US2289350A - Method of reconditioning furnace tubes - Google Patents

Description

July 14, 1942. E. s. DIXON ETAL H METHOD OF RECONDITIONING FURNACE TUBES Filed Dec. :29,v 1937 K2 P & Ask- +0) W INVENTORS 'IEII llll llllll.

fih H- W ATTORNEY Patented July 14, 1942 UNITED STATES PATENT OFFICE METHOD OF RECONDITIONING FURNACE- TUBES Application December 29, 1937, Serial No. 182,174

13 Claims. (01. 196-69) Our invention relates to the cleaning of tubes and more particularly to the removal of coke or carbonaceous deposits from tubes employed in the conversion of hydrocarbon fluids.

When hydrocarbon fluids are subjected to elevated temperatures while flowing through banks of tubes suitably positioned within a furnace, deposition of coke or carbonaceous material upon the inner walls of the tubes comprising the heating coil is commonly encountered. The extent to which such deposition of coke or carbonaceous material within the tubes will take place is dependent upon the characteristics of the hydrocarbon fluid undergoing treatment and time and temperature conditions of the treatment, but it is nevertheless invariably encountered to a greater or lesser degree regardless of the cleanliness of the material treated. The problem of coke deposition is especially prominent in processes wherein the hydrocarbon fluids treated are subjected to temperatures at which decomposition of constituents of the material processed takes place, as for example, in the cracking of hydrocarbon oils. By reducing the available crosssectional area of the tube and greatly reducing the efliciency of heat transfer through the walls of the tube, such deposits upon the inner walls of the tube effectually impede the operation of the process.

Such tubes must therefore be subjected to periodic cleanings to remove the objectionable deposits. Various methods have been proposed for the removal of carbon from the inner walls of tubes but in practicing these methods it has been found that substantially complete removal of the coke-like deposits can be effected only by methods involving substantially complete oxidation of the deposit, resulting in temperatures harmful to the tube Wall or by supplementing the other previously disclosed methods by a removal of a considerable remaining portion of the deposit by means'of mechanical coke knockers or. by means of localized burning. Such a supplementary operation necessitates the removal at each cleaning of headers, return bends or the like. methods are ineflicient, not only because of the time lost inwaiting for the tubes to cool to a temperature at which headers, return bends or the like can be removed, but also because the frequent removal of headers return bends or the like materially aflects the life of the mechanical joints.

Our invention has for an object the provision of a method whereby coke or carbonaceous materials deposited upon the inner walls of tubes or These coils employed in the conversion of hydrocarbon fluids can be efficiently and substantially completely removed in a single complete operation thus eliminating the necessity of the supplementary mechanical operation and the removal of headers, return bends or the like.

Our invention has for another object substantially complete removal of coke-like deposits from tubes by a method which completely eliminates damage of breakage due to mechanical shock at low temperatures.

A further object of our invention is the provision of a method for the removal of carbonaceous deposits from fluid heating tubes wherein severe temperature conditions, harmful to the tubes, are avoided.

Another object of our invention is to provide a method for the reconditioning of tubes previously employed for the conversion of hydrocarbon fluids, such as the cracking of hydrocarbon oils, in which method a simultaneous decoking and heat treatment of the tubes is effected which relieves any strains produced in a foregoing conversion operation and results in a decided reduction of tube failures during subsequent conversion operations.

An additional object of our invention is the relieving of any embrittlement of the tubes and fittings.

We have found that when tubes or coils positioned in a furnace and containing a deposit of coke or carbonaceous material, such as is deposited during the conversion of a hydrocarbon fluid, are gradually raised to and maintained for a suitable time at a critical temperature, while steam is passed through the tubes, there is effected a breaking of the tube-to-carbon bond and spalling of the coke-like deposit is initiated. We have further found that by subsequent judicious control of the furnace temperatures, the judicious addition of small amounts of air. and the control of the direction of flow of the steam and air, coke deposits can be emciently and substantially completely removed. The added air is thought to promote spalling of the coke which is blown out with the steam as a granular mass or as a powder. Actual burning of the coke may be carried On to some extent but only to a limited extent and as a last resort.

According to our invention coke or carbonaceous material, deposited upon the inner walls of tubes or coils during the process of converting hydrocarbon fluids, is removed from these tubes or coils in conformity with the method comprising the following steps:

(a) Raising the tube temperature to 1250 F.- 1450" F., preferably to about 1350 F., while pass ing steam through the tubes positioned in the furnace.

(b) Maintaining the tube temperature at 1250 F.-l450 F., preferably at about 1350 while a substantial spalling of coke is taking place. During this period small amounts of air may be added to the steam to promote spalling, but burning of coke to any appreciable extent within the tubes is avoided.

(c) When the greater part of the spalled coke has been blown out, gradually lowering the furnace temperature and while doing so adding air in slowly increasing proportions of air to steam until finally air alone is being passed through the tubes. The furnace temperature is cooled to such a point that the tubes are at the lowest temperature at which the added air will effect decomposition of coke in the tubes. In practice we have found this temperature to be in the neighborhood of 1100 F.

(d) Raising the tube temperature to a point immediately below that at which scaling or change in the metal structure of the tube might result and continuing the flow of air in order to eifect decomposition of any carbonaceous material whichmay have survived the spalling operation. This temperature maybe of the order of 1250 F. to 1450" F. or higher depending upon the chemical composition of the tube metal.

(e) As a final step, reversal of the flow of air may be made in each direction in order to insure the thorough scavenging of all loose material and to efiect the uniform distribution of the high temperatures necessary to burn away any remaining coke.

The method of decoking tubes in accordance with our invention will be more fully understood from the following description read in connection with the accompanying drawing which shows a diagrammatic representation of an oil cracking coil, suitably positioned in a furnace, and provided with one form of equipment suitable for coke removal according to the method of the invention.

Referring to the drawing, coil 2 represents a single, or several banks, of tubes such as employed in the conversion of hydrocarbon fluids, for example, the cracking of hydrocarbon oils, and which are suitably positioned in a furnace I. It is not intended that the representations of furnace I' and coil 2- be limited to any specific type used in a specific process, for the method of the invention is applicable to any of the many types of heating coils used in the conversion of hydrocarbon fluids and is not limited in any manner by the nature of the process employed to effect the conversion of the hydrocarbon fluid.

During the process of conversion the hydrocarbon fluid to be processed is forced, by means not shown in the drawing, through line 3 controlled by valve 4 into the coil 2. The hot products of conversion leave coil 2 through line 5 controlled by valve 6. When deposition. of coke or carbonaceous material upon the inner walls of the tubes comprising coil 2 becomes such as to necessitate reconditioning of the tubes or for other reasons, the unit is shut down in a conventional manner. Upon completion of this phase of the operation valves 4 and 6 are closed to isolate the heating coil from the rest of the unit.

Steam lines I, 8, 9 controlled by valves I 0, II, I2 and air lines I3, I4, I5 controlled by valves I6, I'I, I8 are manifolded to lines I9, and 2| connected respectively to the inlet, an intermediate point of, and the outlet of coil 2. Lines I9 and 20 are controlled by means of valves 22 and 23 respectively. Lines I9, 20 and 2| establish communication between coil 2 and line 26 controlled by valves 24 and 25.

To initiate the reconditioning operation of the tubes after the particular conversion process is discontinued, the furnace temperature is gradually raised to a temperature of from 1250 F. to 1450 F., preferably about 1350 F. Valves 4, 6, 22 and 23 are closed, valves 24 and 25 are opened, and steam is admitted through line I controlled by valve IE! and passed through line I9 into coil 2. Steam leaves the outlet of coil 2 and flows through lines 2I and 25 to be exhausted through line 21 to a stack or other suitable place of disposal. An elevated steam pressure, preferably above lbs. per sq. in., and preferably of about to lbs. per sq. in. at the inlet of the coil is maintained during this initial phase of the operation. When these preliminary heating conditions are carefully maintained for a period of time, usually not exceeding about three hours, a spalling action is initiated comprising the breaking of the tube-to-coke bond and the disintegration of the carbonaceous deposit into a broken mass which may vary in nature from a granular to a powdery material.

A substantial portion of the coke deposit becomees detached from coil 2 during this spalling operation and is blown from the coil by the steam which also functions as a vehicle for the spalled coke which it eliminates from coil 2 through lines 2|, 26 and 21. It is sometimes found necessary to promote spalling by the intermittent addition of small amounts of an oxygen-containing gas, for example air which, in this particular stage of the cleaning operation, is introduced through line I3 controlled by valve IS. The amount of air required to promote spalling varies with the type of deposit; often none is needed and at other times addition of as much as ten per cent or more of the steam passes through coil 2 is required to initiate the desired effect. It should be pointed out thatthis addition of oxygen-containing gas is essentially to initiate and promote the spalling effect peculiar to this temperature range. The manner in which this addition of the oxygen-containing gas initiates and promotes the spalling action is not definitely known. However, it is thought that the spalling of coke is due in measure to the difference existin; between the value of. the coefficient of expansion of the metal of the tube Wall and that of the coefiicient of expansion of the carbonaceous deposit and to the differences in the coefficients of expansion of varying sections of the carbonaceous deposit itself, and it is believed that upon introducing a limited quantity of an oxygen-containing gas into coil 2 at the operating conditions, the restricted combustion of coke with its inherent sudden temperature change effects the breaking of the tube-to-carbon bond and the simultaneous breaking-up of the carbonaceous deposit. This mechanical disintegration of the carbonaceous deposit is then accelerated without further combustion. of coke by the sudden cooling effect of the steam flowing through the coil, in the absence of any oxygencontaining gas, at a temperature low-er than that of the metal of the tube wall. This mechanical disintegration of the carbonaceous deposit induced, if necessary, by the combustion of only a small fraction of the deposit, supplemented by lated to prevent erosion of the lines.

the further loosening effect of the steam flow is believed to constitute the major part of the spalling action by which means substantially all of the carbonaceous deposit is removed from the tube in solid form as a powdery or granular mass that has undergone neither chemical change nor change of state. Once the spalling action is well under way the steam pressure should be regu- Regulation is based on Visual observation of the quantity and character of the coke discharged. Discharge of coke, exhausted steam and air leaving the systemthrough line 21 is effected within a suitable receptacle or stack and subsequent reaction of the discharged materials is prevented by the addition of a quenching medium, such as water, through line 29 controlled by valve 28. When the spalling action ceases, substantially all of the coke deposit will have been removed from coil 2. Various reversals of the direction of flow through the coil of the steam fiow may be made to promote complete scavenging of loose material. a

The phase of the cleaning operation subsequent to the spalling operation serves to remove by combustion any residual coke within coil 2 which may have escaped the primary phase of the operation. The transition from the spalling operation to the second stage is efiected gradually without interruption of the process. When the spalling action begins to decrease in intensity, which will usually be about two hours after spalling has been initiated, the temperature of the furnace is gradually reduced while admitting an oxygen-containing gas, for example air, in progressively increasing quantities into coil 2 through lines I3 and I9 and simultaneously decreasing the flow of steam through coil 2 by gradually closing valve Hi. When spalling has ceased completely the furnace temperature is allowed to drop to 1000" F.-1200 F., and valve ii] is closed gradually until only the oxygencontaining gas is passed through coil 2, the tubes being kept under careful observation. O'ccasional additions of steam may be made to the system through line I during this phase of the operation to prevent local overheating of the tubes. The passage of the oxygen-containing gas is continued until there is no evidence of coke-combustion within coil 2. The furnace temperature is then gradually raised to approximately 1250 F. while the air flow is continued to remove any coke that may not have been affected at the lower temperatures. When it is apparent that there is no further coke-combustion within coil 2 with the furnace temperature maintained at about 1250 F. the flow of the oxygen-containing gas through coil! is reversed by closing valves i6, 24, 25 and opening valves l2 and 22. The oxygen-containing gas admitted to the system through line 9 controlled by valve l2 will flow through line 2| into coil 2 to burn out the last remnants of coke deposit. By thus reversing the flow, final remnants of coke deposits, which may have remained in coil 2, for example in that part of coil 2 which was the cooler extremity during the previous fiow, will be effectively removed. When all evidence of coke combustion Within the tubes has finally ceased, as indicated by visual observation or by chemical test of the effluent gas for carbon dioxide the furnace fires are extinguished while the flow of the oxygencontaining gas is continued.

The method of reconditioning tubes in accordance with the invention is of course not limited to the specific steps mentioned in the illustrative operation of the method. To obtain and maintain the critical conditions given for the spall ing operation, and to effect the subsequent removal of final traces of coke deposit, it may be desired and sometimes be advantageous to depart or enlarge upon the specific steps of the operation as described without departing from the spirit and scope of the invention. One of such possible additional steps is shown in the drawing. A line 20 controlled by valve 23 leads from an intermediate point of the coil into line 26. A steam line 8 controlled by valve II and an air line H controlled by valve H are manifolded into line 20. The intermediate point of the coil 2, at which connection with line 20 is made, is preferably a point immediately preceding that section of coil 2, at which the carbonaceous deposit is heaviest. By closing valve 23, and suitable manipulation of valves I I and I1, air or steam or both may be introduced into coil 2 to assist in any phase of the operations mentioned in the illustrative example. The method may further be modified by interrupting any phase of the operation by closing of valve 23, opening of valves 22, 24 and 25 and the introduction of air or steam or both into coil 2 through line 20 to effect the discharge of coke, ash or both from coil 2 through line-2| and line l9 simultaneously.

Other modifications, such as the interruption of any of the steps comprised in the illustrative example of the invention for the purpose of passing only steam through a part or all of coil 2 in order to heat or cool any part or all of coil 2, will, of course, be apparent to one skilled in the art. It is not intended that such obvious modifications be considered beyond the scope of our invention, nor is it intended that our method be limited by omission of any obvious modificatory steps from the illustrative example.

The steps of reconditioning hydrocarbon fiuid heating tubes, according to the invention, comprising the gradual heating of these tubes to the critical temperature range in which spalling of the carbonaceous deposit is brought about as set forth in the illustrative example, their maintenance at these temperatures within the prescribed limits of time and their subsequent slow cooling, comprise a very beneficial heat treatment resulting in the release of contained stresses and embrittlement and a great reduction in the number of tube failures. This feature of the reconditioning operation is an important one and is furthered by judicious control of the flexible time element of the reconditioning process. We do not intend to limit ourselves to any specific temperature ranges or steam and air pressures and we prefer to work at the highest temperatures to which it is practicable to heat the tubes.

In practicing the method of reconditioning heating tubes in accordance with the invention man modifications and variations may be made without departing from the spirit of the invention and therefore only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. The method of removing carbonaceous material from tubes positioned in a heater previously employed for heating a hydrocarbon fluid to a temperature at which carbonaceous material is deposited upon the inner walls of said tubes which comprises the steps of subjecting the tubes by means of external heating to an elevated temperature, continuously passing steam throughsaid tubes while at an elevated temperature to effect spa'lling of the carbonaceous deposit within said tubes and effect removal of carbonaceous material as a granular or powdered mass, periodically adding controlled quantities of gas containing free oxygen to said steam to promote further spalling of said carbonaceous deposit, maintaining the temperature conditions and the fiow of steam through said tubes until the spalling action substantially ceases, reducing the temperature to which said tubes are subjected, continuously passing gas containing free oxygen through said tubes while at a, lower temperature, gradually raising the temperature to which the tubes are exposed while continuing said flow of gas containing free oxygen, thereafter reversing said flow of gas containing free oxygen and graduall reducing the temperature to which the tubes are exposed.

2. The method of removing carbonaceous material from tubes which have been previously employed to heat a hydrocarbon fluid to a temperature at which carbonaceous material is deposited upon the inner walls of the tubes which comprises externally heating said tubes to an elevated temperature, continuously-passing steam through said tubes to effect spa-lling of the carbonaceous material within said tubes and efiect removal of carbonaceous material as a granular or powdered mass, intermittently adding controlled quantities of gas containing free oxygen to said steam to further promote spalling of said carbonaceous material, maintaining said temperature conditions and steam flow until the spelling action substantially ceases, thereafter slowly increasing the proportion of gas containing free oxygen to steam while gradually lowering the temperature of said tubes to the lowest temperature at which said added gas containing free oxygen will efiect decomposition of said carbonaceaus material within said tubes until finally gas containing free oxygen alone is passed through said tubes, raising the temperature of said tubes to a temperature below that tem perature at which scaling or change in the metal structure of said tubes results and maintaining the flow of said gas containing free oxygen to substantially completely remove any remaining carbonaceous material.

3. The method of removing carbonaceous material from tubes positioned in a heater previously employed for heating hydrocarbon fluids to a temperature at which carbonaceous material is deposited upon the inner walls of said tubes which comprises the steps of raising the temperature within said heater to 1250" F. to 1450 F., continuously passing steam through said tubes to effect spalling of the carbonaceous material within said tubes, periodically adding controlled quantities of gas containing free oxygen to said steam to further promote spalling of said carbonaceous material, maintaining the temperature conditions and flow of steam until substantially complete removal of the carbonaceous material from said tubes as a granular mass or powder has been effected, reducing the furnace temperature to about 1100 F., continuously passing gas containing free oxygen through said tubes while the heater is maintained at a temperature of about 1100 F., gradu ally raising the heater temperature to about 1250 F. while maintaining said flow of gas containing free oxygen through said tubes, thereafter eversing said fiow of gas containing free oxygen and reducing the heater temperature.

4. The method of removing carbonaceous material from tubes positioned in a heater previously used for heating a hydrocarbon fluid to a temperature at which carbonaceous material is deposited upon the inner walls of said tubes which comprises the steps of subjecting said tubes by means of external heating to a temperature of about G F., continuously passing steam through said tubes while at a temperature of about 1350 F. to eiiect spelling of the carbonaceous deposit within said tubes, intermittently adding controlled quantities of gas containing free oxygen to said steam to further promote the spalling of said carbonaceous deposit, maintaining the temperature conditions and the flow of steam through said tubes until the spalling action ceases, decreasing the tube temperature to about 1160 F. while continuously adding gas containing free oxygen in increasing proportions to said steam flowing through said tubes, gradually raising the tube temperature to about 1250" F. while passing only gas containing free oxygen through said tubes, thereafter reversing said flow of gas containing free oxygen through said tubes and lowering the heater temperature while continuing said flow of gas containing free oxygen through said tubes.

5. The method of removing carbonaceous deposits from fluid conduits in an oil heater previously used for heating hydrocarbon fluids to cracking temperatures which comprises the steps of subjecting the conduits by external heating to a temperature of about 1350 F., continuously passing steam through said conduits while at a temperature of 1350" F. to effect spalling of the carbonaceous deposit within said fluid conduits, intermittently adding air to said steam to further promote spelling of said carbonaceous material, maintaining the conduit temperature of about 1350 F. while continuing said flow of steam and intermittent additions of air to eiiect substantially complete removal of said carbonaceous material from said conduits as a granular mass or powder, gradually lowering the heater temperature to about 1lC-0 F. while continuously adding air in gradually increasing quantities to said steam, raising the temperature of the conduits to about 1250 F. while passing only air therethrough, thereafter reversing said flow of air and maintaining the rever ed flow of air while lowering the conduit temperature.

6. The method of reconditioning tubes in an oil heater previously used for heating hydrocarbon fluids to conversion temperatures comprising the steps of gradually raising the temperature within the heater to about 1350 F. while passing steam through said tubes, maintaining said heater at about 1350 F. and continuing the flow of steam to efiect substantially complete removal of the carbonaceous deposit from said tubes, decreasing the heater temperature to about 1100 F. while continuously adding air in gradually increasing amounts to said steam, gradually raising the heater temperature to about 1250 F. while passing only air through said tubes, reversing the flow of air through said tubes and maintaining the reversed flow of air while reducing the furnace temperature.

7. The method of removing carbonaceous material from tubes positioned in a heater previously employed for heating hydrocarbon fiuids to a temperature at which carbonaceous material is deposited upon the inner walls of said'tubes which comprises the'steps of raising the temperature within said heater to about 1250 F.'-1l50 F., continuously passing steam through said tubes to effect spalling of carbonaceous material within said tubes and removing carbonaceous material as a powdered or granular mass from said tubes, maintaining the temperature and flow of steam until the spalling action substantially ceases, reducing the tube temperature to about 1100 F. while decreasing the flow of steam and adding gas containing free oxygen to the steam to cause burning of carbonaceous material adhering to said tubes, stopping the flow of steam and passing only gas containing free oxygen through said tubes while maintaining the temperature to burn off carbonaceous material adhering to said tubes, gradually raising the temperature of the tubes to about 1250 F.-1450 F. while maintaining the flow of gas containing free oxygen through said tubes to burn off substantially all of the remaining carbonaceous material adhering to said tubes not removed by the burning at the lower temperature.

8. The method of removing carbonaceous material from tubes and simultaneously conditioning said tubes which have been previously employed to heat a hydrocarbon fluid to a temperature at which carbonaceous material isdeposited upon the inner walls of the tubes which comprises externally heating said tubes to an elevated temperature, continuously passing steam through said tubes to effect spalling of the carbonaceous material within said tubes and removal of carbonaceous material as a powdered or granular mass, maintaining the temperature and the flow of steam until the spalling action substantially ceases, lowering the temperature of said tubes While decreasing the flow of steam through said tubes and adding gas containing free oxygen to the steam flowing through said tubes to effect controlled burning of carbonaceous material adhering to said tubes, stopping the flow of steam and increasing the amount of gas containing free oxygen passing through said tubes while maintaining the tube temperature at the lowest temperature at which the gas containing free oxygen will burn off carbonaceous material in said tubes, raising the temperature of said tubes to a temperature below that temperature at which scaling of said tubes occurs and maintaining the flow of gas containing free oxygen to substantially completely remove any carbonaceous material adhering to said tubes after the burning off at said lower temperature.

9. The method of removing carbonaceous deposits from tubes positioned in a heater and previously used to heat hydrocarbon fluids to a temperature at which carbonaceous material is deposited upon the inner walls of the tubes which comprises raising the temperature in the heater to about 1350 F., continuously passing steam through said tubes while said heater is maintained at a temperature of about 1350 F. to effect spalling of the carbonaceous deposit, adding a gas containing free oxygen to the steam to further promote spalling of the carbonaceous material within said tubes, maintaining the temperature conditions and flow of steam until spalling ceases, reducing the heater temperature to about 1100 F. and passing a gas containing free oxygen through said tubes, gradually raising the heater temperature to about 1250 F. while continuing the flow of gas containing free oxygen and reversing the flow of the gas containing free oxygen and reducing the heater temperature.

10. The method of removing carbonaceous deposits from tubes positioned in a heater and previously used to heat carbonaceou fluids to a temperature at which carbonaceous material is deposited upon the inner walls of the tubes which comprises subjecting said tubes to elevated temperature while continuously passing steam through said tubes to effect spalling of the carbonaceous deposits within said tubes and effect removal of carbonaceous material as a granular or powdered mass, intermittently adding gas containing free oxygen to said steam in a quantity adequate to promote the spalling action but insuflicient in quantity to effect burning of the coke to any substantial extent, maintaining said temperature conditions and steam flow until the spalling action substantially ceases, thereupon reducing the temperature to which aid tubes are subjected, continuously introducing a gas containing free oxygen to said tubes while at a lower temperature, and gradually raising the temperature to which the tubes are subjected. while continuing the flow of said gas containing free oxygen to effect controlled combustion of carbonaceous material adhering to said tubes.

11. The method of removin carbonaceous deposits from tubes positioned in a heater and previously used to heat carbonaceous fluids to a temperature at which carbonaceous material is deposited upon the inner walls of the tubes which comprises subjecting said tubes to elevated temperature while continuously passing steam through said tubes to effect spalling of the carbonaceous deposits within said tubes and effect removal of carbonaceous material as a granular or powdered mass, continuing the application of elevated temperature and continuing the flow'of steam through said tubes until the spalling substantially cease-s, thereupon reducing the temperature of said tubes and. introducing a gas containing free oxygen thereinto, continuing the introduction of said gas containing free oxygen while raising the temperature of said tubes sufficiently to effect controlled combustion of carbonaceous material adhering to said tubes but be- 10W the temperature applied in the spalling action.

12. The method of removing carbonaceous deposits from tubes positioned in a heater and previously used to heat carbonaceous fluids to a temperature at which carbonaceous material is deposited upon the inner walls of the tubes which comprises subjecting said tubes to a furnace temperature within the range of approximately 1250 F. to 1450 F. while continuously passing steam through said tubes to effect spalling of the carbonaceous deposits within said tubes and effect removal of carbonaceous material as a granular or powdered mass, continuing the application of said temperature and continuing the flow of steam through said tubes until the spalling action substantially ceases, thereupon reducing the furnace temperature applied to said tubes while holding said temperature approximately within a range of 1000 F. to 1200 F. and passing a gas containing free oxygen through said tubes and gradually raising the furnace temperature to about 1250 F. while continuing the flow of gas to effect spallin of the carbonaceous deposits within said tubes and efi'ect removal of carbonaceous material as a granular or powdered mass, continuing the application of said temperature and continuing the flow of steam through said tubes until the spalling action substantially 10 ceases, thereupon reducing the furnace temperature applied to said tubes while holding said temperature approximately within a range of 1000" F. to 1200 F. and passing a gas containing free oxygen through said tubes and gradually raising the furnace temperature to about 1250 F. while continuing the flow of gas containing free oxygen to effect controlled combustion of carbonaceous material adhering to said tubes.

ENSLO S. DIXON.

THOMAS E. GARRARD.

HAROLD A. BARR.

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