vary significantly. When such temperature changes are neither intended nor apprehended, subsequent control of glass forming and processing operations is impaired. Gob size and homogeneity, for example, are of prime importance particularly where crystal glass is involved but also in the case of common bottle glass.

The use of standard alumina-sheathed rare-metal thermocouples of the platinum-rhodium versus platinum (Pt- Rh/Pt) variety, designated type R (13 wt-% Rh) or type S (10 wt-% Rh) by the Instrument Society of America (ISA) , for measuring furnace crown temperatures has for some time been common practice. However, such types R and S conventional thermocouple assemblies show inherent shortcomings because of several factors, such as:

(i) They show inherent lack of strength, due to the relative brittleness of the recrystallized alumina sheath, particularly where transverse forces are involved, (ii) The hot furnace atmosphere above the molten glass contains exhaust gases from the combustion of the fuel and vapour phases from the glass, both of which can chemically attack the alumina sheath.

In both cases above, penetration of the sheath by glass and/or gas, either when fractured as in (i) or corroded as in (ii) , can cause contamination of, and consequent intolerable loss of measurement accuracy, in the Pt-Rh/Pt thermocouple. (iii) The relatively high cost of rare-metal thermocouples compared with base-metal thermocouples.

The above problems are being increasingly exacerbated by the burgeoning number of special refractory materials being used in glass furnace linings and by some of the more exotic glass additives, both of which can be chemically incompatible with alumina and platinum and its alloys. The more recent use of platinum, or a zirconia grain- stabilised platinum-rhodium alloy, as a sheathing material for alumina-sheathed Pt-Rh/Pt thermocouples, in the form of an elongated thimble, has proved to be a fairly successful move in overcoming some of the technical problems described above.- However such thimbles, to be effective, must be of the order of 0.5 to 0.8 mm in wall thickness and are consequently very expensive.

A typical such thermocouple is illustrated in Figure 1, in which the following features are identified:

1.1. Thermocouple headcover

1.2. Connector head

1.3. Conductor terminals

1.4. "Fish-spine" insulators 1.5. Inconel (INCO Group trade mark) heat- shield tube

1.6. Pure recrystallized alumina thermocouple sheath

1.7. Thermocouple conductor wires 1.8.. Platinum sheath ("thimble")

1.9. Heat-resistant rope

1.10. Heat-resistant washers

1.11. Six-bore recrystallized alumina thermocouple insulator tube 1.12. Adaptor

1.13. Cable gland

1.14. Thermocouple extension wires

The number of failures of alumina-sheathed rare-metal thermocouples, due to mechanical stress and/or chemical contamination, is reportedly reduced by the use of such thimbles . This has been the case particularly where the temperature measurement and control arrangements involve the use of thermocouples actually immersed in the hot molten glass in the furnace, forehearth and feeder channels. Such sensor assemblies may contain multiple thermocouples having measuring-thermojunctions so arranged as to enable temperatures to be measured at various depths in the glass (see Figure 1) . The high temperatures and aggressive glass conditions make it necessary for alumina- sheathed Pt-Rh/Pt thermocouples to be protected by Pt or Pt-alloy thimbles below the glass line.

The glass thermocouple assembly described above, whilst being the best technical solution so far developed to the problem of accurately knowing hot glass temperatures, yet has a number of residual shortcomings which remain problematic in glass manufacturing technology. These problems arise because of factors such as:

(a) The Pt-Rh/Pt thermocouples and, particularly, the Pt- alloy thimbles are becoming increasingly and prohibitively more expensive.

(b) Even though alloying platinum by rhodium addition makes for a stronger thimble, rhodium likewise is most expensive and tends to unacceptably colour the glass under certain conditions.

(c) It is essential to avoid this colouring problem, (b) above, by omitting rhodium from the thimble, where high quality optical or crystal glasses are concerned. Unfortunately this introduces the further problem that molten glass may 'wet' the surface of pure platinum. This wetting phenomenon causes glass to adhere to the metal, thus further decreasing the already poor flow characteristics of the hot glass past the thermocouple sensor probe; it also reduces the thermal response sensitivity of the sensor.

(d) Reducing this wetting problem, (c) above, which can be done by alloying platinum by gold addition, only produces a thimble of lower melting point and lesser strength.

(e) All the above platinum-group metals and alloys exhibit softening (and thus reduced erosion resistance of thimbles to the flowing glass) with the effluxion of time at high temperature. This causes consequent reduction in strength due to recrystallization and grain growth. In addition, in the case of both thermocouple conductor wires and thimbles, unacceptable deformation occurs by high-temperature creep. The resultant limitation of life of the expensive thimbles can be reduced by the use of zirconia grain-stabilized Pt-base alloys, but only with the significant penalty of a further increase in an already prohibitive cost. (f) To minimise the high cost of glass thermocouples incorporating Pt-alloy thimbles and thermocouples, it is common practice to attach a shortened platinum thimble to a co-linear extension tube fabricated from a conventional base-metal alloy like Inconel (see Figure 1) . This extension tube passes up through the combustion gas space above the glass line and through the wall or ceiling of the furnace to the cooler ambient environment outside. Conventional base-metal alloys (like Inconel) , characterized by an optimized combination of lower cost and oxidation resistance, often are inadequate. The extension tube may even fail prematurely by high-temperature corrosion, particularly near the junction with the rare-metal thimble. Thus the life of the whole thermocouple assembly can be terminated prematurely.

As is well known in the art, the manufacture of MIMS cable or of individual MIMS thermocouple sensor structures begins with matched thermocouple wires surrounded by non- compacted mineral oxide powder held within a metal tube. By rolling, drawing, swageing, or other mechanical reduction processes, the tube is reduced in diameter by the required amount and the insulation is compacted around the wires. The conventional product of MIMS structure is illustrated diagrammatically in Figure 2. In Figure 2, which shows a perspective view of the conventional materials of construction, 2.1 is the integral sheath, usually made of stainless steel or Inconel; 2.2 is the mineral insulation, usually a mixture of mineral oxides essentially of about 96 wt.-% MgO and 4 wt.-% Si0₂; and 2.3 are the thermoelement conductor wires usually of the ISA (Instrument Society of America) type K variety.

Unfortunately, the conventional design concept of the MIMS thermocouple is not suited to the measurement of hot molten glass temperatures in the manufacture of glass products. This is because:

(i) The conventional sheath materials, particularly the stainless steels, will not withstand exposure in molten glass and in the associated combustion atmosphere at the temperatures involved (up to about 1200°C) . (ii) The conventional thermocouple conductor wires - ISA type K - will likewise not withstand the highest temperatures and longest times encountered in the glass industry.

In the cases of both (i) and (ii) above excessive high- temperature corrosion, mainly oxidation causing premature failure, is the reason for the unsuitability of the alloys.

(iii) The thermoelement conductor wires can be contaminated by chemical elements which thermally diffuse through the compacted insulant material from dissimilar sheath alloys. The resultant changes in the chemical compositions of the conductor alloys can cause substantial changes in their thermoelectromotive forces. Such changes in thermal emf are analogous with and algebraically additive to those caused by the high-temperature oxidation of these alloys, (iv) The thermoelement conductor wires, particularly the negative wire, may fail mechanically because of substantial alternating strains imposed during thermal cycling. These strains are caused primarily by longitudinal stresses which arise because of substantially different temperature coefficients of linear expansion of the thermoelements and of dissimilar sheath materials.

It is clear from the foregoing that the principal problem in the measurement of high temperatures using a thermocouple of conventional MIMS construction is thermoelectric instability, hence measurement uncertainty. It is equally clear that this thermal emf instability results primarily from the use of dissimilar and unsuitable alloys for both sheath and thermoelement conductors. This problem has arisen because sheath and thermoelement materials have hitherto been chosen independently of each other to match, respectively, the environment of exposure and the calibration of existing pyrometric instrumentation. In contrast to the prior art, the MIMS thermocouple of the present invention has been designed as a truly integral system. The choice of materials for its principal components - sheath, thermocouple, insulant, and filling gas - has been made only after a proper consideration of the inter-related properties of all of them. A discussion of how the problems which plague MIMS thermocouples of conventional design can best be overcome has been given by the present inventor in Australian Patent Applications 80105/87 of 23rd October, 1987, and 12149/88 of 19th February, 1988. The above specifications disclose a novel MIMS cable incorporating -

(a) novel thermoelement conductor alloys known as NIOBELL- P (positive) and NIOBELL-N (NIOBELL is a trade mark of Nicrobell Pty. Ltd.) or, alternatively, standard thermoelement conductors which are ISA type N alloys .

(b) a novel series of sheath alloys known as NICROBELL (trade mark of Nicrobell Pty. Ltd.) of improved thermomechanical and thermochemical properties over conventional MIMS sheath alloys.

The NICROBELL-sheathed MgO-insulated NIOBELL P/N (or type N) MIMS thermocouple cable is the subject of the above-mentioned patent specifications 80105/87 and 12149/88. These specifications set out the conceptual nature and inventive rationale of the above type N MIMS systems; they do not, however, make any reference to the specific thermocouple structures nor the thermocouple sheathing and protection devices for molten glass temperatures which are claimed as novel in the present specification. For all these reasons, in particular the prohibitive cost of the conventional glass thermocouple described above, it is essential that novel thermocouple concepts, designs and structures be introduced. One alternative approach, described in our earlier patent application No. WO-88/02106, provides a MIMS thermocouple in which the sensor sheath is made of NICROBELL, FECRALY, NICRALY or COCRALY base metal alloy, and the sheath is coated with a refractory metal oxide or compound which is overcoated with a layer of platinum or platinum alloy. However, the thermocouples of this specification still require precious metal, and therefore suffer a cost disadvantage. The present invention therefore seeks to provide a thermocouple for use at high temperature which reduces or avoids the use of precious metals.

SUMMARY OF THE INVENTION The invention provides a thermocouple suitable for use at high temperatures in a hostile environment such as molten glass, comprising thermocouple conductors held within a thermocouple sheath, wherein the thermocouple is contained within a protection tube which comprises a ceramic compound having an appropriate combination of intrinsic strength, toughness, and chemical stability at high temperature.

The novel glass-temperature thermocouple of this invention may be of the integrally metal-sheathed mineral- insulated (MIMS) format and structure. The MIMS format, which features improved materials and structure, enables optimal achievement of the necessary performance characteristics. In one aspect, the present invention provides a mineral-insulated, metal-sheathed (MIMS) thermocouple suitable for use at high temperatures in a hostile environment such as molten glass, comprising thermocouple conductors surrounded by mineral insulation held within a thermocouple sheath, wherein the MIMS thermocouple is contained within a protection tube which comprises a fine structural ceramic compound having high intrinsic strength, toughness, and chemical stability at high temperature. Optionally the protection tube is coated with a layer of a protective material selected from the group consisting of a rare metal, a rare metal alloy, a refractory metal oxide, or a metal aluminide. A combination of two or more of these protective materials may be used. Optionally there is a layer of compacted ceramic oxide insulant between the thermocouple and the protection tube.

Preferably the fine structural ceramic compound is selected from the group consisting of silicoaluminoxy- nitrides (Sialons), amorphous alumina, partially stabilized zirconia (PSZ) , silicon nitride, and silicon carbide. Most preferably the ceramic compound is a Sialon, PSZ, or silicon nitride. In some circumstances where the conditions obtaining in the molten glass during manufacture are not overly severe, for example where glass temperatures and flow rates are not excessively high, a less preferred option involves the use of conventional ceramic compounds such as pure recrystallised alumina for thermocouple protection tubes.

Preferably the MIMS thermocouple sheath is made of the nickel-base alloy NICROBELL.

Preferably the thermocouple conductor alloy is selected from the group consisting of the nickel-base alloys NIOBELL and type N alloys. In an alternative aspect, the invention provides for use in particularly demanding conditions a thermocouple (which may be MIMS) of the rare metal type, selected from the group consisting of ISA types R, S or B, wherein the thermocouple sheath is contained within a protection tube which comprises a fine structural ceramic compound as defined above.

The MIMS thermocouple system and specific sensor structures of the present invention are very well suited to glass-temperature thermocouple sensors. The specification claims specific concepts, designs and structures of novel glass-temperature thermocouples.

In particularly preferred embodiments, the new concepts of the present invention feature relatively inexpensive base-metal alloys for the thermocouple conductors and structural fine ceramic materials for the protection tubes.

The ceramic sheath materials chosen show an optimum combination of strength, toughness and high chemical stability at the high-temperatures and under the aggressive environmental conditions prevailing within the glass furnace during the manufacturing processes involved.

Furthermore, the base-metal thermocouple incorporated in the design and structure of the novel thermocouple sensor shows ultra-high thermoelectric stability, such as is exhibited by rare-metal Pt-alloy thermocouples, over the range of temperatures involved.

It is conceivable that, under the most extreme conditions of high temperatures and corrosive atmospheres that can be encountered in glass furnaces, the NIOBELL and type N thermocouple alloys and the protective sheath alloys NICROBELL may not provide, per se, the very high environmental stability and longevity that would be demanded for the longest of the glass furnace campaigns that can be envisaged. In this case one can revert to the use of standard types R or S rare-metal thermocouples, or the newer ISA type B system (Pt-30Rh/Pt-6Rh) which has improved emf-stability and strength over types R and S. Whichever type of thermocouple is chosen to match the relevant conditions, it is essential that it be protected from the mechanical and chemical ravages of the glass furnace environment by a protection tube material which shows the required high values of thermomechanical properties and resistance to high temperature corrosion that would be demanded.

Such a protection tube material is one of the family of newly developed fine structural ceramic compounds. These materials have intrinsic high strength and toughness at high temperatures as well as high chemical stability. Examples of these fine structural ceramics include amorphous alumina A1.0₃, partially stabilised zirconia Y₂0₃- Zr0₂ (commonly known as PSZ) , silicon nitride Si₃N₄, silicon carbide SiC, and the silicoaluminoxynitrides Si_aAl_bO_cN_d (commonly known as 'Sialon'). Of these compounds Y₂0₃-

Zr0₂, Si₃N₄ and Sialon are particularly suitable for use in the present invention.

PSZ shows high values of strength and toughness because of the presence of small quantities of oxides such as CaO and Zr0₂ which stabilise the high-temperature cubic crystal structure. By appropriate heat-treatment, strengths can be increased up to about 700 MPa. si.N₄ has high strength over a wide temperature range, good thermal shock resistance, and high resistance to wear and corrosion. Components consisting of silicon nitride are more resistant (than metals and oxide ceramics such as alumina) to high temperatures, thermal shock, erosion and chemical degradation. A structural ceramic material particularly preferred for the requirements of the present invention is Sialon. This material is, in fact, a chemical 'alloying' of some of the other ceramic compounds mentioned above. The phase relationships of the ceramic 'alloy' system

Si₃N₄-Si0₂-Al₂0₃-AlN (refer K.H. Jack, J. Mat. Sci, 11, (1976) 1135) reveal a compositional zone of general formula Si₆__χAl_χO_χN₈__χ known as beta-Sialon. Although there are other Sialon-type compounds possible, most effort has been focussed on the fabrication of beta-Sialon because of its superior creep and oxidation resistance properties at high temperature.

Many useful engineering shapes, such as the closed-end tubes required in the present invention, can be fabricated by mixing and sintering a suitable powder mix of the above ceramic constituents . The formation of the beta- Sialon compound takes place on sintering at 1600° to 1800°C. A selection of relevant physical and mechanical properties at room temperature (after N.E. Cother and P. Hodgson, Trans. J. Brit. Cera . Soc. 81 (1982), 141) are given in the following table - Mechanical properties Modulus of rupture (MPa) Tensile strength (MPa) Hardness (VHN, GPa) Fracture toughness (MN/m³²) Thermal Properties Thermal expansion coefficient (1/K) Specific heat (J/g.K) Thermal conductivity (J/cm.sec.K) Thermal shock resistance (ΔT) Electrical resistivity (ohm.cm)

These properties, together with the known resistance of Sialon to chemical attack at high temperatures by most non-ferrous metals, slags, fused salts and hot molten glass, make it a most suitable protection sheath material for the present invention. The superiority of Sialon over other structural ceramics with reference, by way of example, to one particular property, is illustrated in Figure 3. In Figure 3, Sialon refers to Syalon 101 (a trade mark of Lucas Cookson Ltd.)

Example 1

The integral compacted MIMS thermocouple sensor of this example is fabricated using existing manufacturing procedures. They begin with thermoelectrically matched thermoelement wires fabricated in the form of a tri-level thermocouple (of the NIOBELL or ISA type N variety) surrounded by non-compacted ceramic oxide insulating powder held within a metallic alloy tube of NICROBELL alloy. By rolling, swageing, drawing, or other suitable mechanical reduction processes the alloy tube may be reduced in diameter until the insulation powder is compacted around the thermocouple wires. The manufacturing process parameters are adjusted so that the ratios of sheath diameter to wire-size and to sheath-wall thickness offer an optimal balance between minimum wall-thickness for adequate life and for strength, and also suitable insulation spacing for effective insulation resistance at elevated temperatures. A most important feature of the fabrication process is that considerable attention is given to the initial cleanliness and chemical purity of the components and to retention of a high degree of cleanliness and dryness particularly of the insulant throughout fabrication.

A diagrammatic conceptual illustration of this embodiment is given in Figure 4. In Figure 4 the following features are identified.

4.1. Thermocouple headcover, connector head, conductor terminals, etc. as individually identified in Figure 1.

4.2. Tri-level thermocouple assembly.

4.3. Sensor sheath of a structural ceramic.

4.4. Compacted ceramic oxide insulant powder. (optional)

4.5. Molten glass/combustion gas interface. T. Top measuring-thermo unction (of "tri-level" group of three) . M. Middle thermo unction. B. Bottom thermojunction.

Example 2

In this example, the mode of manufacture and design of the thermocouple sensor, per se, is the same as in Example 1.

In this case, which is germane to the most extreme conditions of high temperatures and corrosive atmospheres that can be encountered in a glass furnace, the structural ceramic sheathing tube (item 3 in Figure 4) may not provide the very high environmental stability and longevity that would be demanded for the longest of the glass furnace manufacturing campaigns that can eventuate. This problem is overcome by applying a suitable relatively thin but strongly adherent coating to the protection tube of a refractory metal, alloy, or chemical compound which will satisfactorily withstand the extreme conditions described above. Suitable coatings, applied singly or in combination, include an optimal thickness of a rare metal, a rare metal alloy, a refractory metal oxide such as alumina, or a metal aluminide, etc. The thickness of such a metal coating is much less, of the order of one- tenth, of that required for the corresponding thimble of the prior art. Other types of protective coating would also be suitable, and would be known to the person skilled in the art.

In this example, the methods of coating deposition would include electrodeposition from aqueous solutions or fused salts, vacuum or air thermal plasma spraying, or other thermal spraying, physical or chemical vapour deposition, etc. Other suitable deposition processes will be known to the person skilled in the art.

Example 3

In this example the general mode of manufacture and design of the thermocouple sensor is the same as in Example 1 or Example 2. In this case, however, which is also germane to the very most extreme conditions of high temperatures and extended production campaigns that can be encountered in a glass furnace, it is necessary to revert to the use of standard ISA types B, R or S thermocouples. Here it is not feasible to compact the mineral insulant by a tube reduction process, but rather an optional process such as tamping is employed to avoid damage to the rare-metal thermocouple assembly.

Example 4 In this example the general mode of manufacture and design of the thermocouple sensor is the same as in Example 1, Example 2, or Example 3.

In this case, however, which is germane to the operating conditions that apply in general in a glass melting furnace, the thermocouple protection tube chosen is fabricated from a group comprising conventional ceramic compounds such as pure recrystallised alumina.

It will be clearly understood that the invention in its general aspects is not limited to the specific details referred to hereinabove.

Claims

CLAIMS :

1. A thermocouple suitable for use at high temperatures in a hostile environment such as molten glass, comprising thermocouple conductors held within a thermocouple sheath, wherein the thermocouple is contained within a protection tube which comprises a ceramic compound having an appropriate combination of intrinsic strength, toughness, and chemical stability at high temperature.

2. A thermocouple according to claim 1 in which the ceramic compound is a conventional ceramic compound such as pure recrystallised alumina.

3. • A thermocouple according to claim 1 in which the ceramic compound is a fine structural ceramic compound.

4. A thermocouple according to claim 3 in which the fine structural ceramic compound is chosen from one or more of the group consisting of silicoaluminoxynitrides

(Sialons) , amorphous alumina, partially stabilized zirconia (PSZ) , silicon nitride, and silicon carbide.

5. A thermocouple according to any one of claims 1 to 4 in which the protection tube is coated with a layer of a protective material selected from the group consisting of a rare metal, a rare metal alloy, a refractory metal oxide, a metal aluminide, or a combination of two or more of these protective materials.

6. A thermocouple according to any one of claims 1 to 5 in which the thermocouple sheath is made of the nickel-base alloy NICROBELL, and the thermocouple conductors are composed of alloys selected from the group consisting of the nickel-base alloys NIOBELL and type N alloys.

7. A thermocouple according to any one of claims 1 to 5 for use in particularly demanding conditions which comprises a thermocouple of the rare metal type, selected from the group consisting of ISA types R, S or B.

8. A thermocouple according to any one of claims 1 to 7 which is a mineral insulated, metal sheathed (MIMS) thermocouple comprising thermocouple conductors surrounded by mineral insulation held within the thermocouple sheath.