US2769563A - Insulated skirt supported vessels - Google Patents

Description

Nov. 6, 1956 N. A. WElL 2,769,563

INSULATED SKIRT SUPPORTED VESSELS Filed June 22, 1955 INVENTOR.

NICHOLAS A. W-EIL BY 5', H. 1/. i, awn/cu ATTORNEYS United States Patent INSULATED SKlRT SUPPORTED VESSELS Nicholas A. Wail, Middle Village, N. Y., assignor to The M. W. Keilogg Company, Jersey City, N. 3., a corporation of Delaware Application June 22, 1955, Serial No. 517,231

8 Claims. (Cl. 220-9) This invention relates in general to insulated vessels which in use are subjcted to cyclic temperature swings, and in particular to such insulated vessels which are carried on metal support members and connected thereto by fusion welded joints.

It is usual to support large, heavy vessel shells on skirts that are attached to, and carried on, foundations suitable to support the vessels in the intended uses. 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 surface of the vessel. When these vessels are used in operations which involve elevated temperatures, it is common practice to insulate the vessels and their supporting skirts in order to conserve heat and thereby maintain the efiiciency of the operation. It is the usual practice to cover with insulation of a suitable character and thickness, all exposed surfaces including the complete inside surface of the skirt and the crotch formation at the juncture of the vessel and the skirt. Thus, practically all of the heat that flows from the vessel to the skirt and vice versa, must pass through the skirt weld and the metal immediately adjacent thereto. These conventional insulated vessels are satisfactory if in use they remain at substantially constant elevated temperatures for long periods of time and are not subject to rapid cyclic temperature changes. If, however, comparan'vely rapid cyclic swings from one to another of Widely separated temperatures are involved, considerable trouble is experienced and periodic expensive weld repair is required. The cyclic operations mentioned not infrequently result in cracking of the connecting skirt welds.

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 therein. The conventional delayed coking 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 and then falls rapidly by reason of water quenching to atmospheric. The severely cyclic nature of this operation subjects the skirt attachment weld to destructive stresses. It has been found that stresses which exceed the yield point of the steel out of which these vessels are made can be developed if the axial thermal gradient exceeds 10 F. per inch. Examination of actual operating delayed coking installations employing conventional insulated vessels with conventional skirt attachment welds indicated axial gradient values at the skirt welds in excess of F. per inch as not unusual.

Cracking of the skirt welds 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 penetrates both circumferentially and radially inwardly. Often the cracking penetrates the entire weld 2,769,563 Patented Nov. 6, 1956 and vessel shell thickness so that its presence is detected by oil seepage. Cracks have been found to 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 the 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 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 of the weld. With the simple, conventional fillet welds and conventional insulation, the level of this calculated tensile stress 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 ves sels, 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 foundation up the skirt and through the skirt weld to the vessel Wall so that the reverse plasic 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 is fatigue cracking after a sufiicient number of cycles, the cracks originating on the tensile outer face of the weld. The fatigue origin of the skirt weld cracking is further indicated by the fact that in cases where the damaged weld was repaired in the conventional manner, cracking of the new repair weld proceeded with almost clockwise predictability after a period of time.

It is a principal object of this invention to provide an insulated, skirted vessel such that fatigue producing stresses at the weld joining the skirt to the vessel are materially reduced and the tendency to develop fatigue cracks in the Weld over extended periods of time is practically eliminated.

It is also a principal object of this invention to provide an insulated, skirted vessel such that the axial temperature gradients in the region of the weld joining the skirt to the vessel are materially reduced with consequent material reduction in the tendency to develop fatigue cracks in the Weld over extended periods of use of the vessel.

It is also a principal object of this invention to provide an insulated, skirted vessel in which the insulation is so arranged in the region of the weld joining the skirt to the vessel that the axial thermal gradients at said region, but particularly in the skirt, are reduced materially below values which produce destructive stresses in the weld due to heat flow, to the end that the tendency to produce fatigue cracks in the weld over extended periods of use of the vessel is practically eliminated.

It is a still further principal object of this invention to provide an insulated, skirted vessel in which the insulation is omitted from the surface :of the skirt and the surface of the vessel in the crotch region adjacent the weld uniting the skirt to the vessel and said region closed to the atmosphere by a heat barrier whereby heat in flowing from the vessel to the skirt and vice versa will not only flow through the Weld uniting the skirt to the vessel but will also flow across said closed region by radiation and convection, to the end that the axial heat gradient in the zone of saidweld, but particularly at the upper end of the skirt, is materially reduced and the tendency to produce fatigue cracks in the weld over extended'periods of use of the vessel is practically eliminated.

The further objects, advantages, and features of my manner in which the vessel is insulated at the region of the juncture of the skirt and the vessel wall; and

Fig. 3 is a fragmentary view similar to Fig. 2 illustrating a different positioning of the skirt end relative to the vessel wall.

The insulated, skirted vessel of this invention is of general application and is suited for use in operations involving elevated temperatures where safety and vessel life are primary considerations. The novel vessel of this invention is particularly useful in operations which involve conditions severe enough to generate fatigue producing stresses in the skirt weld of the conventional skirted vessel. The novel vessel of the invention finds its most important use in the materials processing and treating industries'whereinelevated temperatures are employed and especially to those operations in said industries which involve cyclic swings from one to another, and vice versa, of widely separated temperatures. The conventional delayed coking processes are typical of processes in which the vessels of the present invention are especially useful; than by way of limitation, the novel skirted, insulated vessel of this invention will be disclosed in detail as a coke drum, or similar vessel of the character used in conventional delayed coking processes.

Conventional delayed coking processes are batch operations which 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 drum is brought down to ambient temperature. The quenching portion of the cycle is of comparatively short duration and takes some eight hours and'less to complete. 'Barring shutdowns due to failures or repair, the cycle is repeated continuously.

Referring now to the drawings:

, A coke drum suitable for carrying out the con- Thus, by way of illustration rather ventional delayed coking process, is shown in Fig. 1. I

The coke drum 16 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 as is usual, includes various manholes, nozzles, lines, etc., for introducing the heated coke producing materials, for re-' moving vaporous products and the formed coke and for supplying the quenching water. These manholes, nozzles, lines, etc., have not been shown as they are conventional and are not necessary for the proper disclosure and understanding of the present invention. The drum'10 in use is vertically disposed and is mounted on the supporting 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 15 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 circumferential fusion weld 16. Typically, a coke drum 10 in the erected condition is.

some 70 feet high and 19 feet in diameter and will yield about 450 tons of coke per coking cycle. Such a coke drum 10 and its skirt 14 are approximately 1 inch thick with the skirt 14 some 4 or 5 feet long; both the drum 1t and the skirt 14 are made of carbon steel.

In order to prevent heat loss to the atmosphere and the consequent reduction in the efficiency of the coke forming process, the whole external surface of the drum 10 and the whole external surface of the skirt 14 are covered with a thick layer of insulatio'n' 17, usually about 4 inches thick. The external surface of the cone 13 within the skirt 14 and the internal surface of the skirt 14 are likewise covered with a thick layer of insulation 17 with the exception of a zone of each of said surfaces in the crotch adjacent the juncture of the skirt 14 and the drum 10 which is left uncovered as explained hereinafter.

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 drum 1% at the site. The circumferential weld employed to unite the skirt 14 to the drum may bein the form of a;

conventional fillet weld, that is, a weld deposited by arc welding or gas welding on the shoulder formed by the horizontal top surface of the skirt 14 and the contiguous vertical, or inclined, surface of the drum 10 and extending along said contiguous surface. However, in accordance with the teachings of the present invention and in order to better secure the results flowing from the use of the invention, said circumferential weld is preferably. of the form of the weld 16 shown in Fig. 2, When the vessel and skirt arrangement is such as is shown in Fig. 3,

and a fillet portion 19 which is integral withthe top of the portion 18 and extends upwardly along the cylindrical section 11 a distance also approximately equal to the thickness of the skirt 14. The preferred vertical leg of the weld 16 is inthe order of twice the thickness of the skirt 14. With a cokedrum 10 dimensioned as above, a weld 16 with a vertical leg 1.8 times the thickness of the skirt 14 is fully satisfactory. The weld 16 is also characterized by the fact that the portion 18 penetrates into the skirt 14 at an angle approximating 45- and changes its angle sufiiciently at its root to provide complete fusion and penetration into the metal of the skirt 14' and the cylindrical section 11 at said root. The weld deposit 16is further characterized by the fact that the length of its throat is equal to the thickness of the skirt 14.

7 While the position of the end of the skirt 14 against '7 V the side of the cone 13, as shown in Fig. 3, is not considered the best arrangement, it is a not unusual arrangement. With the skirt 14 positioned against the cone 13 as shown in Fig. 3, the weld metal nugget is limited to;

that one that can be deposited between the top horizontal surface of the skirt 14 and the contiguous surface of the cone 13. To obtain the best results, the nugget 16 is deposited to completely fillv the space between the top of the skirt 14 and the surface of the cone 13and it is carried up so that its exposed surface approximates a continuation of the surface of theskirt 14. 'The nugget 16a is made to have a vertical leg of such length that the length of the nugget 16 along the inclined surface of the cone 13 is in the order of twice the thickness of the skirt 14. With a coke drum 10 dimensioned as above,

said length is preferably 1.8 times the thickness. of the skirt 14. Y

Y. The weld metal nugget 16, or 16a, is preferably deposited from the end of a fusing metal electrode under the influence of an electric arc' discharge in a conventional manner. After the welding, the exposed surface of the weld nugget, weld 16 or weld 16a, is ground to eliminate all projections, undercuttings, pits, and other stress;

raisers. Likewise, the toe areas of the weld, weld 16 or weld" 16a, are ground to secure smooth blending of the metal of the weld nugget into the contiguous metal of the skirt 14 and the drum 10f The welds 16 and 1611 result in improved heat distribution at the joint of the skirt .14 and the drum 10 with a consequent better stress distribution at the Weld area. Furthermore, these welds 16 and 16a, because of their long throats, do not act as restrictionsin the flow of heat from the cylindrical section 11 to the skirt I4 and vice versa, and thus improve the stress distribution.

As stated, the inside surface of the skirt 14 and the outside surface of the cone I3 are also covered with a depth of insulation 17. However, a band of each of these surfaces in the crotch region formed where the skirt 14 and the drum come together, is left uncovered. To close the mouth of the triangular annulus defined by said non-insulated bands of the surface of the skirt 14 and the cone 13 to thereby form the chamber 21, a ring sheet 20 is welded to either the skirt 14 or the cone 13, but preferably to the cone 13 as shown, by deposition of circumferential fillet welds 22. After the ring 20 is welded in place, insulation 17 is packed against its bottom face to seal the said chamber 21. With a coke drum 10 dimensioned as indicated above, the chamber 21 has a vertical le of about 20 inches and a horizontal leg of about 8 inches. The size of the chamber 21 depends on the size of the vessel and the duty to which the vessel is to be subjected. In any case, the chamber 21 should be large enough to assure a sutficient heat fiow from the drum 10 to the skirt 14 or vice versa, by radiation thereacross and by conduction therethrough, to materially reduce the axial thermal gradients in the zone of the weld 16 and particularly in the upper end of the skirt 14. The axial thermal gradients in the skirt 14 are generally the chief source of excessive stress at the skirt weld in conventional skirted vessels. It was found from a comparison of data derived from a coke drum 10 having a chamber 21 with insulation omitted from the internal walls thereof in accordance with this invention, and data derived from a coke drum of conventional construction and insulation arrangement, that a reduction of stresses at the skirt weld in the order of 2.3 times is obtainable by the use of insulation arrangements of the invention.

While the size of the chamber 21 will vary as the size of the drum It varies, considerable variation is possible with a drum 10 of a particular size without greatly changing the results obtained. It was found that with a drum 1-3 dimensioned as indicated, an important reduction in stresses at the skirt 14 was obtained when the chamber 21 had a vertical leg in the order of 10 times the thickness of the skirt 14. Upon increasing said vertical leg it was found that the maximum stress reduction was obtained when said leg reached a length in the order of 20 times the thickness of the skirt 14. Further increase of leg length showed no significant improvement. When said leg length reached the order of 30 times said thickness, the chamber 21 began to seriously diminish the space available for the necessary apparatus elements at the bottom of the drum 10.

Although many changes can be made by those skilled 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.

I claim:

1. A vessel adapted in use to be subjected to elevated temperatures comprising a shell having a bottom section of diminishing diameter, a supporting skirt member having an end positioned on said vessel in the region of said bottom section, a weld metal deposit at said end uniting said skirt member to said vessel, and a depth of insulation forming a heat barrier between said vessel and the atmosphere and said skirt member and the atmosphere, said skirt member and said vessel providing a crotch formation at their juncture of increasing area in the direction of the bottom of the vessel, said insulation applied to substantially the whole internal surface of said skirt below a level below said juncture at least approximately 10 times the thickness of said skirt member, said insulation applied to substantially the Whole external surface of said bottom section inward of said skirt member to said level to provide an annular space at said crotch formation '6 in part defined by exposed surface of said vessel and said skirt for heat flow by radiation and convection from one to the other of said vessel and said skirt members.

2. A vessel as defined in claim 1, in which said level is below said'juncture a distance at least approximately ten times but not greater than approximately thirty times the thickness of said skirt.

3. A vessel as defined in claim 1, in which said level is below said juncture a distance in the order of twenty times the thickness of said skirt.

4. A vessel adapted in use to be subjected to elevated temperatures, comprising a shell having a bottom section of diminishing diameter, a supportingskirt member having an end positioned on said vessel in the region of said bottom section, a weld metal deposit at said end uniting said skirt member to said vessel, a depth of insulation forming a heat barrier between said vessel and the atmosphere and said skirt member and the atmosphere, said skirt member and said vessel providing a crotch formation at their juncture of increasing area in the direction of the bottom of the vessel, said insulation applied to substantially the whole internal surface of said skirt below a level a substantial distance from said juncture, said substantial distance at least approximately ten times the thickness of said skirt member, said insulation applied to substantially the Whole external surface of said bottom section inward of said skirt member to said level, and a battle member supported from one of said bottom sections and said skirt member at said level to substantially close said crotch formation at said level to thereby provide a chamber defined by said bafiie and the exposed surface of said vessel and said skirt member for heat flow by radiation and convection from one to the other of said vessel and said skirt member, said insulation applied to cover the external surface of said baffle member.

5. A vessel adapted in use to be subjected to elevated temperatures comprising a shell having a bottom section of diminishing diameter, a supporting skirt member having an end positioned on said vessel in the region of said bottom section, a weld metal deposit at said end uniting said skirt member to said vessel, said weld metal deposit having a throat length substantialy equal to the thickness of said skirt member and extending along said vessel a distance in the order of twice the thickness of said skirt member, and a depth of insulation forming a heat barrier between said vessel and the atmosphere and said skirt member and the atmosphere, said skirt'member and said vessel providing a crotch formation at their juncture of increasing area in the direction of the bottom of the vessel, said insulation applied to substantially the whole internal surface of said skirt member below a level a substantial distance from said juncture, said insulation applied to substantially the whole external surface of said bottom section inward of said skirt member to said level to provide a chamber at said crotch formation in part defined by exposed surface of said vessel and said skirt for heat flow by radiation and convection from one to the other of said vessel and said skirt member.

6. A vessel adapted in use to be subjected to elevated temperatures comprising a shell having a cylindrical body section, a bottom section of diminishing diameter closing said cylindrical section, a cylindrical supporting skirt member encircling said cylindrical body section and having an end contacting a portion thereof immediately above the upper end of said bottom section, a weld metal deposit at said end uniting said skirt member to said cylindrical body section, a depth of insulation forming a heat barrier between said vessel and the atmosphere and said skirt member and the atmosphere, said skirt member and said vessel providing a crotch formation at their juncture of increasing area in the direction of the bottom of said vessel, said insulation applied to substantially the whole internal surface of said skirt below a level at a distance from said juncture at least approximately ten times the thickness of said skirt member, said insulation 7 said crotch formation at said level, said insulation applied to the external surface of said bafile, the exposed surface of said skirt member, said bottom section and said baffle defining a chamber for heat flow by radiation and convection from one to the other of said vessel and said a skirt member.

7. A vessel as defined in claim 6 in which said weld deposit has a throat length substantially equal to the thickness of said skirt and a leg along said cylindrical section in theorder of twice the thickness of said skirt.

8. A vessel adapted in use to be subjected to elevated temperatures, comprising a shell having a bottom section of diminishing diameter, a supporting skirt member having an end around said vessel and positioned in contact with said bottom section, a weld metal deposit at said end uniting said skirt member to said bottom section, said weld metal deposit having a throat length substantiallyequal to the thickness of said skirt member and extending along said bottom section a distance in the '8 7 order ofltwi'ce the thickness of said skirt member, a depth of insulation forming a heat'barrier between said vessel and the atmosphere and said skirt member and the atmosphere,vv said skirt member and said bottom section providing a crotch formation at their juncture of increasing width in the direction of the bottom of the vessel, said insulation applied to substantially the Whole internal sur" face of said skirt below a level at a distance from said juncture at least '10 times but not in excess of 30 times the thickness of said skirt member, said insulation applied to substantially the whole internal surface of said bottom section inward of said skirt member to said level, and a bafile member supported from one of said bottom sections and said skirtfat said level to substantially close said crotch formation at said level, said insulation applied defining a chamber for heat flow by radiation and convection from one to another of said bottom section and said 20 skirt member.

No references cited.

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