US2822257A

US2822257A - Method and apparatus for controlling blast furnaces

Info

Publication number: US2822257A
Application number: US516848A
Authority: US
Inventors: Ralph T Hanna; Carl G Hogberg
Original assignee: United States Steel Corp
Current assignee: United States Steel Corp
Priority date: 1955-06-21
Filing date: 1955-06-21
Publication date: 1958-02-04
Anticipated expiration: 1975-02-04

Description

Feb. 4 1958 R. T. HANNA ETAL 2,822,257

METHOD AND APPARATUS FOR CONTROLLING BLAST FURNACES Filed June 21, 1955 2 Sheets-Sheet l hrs-K 1- THROTTLE l5 ,8 COIVTROL MIXER mm: CONTROL ausr vowus co/vmaL f 26 PRESSURE IND/CA ran PR5 881m CHANGE INDICATOR 40mm m rs P n "70, 0,127?" TEMPERATURE cor/mo;

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RALPH r. HANNA and CARL a. HOGBERG,

their AIM/(76y.

Feb. 4 1958 R. T. HANNA ET AL 2,322,257

METHOD AND APPARATUS FOR CONTROLLING BLAST FURNACES Filed June 21, 1955 2 Sheets-Sheet 2 HOURS RALPH 7'. HAN/VA 0/70 CARL 6. HOGBERG,

8Y1 domwaa/Oza their Attorney.

United States Patent METHOD AND APPARATUS FOR CONTROLLING BLAST FURNACES Ralph T. Hanna, Fairless Hill, and Carl G. Hogberg,

Glenshaw, Pa., assignors to United States Steel Corporatron, a corporation of New Jersey Application June 21, 1955, Serial No. 516,848 Claims. (Cl. 75-41) This invention relates to an improved method of operating a blast furnace.

Within limits, a blast furnace becomes more eflicient as the volume and temperature of its hot air blast increase. However, furnace pressure is a function of factors which include this volume and temperature, and excessive pressure interferes with proper descent of the burden and may cause it to hang and later slip, and thus lead to serious difliculties. Therefore, the maximum blast volume and temperature are limited by extreme pressure conditions that just approach hanging. It is customary to observe the absolute pressure within a blast furnace and control the volume and temperature to maintain this pressure at a safe value, but all previous controls with which we are familiar rely exclusively on absolute pressure values. Such controls commence cutting back the volume or temperature only after the absolute pressure surpasses a designated value, which must be far enough below the maximum safe value or critical to avoid difiiculties. The margin between the pressure value at which the control operates and the critical value of course represents a direct efliciency loss.

An object of our invention is to provide an improved operating method which enables the blast volume and temperature to be maintained at higher values closer to the critical without risking hanging.

A further object is to provide an improved operating method wherein the blast volume and temperature are controlled in accordance with an additional phenomenon, namely rate of change of furnace pressure.

A further object is to provide an improved operating method wherein either the blast volume or temperature can be continually varied according to a pattern to produce maximum efliciency, that is, periodically one or the other is increased until pressure conditions approach the danger point and then is cut back to a safe starting point.

In the drawing:

Figure 1 is a schematic showing of a blast furnace equipped with means for controlling the blast volume and temperature in accordance with the rate of pressure change; and

Figure 2 is a graph, not to scale, showing the general characteristics of the temperature and volume of the blast of a furnace equipped as shown in Figure 1 and operated according to our invention.

Our invention is based on our discovery that a rapid rate of pressure increase usually foretells furnace conditions which cause hanging, irrespective of the absolute pressure. We have observed that considerably higher absolute pressure can be tolerated than previously has been considered possible, provided this high pressure is not preceded by an unduly rapid rate of pressure increase. Of course absolute pressures are not neglected altogether, but the absolute pressure at which the control means cuts back the volume or temperature can be considerably higher than that used previously. It is diflicult to ascribe definite values at which blast furnace pres- 2,822,257 Patented Feb. 4, 1958 sures, volumes and temperatures are maintained, since these values involve so many other variables, notably the characteristics of the rawmaterials and the furnace design. For example, considerably higher values are prevalent in the Birmingham, Alabama district than in the Pittsburgh, Pennsylvania district. Consequently we are able only to furnish typical examples of the way in which our control operates, and point out that the same principles apply where the absolute values are considerably different.

Figure 1 shows a typical blast furnace installation which includes a furnace 10, a plurality of stoves 12, a blower 13, and cold and

hot blast lines

14 and 15. The furnace has the, usual bustle pipe 16 and tuyeres 17. In accordance with usual practice, the blower 13 directs cold air through the cold blast line, into the stoves, and thence through the hot blast line, bustle pipe and tuyeres and into the lower portion of the furnace. The temperature of the blast entering the furnace is commonly about'700 to 1600" F., the average being about 1000 to 1200 F. The stoves are capable of heating the blast considerably higher than the maximum temperature at which the furnace operates smoothly. Therefore, aportion of the blast goes directly from the cold blast line 14 to the hot blast line 15 via a by-pass 18. An adjustable mixer valve 19 is mounted in this by-pass to control the portion of the blast which takes this route and thus control the temperature of the blast reaching the furnace. The mixing valve has a mechanical control 20 for varying its opening. The blower 13 has a throttle control 21 for varying the blast volume, which commonly is about C. F. M. per square foot of hearth area.

Also in accordance with usual practice, temperature and

pressure indicators

22 and 23 are connected into the hot blast line 15 to show the temperature and absolute pressure of the blast entering the furnace. The temperature indicator 22 operates an automatic temperature control 24, which in turn operates the mixing valve control 20 to maintain the temperature at any desired value. The individual indicators and controls are well known instruments and hence are not shown in detail. In a typical blast furnace having a hearth diameter of 28 feet and lacking our control, the normal operating conditions may be:

The critical pressure which causes hanging may be 24 p. s. i. g., which would appear under such conditions as:

blast volume O. F. 96, 000 90, 000 93,000 blast temperature-.. F. 1, 400 1, 600 1, 500 blast pressure p. s. i. g... 24 24 24 To insure against hanging it is necessary to commence cutting back the temperature or volume whenever the pressure commences to exceed the normal value of 20 p. s. i. g. The four pound margin is needed as a safety factor, but of course represents an efficiency loss.

In accordance with our invention, an indicator 25 which shows the rate of pressure change in the hot blast line is connected with the pressure indicator 23. In its simplest form the indicator 25 can be a recorder which continuously plots the pressure. The rate of change can be determined by observation of the slope of the pressure curve. As long as the curve is a substantially straight line, there is no appreciable rate change, but if the curve swerves upwardly, the rate is increasing. Whenever the rate increase becomes excessive, the operator can manually adjust either the temperature control 24 or a volume control 26 connected to the throttle consubstantially higher than in conventional practice.

trol 21 to cut back the temperatureor volume, even though the absolute pressure remains below the value at which these factors would be cut back. Nevertheless weprefer to employ an indicator which'automatically cuts back these factors in response to excessive rates of pressure increase. Automatic regulators for this purpose are simple adaptations of well known commercial instruments. As previously pointed out, we set the controls so that the absolute pressure at which they operate is In the aforementioned furnace we might commence cutting back when the pressure rise reached a rate of 1 p. s. i. g. per minute, although absolute pressure could go as high as 24 p. s. i. g. t I

In operating the furnace we prefer gradually to increase the blast volume while maintaining the blast temperature substantially constant. Under such conditions the pressure of. course rises. Before hanging occurs, the rate of pressure rise suddenly commences to increase. At this point we cut back the volume or temperature or both to a safe value and repeat the sequence. Thus we are able to operate the furnace at pressures extremely close to the critical without risking hanging.

As a specific example of our invention, the volume of blast delivered to a furnace was set at 80,000 C. F. M. and the temperature at about 1250 F. The blast pressure was about 20 p. s. i. Over a /2 hour period the volume was gradually increased to about 92,000 C. F. M., as indicated by curve A in Figure 2. At first there was no noticeable increase in the rate of pressure rise even though the absolute pressure was rising. After about 4 hours the rate of pressure rise commenced to increase, as shown by curve B in Figure 2. After 5% hours the blast volume was cut back to about 85,000 C. F. M. and the blast temperature briefly lowered to about 1150 F., as indicated by curves A and C. Again the volume and temperature were increased. This time the rate of pressure rise commenced to increase sooner, since the starting volume was higher, but otherwise the steps were repeated according to the same pattern. Although the curves show both the volume and temperature factors decreased in response to pressure rate increases, it is apparent that largely equivalent results can be obtained by decreasing only one of these factors.

' When the furnace was operated as just described, no hanging occurred although the conditions just approached hanging when the volume and temperature were cut back. The furnace efliciency was improved because the volume and temperature factors were kept above the maxima at which the furnace operates smoothly in the absence of our control.

While we have shown and described only a single embodiment of our invention, it is apparent that modifications may arise. Therefore, we do not wish to be limited to the disclosure set forth but only by the scope of the appended claims.

We claim:

1. In the operation of a blast furnace, wherein a preheated air blast is applied to the lower portion of the furnace and the pressure within this portion is a function of factors which include volume of the blast and temperature of the blast, a method of maintaining this pressure at absolute values close to the critical that causes hanging comprising continuously measuring the rate of pressure change, increasing at least one of said factors as long as no increase occurs in said rate regardless of increases in the absolute pressure up to the limit which itself causes hanging and cutting back one of said factors when an increase is detected in said rate.

2. In the operation of a blast furnace, wherein a pre heated air blast is applied to the lower portion of the furnace and the pressure within this portion is a function of factors which include volume of the blast and temperature of the blast, a method of maintaining this pressure at absolute values close to the critical that causes hanging comprising periodically gradually increasing at least one of said factors from a starting point substantially below that which produces a critical pressure and thereby gradually increasing the absolute pressure, continuously measuring the rate of pressure change as the absolute pressure increases, continuing to increase this factor as long as no increase occurs in said rate regardless of increases in the absolute pressure up to the limit which itself causes hanging, and cutting back this factor when an increase is detected in said rate.

3. A method as defined inclaim 2 in which the factor periodically increased and cut back is the volume of the blast.

'4. A method as defined in claim 2 in which the factor periodically increased and cut back is the temperature of the blast.

5. A method as defined in claim2 in which both the volume and temperature of the blast are periodically raised and cut back.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

2. IN THE OPERATION OF A BLAST FURNACE, WHEREIN A PREHEATED AIR BLAST IS APPLIED TO THE LOWER PORTION OF THE FURNACE AND THE PRESSURE WITHIN THIS PORTION IS A FUNCTION OF FACTORS WHICH INCLUDE VOLUME OF THE BLAST AND TEMPERATURE OF THE BLAST, A METHOD OF MAINTAINING THIS PRESSURE AT ABSOLUTE VALUES CLOSE TO THE CRITICAL THAT CAUSES HANGING COMPRISING PERIODICALLY GRADUALLY INCREASING AT LEAST ONE OF SAID FACTORS FROM A STARTING POINT SUBSTANTIALLY BELOW THAT WHICH PRODUCES A CRITICAL PRESSURE AND THEREBY GRADUALLY INCREASING THE ABSOLUTE PRESSURE, CONTINUOUSLY MEASURING THE RATE OF PRESSURE CHANGE AS THE ABSOLUTE PRESSURE INCREASES, CONTINUING TO INCREASE THIS FACTOR AS LONG AS NO INCREASE OCCURS IN SAID RATE REGARDLESS OF INCREASES IN THE ABSOLUTE PRESSURE UP TO THE LIMIT WHICH ITSELF CAUSES HANGING, AND CUTTING BACK THIS FACTOR WHEN AN INCREASE IS DETECTED IN SAID RATE.