Method of welding a skirt to a cylindrical vessel

Jan. 10, 1961

N. A. WEIL

METHOD OF WELDING A SKIRT TO A CYLINDRICAL

2,967,352

VESSEL

Filed June 22, 1955

FIG. I

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ATTORNEYS

United States Patent Office

2,967,352

Patented Jan. 10, 1961

2,967,352

METHOD OF WELDING A SKIRT TO A CYLINDRICAL VESSEL

Nicholas A. Weil, Middle Village, N.Y., assignor to Ths M. W. Kellogg Company, Jersey City, N.J., a corporation of Delaware

Filed June 22,1955, Ser. No. 517,232

Claims. (Cl. 29—479)

This invention relates in general to fusion welding and in particular to the provision of a welded joint between the shell of a vessel and a support member which connects the vessel to its foundation.

It is usual to support large, heavy vessel shells on skirts that are attached to, and carried on, foundations suitable to support the vessel in the intended use. It is also usual to unite the vessel shells to the supporting skirts by fusion welds that are deposited circumferentially as fillet welds around the vessel shells. These fillet welds are deposited on the shoulder defined by the horizontal end surface of the skirt and the contiguous vertical surface of the vessel. The throat of such fillet welds must always be less than the thickness of the skirt. Therefore, regardless of the length of the vertical leg of the weld, these fillet welds always produce a restricted path for stress and heat flow between the vessel and the skirt. This conventional form of attachment is satisfactory only if the vessels remain at substantially constant temperatures, whether ambient or elevated, for long periods of time and are not subjected to rapid cyclic temperature changes. However, if the vessels are used in processes which include a comparatively rapid cyclic swing from one to another of widely separated temperatures, the conventional form of attachment is unsatisfactory and a source of considerable trouble and expense. The cyclic operations mentioned not infrequently result in cracking of the connecting skirt weld.

The conventional delayed coking process is an example of a process having a temperature swing cycle of such severity as to render the conventional skirt attachment unsatisfactory for the vessels used. This process is a batch operation running on a cycle which varies from 24 to 48 hours. During the operating portion of the cycle the temperature of the equipment rises to about 800° F. and then falls rapidly by reason of water quenching to atmospheric. The severely cyclic nature of this operation subjects the skirt attachment weld to severe stresses. It has been found that stresses which exceed the yield point of the steel out of which the vessels are made can be developed if the axial thermal gradient exceeds 10° F. per inch. Examination of actual operating delayed coking installations indicated axial gradient values at the skirt weld in excess of 20° F. per inch as not unusual.

Cracking of the skirt weld commonly begins as small incipient cracks on the outer surface of the weld, generally at the roots between the weld beads, and as time goes on, penetrate both circumferentially and radially inwardly. Often the cracking penetrates the entire weld and vessel shell thickness so that its presence is detected by oil seepage. Cracks have been found that run entirely around the girth of the vessel and in instances, the cracks actually separated the vessel from the skirt support with the vessel sitting loosely on the skirt. The reasons for the cracking of conventional skirt welds are generally known. During the coking portion of the operating cycle a heavy heat flow exists conducting heat from the vessel wall through the skirt weld to the skirt and down the skirt to the foundation. The resulting gradients, in the order of 10° F. and more, induce high bending moments at the skirt attachment weld. The nature of this bending is such that a tensile stress is present at the outer surface 5 of the weld. With the simple, conventional fillet weld, the level of this calculated tensile strength is considerably in excess of 100,000 p.s.i.

Stresses in the order of 100,000 p.s.i. cannot be carried by the material out of which the coke drums, or vessels, 10 are ordinarily fabricated. Instead, the weld metal yields a small amount to accommodate the rotation tendency of the skirt. When the vessels are brought down to ambient temperature during quenching, a heavy heat flow also exists. At this time the heat flow is from the founda15 tion up the skirt and through the skirt weld to the vessel wall, so that the reverse plastic flow and rotation takes place, mitigated to a certain degree by the previously established locked-up stresses. With every operating cycle this high amplitude stress reversal continues. The result 20 is fatigue cracking after a sufficient number of cycles, the cracks originating on the tensile outer face of the weld. The fatigue origin of skirt weld cracking is further indicated by the fact that in cases where the damaged weld was repaired in the conventional manner, cracking of 25 the new repair weld proceeded with almost clockwise predictability after a period of time.

It is a principal object of this invention to provide a welded joint, and method for making it, between a vessel and its supporting skirt, such that the tendency to develop ³⁰ fatigue cracks in the joint over extended periods of use of the vessel is markedly reduced, if not entirely eliminated.

It is also a principal object of this invention to provide a novel welded joint, and novel method for making it, be³⁵ tween a vessel and its supporting skirt, said vessel in use subjected to cyclic conditions that tend to the development of fatigue causing stresses in the metal joining it to the skirt, said novel joint and method such that said tendency to develop fatigue is greatly reduced, if not com⁴⁰ pletely eliminated.

It is also a principal object of this invention to provide a novel skirt weld, and novel method for producing it, uniting a vessel to its supporting skirt, said vessel subjected in use to wide cyclic temperature changes, said ₄_ novel weld and method such that a large area is provided for the flow of heat from the vessel to the skirt, and vice versa, whereby better heat distribution is effected and the stresses developed in the weld metal materially reduced to the end that the cracking of the weld metal due to ₅₀ fatigue over extended periods of use of said vessel is markedly reduced, if not completely eliminated.

It is a further principal object of this invention to provide a novel skirt weld, and novel method for making it, uniting a vessel to its supporting skirt, said vessel sub55 jected in use to wide cyclic temperature changes, said novel weld and method such that a large area is provided for the flow of heat from the vessel to the skirt, and vice versa, and the contour of the weld is shaped to eliminate potential stress raisers, whereby the stresses developed _6Q in the weld metal are materially reduced to the end that the tendency to develop fatigue cracks in the weld metal over extended periods of use of said vessel is markedly reduced, if not completely eliminated.

The further objects, advantages, and features of my _6g invention will become apparent from a consideration of the following detailed description thereof taken with the accompanying drawings, in which:

Fig. 1 is a front view showing a vessel supported on the skirt which connects the vessel to its foundation;

₇₀ Fig. 2 is a fragmentary sectional view showing the skirt with its prepared upper edge applied to the vessel prior to welding; and

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Fig. 3 is a fragmentary sectional view showing the skirt welded to the vessel.

The novel welded joint, as well as the novel method for making it are of general application and may be used wherever it is desired to provide a joint of great strength with a high factor of safety. When the parts to be united are not in use subjected to conditions so severe and cyclic as to generate stresses in the weld metal inducive to fatigue cracking thereof, the added cost of the novel joint may not be justified, and the conventional fillet welds or the equivalent are indicated. However, when the parts to be united are used under conditions so severe that fatigue producing stresses are developed in the weld metal, the joint and method of this invention are particularly applicable. The high stresses leading to fatigue cracking may result from wide cyclic temperature changes or cyclic loading changes due to pressure, or other causes.

The novel welded joint, and method for forming it, of the invention are particularly applicable to the attachment of the supporting skirts to the heavy vessels used in the materials processing and treating industries, and are especially applicable to those vessels which in use are cyclicly swung from one to another and vice versa, of widely separated temperatures. The coke drums, or vessels, used in the conventional delayed coking process are typical of the vessels to which the present invention applies. Thus by way of illustration, rather than by way of limitation, the novel joint and method of making it, of the invention will be disclosed in detail in connection with a coke drum and the attaching of a supporting skirt thereto.

The conventional delayed coking process is a batch operation which is run on a 24 to 48 hour cycle. During the coke forming portion of the operating cycle the coke drum attains a temperature of 800° F. and more. During the water quenching portion of the cycle the coke drum is brought down to ambient temperature. The quenching portion of the cycle is of comparatively short duration and takes some 8 hours and less to complete. Barring shutdowns due to failures or for repair, the cycle is repeated continuously.

Referring now to the drawing:

A coke drum 10 for carrying out the conventional delayed coking process is shown in Fig. 1. The drum 10 includes a cylindrical body section 11 whose upper end is closed by a dished head 12 and whose lower end is closed by a cone 13. The drum 10 includes various nozzles, manholes, etc., for introducing the heated coke producing material therein, for removing of the vaporous products and formed coke therefrom, and for supplying the quenching water thereto. These manholes, nozzles, etc. have not been shown as they are conventional and are not necessary for a proper understanding. of the present invention. The drum 10 in operation is vertically disposed and is mounted, as shown, on the skirt 14. The skirt 14 is cylindrical in form and closely encircles the lower end of the cylindrical body section 11. The bottom of the skirt 14 is connected in any suitable conventional manner to the foundation 16 which as. is usual may be of masonry, concrete, or similar material. The skirt 14 is joined to the cylindrical body section 11 by the fusion weld 15. In operative condition, the whole external surface of the drum 10 and the whole external and internal surface of the skirt 14 are covered with a thick layer of suitable insulation. The insulation has been omitted from the drawings as it is not necessary to the explanation of the invention. The skirt 14 may be attached to the drum 10 prior to the erection of the drum 10 at the site or it may be applied to the drum 10 during the erection of the vessel at the site. The coke drum 10 usually is some 70 feet high and 19 feet in diameter and will yie’d some 450 tons of coke per coking cycle. The coke drum 10 and the skirt are approximately one inch thick and are usually made of carbon steel.

In preparation of the welding operation, the skirt 14 has the end 17 thereof, the end that is to be its upper end when the skirt 14 is positioned on the foundation 16, machined, flame cut, or otherwise shaped to provide the welding groove 18 shown in Fig. 2 when the skirt 14 is positioned against the vertical surface of the cylindrical body section 11. The shaped end 17 includes an inclined chamfer 19 which ends in the curved chamfer 20. The chamfer 18 is not carried to the outside face of the skirt 14 but rather begins at a distance b inwardly therefrom. It has been found in practice that if the chamfer 19 begins at the outside face of the skirt 14, cracking may take place in the region of the juncture of the end 17 and the metal of the weld 15, due to undercutting at the edge of the weld. By making the distance b about i/s of an inch and thus avoiding a chisel-like edge at the end 17, cracking is effectively eliminated. Values of the distance b substantially in excess of Vs of an inch, while they too assure a crack free weld toe, are usually not desirable as they render the groove 18 too shallow. Values of b substantially less than ’/a of an inch do not effectively eliminate the cracking.

In order to fully obtain the novel results of the invention, the throat of the weld 15, in this case the distance from the vertical leg 21 at the weld 15 to the toe of the weld 15 at the end 17 measured on a line normal to the leg 21, should be at least equal to the thickness of the skirt 14. This can be accomplished by making the vertical weld leg 21, shown in Fig. 3, sufficiently long. The longer the weld leg 21 relative to the thickness of the skirt 14, the better the heat flow path. However, as the portion of the weld leg 21 forming part of the deposit in the groove 18 is made longer, the lower end of the groove 18 becomes narrower and more inaccessible. It is found as a practical matter that the best results are obtained when the chamfer 19 approaches the diagonal of a square, that is, the chamfer 19 is at an angle in the order of 45° to the horizontal. With this, angle of chamfer the portion of the vertical leg 21 in the groove 18 will approach but be somewhat less than the thickness of the skirt 14. If the chamfer 19 is carried to the inside surface of the skirt 14, the weld groove 18 will have a steep sided, inaccessible bottom, so that the obtaining of a satisfactory joint at the root of the weld would be difficult even with very small electrodes. To provide a weld groove bottom that is readily accessible to the electrode or other welding instrumentality usually employed, the chamfer 19 terminated in the curved chamfer 20. With the skirt 14 of a thickness in the order of 1 inch and with the chamfer 19 cut at an angle in the order of 45°, a curved chamfer 20 with a radius of % of an inch is satisfactory.

After the end 17 has been shaped, the skirt 14 and the vessel 10 are relatively positioned as indicated in Fig. 2 preparatory to welding. The welding may be carried out in any convenient manner either as. an arc operation or as a gas operation. The welding is, however, preferably performed by striking an electric arc from the end of a fusible metal electrode.. While the conventional flux submerged arc operation is usable, it is preferred to deposit at least the first several beads by hand welding. After the vessel 10 and the skirt 14 are positioned as indicated, the welding operation is begun, care being taken to deposit fusing weld metal into the very bottom of the groove 18 and to secure proper penetration into the metal defining the bottom of groove 18. After this is done, further beads are deposited until the whole of groove 18 is filled. In order to improve the heat distribution and to reduce the stresses that result from the heat flow, the vertical leg 21 of the weld should be long enough to insure a throat dimension equal to the thickness of the skirt 14.. It is found that the best results commensurate with efficiency and economy are obtained if the vertical leg 21 of the weld 15 is made approximately twice the length of the

2,967,352 thickness of the skirt 14. At present a vertical leg equal to 1.8 times the thickness of the skirt 14 is preferred. To achieve this result a fillet weld is deposited which extends upwardly above the weld metal which fills the original groove 18. As indicated in Fig. 3, these further 5 weld deposits are made in a manner to approximate the usual fillet weld contour. By this procedure a weld IS is produced which is a groove weld combined with a fillet weld.

When the welding operation is completed, the surface 10 of the weld 15 as well as the toe areas of the weld 15 are ground to eliminate all stress raisers which may be in the form of projections, pits, undercuttings, etc. During the grinding operation not only is the exposed surface of the weld 15 ground to provide a smooth surface 15 but the grinding is carried out into the toe areas of the weld to obtain a smooth blending of the weld metal at said toe areas into the contiguous metal of the skirt 14 and the vessel 10.

Although many changes can be made by those skilled 20 in the art without departing from the scope of the invention, it is intended that all matter contained in the above description and appended claims and shown in the accompanying drawings shall be interpreted as illustrative and not limitative. 25