US1988418A - Method of securing tubes in tube sheets and article produced thereby - Google Patents

Description

Jan. 15, 1935. A. J GERMAN Patented Jan. 15, 1935 UNITED STATES PATENT oFFicE METHOD OF SECURING TUBES IN TUBE lSIIiEETS AND ARTICLE PRODUCED THERE- Abraham J. German, Waterbury, Conn., assignor to Scovill Manufacturing Company, Waterbury, Conn., a corporation of Connecticut Refiled. for abandoned application Serial No.

459,140, June 4, 1930. This application November 1, 1933, Serial No. 696,130

6 Claims.

cured at their ends to tube sheets, the tubes and tube Sheets together with the outside walls of' the vessel serving to separate one of the fluids flowing through the apparatus from the other. In such apparatus it has always been important to provide a tight joint between the tube and tube sheet in order to prevent leakage which Would cause one fluid to be contaminated by the other. In recent years, however, the problem has become much more difficult in some types of heat exchange apparatus due to the great difierentials of pressure which are now maintained. For example, in condensers used with steam turbines, a vacuum of twenty-nine inches is frequently maintained on the exhaust steam, While a pressure approximating thirty pounds per square inch is maintained on the cooling medium, the total differential of pressure being eflective to force the cooling medium through any leak which may exist between the tube and tube sheet. Inasmuch as salt water is frequently used as the cooling medium, a leak may cause serious difiiculties.

Similarly, inheat exchange' apparatus used in modern oil refining processes, difierentials of pressure in the neighborhood of eight hundred pounds are sometimes maintained.

Various methods of Securing a tight joint have been proposed and have been used with varying degrees of success. The most common methods used commercially at the present time are the ferrule and packing method, and the expanding, flaring and beading method.

While the ferrule and packing method is fairly satisfactory when used at the outlet ends of the tubes, it has not been generally satisfactory when used at the inlet ends of the tubes, because the turbulence set up by the ferrule releases air from solution which causes severe corrosion near the inlet ends of the tubes. Another difiiculty is that the packing deteriorates fairly rapidly.

The expanding, flaring and beading method eliminates the turbulence difiiculty, but is also unsatisfactory because, first, the maximum holding power obtainable is less than that which is desirable, and second, the expansion necessary to obtain this maximum is sufficient to strain, and

'therefore weaken the metal, making it more liable to corrosion, and third, unskilled workmen, 'in an effort to obtain maximum holding Power, frequently expand the tubes excessively, thus weakening the tube excessively and reducing its holding power below the maximum otherwise obtainable.

In some types of heat exchange apparatus it has been the practice to use the expanding fiaring and beading method at the inlet ends of the tubes, and the ferrule and packingmethod 'atv the outlet ends. Recent .developments in heat exchange design have indicated the desirability of using the expanding method at both ends of the tube, but when this is done, an extremely strong joint must be provided between tube and tube sheet, because of Vthe stresses developed bylongitudinal expansion and contraction of the tubes caused by sudden temperature changes which sometimes occur in heat exchangers.

It is an object of the present invention to overcome these difficulties andto provide a method of Securing tubes to tube sheets which will provide holding power in excess of the strength of thetube, but without` expanding the tubes to an extent sufficient to strain the metal. 'I 'he phrase strength of the tube as used in this specification is used to denote the ability of a tube to withstand stresses, either tensionalor coinpressive, applied longitudinally of the tube without collapsing or breaking. Thus, when I say that I provide holding. power in excess of the strength of the tube I mean that the resistance to slippage at the joint between tube and tube Sheet is sufcient to hold the tube firmly, even when the stresses applied to the tube by expansion or contraction thereof are sufiicient to cause the tube to collapse or break. It will be observed, therefore, that the strength of the tube iS related to its wall thickness, for obviously a thick walled tube will have greater strength than a thin walled tube.

It will be obvious that theoretically if' a tube could be supported against longitudinal displacement in a tube sheet by an annular shoulder or fiange having a width equal to the wall thickness of the tube, the holding power of such a fiange would be greater than the strength of the tube. That is, if the tube was subjected to compressive stress, vthe tube would collapse before being displaced. It will also be obvious that'the width of the said annular fiange might be decreased to a point where the holding power of the fiange would be approximately equal to the strength of the tube when subjected to compressive stress, the

exact width of the fiange required being dependent on the material from which the tube is made.

It is impractical in the construction of heat exchange apparatus to provide such flanges, however, and accordingly by the present method, I propose to provide an interengagement between the wall of the tube and the wall of the tube sheet aperture of such nature that the total width, measured transversely of the tube, of the interengaging parts will be approximately equal to or will exceed the width of the theoretical flange required to hold the tube against displacement when subjected to the maximum compressive stress possible without collapsing the tube.

In order to provide the desired interengagement, I proposeto form a large number of parallel annular grooves in the wall of the tube sheet aperture, and to expand the tube slightly (but to a less degree than ordinarily required) in order to cause the metal of the tube to protrude into the grooves in the tube sheet wall to form a large number of locking rings which engage the walls of the grooves thus providing thedesired interengaging surfaces.

Were it not for the fact that excessive expansion of the metal of the tubes causes strains in the metal which weaken the metal structure, thus making it more susceptible to corrosion, it will be obvious that interengaging surfaces of the desired width might be obtained with two or three grooves only. It is well known, however, that a relatively slight degree of expansion is suflicient to produce undesirable strains in the metal, and accordingly I propose to conflne the expansion Within limits within which no undesirable strains will be produced, and to secure interengaging surfaces of the desired total width by increasing the number of grooves, thus providing a large number of relatively narrow interengaging surfaces, the total width, however, being sufiicient to prevent displacement of the tube when subjected to the maximum longitudinal stresses which can be withstood by the tube.

I have found that if the expansion of the tube is kept below that which is suflicient to cause the tube metal to protrude into the tube sheet grooves to a depth not exceeding two thousandths of an inch (.002") and preferably not exceeding one and one-half thousandths of an inch (.0015"), the metal will not be strained sufficiently to. make it especially susceptible to corrosion, and I prefer, therefore, to keep the expansion within these limits; The permissible degree of expansion varies with the wall thickness of the tube, however, for in the case of thin walled tubes a less degree of expansion may be permissible than in the case of a thicker walled tube. By reference to the drawing it will be observed that the degree of expansion threin shown, and which I have found to be especially effective, is such that the distance to which the tube metal is caused to-protrude into the grooves does not exceed one-tenth of the wall thickness of the tube. With any metal the degree of expansion which can be practiced without strain can be readily determined, and in each case I prefer to keep the expansion below that limit.

Having thus established the limit of permissible expansion, the number of grooves necessary to provide interengaging surfaces of suicient width to providethe requisite holding power may be easily determined. The cumulative depth need not exceed the wall thickness of the tube and may be as much less as may be determined, experimentally or otherwise, to be necessary to provide adequate holding power.

With some types of tubes it is found that the cumulative depth of the interengaging surfaces required to provide the desired holding power may be approximately equal to one-half the wall thickness of the tube. On that assumption, if the wall thickness is five hundredths inches (.05"-) and the depth of the looking rings on the tube is one and one-half thousandths inches (.0015"), it will be apparent that approximately sixteen grooves and looking rings will be suflicient. If the wall thickness is greater or less the number of grooves required will be proportionately greater or less.

Referring to the drawing, wherein I have illustrated the preferred embodiment of the invention in a longitudinal cross section through a tube and tube sheet, Figure 1 being approximately .twice normal size, and Figure 2 being a greatly enlarged fragmentary section, the tube sheet 1 is provided with a plurality of parallel annular grooves 2 which serve the functions described above. Because of the fact that the metal of the tube only needs to be caused to protrude into the grooves to a relatively slight extent, the

` grooves may be fairly shallow, and in ordinary cases a depth of approximately five thousandths inches (.005") is sufiicient to receive the tube metal, and still leave some clearance, as is desirable, for it is not intended to cause the tube metal to entirely fill the grooves.

I have found it convenient to make the grooves in the shape illustrated in the drawing With their side walls'3 inclined at angles of about degrees to the wall of the tube sheet, but it will be understood that grooves of different shapes may be used within the scope'of the invention.

.The spacing of the grooves one from another is dependent on the number of grooves required to provide adequate holding power and also upon the thickness of the tube sheet, for it Will be obvious that if twelve grooves are required, they must be spaced more closely if the thickness of the tube sheet is only 1/2" than they need be if the thickness of the tube sheet is 1". With many types of tubes I have found it possible to space the' grooves as closely as forty to the inch, and indeed, I prefer to so space the grooves in most cases, and, if the thickness of the tube sheet is sufiicient to accommodate more grooves than'necessary to provide the requisite holding power, I can eitherv limit the number of closely spaced grooves, or I can use extra grooves, even though they may serve no useful purpose. VThe width of the grooves depends to some extent upon the spacing of the grooves, but to a greater extent upon the hardness of the metal of the tube. Obviously, if the grooves are spaced forty to the inch, the grooves could not be more than 1/40" wide, but I prefer to provide a land 4 between each groove thus reducing the width of the grooves somewhat. 4With conventional tubes for heat exchangers, I have found that grooves approximately .0154" wide, having lands approximately .0096" wide between give satisfactory results. The width of the grooves should be somewhat greater in the case of relatively hard metals than for relatively soft metals.

There is also a relationship between the width of the grooves and lands and the wall thickness of the tube. Thus, by reference to the drawing it will be observed that the width of each of the grooves, and the width lof each of the lands does not exceed one-third of 'the wall thickness of the tube.

It will be observed that since the grooves are parallel, each individual groove serves tov provide a seal against leakage between the tube and tube sheet, thus providing an exceptionally tight joint for use Where a wide difierential of pressure exists.

It will be understood that the invention may be variously modified and embodied within the scope of the subjoined claims.

I claim as my invention:

1. Method of 4Securing tubes in an apertured tube sheet which comprises forming a large number of parallel annular grooves in the tube sheet aperture, and expanding the tube to cause the tube metal to protrude slightly into said grooves, the amount of said expansion being less than that required to change substantially the metal structure of the tube, the distance to which said tube metal is caused to protrude into said grooves vbeing less than one-tenth of the wall thickness of the tube, and the cumulative depth of the protruding portions of said tube being greater than one-half of 'the wall thickness of the tube..

2. Method of `Securing tubes in an apertured tube sheet which comprises forming a large number of parallel annular grooves in the tube sheet aperture, and expanding the tube to cause the.

tube metal to protrude slightly into said grooves, the amount of said expansion being less than that required to change substantially the metal structure of the tube, and the distance to which said tube metal is caused to protrude into saidv grooves being less than one-tenth of the wall thickness of the tube.

3. As an article of manufacture, in combination, a tube sheet having an aperture therein, said aperture being provided with a largeV number of relatively shallow independent circumferential grooves, each separated from the other by a land, and a tube having. a normal outside diameter slightly less than the diameter of the aperture of the tube sheet, the width of each of said grooves being less than one-third of the wall thickness of the tube, said tube being expanded within said aperture against the lands to form a plurality of locking rings extending into said grooves to a distance less than one-tenth of the wall thickness of the tube.

4. As an article of manufacture, in combination, a tube sheet having an aperture therein, said aperture being provided with a large number of relatively shallow independent circumferential grooves, each separated from the other by a land, and a tube having a normal outside diameter slightly less than the diameter of the aperture of the tube sheet, the width of each of said grooves being less than one-third ofthe wall thickness of the tube,'said tube being expanded within said aperture against the lands to an extent sufiicient to form a plurality of locking rings extending into said grooves, but without altering substantially the metal structure of the tube.

5. As an article of manufacture, in combination, a tube sheet having an aperture therein, said aperture being provided with a large number of relatively shallow independent circumferential grooves, each separated from the other by a land, and a tube having a normal outside diameter slightly less than the diameter of the aperture of the tube sheet, .the width of each of said lands being-less than one-third of the wall thickness of the tube, said tube being expanded within said aperture against the lands to form a plurality of locking rings extending into said grooves to a distance less than one-tenth of the wall thickness of the tube.

6. As an article of manufacture, in combination', a tube sheet'having an aperture therein, said aperture being provided with a large number of relatively shallow independent circumferential grooves, each separated from the other by a land, and a tube having a normal outside diameter slightly less than the diameter of the aperture of the tube sheet, the width of each of said grooves being less than one-third of the wall thickness of the tube, the width of said lands being less than one-third of the wall thickness of said tube, said tube being expanded within said aperture against the lands to form a plurality of locking rings extending into said grooves to a distance less than one-tenth the wall thickness of the tube.

' ABRAHAM J. GERMAN.

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