Repair of thermoelectric converters: welding of thermocouples


Repair of thermoelectric converters

Survey of thermoelectric converters

The thermoconverter is taken to separate pieces, clean them from dirt and carefully the vyyasneyoniya of the condition of thermoelectrodes and their working end, clips on the insert of the head and the insert, the ceramic insulating insert (glass) for the working end of the thermocouple, the protective tube examine on purpose.

Схема термоэлектрического преобразователя

Scheme of the thermoelectric converter.

At survey of thermocouples at which thermoelectrodes are made of ignoble metals or alloys (copper, the thaw, the chromel, alumel, etc.) check lack of cross cracks which sometimes appear as a result of long operation of the thermoconverter at temperatures, high for thermoelectrodes, or owing to frequent alternate changes of temperature of the studied environment.

Emergence of cracks in thermoelectrodes can be also the consequence of mechanical stresses from the wrong rationing of the thermoconverter. So, use of two-channel insulators at thick thermoelectrodes often leads to the exit of thermoconverters out of operation. Inadmissibly, that the thermocouple which is especially manufactured of thick thermoelectrodes rested the working end against the bottom of the protective tube or the insulating ceramic insert (glass).

At external survey of thermocouples which thermoelectrodes are produced from precious metals or alloys (platinum, platinorodiya, etc.) check absence for surfaces "пересечек" — small deepenings (as from blow by the knife). At their detection thermoelectrodes in places where "peresechka" are noticed, break off and weld

Annealing of thermocouples from precious metals

Способ изготовления термопар

Way of production of thermocouples.

In serviceable conditions at very high temperatures it is always possible to protect platinorodiyevy and platinum thermoelectrodes from impact on them of the recovery gas environment (hydrogen, carbon oxide, hydrocarbons), hostile gas environment (carbonic acid) in the presence of vapors of iron oxides, magnesium and silicon. The silicon which is present almost at all ceramic materials represents the greatest threat for platinorody-plane-tree thermoconverters. Thermoelectrodes of these thermoconverters easily absorb it with formation of silicide of platinum. There is the change termo-EDS, the mechanical durability of thermoelectrodes decreases, sometimes they completely collapse in connection with the arisen fragility. Adverse impact is exerted by presence of coal materials, for example graphite as in them there are impurity of silicon dioxide which at high temperatures in contact with coal is easily recovered with release of silicon.

For removal of pollutants from termoelektroyod of precious metals or alloys of the germopara podyovergat to annealing (calcinating) within 30-60 min. elektyorichesky current on air. For this purpose thermoelectrodes exempt from insulators and suspend on 2 shtayotiva then degrease by means of the tampon, smoyochenny pure ethyl alcohol (1 g of alcohol on each sensing device). The free ends of thermoelectrodes connect to the network circuit voltage of 220 V or 127 V with a frequency of 50 Hz. The current necessary for annealing, regulate by means of the voltage adjuster and kontroyolirut according to indications of the ampermeter.

Sensing devices of thermoconverters with the grayoduirovochny characteristic of software (platinorodiya — the playotin) with thermoelectrodes with a diameter of 0,5 mm anneal at current 10-10,5 A [temperature (1150 + 50) °C], chuvstviyotelny elements with the grading characteristic of the PR-30/6 type [platinorodiya (30%) - platinorodiya (6%)] otzhiyogat at current 11,5-12 A [temperature of (1450±50) °C].

During annealing thermoelectrodes wash out brown. For this purpose on tin or any other plate fill to the drill and then move the plate along the heated thermoelectrode so that it was submerged in the drill (not to forget about conductivity of the plate). It is enough to carry out 3-4 times by the plate with brown along the thermoelectrode that platinorodiya and the playotin were clean, without surface pollution.

Also the different way can be recommended: on the raskayolenny thermoelectrode drills alloy the drop, allowing this drop to roll down freely.

Upon termination of annealing current is smoothly reduced to nuyolevy value during 60 pages.

After cleaning remained on thermoelectrodes to the drill delete: large drops — mechanically, and the weak remains — washing in the distilled water. Then the thermocouple is annealed again. Sometimes brown and annealing byyovat washings insufficiently as thermoelectrodes nevertheless остают­ся rigid. It indicates that platinum absorbed silicon or other not burning down elements and cleaning at refinery where direct termoyoelektroda is necessary. Also arrive if on thermoelectrodes there are surface pollution.

Check of uniformity of thermoelectrodes

At praktiyochesky use of the thermoconverter some difference of temperatures along length of its thermoelectrodes obnaruyozhivatsya always. The working end to the termopreobrazovata usually is located in the zone of the highest temperature: for example, in the center of the flue. If to move the certain measuring instrument of temperature, for example the working end of the thermoconverter (connected to other millivoltmeter), along thermoelectrodes of the first thermoconverter in the direction from the worker by the free ends, then reduction of temperature in process of removal from the center of the flue to its walls will be noted.

Многозонная кабельная термопара

Multizonal cable thermocouple: 1 - seals; 2 thermoelectrodes; 3 - cover; 4 - plug; 5 - sealant; 6 - conclusions of thermoelectrodes.

Each of thermoelectrodes longwise usually has heterogeneity (inhomogeneity) — not significant difference as a part of alloy, the peening, mechanical stresses, local pollution, etc. affect.

As a result of nonuniform distribution of temperature along thermoelectrodes and their heterogeneity in the thermoelectric chain arise inherent in points of heterogeneity of thermoelectrodes own termo-EDS which part is summed up, the part is subtracted, but all this leads to distortion of result of temperature measurement.

For the purpose of heterogeneity influence reduction each thermoelectrode of thermocouples from precious metals, osobenyono exemplary, after annealing is checked for uniformity.

For this purpose the straightened checked thermoelectrode is entered in not included small tubular электро­печь, capable when heating to create the local thermal field. Attach the negative clip of the sensitive zero galvanometer to the positive thermoelectrode, to the positive clip of this galvanometer connect the positive clip of the source of adjustable tension (SAT), and to the negative clip of IRN — the negative thermoelectrode of the thermocouple. Such inclusion of IRN gives the chance to compensate (to counterbalance) termo-EDS thermocouples with tension from IRN. Not to damage the sensitive zero galvanometer, at first instead of it include more rough zero galvanometer, make compensation termo-EDS, then interchange the position of zero galvanometers and carry out okonchatelyyony compensation termo-EDS, using rheostats of smooth regulation of IRN and sensitive zero гальвано­метр.

Turn on the electric furnace, create local warming up of the isyosleduyemy thermoelectrode and slowly stretch it via the furnace on all its length. At uniformity of metal or alloy of the thermoelectrode the index of the zero galvanoyometr will be on the zero mark. In case of heterogeneity of the thermoelectrode wire the index of the nuyolevy galvanometer will deviate to the left or to the right from the nuyolevy mark. The heterogeneous site of the thermoelectrode is vyreyozat, the ends weld and check the seal for the odnorodyonost.

With insignificant heterogeneity when additional termo-EDS does not exceed the half of the doyopuskayemy error for termo-EDS this couple, the site of the thermoelectrode is not cut out and do not reckon with the specified neodyonorodnost.

Preparation of thermoelectrodes for welding

If length remained not burned down thermoelectrodes allows, instead of the destroyed working end make new.

Таблица родов и классов материала

Table of childbirth and classes of material.

If there is the opportunity to manufacture the thermocouple of new thermoelectrodes, very carefully proyoveryat compliance of material of thermoelectrodes to the izgotavliyovayemy thermoconverter to be convinced of its quality.

For this purpose on the basis of normative documents ustayonavlivat the material sort, its technical harakteristiyoka, test dataa of the material DTC (Department of Tekhniyochesky Control) of Manufacturer. At compliance of these data to technical requirements material can be used; otherwise it is put on trials.

For check of uniformity from the bay of material otreyozat the piece of the thermoelectrode length exceeding neobkhoyodimy for production of the thermoconverter then by means of clips connect short copper bonding wires to the ends of the thermoelectrode. Clips lower in heat-insulating vessels with the thawing ice (0 °C) and define uniformity of material of the thermoelectrode.

For definition of the sort of material and its class from the bay cut off about 0,5 m of the thermoelectrode and weld it with the same piece of the platinum wire of the Extra brand. The working end of the received thermocouple is placed in the steam thermostat with the temperature of 100 °C, and the free ends take away in heat-insulating vessels with the thawing ice (0 °C) and connect copper wires with potentsioyometry. On termo-EDS, the developed thermocouple, opredeyolyat the sort and the class of material (tab. 1).

The chromel from алюмеля differs in appearance slightly, however the chromel is firmer, than alumel that easily is defined when bending, and, besides, alumel магнитен unlike the non-magnetic chromel.

The material meeting technical requirements is cut on pieces of the corresponding length and weld from them the thermocouple.

When cutting length of the thermoelectrode has to be несколь­ко bigger, than it is necessary for placement in the armature: it is necessary for fixing of the ends of thermoelectrodes in thermoconverter head clips.

Welding of thermocouples in the electric arc

At the izgotovleyoniya of thermoconverters with standard graduirovochyony characteristics material of both thermoelectrodes is selected surely one class. In this case, for example, the chromfir thermoelectrode belonging to the I class together with the alyumelevy thermoelectrode also of the I class forms the thermocouple with the same termo-EDS as couples from chromfir and alyumelevy thermoelectrodes of II. III or IV classes (tab. 1).

Рисунок 1. Установка для электросварки термопар в электрической дуге при помощи трансформатора

Figure 1. Installation for electric welding of thermocouples in the electric arc by means of the transformer.

Most often for welding of platinorodiyevy, platinum, copper, iron and kopelevy thermoelectrodes the alternating-current electric arc from network of napryayozheniy 24 Century ispolyyozutsya. The specified tension is received from the transforyomator 220/12 B, 500 W, having connected secondary winding of the transformer consistently (having removed predvariyotelno the casing). The boiler transforyomator is suitable for this purpose.

The installation given on fig. 1 is suitable not only for welding of thermocouples, but can be applied also to recovery calcinating (annealing) of thermocouples from precious metals.

Switching circuits of vtorichyony transformer winding with 12 on 24 V are given by fig. 2

When welding in the electric arc the welded thermoelectrodes from ignoble metals on one end twist, and on another — connect to one of clips of secondary winding, and connect the graphitized electrode to the second clip; then the graphitized electrode is pribliyozhat to the place of twist. The formed electric arc melts metal. At achievement of reliable welding, the graphitized electrode is quickly deleted.

Рисунок 2. Схема включения вторичных обмоток трансформатора

Figure 2. Scheme of inclusion of secondary winding of the transformer.

Platinorody-platinum thermocouples weld directly in playomen of the lit electric arc, without concerning thermoelectrodes of graphitized electrodes (in order to avoid the naugleroyozhivaniye of thermoelectrodes). Installation for welding on postoyoyanny current is given in fig. 3

The twist of thermoelectrodes from ignoble metals before welding is strewed brown with sand. When welding platinorody-platinum thermoelectrodes of topping brown it is not required, however with brown welding goes quicker.

During welding apply goggles to the preyodokhraneniye of eyes from the bright luminescence of the electric arc.

The long short-circuit current heats transformer winding to isolation carbonization that leads to burn-out of the wire winding. Therefore primary winding of the transformer is protected the fusible predokhraniyotel so that at the overload it fused, disconnecting network of the power supply of the transformer.

Рисунок 3. Схема установки для сварки платинородий-платиновых термопар в электрической дуге

Figure 3. The scheme of installation for welding of platinorody-platinum thermocouples in the electric arc.

Usually on the electric arc voltage drop is observed about 30 Century. Therefore at direct turning on of the welding device in network of constant or alternating-current 220 V (fig. 1) it is useful only the small part of the elektriyochesky energy consumed by the device is used. Other electric power prevrayoshchatsya in the unproductive warmth which is marked out proyovoloky the rheostat.

The diseconomy of work of installation at neposredstvenyony inclusion of the electric arc in lighting network is obvious, but in view of convenience this method finds the priyomeneniye when welding platinorody-platinum thermocouples.

Welding in the ball of platinorody-platinum thermocouples is made in the electric arc with coals with a diameter of 8-15 mm. The cut thermoelectrodes connect verevochyony twist and weld the end. After welding of the termoyoelektroda untwist and examine the place of welding. When welding it should not be formed "necks" about the ball. In case of its detection at survey naked eye thermoelectrodes weld again.

Welding of thermocouples from ignoble metals in the salt electrowelding machine

Installation for welding in hloyoristy barium is given in fig. 4

Рисунок 4. Установка для электросварки термопар в хлористом барии

Figure 4. Installation for electric welding of thermocouples in chloride barium.

  1. The cut thermoelectrodes, according to their class, select in couples and their ends intended for welding, carefully clean from oxides on length of 25-30 mm in the mechanical way or etching in structure solution: 49% of HN03, 49% of NSL and 2% of Sisl2.
  2. Make etching of thermoelectrodes in the exhaust cabinet. After etching thermoelectrodes wash out the snayochal in hot, and then in the cold flow water.
  3. The cleaned working ends of pair thermoelectrodes connect: the chromel — alumel, the chromel — the thaw, iron — the thaw with a diameter of 3-3,2 mm — rope twist; хро­мель — alumel, the chromel — the thaw, iron — the thaw, copper — the thaw with a diameter up to 1,5 mm — cable twist, and alumel and the thaw okruchivat around the chromel, iron or copper. The type of the working ends of thermocouples after their welding is given in fig. 5
  4. Make welding of the smoothed-out and twisted thermoelectrodes in the salt electrowelding machine. I will merge chemically pure chloride barium serves.

The welding machine (fig. 5) is prepared for work as follows: in the ceramic glass on 2/3 of its heights fill up the coal krupka of the UK1 brand. Submerge the graphite or coal crucible on 2/3 of its heights in the filled coal krupka. After installation of the crucible in it fill up salt — chloride barium and pass through it electric current why chloride barium melts.

Рисунок 5. Рабочие концы термопар

Figure 5. Working ends of thermocouples.

Current is switched off and the twist of thermoelectrodes is submerged in fused salt on depth about 15 mm then current is included again and the working end of the termopreobrazoyovatel for its flow is smoothly lifted to the poverkhyonost of fused salt. When on the welded end the large drop of the melted metal is formed, odnovreyomenno switch off current and the welded end is submerged in salt on depth of 15 mm where maintain during 2-5 with then quickly transfer to the bathtub with the flow cold water, and the stuck salt recoils. Then carefully delete the gumboil remains then the place of welding is washed out again, but already in hot water (60-80 °C).

As shows experience, at bad cleaning of the working end of the thermocouple of gumboil the term of its service is considerably reduced.

Welding of thermocouples in coal powder

Especially ravnoyomerny seals receive when welding thermocouples in coal or anthracitic powder. This device (fig. 6) can be used also for heating of normal soldering irons. As electrode filler ispolyyozut the shredded remains of coal brushes from электро­машин or grains of anthracite of 1-2 mm in size.

On alternating current to make welding quite difficult therefore in the presence of the direct current it is given preference.

Welding of thermocouples on the acetylene torch

Рисунок 6. Устройство для сварки термопар в угольном порошке

Figure 6. The device for welding of thermocouples in coal powder.

 Thermocouples from ignoble metals and alloys well сваривают­ся on the acetylene torch under the layer drills and sand. For this purpose on the working end fill to the drill, enter twist into the flame of the torch and warm up. After drill melts and will fill in the place of welding, the working end is brought out of the flame of the torch and submerged in quartz sand.

At the same time the working end is covered with the lamina the shlka that promotes good welding. Then the working end is brought in the flame of the torch again and make welding then the seal which did not cool down still is submerged in water and the drill easily gives in to separation from thermoelectrodes.

Reinforcing of thermoelectrodes

Quality indicators of isolation of thermoelectrodes from each other and from the protective tube are important for all types of the thermoconverters working at temperatures of 1200-1300 °C. Change of electrical conductance of insulating materials at such temperatures can cause short circuit of thermoelectrodes that will lead to errors when determining temperature. If the thermoconverter is located in the electric furnace where because of electric conductance of lining there is the leakage of current, then it can distort results of measurement in general. For example, when determining the grading characteristic of model thermoconverters already when heating to 1000 °C it is necessary to avoid contact of the cover of the thermoconverter with the spherical pipe of the furnace.

From these positions rather high requirements are met for thermocouples with the grading characteristic of PR-30/6 (platinorody 30% — platinorody 6%) insulating two-channel tubes from aluminum oxide, for thermocouples with the grading characteristic of software (platinorodiya — platinum) up to 1200 °C — the tube from pirometriyochesky porcelain or aluminum oxide, higher than 1200 With — from aluminum oxide. Diameter of tubes - 3-5 mm, length - 300-50 mm, diameter of channels - not less than 0,9 mm. It is necessary to place thermoelectrodes only of one type in the same canal.

Пример условного обозначения каната

Rope identification number.

For protection of platinum thermoelectrodes from pollution by rhodium insulating tubes, earlier приме­нявшиеся for isolation of platinorody-platinum or platinorodiyevy thermocouples, previously process: pickle in hot "aqua regia", proyomyvat the distilled water and calcinate at 1200 °C.

Thermoelectrodes with the grading characteristic HA (the chromel — alumel) and HK (the chromel — the thaw) isolate the short single-channel ceramic beads, the nayoprimer from fire-resistant porcelain with the raised alumina soderyozhaniye (pyrometric porcelain). Thermocouples of the special purpose reinforce special isolation. So, the chromel — thermocouple alumel for aluminum alloys is isolated the asbestos which is put on thermoelectrodes in the form of the stocking from the asbestos cord.

Thermoelectrodes of the thermoconverter, otremontiroyovanny or the given welding spoyosob made by one of, isolate from each other and attach to head insert clips. At the repaired thermoconverters clips and their screws before assembly carefully clean from dirt and oxides.

The assembled thermoconverter, without outside zashchityony tube and the head, arrives in actual fact for the purpose of clarification of its suitability to operation.

Repair of protective tubes

 Protective tubes predokhrayonyat thermoelectrodes from environmental activity, i.e. from gases, harmful to them, or vapors of metals and also from influence of high temperature of the environment. That measurement in general could be carried out, the protective tube has to be made of rather strong and chemically resistant material. The most often protective tubes are produced from pyrometric porcelain, kvaryotsevy glass and metal (iron, heat-resistant became).

Repair of the destroyed or damaged porcelain or quartz protective tubes comes down to their replacement and is not considered below, and repair of metal tubes — to tea leaves of the damaged places.

Metal protective tubes clean from rust, dirt and oil then the damaged places zavariyovat on the electric welding machine.

At tea leaves of the damaged places or welding of protective tubes from steel of brand of St.Z apply electrodes of types 1512 with the thin covering.

At tea leaves of the damaged places or welding of protective tubes from the acid-resistant heat resisting and heat-resistant stayola apply electrodes with the thick covering.

Current intensity when welding and diameter of electrodes is vybiyorat according to the data given below:

Thickness of the wall of the welded tube

fittings, mm                      1 2 3 4-5

Current intensity,      A 35-45 60-70 80-120 100-120

Diameter of electrodes, mm....       2 2-3 3-4 4

Lower limits of current intensity and diameters of electrodes belong to thinner parts welded on the main protective tube (welding of patches, the tube bottom, building of the tube); upper limits belong to more massive welded parts (flanges, the union).

When welding parts more than 3 mm thick on the direct current to them attach the clip having the designation "+", and to the electrode — the clip with the sign "—", i.e. welding happens at direct polarity.

Welding of thin products (material thickness less than 3 mm) is conducted, having attached the clip "+" to the electrode, and "—" to the part, i.e. welding is made at the return polarity.

Before welding to the protective tube up to 2 mm thick the union is heated to temperature of 250-300 °C.

In order to avoid burn-throughs of thin-walled parts the electric arc detain on more massive parts: for example, when welding the union with the tube the arch detain on the union.

High heat resistance is reported to steel tubes by alitizing, i.e. surface saturation of the tube aluminum. Process of alitizing is conducted, for example, by heating of the tube in the aluminum powder mixed by aluminum oxide in the presence of chloride ammonium at the temperature about 1000 °C.

The layer of solid solution of aluminum which is formed on the tube surface in steel has the surface heat-resistant film of A1203, but at long heatings, over 900 °C, aluminum diffuses in thickness of the tube.

Protective tubes of thermoconverters put on water pressure test. Test pressure is created the hydraulic press and given in the protective tube.

Test pressure variously: 0,5 MPas (5 kgfs/cm2) - for thermoconverters without union and 6 MPas (60 kgfs/cm2) — with the union.

In the armature apply fire-resistant putty of the following structure to fastening of porcelain protective tubes of platinorody-platinum thermocouples (in mass parts): dry clay — 500, chamotte ground — 400, feldspar ground — 100 and liquid glass to desirable density.

At putty production clay, chamotte and feldspar have to be dried up, mixed, small pounded and sifted through the sieve having 900 openings on 1 cm2.

The received powder at the use is pounded in liquid glass.

Drying of putty is made on air during 48 h.

Such putty maintains heating to 800 °C.

Assembly of thermoconverters

 Having finished production, repair and check of parts, start assembly of all thermoconverter.

At the same time make the following operations:

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