US3393731A

US3393731A - Pressure vessel

Info

Publication number: US3393731A
Application number: US638063A
Authority: US
Inventors: Irwin R Friedman; George P Staats
Original assignee: Trane Co
Current assignee: Trane US Inc
Priority date: 1967-05-12
Filing date: 1967-05-12
Publication date: 1968-07-23
Anticipated expiration: 1985-07-23

Description

United States Patent Oflice 3,393,731 PRESSURE VESSEL Irwin R. Friedman and George P. Staats, La Crosse, Wis., assignors to The Trane Company, La Crosse, Wis., a corporation of Wisconsin Filed May 12, 1967, Ser. No. 638,063 8 Claims. (Cl. 165-158) ABSTRACT OF THE DISCLOSURE A pressure vessel assembly having at least two opposed end walls of curved cross section wherein the end walls Background of the invention In designing vessels to contain relatively high fluid pressures where size and weight are important considerations, various means have been employed to reduce the thickness of the materials of which the vessel is constructed. The superior pressure containing characteristics of curved wall sections are known. Even where curved wall sections are employed, however, there exists a problem of minimizing the bending stresses on the curved walls in order that walls of minimum thickness may be employed. When a cover plate is bolted over an access opening in the end of a curved pressure vessel, bending moments introduced in the cover plate and in the vessel portion to which it is attached normally necessitate thick cover plates and reinforcing thicknesses of metal at the joints between the cover plate and the vessel. Thick, heavy cover plates are diflicult to remove, and often require special lifting gear.

Summary of the invention Having in mind the foregoing problems associated with the design of pressure vessels, we have designed a pressure vessel utilizing the pressure containing strength of curved sections in an essentially rectangular shaped vessel in such a way as to minimize material requirements and weight. This is accomplished by placing as many members in pure tension as possible and by eliminating or reducing bending moments by balancing tension forces. According to our invention, bending forces on curved wall sections are minimized by utilizing tension carrying tie bar members to secure one edge of the curved wall sections to a back plate. The curved walls and tie bar members are interconnected by means of flange bars to which both the curved walls and the tie bar members are welded. The flange bars also serve to support a removable cover plate which is bolted to the flange bars. The fluid pressure acting outwardly on the flange bars through the cover plate bolts is balanced by the counteracting tension forces in the tie bars and in the curved end sections so that there is substantially no net bending force acting on the curved wall sections. This permits the use ofv curved wall sections of minimum thickness.

Another advantageous feature of our improved pressure vessel lies in the selection of the angle of contact and contact location of the curved wall sections on the flange bars so that torsional moments acting on the flange bars are minimized. Also, the required thickness 3,393,731 Patented July 23, 1968 for the removable cover plate is substantially reduced and the overall strength of the pressure vessel assembly is improved by utilizing a baflle plate which separates the vessel into two separate fluid chambers to secure the cover plate to a back plate.

These design features are particularly advantageous when applied to a rectangular chamber having a small depth compared to its length. We have incorporated these features in a pressure vessel of such a shape, which is particularly adapted for use as a header on a shell and tube heat exchanger.

These and other features and advantages of our invention will become readily apparent as the following description is read in conjunction with the attached drawmgs.

Brief description of the drawings FIGURE 1 is a front elevation view of the improved pressure vessel of this invention.

FIGURE 2 is a vertical cross-section view taken along line 2-2 of FIGURE 1.

FIGURE 3 is a perspective view of the pressure vessel with the front cover plate removed.

FIGURE 4 is a blown-up section view showing the various force vectors acting on one of the flange bars which joins the front cover plate, one set of the tie bars and one of the curved end walls together.

Description of the preferred embodiment Our improved pressure vessel is shown in FIGURES 1 through 3 as a header 2 closing one end of a heat exchanger comprised of a cylindrical shell 1 and a plurality of tubes 5 extending lengthwise thereof. Tube sheet 4 is welded or otherwise secured to one end of shell 1, and is provided with a plurality of holes through which tubes 5 pass. Tube sheet 4 serves as the substantially planar back wall of rectangular-shaped header assembly 2. In fabricating header assembly 2, longitudinally extending

flange bars

20 and 22 are first welded lengthwise between the

front edges

16 and 18 of flat side walls 6 and 8 respectively. Flange bar 34 is also welded between the front edges of side plates 6 and 8 parallel to and between

flange bars

20 and 22. Baflle plate 32 is then welded to the inside edge of flange bar 34 and to opposed side walls 6 and 8 along its side edges. Baffle plate 32 extends generally perpendicular to side plates 6 and 8, and forms therewith a generally H-shaped subassembly. This entire subassembly is then secured to one end of the heat exchanger by welding the inner edges of side plates 6 and 8 and baffle plate 32 to tube sheet 4.

Next, a plurality of spaced apart tie bars 28 are welded across the top of heat exchanger header assembly 2 between tube sheet 4 and flange bar 20. A second set of spaced apart tie bars 30 are secured between tube sheet 4 and bottom flange bar 22. Each of the side plates 6 and 8 are curved along their top edges 9 and 9a and

bottom edges

10 and 10a. Two opposed

end walls

12 and 14 of curved cross-section extend across the top and bottom of header assembly 2 between side walls 6 and 8.

End walls

12 and 14 are curved to conform to curved top and

bottom edges

9, 10 and 9a and 10a of side plates 6 and 8 to which they are welded. Curved

end walls

12 and 14 are also welded along their back edges to tube sheet 4.

Spaced apart

flange bars

20 and 22 are coplanar with

front edges

16 and 18 of side plates 6 and 8, and define therewith a front access opening. This access opening is normally closed by front cover plate 24 which is removably fastened to

flange bars

20 and 22 to

front edges

16 and 18 of side plates 6 and 8 by bolts 26. Cover plate 2-4 is also fastened to intermediate flange bar 34 by bolts 36.

Curved

end walls

12 and 14 are preferably cylindrically shaped, and have a small radius of curvature so that the lightweight materials of which they are made can contain relatively high pressures. Three-sixteenth inch to 4 inch plates were used for

curved end walls

12 and 14 in place of the l to 1 /2" plates normally required in conventional designs having flat end walls.

Flange bars

20 and 22 serve as structural members interconnecting the front edges of

curved end walls

12 and 14 to the outer ends of

tie bar members

28 and 30. The major portion of the outward thrust generated by the pressure of the fluid within header assembly 2 one end walls '12 and 14 is thus carried by

tie bars

28 and 30. In order to effectively eliminate any significant bending moment on

curved end walls

12 and 14, the tension forces acting on the front edges of

end walls

12 and 14 are balanced. The manner in which this is accomplished can best be understood by referring to the force vectors illustrated in FIGURE 4. The reaction tension force F acting through end wall 12 has been resolved into its vertical and horizontal components F and F The fluid pressure acting outwardly on cover plate 24 through bolts 26 is represented by force vector P and the reaction tension force acting through tie bars 28 is shown by vector F Vertical component P of F is balanced by a shear force P on bolts 26 transmitted to cover 24 through the bolts. The angle on at which curved end wall 12 is connected to flange bar 20 is so selected that the horizontal force component P of wall 12 combined with tension force F in bars 28 equals the opposing force F through bolts 26. This relationship of tension forces may be expressed algebraically by the equations:

With a radius of curvature of 127 for

end walls

12 and 14, angle a at which they contact

flange bars

20 and 22 will be approximately 30. This balancing of tension forces effectively precludes any net bending stress acting on curved end -

walls

12 and 14. This permits these walls to be designed with a view towards only the membrane stress resulting from the fluid pressure within header assembly 2, and thus to be relatively thin.

The point of contact of

curved end walls

12 and 14 on

flange bars

20 and 22 respectively is selected so as to minimize the net torsional stress acting on these flange bars. The net torsional stress acting on flange bar 20', for example, can be expressed algebraically by summing the moments about a particular point on bar 20. With end wall 12 contacting the top of flange bar a distance r from the front face of bar 20, the sum of the moments about point 0 in the vertical center of the front face of bar 20 will be:

Sum of wherein forces F and F are acting at distances 1' and r from point 0, and Me is the torsional moment of the front cover acting on the flange bar 20 as a result of the fluid pressure Within header 2. The distance 1' and the angle or for

cylindrical end walls

12 and 14 for a particuular size header assembly are selected so that the net torsional stress on

flange bars

20 and 22 will be a minimum value. In manufacturing and assembly 2, a tolerance is established for distance 1' which will produce a net torsional load on

flange bars

20 and 22 which does not exceed the minimum torsional strength for which they are designed. By minimizing the torsional stress acting on

flange bars

20 and 22, the dimensions for these members may be determined primarily by bolting and gasketing requirements rather than on the basis of strength considerations. This design feature is particularly useful on long, narrow pressure vessels having relatively

long flange bars

20 and 22 on which the cumulative torsional stress would be very great. Balancing torsional moments by means of the aforesaid design arrangement of

curved end walls

12 and 14 with respect to

flange bars

20 and 22 permits relatively small flange bars to be used in place of the thick, heavy bars which would normally be required on header assemblies of such a shape. For example, with a design pressure of p.s.i. and a rectangular shaped header approximately 56 inches long and 17% inches deep, flange bars 1 /2 inches thick may be used. If the angle of contact a for end walls "12 and 14 of a header of such size is 30, then dimension r will be about 1% inches.

In the example given above showing the torsional moments acting on flange bar 20, a minimum net torsional stress was achieved by having the thrust load carried by tie bars 28 act along bottom face 21 of flange bar 20. For this purpose, and for welding convenience, tie bars 28 are shown lying against face 21 of tie bar 20. The same result could be obtained by abutting tie bars 28 against back face 23 of bars 20 in the same plane as bottom face 21. However, this would require cutting tie bars 28 to a particular length so that they fit exactly in the space between tube sheet 4 and back face 23 of tie bar 20. It is possible that in order to obtain the proper balancing of torsional moments for various size pressure vessels, tie bars 28 would have to be welded farther up along back face 23 of flange bar 20 intermediate its top and bottom faces. Of course, the same considerations apply to the determination of the proper point for welding tie bars 30 to flange bar 22.

When our improved pressure vessel is employed as a header assembly as illustrated, bafile plate 32 serves to divide the header interior space into inlet and outlet fluid chambers. Flanged fluid inlet and

outlet fittings

38 and 40 are provided in flat side wall 8 above and below dividing baffle 32. Side walls 6 and 8 could also be cylindrically shaped, as are

end walls

12 and 14. However, this would make it more difficult to secure

fittings

38 and 40 to one of the side walls. This arrangement wherein the fluid connections and cover plate are in different walls is the normal marine type of water box design which permits access to tubes 5 for cleaning and replacement without disturbing the pipes connected to

fittings

38 and 40. Heat exchange fluid is directed into header 2 through inlet fitting 40, down through tubes 5 in One direction inside of shell 1, back through tubes 5 in the opposite direction into the upper part of header 2 above chamber separation bafile 32 and out through fitting 38. A second heat exchange fluid is directed through shell 1 in thermal contact with tubes 5 in the well known manner. Since baffle plate 32 is welded to flange bar 34 which in turn is bolted to front cover plate 24, plate 32 also acts as a thrust carrying member lending support to cover plate 24. By thus reducing the strength requirements for cover plate 24, its thickness may be reduced to half that normally required. Cover plate 24 will thus be sufliciently lightweight that it can be removed without the aid of special overhead hoists. Also, the tie bar effect of plate 32 further reduces the torsional moment Me of cover 24 on

flange bars

20 and 22 as well as the thrust load on

bars

28 and 30.

A pressure vessel fabricated as described above has the particular advantage of high strength coupled with light weight. By placing as many members as possible in pure tension and minimizing bending and twisting stresses by balancing tension forces, material requirements are greatly reduced. The H-frame assembly formed by side plates 6 and 8 and bafile plate 32 produces an extremely rigid structure. The mutual support which tube sheet 4 and front cover 24 provide for each other through this interconnecting I-I-frame assembly permits the thickness of the tube sheet and the front plate to be drastically reduced.

Although we have illustrated and described our pressure vessel as a header for use on a heat exchanger, the design principles pointed out above could be applied to any vessel and could be adapted to a symmetrical design bolted together in the middle.

Various modifications will occur to those skilled in the art which we contemplate will be within the spirit and scope of our invention as defined by the following claims.

We claim:

1. In combination with a heat exchanger comprising a shell and a plurality of tubes extending therein, a generally rectangular header assembly comprising:

a tube sheet secured to and closing one end of said shell defining the back wall of the header assembly;

a substantially planar front wall;

two opposed side walls extending between said front and back walls, each of said side walls being curved along the opposed edges thereof which extend between said front and back walls;

two opposed end walls of curvilinear cross section, said end walls being curved to conform to said curved edges of said side walls and extending lengthwise of said header assembly between said opposed side walls in mating, abutting relationship therewith;

a first set of spaced apart tie bar members secured at one end to said back wall;

first means securing the other end of each of said first set of tie bar members to the front edge of one of said curved end walls;

a second set of spaced apart tie bar members secured at one end to said back wall;

and second means securing the other end of each of said second set of tie bar members to the front edge of the other one of said curved end walls.

2. Apparatus as defined in claim 1 wherein:

said first and second means securing the other end of each of said tie bar members of said first and second sets to the front edge of each of said curved end walls comprise first and second spaced apart flange bars extending lengthwise of said header assembly between said side walls, said front edge of said one of said curved end walls and each of said first set of tie bar members being welded to different faces of said first flange bar, and said front edge of said other one of said curve end walls and each of said second set of tie bar members being welded to different faces of said second flange bar;

and wherein said planar front wall is secured to said first and second flange bars.

3. Apparatus as defined in claim 2 wherein:

each of said first set of tie bar members lies against a face of said first flange bar which is opposite the face to which said front edge of said one of said curved end walls is welded;

and wherein each of said second set of tie bar members lies against a face of said second flange bar which is opposite the face to which said front edge of said other one of said curved end walls is welded.

4. Apparatus as defined in claim 2 wherein:

the contact angle and point of contact of said front edges of each of said curved end walls with respect to the faces of said first and second flange bars to which said front edges are welded are such that the net torsional stress on each of said flange bars approximates zero.

5. Apparatus as defined in claim 2 and further includa third flange bar positioned between and parallel to said first and second flange bars, said planar front wall being secured to said third flange bar;

a bafile plate extending generally longitudinally of said header assembly, said baflle plate being secured along its edges to said third flange bar, said back wall and said side walls;

and fluid inlet and outlet openings in said back wall and in one of said side walls, said inlet opening being located on the opposite side of said baflle plate from said outlet opening, whereby said baffle plate serves both as a separation member dividing said header assembly into fluid inlet and outlet chambers and as a thrust carrying member lending support to'said front wall.

6. Apparatus as defined in claim 5 wherein:

said planar front wall is in the form of a lightweight cover plate removably secured to said first, second and third flange bars.

7. Apparatus as defined in claim 1 wherein:

said first and second means securing said other ends of said first and second sets of tie bar member to said front edges of said curved end walls comprises first and second flange bars extending lengthwise of said header assembly between said side walls, said first flange bar being welded to said front edge of said one of said curved end walls and to said other end of each of said first set of tie bar members, and said second flange bar being welded to said front edge of said other one of said curved end walls and to said other end of each of said second set of tie bar members;

and wherein said flange bars are coplanar with the front extremities of said side walls and define therewith an access opening;

and wherein said planar front wall is in the form of a cover plate removably secured over said access opening to said flange bars.

8. In combination with a heat exchanger comprising a shell and a plurality of tubes extending therethrough, a header assembly comprising:

a tube sheet secured to and closing one end of said shell;

planar side wall means and curvilinear end wall means secured to said tube sheet and extending forwardly therefrom, the forward extremities of said side wall means and said end wall means terminating in flange means which define a front access opening;

a cover plate removably mounted on said flange means over said access opening;

means dividing said header assembly into two separate fluid chambers, said means comprising a baflle plate secured at its back edge to said tube sheet;

means securing the forward edge of said baflle plate to said cover plate, whereby said baffle plate serves as a thrust carrying member supporting said cover plate;

and tie bar members secured between said tube sheet and said flange means at the forward extremity of said end wall means.

FOREIGN PATENTS 12/1952 Great Britain. 12/1959 Great Britain.

ROBERT A. OLEARY, Primary Examiner.

A. W. DAVIS, JR., Assistant Examiner.