US2754993A - Vessel head reinforcing ring and method of pre-stressing - Google Patents

Description

July 17, 1956 F. MAKER 2,754,993

VESSEL HEAD REINFORCING RING AND METHOD OF PRE-STRESSING Filed Oct. 25, 1953 2 Sheets-Sheet 1 DISCONTINUITY PRESSURE COMPRESSION RING STRESS STRAIN INVENTOR FRANK L. MAKER BY MM...

ATTORNEYS July 17, 1956 F. MAKER 2,754,993

VESSEL HEAD REINFORCING RING AND METHOD OF PRES-STRESSING Filed Oct 23. 1953 2 Sheets-Sheet 2 l0 /6 SHELLx /5 PARTITION 6 HI H [6 HEAD PRESSURE SPACE Low PRESSURE SPACE l5 ,4 l4

INVENTOR FRANK L. MAKER A MM MM ATTORNEYS VESSEL EEAD REENFQRQEZNG RENE AND MEET-1GB F PRE-STRESSHNG Frank L. Maker, Concord, Calif assignor, lay mesne asslgnments, to the United States of America as represented by the United States Atomic Energy Commission Application Qctober 23, 1953, Serial No 38$,tl09

11 Qlaims. (Cl. 22d--71) This invention relates to the construction and operation of sheet metal vessels, and particularly refers to the re ,inforcing of the juncture of the cylindrical shell of a vacuum vessel with a segmental semi-spherical or conical head which is concave toward the pressure to which such a structure is subjectedandalso refers to a procedure for pre-stressing such a structure and reducing to a minimum the localizedstresses at the junction.

Heretofore, structures of this type .have been reinforced by.rigid vcompression rings, either external to the shellheadjuncture or projecting radially inwardly therefrom. This invention comprehends broadly the utilizationof a composite ring, one part secured rigidly to the shell at or adjacent to the juncture, and the other part selectively movable with respect to the shell or to the first part of the ring. These cooperateto control the stresses that are set up by changes in pressure within the vessel by operations carried out therein, for example, when reducing the internal pressure to very low values for process conditions and later admitting atmospheric air or other gases when the processmust be stopped for clean-out, repairs, or the like. Such pressure changes mayoccur-cyclically at relatively frequent intervals and may introduce unfavorable stress Conditions leading to fatigue failure, as will be pointed out in further detail below.

It is anIobject-of this'invention to provide an improved reinforcing structure as well as a method of selectively prestressing a juncture of a concave head or diaphragm with a cylindrical shell, and specifically the shell-head junetureof a welded vessel for vacuum or pressure service.

Another object is'to reduce the stresses in a structure of this type that is subject to repeated or cyclic pressure conditions which would otherwise cause premature weakening or possible failure. of certain parts.

These and other objects and advantages will be further apparent from thefollowing description and the attached drawings, whichform a part of this specification and illustrate a preferred arrangement embodying the invention and its method'of utilization, together with several alternatives.

In the drawings, Fig. lis a diagrammatic cross sectional view of the junction between a cylindrical shell and a semi-spherical head of a vacuum vessel showing the type of deformation which tends to occur with conventional reinforced structures in this service. Fig. 2 is a stressstrain diagram representing the conditions of the material of the headrepresented by Fig. 1, together with anindication of the stress reduction attained by utilizing this invention. Fig. 3 is a schematic diagram similar to Fig. 1, representing the reduced shell deflection attainedby the improved method and apparatus. Fig. 4 is a longitudinal sectional view of a preferred form of shell-head juncture reinforcing ring structure and adjusting means therefor embodying this invention. Fig. 5 is an end elevational view on line V-V of the arrangement .ofFig. 4. .Fig. 6

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is a diagrammatic longitudinal sectional View of a .compartmented vessel, part of which is at a higher pressure than the remainder, to which the arrangements of Figs. 4 and 5, and Fig. 9 have been applied to reinforce the shellhead and shell-partition junctures, respectively. Figs. 7, 8 and 9 are longitudinal sectional views of alternative arrangements to that of Figs. 4 and 5.

Among the advantages of a reversed spherical segmental head for a cylindrical vesselforvacuurn service are saving in space required, less material than an elliptical or .outwardly dished head and moreuniform stress distnbution except near the edge or periphery of the head wherediscontinuity stresses exist in a narrowband. The spherical or semi-spherical segment has a membrane tensile stress tangent to the head at its outer edge, which may .beresolved intoaxial and radial components. Theaxialcomponent has as its reaction the longitudinal compressive stress set up in the cylindrical shell. However,the.membrane forces in the shell have no radial component that will serve as a reaction to the radial component of the head tensile stress, except as theyare developed by .circumferential compressive stresses .in the cylindrical shell. These may easily become very high with usual proportions of such vessels, and it may be necessary to usea compression ring at the shell-headjuncture to supply the radial reaction force to the head without excessive compressive stress. The ring must also bestable.againstcollapse from external load due to atmospheric air pressure.

Sucha compression ring, in order to. develop an outward radial reaction, must be under compression, and this means that .itmust decrease slightly in .diameter under load. The spherical head, on the other hand, underthe effect of atmospheric pressure, tends toincrease inradius.

This produces a discontinuity that must be met by transverse bending in the edge of the head, as indicated diagrammatically in Fig. .1.

As shown-in Fig. 1, thecompression ring moves radially inwardly under the effect .ofthe tension from the head. The edge of the headat the point whereit is attached to the .ring is also under compression and moves in radially, but, as the distance from thepointof attachmentof the headto the. compression ring increases, the circumferential compressionin the edge of the head becomes less. Asa result, the .head takes up the shape shown in the'dotted line. of that figure.

The discontinuitydiscussed above is made up of two elements, that due to themembrane displacement of the spherical head, and that due to displacement of the compression ring necessary to develop the radial reaction to resist the radial component of the tension in the head. When there is a differential pressure actington the concave side of an actual head'or across a diaphragm or-partition (Fig. 6), such aphysical discontinuity cannotexist, because the spherical head, the shell and the compression ring are welded orotherwise secured together. Instead, the edge of the head is bent transversely, as shown in Fig. l.

The 'framing of the compression ring into the cylindrical shell is so stiff that it may be assumed the compressionring will not rotate. Therefore, the tangent to the'head plate at its juncture with the compression ring will remainrfixed in direction. This will resultin-a maximum bending moment at the juncture, with accompanying bending stresses of considerable magnitude.

*If the fabrication processes left the material free of stress (which could only be approximated by stress-relieving the-entirestructure after welding), thetalgebraic sum of the membrane stress andbending stress can bekept within-reasonable limits by making the compression ring of suflicient cross-section. Actually, however, there will always be residual stresses from weld shrinkage that, in local areas, may approach the yield point. The first time the vessel is subjected to vacuum, the combination of stresses produced by the vacuum with the residual welding stresses will cause the yield point to be reached in such local areas as have residual stresses of considerable magnitude. The design will have a factor of safety provided so the computed stress will be only a fraction of the yield stress, normally from one-half to twothirds of the yield point.

Assume for this discussion that at these points where discontinuity stresses exist, the computed stress is twothirds of the yield point. It follows that at any point where the residual stress is more than one-third of the yield point, the yield point will be reached and plastic flow will occur during the vacuum test. Upon returning to atmospheric pressure, the stress will then back down or decrease by an amount equal to two-thirds of the yield stress, leaving a residual stress of one-third of the yield stress. At each subsequent application of vacuum loading, the stress will return to the yield stress and go back to one-third of the yield stress. This condition is conducive to fatigue failure in a much lower number of cycles than would be required for the same range of stress extending from zero to two-thirds of the yield point (in fact the latter condition could be withstood indefinitely). These conditions are illustrated in diagram of Fig. 2, in which the line OH represents a strain occurring up to the yield point and line H-] represents plastic strain without increased stress. The line 1-K represents the decrease of stress when the vacuum loading is removed.

The present invention proposes that, after the first application of vacuum loading of the shell, ring and head, the reinforcing ring that is attached to the cylindrical shell is to be placed in tension so that it is increased in radius. This would then produce a discontinuity in the opposite sense to that produced by the vacuum loading, and, on the stress-strain diagram of Fig. 2, it corresponds to bringing the combined membrane and discontinuity stress at the juncture of head and reinforcing ring down to the point L. This can be done by making the ring in two parts, which may conveniently be designated a main and an auxiliary ring, and which will now be described, in connection with Figs. 4 and 5 of the appended drawings.

In those figures, reference numeral designates a cylindrical metal shell to which the semi-spherical metal head 11, subjected to pressure, is joined by means of a weld 12. The end 13 of shell 10, in this example, extends beyond the weld 12 and receives a radial member or web 14 which has a transverse flange 15 to form a main ring which extends completely around the inner face of shell 10. Radially spaced from the inner face of flange 15 is an auxiliary or compression ring 16, which is kept in alignment with the main ring by means such as circumferentially-spaced washers or blocks 17 and 18, held in position by threaded studs 19 and nuts 20 and 21.

To place the main ring comprising 13, 14 and 15 in tension, and the auxiliary ring 16 in compression, as outlined above, a pair of metal wedges 22 and 23, meeting along inclined face 24, are received on each stud 19 to be positioned in the annular space between flange 15 and ring 16. Leaving nut 20 loosened, nut 21 is tightened on stud 19, compressing wedges 22 and 23 between nut and washer 17. The slippage between the wedges along face 24 will widen the annular space between flange 15 and ring 16 and accomplish desired result of stretching the main ring and compressing the auxiliary ring. After the adjustment is complete, nut 20 may be tightened against washer 18 to prevent transverse displacement of the main and auxiliary rings.

For a vessel that has not yet been put under vacuum, the auxiliary ring is put into place, but with the wedges left loose so that the main ring can decrease in diameter without stressing the auxiliary ring. If the full vacuum load is then put on the vessel, all points with residual stresses of sufficient magnitude will be plastically deformed when the stresses reach the yield point. The effect will then be to iron out all of the excessive residual stresses, as a relatively large deformation can take place without causing the stress to rise appreciably above the yield point. The vessel is then restored to atmospheric pressure and the nuts and bolts passing through the wedges are tightened up uniformly all around the ring. This puts the main ring into tension and at the same time puts the auxiliary ring into compression while there is no vacuum on the vessel.

The amount of tension put into the main ring can be adjusted to any desired value. If a case is considered in which the cross-section of the auxiliary ring is equal to that of the main ring, it will be appropriate to stretch the main ring so that its radial displacement outward while under atmospheric pressure is equal to one-quarter of the radial displacement noted when the vessel is subjected to vacuum loading without the support of the auxiliary rings. After having pre-stressed the main and auxiliary ring in this manner, vacuum loading may now be applied. For the assumed case, the radial displacement of the compound ring will now be only one-half as great as was the case for the main ring only, since the total cross-sectional area of main and auxiliary rings is now twice as great. Under the vacuum loading, the component of the discontinuity resulting from the vacuum loading is then only one-quarter as great as on the first vacuum test with the auxiliary ring not acting.

In Fig. 3, the approximate forms of the successive displacements of the edge of shell 11 are shown. Curve 0 indicates the position of shell 11 after fabrication and initially at atmospheric pressure. Curve 1 shows the approximate displacement at the edge of the head 11 resulting from the first application of vacuum to the vessel and without the support of the auxiliary ring 16. Curve 2 shows the bending of the head 11 resulting from the deliberate pre-stressing of the main and auxiliary rings at atmospheric pressure, as described above, and curve 3 shows the approximate bending of the head under the subsequent vacuum loading. In this example, for the assumed case, the total movement (2--3) of the head edge after the pre-stressing of the rings is completed is only one-half as great as the total movement (0-1) or bending of the head 11 with only the main rings 13, 14, 15 in operative condition on the stress-strain diagram of Fig. 2. The stress range in the head 11, with both rings in the pre-stressed condition, would be represented by the distance L-M.

In the foregoing discussion it has been assumed that the main and auxiliary rings were of the same cross-sectional area. This is not necessarily the most economical arrangement. Also, it is not necessarily true that the total cross-section of main and auxiliary rings should be twice as great as would be provided with the ordinary compression ring alone. To explore this condition, computations will have to be made for each specific case.

Alternative arrangements of main and auxiliary rings are shown in Figs. 7, 8 and 9 wherein similar reference numerals designate corresponding elements. In Fig. 7, the auxiliary or compression ring is formed in two parts 161 and 162, which are initially loosely positioned on each side of web 14 by bolts 26 passing through slots 27. Studs 28 threaded at 29 into rings 161 and 162 extend through holes 30 in inner flange 15 of the main ring and are adapted to be tightened by nuts 31 to place rings 161 and 162 in compression after the initial vacuum loading of the vessel, as described above. Bolts 26 may be drawn snug to maintain rings 161 and 162 in position.

In Fig. 8, the auxiliary rings 161 and 162 are similarly placed as in the arrangement of Fig. 7, but are provided with thrust bolts 32 threaded at 33 through the rings to be received into sockets 34 on the lower face of flange 13. These act to place the rings in compression after the initial vacuum loading of the vessel. Bolts 26 similarly maintain the rings in position on each side of web 14.

Fig. 9 illustrates another alternative in which auxiliary ring 16 is externally niountedout'side of flange 13 and is aligned and also placed in compression at the desired time of operation by means of tension bolts 35 passing through holes 36 in flange 13.

Any of these arrangements saisfactorily reduces the discontinuity bending stresses at the juncture of head and main ring and insures that the maximum stresses vary over a range of compression to tension that is well within the yield point and fatigue limit. The main ring will have a circumferential tension at atmospheric pressure and a circumferential compression under vacuum loading. The auxiliary ring will have an initial circumferential compression (which is necessary to cause the prestressing of the main ring) and under the vacuum loading will have this compressive stress increased. There will be one limitation on the design, which will be the maximum compressive stress that is considered desirable for this compression ring. The stress system for these auxiliary rings is very simple, and it will be reasonable to allow a somewhat higher stress on this account than for locations where uncertainties exist. A compressive stress as high as 20,000 pounds per square inch would not be unreasonable. This is actually less than provided for short columns by the usual structural specifications. The other design limit will concern the magnitude of the discontinuity plus membrane stress in the head-main ring juncture during the initial vacuum test when the auxiliary rings are not acting. The magnitude of this stress is more difiicult to fix. If the initial conditions were those of a vessel having no residual stresses, it would be reasonable to run this up to the yield stress, since the rings can be so proportioned as to insure that thereafter the stresses will not exceed one-quarter of the yield stress. However, if there is taken into account the fact that there will necessarily be local conditions where there is considerable residual stress from welding, application of too much additional loading might require to great an amount of plastic yielding. It is probably desirable, therefore, to keep this computed stress somewhat below the yield point. A value of 25,000 pounds per square inch would appear reasonable. This amounts to about 65 percent of the yield stress for steel with 70,000 pounds per square inch ultimate strength such as A212 Grade B Flange Quality. Assuming these criteria, the desirable proportions of the main and auxiliary rings can be computed for any case.

A system of pro-stressed compression rings offers the advantage of giving conditions similar to those that result in an ordinary pressure vessel having discontinuity stresses (which all pressure vessels have) resulting from applying a test pressure in excess of the normal working pressure, without making it necessary to go to extreme lengths, such as putting on a temporary additional head, to actually secure the additional pressure for testing. The stresses in each working cycle, after pre-stressing the rings, will in all cases be well within the yield stress, making the structure safe against fatigue failure.

In the foregoing descriptions, the term head has been generally used to designate an end closure. It is intended that the term head should also apply to a diaphragm or partition extending across a cylindrical shell to separate adjacent chambers or compartments between which pressure differences will exist. For example, a higher pressure compartment may be separated by a concave partition or diaphragm from a compartment of much lower pressure, as shown in Fig. 6. For convenience, the form of auxiliary ring 16 for such a juncture could be that of Fig. 9, so that adjustment of the compression ring could be made from the outside of shell 10.

If desired, substantially the reverse of the tensioncompression rings may be applied to a convex head or partition, which would, in circumstances apparent to one skilled in this art, give a comparable stress reduction as those described herein for concave heads.

Accordingly, although specific examples illustrating this invention have been illustrated and described, numer- '6 bus modifications and changes could be made without departing therefrom, and all those that fall within the scope of the appended claims are intended to be embraced thereby.

I claim:

1. In combination with a sheet metal vessel subject to pressure changes and having a concave head welded to a cylindrical shell, stiffening means for said shell spaced from its juncture with said head, said means comprising a main ring rigidly secured to the inside of said shell and beyond its juncture with said head, an auxiliary ring positioned adjacent to said main ring, and circumferentially spaced means for selectively connecting. said rings to place said main ring under tension and said auxiliary ring under compression.

2. The combination according to claim 1, in which said main ring comprises a web secured to said shell and a transverse flange for said web, said auxiliary ring being positioned adjacent said flange and selectively connected thereto.

3. The combination according to claim 1 in which said main ring comprises a web secured to said shell and a transverse flange for said web, said auxiliary ring being in two parts, one part positioned on each side of said web adjacent said flange and selectively connected to said flange and said web.

4. The combination according to claim 1 in which said auxiliary ring is radially spaced inwardly from said main ring and is connected thereto by circumferentially spaced expansion members.

5. The combination according to claim 1 in which said auxiliary ring is connected to said main ring by threaded members selectively adjustable to stretch said main ring and compress said auxiliary ring.

6. In combination with a sheet metal vessel subject to pressure changes and having a concave head welded to a cylindrical shell and circumferential stiffening means secured in said shell facing the concave side of said head and spaced from said weld, means for reducing stress in said head adjacent to said weld due to pressure changes in said vessel, comprising a ring selectively connected to said stiffening means, and means for introducing compressive stresses in said ring.

7. A method of reducing stresses in a sheet metal vessel subject to pressure changes, said vessel having a cylindrical shell welded to a concave head and provided with integral circumferential stiffening means spaced outwardly from said weld, comprising the steps of reducing the pressure within said vessel to introduce stresses in said head adjacent to said weld that will cause plastic deformation thereof, and subsequently introducing and maintaining tension in said stiffening means.

8. A method of reducing stresses in a sheet metal vessel having a concave head to separate a zone of higher pressure from a Zone of lower pressure, said vessel having a cylindrical shell welded to said head and provided with circumferential stiffening means spaced from and facing said head, comprising the steps of increasing the differential pressure across said head to introduce tension stresses in said head adjacent to said weld that will cause plastic deformation thereof, reducing said pressure differential, and subsequently introducing and maintaining tension in said stiflening means.

9. The combination according to claim 6, in which said stiffening means comprises a web extending inwardly from said shell, and said ring comprises a member encircling said shell and aligned with said web, and means selectively connecting said shell and said member adjacent to said web, for introducing tension in said web and compression in said ring member.

10. The combination according to claim 6, in which said stiffening means comprises a web extending inwardly from said shell, said ring being in two parts, one part positioned on each side of said web and means selectively connecting the inside of said shell adjacent said web and each of said ring parts for introducing tension in said web and compression in said ring parts.

11. In a sheet metal vessel subject to pressure changes and having a generally concave head welded to a cylindrical shell at the junction therewith and circumferential stiffening means secured to said shell adjacent to said juncture, said stiffening means tending to become smaller in diameter due to pressure changes in said vessel, the combination comprising a ring selectively connected to said stiffening means, and means for introducing sutficient compressive stresse in said ring to produce tension stresses in said stiffening means by increasing the diameter thereof.

References Cited in the file of this patent UNITED STATES PATENTS Hawthorn et a] Dec. 22, Emery Apr. 28, Guibert June 13, Loffier Nov. 19, McBride Feb. 25, Merriam Feb. 25, Gerstenberg Dec. 14,

FOREIGN PATENTS Netherlands Oct. 15,

Claims (1)

1. 6. IN COMBINATION WITH A SHEET METAL VESSEL SUBJECT TO PRESSURE CHANGES AND HAVING A CONCAVE HEAD WELDED TO A CYLINDRICAL SHELL AND CIRCUMFERENTIAL STIFFENING MEANS SECURED IN SAID SHELL FACING THE CONCAVE SIDE OF SAID HEAD AND SPACED FROM SAID WELD, MEANS FOR REDUCING STRESS IN SAID HEAD ADJACENT TO SAID WELD DUE TO PRESSURE CHANGES IN SAID VESSEL, COMPRISING A RING SELECTIVELY CONNECTED TO SAID STIFFENING MEANS, AND MEANS FOR INTRODUCING COMPRESSIVE STRESSES IN SAID RING.

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