CAST IRON

MONITORING HOT-BLAST TEMPERATURE ON A BLAST FURNACE BY A CONTACTLESS METHOD E. M. Sherman, V. M. Yanchevskii, and A. B. Vorkunov

UDC 669.162.238.21:681.2

The improvements in blast-furnace technology in world practice during the last 25 years have led to a change in the parameters of the hot blast. For example, the excess pressure of the blast has more than doubled and has reached 300-400 kPa. Blast temperature has increased by a factor of 1.5 and reached 1400~ The composition of the blast has also undergone changes -- moistened air was substituted for atmospheric air, and later the blast was enriched with up to 35-40% oxygen. Here, the new technology required highly accurate stabilization and measurement of the temperature of the hot blast. Under these conditions, the traditional contact method of measuring blast temperature ceased to satisfay technical demands, due to the relatively short life of thermoelectric thermometers in the aggressive working medium, their high inertia, and the high cost and short supply of the materials used. Work was done abroad and in the USSR on use of the contactless method of measuring hot blast temperature. The main difference in this method is the location of the transducer which converts temperature into an electrical signal outside the working medium of the object and the fact that the transducer is not subjected to its destabilizing factors. This circumstance ensures high stability for the calibration curve of the pyrometric transducer and independence of its life and reliability on the temperature level and eliminates the need to consume expensive metals and scarce materials. The contactless method is fairly complicated to use in its purest form to directly measure hot blast temperature, since the radiant energy of the gaseous medium of t h e b l a s t is due to the presence of water vapor in the blast and is at a relatively low level. A unit has been developed abroad ("Siemens" company, Federal Republic of Germany) whereby the pyrometric transducer (Fig. i) is sighted on a special sighting body of fairly complicated form. The body is installed in the hot blast line, and its platform is located in the turbulent part of the flow. The shape of the body, made of the same refractory material as the lining of the inside of the blast line, ensures that the temperature of its surface which faces the pyrometer will approach the temperature of the hot blast. The material of the body is impregnated with a special composition which increases the emissivity of its surface. This measurement method can only conditionally be called a contactless method, since the sighting body -- an element of the measurement circuit -- is in the flow of the working medium and is subjected to its aggressive action just like the With respect to reparability~ thermometer in the contact method. It is understood that the reliability of this method depends on the service life of the sighting body. The life of the body is shorter, the greater the aggressiveness of the working medium. With respect to repairability, this measurement circuit is inferior to the since failure of the sighting body would require a long shutdown of the furnace the hot blast line to repair it. In connection with this, the life of the body shorter than the life of the lining of the hot blast line -- a requirement which realistic using the present design.

contact method, and cooling of should be no is hardly

The solution to this problem would be more complete if we could succeed in measuring blast line. The reliability of such measurements would be independent of the parameters of the hot

"Chermetavtomatika" Scientific-Industrial Association. All-Union Scientific-Research Institute of Automation and Mechanization in Ferrous Metallurgy (VNIIAchermet). Translated from Metallurg, No. 6, pp. 12-14, June, 1983.

0026-0894/83/0506-0199507.50

9 1984 Plenum Publishing Corporation

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Fig. i. Scheme for measurement of blast temperature with an "Ardometer" pyrometer: i) telescope of "Ardometer" pyrometer; 2) protective fittings of pyrometer; 3) stop valve; 4) sighting body.

blast, and malfunctions could occur only in the component parts of the pyrometric instrument. These problems could be corrected without stopping the furnace. In terms of its dynamic characteristics, this measurement scheme is the equal of the scheme employing the sighting body, since it registers a steady temperature only after a steady-state regime of heat transfer from the blast flow to the inside surface of the lining of the hot blast line is established. A natural shortcoming of this method is that the temperature difference between the blast and the inside surface of the lining is greater than the temperature difference between the blast and the sighting body, since under steady-state conditions the latter registers an intermediate value between the blast temperature and the temperature of the inside surface of the hot blast line. However, this deficiency can be considered unimportant in practice. It is well known that the temperature of the inside surface of the lining (for brevity, the wall temperature twa) under steady-state conditions is always lower than the temperature of the hot blast tb due to heat loss from the blast through the imperfect thermal insulation of the lining to the environment. This temperature difference (tb -- twa) depends on the optimality of the thermal insulation of the hot blast line, the parameters of the blast, and the ambient temperature. These quantities vary within limited ranges which are characteristic of a given furnace. If tb -- twa fluctuates within narrow ranges, then its mean can be taken as a correction for the readings of a pyrometric unit with a pyrometer sighted on the surface of the lining. Here, the small deviations from the mean are part of the blast temperature measurement error. The value of tb -- twa can be calculated for each group of blast parameters and ambient temperature and each specific design of hot blast line through the simultaneous solution of three equations which describe the heat transfer process.* Here, the calculation error turns out to I-2~ which supports the empirical data. Analysis of this system of equations shows that the difference (tb -- twa) has its greatest and least values at limiting values of all of the blast parameters, except temperature, and at limiting values of the ambient temperature. The difference is maximal at minimum values of the wind rate, steam partial pressure, and ambient temperature. The converse also holds. This allows us to easily calculate the maximum and minimum temperature difference at all possible values of the parameters within their working ranges.

*E. M. Sherman, A. M. Kaufman, and S. L. Gerasimova, "Dependence of the error of measurement of a gas flow by a thermoelectric thermometer on its parameters," Manuscript submitted to All-Union Institute of Scientific and Technical Information, bibliographic reference "Deposited Letters," No. 2, 128, No. 7D/86 (1978). 200

Fig. 2

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Fig. 2. Installation of pyrometers at the hot blast line of a blast furnace. Fig. 3. Installation of APZ-7209 mounting on the hot blast line (remaining notation in text): 3) manometer; 4) valve for releasing air into atmosphere.

Calculation of t b -- twa by this method for a modern hot blast line on a large furnace (No. 6 at the Novolipetsk Metallurgical Plant) in the working range of the blast parameters and ambient temperature showed that the fluctuations in this difference did not exceed •176 while the mean value was within 12-15~ The range of t b -- twa is due to the good thermal insulation of the lining, made of four layers of refractories. The radiation of the hot blast line through a small hole in the lining (40-mm diameter) is characterized by a very high emissivity close to that of a blackbody. Along with the radiation of the internal surface of the lining, additional radiation from the water vapors is introduced into the flow. This lowers the error of the blast temperature measurement by the contactless method. This circumstance makes it possible to measure blast temperature by the proposed scheme (Fig. 2), put into effect on furnace No. 6 at the Novolipetsk plant with an ITP-7209 pyrometric temperature meter. It was developed by the "Chermetavtomatika" ScientificIndustrial Association to replace the earlier UTD-7069 for automatic monitoring of the temperature of the area under the dome of blast furnace stoves. This change was related to a change in the instrument b a s e . The meters and their installation are otherwise similar. The ITP-7209 consists of two independent pyrometric units and an ShPE-7209 auxiliary pneumoelectric equipment cabinet. Each unit has two transducers -- a PPT-12i-01 pyrometric transducer a n d a P V V - 0 1 1 1 1 measurement transducer, part of the APIR-S system. It also has a protective pyrometric APZ-7209 mounting for installing the pyrometric transducer on the object. The mounting is equipped with a quartz illuminator which hermetically encloses the cavity of the sighting channel in the object. The pyrometric transducer is sighted on the surface being monitored and receives its radiation, which passes through the sighting channel and the illuminator. Air or nitrogen is supplied from the equipment cabinet into the mounting to protect the transducer from contamination by dust particles in the working medium. The air or nitrogen flow is blown over the illuminator surface turned toward the object and blows the solid particles out through the sighting channel. 201

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Fig. 4. Hot blast temperature curve obtained with an ITP-7209 meter.

The APZ-7209 mounting also protects the transducer from atmospheric deposits and dust, ensuring normal conditions for its operation. The mounting is equipped with a locking device which makes it possible to safely and simply replace an illuminator with a contaminated surface with a new illuminator. The ITP-7209 meter is supplied with clean, dry air or nitrogen. The gas enters the equipment cabinet, where the air undergoes fine cleaning and reduction and is directed into the APZ-7209 protective mounting. The air has a maximum dew point of --40~ so that the surface of the illuminator is free of condensate which might precipitate from the working medium. As a result, the adhesive capacity of the dust particles is reduced. The air or nitrogen pressure is automatically maintained so that its drop relative to the pressure of the working medium is a constant 80-100 kPa. The pressure drop is stabilized with regulators installed in the cabinet which receive a pressure pulse from the working medium. The organization of a flow of protective gas onto the illuminator surface, stabilization of the pressure drop of the protective gas, and the high purity and low moisture content of the gas ensure long (at least 3 months) continuous operation of the illuminator under blast-furnacestove conditions. The presence of two pyrometric units ensures the possibility of simultaneous and independent monitoring of temperature in two zones of the object. When the pyrometric transducers are sighted on one or two zones close to each other, one unit can be used as a monitoring unit (the locking device on its mounting is always closed and separates the illuminator from the working medium), while the second can be used as working unit for automatic temperature control. Its readings can be periodically checked against the readings of the monitoring unit, which is connected for this period. Pyrometric transducer PPT-121-01 has a sighting index of 1/50 with an inlet aperture with a diameter of about 25 mm, which for normal operating conditions requires a sighting channel with a minimum inside diameter of 25 mm and a length up to i000 mm. This makes it possible to use the channel in a hot-blast-line lining intended for thermoelectric thermometer installation to sight the transducer. We succeeded in installing the APZ-7209 mounting (Fig. 3) on a hot blast line without shutting down the furnace thanks to the use of a stop valve on an AUT-7131 fitting of a U-7131 202

unit installed earlier. Here, we removed the lock chanber of the AUT-7131 and, with the stop valve closed, replaced it with an adapter flange (2). The APZ-7209 mounting (i) was then connected to the flange. After the Du-80 stop valve was opened, the PPT-121-01 pyrometric transducer, installed in the mounting, was sighted through the quartz illuminator in the mounting coax• with the channel in the lining onto its inside surface. The PVV-01111 measurement transducer converts the emf of the PPT-121-01 pyrometer into a 0-5-mA current signal which is linearly related to the temperature being measured. This signal can be used in an automatic system to stabilize the temperature of the hot blast and can be recorded on an appropriate KSU-series recorder (such as the KSU2-O03). Figure 4 shows a curve of the meter with a blast temperature record. Blast furnace No. 6 at the Novolipetsk plant is currently operated with a closed mixing valve (without automatic temperature stabilization), which explains the periodic change in blast temperature. Comparison of the readings of the recorders of the U-7131 and the ITP-7209 meter did not reveal any consistent discrepancies between them. The instrument error of the meter does not exceed •176 in connection with the fact that the permissible error of the PPT-121-01 transducer is •176 When a class III standard transducer is used, this error can be reduced to il5~ The positive results gained in measuring hot blast temperature on furnace No. 6 at the Novolipetsk plant with the ITP-7209 show that the blast temperature on smaller furnaces can be similarly monitored. A section of the hot blast line with a four-layer lining can be used for this purpose. For example, one could use the section where the line constricts, which is designed to improve mixing of the cold b l a s t a n d the blast coming from the stove. The above method of measuring the temperature of the hot blast has yet another advantage in that the radiation of the blast line cavity corresponds to the average temperature of the blast, not to the temperature of some limited zone of the flow where the thermometer tip would be located. This fact is important when there is poor mixing of the two flows (hot and cold). In this case, the contact method of measurement may not provide the required accuracy. The possibility of realizing the contactless method of measuring hot blast temperature with the ITP-7209 meter, together with the existence of a proven method of measuring the temperature under the dome of blast furnace stoves using similar equipment, will in the future make it possible to completely eliminate the use of thermoelectric thermometers with thermocouples made of platinura-rhodium alloys from blast furnaces. With the replacement of the contact method of temperature measurement in the combustion chambers of the stoves by the contactless method, the thermoelectric thermometer will still be used as a monitoring device or as a control element for low temperatures (up to 9000C). In the last case, it will be quite acceptable to use inexpensive thermoelectric thermometers with chromel--alumel thermocouples. The consumption of these thermocouples will be low due to the low frequency of their use. A precondition of the introduction of the ITP-7209 meter is the presence of a source of clean and dry compressed air or nitrogen with a pressure 150 kPa greater than the pressure of the hot blast. The consumption of the air or nitrogen changes with a change in the pressure of the hot blast; the maximum consumption by the meter, with the simultaneous supplying of both APZ-7209 mountings, is 50 m3/h. The advantages of the contactless method of temperature measurement in the hot blast channel of blast furnaces makes it more urgent that all blast-furnace shops in the sector be equipped with sources of clean, dry air or nitrogen with the required parameters. The lack of such is currently preventing the broad adoption of the ITP-7209 meter on blast furnaces.

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