US2289351A

US2289351A - Method of cleaning heater tubes

Info

Publication number: US2289351A
Application number: US266336A
Authority: US
Inventors: Enslo S Dixon; Thomas E Garrard; Harold A Barr
Original assignee: Texaco Inc
Current assignee: Texaco Inc
Priority date: 1939-04-06
Filing date: 1939-04-06
Publication date: 1942-07-14
Anticipated expiration: 1959-07-14

Description

E. S. DIXON ETAL METHOD OF CLEANING HEATER TUBES July 14, 1942.

Filed April 6, 1939 lllllll lllllr.

llllllllllll f lllllll .llllll v a INVENTORS [ll 5L0 SID/X0 THOMAS E: GARRA/PD BY #40042 A. an?

ATTORNEY Patented July 14, 1942 2,289,351 METHOD or CLEANING HEATER TUBES Enslo S. Dixon, Thomas E. Garrard, and Harold A. Barr, Port Arthur, Tex., assignors, by mesne assignments, to The Texas Company, New York, N. Y., a corporation of Delaware Application April 6, 1939, Serial No. 266,336

16 Claims.

This invention relates to the cleaning of heater tubes and more particularly to the removal of coke or carbonaceous deposits from heater tubes employed in the conversion of hydrocarbon fluids. c

When hydrocarbon fluids are subjected to elevated temperatures while flowing through banks of heater tubes suitably positioned within a. furnace, deposition of coke or carbonaceous material upon the inner walls of the heater tubes is commonly encountered. The extent to which such deposition of coke or carbonaceous material within the tubes will take place is depend ent 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 totemperatures at which decomposition of hydrocarbon constituents takes place, as for example, in the cracking of hydrocarbon oils. By reducing the available cross-sectional area of the heater tubes and greatly reducing the efficiency of heat transfer through the walls of the tubes, such deposits upon the inner walls of the tubes effectually impede the operation of the process.

Heater tubes must therefore be subjected to periodic cleanings to remove the objectionable carbonaceous 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 burning out the deposit, resulting in temperatures harmful to the tube wall or by supplementing known methods by a removal of a considerable remaining portionof the deposit by means of mechanical coke knockers. Such a supplementary operation necessitates the removal at each cleaning of headers, return bends or the like. methods are inefficient, not only because of the time lost in waiting forthe tubes to cool to a temperature at which headers, return bends or the like can be removed, but also because the frequent removalof headers, return bends or the like materially affects the life of the mechanical joints. 7

According to our invention coke or carbonaceous material deposited upon the inner walls of tubes or cells employed in the conversion of hydrocarbon fluids can beefficiently, quickly 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.

These Substantially all of the coke-like deposits are removed from tubes by our method which completely eliminates breakage due to mechanical shock at low temperatures. Also carbonaceous deposits are removed from fluid heating tubes by our method wherein severe temperature conditions, harmful to the heating tubes, are avoided. When decoking utilizes temperatures above about 1250" F. in the heater, a thorough stress relieving and removal of embrittlement of chromium alloy heater tubes is affected.

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 an elevated 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. The coke-like deposit is removed from the tubes as a powdered or granular mass by the steam. Small amounts of air may be added to the steam to promote spalling of the coke which is blown out with the steam as a granular mass or as a powder. By this spalling step the greater part of the carbonaceous deposit is rapidly removed from the heater coil.

After the larger part of the carbonaceous deposit is removed by the spalling or disintegration step, the next step is a controlled combustion or oxidation of the remaining portion of the carbonaceous deposit and this is effected by mixing steam and air or other oxygen-containing gas. Toward the end of the operation air alone is preferably passed through the coil. By this combination of steps, the carbonaceous deposit is rapidly and efficiently removed without injury to the heater tubes or coil and without the necessity of dismantling the heating apparatus.

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 including, generally, the following steps:

(a) After the furnace has been shut down, the heater tubes are steamed free of oil in the usual way while the furnace is being heated up.

(b) The furnace is then fired to about 1000 to 1350 F, while steam is passed through the tubes positioned in the furnace.

(c) The furnace temperature is maintained at about 1000 to 1350 F. while a substantial spalling of coke is taking place. During this period the coke or carbonaceous material is removed from the heater tubes in solid form, either as a powder or in granular form. During the removal of this coke, small amounts of air may be added to the steam to promote spalling, but burning of the coke or carbonaceous material to any appreciable extent is avoided. Reversals of the flow of steam may be made when the flow of carbonaceous particles from the tubes has decreased substantially.

(d) When the greater part of the spalled coke or carbonaceous material has been blown out, small amounts of air or air in slowly increasing proportions of air to steam is added until in the final stages of the operation, air alone is being passed through the tubes. Usually a furnace temperature below the upper limit is used so that the tubes are at the lowest temperature at which the added air will effect decomposition of the coke or carbonaceous material in the tubes.

(e) Toward the end of the operation when most of the coke or carbonaceous material has been removed from the tubes, air alone is passed through the tubes for burning out the residual amounts or portions of the coke or carbonaceous material adhering to the inner walls of the tubes.

(1) Reversals of the flow of air through the tubes may be made in each direction in order to insure the thorough scavenging of all loose material and to effect the uniform distribution of the high temperatures necessary to burn away any remaining coke.

(Q) As a final step, water under pressure is preferably pasesd through the tubes to flush out coke ash.

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 a conversion coil, suitably positioned in a furnace, and provided with one form of equipment suitable for coke removal according to our invention Referring now to the drawing, the reference character I designates a furnace. A coil 2 represents a single bank or several banks of tubes such as are usually employed in the conversion of hydrocarbon fluids, for example, the cracking of hydrocarbon oils, and which are suitably positioned in the furnace I. It is not intended that the apparatus including furnace l and coil 2 be limited to the form shown in the drawing, as our method is applicable to any of the many types of heating coils or tubes and furnaces used in the conversion of hydrocarbon fluids and is not limited in any manner by the nature of the process employed to eifect the conversion of the hydrocarbon fluid.

The hydrocarbon fluid to be treated 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 cleaning 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 B are closed to isolate the heating coil from the rest of the unit.

Steam lines 1, 8, 9 controlled by valves 10, II, I 2, respectively, and air lines I3, l4, l5 controlled by valves IS, IT, l8, respectively, are manifolded to lines I9, and 2|, respectively. Lines [9, 20 and 2|, are connected respectively to the inlet line 3, an intermediate point of the coil 2 near the inlet, and the outlet line 5 of coil 2.

Lines

19 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.

As shown in the drawing another steam line 21 having valve 28 and another air line 29 having valve 30 are manifolded into line 3| which communicates with the coil at the latter portion thereof and near the outlet end of the coil. The number of sets of air and steam lines used depends on the size of the coil 2. As shown in the drawing, line 20 communicates with the coil 2 at about one-third of the Way through the coil and line 3| communicates with the coil 2 at about two-thirds of the way through the coil. With smaller coils, line 3| may be omitted and with larger coils, additional sets of steam and air lines may be used, if desired.

One method of cleaning tubes will now be given. After the conversion operation is discontinued,

valves

4, 5, 22 and 23 are closed and valves 24 and 25 are opened. At first steam is passed through lines 7 and I9 and through coil 2 to steam out oil in the coil. The furnace temperature is raised during the steaming out step. The furnace is then further heated to a temperature of about 1000 F.l350 F., and during this time the steam at an elevated pressure is passed through lines I and I9 and through the coil 2. For example, in one case the furnace temperature was raised from 700 F., to 1300 F., in one hour while steam at about '70 pounds per square inch was being passed through the heating coil 2. Heater coils which have been subjected to severe cracking conditions require the higher temperatures while lower temperatures will be adequate for heaters for crude stilling heaters. With crude stilling heaters the heating may be more gradual to effect spalling of the carbonaceous deposit. In some instances higher temperatures than 1350 F. may be necessary. Because of the tendency of steel heater tubes to scale at elevated temperatures, care is taken not to overheat the tubes. Higher temperatures could be used if metal tubes were used which did not scale and the spalling operation could then be hastened.

Thermocouples are disposed in the furnace to determine the maximum temperatures to which the tubes will be subjected by external heating. Steam leaves the outlet of coil 2 and flows through lines 2! and 26 to be exhausted through line 32 to a stack 33 or other suitable place of disposal. An elevated steam pressure is maintained during this initial phase of the operation. The velocity of steam through the coil 2 should not be so large as to result in erosion of the return bends, etc., of the coil. Steam pressures up to about pounds per square inch have been used but usually lower pressures are used. Instead of first passing steam through line I at the beginning of the operation, steam may be passed through line 9.

When these preliminary heating conditions are carefully maintained for a period of time, usually about an hour and not exceeding about eight hours, a spalling action is initiated comprising the breaking of the tube-to-coke bond and the distintegration of the carbonaceous deposit into a broken mass which may vary in nature from a granular to a powdery material. After spalling of the carbonaceous material begins, the rate of steam flow is reduced to prevent erosion of headers as will be pointed out hereinafter.

A substantial portion of the coke deposit be comes detached from coil 2 during this spalling operation and is blown from the coil as coke particles by the steam which also functions as a vehicle for the spalled coke which it eliminates from coil 2 through lines 21, 26 and 32. The flow of steam through the coils may be reversed one or more times. It is sometimes found necessary to promote spalling of the coke by the intermittent addition of small amounts of an oxygencontaining gas, for example air, to the steam. The oxygen-containing gas in this particular stage of the cleaning operation is passed through line I3 controlled by valve I6 and introduced into the inlet end of coil 2 by line [9. 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 passed through coil 2 is required to initiate the desired efiect.

It should be pointed out that this addition of oxygen-containing gas at this point in the process is essentially to initiate and promote the spalling effect or mechanical disintegration of the carbonaceous deposit. 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 existing between the value of the coefficient of expansion of the metal of the tube wall and that of the coefficient 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 oxygencontaining gas into coil 2 at the operating conditions, the restricted combustion of coke with its inherent sudden temperature change efiects 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 eifect of the steam flowing through the coil, in the absence of any oxygen-containing gas, at a temperature lower 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 the further loosening eifect 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. It is believed that in this action the carbonaceous deposit is dried of volatile material, hardened and embrittled, and then cracked by the drying and shrinkage.

Once the spalling action iswell under way the steam pressure is reduced to prevent erosion of the lines and headers. Regulation of the steam pressure is based on visual observation of the quantity and character of the coke discharged through sample line 34 or other vent. One method for regulating steam pressure to prevent erosion is practiced as follows but other methods may be used. From time to time the operator reduces the inlet pressure to such a pressure as will continue to bring out coke particles and then raises the pressure until the coke particles flowing out begin to show a definite increase in percentage of fines, indicating that abrasion has started. This is the maximum pressure or highest permissible velocity to be used at this stage of the cleaning operation. 7

At times it has been found that less than 10 pounds per square inch. inlet steam pressure is sumcient to bring a copious flow of coke particles. ranging in size from dust to pea size. If coke flow stops at: the lowered pressure, the steam. pressure is increased at intervals to maintain a. flow of coke particles. Discharge of coke and exhausted steam leaving the system through line 32 is elfected Within stack 33, sewer or other suitable means and subsequent reaction of the discharged materials is prevented by the addition of a quenching medium, such as water, through line 36 controlled by valve 31. When the flow of coke decreases, small amounts of air may be added to the steam to maintain removal of coke particles at the desired rate- When the flow of coke lessens materially, reverse flow of steam is preferably used. Valve H] in steam line 1 and valve 24 in line 26 are closed and valve I 2 in steam line 9 and valve 22 in line l9 are opened so that steam under pressure is forced through the coil 2 in a reverse direction. Small amounts of air may be added to, the steam for short periods of time to promote spalling and removal of coke as a powder or in a granular form. The addition of air promotes spalling to such an extent at times that it is necessary to reduce air and steam pressures to prevent erosion of headers. Various reversals of the, direction of flow of steam or steam and air through the coil 2 may be made to effect complete scavenging of loose coke or carbonaceous material from the tubes.

When the spalling action ceases, a large part of the coke deposit will have been removed from coil 2. When the spalling action has ceased and the flow of particles of carbonaceous material has substantially stopped, which will usually be about 4-6 hours after spalling has been initiated, additional amounts of oxygen-containing gas, for example air, are introduced into the steam flow as will be presently described. Usually the spalling action stops after about 4-6 hours but this time may vary with different carbonaceous deposits. During this time the furnace temperature: is maintained at about 1000-1350 F. By this first step, we rapidly remove the greater part of the carbonaceous deposit in the coil or tubular heater by spalling or disintegration of the carbonaceous deposit and the detached particles of carbonaceous material are blown from the coil as such. The spalling step forms an essential part of our invention.

The next step of the cleaning operation subsequent to the spalling operation serves to remove by controlled combustion or oxidation any residual coke adhering to the walls of coil 2 which may have escaped the primary phase of the operation. Controlled combustion is obtained by controlling the relative amounts of steam and air. The air may be admitted in increasing quantities into coil 2 through lines 13 and I9 while simultaneously decreasing the flow of steam through coil2 by gradually closing valve Ill. The transition from the spalling operation to the second stage is effected gradually without interruption of the process. During controlled combustion of the carbonaceous deposit, the furnace is preferably kept at as low a temperature as will insure combustion of the carbonaceous deposit. Some carbonaceous deposits are highly inflammable and a low temperature may be used, but other types of carbonaceous deposits ore diifieult to burn and temperatures up to 1300 F., are necessary to start and maintain combustion. The actual temperatures used will vary between about 1000 F.-1300 F., and will depend on any one job on the observation of the operator at that time as to the ease of igniting of the carbonaceous deposit. For example, in one instance the temperature of the furnace for controlled combustion of the carbonacecus deposit in the heater tubes was about 1200 F., to 1280 F. For controlled combustion of carbonaceous deposits from heater tubes in crude preheating furnaces on stilling units, lower temperatures than those just given may be used.

This operation is used to burn the patches of coke or carbonaceous material which do not loosen or blow out in the spalling operation. The operator controls the amount of air in the steam flow by watching the reddening of the tubes to prevent local overheating of the tubes. Various tubes and patches or portions of tubes will be seen to fire up and cool down successively along the tube flow from inlet to outlet and at length it will be apparent that the coke or carbonaceous material has been burned from all the tubes. Reversals of flow of steam and air may be used. Then increasingly richer air mixtures are used and finally when all steam flow is off and only air is flowing through the tubes a flue gas test at the vent or sample line should show no CO2 or only a trace of CO2 as will be presently described.

When it is apparent that the carbonaceous material has been substantially all burned out with the use of steam and air, the furnace temperature is maintained between about 1000-1350 F. and valve ID in steam line 1 is closed gradually until only the oxygen-containing gas is passed through lines I3 and I 9 and coil 2, the tubes being kept under careful observation. Occasional additions of steam may be made to the system through line 1 during this phase of the operation to prevent local overheating of the tubes. The passage of the oxygencontaining gas is continued until there is no evidence of coke-combusion within coil 2. To determine whether or not coke combustion is completed, a glowing splinter can be used at the outlet to see if oxygen or CO2 is coming out or a gas test may be made to determine the amount of CO2 in the gas coming out of the outlet of the coil. Where unusually large amounts of coke are to be burned out the furnace temperature is lowered to about 1100 F. or lower but usually higher temperatures are used.

When it is apparent that there is no further burning or combustion of coke within coil 2 with flow of air in one direction and with the furnace temperature maintained at about 1000-1350 F., the flow of the oxygen-containing gas through coil 2 is preferably reversed by closing valves I6, 24, 25 and opening valves l8 and 22. The oxygen-containing gas admitted to the system through line l5 controlled by valve l8 will flow through line 2I int coil 2 and through coil 2 to burn out any last remnants or patches of coke deposit, the hot gases passing through line H) to the stack 33. 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 flow, will be effectively removed. The reversal of air flow may be repeated, if desired. However, in some cases no reversal of air flow will be necessary.

Before or after reversals of flow of air or air and steam, it is advisable to flush out any coke ash with steam alone. It is also within the contemplation of our invention to cut off the air after about one hour of controlled burning with air and steam and fiush the coil out with steam only to clean out the coke ash and clean ofi patches of coke which might be covered with ash as the ash would tend to inhibit further burning of the coke or carbonaceous deposit. The flushing step may be repeated at intervals, if desired.

When coke combustion within the tubes has finally ceased, as indicated by visual observation or by chemical test of the eiiluent gas for carbon dioxide, the furnace fires are extinguished While the flow of the oxygen-containing gas may be continued for a relatively short period. The air flow is then cut ofi and steam may be passed through the coil, reversing the flow to flush out coke ash. When the furnace is cooled to about 400-600 F. the coil is preferably flushed out with water under about 250 pounds per square inch pressure in each direction until the Water runs clean.

The method of cleaning 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 spalling operation, and to efie-ct 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. The line 20 controlled by valve 2-5 leads from an intermediate point of the coil 2 into line 2%. Steam line 3 controlled by valve I I and air line I4 controlled by valve ll are manifolded into line 23. The intermediate point of the coil 2 near the inlet end of the coil, at which connection with line 2E1 is made, is preferably a point immediately preceding that section of coil 2, at which the carbonaceous deposit is usually heaviest. By closing valve 23, and suitable manipulation of valves H and H, 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 introducing air or steam or both into coil 2 through line 2i! to effect the discharge of coke, ash or both from coil 2 through line 2! and line l9 simultaneously.

As another modification, when the coke or carbonaceous material is to be burned out with an air-steam mixture after the spalling operation, steam only is passed through lines 1 and Hi and coil 2 and then through lines 2! and 2% to the stack 33. With steam flowing through the coil 2 in this manner, air alone is then introduced through lines 29 and 3| through the latter turns of coil 2 and the air is controlled until a show of redness is seen in the latter portion of the coil 2 near the outlet end. When the air-steam ratio is balanced to hold a low redness on the tubes in this section and with steam still flowing through coil 2 from line T, air alone is introduced into an intermediate portion of the coil 2 through lines It and 20, valve 23 being closed, and when the air-steam flow is balanced to show a low redness on the intermediate section of the coil 2, air is finally added through lines l3 and L9 to heat the rest of the coil 2 to a low redness. As an alternative, air and steam may be introduced through lines 29, 2! and 3| to the latter portion of the coil, then air and steam may be introduced through lines I4, 8 and 20 to an intermediate portion of the coil and finally air and steam may be passed through the inlet of the coil and through lines [3, 1 and I9 while watching the tubes of the coil 2 to maintain them at a low redness and to prevent overheating of the tubes.

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.

The steps of removing carbonaceous material from and reconditioning of hydrocarbon fluid heating chromium alloy tubes, according to our invention, comprising the gradual heating of these tubes to an elevated temperature above about 1250 F. in which spalling of the carbonaceous deposit is brought about as set forth in the illustrative example, their maintenance at an elevated temperature 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. ing operation is an important one and is furthered by judicious control of the flexible time element and temperature of the reconditioning process.

Our invention may also be used for decoking and cleaning other heaters or the like, such as furfural refining heaters and naphtha and tar cooling coils.

In practicing the method of cleaning and reconditioning heating tubes in accordance with the invention, many modifications and variations may be made without departing from the spirit of our invention. 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 without causing scaling thereof.

This application is filed as a continuation-inpart of our application Ser. No. 182,174, filed December 29, 1937, for Method of reconditioning furnace tubes.

We claim:

1. A 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 raising the temperature within said heater to an elevated temperature between about 1000 F. and about 1350 F. and continuously passing steam through said tubes to efiect spalling of the carbonaceous material within said tubes and removal of carbonaceous material as a powdered or granular mass from said tubes, maintaining the temperature between about 1000 F. and about 1350 F. and maintaining the flow of steam through said tubes until the flow of carbonaceous material from said tubes has substantially stopped, adding oxygen-containing gas to the steam to eflect controlled burning of remaining portions of carbonaceous material in said tubes and finally completely stopping the flow of steam and passing only oxygen-contain-' This feature of the reconditioning gas through said tubes to burn off residual portions of carbonaceous material adhering to said tubes.

2. A method of removing carbonaceous material from tubes positioned in a heater previously employed for heating hydrocarbon fluids to temperature at which carbonaceous material is deposited upon the inner walls of said tubes which comprises raising the temperature within said heater to an elevated temperature of about 1000-1350 F. and 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 from said tubes, adding a relatively small amount of oxygen-containing gas continuously to the flow of steam to effect further spalling and removal of carbonaceous material from said tubes without any substantial burning of the carbonaceous material in said tubes, maintaining the flow of steam and added amounts of oxygen-containing gas through said tubes until the carbonaceous material has been substantially completely removed from said tubes, increasing the amount of oxygen-containing gas in the steam to effect controlled burning of remaining portions of carbonaceous material in said tubes and finally completely stopping the flow of steam and passing only oxygen-containing gas through said tubes to burn off residual portions of carbonaceous material adhering to said tubes.

3. A method of removing carbonaceous material from a tube 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 tube which comprises raising the temperature within said heater to an elevated temperature between about 1000 F. and about 1350 F. and continuously passing steam and air through said tub to effect spalling of the carbonaceous material within said tube and removal of carbonaceous material as a powdered or granular mass from said tube, maintaining the temperature between about 1000 F. and about 1350 F. and maintaining the flow of steam and air through said tube at a temperature lower than that of the tube Walls until the carbonaceous material has been substantially completely removed from said tube, and adding increasing amounts of air to the steam to effect controlled burning of remaining portions of carbonaceous material in said tube.

4. A method as defined in claim 3 wherein the flow of steam and air through said tubes is re versed at intervals to eifect further spalling of the carbonaceous material and removal of carbonaceous material as a powder or granular mass.

5. A method of removing carbonaceous material from tubes positioned in a heater previously employed for heating hydrocarbon fluids to a temperature at which carbonaceous mate-rial is deposited upon the inner walls of said tubes which comprises raising the temperature within said heater to an elevated temperature of about 1000-l350 F. and continuously passing steam and air through said tubes to effect spalling of the carbonaceous material within said tubes and removal of carbonaceous material as a powdered or granular mass from said tubes, maintaining the temperature between about 1000 F. and about 1350 F. and maintaining the flow of steam and air through said tubes until the flow of carbonaceous material from said tubes has substantially stopped, adding increasing amounts of air to the steam to eiiect controlled burning of remaining portions of carbonaceous material in said tubes and finally stopping the flow of steam and passing only air through said tubes to burn ofi residual portions of carbonaceous material adhering to said tubes.

6. A 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 raising the temperature within said heater to about 1000 F.1350 F. and 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 from said tubes, maintaining the temperature at about 1000 F.1350 F. and maintaining the flow of steam through said tubes until the carbonaceous material has been substantially completely re moved from said tubes, adding oxygen-containing gas to the steam to effect controlled buming of remaining portions of carbonaceous material in the tubes and finally stopping the fiow of steam and passing only oxygen-containing gas through said tubes to burn off residual portions of carbonaceous material adhering to said tubes and thereafter reversing the flow of oxygen-containing gas to effect burning of any remaining portions of carbonaceous material adhering to said tubes.

'7. A 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 raising the temperature within said heater to about1000 F.-l350. F. and continuously passing steam through said tubes to effect spalling of the carbonaceous material within said tubes and removal of carbonaceous material as a powdered r granular mass from said tubes, maintaining the temperature between about 1000 F. and about 1350 F. and maintaining the flow of steam through said tubes until the flow of carbonaceous material from said tubes has substantially stopped, then adding small amounts of oxygen-containing gas to the steam to promote further spalling of carbonae ceous material within said tubes and removal of solid carbonaceous material from said tubes, add,

ing increasing amounts of oxygen-containing gas to the steam to effect controlled burning of the carbonaceous material in the tubes and finally stopping the flow of steam and passing only oxygen-containing gas through said tubes to burn off residual portions of carbonaceous material adhering to said tubes.

8. A 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 whichcomprises raising the temperature within said heater to about l000 F.-1350 F. and 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 from said tubes, maintaining the temperature between about 1000 F. and about 1350". F. and maintaining the flow of steam through said tubes until the fiowof carbonaceous material from said tubes has decreased, reversing the flow of steam through said tubes to effect further spalling and removal of carbonaceous material, then again reversing the flow of steam through said tubes while maintaining the heater temperature between about 1000 F. and about 1350 F., then decreasing the flow of steam and adding oxygencontaining gas to the steam to effect controlled burning of the carbonaceous material in the tubes and finally stopping the flow of steam and passing only oxygen-containing gas through said tubes to burn off residual portions of carbonaceous material adhering to said tubes.

9. In a 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, the steps which comprise raising the temperature within said heater to about 1000 F.1350 F. and continuously passing steam through said tubes at a temperature lower than that of the tube walls to effect spalling of the carbonaceous material Within said tubes and removal of carbonaceous material as a powdered or granular mass from said tubes, maintaining the temperature in said heater between about 1000 F. and about 1350 F. and maintaining the flow of steam through said tubes at the lower temperature until the flow of carbonaceous material from said tubes has substantially completely stopped, adding oxygen-containing gas to the steam in a quantity suflicient to efiect controlled burning of carbonaceous material adhering to the tubes and finally passing only oxygen-containing gas through said tubes to eiiect burning of the remaining carbonaceous material in the tubes.

10. A 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 raising the temperature within said heater to about 1000 F.-l350 F., continuously passing steam at a lower temperature than the tube walls through said tubes to effect spalling of the carbonaceous material within said tubes and removal of the spalled carbonaceous material as a powdered or granular mass from said tubes. maintaining an elevated temperature within said heater and maintaining the flow of steam at the lower temperature until the flow of carbonaceous material from said tubes has substantially stopped, reversing the flow of steam through said tubes to effect further spalling and removal of carbonaceous material from said tubes, again reversing the flow of steam through said tubes when the flow of carbonaceous material from said tubes has substantially stopped to eiiect further spalling and removal of carbonaceous material from said tubes, adding small amounts of air to the flow of steam to promote further spalling and removal of carbonaceous material from said tubes, and then adding increased amounts of air to the steam in quantities sufficient to effect controlled burning of carbonaceous material in said tubes.

11. A 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 raising the temperature within said heater to about 1000 F.-1350 F., continuously passing steam through said tubes to effect spalling of the carbonaceous material within said tubes and removal of the spalled carbonaceous material as a powdered or granular mass from said tubes, maintaining an elevated temperature within said heater and the flow of steam until the flow of carbonaceous material from said tubes has substantially stopped, reversing the flow of steam through said tubes to effect further spalling and removal of carbonaceous material from said tubes, again reversing the flow of steam through said tubes when the flow of carbonaceous material from said tubes has substantially stopped to effect further spalling and removal of carbonaceous material from said tubes, adding small amounts of air to the flow of steam to promote further spalling and removal of carbonaceous material from said tubes, and then adding increased amounts of air to the steam in quantities suflicient to effect controlled burning of carbonaceous material in said tubes and then stopping the flow of steam and passing only oxygen-containing gas through said tubes to burn off any carbonaceous material adhering to said tubes.

12. The method of cleaning and reconditioning tubes in an oil heater previously used for heating hydrocarbon fluids to conversion temperatures comprising raising the temperature within the heater to about 1000 F.-1350 F. while passing steam through said tubes, maintaining said heater between about 1000 F. and about 1350 F. and continuing the flow of steam to effect substantially complete removal of the carbonaceous deposit from said tubes as a powder or granular mass, continuously adding air in gradually increasing amounts to said steam to effect controlled burning of the carbonaceous deposit and finally passing only air through said tubes to remove remaining portions of carbonaceous deposit by burning.

13. A method of removing carbonaceous material from tubes positioned in a heater provided with an inlet and an outlet and 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 subjecting said tubes to external heating by raising the heater to a temperature of about 1000" F.-1350 R, continuously introducing steam through said inlet and passing steam through said tubes and said outlet while said tubes are at an elevated temperature to effect spalling of the carbonaceous material within said tubes and removal of carbonaceous material as a powder or in granular form from said tubes,'maintaining the temperature and the flow of steam through said tubes until the spalling action substantially stops and the flow of carbonaceous material from said tubes substantially stops, continuing the flow of steam through said inlet and said tubes while introducing oxygen-containing gas into said tubes nearest said outlet to effect controlled burning of the remaining carbonaceous material in said tubes nearest said outlet, continuing the flow of steam and then introducing oxygen-containing gas to said tubes adjacent said inlet to effect controlled burning of the carbonaceous material in said tubes adjacent said inlet, and continuing the flow of steam through said inlet and tubes and then introducing oxygen-containing gas into said inlet to efiect controlled burning of the carbonaceous material in the remaining tubes.

14. A 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 raising the temperature within said heater to an elevated temperature between about 1000 F. and about 1350" F. and passing steam at a given velocity through said tubes to effect spalling of the carbonaceous material within said tubes and removal of carbonaceous material as a powdered or granular mass from said tubes, reducing the velocity of the flow of steam after the spalling action has commenced and continuing the flow of steam at the reduced velocity while maintaining the temperature within the heater between about 1000 F. and 1350 F. until the flow of carbonaceous material from said tubes has substantially stopped, adding oxygen-containing gas to the steam to effect controlled burning of remaining portions of carbonaceous material in said tubes and finally completely stopping the flow of steam and passing only oxygencontaining gas through said tubes to burn off residual portions of carbonaceous material ad hering to said tubes.

15. A 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 raising the temperature within said heater to an elevated temperature between about 1000 F. and about 1350 F. and passing steam at a given velocity through said tubes and at a temperature lower than that of the tube walls to effect spalling of the carbonaceous material within said tubes and removal of carbonaceous material as a powdered or granular mass from said tubes, reducing the velocity of the flow of steam after the spalling action has commenced and continuing the flow of steam at the reduced velocity and at a temperature lower than that of the tube walls while maintaining the temperature within said heater between about 1000 F. and 1350 F. until the flow of carbonaceous material from said tubes has substantially stopped, adding oxygen-containing gas to the steam to effect controlled burning of remaining portions of carbonaceous material in said tubes and finally completely stopping the flow of steam and passing only oxygen-containing gas through said tubes to burn ofi residual portions'of carbonaceous material adhering to said tubes.

16. A method for removing carbonaceous material from tubes positioned in a heater previous-- ly used for heating a hydrocarbon fluid to a temperature at which carbonaceous material is de posited upon the inner walls of said tubes which comprises raising the heater to an elevated temperature, passing steam through said tubes to effect spalling of the carbonaceous material within said tubes and removal of carbonaceous material from said tubes as a powdered or granular mass, maintaining the heater at elevated temperature and continuing the flow of steam until the spalling action substantially stops, then adding air to the steam to effect controlled burning of portions of the carbonaceous material adhering to said tubes, then stopping the flow of steam and passing only air through said tubes to burn oiT residual amounts of carbonaceous material adhering to said tubes.

ENSLO S. DIXON. THOMAS E. GARRARD. HAROLD A. BARR.