US2966340A

US2966340A - Joining tubes to tube sheets

Info

Publication number: US2966340A
Application number: US644805A
Authority: US
Inventors: Edward C Chapman
Original assignee: Combustion Engineering Inc
Current assignee: Combustion Engineering Inc
Priority date: 1957-03-08
Filing date: 1957-03-08
Publication date: 1960-12-27
Anticipated expiration: 1977-12-27
Also published as: FR1181125A; DE1093391B; BE561597A

Description

Dec. 27, 1960 E. c..cHAPMAN JOINING TUBES To TUBE 'SHEETS Filed March 8, 1957 lllll.

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United States Patent O JOINING TUBES T TUBE SHEETS Edward C. Chapman, Chattanooga, Tenn., assignoi to `Combustion Engineering, Inc., New `ttor-k, NX., a corporation of Delaware Filed Mar. 8, 1957, Ser. No. 644,805

Claims. (Cl. 257-236) This invention relates to the attachment of tubes to tube sheets, headers or the like in heat exchangers and has specific relation to a connecting arrangement for overcoming the problem of stress corrosion cracking experienced with certain materials under certain conditions.

The term stress corrosion cracking refers to the cracking of metal that is in a stressed condition and which is contacted by certain chemicals which attack the stressed metal in a manner to produce cracking. The exact conditions necessary for stress corrosion cracking to progress have never been accurately established but the variables that are involved are the concentration of the chemicals in contact with the metal and the amount of stresses present in the metal. One instance of stress corrosion cracking which is presently causing great concern in the eld of heat exchangers is the stress corrosion cracking of austenitic stainless steel when operating under stresses in water containing chlorides, such as NaCl, CaClz and MgCl2. Austenitic stainless steel has been known to crack under stress when the water contains not more than 18 parts per million of chlorides and when evaporation of the water is taking place and there are crevices in which the chlorides may concentrate (as may prevail with conventional tube connecting arrangements) an unlimited degree of concentration may be obtained in these crevices with a very small concentration of chlorides being present in the water.

In connecting tubes to tube sheets, headers or the like, the tubes are generally expanded into tight engagement with the wall of the opening in the tube sheet with the extremity of the tube being seal welded to the tube sheet. These operations of course stress the tube metal at this connection so that if the conditions are right (i.e., the right metals and the right chemicals present) stress corrosion cracking of this stressed metal may take place. One situation where this is the case is where austenitic steel tubes are connected with ferritic steel tube sheets in a steam generator organization wherein the water that is evaporated contains chlorides (as most boiler water does) and is in contact with the tubes. Ferritic steel -is not subject to stress corrosion cracking in the presence of chlorides while, as previously mentioned, austenitic stainless steel is. In such an instance it is essential that there be no crevices at the tube connection in which chlorides may concentrate in contact with the austenitic steel tube. If conventional tube connections are used it is more than likely that the expanded joint between the tube and tube sheet will open up to at least a minor extent since the coefficient of the expansion of the tube and the tube sheet are different. Once the expanded joint is slightly open a situation prevails where stress corrosion cracking of the tube will undoubtedly occur. As an example of a situation involving the problem, there are presently being built steam generators wherein the heating medium is conveyed through tubes and the water that is evaporated surrounds the tubes with the design calling for 5/s inch outside diameter austenitic stainless steel tubes to be connected to a ferritic steel tube sheet 8 inches thick and with the design being such that accessibility to the tubes is very diicult rendering it almost impossible to obtain a good job of expanding the tubes into engagement with the bore throughout the thickness of the tube sheet.

The present invention eliminates the possibility of stress corrosion cracking of the tube connection in instances such as the one just mentioned and in accordance with the invention, in connecting a tube, which is subject to stress corrosion cracking in the presence of certain chemicals that exist in the environment in which the tube is to be used, to a tube sheet or the like, which is not subject to such cracking in the presence of these chemicals, a sleeve is positioned over the tube end that is to be connected to the tube sheet and is brazed to the tube throughout the overlying surfaces of the sleeve and tube. This sleeve is of a material which is not subject to stress corrosion cracking in the presence of these chemicals and after being thus secured to the tube, the tube end and sleeve are placed in a suitable bore provided in the tube sheet and by rolling or similar method the tube and sleeve are expanded so that the sleeve is in tight engagement with the bore. The extremity of the tube and sleeve are then seal welded to the tube sheet.' With this construction, if the expanded joint between the sleeve and tube sheet should leak stress corrosion cracking, as a result of concentration of the chemicals in this joint, will not occur since the material of the sleeve and tube sheet are not subjected to such cracking and since the sleeve and tube are integrally brazed'together there will be no chance for the formation of crevices between these members in which the chemicals could possibly con-` centrate and cause cracking of the tube.

It is the object of this invention to provide an im.- proved tube and plate or the like connection particularly adaptable for use in evaporators and organized to prevent stress corrosion cracking of the-tube at the connection when the Itube is made of a material susceptible to such cracking while the tube sheet is not succeptible to such cracking.

Other and further objects of the invention will become apparent to those skilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises an arrangement, construction and combination i of the elements of the inventive organization in such a manner as to attain the results desired as hereinafter more particularly set forth in the following detailed description of an illustrative embodiment, said embodiment being shown by the acompanying drawing wherein:

Fig. l is a diagrammatic representation in the nature of a vertical section through a heat exchanger, which takes the form of a steam generator, and wherein tubes are interconnected with spaced tube sheets with the interconnection of the t-ubes with the tube sheets being in accordance with`the present invention.

Fig. 2 is an enlarged sectional view of the connection of one of the tubes with one of the tube sheets.

Referring now to the drawing, wherein like reference I characters are used throughout to designate like elements, the heat exchanger shown in Fig. l comprises a housing or shell 10 within which is provided the spaced tube sheets 12 with these tube sheets being spaced from the stantially lled with waterwhich is supplied through conf duit 24 with this water of course surrounding the exterior of tubes 18. Sufficient heat is imparted from the heating medium iowing through the tubes so that a portion of the water in space 26 is converted to steam with the steam passing upward through connecting'` conduits 23 into. drum 3.0 where any residual water is separated fromr the steam and returned to space 26 through conduit 32 with. the steam leaving the upper portion of the drum through conduit 34.

In the illustrative organization the tube sheets I2 are of ferritic steel while thetubes 18 are of austenit'ic stainlessV steel so thatit even minor amounts of chlorides are presentv in the water in space 26 stress. corrosion cracking of the tubes at the end connections with the tube sheets would prevailif there are crevices present which will permitA the chlorides to concentrate in engagement with the tubes. Most boiler water contains suficient chlorides to cause stress corrosion cracking of the austenitic steel tubes under these circumstances with marine boilers generally having rather large amounts of chlorides in the boiler waten As best shown in Fig. 2, the ends of the tubes 18 are provided with a sleeve 36 disposed thereover with this sleeve being of a material which is not affected by chlorides or in other words, is not subjected to stress corrosion cracking as a result of chlorides being present in the Water in space 26. Examples of material of which this sleeveV may be made are; ferritic steel and non-ferritic-alloys high in chrome content, although it should be understood that any material may be employed which will withstand the operating conditions that prevail and which is not subject to stress corrosion cracking as is the tube. Sleeve 36 is brazed to tube 18 so that the overlying surfaces of the sleeve and tube are integrally bonded together throughout their entire area. Any one of a variety of metals or alloys may bev used as the tiller, indicated as 39, in brazing the sleeve and tube together with an alloy consisting of 991/2% silver and 1/2%. lithium having been found particularly satisfactory. Since most of the desirable lfiller materials, such as the one mentioned, are suiciently remote from austenitic stainless steel in the electromotive series so as to produce troublesome electrolysis between this ller metal and the tube the tiller metal is sealed from the water in space 26 by means of seal Weld 38 which is preferably of a material generally corresponding to tube 18.

After the sleeve 36 is brazed to the end of tube 18 the tube and sleeve are positioned within bore 40 provided in thetube sheet 12. 'Ihe end of the tube and the sleeve are then expanded in the conventional manner so that sleeve 36 is in tight engagement with the wall of bore 40. Thereafter the tube and sleeve are seal welded to the tube sheet by means of weld 42 which extends throughout the circumference of the tube and sleeve.

By connecting tube 18 to tube sheets 12 in this manner the only possible place that a crevice may develop is between the sleeve 36 and `tube sheet 12 and since both of: these members are of a material which is not affected by the presence of chlorides in the Water in space 26 and accordingly is not subjected to stress corrosion cracking as the result of building up of concentrations of chlorides in these crevices the problem of stress corrosion cracking. in such an organization is overcome. Because sleeve 36 and tube 18 are bonded together throughout their entireoverlyingl area the juncture of the sleeve and tube is not subjected to the development of leaks or the formation of crevices. lt is essential that the sleeves and tubes be joined together throughout their overlying areas in order to obtain this result since if a portion of the overlying area were not brazed together the unbrazed area would have a tendency to propagate due to the notch effect in the presence of thermal fatigue stresses. By integrally bonding the sleeve and tube together in this manner good heat conductivity between these elements is had which minimizes thermal stresses.

While- -I haveA illustrated and described` apreterred emd bodiment of my invention it is to be understood that such is merely illustrative and not restrictive and that varia` tions and modifications may be made therein without departing from the spirit and scope of the invention. I therefore do not wish to be limited to the precise details set forth but desire to avail myself of such changes as fall within the purview of my invention.

What I claim is:

l. In an organization of the type described an austenitic steel tube, a ferritic steel sleeve disposed on` the end of the tube and brazed to the tube throughout the overlying area of the tube and sleeve with the filler metal being such as to produce electrolysis between it and the tube in the presence of Water, the end of the sleeve remote from the tube end being seal welded to the periphery of the tube by a material which will not produce electrolysis, a ferritic steel plate having a bore therein, said sleeve and tube being disposed in said bore with the tube extending from one end ofthe bore and the end ofthe tube and sleeve generally corresponding, with the face of the plate at the other end. of the bore and with said sleeve and tube being expanded. so the sleeve is in tight engagement with the bore, said sleeve and tube being Welded to said one face of the plate with the weld extending V.asggughout the circumference of the. sleeve and tube.

2. In an organization of the type described a tube of a material subjected to stress corrosion cracking in the presence of certain chemicals, a sleeve disposed on the end of the tube and comprised of a material that is substantially unaffected by such chemicals and which is. suffi.- ciently close to the metal of the tube in the electromotive series as to obviatey troublesome electrolysis, said sleeve and tube being integrally bonded together throughout their overlying surfaces, a plate also of a material substantially unaiected by such chemicals and having a. bore extend-ing therethrough, said sleeve and tube being disposed in said bore so the tube extends from one end thereof and being expanded so the sleeve is in tight engagement with the bore, the tube and sleeve being welded to the plate at the other end of the boreand throughout the circumference of the tube and sleeve and a seal Weld disposed -about the end of the sleeve corresponding with said one end of the bore to prevent electrolysis between the material bonding the sleeve to the tube and the tube.

3. In an organization of the typeV described a ferritic steel tube sheet having a bore therein for receipt of a tube, an austemtie steel tube, a metallic sleeve disposed over the end of the tube and brazed thereto, with the brazing material being disposed throughout the entire overlying area of the sleeve and tube, said metallici sleeve being `of a material that is not subjected to stress corrosion` cracking in the presence of chlorides, the end of the tube with the sleeve secured thereto being disposed inY said bore and expanded so the sleeve is in tight en gagement with the wall of the bore with the remainder of the tube. extendinggfrom one end ofthe bore, the sleeve being seal welded tothe tube sheet at the. other end. of the bore.

4. In a heat exchanger a plate into which the end of a tube isY connected with theA tube being of a material subjected to stress corrosion cracking from they presence of certain chemicals in water `in contact with the tube, andk the plate beting. of a material that is substantially unaiected by the presence of such chemicals, the. tube being adapted to have a heating medium conveyed therethrough and water containing said certain chemicals disposed thereabout and evaporated by the applicationI of heat from said medium, said end of the tube having a sleeve disposed thereabout and brazed thereto throughout the overlying surface of the sleeve and tube with the sleeve being'` of a materialV substantially unaffected by said chemicals and which is sufficientlyv close to the metal of the tube in the eleetromotive series as to obviate troublesome electrolysis,. said sleeve being disposed witte in a suitable bore provided in the plate with the end of the tube adjacent one end of the bore, said sleeve and tube being welded to said plate at said one end of the bore with this weld extending across the surface of the sleeve and throughout the circumference of the tube and sleeve, and a seal weld disposed about the end of the sleeve remote from said one end of the bore to prevent electrolysis between the brazing material and the tube.

5. In an organization of the type described, a plate having a bore extending therethrough, said plate being of ferritic steel, an austenitic steel tube having one end received within said bore with said tube being adapted to have water containing chlorides disposed thereabout, said end of the tube having a sleeve disposed thereon which extends at least the full length of the bore with said sleeve being of a material that is unaffected by chlorides, said sleeve and tube being brazed together throughout their overlying surfaces and said sleeve and tube being expanded so the sleeve is in tight engagement with the Wall of the bore while the tube remains spaced from said Wall, the end of said sleeve adjacent said one end of the tube being circumferentially welded to the plate, and a =seal weld disposed about the end of the sleeve remote from said one end of the tube to prevent electrolysis between the brazing material and the tube.

References Cited in the file of this patent UNITED STATES PATENTS 1,438,596 Harding Dec. 12, 1922 1,856,618 Brown May 3, 1932 2,209,974 Jacobus Aug. 6, 1940 2,368,391 Young Jan. 30, 1945 2,678,224 Kooistra May 11, 1954 2,745,797 Long May 15, 1956