Abstract:
A pressure vessel includes: a body comprising a cylindrical section; and a skirt comprising a hip, a leg, and a restraint. The hip formed with or attached to the body and has a profile for receiving an upper portion of the leg. The profile is oversized relative to the leg upper portion, thereby defining a radial clearance between the hip and the leg to account for thermal cycling of the body. The restraint fastens the hip and the leg while allowing limited movement between the hip and the leg to account for the thermal cycling.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention generally relate to a pressure vessel skirt for accommodating thermal cycling. 
     2. Description of the Related Art 
     Delayed petroleum coking is a process in which a petroleum fraction is heated to a temperature at which it thermally decomposes to provide a solid coke product and a hydrocarbon distillate product. In general, a liquid petroleum feed stock is first distilled until the lighter ends have been recovered and a heavy residuum remains. This heavy residuum of heated pitch and cat-cracked heavy or cycle oil is charged to the bottom of a structure called a coke drum. 
     Coke drums are vertically-disposed pressure vessels that are commonly twelve to thirty-two feet in diameter with a thirty to over eighty-feet tall cylindrical section. A coke drum typically has a conical bottom section to provide uniformity of support stresses in the structure, and is supported by a skirt that is welded to or near a transition knuckle between the cylindrical section and the conical bottom section. 
     In the coke drum, the heavy residuum is further heated to about one thousand degrees Fahrenheit and undergoes extensive and controlled cracking and coking under high-pressure conditions. A cracked lighter product rises to the top of the coke drum in a process called steam stripping and is drawn off. 
     A heavier product remains and cracks to coke, a solid, coal-like substance. The coke is usually purged with steam to remove any remaining volatile components. After the cracking and coking process is complete, quench water is introduced and high-pressure water jets are used to cut away and remove the coke. The water reduces the temperature in the drum to around two hundred degrees Fahrenheit or less before a new cycle begins. In order to increase production speed, the quenching operation is often done as quickly as possible. The cycle time for a vessel is typically 48 hours or less. 
     The heating and quenching cyclic operations of vessels such as coke drums cause deterioration of the structure over time. Vessels subjected to such extreme thermal cycling may experience a failure in the area where the support skirt is welded to the vessel shell. Cracking and structural failure of the support skirt, the vessel wall, and/or the attachment weld may occur. 
     A structure that is more resistant to cracking and other fatigue-related failures would be advantageous. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention generally relate to a pressure vessel skirt for accommodating thermal cycling. In one embodiment, a pressure vessel includes: a body comprising a cylindrical section; and a skirt comprising a hip, a leg, and a restraint. The hip formed with or attached to the body and has a profile for receiving an upper portion of the leg. The profile is oversized relative to the leg upper portion, thereby defining a radial clearance between the hip and the leg to account for thermal cycling of the body. The restraint fastens the hip and the leg while allowing limited movement between the hip and the leg to account for the thermal cycling. 
     In another embodiment, a method for retrofitting a pressure vessel includes: severing a lower portion of a skirt of the pressure vessel from an upper portion of the skirt; fastening or welding an adapter to the severed lowered portion; and supporting the pressure vessel by engaging the adapter with a profile of the skirt upper portion. 
     In another embodiment, a method for retrofitting a pressure vessel includes: severing a lower portion of a skirt of the pressure vessel from an upper portion of the skirt; replacing the severed lowered portion with a leg having a diameter less than the severed lower portion; and supporting the pressure vessel by engaging the leg with a profile of the skirt upper portion 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  illustrates a coke drum, according to one embodiment of the present invention. 
         FIGS. 2A-2C  illustrate skirts for the coke drum, according to other embodiments of the present invention.  FIG. 2D  illustrates a retrofit skirt for the coke drum, according to another embodiment of the present invention. 
         FIGS. 3A-3C  illustrate restraints for the skirts, according to other embodiments of the present invention. 
         FIG. 4  illustrates a segmented skirt, according to another embodiment of the present invention. 
         FIG. 5A-5D  illustrates bearings for the skirts, according to other embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a coke drum  1 , according to one embodiment of the present invention. The coke drum  1  may include a cap  2 , a cylindrical section  3 , a bottom  4 , a skirt  10 , an inlet, an outlet, and a utility port. A body  2 - 4  of the drum  1  and the skirt  10  may be made from a metal or alloy, such as steel. The steel may be plain carbon, low alloy, or stainless. Alternatively, the metal or alloy may be special, such as a high alloy steel, a nickel based alloy, aluminum, aluminum based alloy, titanium, or a titanium based alloy. The skirt  10  or one or more members thereof may be made from the same or different metal or alloy than the body  2 - 4 . The coke drum  1  may be vertically oriented. The cap  2  and bottom  4  may each be welded or fastened to the cylindrical section  3 . The cap  2  may be hemispherical, hemi-ellipsoidal, or torispherical. Alternatively, the cap  2  may be a blind flange. The bottom  4  may be conical or a polygonal approximation thereof. 
     Alternatively, the skirt  10  may be used with any other kind of pressure vessel subject to thermal cycling, such as a heat exchanger, boiler, or reactor. The pressure vessel may have a design pressure greater than, equal to, or less than fifteen pounds per square inch gage (psig). The pressure vessel may or may not be designed according to local jurisdictional code, such as ASME Section VIII. The pressure vessel may have two caps (or blind flanges) instead of a cap and bottom. The caps and cylindrical section may each include a respective flange assembly (not shown). Each flange assembly may include a flange welded to the cylindrical section  3 , a flange welded to the respective cap and bottom, a gasket (not shown), and fasteners (not shown) for connecting the two flanges, such as bolts or studs and nuts. Each flange assembly may be made from any of metals or alloys, discussed above. 
     A length to diameter ratio of the cylindrical section  3  may range from two to five. A thickness of the cylindrical section wall may be greater than or equal to one-half inch, such as ranging from three-quarters of an inch to three inches. The skirt  10  may be connected to the body  2 - 4  at or adjacent to a knuckle  5 , such as by welding. 
       FIGS. 2A-2C  illustrate skirts  10   a - c  for the coke drum  1 , according to other embodiments of the present invention. Each skirt  10   a - c  may include a hip  11  and a leg  12 . Each of the hip  11  and leg  12  may be a ring (or segmented approximation thereof, see  FIG. 4 ). The hip  11  may be formed integrally with or welded to the body  2 - 4  at or adjacent to the knuckle  5 . Alternatively, the hip  11  may be fastened to the body  2 - 4 . The leg  12  may be connected to a foundation  6  (i.e., footing or slab) via anchors, such as chairs  7  (only one shown). The leg  12  may be fastened to each chair  7 . The hip  11  may have a profile, such as a cup  13 , formed in a lower surface thereof for receiving an upper portion of the leg  12 . The hip  11  may be supported by resting on the leg  12 , such that the body  2 - 4  retains at least limited freedom to thermally expand and contract. The hip cup  13  may be oversized relative to the leg upper portion, thereby forming a radial clearance  14  therebetween. The radial clearance  14  may accommodate the thermal cycling of the body  2 - 4 . The hip  11  may be located about the knuckle  5  such that a mid-wall diameter Dv of the cylindrical section  3  is greater than, less than, or equal to a mid-wall diameter Ds of the skirt leg upper portion. 
       FIG. 2D  illustrates a retrofit skirt  10   d  for a coke drum having a conventional skirt  10   o , according to another embodiment of the present invention. The retrofit skirt  10   d  may be made using the conventional skirt  10   o . The conventional skirt  10   o  may be disconnected from the foundation  6 . The coke drum may be lifted and suspended. A lower portion of the conventional skirt  10   o  may then be severed from an upper portion of the skirt, thereby forming the retrofit hip. An adapter  15  may then be fastened  16  or welded  17  to the severed lowered portion, thereby forming the retrofit leg. The adapter  15  may have a diameter less than the conventional skirt in order to correspond to the retrofit hip. The retrofit leg may then be reconnected to the foundation  6 . The coke drum may then be lowered onto the retrofit leg such that the adapter  15  engages the profile of the retrofit hip, thereby supporting the coke drum. 
     Alternatively, instead of adapting the severed lowered portion, a new leg may be constructed having the diameter of the adapter  15 . 
       FIGS. 3A-3C  illustrate restraints  18   a - c  for the skirts  10   a - d , according to other embodiments of the present invention. Any of the skirts  10   a - d  may further include any of the restraints  18   a - c . The restraints  18   a - c  may further connect the hip  11  and the leg  12  to secure the body  2 - 4  against toppling due to wind loading. The restraints  18   a - c  may also provide for at least limited freedom of the body  2 - 4  to thermally expand and contract. Each restraint  18   a - c  may include sets of members spaced around the hip  11  and the leg  12 . 
     Each set of the first restraint  18   a  may include a pair of fasteners, such as bolts  19   a,b , one bolt  19   a  connected to the hip  11  and another bolt  19   b  connected to the leg  12 . The bolt connections may be welded or threaded. The bolt pair  19   a,b  may be connected by a band  20  to accommodate the radial movement between the hip  11  and the leg  12  while preventing vertical separation of the bolt pair  19   a,b.    
     Each set of the second restraint  18   b  may include a fastener, such as a bolt  21 , connected to the leg  12 , such as by a threaded connection, and a keyhole  22  formed through an outer wall of the hip  11 . A shaft of the bolt  21  may extend through the keyhole  22 . The keyhole diameter may be substantially greater than the bolt shaft diameter to accommodate radial movement between the hip  11  and the leg  12  while preventing relative vertical separation therebetween. 
     Each set of the third restraint  18   c  may include a fastener, such as a bolt  23 , connected to the hip  11 , such as by a threaded connection, and a bracket  24  connected to the leg  12 , such as by a weld. The bracket  24  may have a keyhole formed through an outer portion thereof. A shaft of the bolt  23  may extend through the keyhole. The keyhole diameter may be substantially greater than the bolt shaft diameter to accommodate radial movement between the hip  11  and the leg  12  while preventing relative vertical separation therebetween. 
       FIG. 4  illustrates a segmented skirt, according to another embodiment of the present invention. Any of the skirts  10   a - d  may include a segmented  11   a - c  hip and/or a segmented leg  12   a - c.    
       FIGS. 5A-5D  illustrates bearings  30   a - d  for the skirts  10   a - d , according to other embodiments of the present invention. The bearings  30   a - d  may minimize abrasion to the hip and/or leg resulting from the radial movement therebetween. Any of the skirts  10   a - d  may further include any of the bearings  30   a - d.    
     A first bearing  30   a  may include a liner  31  coating the hip cup  13  and a tip  32  coating a top of the leg  12 . Each of the liner  31  and the tip  32  may be made from an abrasion resistant material, such as Babbitt metal, ceramic, ceramic-metal composite (aka cermet), bi-metal, or lubricant infused alloy composite. The liner  31  and tip  32  may be made from the same or different material. If different, the liner material may be harder than the tip material and the tip may be made from a softer abrasion resistant material or a sacrificial material, such as a soft metal or alloy. The abrasion resistant material may be thermally insulative or conductive. If metallic, the liner  31  and tip  32  may be welded onto the respective hip cup  13  and leg top. If ceramic, the liner  31  and tip  32  may be painted and cured or thermally sprayed onto the respective hip cup  13  and leg top. Alternatively, the bearing  30   a  may include only either one of the liner  31  and the tip  32 . 
     A second bearing  30   b  may include a tip  33  received in a groove  34  formed in a top of the leg  12  and may or may not include the liner (not shown) coating the hip cup  13 . The tip  33  may be a ring or (or segmented approximation thereof) depending on the leg  12 . The tip  33  may form a sliding or interference fit with the groove  34 . The tip  33  may be made from the abrasion resistant material. 
     A third bearing  30   c  may include a tip  35  welded  36  to a top of the leg  12  and may or may not include the liner (not shown) coating the hip cup  13 . The tip  35  may be a ring or (or segmented approximation thereof) depending on the leg  12 . The tip  33  may be made from one of the metallic abrasion resistant materials. 
     A fourth bearing  30   d  may include a cap  37  disposed on a top of the leg  12  and may or may not include the liner (not shown) coating the hip cup  13 . The cap  37  may be a ring or (or segmented approximation thereof) depending on the leg  12 . The cap  37  may form a sliding or interference fit with the leg  12 . The cap  37  may be made from the abrasion resistant material. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.