US3198855A - Method of operating soaking pits - Google Patents

Description

Aug. 3, W SUYDAM METHOD OF OPERATING SOAKING PITS Filed April 24, 1962 5 Sheets-Sheet 1 "4%,MM IIIIIIIIIII *I ATToRNEYs.,

3 Sheets-Sheet 2 WMDOI Z. NZZ.

ATTORNEYS.-

Aug. 3, 1965 w. sUYDAM METHOD OF OPERATING SOAKING' P ITS Aug. 3, 1955 wysUYDAM 3,198,855

METHOD OF OPERATING SOAKING FITS Filed April 24, 1962 3 Sheets-Sheet 3 INVEN TOR. WALTER SUYDAM.

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United States Patent 3,l98,855 METHGD 0F @PERATENG SAKING PllS Walter Suytlani, Mount Lebanon Township, Allegheny County, Pa., assigner to Loftus Engineering Corporaiou, a corporation of Maryland Filed Apr. 24, 1962, Ser. No. l89,820 lt) Claims. (Cl. 25E-'52) This application is a continuation-impart of my application Serial No. 62,150, tiled October l2, 1960, which in turn is a contnuation-in-part of my application Serial No. 36,012, filed lune 14, 1960, both now abandoned.

My invention relates to industrial furnaces, particularly to furnaces for heating articles of metal to working temperature, and more particularly to a method of firing and operating the same.

Pit type furnaces are widely used throughout the steel industry for heating ingots of steel to rolling or forging temperature. Pit type furnaces are commonly known as soaking pits, and in exemplary Way the invention will be described as it is applied to and practiced in the operation of soaking pits for heating ingots of steel to rolling ternperature.

In the operation of a soaking pit the steel ingots to be heated are usually positioned on end and in spaced-apart relation in the furnace chamber, the chamber having a top cover that is removable to permit of the charging and removing of ingots.

When a soaking pit has been charged the cover is seated in sealed position upon the top of the furnace chamber, and a combustion system functions to deliver lluid fuel and combustion air through one or more burners into the furnace chamber, filling the chamber with llames and hot products of combustion. The waste products of combustion escape by Way of one or more outeo ports opening from the furnace chamber into a flue system that leads to a stack. Usually a furnace pressure controller actuates a damper in the flue system so to regulate the flow of the waste combustion products that a superatrnospheric pressure is maintained in the furnace chamber as long as the cover is closed and while the ingots are being heated to and soaked at specified temperature for rolling, say to a temperature in a typical case of between 2200o F. and 24U0 F.

When a charged soaking pit begins an ingrat-heating cycle the combustion system of the pit operates at maximum normal rated capacity to deliver all heat into the furnace chamber required to bring the ingots substantially to rolling temperature as fast as possible Without melting or washing the surfaces of the ingots. When the ingots approach rolling temperature the rate of ring the pit is reduced to a point at which the pit will be maintained at the proper temperature for thermally soaking the ingots, whereby the temperature throughout the bodies of all of the ingots in the pit is substantially uniform. The ingots are held at soaking temperature until the rolling mill is ready to receive them, at which time the pit cover is opened and the ingots are removed one by one to serve the mill. The cover is returned to closed position between the removal of successive ingots in order to minimize loss in pit temperature.

ln years gone by soaking pits were fired by means of low-velocity, luminous-flame burners; that is to say, burners which jet the fuel and air into the furnace chambers of the pits at velocities in the order of 50 feet per secon l. Automatic fuel-to-air regulator equipment was provided to hold the ratio of air to fuel at predetermined value, usually with the air proportioned in excess of that theoretically required for complete combustion of the fuel. A serious difliculty was experienced in the operation of these soaking pits, due to the fact that when the firing of a pit was reduced to soaking rate it was practically impossible to generate sufficient quantities of hot products of combustion to maintain throughout all portions of the furnace chamber a circulation of hot gases that would establish desirable temperature uniformity within the chamber, this ditliculty being particularly serious in soaking pits fired with a gaseous fuel. As a consequence it has been practically impossible to soak the ingots with the desired consistency or uniformity of temperature between their tops and bottoms; furthermore, the ingots in the corners of the chamber soak-out at temperatures lower than the ingots in the medial portions of the chamber.

Various expedients have been tried to overcome this objectionable condition of pit operation. In some cases the hot Waste gases flowing from the pit have been bypassed from the flue system and reintroduced to the furnace chamber, together with the fuel and combustion air delivered at soaking rate. ln this Way an adequate volume of hot gases was maintained in the pit chamber during the soaking cycle. While this practice is theoretically sound it has not been entirely satisfactory in its results. Furthermore, the practice involves a high initial cost of pit construction, requiring additional ducts, instruments, and controls to effect the essential recirculation'of hot Waste gases and fuel costs per ton of ingots heated and soaked are higher than otherwise need be.

ln other cases it has been proposed to provide a pit with a multiplicity of burners, which .burners are operated at reduced firing rate during the soaking cycle, as evidenced by United States Letter Patent No. 2,776,827, granted January 8, 1957, on the application of one Frank H. Graham. But in these cases too the operation of the pits has left much to be desired, since during the soaking cycle the llames and products of combustion developed by the burners, while delivering adequate fuel and air theoretically to keep the furnace chambers at soaking temperature, are not adequate or energetic enough to maintain uniform temperature conditions throughout the chamber.

ln the year 1957 the assignee of record of the invention herein disclosed made the first installation of a soaking pit fired with a high-velocity gas burner, as distinguished from a low-velocity burner. The characteristics of highvelocity burners will be described in the ensuing speciication.

With this state of the art in mind, it will be noted that my present invention consists in the discovery that soaking pits may be tired with a plurality of gas burners, preferably all of which but essentially at least one of which is a high-velocity burner, the said one high-velocity burner being so arranged that, when the other burners are shutoff or fired at reduced capacity, the high-velocity burner may be continued in operation at substantially rated maximum capacity, not oni to maintain the temperature of the pit at soaking value but also to provide an adequate volume of flames and products of combustion circulating and recirculating within the furnace chamber to establish and hold throughout the chamber substantially uniform temperature conditions. It is important to note that during the soaking operation the high-veloctiy burner may deliver fuel and air into the pit at substantially the same selected, or predetermined, automatically regulated normal ratio of fuel-to-air as it does during the heating cycle of pit operation. y

lt is also important to note that my present invention may be applied either to pits in which the air for combustion is preheated, as previously mentioned, or to pits in which the air for combustion is not preheated.

An embodiment of the invention is illustrated in the accompanying drawings, in which FIG. l is a view in sectional plan of a soaking pit installation including two pit furnaces, the plane of section of this ligure being indicated at I l in FlG. 2.

FIG. 2 is a view in longitudinal section of the pit installation, as seen on the plane II-II in FIG. 1;

FIG. 3 is a graph illustrating pit and ingot temperatures compared with fuel flow during the heating and soaking cycles of pit operation;

FIG. 4 is a View in vertical section of one of several types of burners which may be operated asa high-velocity burner;

FIG. 5 is a view of the burner in elevation, as seen from the left of FIG. 4;

FIG. 6 is a fragmentary view in sectional plan of a single soaking pit, illustrating that the three burners shown in FIG. 1 may be consolidated or constructed in the form of two burners;

FIG. 7 is a schematic view in vertical section, illustrating the consolidation or incorporation of the three burners in a single burner structure; and

FIG. 8 is a view of the latter burner as seen from the left of FIG. 7.

Referring to the drawings, each of the pit chambers A and B comprises a substantially rectangular body having four side walls 2, 3, 4 and 5, a hearth 6, and a cover 7. The walls 2 and 4 are sometimes called they end walls of the chamber, `while the walls 3 and 5 are more properly known as the side walls. The 4hearth and side walls are constructed of refractory and insulating materials Ithat are externally supported and bound by steel plate and steel buckstays in accordance with common practice. The reference numeral '7 is immediately applied to the refractory arch of the cover, which arch is supported and carried by a frame S of steel. Conventional mechanism (not shown) is provided to raise and lower each cover and to move it to and from seated position upon the pit chamber when during the operation of the associate pit ingots are to be charged or withdrawn. It is needless to involve this specification with a description of well-known details of soaking pit construction, other than to the extent they have a direct bearing on my present invention.

A plurality of burners is provided in the Walls of the pit chamber, in this case, and advantageously so, a transverse row or line of three burners 9, I() and 11 is incorporated in the end wall 4, as shown in the drawings.

Gaseous fuel, such as natural gas, or coke oven gas, or a mixture of fuel gases, is delivered through a feed pipe 12 to each burner, and the feed pipes lead from a fuel manifold 13 for the three burners of each pit. The fuel manifold 13 of each pit is connected by a pipe 14 to a fuel supply line 15, each pipe 14 including valve means 16 for regulating the flow of fuel to the group of burners of each pit for pit temperature control, while a valve 17 is provided in each feed pipe 12, whereby the fuel ow to the individual burners may be shut off when desired. The burners in service are adapted to deliver flaming jets of mixed fuel and air into the upper portions of the furnace chamber of each pit, and the products of combustion escape from the lower portion of each furnace chamber by way of one or more outgo ports I8 in the burner wall 4 and a flue l that opens into a stack 2t). The stack 26 serves both pits. In the flue 19 of each pit a vertically adjustable damper 21 is operable in known way to regulate the effective area of the flue and establish the desired pressure of the hot gases prevailing in the furnace chamber.

In the soaking pit installation shown in FIGS 1 and 2 of the attached drawings two sets 22 of metallic recuperators are positioned in the path of ow of the hot waste gases from each pit to the stack 24). Each pit is provided with a blower 23 (shown in FIG. 1, but omitted in FIG. 2) that draws air from the open atmosphere and delivers it through a duct 24 to a header 25, from header 25 through pipes 26 to the inlet of each set of recuperators 22, from the outlet of each set of recuperators 22 through a duct 27 to a header 2S (FIG. 2), from header 28 through an uptake 29 to a header 39, and from header 3l) through feed pipes 31 to the burners 9, 10 and 11,

begins to cut-back the total fuel input.

4. severally. r.The uptake 29 and feed pipes 31 include the usual flow-controlling valves, with which this specification need not be involved. A typical burner is illustrated in FIGS. 4 and 5, and in FIG. 4 the air inlet of the burner is shown at 37.

In conventional manner the temperature of the air passing through the recuperators on its way to the burners is raised several hundred degrees F., as by means of heat transferred from the hot combustion products owing to the stack. As a consequence the burners, receiving this preheated combustion air, function with high thermal efficiency in burning the fuel and developing the required temperature in the furnace chambers of the pits.

When a furnace chamber has been charged with ingots to be heated and the cover 7 closed, the fuel flow to the three burners is initiated and gradually increased, as indicated by curve portion a of the graph shown in FIG. 3. When the fuel flow reaches percent of the normal maximum rated capacity of the burners, it is there held, as indicated by curve portion b, until the pit or furnace chamber temperature approaches the control point temperature of 2400 F. in thiscase, as illustrated by curve e. At about this time the temperature control valve 16 When the fuel input is reduced to about two-thirds of the normal maximum input the central burner is shut off, leaving the outer two burners in normal operation, giving the inner body portions of the ingots time in which to approximate the 2300 F. temperature (see curve f) of the outer body portions of the ingots, preparatory to the initiation of the soaking cycle per se. Then, the fuel flow is further gradually reduced to a point of about one-third of normal maximum input, at which time the outer two burners are shut-off, and firing of the central burner is reinstated. Note curve c. Thereafter the fuel llow is established in the central or soaking burner at about 15 to 30 percent of the normal maximum rated capacity of all burners. Note curve portion d and further note that during the soaking cycle the control point temperature is not substantially exceeded, as indicated by the horizontal portion of curve e. In other words the ingots are soaked substantially at and within the limit of the control point temperature. It will be understood that the rate of firing of the central burner during the soaking cycle generates enough heat to compensate for the heat the pit loses by radiation; that is, enough heat is generated during the soaking cycle to maintain the furnace chamber at substantially 240G F., whereby the ingots thermally soak at the rolling temperature until uniform temperature is established throughout the bodies of the ingots, at which time the mgots are ready to be withdrawn from the pit for rolling.

It also will be understood that the valves in the fuel and combustion air piping that supplies the burners may bemanually adjusted to establish and maintain the desired firing rate of each pit, and to hold the fuel-to-air ratio at normal value, but in practice these valves are operated through automatic control apparatus of known construction, not shown. When the cover '7 of a pit is opened to charge or withdraw ingots the flow of fuel and combustion air is automatically either shut-off or reduced to pllot flow in the soaking burner 10, and when the cover 1s reclosed the desired ow is restored, first three burners, then two, and then one. In practice thermally responsive instruments or control apparatus are coordinated with the fuel and air flow-controlling mechanism to regulate the fuel llow in accordance with the curve a, b, c, d, shown in FIG. 3, and, as said, the controlled variations in fuel flow are accompanied by corresponding variations in combustion air flow, whereby a predetermined normal fuel-to-air ratio is maintained. It is desirable to operate any soaking pit with a minimum of air in excess of that required for theoretically perfect combustion, and where the fuel is natural gas it has been found that 10% excess are appropriately sized to meet the capacity of each burner. The burners 41 and 42 are individually subject to fuelto-air ratio control and to instruments and controls similar to those associated with the three burners first described, whereby the specified method of this invention may be practiced.

By way of further modification, the three burners may be incorporated in a single burner structure 47, as illustrated schematically in FIGS. 7 and 8. The burner is provided with three fuel pipes 49 each having a fuel shut-off valve 50, and the three fuel pipes may be connected in common to a fuel supply line 51 having a butterfly flow or temperature control valve 52. The fuel pipes extend through the refractory baffle 48 of the burner. The burner includes three independent combustion air chambers 53, 54 yand 55 defined by the outer body 58 of the burner and the annular partitions 59 and 60, and each of the air chambers is individually supplied with combustion air by a duct 56 provided with a valve 56, and the three air ducts 57 may be connected in common to an air-supply header (not shown) having a conventional air-iloW-controlling valve. The baffle 48 includes an annular series of orifices or passages 61 that delivers the high-velocity air from chamber 53 for combustion of the fuel delivered by one of the fuel pipes 49; the baffle 48 includes an annular series of orifices 62 that delivers high-velocity air from chamber S4 for the combustion of fuel delivered by the second of the fuel pipes 49; and the baffle 48 includes a plurality of orifices 63 that delivers high-velocity air for the combustion of fuel delivered by the third of said three fuel pipes 49. Thus, it will be perceived that the burner 47 comprises in practical effect three burners that may be operated to practice the method of the claims appended hereto. Indeed, it is conceivable that the three fuel pipes 49 may be provided as a single fuel pipe in which the fuel supply is controlled in such way with respect to the flow of combustion air through the three series of orifices 61, 62 and 63 as to provide in effect three burners that may be operated in accordance with the method herein disclosed and defined.

Within the terms of the appended claims it will be understood that various burner arrangements or structures other than those described may be provided in one or more of the end or side walls of a pit structure within the scope of the present invention, so long as at least one burner is adapted to deliver a jet of gaseous fuel and one or more jets of high-velocity combustion air at substantially normal fuel-to-air ratio for the uniform soaking of ingots that have been heated, as described in this specification.

I claim:

l. The method herein described of heating a plurality of metal ingots and thermally soaking them in a furnace chamber to bring them to a condition to be Worked, said method comprising the step of firing said chamber through fuel-gas and air passages opening directly into said furnace chamber and adjusted to a predetermined normal operating ratio to effect an initial heating of the ingots, thereafter reducing the volume of air and gas delivered directly into the chamber while maintaining said normal operating air to gas ratio but maintaining the air at a lineal velocity through at least one passage of at least 200 feet per second to thereby secure an energetic and substantially uniform distribution of flame and hot gases through the chamber while soaking the ingots substantially at control point temperature and while avoiding any substantial variation in the ratio of air to gas introduced into the chamber.

2. In the operation of a gas-tired soaking pit for heating metal ingots to rolling temperature wherein the cycle of operation includes charging the ingots into the furnace, heating them rapidly to a predetermined temperature in an initial phase of the cycle in an atmosphere of burning gas and air in which there is a predetermined normal airto-gas ratio, and thereafter subjecting the ingots to a soaking phase to secure substantially uniform penetration of heat therethrough, the step of reducing oxidation of the ingots during the soaking phase While maintaining generally uniform heat distribution in the pit by reducing the supply of combustion air and gas used in the initial phase of the ingot heating cycle and supplying air and gaseous fuel to the pit in the same normal ratio used in the initial phase through a high velocity burner operating with an air discharge velocity of not less than 200 feet per second whereby the high kinetic energy of the burning air and gaseous fuel from the burner renders unnecessary the dilution of the gases in the pit with air in excess or the said normal ratio to secure adequate diffusion of the flame and hot gases through the pit.

3. In the operation of a gas-red soaking pit for heating metal ingots to rolling temperature wherein the cycle of operation includes charging the ingots into the furnace, heating them rapidly to a predetermined temperature in an initial phase of the cycle in an atmosphere of burning gas and air in which there .is a predetermined normal airto-gas ratio and thereafter subjecting the ingots to a soaking phase to secure substantially uniform penetration of heat therethrough, the steps of effecting the initial phase of the heating cycle by firing the pit with a plurality of gaseous fuel burners while supplying combustion air and fuel gas directly into the pit in a predetermined normal ratio, thereafter in the soaking phase supplying combustion air and gas directly into the pit from a single high velocity burner in which air and gas are maintained in the same normal ratio and idling the other burners, the air being discharged from the high velocity burner into the pit at a velocity of not less than 200 feet per second whereby the high kinetic energy of the resulting jet of air and gaseous fuel from the burner renders unnecessary the introduction of air in excess of said normal ratio to secure diffusion of the flames through the pit.

4. The method herein described of heating and thermally soaking ingots in a soaking pit, which method comprises firing the pit with a plurality of burners having passages for the flow of fuel and air directly into the pit at a selected and automatically regulated normal ratio of fuel to air and distributing with substantially uniform effect throughout the pit the flames and hot products of combustion and thereby heating the ingots until the surface portions of the ingot bodies substantially reach working temperature, and thereafter reducing the firing rate of the several burners until one burner is firing with the fuel and air flow maintained substantially at said normal ratio and with the combustion air propelled from its air passages at a velocity of at least 200 feet per second for uniformly soaking said ingots at drawing temperature.

5. The method herein described of heating metal ingots in a soaking pit which comprises firing the pit through fuel and air passages opening directly into said pit, from which air passages the air is delivered at a velocity of at least 200 feet per second, regulating at substantially predetermined normal value the ratio of the fuel to air flowing through said passages while effecting a substantially uniform distribution of flames and hot products of combustion throughout said chamber for heating said ingots substantially t-o working temperature, and thereafter re ducing the total supply of fuel by at least two-thirds and correspondingly reducing the number of said air passages in service while substantially maintaining said normal ratio of fuel to air and the flow of air at a velocity of at least 200 feet per second for thermally soaking said ingots.

6. The method herein described of heating a plurality of metal ingots in a soaking pit which comprises firing said pit with gaseous fuel and combustion air jets delivered directly into the pit, the air being propelled at a velocity of at least 200 feet per second, regulating at substantially predetermined normal value the ratio of the fuel to air flowing while effecting a substantially uniform distribution of flames and hot products of combustion with relatively high kinetic energy throughout the pit for heating said ingots while free from washing to a temperature approaching working temperature, then reducing the total suply of fuel and air by at least one-third until the ingots substantially reach working temperature, and thereafter reducing the total supply of fuel by at least two-thirds and correspondingly reducing the number of said air passages in service while substantially maintaining said normal ratio of fuel to air and the ilow of air at a velocity of at least 20() feet per second for thermally soaking said ingots with the kinetic energy of the llames and hot products of combustion maintained at approximately one-third of the kinetic energy first mentioned.

7. The method of heating a plurality of ingots and thermally soaking them in a furnace chamber to bring them to a condition to be worked, said method comprising the steps of firing said chamber through fuel-gas and air passages opening directly into said furnace chamber to eiect an initial heating of the ingots while controlling at a predetermined normal Value the ratio of the gas to air flowing, thereafter reducing the number of said air passages through which air flows directly into the furnace chamber and reducing the volume of the gas flowing while maintaining substantially at said value the ratio of gas to air and while maintaining the air flow at a velocity of at least 200 feet per second Ito thereby secure an energetic and substantially uniform distribution of flame and hot products of combustion through the chamber for soaking the ingots substantially at and within :the limits of the control point temperature.

S. The method of heating a plurality of ingots and thermally soaking them in a furnace chamber to bring them to a condition to be worked, said method comprising the steps of firing said chamber through fuel-gas and air passages opening directly into said furnace chamber to effect, with a relatively high kinetic energy of the llame and hot products of combustion, an initial heating of the ingots while controlling at a predetermined normal value the ratio of the gas to air flowing, thereafter reducing the number of said air passages through which air flows directly into furnace chamber and reducing the volume of the gas flowing while maintaining substantially at said Value the ratio of gas to air and while maintaining the air flow at a velocity of at least 200 feet per second to thereby secure in the chamber a substantially uniform distribution of flame and hot products of combustion having a kinetic energy of approximately one-third said relatively high energy for soaking the ingots substantially at and within the limit of the control point temperature.

9. The method of heating ingots in a soaking pit utilizing an initially high tiring rate followed thereafter by a period of soaking at a low firing rate, said method comprising injecting fuel gas into the pit and mixing and burning it with an air stream having an entering velocity of at least 200 linear feet per second during the high llring step and with the gas and air at a predetermined normal ratio, and thereafter effecting soaking at the low firing rate by reducing the volume of fuel gas to below an average of one-third the volume at the high rate and correspondingly reducing the Volume of air to maintain the ratio of gas to air substantially the same as at the high firing rate but maintaining the entering velocity of the combustion air at substantially the same linear speed of at least 200 feet per second while otherwise leaving unchanged the introduction of gases into the soaking pit whereby maximum kinetic energy of that flame which remains at the lower combustion rate is substantially onethird its kinetic energy at the higher combustion rate.

10. The method of heating ingots in a soaking pit utilizing an initially high firing rate followed thereafter by a period of soaking at a low tiring rate, said method comprising injecting fuel gas into the pit and mixing and burning it with an air stream having an entering velocity of at least 200 linear feet per second during the high tiring step and with the gas and air at a predetermined normal ratio, and thereafter effecting soaking at the low tiring rate by reducing the volume of fuel gas to within an average of one-third the volume at the highrate and correspondingly reducing the volume of air to maintain the ratio of gas to air substantially the same as at the high firing rate but maintaining the entering velocity of the combustion air at substantially the same linear speed of at least 200 feet per second while otherwise leaving unchanged the introduction of gases into the soaking pit whereby maximum kinetic energy of the tlame is obtained during the lower combustion rate with said normal ratio of gas to air.

Claims (1)

1. 3. IN THE OPERATION OF A GAS-FIRED SOAKING PIT FOR HEATING METAL INGOTS TO ROLLING TEMPERATURE WHEREIN THE CYCLE OF OPERATION INCLUDES CHARGING THE INGOTS INTO THE FURNACE, HEATING THEM RAPIDLY TO A PREDETERMINED TEMPERATURE IN AN INITIAL PHASE OF THE CYCLE IN AN ATMOSPHERE OF BURNING GAS AND AIR IN WHICH THERE IS A PREDETERMINED NORMAL AIRTO-GAS RATIONA DNTHEREAFTER SUBJECTING THE INGOTS TO A SOAKING PHASE TO SECURE SUBSTANTIALLY UNIFORM PENETRATION OF HEAT THERETHROUGH, THE STEPS OF EFFECTING THE INITIAL PHASE OF THE HEATING CYCLE BY FIRING THE PIT WITH A PLURALITY OF GASEOUS FUEL BURNERS WHILE SUPPLYING COMBUSTION AIR AND FUEL GAS DIRECTLY INTO THE PIT IN A PREDETEERMINED NORMAL RATIO, THEREAFTER IN THE SOAKING PHASE SUPPLYING COMBUSTION AIR AND GAS DIRECTLY INTO THE PIT FROM A SINGLE HIGH VELOCITY BURNER IN WHICH AIR AND GAS ARE MAINTAINED IN THE SAME NORMAL RATIO AND IDLING THE OTHERE BURNERS, THE AIR BEING DISCHARGED FROM THE HIGHVELOCITY BURNER INTO THE PIT AT VELOCITY OF NOT LESS THAN 200 FEET PER SECOND WHEREBY THE HIGH KINETIC ENERGY OF THE RESULTING JET OF AIR AND GASEOUS FUEL FROM THE BURNER RENDERS UNNECESSARY THE INTRODUCTION OF AIR IN EXCESS OF SAID NORMAL TO SECURE DIFFUSION OF THE FLAMES THROUGH THE PIT.

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